09-11-2012

Rome, February 14, 2013

Status of the Project

Report on the first yearactivities

Short Wave critical Infrastructure Network based on new Generation of high survival radio communication system

With the support of the Prevention, Preparedness and Consequence Management of Terrorism and other Security-related Risks ProgrammeEuropean Commission - Directorate-General Home Affairs

PHY layer architectureof the SWING system

1) Selection of the modulation technology

2) System design for voice transmission

3) System design for data transmission

Selection of the modulation technology

Most military HF standards employ a serial-tone waveform with a powerful FEC code and temporal interleaving to exploit the time-diversity of the HF channel

The use of a temporal interleaver with an interleaving depth greater than the HF channel coherence time poses a serious problem in terms of overall link latency

The alternative approach to increase the system reliability is to exploit the frequency diversity offered by the multipath phenomenon

Selection of the modulation technology

In this case the transmission bandwidth must greatly exceed the channel coherence bandwidth and the received signal will be affected by ISI

The common approach to mitigate ISI in serial-tone waveforms is the use of a channel equalizer in the form of a tapped delay line. In case of severe multipath distortion, the number of required taps is very high and the equalizer cannot be implemented with affordable complexity

Multi-tone transmission in the form of OFDM is the most appropriate technology for low-complexity multipath mitigation

Advantages of the OFDM technology

The channel distortion appears as a multiplicative factor which can be compensated for through a bank of complex multipliers

Increased spectral efficiency due to partially overlapping subbands in the frequency domain

Simple digital implementation by means of DFT/IDFT operations

Increased resilience against narrowband interference, which only hits a small portion of the signal spectrum

Possibility of adaptively selecting the constellation size on each subband (autobaud capability)

Requirements of the digital voice link

1) It will support interactive voice communications. Interactivity is a basic design constraint

2) The maximum accepted delay is around 120 ms so as to guarantee a whole delay observed by the user below the subjective limit of 250 ms

3) Temporal interleaving cannot be used due to the strict requirement in terms of overall delay

4) In order for the system to be applicable to commercial vocoders, the bit rate should be 2400 bps with a BER lower than 10-2

5) A fixed 4-QAM constellation is used (no autobaud capability)

Guidelines for the design of the digital voice link

The signal bandwidth B must exceed the channel coherence bandwidth so as to capture most of the frequency diversity offered by the HF channel

B Bcoh

The subcarrier spacing f must be much smaller than the channel coherence bandwidth Bcoh so as to make the channel response nearly flat over each subcarrier and much larger than the Doppler spread in order to avoid significant channel variations over one OFDM block

5 Hz : BDoppler f Bcoh : 500 Hz

Design of the main system parameters

The sampling frequency fs is fixed to 14.4 kHz, which seems reasonable for implementation on commercial HW platforms

The IDFT/DFT size is fixed to N=256. This value results into a subcarrier distance f =56.25 Hz

Assuming a maximum delay spread max =5 ms, the number of samples in the cyclic prefix is fixed to Ng= maxfs=72

The number of modulated subcarriers is Nu=171, while the number of null subcarriers placed at the spectrum edges is Nv=N-Nu=85

The signal bandwidth is B=Nu f = 9600 Hz

Pilot insertion in the voice linkA total of 35 pilot subcarriers are inserted in each OFDM block for channel estimation

This results into 136 data subcarriers divided into 34 chunks, each containing 4 data subcarriers. The baud rate is 5970 baud

B=9600 Hz

frequencypilot data

Upper band Lower band

Transmitter structure for the voice link

informationbits FEC Mapper OFDM

modulatorInterleaverSubcarrierallocation

FEC is accomplished by means of the industry-standard convolutional encoder with rate 1/2 and constraint length 7

Bit interleaving is accomplished by means of a block interleaver matrix

Interleaved bits are mapped onto 4-QAM symbols without any autobaud capability

Subcarrier allocation: mode I

B=9600 Hz

frequencypilot data

Upper band Lower band

Mode I is suggested in case of harsh channel conditions

A total of 136 coded bits are mapped onto 68 channel symbols, which are next repeated and allocated over each subband (repetition factor Rf=2)

Subcarrier allocation: mode II Mode II is suggested in case of better channel conditions

A total of 272 coded bits are mapped onto 136 channel symbols, which are next allocated over the 136 available data subcarriers without any repetition

Tab 1 Ğ Transmission parameters for Mode I and Mode II voice links

PARA METER MODE I MODE II

Number of information bits 60 128

Number of flush bits 8 8

Number of coded bits 136 272

Interleaver matrix dimensions 17 x 8 17 x 16

Number of coded symbo ls 68 136

Bit rate (bit/s) 2634 5620

Requirements of the data link

1) The data link provides non-delay sensitive services, meaning that we can relax the interactivity constraint

2) Channel coding and frequency interleaving are necessary to provide sufficiently low packet error rate

3) The signal bandwidth is chosen large enough so as to provide the system with the desired frequency diversity

4) CRC and ARQ are requested for error-free packet delivery

5) An autobaud capability is employed to adaptively select the most appropriate constellation

Design of the main system parameters

The subcarrier distance is f =56.25 Hz as in the voice link

To account for the severe constraint on the achievable data rate, the IDFT/DFT size is fixed to N=2048.

Assuming a maximum delay spread max =5 ms, the number of samples in the cyclic prefix is fixed to Ng= maxfs=576

The number of modulated subcarriers is Nu=1728, while the number of null subcarriers placed at the spectrum edges is Nv=N-Nu=320

The signal bandwidth is B=Nu f = 97.25 kHz

PARA METER VAL UE

IDFT/DFT size 2048

Subcarrier spacing 56.25 Hz

Length of the useful part of the OFDM block 17.78 ms

CP length 5 ms

Length of the extended OFDM block 22.78 ms

Number of virtual carriers 320

Number of cyclic prefix samples 576

Number of mod ulated subcarriers 1728

Row bandwidth 115.2 kHz

Channel bandwidth 97.2 kHz

Main system parameters

Pilot grid for the data link The available subcarriers are divided into clusters, where each

cluster contains 9 subcarriers and spans over 3 adjacent OFDM blocks

In each cluster there are 8 pilot symbols and 19 data subcarriers

A total of 192 clusters are present in three adjacent OFDM blocks, corresponding to 3648 data subcarriers

pilot data

frequency

time symbol 3k+1

symbol 3k

symbol 3k+2

Transmitter structure for the data link

A 16-bit CRC is appended to each data packet

FEC and bit interleaving as in the voice link

The overall bandwidth is divided into 8 subbands, each containing 24 adjacent clusters and 456 data subcarriers. A different constellation size can be used on different subbands (autobaud)

The interleaved bits are mapped onto 4QAM, 16QAM or 64QAM constellation symbols, which are transmitted within one single subband.

CRC16-bit

datapacket FEC Interleaver Mapper

Subcarrierallocation

OFDMmodualtor...from other

subbands

Data link waveformsTab 1 Ğ Parameters for the six transmission modes of the SWING data link

PARA METERS MODE I MODE II MODE III MODE IV MODE V MODE VI

Constellation 4-QAM 4-QAM 4-QAM 4-QAM 16-QAM 64-QAM

Repetition factor 8 4 2 1 1 1

Data packets over the 8 subbands

1 2 4 8 16 24

Information bits over the 8 subbands

432 864 1728 3456 6912 10368

Flush bits over the 8 subbands

8 16 32 64 128 192

CRC bits over the 8 subbands

16 32 64 128 256 384

Coded bits over the 8 subbands

912 1824 3648 7296 14592 21888

Channel symbols over the 8 subbands

456 912 1824 3648 3648 3648

Interleaver matrix dimension

48x19 48x19 48x19 48x19 48x38 48x57

Bit rate (kbit/s) 6.322 12.644 25.288 50.576 101.152 151.728

HF Channel Model

Channel type Mid-latitude disturbed

Mid-latitude moderate

Mid-latitude good

Delay spread(ms)

2.0 1.0 0.5

Doppler spread (Hz) 1.0 0.5 0.1

Coherence bandwidth can range from less than 100 Hz to more than 20 kHz

Coherence time can range from 1 second to more than 10 seconds

Voice link with moderate channel condition

Voice link with good channel condition

Data link with moderate channel condition