Elsevier

Physical Communication

Volume 1, Issue 3, September 2008, Pages 229-235
Physical Communication

Full length article
Turbo equalization receivers for evolved GSM/EDGE radio access network using QAM modulation

https://doi.org/10.1016/j.phycom.2008.07.001Get rights and content

Abstract

For evolution of the GSM/EDGE radio access network (RAN), the use of higher order modulation like 16- and 32-ary quadrature amplitude modulation (QAM) is considered in standardization for increased peak data rates and reduced transmission delays. In this paper, an optimized receiver design for different packet data transmission schemes is proposed. Turbo coding and turbo equalization is discussed for improved power efficiency and interference robustness. An efficient complexity reduction of the equalizer enables the usage of turbo equalization at a complexity comparable to that of separate equalization and decoding for turbo-coded transmission.

Introduction

In order to increase the peak data rates and to reduce transmission delays in the GSM/EDGE system, higher order modulation like 16- and 32-ary quadrature amplitude modulation (QAM) and double (dual) symbol rate (DSR) [1] are currently discussed for standardization [2]. In this paper, we focus on higher order QAM modulation, but the results can also be extended to DSR in a straightforward way. Furthermore, an extension of channel coding in GSM/EDGE 1 is discussed currently based on turbo coding of UMTS Terrestrial Radio Access Network (UTRAN) [3].

A performance analysis of turbo-coded transmission with 16QAM can be found in [4], where in addition the influence of transmission impairments on the performance as well as the gain in terms of network throughput is shown. In this paper, we investigate turbo equalization [5] applied to the conventional convolutionally coded transmission schemes as competitor for the turbo-coded transmission schemes, and compare performance of different reduced-complexity (inner) equalizers 2 for the 16QAM and 32QAM packet data transmission schemes. An advantage of the turbo equalization approach is, that the conventional coding format can be preserved, so that even conventional receivers (separate equalization and decoding) are applicable in the system. Optionally, the receiver may apply a varying number of equalization and decoding iterations, depending on the signal quality (e.g. signal-to-interference-plus-noise ratio (SINR)).

Several turbo equalization schemes for time division multiple access (TDMA) systems based on PSK modulation can be found in the literature, e.g. [7], [8], [9], [10], [11], [12], [13], [14]. For turbo equalization, the (inner) equalizer receives extrinsic a priori information input from the outer channel decoder and provides extrinsic a posteriori information output to the decoder. The decoder processes the input from the equalizer and produces extrinsic a posteriori information on the coded bits, so that equalization performance improves from iteration to iteration. As (inner) equalizer, reduced-complexity variants of the BCJR algorithm [15], [16], [17] are selected, which are based on joint reduced-state sequence estimation (JRSSE) with Ungerboeck set partitioning [18]. We show that a large complexity reduction (compared to the full-state equalizer) is feasible, similar to 8PSK modulation [19]. Furthermore, a simplified minimum mean-squared error (MMSE) soft-output detector [7], [8], [9], [10] with soft cancellation of pre- and postcursor intersymbol interference (ISI) extended to higher order modulations can be applied for subsequent turbo equalization iterations. For some of the transmission schemes considered, extrinsic information transfer (EXIT) chart [20] convergence analyses are performed.

The paper is structured as follows. The system model is introduced in Section 2, and the receiver equalization algorithm is presented in Section 3. In Section 4, performance is analyzed and simulation results are given for all currently considered GSM/EDGE modulation and coding schemes (MCSs).

Section snippets

System model

The system model in equivalent discrete-time complex baseband representation is shown in Fig. 1. After encoding of the source information bits ds[k] and successive interleaving, the encoded bits d[k] are mapped to linear modulation symbols a[k] and 4 bursts3 of NS=120 symbols each are transmitted over the channel with impulse response h[k] of order qh, which includes transmit pulse shaping and receive

Reduced-complexity BCJR equalization

Reduced-complexity variants of the BCJR algorithm [15], [16], [17] based on reduced-state sequence estimation (RSSE) with Ungerboeck set partitioning [18] are employed as (inner) component for turbo equalization in order to obtain a low-complexity equalizer with reduced number of (hyper)states. Only a low number of states compared to joint maximum-likelihood sequence estimation (JMLSE) may be allowed. A survivor map is created within the forward recursion, so that the selected states of all

Simulation results

For Monte-Carlo simulations of the proposed transmission schemes, the GSM/EDGE typical urban (TU) channel profile is used (qh=5). Perfect channel knowledge is assumed. Using the conventional training sequence based channel estimation of GSM/EDGE, performance typically degrades by 1.2–1.5 dB compared to ideal channel knowledge. In [14] it has been shown that the inaccuracy of the estimated channel impulse response may be reduced by including channel re-estimation in the iterative equalization

Conclusion

Different packet data transmission schemes for GSM/ EDGE have been analyzed in this paper. It could be observed that turbo equalization is an interesting candidate as a receiver algorithm for the convolutionally coded schemes of the evolved GSM/EDGE standard at moderate complexity increase. Compared to the turbo-coded transmission without turbo equalization, similar performance is already observable after the first turbo equalization iteration for most considered transmission schemes. With more

Acknowledgments

The authors would like to thank Thomas Wagner, Jürgen Petersen, and Hans Kalveram of Philips/ NXP Semiconductors for their support and discussions.

Patrick Nickel received his diploma degree in electrical engineering from the University of Technology Darmstadt, Germany, in 2002, and is currently working towards his Ph.D. degree at the University of Erlangen–Nuremberg. His research interests include interference cancellation methods for mobile radio receivers and turbo equalization algorithms.

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    Patrick Nickel received his diploma degree in electrical engineering from the University of Technology Darmstadt, Germany, in 2002, and is currently working towards his Ph.D. degree at the University of Erlangen–Nuremberg. His research interests include interference cancellation methods for mobile radio receivers and turbo equalization algorithms.

    Wolfgang Gerstacker was born in Nuremberg, Germany, in 1966. He received the Dipl.-Ing. degree in electrical engineering, the Dr.-Ing. degree for a thesis on equalization concepts for fast digital transmission over twisted pair lines, and the Habilitation degree (Venia Legendi) for work on digital transmission concepts for the EDGE mobile communications system from the University of Erlangen–Nuremberg, Erlangen, Germany, in 1991, 1998, and 2004, respectively.

    From 1992 to 1998, he was a Research Assistant at the Telecommunications Institute of the University of Erlangen–Nuremberg. From 1998 to 2002, he was a Consultant for mobile communications and an External Lecturer in digital communications at the University of Erlangen–Nuremberg. From 1999 to 2000, he was a Postdoctoral Research Fellow for a six-month period at the University of Canterbury, Christchurch, New Zealand, sponsored by a fellowship from the German Academic Exchange Service (DAAD). Since 2002, he has been a Senior Researcher and Lecturer at the Institute for Mobile Communications of the University of Erlangen–Nuremberg. His current research interests include wireless communications, detection, equalization and parameter estimation, blind techniques, space-time processing, OFDM, MIMO systems, LTE and WLANs.

    In 2001, he was a co-recipient of the Research Award of the German Society for Information Technology (ITG). For work on single antenna interference cancellation for GSM, he was a co-recipient of the EEEfCOM Innovation Award 2003 and of the Vodafone Innovation Award 2004. He is a Member of the Editorial Board of EURASIP Journal on Wireless Communications and Networking and has served as a TPC Member of various conferences such as ICC, Globecom, and WCNC. He is an External Lecturer at the University of Kiel.

    Christoph Reck was born in Forchheim, Germany, in 1981. He received his diploma degree in electrical engineering from the University of Erlangen–Nuremberg in April 2007 for a thesis on high order modulation and turbo coding for GSM/EDGE. He is currently working with Institute for High Frequency Technology of the University of Erlangen–Nuremberg. His research topics are algorithms for direction of arrival estimation from secondary surveillance radar signals, phased antenna arrays and analog/digital receiver hardware.

    Wolfgang Koch was born in Hannover in 1949. He received the Dipl.–Ing. degree and the Dr.–Ing. degree in communications from Technical University of Hannover, Germany, in 1976 and 1982, respectively. From 1983 to 1995, he was with Philips Kommunikations Industrie AG in Nuremberg, where he participated in GSM standardization for 4 years, worked on development of the UMTS system and served as a head of department of basic research in cellular infrastructure systems.

    From 1995 to 2001, he was with Ericsson Eurolab Germany GmbH in Nuremberg as a senior expert in mobile communications and head of research. Since 2001, he holds the Chair of Mobile Communications of the University of Erlangen–Nuremberg. His research interests include mobile communications systems such as GSM/EDGE, UMTS, WLANs, wireless transmission, radio access networks, radio resource management, and systems with multiple antennas (MIMO systems).

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