Abstract—Alongside with on-going further WCDMA

development, work on Evolved Universal Terrestrial Radio Access Network (EUTRAN), also known as Long Term Evolution (LTE), has been initiated in 3GPP. The objective of EUTRAN is to develop a framework for the evolution of the 3GPP radio-access technology towards wider bandwidth, lower latency and packet-optimized radio-access technology with peak data rate capability up to 100 Mbps. For the uplink direction, Single Carrier – FDMA (SC-FDMA) has been chosen as the multiple access technology. This paper introduces the uplink technology, the current state of progress in 3GPP as well as expected schedule for actual specification availability and describes a couple of key features, channel dependent frequency domain scheduling and Multi-User MIMO in more detail. System performance results for channel dependent frequency domain scheduling are presented as well.

Index Terms—3GPP, EUTRAN, LTE, SC-FDMA

I. INTRODUCTION

N order to prepare for future needs, the 3rd Generation Partnership Project (3GPP) has initiated activity on the Long Term Evolution (LTE) of UTRAN (Universal Terrestrial

Radio Access Network) [1], which is aiming clearly beyond to what the WCDMA can do with High Speed Downlink Packet Access (HSDPA) or High Speed Uplink Packet Access (HSUPA) in case of uplink.

Based on the requirements in [2], it was soon understood in 3GPP that something new was needed, especially for the support of large range of different bandwidth variants. Following the discussion, 3GPP decided in December 2005 to adopt the for the uplink direction the Single Carrier FDMA (SC-FDMA) due to the good performance in general and superior properties in terms of uplink signal Peak-to-Average Ratio (PAR), or especially when observed as a function of Cubic Metric (CM) [3], when compared to OFDM in the uplink. In the downlink direction the solution was OFDM, mainly due to the simplicity of the terminal receiver in case of large bandwidths in difficult environment.

This paper covers the uplink physical layer development and addresses the initial findings on the uplink performance. Section 2 covers the remaining work plan and the expected schedule of the LTE standardization. Section 3 looks at the uplink physical layer details. Two features that differentiate LTE from other radio systems, channel dependent frequency domain scheduling and Multi-User MIMO are introduced in Sections 4 and 5, respectively. In Section 6 system performance results for channel dependent frequency domain scheduling are presented. Conclusions are drawn in section 7.

II. EUTRAN STANDARDIZATION SCHEDULE

The targeted schedule for LTE standardization in 3GPP is illustrated in Fig. 1. Following the creation of the work item for actual specification work, 3GPP has defined the target date for the detailed (stage 3 in 3GPP terms) specification availability by September 2007. The work item phase was closed in September 2006 and the actual specification work is currently ongoing. The stage 2 (not yet implementation details) level specification is targeted to be approved in March 2007. The expected specification release is Release 8, but that is to be confirmed later as 3GPP does only decide the actual release once the work is completed. Terminal and BTS performance requirements are expected to take until December 2007. According to current estimates, first commercial LTE products should be out in the market by 2009/2010.

Fig. 1. The estimated schedule of LTE standardization.

EUTRAN Uplink Performance Timo Lunttila Jari Lindholm Kari Pajukoski Esa Tiirola Antti Toskala Nokia Siemens

Networks Nokia Siemens

Networks Nokia Siemens

Networks Nokia Siemens

Networks Nokia Siemens

Networks P.O. Box 301 P.O. Box 301 P.O. Box 319 P.O. Box 319 P.O. Box 301

00045 NOKIA GROUP FINLAND

00045 NOKIA GROUP FINLAND

90651 OULU FINLAND

90651 OULU FINLAND

00045 NOKIA GROUP FINLAND

timo.lunttila@nokia.com jari.o.lindholm@nokia.com kari.pajukoski@nokia.com esa.tiirola@nokia.com antti.toskala@nokia.com

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III. UPLINK MULTIPLE ACCESS TECHNOLOGY

The chosen SC-FDMA solution is based on the use of cyclic prefix to allow for high performance and low complexity receiver implementation in the base station (eNodeB) [4]. As such the receiver requirements are more complex than in the case of OFDMA for similar link performance but this was not considered to be a problem in the base station.

The available spectrum is divided between users so that continuous frequency band is allocated for a single user. This approach is often referred as blocked or localized SC-FDMA. Earlier in the standardization process the so called distributed or interleaved FDMA [5] was also considered as an alternative, but due to slight performance disadvantages caused by the requirements channel estimation accuracy it was not included into the standard. The general SC-FDMA transmitter and receiver concept with frequency domain signal generation and equalization is illustrated in Fig. 2.

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Fig. 2. SC-FDMA transmitter and receiver chains with frequency domain equalization.

With FDMA the type of modulation applied will impact the

peak-to-average ratio (PAR), but with OFDMA the large number of parallel sub-carriers typically makes the PAR 2-6 dB higher than with FDMA. The difference is largest with modulations like pi/2-BPSK where “traditional” QAM transmitter can benefit from the low envelope variations.

For the uplink considerations the cell edge operation is highlighted due operator requirements. The more uniform the performance, the better service availability for end user. Hence a few dBs improvement in PAR is very essential to achieve high

data rate availability close to cell edge. The OFDMA and SC-FDMA uplink CM comparison is shown

in Fig. 3, indicating the difference depending on the modulation being used. Using pi/2-BPSK the difference was up to 4 dB in the favor of SC-FDMA. This leads to having a better link budget when compared with OFDMA and allows smaller power amplifier back-off and more power efficient devices.

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Fig. 3. SC-FDMA CM compared to OFDMA uplink CM.

A. Physical Layer Parameters

A number of bandwidth options have been planned for LTE uplink ranging up to 20 MHz. To ensure fast progress in standardization, the layer one specification work is conducted in a bandwidth agnostic manner and 3GPP RAN WG#4 will define in parallel the actual RF bandwidths and the number of sub-carriers for each option [6].

All bandwidths have the same transmission time interval (TTI), which has been agreed to be 1.0 ms. This was chosen to enable very short latency with L1 Hybrid ARQ combined with good cell edge performance. The channel coding in LTE is based on turbo codes.

Other key parameters have relationship with the multiple access method, such as the 15 kHz sub-carrier spacing of OFDM. This selection is a compromise between support of high Doppler frequency, overhead from cyclic prefix, implementation imperfections etc. To optimize for different delay spread environments, two cyclic prefix values are to be used.

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IV. CHANNEL DEPENDENT FREQUENCY DOMAIN SCHEDULING

One of the most attractive features in SC-FDMA is the chance to flexibly schedule user data traffic in frequency domain. The principle of frequency scheduling in LTE is presented in Fig. 4. The available spectrum is divided into resource blocks (RB) consisting of 12 adjacent sub-carriers. The duration of a single RB is 1 ms. One or more neighboring RBs can be assigned to a single user by the base station and multiple users can be multiplexed within the same frequency band on different resource blocks.

In order to optimize the use of frequency spectrum the base station utilizes the so called sounding pilots sent by the UEs. Based on the channel state information estimated from the sounding pilots the base station can divide the available frequency band between the UEs. The spectrum allocation can be changed dynamically as the propagation conditions fluctuate. The base station can be configured to use the channel state information for e.g. maximizing cell throughput or favoring cell-edge users with coverage limitations.

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Fig. 4. The principle of frequency scheduling in LTE.

In some use cases with large number of simultaneous connections like e.g. VoIP, the number of available sounding pilot patterns might not be large enough. In order to support such scenarios, there is also the possibility to utilize frequency diversity by employing frequency hopping. The UEs can hop between frequencies within allocated band according to some predefined pattern. Hopping can take place on a 0.5 ms interval.

An exemplary frequency domain scheduling scheme is presented in Fig. 5. The outer parts of the spectrum are reserved for frequency hopping UEs in order to maximize the scheduling flexibility and the data rates possible for other UEs. The RB indexes 1-4 and 22-25 are allocated for frequency hopped UEs while RBs 5-21 are allocated for UEs having channel dependent scheduling.

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Fig. 5. An exemplary resource allocation utilizing both channel dependent frequency domain scheduling and frequency hopping.

V. MULTI-USER MIMO

Another special feature used in LTE for boosting up bit rates is the Multiple-Input Multiple-Output (MIMO) transmission. MIMO transmission can be divided into multi-user and single-user MIMO (MU-MIMO and SU-MIMO, respectively). The difference between the two is that in SU-MIMO all the streams carry data for/from the same user while in the case of MU-MIMO the data of different users is multiplexed onto a single time-frequency resource. For LTE uplink, it is still uncertain whether SU-MIMO will be included in the standard or not. Thus, here we concentrate on MU-MIMO.

For LTE uplink, both 2x2 and 4x4 antenna options and being considered. With four transmit and receive antennas the peak spectral efficiency for a given frequency band is in principle four time higher than in single stream case.

The basic principle of uplink MU-MIMO with 2x2 antenna configuration is depicted in Fig. 6. Each of the two UEs transmits a single data stream simultaneously using the same frequency band. The eNodeB receives the transmitted signals with two antennas. The reference signals of the UEs are based on CAZAC sequences which are code multiplexed using cyclic shifts [7]. This enables accurate channel estimation, which is crucial in MIMO systems. Using the channel state information, the eNodeB can separate and decode the both streams.

Uplink MU-MIMO also sets requirements for the power control. In the Single-Input Single Output (SISO) case, due to the nature of FDMA rather slow power control is sufficient. When several users are multiplexed on the same frequencies, the near-far problem well known from CDMA-based systems arises.

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Fig. 6. The basic principle of LTE uplink MU-MIMO with 2x2 antenna configuration.

VI. PERFORMANCE OF FREQUENCY DOMAIN SCHEDULING

With the use of SC-FDMA especially the cell edge performance has a good potential for improvement over the WCDMA (HSUPA or E-DCH) uplink. Besides the improved PAR properties, the uplink users maintain the intra-cell orthogonality, which improves the link budget for the users at cell edge. This chapter studies the performance of frequency domain channel dependent scheduling which also has positive impact to the cell edge performance.

In the system simulations it is assumed that that there are a total of 16 users per 5 MHz in each cell regardless of the scheduling scheme. The scheduling bandwidth equals 1.25 MHz (i.e., 4 resource blocks of 315.5 kHz1). This corresponds to four users per scheduling bandwidth. The distributed sounding pilot is transmitted in such a way that it covers the whole scheduling bandwidth (1.25 MHz) allowing user-specific CQI measurement separately for each sub-band. It is noted that in the coverage limited case for UL a relatively narrow transmission bandwidth for the sounding pilot, such as 1.25 MHz, provides the best trade-off between the achievable frequency/multi-user diversity and a sufficient CQI measurement accuracy.

We consider three different cases in the system simulations. The first case represents FDM multiplexing without channel dependent scheduling (i.e., Round Robin). In the second case, illustrated in Fig. 7 the scheduling bandwidth consists of four adjacent sub-bands, each 312.5 kHz.

1 The simulations are based on an early assumption of the physical resource

block size (18 sub-carriers)

Fig. 7. Scheduling over adjacent sub-bands.

The third case, shown in Fig. 8 is also based on FDM, but the

different sub-bands are distributed over the 5 MHz frequency band. An advantage of this arrangement is that it can utilize the frequency diversity provided by the whole system bandwidth, not only the scheduling bandwidth as in case of Fig. 7. It is noted that in single-carrier system scheduling over non-adjacent sub-bands can be supported having frequency hopping reference (sounding) signal.

Fig. 8. Scheduling over non-adjacent sub-bands. Notice that only one of the sub-bands is used at the time by same UE.

In the system simulations we consider the extreme coverage limited case, namely Case 3 specified in 3GPP [8]. In Case 3 the inter-site distance (ISD) equals to 1732 m and there is an additional penetration loss of 20 dB included in the path loss. The system is assumed to be fully loaded with frequency reuse of 1/1.Results are shown for full buffer and 16 UEs per sector per 5 MHz. In this study the power control target has been varied to favor either cell edge users or average sector capacity and also some intermediate points have been calculated. Link adaptation including HARQ with Chase Combining was explicitly implemented in the simulator. Link-to-System mapping was done using AVI interface. AVI curves were simulated assuming practical FDE receiver and realistic channel estimation algorithms. In this simulation the interference control was switched OFF. The detailed simulation assumptions are found in [9].

The simulation results in Fig. 9 show the relationship between average sector throughput and cell edge throughput. Results show that channel dependent frequency domain scheduling provides significant user throughput gain at the cell edge. Scheduling over non-adjacent sub-bands performs clearly better than scheduling over adjacent sub-bands. This is due to the fact that the degree of frequency diversity is much higher in the former case.

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Fig. 9. System performance with channel dependent frequency domain scheduling over non-adjacent frequency blocks.

VII. CONCLUSIONS

3GPP is developing further the specifications for the LTE with the target date for the detailed specification availability is September 2007. The uplink performance is getting very competitive with the use of SC-FDMA based solution. The 3GPP requirements for LTE target for 2-3 times the capacity of the HSPA uplink reference scenario [2].

This paper reviews the current development in 3GPP LTE uplink standardization and highlights a couple of features which enable the target bit rates of up to 50 Mbps: channel dependent frequency domain scheduling and Multi-User-MIMO. System level performance results for channel dependent frequency domain scheduling show significant gain in user throughput gain at the cell edge.

REFERENCES [1] Toskala, A., Holma, H., Pajukoski, K., Tiirola, E., “UTRAN long term

evolution in 3GPP”, The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications 2006, PIMRC’06, Helsinki, Finland

[2] 3GPP technical Report, TR 25.913 version 2.1.0 “Requirements for Evolved UTRA and UTRAN”, 3GPP TSG RAN#28, Quebec, Canada, June 1-3, 2005, Tdoc RP-050384.

[3] Holma, H., Toskala, A., “HSDPA/HSUPA for UMTS”, Wiley, 2006. [4] Czylwik, A., “Comparison between adaptive OFDM and single carrier

modulation with frequency domain equalization”, IEEE Vehicular Technology Conference 1997, VTC-97, Phoenix, pp. 863-869.

[5] Sorger, U., De Brock, I., Schnell, M., “Interleaved FDMA –A New Spread Spectrum Multiple Access Scheme”, IEEE Globecomm 1998.

[6] Nokia, “On specifying the variable bandwidth property for E-UTRAN”, October 2006, Seoul, South Korea, 3GPP Tdoc R1-062815

[7] Nokia, “UL MIMO reference signal structure”, Novermber 2007, Riga, Latvia, 3GPP Tdoc R1-063370

[8] 3GPP technical Report, “TR25.814 v7.1.0 Physical Layer Aspects for Evolved UTRA '', 3GPP TSG RAN#33, Palm Springs, USA, September 19-22, 2006

[9] Nokia, “Channel dependent scheduling in E-UTRA uplink and Text Proposal”, Denver, USA RAN WG1#44, 3GPP Tdoc R1-060295

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