cogeu cogeu d5.3 cogeu tvws transceiver integration

COGEU

FP7 ICT-2009.1.1

COgnitive radio systems for efficient sharing of TV white spaces

in EUropean context

COGEU D5.3

COGEU TVWS Transceiver Integration

Contractual Date of Delivery to the CEC: February 2012

Actual Date of Delivery to the CEC: 29th February 2012

Author(s): Tim Forde (TCD), Colman O’Sullivan (TCD), Linda Doyle (TCD), Rogério Pais

Dionisio (IT), Jose Carlos Ribeiro (IT), Paulo Marques (IT), Jonathan Rodriguez (IT), Filipe Alves

(IT), Pawel Kryszkiewicz (PUT) Participant(s): TCD, IT, PUT

Workpackage: WP5

Est. person months: 14

Security: PU

Nature: Report

Version: 1.0

Total number of pages: 39

Abstract: This deliverable reports on the work performed in COGEU tasks T5.2, T5.3 and T5.4. It describes the implementation and integration of the PMSE sensing, spectrum shaping, rendezvous technique and geolocation database access to form a COGEU TVWS transceiver which can be used to safely and efficiently access the available whitespace spectrum.

Keyword list: TVWS, Transceiver, SDR, OFDM, USRP, IRIS, Sensing, Spectrum Shaping, Rendezvous, Matlab, Labview, Geolocation database, PMSE

COGEU D5.3 – COGEU TVWS Transceiver Integration

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Executive Summary

This deliverable addresses and fulfils tasks T5.2 (integration of sensing and database access), T5.3 (design of spectrum shaping algorithms) and T5.4 (design of rendezvous techniques). It details the implementation and integration of algorithms for spectrum sensing, spectrum shaping and for rendezvous in a TVWS prototype transceiver. It also details the integration of the transceiver with the geolocation database. The bulk of the work that is described in this deliverable concerns very practical, laborious and engineering-oriented elements; software coding, hardware familiarization, lab and field testing. The main output of this deliverable is the prototype TVWS transceiver. The key conclusions of this deliverable are summarized here: TVWS Transceiver PMSE Sensing A Programme Making and Special Events (PMSE) sensing module was developed for the TVWS transceiver based on three sensing algorithms; Covariance Absolute Value (CAV), Blindly Combined Energy Detection (BCED) and Energy Detection (ED). The purpose of the sensing platform is to identify the central frequency and bandwidth of multiple wireless microphones (WMs) present in a DVB-T channel(s) indicated to the sensing module to be free by the geolocation database. This feature is crucial to allow the TVWS transceiver access the unused spectrum in a safe manner, using spectrum shaping techniques to allow coexistence between TVWS devices and PMSEs systems in the same DVB-T channel(s). The sensing module was implemented on the TVWS SDR using the National Instruments Labview system. Labview allows for ease of graphical programming combined with functionality that allows control of the USRP radios. The sensing module also makes use of the National Instruments (Ettus LLC) USRP WBX radio frontend daughter-boards which allow it to receive signals in the TV bands of interest. The sensing module allows the user to select a TV channel which has been indicated as being available for use (i.e. by looking up the TVWS geolocation database) and then to scan the channel for the presence of unknown local WMs that may be operating in the channel. The sensing module was evaluated in practice in a real setting; the CAV and BCED algorithms were found to out-perform the ED algorithm. This practical evaluation confirmed the simulation results. TVWS Transceiver Spectrum Shaping Spectrum shaping assists in protecting primary users transmissions in bands adjacent to COGEU TVWS transceiver transmissions. Two shaping mechanisms are employed in the TVWS plat form; windowing and cancellation carriers as they provide flexible spectrum shaping capabilities that suit a range of coexistence scenarios, i.e. PMSE or DVB-T neighbours. The spectrum shaping algorithms were integrated within the Iris SDR as Iris has been purposefully designed for the kind of runtime reconfiguration required by cognitive radio systems. As the shaping algorithms are contained within OFDM modulator and demodulator components, they are wholly integrated within Iris, and were therefore implemented largely in C++. The windowing algorithm was implemented wholly in C++, while the cancellation carriers algorithm currently employs the popular Linear Algebra PACKage (LAPACK) Fortran libraries for its linear algebra calculations. Following extensive testing, the shaping algorithms were successfully integrated into new feature-rich OFDM modulator and demodulator components to form part of the COGEU TVWS transceiver. A link comprising two USRP-enabled t ransceivers was set-up so that over-the-air transmission could be demonstrated.


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TVWS Transceiver Rendezvous The mechanism employed for rendezvous in the COGEU TVWS transceiver is cyclostationary signatures. Rendezvous enables Master and Slave TVWS devices to find each other in the unchannelised and dynamic TVWS spectrum. The cyclostationary rendezvous was integrated into Iris. Its functionality is spread across the COGEU OFDM modulator and demodulators, as well as the CycloSignatureDetector component and the rendezvous controller. The rendezvous module has been implemented in such a way that it allows for external control of the Master TVWS radios which is a necessary feature for the COGEU model. Taking input from the Geolocation database, or another entity, the centre frequency and bandwidth of operation can be dictated to a TVWS Master radio which will then embed the appropriate cyclostationary signatures into its transmitted waveforms at the appropriate frequencies.

Integration of Geolocation Database and TVWS Transceiver Radio Components

The database access, sensing and t ransmission modules of the COGEU TVWS transceiver were successfully integrated into a single transceiver involving both one and two host-laptops configurations.


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Table of Contents 1- Introduction ........................................................................................................................ 5

2- Integration of Spectrum Sensing ................................................................................. 6 2.1- Introduction........................................................................................................................................................... 6 2.2- COGEU S pectrum Sensing ............................................................................................................................... 6

2.2.1- Covariance-based detection....................................................................................6 2.2.2- Eigenvalue-based detection ....................................................................................6 2.2.3- Threshold computation ...........................................................................................7

2.3- Implementation..................................................................................................................................................... 7 2.3.1- Dependencies ........................................................................................................7 2.3.2- Coding, Interface....................................................................................................7 2.3.3- Control and Feedback ............................................................................................8

2.4- Integration and Evaluation – Algorithm Validation ................................................................................ 11 2.5- Summary .............................................................................................................................................................. 12

3- Integration of Spectrum Shaping ............................................................................... 13 3.1- Introduction......................................................................................................................................................... 13 3.2- COGEU S pectrum Shaping ............................................................................................................................ 13 3.3- Implementation................................................................................................................................................... 14

3.3.1- Dependencies ......................................................................................................14 3.3.2- Coding, Interfaces ................................................................................................15 3.3.3- Control and Feedback ..........................................................................................16

3.4- Integration and Evaluation ............................................................................................................................. 17 3.5- Shaping Applications ........................................................................................................................................ 18 3.6- Summary .............................................................................................................................................................. 21

4- Integration of Rendezvous ........................................................................................... 22 4.1- Introduction......................................................................................................................................................... 22 4.2- COGEU Rendezvous ......................................................................................................................................... 22 4.3- Implementation................................................................................................................................................... 23

4.3.1- Dependencies ......................................................................................................23 4.3.2- Coding, Interfaces ................................................................................................23 4.3.3- Control and Feedback ..........................................................................................25

4.4- Integration and Evaluation ............................................................................................................................. 27 4.4.1- Frequency acquisition of a fixed-bandwidth link ......................................................27 4.4.2- Unknown bandwidth link .......................................................................................27 4.4.3- Changing signal ...................................................................................................27

4.5- Rendezvous Applications ................................................................................................................................. 28 4.6- Summary .............................................................................................................................................................. 28

5- Integration of Geolocation Database and TVWS Transceiver ............................ 29 5.1- Introduction......................................................................................................................................................... 29 5.2- System setup ........................................................................................................................................................ 29 5.3- Interface................................................................................................................................................................ 31 5.4- Integration Testing ............................................................................................................................................ 31 5.5- Summary .............................................................................................................................................................. 33

6- Conclusions ..................................................................................................................... 34

References ............................................................................................................................ 36


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1- Introduction

This deliverable relates to the integration of aspects of a TVWS Transceiver prototype which has been developed by COGEU. This deliverable addresses and fulfils tasks T5.2 (integration of sensing and database access), T5.3 (design of spectrum shaping algorithms) and T5.4 (design of rendezvous techniques). It details the implementation and integration of algorithms for spectrum sensing, spectrum shaping and for rendezvous in a TVWS prototype transceiver. It also details the integration of the transceiver with the geolocation database. It should be noted that the bulk of the work that is described in this deliverable concerns very practical, laborious and engineering-oriented elements; software coding, hardware familiarization, lab and field testing. This work also involved two week-long workshops involving the relevant partners. The main output of the work described in this deliverable is the actual working prototype COGEU TVWS transceiver, early versions of which have been publicly demonstrated through 2011. The scientific and engineering basis for separate aspects of this system have been largely independently developed, and reported on, in other deliverables. An extensive study and simulation of several Programme Making and Special Events (PMSE) spectrum sensing algorithms was reported in Deliverable 4.2 [1]. Algorithms and systems for both spectrum shaping and rendezvous were reported in Deliverable 5.2 [18]. The focus of this deliverable is to document how the components have been integrated into a single transceiver and, where applicable, how they relate to each other within that transceiver. In Chapter 2 the development of the PMSE sensing module for the TVWS transceiver is described. The sensing module is based on three sensing algorithms; Covariance Absolute Value (CAV), Blindly Combined Energy Detection (BCED) and Energy Detection (ED). The sensing module was implemented on the TVWS SDR using the National Instruments Labview system. The sensing module also makes use of the National Instruments (Ettus LLC) USRP WBX radio frontend daughter-boards which allow it to receive signals in the TV bands of interest. Chapter 3 describes the implementation and integration of the spectrum shaping algorithms. Spectrum shaping assists in protecting PU t ransmissions in bands adjacent to COGEU TVWS transceiver transmissions. Two shaping mechanisms are employed in the TVWS plat form; windowing and cancellation carriers. As part of the COGEU TVWS transceiver, new feature-rich OFDM modulator and demodulator components were implemented in the run-time reconfigurable Iris SDR. The shaping algorithms are integrated into these components. In Chapter 4 the implementation and integration of the mechanism employed for rendezvous in the COGEU TVWS transceiver, cyclostationary signatures, is described. Rendezvous enables Master and Slave TVWS devices to find each other in the unchannelised and dynamic TVWS spectrum. The cyclostationary rendezvous was integrated into the Iris SDR. Its functionality is spread across the COGEU OFDM modulator and demodulators, as well as the CycloSignatureDetector component and the rendezvous controller. The rendezvous module has been implemented in such a way that it allows for external control of the Master TVWS radios which is a necessary feature for the COGEU model.

In Chapter 5 we detail the integration of each of the components described in the previous chapters in to a single TVWS transceiver. As noted, the PMSE sensing and geolocation database access for the TVWS transceiver was implemented in National Instruments Labview. The radio-chain was implemented separately on the Iris SDR platform. As a result, steps were necessary to integrate these two plat forms and facilitate data-passing between them. This chapter discusses these steps as they were performed, and explains the interfaces designed and deployed.


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2- Integration of Spectrum Sensing

2.1- Introduction

Spectrum sensing plays an important part of the COGEU TVWS transceiver’s ability to safely access spectrum while protecting the incumbent, but transient, PMSE users. Since DVB-T is quite stable and protected by the geo-location database access authorization, we are principally concerned with PMSE such as wireless microphone (WM) users. Extensive work has been carried out in WP4 on the exploration and identification of appropriate sensing algorithms. This chapter details the work that has been progressed to implement and integrate the sensing algorithms with the transceiver in a standalone fashion (related with T5.2).

2.2- COGEU Spectrum Sensing for PMSE

An extensive study and simulation of several sensing algorithms, reported in Deliverable 4.2 [1], have shown that advanced methods, in particular Covariance Absolute Value (CAV) [2] and Blindly Combined Energy Detection (BCED) [3], presented a superior performance compared to other algorithms, for PMSE detection. Their performances were measured against the Energy Detection (ED) algorithm [4], and confirmed that the higher complexity of the CAV and BCED algorithms results in significant sensing gains compared to ED alone. Metrics used to evaluate each algorithm are the probability of false alarm (PFA), the probability of detection (PD) and the Receiver Operating Characteristic (ROC).

2.2.1- Covariance-based detection

The statistical covariance matrices of signal and noise are generally different. Thus this difference is used in the proposed methods to differentiate the signal component from background noise, i.e. this technique is based on measuring the whiteness or correlation level of the covariance matrix. In practice, there are only a limited number of received signal samples. Hence, the detection methods are based on the sample covariance matrix,

𝑅𝑥 [𝑁𝑠] = �𝜆[0] ⋯ 𝜆[𝐿 − 1]⋮ ⋱ ⋮

𝜆[𝐿 − 1] ⋯ 𝜆[0]�, ( 2.1 )

where

𝜆[𝑙] =1𝑁𝑠

� 𝑥[𝑚]𝑥[𝑚 − 𝑙] , 𝑙 = 0,1, … 𝐿 − 1,

𝑁𝑠−1

𝑚=0

( 2.2 )

are the sample autocorrelations of the received signal 𝑥[𝑛] and L is the smoothing factor. Test statistics are constructed directly from the entries of the sample covariance matrix and generally are given as

𝑑 = (𝐹_1 (𝑟_𝑚𝑛 )) ⁄ (𝐹_2 (𝑟_𝑚𝑚 ) ) ( 2.3 ) where F1 and F2 are two functions and rmn are the elements of the sample covariance matrix Rx. There are many ways to choose the two functions. The CAV test statistic is,

𝑑𝐶𝐴𝑉 = � � |𝑟𝑛𝑚 |𝐿

𝑚=1

𝐿

𝑛=1

� |𝑟𝑚𝑚 |𝐿

𝑚=1

� ( 2.4 )

2.2.2- Eigenvalue-based detection

The eigenvalue-based detection algorithms compute the eigenvalues 𝜆1 ≥ 𝜆2 ≥ ⋯ ≥ 𝜆𝐿, from the covariance matrix ( 2.1 ). The BCED test statistic is:

𝑑𝐵𝐶𝐸𝐷 = 𝜆1 � 𝜆𝑚

𝐿

𝑚=1

� ( 2.5 )


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2.2.3- Threshold computation

PMSE signals are detected, comparing the test statistics from CAV or BCED with a threshold. Threshold can be computed theoretically using the following equation for CAV [5],

𝑇𝐻𝐶𝐴𝑉 =1 + (𝐿 − 1)� 2

𝑁𝑠𝜋

1 − 𝑄−1�𝑃𝑓𝑎 ��2𝑁𝑠

( 2.6 )

and for BCED [6] as:

𝑇𝐻𝑀𝐸𝑇_𝐵𝐶𝐸𝐷 =��𝑁𝑠 + √𝐿�

2

𝑁𝑠⎝

⎜⎛

1 +��𝑁𝑠 + √𝐿�

23

(𝑁𝑠𝐿)16

𝐹2−1�1 − 𝑃𝑓𝑎�

⎠

⎟⎞

( 2.7 )

From simulation results, we observe poor performance of the sensing algorithms, using thresholds computed according to ( 2.6 ) and ( 2.7 ).Therefore, we changed the methodology and compute the threshold based on a heuristic method, as described in [1]:

1. Compute the test statistic of the sensed channel, with no primary user signal (noise only channel).

2. Repeat the measurement and create a histogram.

3. Compute the complementary cumulative density function (CCDF).

4. From the CCDF, search for the threshold value associated with the desired probability of false alarm (PFA).

2.3- Implementation of the PMSE sensing algorithms

2.3.1- Dependencies

The sensing module relies on a software-defined radio (SDR), a GPS device and a host PC. The SDR platform is based on USRP2 hardware prototyping [7]. The USRP2 is supported by a WBX daughterboard. WBX daughterboard is a two antenna module (TX/RX, RX2) tunable from 18 MHz to 2.2 GHz. The same WBX daughterboard may be used for data transmission or reception. During sensing, the frequency scanning is fast enough to allow frequency hopping and sensing in less than 100 ms. The GPS device is independent and is connected to the host PC by a Bluetooth connection. Since PMSE signals presence is not guaranteed in the demonstration sites, commercial tunable FM wireless microphones are also used, as primary users. This way, we can define a variety of sensing scenarios and measure the performance of sensing algorithms, for indoor and outdoor tests, under different propagation conditions. Wireless microphones operate in predefined TV bands of 30 MHz bandwidth, sub-divided in 24 channels.

2.3.2- Coding, Interface

Labview, from National Instruments, was used to program the software application for PMSE sensing and interact with the hardware used for sensing. Labview combines a graphical programming language with the capability to create user-friendly user interfaces, which makes it a preferred choice compared with other software solutions. This particular combination of hardware and software was chosen due to the recent development of USRP2 drivers for Labview [8]. Figure 1 shows a diagram of the sensing platform.


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Labview has many pre-defined objects and functions for math, flow control, conditional or Boolean operations, among many others. However, to increase speed and reduce complexity of the program, all sensing algorithms (CAV, BCED and ED) are coded in C++ and integrated as new Labview functions.

Figure 1 - Structural and functional diagram of the sensing tool.

2.3.3- Control and Feedback

The sensing program is divided in three functional pages:

1. Pre-configuration

This page is reserved for parameters that are only adjusted during start up, “e.g.” enable or disable GPS receiver, FFT parameters, or input parameters for blind sensing algorithms.

2. Setup interface

This is the main setup page. As shown in Figure 3, the user may define operational modes: “Standard operations” when sensing is done for all available channels, or “ROC operation” (ROC- Receiver Operation Characteristic, i.e. Probability of Detection vs. Probability of False Alarm) , when sensing is made only for the selected channel, varying the PFA (Probability of False Alarm). Figure 2a and Figure 2b shows a functional diagram for each method, respectively.

In normal operation, sensing is done from a list available channels, sent by the geolocation database. Here, the PFA must be entered by the user. If wireless microphones are present in one or more channels, they must be marked in the “Wireless Microphone Booking” bar (point and click over the green channel indicator, as depicted at the bottom of Figure 3).

a)


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b)

Figure 2 – Flux diagram of sensing for a) Standard operations and b) ROC operations.

The ROC operation is performed using a default vector of PFA values. The vector comprises a set of 8 values logarithmically spaced from 0.1% to 22%. Thresholds are defined using the method described in [1] or can be loaded from a previous saved file. The measurement results are saved in spread sheet files for post-processing.

From the setup interface, the user can also define sensing parameters, such as detection threshold or sensing time. Following an initial configuration stage, the GUI triggers the communication process between the host PC and the USRP2. Local coordinates are acquired from a GPS receiver (if no GPS signal is available, coordinates can be inserted manually or using A-GPS, as described in Chapter 5). Using the Google Maps API, a map is displayed, centred on the sensing device location. The preferred coverage area can be simulated in the map by introducing the coverage radius. After a query from the DVB-T geo-location database, a list of all available TV channels is received and displayed as LED indicators with different colours and symbols: red (cross symbol) means that the channels is already occupied by a DVB-T channel, and green (check symbol) means that the channel is free of DVB-T signals. Each channel reserved for PMSE usage is represented by a WM symbol.

Figure 3 - Setup interface of the PMSE sensing test platform.

3. Sensing interface

After pressing the RUN button from the setup interface, the USRP starts the threshold computation for each sensing algorithm, using a heuristic method as described in [1]. The operation ends after 1000 runs. This operation must be done without any PMSE signal present. Next, USRPs are ready to start sensing on free channels specified by the geo-location database.


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For each sensing algorithms, the result of sensing is compared to the respective threshold. If the result is above the threshold and if a WM is booked in that channel, status is set to ‘detection’ (colour black), and if there’s no previous booking of a WM, then status is set to ‘false alarm’ (colour yellow). On the other hand, i f the result from sensing is below the threshold and there is a WM booking for that channel, status is set to ‘miss detection’ (colour red), if there is no booking of a WM, the status is ‘free channel’, (colour green), as depicted in Figure 4.

Figure 4 – LED bar control logic.

This method is continuously repeated and produces statistical results, dependent on preliminary information about any WM booking on free channels, i.e. without DVB-T signal: If a PMSE is booked for one channel, the measurement results for that channel, after sensing, will be ‘detection’ or ‘miss detection’. Otherwise, if no PMSEs are booked for that channel, measurement results will be ‘free channel’ or ‘false alarm’. All results from measurements are presented in real time bar graphs and also saved in a spreadsheet file for post-processing. Table 1 summarises the colour code used and their meaning.

Table 1 - Colour code used to identify sensing results

Colour Status Green Free channel Yellow False alarm Red Miss detection Black Occupied Channel Grey Occupied with DVB-T

The sensing GUI is represented in Figure 5. The sensing interface automatically presents the power spectrum for each DVB-T channel sensed, the total sensing time, the channel’s number being sensed, the number of samples acquired, the thresholds and ratio values from each algorithm, and the SNR, which is estimated as described in Deliverable 4.2 [1].


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Figure 5 - GUI of the PMSE sensing testbed with 3 algorithms implemented in a USRP2 (SDR

commercial platform).

2.4- Integration and Evaluation – Sensing Algorithms Validation

We tested the functionality of the sensing platform on the floor of a university building. The primary system (PMSE) is located inside an empty auditorium, as shown in Figure 6(a). Sensing was done in two distinct places: inside the library (L1) and outside the school walls (L2), with non-line-of-sight propagation between the sensing device and the primary system. The distance between both locations and the wireless microphone was 45m. The threshold of each sensing algorithms was measured with a heuristic method described in [1]. After the WM is switched on, the platform is programmed to automatically sense a DVB-T channel during one hour, with sensing time of 100 ms. This process is repeated several times, depending on the probability of false alarm (between 1% and 22%), the number of channels to be sensed and the WM operation mode (mute or soft speaker mode). Figure 6(b) and Figure 6(c) present ROC for locations L1 and L2, respectively, and show that WMs in silent mode are easier to detect. This is due to the high peak correlation of the FM carrier without a modulation signal. Also, there is a significant improvement in the PD in all scenarios and locations, using the proposed detection algorithms instead of ED algorithm. These measurements confirm the simulation results from [1].


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a)

b)

c)

Figure 6 - a) Building plant where field measurements were realized; Measured ROC curves from: b) locations L1 and c) Location L2.

2.5- Summary

A Programme Making and Special Events (PMSE) sensing module was developed for the TVWS transceiver based on three sensing algorithms; Covariance Absolute Value (CAV), Blindly Combined Energy Detection (BCED) and Energy Detection (ED). The purpose of the sensing platform is to identify the central frequency and bandwidth of multiple wireless microphones (WMs) present in a DVB-T channel. This feature is crucial to allow the TVWS transceiver access the unused spectrum in a safe manner, using spectrum shaping techniques to allow coexistence between TVWS devices and PMSEs systems in the same DVB-T channel. The sensing module was implemented on the TVWS SDR using the National Instruments Labview system. Labview allows for ease of graphical programming combined with functionality that allows control of the USRP radios. The sensing module also makes use of the National Instruments (Ettus LLC) USRP WBX radio frontend daughter-boards which allow it to receive signals in the TV bands of interest. The sensing module allows the user to select a TV channel which has been indicated as being available for use (i.e. by looking up the TVWS geolocation database) and then to scan the channel for the presence of unknown WMs that may be operating in the channel. The sensing module was evaluated in practice in a real setting; the proposed detection algorithms were found to out -perform the ED algorithm. Field trials confirmed the simulation results.

Wireless Microphone (Tx) Receiver (Rx) Sensing device (L)

Tx Rx

L1

L2


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3- Integration of Spectrum Shaping

3.1- Introduction

The shaping algorithms that are employed in the COGEU TVWS plat form were introduced and described in D5.2 [18]. Spectrum shaping assists in protecting PU (Primary User) transmissions in bands adjacent to COGEU TVWS transceiver t ransmissions. A new spectrum shaping method was required as typical digital or analogue filtering does not fulfil all the requirements of the COGEU TVWS Transceiver. The two main features required are flexibility (i.e. the ability to comply with various restrictions on and rapid changes to the spectrum mask, and to respond to changes in the positions of primary users to be protected) and low computational complexity. A short summary of the spectrum shaping mechanism described in D5.2 is recapped in this chapter. This chapter describes how the spectrum shaping algorithms were implemented and integrated into the transceiver.

3.2- COGEU Spectrum Shaping

Two shaping mechanisms are employed in the TVWS platform; windowing and cancellation carriers. Windowing involves using pulse-shaping to reduce the aggregate out-of-band emissions (improving the roll-off) of the transmitted waveform. This reduces interference to transmissions in adjacent frequencies. It cyclically prolongs OFDM symbols in the time-domain, using a raised-cosine window to multiply the β last samples of extended symbols. (See Figure 7.) This reduces the incidence of rapid value changes between consecutive OFDM symbols. In the frequency domain it manifests as strong suppression of sidelobes more distanced from subcarrier mainlobe. Though windowing comes with a throughput penalty, this can be minimised by allowing consecutive symbols to overlap over β samples.

Figure 7 Representation of windowing where N is the orthogonality period-length, Ncs is the cyclic suffix length, Ncp is the cyclic prefix length and β is the hanning window extension length.

Cancellation carriers on the other hand provide for a more immediate reduction in the interference caused adjacent to OFDM subcarriers. It works by dedicating a set of edge data-carriers to cancellation of OOB radiation. Carefully optimised values are applied to these subcarriers, resulting in a reduction in the aggregate sidelobes of the waveform’s data carriers in a chosen optimisation region (i.e. nearby PU bands). A graphical example is shown in Figure 8. The resulting increase in roll-off, allows transmissions to take place closer in frequency to other devices, without increasing the interference impact to them. This technique may also be used for non-contiguous transmission, in which a notch in the waveform is created by the nulling of subcarriers. In general, a naïve implementation of such a notch would simply null the carriers. However, distortions owing to intermodulation effects, the combination of out-of-band interference from both sides of the notch, and imperfect transmitters and receivers lead to increased interference in the notch, particularly close to the edge. Applying cancellation carriers to the edge of the notch reduces these effects, mitigating the interference to any transmissions in the notch, such as a PMSE device.

NCP N NCS

β β


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Figure 8 - Diagram of the insertion of one CC at the edge of a data subcarrier block.

Windowing provides relatively strong OOB suppression of components far from the transmission band of the shaped signal, while Cancellation Carriers (CCs) provide stronger suppression closer to the shaped signal edge. This means that a combination of both best serves the COGEU TVWS transceiver, providing it with flexible spectrum shaping capabilities to suit a range of coexistence scenarios. Both mechanisms were motivated and described in greater detail and with recourse to their mathematical forms, in D5.2 [18].

3.3- Implementation

3.3.1- Dependencies

As we described in D5.1 [19], we use a highly reconfigurable SDR to implement and prototype new radio systems. The key objective of the Iris system is that it enables run-time reconfiguration of the node, which contrasts with the design objectives of both GNU Radio and SCA; although GNU Radio now too supports run-time reconfiguration, this was not a core design feature in its initial architecting. Runtime reconfigurability permits an IRIS node to seamlessly reconfigure itself in response to changes in its environment. Iris contains existing simple OFDM modulator and demodulator components in its library. As part of the COGEU TVWS transceiver, new feature-rich OFDM modulator and demodulator components in Iris are implemented. The shaping algorithms (along with the rendezvous features that will be discussed in Chapter 4) are integrated into these components. Similar to the existing simple OFDM components, they are dependent on data input as raw Bytes from some other Iris component(s). In order to effect transmission of the OFDM signal, the modulator is dependent on Iris’s Scaler component to scale the power level of the samples, and the UsrpTx component to interface with the USRP hardware and transmit the samples over the air. Example transmit and receive chains involving simple OFDM modulator and demodulator components, and the additional components required for over the air transmission, can be seen in Figure 9. The modulator and demodulator are physical-layer elements that are agnostic of the system in which they reside. As a result, they may be fed data by a range of higher layer components, from a simple file or media reader to a selection of MAC layer elements. The behaviour of the overall system is highly dependent on which components are used to feed the OFDM components.

n-2 n-1 n n+1 n+2 n+3 n+4 n+5

0

Subcarrier index

Spe

ctru

m

CANCELLATION SUBCARRIER

OPTIMIZATION RANGEDATASUBCARRIERS


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3.3.2- Coding, Interfaces

As the shaping algorithms are contained within the OFDM modulator and demodulator components, they are wholly integrated within Iris, and therefore implemented largely in C++. The windowing algorithm is implemented wholly in C++, while the cancellation carriers algorithm currently employs the popular Linear Algebra PACKage (LAPACK 1

) Fortran libraries for its linear algebra calculations.

Figure 9 - Iris Simple OFDM Modulator and Demodulator shown with the minimum number of components required for successful over-the-air transmission

The modulator component requires containers of Bytes as input. This input is the raw data to be modulated and t ransmitted over the air. Its output is containers of complex baseband (IQ) samples, formatted as C++ complex<float> datatype. Each container output is a complete OFDM frame comprising a set of OFDM symbols, preceded by a WiMax 2

-like preamble. This format was chosen to demonstrate the TVWS transceiver’s ability to operate using methods similar to real world commercial solutions.

The demodulator requires containers of complex<float> as input. It searches through each container of samples, using autocorrelation as per the Schmidl-Cox preamble synchronisation method to attempt to find the preamble of a t ransmitted OFDM frame [21]. The preamble synchronisation method is computationally lightweight, and therefore allows for large gaps between transmissions with little waste of processing resources by the demodulator. If a preamble is successfully located, the transmitted data following it is recovered and demodulated. The successfully recovered data is output as a container of Bytes. Each OFDM symbol within an OFDM frame is constructed by following a series of steps, as indicated in Figure 10 below. The windowing and carrier cancellation shaping algorithms are each separate steps added to this process, and are clearly indicated in the Figure 10. Carrier cancellation applies the required number of cancellation carriers while still in the frequency domain, while the windowing extension is performed once the symbol has been transformed to the time domain.

1 http://www.netlib.org/lapack/index.html 2 http://www.wimaxforum.org


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The shaping algorithms form part of a larger OFDM modulator and demodulator component, but their unique functionality is controllable through Iris parameters. The windowing algorithm accepts extension length (corresponding to β Figure 7.) as a parameter at the modulator side, which sets the length of the windowing extension. A larger extension length leads to a reduction in the amount of out-of-band emission, at a cost of reduced throughput. (This was discussed in D5.2.) At the demodulator side, extension length may be set as a manual parameter, or alternatively, an automatic mechanism may be enabled to attempt to detect the extension length.

Figure 10 - Steps involved in creating an OFDM symbol in the Modulator component, with both shaping algorithms' steps clearly indicated

The carrier cancellation algorithm has several parameters. On the modulator side, the number of cancellation carriers to be employed can be chosen and varied using the numcancellationcarriers parameter. This number can be any value greater than or equal to zero, as long as there are a sufficient number of idle subcarriers to place cancellation carriers on each side of every block of data subcarriers. The ccsoptimisationfactor parameter sets the optimisation factor used to balance the power level of these carriers with degree of suppression they provide. A value of zero indicates cancellation is the primary goal, and power on the CCs need not be limited. An increased optimisation factor means a stricter limit on the power of the CCs. Also variable is the range of active subcarriers, set via the activeBands parameter. The effects of choices of these parameters on the carrier cancellation algorithm were also discussed in D5.2 [18]. All these parameters may be set at radio initialisation-time via the Iris radio configuration xml. At runtime, they may be reconfigured either by modifications to the xml, or by internal reconfigurations triggered by Iris controllers. At the demodulator, the carrier cancellation algorithm requires only the details of the activeBands in order to function correctly. This may either be set manually by a user at runtime, or incorporated as part of the rendezvous mechanism which is discussed in Chapter 4.


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Table 2 - Iris OFDM modulator and demodulator parameters relating to shaping algorithm functionality, assuming a 256-bin FFT, with 200 active subcarriers, 192 for data and 8 pilots.

Parameter Name Purpose Value

range Modulator Windowing shapingExtLength Set windowing extension length 0 – 128 Cancellation carriers numCancellationCarriers The number of cancellation carriers to be

applied to each signal edge ≥ 0

ccOptimisationFactor

A factor for balancing the level of power on cancellation carriers with the degree of OOB suppression they provide

Usually 0 ≤ x ≤ 1

activeBands Indicate the edges of each active part of the signal

subcarrier pairs [-100,100]

Demodulator Windowing shapingExtLength Manually set windowing extension length 0-128 autoDetectShapingExtLength Whether to attempt to automatically

detect the windowing extension length true/false

Cancellation carriers activeBands

Indicate the edges of each active part of the signal

subcarrier pairs [-100,100]

These shaping related parameters are summarised in Table 2 above. Both modulator and demodulator also possess a number of parameters relating to standard OFDM functionality, including the ability to output debug information at each step of the process shown in Figure 10. This allows confirmation of the effects of each step and analysis in an environment such as Matlab.

3.4- Integration and Evaluation

In D5.1, Section 6.1 discussed the standard implementation procedure for algorithms leading to components in Iris. Figure 19 from that deliverable is reproduced here with the addition of new Phases 4 & 5. (See Figure 11, below.) Phase Signal Generation Signal Transmission &

Channel Signal Analysis

1 MATLAB MATLAB channel models MATLAB 2 MATLAB Signal Generator USRP MATLAB 3 MATLAB Signal Generator USRP Iris 4 Iris Direct Connection Iris 5 Iris Iris AWGN Channel Iris 6 Iris USRP USRP Iris

Figure 11 - Typical development path in Iris (Reproduced from D5.1, Figure 19)

Initial testing of the algorithms took place in Matlab. Spectrum shaping performance in terms of power spectral density and their influence on other system parameters such as throughput loss, PAPR increase and SNR decrease, was measured. The results were reported in D5.2. Subsequent to the initial implementation of the algorithms in Matlab, each step was integrated into the OFDM modulator and demodulator separately, and tested on a single host in Iris, as part of a simple radio chain set up as in Figure 12, corresponding to Phase 4 in Figure 11 above. In particular, use of the Visual Studio C++ debugger allowed location and correction of a number of bugs. Next the components were tested using the Iris Channel component, which simulates an AWGN channel (Phase 5 above). In each case, data


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was streamed from FileReader components, and after demodulation written to FileWriter components for verification. At this point any indexing mismatching bugs at the receiver were discovered and corrected.

Figure 12 - A simple test radio chain in Iris, to test modulator and demodulator behaviour

Next, they were tested over the air using USRPs. Two USRPs, equipped with XCVR2450 daughterboards were used, so that transmission could take place in the 5GHz ISM band. The USRPs were set up 1m apart. Initial transmissions also sourced data from a FileReader, and wrote received data using a FileWriter, to allow manual inspection of correct operation. For verification of large-scale packet transmissions, CRC checksums were added to allow aggregation of performance for the transmissions of large numbers of packets. Finally, a simple ARQ based MAC was used to test transceiver communications. Each Data packet was responded to by a short Acknowledgement packet. In this case, each host contained both a receive and a transmit radio chain, similar to the example configuration depicted in Figure 9. This allowed for testing of unforeseen behaviour when operating as a transceiver. This is a worthwhile test to perform, as when operating in both transmit and receive mode, a USRP may int roduce unexpected data into components, causing behaviour that may be difficult to predict in advance. An example of one such problem is a condition in which the switch from transmit to receive mode in the USRP may introduce samples into the receive chain of unusually low magnitude. These samples disrupt the moving average calculations used as part of the preamble detection process, causing it to go into an irrecoverably bad state, from which it must be forcibly reset. In addition to confirming functionality of the link, over the air t ransmissions also allowed visual inspection on a Spectrum Analyser of the reduction in out-of-band emission caused by activation of the shaping features of the modulator. The following general observations were also made as part of the Integration and Evaluation process, which will feed into D5.4:

• The OOB radiation attenuation by Cancellation carriers is strongly affected by the LAPACK library calculation accuracy, specifically in its calculation of the pseudoinverse of a matrix. The strongest improvement was observed in version 3.2.1, which employs a new SVD decomposition algorithm.

• The spectrum analyser parameters must be adjusted carefully as the resolution bandwidth filters are to be narrower than generated spectrum notch. Moreover, the reference level should not allow for spectrum analyser mixer inter-modulation (when the input signal is too strong), but must be kept above noise level.

• The limited linearity of USRP frontend causes uplift of intermodulation distortion tones. In initial

experimentation the approach to circumventing this has been to avoid intermodulations “hiding” the effect of spectrum shaping by making certain all transmissions are at low enough power to utilise the linear region of the USRP PA. Another possibility would be to linearize PA characteristic.

3.5- Shaping Applications

The completed OFDM modulator and demodulators incorporating shaping algorithms can be used to transmit and receive waveforms that induce lower out-of-band emissions, and hence lower interference to transmissions taking place on nearby frequencies. This allows it to coexist with other devices while minimising impact to them, and is a distinct advantage for transceivers such as the COGEU TVWS


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transceiver, operating in an environment in which neighbouring device transmissions must be protected from undue interference. An example of this is the demonstrator which was set up as in Figure 13 below. Three Iris nodes were used, two comprising a transmit-receive pair, to emulate an active primary user link. An audio stream was transmitted to allow the effects of any nearby interference to be immediately noticed. The third node used an OFDM modulator capable of windowing and carrier cancellation shaping. This secondary user (SU) node transmitted in a closely neighbouring frequency, both with and without shaping features enabled. The result was that a sufficiently close interfering signal with shaping disabled would destroy the PU signal (rendering the audio stream completely unintelligible), while with shaping enabled allowed it to remain unaffected. Pictures in Figure 14 show the output of a Rohde & Schwarz FSVR spectrum analyser, set to monitor the transmissions produced by the setup in Figure 13. They show that the difference in interference effects caused by an otherwise identical waveform with shaping enabled vs. shaping disabled can be seen very clearly visually. Note that the significantly wider and higher power signal on the left is the SU transmission, and the narrower, lower-power signal to the right is the PU link.

Figure 13 - Example demonstration using three Iris nodes, each comprising a host and USRP, to emulate a primary user link. This link neighbours a secondary transmitter using the COGEU TVWS OFDM modulator allowing demonstration of the effects of the waveform with and without shaping on a neighbouring link.


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Figure 9 - Spectrum Analyser Screen Capture for the scenario detailed in Figure 13, with shaping disabled in the upper figure, and shaping enabled in the lower figure. The left-hand waveform is the Secondary (SU) Interferer, and the weaker right-hand waveform is the Primary (PU) transmission. Note the significant improvement in SU transmission roll-off with shaping enabled.


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3.6- Summary

Spectrum shaping assists in protecting primary users transmissions in bands adjacent to COGEU TVWS transceiver transmissions. Two shaping mechanisms are employed in the TVWS plat form; windowing and cancellation carriers as they provide flexible spectrum shaping capabilities that suit a range of coexistence scenarios, i.e. PMSE or DVB-T neighbours. The spectrum shaping algorithms were integrated within the Iris SDR as Iris has been purposefully designed for the kind of runtime reconfiguration required by cognitive radio systems. As the shaping algorithms are contained within OFDM modulator and demodulator components, they are wholly integrated within Iris, and were therefore implemented largely in C++. The windowing algorithm was implemented wholly in C++, while the cancellation carriers algorithm currently employs the popular Linear Algebra PACKage (LAPACK) Fortran libraries for its linear algebra calculations. Following extensive testing, the shaping algorithms were successfully integrated into new feature-rich OFDM modulator and demodulator components to form part of the COGEU TVWS transceiver. A link comprising two USRP-enabled t ransceivers was set-up so that over-the-air transmission could be demonstrated. This functionality was publicly demonstrated, at varying degrees of maturity, in 2011; firstly at the 2011 COGEU 1st audit and secondly at the FUNEMS conference in Warsaw, June 2011.


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4- Integration of Rendezvous

4.1- Introduction

The mechanism employed for rendezvous in the COGEU TVWS transceiver is cyclostationary signatures, introduced in D5.2.[18]. While all man-made RF signals have determinable cyclostationary characteristics, a cyclostationary signature is an additionally induced cyclostationary feature or set of features in a waveform, which can be easily detected. This mechanism allows receivers to determine the centre frequency and bandwidth of their intended transmitter, without a-priori knowledge of them. Only one parameter, the cyclic frequency must be agreed in advance, which can be used as a group identifier.

4.2- COGEU Rendezvous

The COGEU rendezvous mechanism employs cyclostationary signatures, as part of the OFDM modulator and demodulator. By transmitting identical data on pre-agreed groups of subcarriers (referred to from here on as mapped subcarriers) cyclostationary features can be induced. Each group of mapped subcarriers creates a distinct cyclostationary feature. The distance between the mapped subcarriers determines the cyclic frequency at which the feature will manifest, while the location of the mapped carriers in linear frequency (the standard frequency measurement considered in RF communications) determines the linear frequency at which the signature will produce a strong correlation peak. A computationally lightweight cyclostationary signature detector may sweep across large ranges of frequencies, correlating for peaks at the specific pre-agreed cyclic frequency of the signature, and will find a strong peak or peaks at the location(s) of the signature(s). In order to determine both the centre frequency and bandwidth of the unknown signal, multiple cyclostationary signatures must be employed. The bandwidth of any possible OFDM waveform is divided into blocks of 64 subcarriers/ FFT bins, and each contains a set of four mapped subcarriers. A transmission will consist of an integer number of such blocks. Then a two-step process for detection of carrier frequency and bandwidth may be employed in a receiving COGEU TVWS transceiver. Step 1 Rough carrier frequency acquisition: The receiving device switches to the lowest possible frequency in the range in which the transmission is expected, and begins to receive. Sets of received samples are processed by the cyclostationary signature detection algorithm, which shifts them by the known cyclic-frequency, and correlates them with the same sets unshifted. This will result in a peak in the event of a cyclostationary signature being present. (See D5.2 for a more detailed explanation of this process.) Each time a number of consecutive samples at a given frequency fails, the frequency is incremented upwards, and samples are taken at the new higher frequency. The result is that the receiver sweeps across frequencies, until the detector successfully locates a peak indicating the presence of the desired signal at or around the current frequency. (See Figure 15.) Step 2 Bandwidth acquisition: Once the carrier frequency has been roughly determined, the frequency of the receiver is reconfigured to position the peak detected in Step 1 in the leftmost portion of the received signal. At this point, further samples are taken and processed by the signature detection algorithm, which counts the total number of detected peaks found within the complete bandwidth. Each individual peak is the signature resulting from the mapped subcarrier set of an individual bandwidth unit in the transmission. Provided the width of a bandwidth unit’s transmission is pre-agreed, counting the number of peaks gives the bandwidth of transmission. With this knowledge, the receiver may begin to receive and demodulate the signal as normal. (See Figure 16.) (Optional) Step 3: As a feature to allow robustness and resilience in the presence of an error in the signature detection process, each individual bandwidth unit can be demodulated independently as a 64 subcarrier OFDM waveform, which can include header information to correct a mis-measurement of the centre frequency and/ or bandwidth of the overall signal. This allows MAC-layer correction of errors in the physical layer signature-detection process.


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Figure 10 - Steps involved in the frequency acquisition process.

Figure 11 - Steps involved in the bandwidth acquisition process.

4.3- Implementation

4.3.1- Dependencies

Similar to the Shaping mechanisms discussed in Chapter 4, the rendezvous mechanisms must also be integrated into the feature-rich OFDM modulator and demodulator Iris components implemented as part of the COGEU TVWS transceiver. As shown in Figure 9 earlier, these components depend on existing Iris components to provide data input, a Scaler component to scale output for transmission, and UsrpTx and UsrpRx components for interface to USRP hardware for transmission and reception respectively. Unlike the earlier shaping modifications, the rendezvous mechanism also requires a CycloSignatureDetector component to contain the cyclostationary signature detection algorithm, to signal the need for frequency reconfigurations, and to pass successfully detected bandwidth information to the demodulator. It also requires a controller to effect these frequency and bandwidth configurations. (The role of controllers in Iris and their relationships with components and parameters was discussed in D5.1.) In a receive chain, the controller will be required both to coordinate the actions of the demodulator and the CycloSignatureDetector, and to issue frequency and bandwidth reconfiguration instructions to the USRP. The frequency acquisition and bandwidth detection elements of the mechanism are not strictly independent, and if one of these parameters is already known, then that portion of the mechanism may be disabled to improve performance. The transmit and receive chain of a radio employing the COGEU rendezvous mechanism are depicted in Figure 13 and Figure 14, respectively.

4.3.2- Coding, Interfaces

This mechanism runs in Iris. In the transmit chain it runs solely in the OFDM modulator component. In a receive chain its functionality is spread across the OFDM demodulator, CycloSignatureDetector component and CycloSignature controller. If the parameters of the transmitter are to change at run time however, a further controller will be required in order to effect these changes.


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Figure 12 - Transmit chain of a radio using the COGEU rendezvous mechanism. Components are highlighted if they are directly involved in the mechanism. Parameters changed by controllers are shown as red arrows.

Figure 13 - Receive chain of a radio using the COGEU rendezvous mechanism. The controllers and components involved in the mechanism are highlighted. Parameters changed by controllers are shown as red arrows, and events subscribed to by controllers shown as green arrows.

As explained in Section 4.2, the OFDM modulator takes the data to be modulated and t ransmitted as input, in the form of a container of Bytes. Its output is a container of complex<float>, each container comprising a complete OFDM frame, shaped according to the parameters detailed in 5.2.3, and with a cyclostationary signature embedded to allow rendezvous. In order to generate the signature, there is a modification to the “Map to Subcarriers” step as shown in Figure 10 earlier, which depicted the steps involved in creating an individual OFDM symbol. The OFDM demodulator accepts containers of complex<float> as input, and where OFDM frames are successfully found, it outputs a container of Bytes which should be identical to the original container of Bytes that was modulated. The CycloSignatureDetector component (as can be seen in Figure 18) should be positioned before the OFDM demodulator in the receive chain. It requires containers of complex<float> as input, and will pass them through unmodified as output.


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Finally, in order to issue reconfiguration instructions, one or more controllers will be required. As their input, actions and operations tie-in with control and feedback of the other components, they will be explained in detail in the next section.

Figure 14 - Steps for OFDM symbol c reation in a modulator, with the placement of the carrier mapping step highlighted, to show the position of the cyclostationary signature embedding in the modulator.


The Rendezvous features comprise a combination of the OFDM modulator, demodulator, CycloSignatureDetector and one or more Controllers. The Controllers act to control system behaviour by changing and measuring Iris Parameters, and responding to Iris events from them.

4.3.3.1 Component Parameters The rendezvous element of the OFDM modulator requires the number of bandwidth units to be set via the numBandwidthUnits parameter, which sets the bandwidth of the signal to be transmitted. It also allows the modulator to be enabled or disabled via an “enable” parameter, which prevents wasteful processing of samples when a signature is not yet detected, and hence a signal is unlikely to be present. The OFDM demodulator’s rendezvous element also requires a numBandwidthUnits parameter to be set, to indicate the bandwidth over which it should receive. This parameter can be set by a controller in response to information at the Detector component.


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Table 3 - Table of Iris Parameters involved in the Cogeu Rendezvous algorithm

Parameter Name Purpose Val range Modulator

numBandwidthUnits The number of bandwidth units to transmit on – effectively determining the active bandwidth of the signal

0-12

enable A factor for balancing the level of power on cancellation carriers with the degree of OOB suppression they provide

Usually 0 ≤ x ≤ 1

Demodulator numBandwidthUnits The number of bandwidth units to receive 0-12 Detector detectThreshold The SCF threshold above which a signature is

deemed detected 0 ≤ x ≤ 1

cyclicFrequency The cyclic frequency on which the signature(s) can be found > 0

minFrequency Start of the range of linear frequencies to sweep > 03 maxFrequency End of the range of linear frequencies to sweep > minFreq3 The CycloSignatureDetector takes a spectral correlation of the received signal, which will be returned as a value between zero and one. The threshold for this value above which a signature is considered to have been successfully detected is set using the detectThreshold parameter. The component also needs to know the cyclic frequency at which the desired signature will be located. This cyclic-frequency is determined by a combination of the mapping distance between the carriers in the OFDM modulator generating the signal, the bandwidth over which the signal is transmitted, and the fftsize of both this detector and of the transmitter. The signature detector also allows selection of the fftsize it employs, which determines the resolution of its signature detection and hence its accuracy. An increase in fftsize will however come with a penalty in the form of an increase in the time taken to detect a signature. More information on the tradeoffs associated with cyclostationary signature detection parameter choice can be found in the more detailed discussion of rendezvous in D5.2 [18] The signature detector also requires the minimum and maximum frequency over which it will search for the signature to be supplied via the minFrequency and maxFrequency parameters. These can be set to the same value to disable frequency acquisition and engage only in bandwidth selection. These parameters are summarised in Table 3. Note that in addition to involving the reconfiguration of these parameters, the COGEU rendezvous mechanism also requires changes to the USRP frequency parameter, to effect changes in frequency.

4.3.3.2 Component Events The rendezvous algorithm components also expose a number of Iris events, which the controllers involved in the algorithm respond to in order to facilitate rendezvous. The modulator exposes no Iris events relating to rendezvous. The demodulator however, exposes a signalLost event, which is triggered when packets have not been successfully received for a pre-set period of time. This event allows the controller know that it should reset the signature detector as the transmitter has likely changed frequency, The detector exposes several events. The changeFrequency event allows it to request that the USRP’s receive frequency be changed by the controller. The frequencyDetected event indicates to the controller that a signature has been found at a precise frequency, and that the centre-frequency of the USRP 3 Note that the USRP must be equipped with a daughterboard capable of receiving at this frequency.


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should be calibrated to this, and OFDM demodulation commenced. The signatureChanged event meanwhile indicates that the detected number of signatures has changed, indicating a change in the bandwidth of the signal, which must be passed to the OFDM demodulator. These events, their purposes, and the actions the controller is expected to perform in response to them are now summarised. • Demodulator:

o signalLost:

Purpose: Indicate that no OFDM frames have been successfully demodulated for a pre-set period of time, implying the transmitter has changed frequency, or been disabled.

Controller response: Instruct the USRP to change receive frequency and start frequency acquisition again.

• Detector:

o changeFrequency: Purpose: Indicate no signature has been detected at this frequency. Controller response: Change USRP frequency

o frequencyDetected Purpose: Indicate that the exact centre frequency of a signature has been detected. Controller Response: Change USRP frequency to the exact centre frequency, set

the Detector to detect bandwidth, and enable the OFDM demodulator. o signatureChanged:

Purpose: The number of signatures detected has changed, indicating a probable change in the bandwidth of the signal.

Controller Response: Reconfigure the demodulator to demodulate the new bandwidth.

4.4- Integration and Evaluation

As the rendezvous mechanism involves two distinct steps, testing as part of the integration was to test these steps separately.

4.4.1- Frequency acquisition of a fixed-bandwidth link

The first step was to test the frequency acquisition part of the mechanism. Following confirmation of the detector algorithm’s functionality in Matlab, testing was moved directly to over the air transmission, as it is difficult to meaningfully simulate the concept of a radio sweeping across frequencies. A simple transmit chain, similar to Figure 17, but without any need for the controller was used to generate fixed bandwidth transmissions with an embedded signature. A simple receiver 1m away, configured as in Figure 18 attempted to locate this transmission and return the correct centre frequency. This step involved tuning the detectThreshold parameter of the detector to suit the radio environment and give consistently accurate results. It also confirmed the detector’s ability to deal with real-world input.

4.4.2- Unknown bandwidth link

The second step was to modify the t ransmit chain to transmit at a bandwidth unknown a-priori to the receiver. Transmissions were made for several values of bandwidth, ensuring that each time the receiver was capable of correctly determining the bandwidth, activating the demodulator and correctly demodulating the signal.

4.4.3- Changing signal

The final test was to more fully test the behaviour of the rendezvous controller. In order to do this, a controller was added to the simple transmitter so that not only would the frequency and bandwidth of the signal be unknown to the receiver at its start -up, these parameters would also be subject to change. At start-up the receiver would acquire frequency and then bandwidth, and begin demodulation. In cases where the transmitter changed bandwidth, the detector signalled this by means of the


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signatureChanged event, and the rendezvous controller informed the OFDM demodulator. In cases where the transmitter changed frequency, the OFDM Demodulator signalled this by means of the signalLost event, and the rendezvous controller initiated frequency acquisition.

4.5- Rendezvous Applications

Rendezvous is extremely useful in any scenario where it is either desirable or impossible for waveform parameters to not be known a priori. TVWS is an excellent example. Given then wide range of possible frequencies on which a device may be transmitting, the lack of a consistently available out-of-band signalling mechanism or even a simple dedicated control channel means that either painstaking exhaustive search is required in order to allow systems to rendezvous – which can be resource intensive both in time and processing terms – or a technique like cyclostationary signatures can be used to speed up the process. This negates any need for a pre-agreed control channel as is sometimes proposed for cognitive radio applications. Furthermore, in addition to the simple cases discussed in the testing and implementation steps above, the rendezvous mechanism can be used for distinguishing between networks. Each network or group of devices may use an agreed cyclostationary signature as an identifier. Signatures can potentially be changed to reflect a device’s changes its network membership, and a device at different stages of network setup might employ different signatures for each phase [20]. It is worth noting that the primary benefit a rendezvous technique such as cyclostationary signatures brings is not to enable functionality that was previously impossible, but rather to significantly enhance the speed and practicality with which an otherwise onerous procedure can occur.

4.6- Summary

The mechanism employed for rendezvous in the COGEU TVWS transceiver is cyclostationary signatures. Rendezvous enables Master and Slave TVWS devices to find each other in the unchannelised and dynamic TVWS spectrum. The cyclostationary rendezvous was integrated into Iris. Its functionality is spread across the COGEU OFDM modulator and demodulators, as well as the CycloSignatureDetector component and the rendezvous controller. The rendezvous module has been implemented in such a way that it allows for external control of the Master TVWS radios which is a necessary feature for the COGEU model. Taking input from the Geolocation database, or another entity, the centre frequency and bandwidth of operation can be dictated to a TVWS Master radio which will then embed the appropriate cyclostationary signatures into its transmitted waveforms at the appropriate frequencies. The integration of this component with the other TVWS transceiver elements is described in the next chapter. This functionality was demonstrated publicly, at its given state of maturity, at the FUNEMS conference in Warsaw, June 2011.


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5- Integration of Geolocation Database and TVWS Transceiver

5.1- Introduction

As detailed in previous chapters, the PMSE sensing and geolocation database access for the TVWS transceiver is implemented in National Instruments Labview. The radio-chain is implemented separately on the Iris software-radio platform. As a result, steps are necessary to integrate these two plat forms and facilitate data-passing between them. This chapter discusses these steps as they were performed, and explains the interfaces designed and deployed.

5.2- System setup

The sequence of operations involved in sensing and transmission across both Labview and Iris are outlined in Figure 20. From now on, they will be referred to as the sensing and transmission module, respectively. Each requires its own USRP frontend. At startup, the sensing module performs initialization by ret rieving the TVWS transceiver’s geographical location using a GPS receiver device. This information is used to query the geolocation database, which supplies a list of channels vacant of TV transmissions. The Labview-based sensing module then commences sensing in all vacant channels, sequentially. A decision engine makes use of the database information and sensing results to select a channel for operation. The sensing module informs the transmission module of this decision and information as to the location and power of any PMSE(s) in the channel. It then goes into an idle state, awaiting further communication from the transmission module. The transmission module starts in an idle state and awaits a message from the sensing module. This message will include the TV channel to transmit on, the acceptable power to transmit it, and the location of any PMSE devices within this channel. Once the message has been received, transmission is commenced for a short interval. After this interval, a message is sent to the sensing module, and a response is awaited.

Figure 20 - Flowchart of operations in the TVWS transceiver comprising Labview-based sensing module and Iris-based transmission module. Startup commences at the GPS step in the upper-left corner.

When the sensing module receives a message indicating the end of a t ransmission, it briefly senses for PMSEs within the selected TV channel. Note that the process of sensing within this single band takes only a very short period of time. Once again a message is then passed to the transmission module providing the status (and change in status) of any PMSE device(s) in the band. This repeats constantly,


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to ensure that PMSE status information is kept up to date, and transmissions are tailored to prevent disruption to PMSE devices. Implementation of this interface in Labview uses the standard UDP socket objects included as part of Labview. Implementation in Iris uses the existing UdpSocketTx and UdpSocketRx components, but also required the implementation of several new components. These were as follows:

1. CogeuSensingParser: A simple component to check await a message from the Sensing module, check it for correct formatting, and activate an appropriate Iris event so that a Controller may trigger the necessary reconfigurations.

2. CogeuSensingController: A controller to respond to the events created by the SensingParser component, and reconfigure Iris to start transmissions on the appropriate frequency and power, as well as to send trigger a message for sending at the end of this burst of transmissions.

3. Gate: A simple component which pauses transmissions until “opened” by the Controller.

4. StoredMessage: A component which sends a preset message when triggered to do so by a controller, used to send messages to inform the sensing module that transmission is completed.

They can be seen laid out in a radio configuration in Figure 21.

Figure 21 Layout of Iris transmission module with components necessary for integration with Labview sensing module. Newly implemented components are highlighted in blue.


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5.3- Interface

Messages are passed between the Labview sensing module and Iris transmission module using the UDP protocol. There are only two formats, one sent by the sensing module, the other sent by the transmission module. They are defined as follows:

• Sent by the Labview sensing module:

o CH : Channel number. o Pow : Maximum power allowed. o WM : Number of wireless microphones. o FC : Central frequency of this wireless microphone. Note this field (underlined above) is

only present if the number following WM is greater than zero, and will be repeated nn times, where nn is the number following WM.

o END : Confirms the end of message. Note: nn is any integer.

• Sent by the Iris transmission module:

o The only information required by the sensing module is notification that transmission of this burst is complete, so a TXEND message is sufficient.

5.4- Integration Testing

The integration of the radio sensing module, the transmission module and the database access occurred at the wireless testbed lab at CTVR, Trinity College Dublin in a week-long workshop during which IT personnel visited Dublin. Prior to this another week-long workshop had been held to finalise the implementation and integration of the shaping elements of the transmission module with personnel from PUT visiting the CTVR lab. As a result of this workshop we were able to demonstrate early, somewhat immature, implementations of the shaping and rendezvous components during 2011. The first demonstration was for the 1st COGEU audit in Brussels, March 2011 during which a shaping and co-existence demonstration was given. The second demonstration occurred at the FUNEMS conference in Warsaw, Jun 2011. At this event rendezvous, shaping and sensing were demonstrated; each element of the transceiver being at different levels of completion. For completion of the integration and development of a prototype transceiver that could demonstrate all of the COGEU functionality, it was necessary to bring the transmission and sensing modules together. As the sensing and transmission modules each require a separate USRP, there were two options for this step in the integration. The first was to split the functionality of the COGEU TVWS transceiver over two hosts, each with a USRP, and connect the hosts via UDP sockets using IEEE802.11. Figure 22 shows a basic outline of this configuration. The second configuration for integration was to co-locate the sensing and transmission modules on a single host. In this configuration, Labview and Iris communicate over UDP sockets on localhost. This configuration is shown in Figure 23.

CHnnPownnWMnnFCnnn… END

TXEND


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Figure 22 - COGEU TVWS transceiver with sensing and t ransmission module split over two hosts, linked via UDP sockets over IEEE 802.11.

Figure 23 - COGEU TVWS transceiver with sensing and t ransmission module operating on the same host with two USRPSs. The two modules communicate via UDP sockets using the localhost interface.

The testing process involved creation of Iris and Labview modules to confirm reliable inter-module communication. Once this was confirmed, the ability of the sensing module to correctly note the changing frequency of a PMSE, and fully inform the Iris transmission module of these changes was tested. This involved multiple changes to the frequency of a test PMSE device, and checking for the appropriate response in Labview, and that same response being correctly passed to and parsed by Iris. A photo of the sensing module detecting a PMSE (and the PMSE signal rendered on a Rohde & Schwarz FSVR Spectrum Analyser) is shown in Figure 24 below. As our prototyping transceiver is based upon the SDR paradigm, and as we are experimenting with some techniques that are process-intensive, the choice of a single or two-host solution depends on the computational capacity that the application demands and the capabilities of the host machine. To put it another way, it is easier to extract more computational capacity from a workstation than it is from a mobile PC laptop. As such, the precise application to which we target the TVWS transceiver impacts the integration option that we choose. This is acceptable in the use of general-purpose-processor (GPP)-based experimental SDR. As we motivated in D5.1 [19], the use of Iris gives us many degrees of freedom and the ability to rapidly prototype many different solutions. If COGEU solutions were being taken to the point of commercialization they could be committed to another less flexible architecture, one that uses FPGAs, DSPs or the like.

LabVIEW/ IRIS

Database access

USRP transmitter

COGEU TVWS Transceiver

USRP

LabVIEW IRIS

USRP Sensing Database access

USRP transmitter

IEEE 802.11

COGEU TVWS Transceiver


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Figure 24 - Photo of testing for integration, taken in the CTVR, Trinity College Dublin lab. The laptop is running the Labview sensing module, and a PMSE is present in the chosen channel, visible on the Spectrum Analyser to the left.

5.5- Summary

The database access, sensing and t ransmission modules of the COGEU TVWS transceiver were successfully integrated into a single transceiver involving both one and two host-laptops configurations.


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6- Conclusions

This deliverable has addressed the fulfilment of tasks T5.2 (integration of sensing and database access), T5.3 (design of spectrum shaping algorithms) and T5.4 (design of rendezvous techniques).

In Chapter 2 described a PMSE sensing module which was developed for the TVWS transceiver based on three sensing algorithms; Covariance Absolute Value, Blindly Combined Energy Detection and Energy Detection (ED). The purpose of the sensing platform is to identify the central frequency and bandwidth of multiple wireless microphones present in a DVB-T channel. This feature is crucial to allow the TVWS transceiver access the unused spectrum in a safe manner, using spectrum shaping techniques to allow coexistence between TVWS devices and PMSEs systems in the same DVB-T channel.

The sensing module was implemented on the TVWS SDR using the National Instruments Labview system. Labview allows for ease of graphical programming combined with functionality that allows control of the USRP radios. The sensing module also makes use of the National Instruments (Ettus LLC) USRP WBX radio frontend daughter-boards which allow it to receive signals in the TV bands of interest.

The sensing module allows the user to select a TV channel which has been indicated as being available for use (i.e. by looking up the TVWS geolocation database) and then to scan the channel for the presence of unknown wireless microphones that may be operating in the channel, i.e., combination of geo-location database access with local sensing. The sensing module was evaluated in practice in a real setting; the proposed detection algorithms were found to out-perform the ED algorithm. This practical evaluation confirmed the simulation results.

Chapter 3 described the integration of the spectrum shaping functionality. Spectrum shaping assists in protecting primary users transmissions in bands adjacent to COGEU TVWS transceiver transmissions. Two shaping mechanisms are employed in the TVWS platform; windowing and cancellation carriers as they provide flexible spectrum shaping capabilities that suit a range of coexistence scenarios, i.e. PMSE or DVB-T neighbours. The spectrum shaping algorithms were integrated within the Iris SDR as Iris has been purposefully designed for the kind of runtime reconfiguration required by cognitive radio systems. As the shaping algorithms are contained within OFDM modulator and demodulator components, they are wholly integrated within Iris, and were therefore implemented largely in C++. The windowing algorithm was implemented wholly in C++, while the cancellation carriers algorithm currently employs the popular Linear Algebra PACKage (LAPACK) Fortran libraries for its linear algebra calculations. Following extensive testing, the shaping algorithms were successfully integrated into new feature-rich OFDM modulator and demodulator components to form part of the COGEU TVWS transceiver. A link comprising two USRP-enabled t ransceivers was set-up so that over-the-air transmission could be demonstrated. This functionality was publicly demonstrated, at varying degrees of maturity, in 2011; firstly at the 2011 COGEU 1st audit and secondly at the FUNEMS conference in Warsaw in June 2011.

In Chapter 4 implementation and integration of the mechanism employed for rendezvous in the COGEU TVWS transceiver, cyclostationary signatures, was described. Rendezvous enables Master and Slave TVWS devices to find each other in the unchannelised and dynamic TVWS spectrum. The cyclostationary rendezvous was integrated into Iris. Its functionality is spread across the COGEU OFDM modulator and demodulators, as well as the CycloSignatureDetector component and the rendezvous controller. The rendezvous module has been implemented in such a way that it allows for external control of the Master TVWS radios which is a necessary feature for the COGEU model. Taking input from the Geolocation database, or another entity, the centre frequency and bandwidth of operation can be dictated to a TVWS Master radio which will then embed the appropriate cyclostationary signatures into its transmitted waveforms at the appropriate frequencies. This functionality was demonstrated publicly, at its given state of maturity, at the FUNEMS conference in Warsaw, June 2011. Finally, Chapter 5 described the process by which all of the elements were brought together; the database access, the sensing, the shaping and rendezvous. One of the key features of the COGEU platform is that we enable controlled access to the TVWS spectrum. Database-controlled TVWS access is emerging as the preferred regulatory approach for cognitive radio access to the TVWS.


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The COGEU geolocation database gives permission to a COGEU TVWS to access a channel or set of channels by providing it with a list of free channels. At startup, the COGEU sensing module performs initialization by retrieving the TVWS transceiver’s geographical location using a GPS receiver device. This information is used to query the geolocation database, which supplies the list of vacant TV channels which then must be checked for the presence of PMSEs before appropriately shaped TVWS transmissions can begin.

The database access, sensing and t ransmission modules of the COGEU TVWS transceiver were successfully integrated in configurations involving both one and two host-laptops.


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7- References

[1] COGEU(ICT-248560), "Sensing algorithms for TVWS operations," Report D4.2, June 2011. [2] Z. Yonghong and L. Ying-Chang, "Covariance Based Signal Detections for Cognitive Radio," in

New Frontiers in Dynamic Spectrum Access Networks, 2007. DySPAN 2007. 2nd IEEE International Symposium on, 2007, pp. 202-207.

[3] Z. Yonghong, L. Ying Chang, and Z. Rui, "Blindly Combined Energy Detection for Spectrum Sensing in Cognitive Radio," Signal Processing Letters, IEEE, vol. 15, pp. 649-652, 2008.

[4] C. Hou-Shin, G. Wen, and D. G. Daut, "Spectrum Sensing for Wireless Microphone Signals," in Sensor, Mesh and Ad Hoc Communications and Networks Workshops, 2008. SECON Workshops '08. 5th IEEE Annual Communications Society Conference on, 2008, pp. 1-5.

[5] Z. Yonghong and L. Ying-Chang, "Spectrum-Sensing Algorithms for Cognitive Radio Based on Statistical Covariances," Vehicular Technology, IEEE Transactions on, vol. 58, pp. 1804-1815, 2009.

[6] A. Kortun, T. Ratnarajah, and M. Sellathurai, "Exact performance analysis of blindly combined energy detection for spectrum sensing," in Personal Indoor and Mobile Radio Communications (PIMRC), 2010 IEEE 21st International Symposium on, 2010, pp. 560-563.

[7] (February 2011). Ettus Research LLC. Available: http://www.ettus.com/ [8] (February 2011). National Instruments LABVIEW, "Universal Software Radio Peripheral (USRP)

Pre-Release Driver for LabVIEW,". Available: http://decibel.ni.com/content/docs/DOC-14531 [9] COGEU(ICT-248560), " Initial Architecture for TVWS Spectrum Sharing Systems," Report D3.2,

December 2010. [10] COGEU(ICT-248560), "Definition of test scenarios for the demonstrator," Report D7.1, May

2011. [11] S. Probasco, G. Bajko, B. Patil, and B. Rosen. (2012). Protocol to Access White Space

database: PS, use cases and rqmts draft-probasco-paws-problem-stmt-usecases-rqmts-00 (IETF ed.). Available: https://http://www.iet f.org/mailman/listinfo/paws

[12] (2012). IETF - The Internet Engineering Task Force. Available: http://www.iet f.org/ [13] (2012). OpenCellId. Available: http://www.opencellid.org/ [14] (2012). Navizon. Available: http://www.navizon.com/howitworks.php [15] (2012). SkyHook . Available: http://www.skyhookwireless.com/ [16] (2012). Google Gears Geolocation API. Available:

http://code.google.com/intl/nl/apis/gears/api_geolocation.html [17] (2012). GSM/GPS/A-GPS modem XT65. Available: http://www.siemens.com [18] COGEU(ICT-248560), "Algorithms for cognitive spectrum shaping and advanced rendezvous,"

Report D5.2, April 2011. [19] COGEU(ICT-2485600), "COGEU transceiver plat form specification ", Report D5.1, September

2010. [20] J. Tallon, T.K. Forde, L.E. Doyle, " Independent Coalition Formation for Dynamic Spectrum

Access Networks", IEEE VTC Magazine Special Issue on Cognitive Radio Application, to appear 2012.

[21] Schmidl, T.M.; Cox, D.C.; , "Robust frequency and timing synchronization for OFDM," Communications, IEEE Transactions on , vol.45, no.12, pp.1613-1621, Dec 1997.

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http://www.navizon.com/howitworks.php�

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List of Tables Table 1 - Colour code used to identify sensing results ...................................................................... 10 Table 2 - Iris OFDM modulator and demodulator parameters relating to shaping algorithm functionality,

assuming a 256-bin FFT, with 200 active subcarriers, 192 for data and 8 pilots. .......................... 17 Table 3 - Table of Iris Parameters involved in the Cogeu Rendezvous algorithm ................................ 26


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List of Figures Figure 1 - Structural and functional diagram of the sensing tool. .......................................................... 8Figure 2 – Flux diagram of sensing for a) Standard operations and b) ROC operations. ........................ 9Figure 3 - Setup interface of the PMSE sensing plat form. ................................................................... 9Figure 4 – LED bar control logic. ..................................................................................................... 10Figure 5 - Sensing interface of the PMSE sensing testbed. ............................................................... 11Figure

6 - a) Building plant where field measurements were realized; Measured ROC curves from: b)

locations L1 and c) Location L2. ............................................................................................... 12Figure 8 - Diagram of the insertion of one CC at the edge of a data subcarrier block. ......................... 14Figure

9 - Iris Simple OFDM Modulator and Demodulator shown with the minimum number of

components required for successful over-the-air transmission .................................................... 15Figure

10 - Steps involved in creating an OFDM symbol in the Modulator component, with both shaping

algorithms' steps clearly indicated ............................................................................................ 16Figure 11 - Typical development path in Iris (Reproduced from D5.1, Figure 19) ................................ 17Figure 12 - A simple test radio chain in Iris, to test modulator and demodulator behaviour .................. 18Figure

13 - Example demonstration using three Iris nodes, each comprising a host and USRP, to emulate a primary user link. This link neighbours a secondary transmitter using the Cogeu TVWS OFDM modulator allowing demonstration of the effects of the waveform with and without shaping on a neighbouring link.............................................................................................................. 19

Figure 14 - Spectrum Analyser Screen Capture for the scenario detailed in Figure 13, with shaping disabled in the upper figure, and shaping enabled in the lower figure

. The left-hand waveform is the Secondary (SU) Interferer, and the weaker right -hand waveform is the Primary (PU) transmission. Note the significant improvement in SU transmission roll-off with shaping enabled. ..................... 20

Figure 15 - Steps involved in the frequency acquisition process. ....................................................... 23Figure 16 - Steps involved in the bandwidth acquisition process........................................................ 23Figure

17 - Transmit chain of a radio using the COGEU rendezvous mechanism. Components are highlighted if they are directly involved in the mechanism. Parameters changed by controllers are shown as red arrows. .............................................................................................................. 24

Figure

18 - Receive chain of a radio using the COGEU rendezvous mechanism. The controllers and components involved in the mechanism are highlighted. Parameters changed by controllers are shown as red arrows, and events subscribed to by controllers shown as green arrows. ............... 24

Figure

19 - Steps for OFDM symbol c reation in a modulator, with the placement of the carrier mapping step highlighted, to show the position of the cyclostationary signature embedding in the modulator. .............................................................................................................................................. 25

Figure

20 - Flowchart of operations in the TVWS transceiver comprising Labview-based sensing module and Iris-based transmission module. Startup commences at the GPS step in the upper-left corner. .............................................................................................................................................. 29

Figure

23 - COGEU TVWS transceiver with sensing and t ransmission module operating on the same host with two USRPSs. The two modules communicate via UDP sockets using the localhost interface. ................................................................................................................................ 32

Figure

24 - Photo of testing for integration, taken in the CTVR, Trinity College Dublin lab. The laptop is running the Labview sensing module, and a PMSE is present in the chosen channel, visible on the Spectrum Analyser to the left. .................................................................................................. 33


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List of Abbreviations ARQ Automatic Repeat-reQuest AWGN Additive White Gaussian Noise BCED Blindly Combined Energy Detection CAV Covariance Absolute Value CCDF Complementary Cumulative Density Function CR Cognitive Radio CRC Cyclic Redundancy Check DSM Dynamic System Management DTV Digital Television DVB-H Digital Video Broadcasting – Handheld DVB-T Digital Video Broadcasting - Terrestrial ED Energy Detection FFT Fast Fourier Transform FM Frequency Modulation FPGA Field-Programmable Gate Arrays GPS Global Positioning System GSM Groupe Spécial Mobile (also, Global System for Mobile communication) GUI Graphical User Interface IEEE The Institute of Electrical and Electronics Engineers ISM Industrial Scientific and Medical (band) LTE Long Term Evolution MAC Medium Access Control OFDM Orthogonal Frequency Division Multiplexing PAPR Peak to Average Power Ratio PMSE Programme Making and Special Events PU Primary User ROC Receiver Operating Characteristic SDR Software Defined Radio SU Secondary User TV Television TVWS TV White Spaces UHF Ultra High Frequency USRP Universal Software Radio Peripheral VHF Very High Frequency WiFi IEEE 802.11 WiMAX Worldwide Interoperability for Microwave Access WLAN Wireless Local Area Network WM Wireless Microphone WP Work Package WPAN Wireless Personal Area Network WWRF Wireless World Research Forum

cogeu cogeu d5.3 cogeu tvws transceiver integration

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