convolutional codes. This result is inferred on the basis of attaining the highest coding gain in the scenarios considered at comparable complexity. âJim Esch.
Prolog to
Space–Time Codes and Concatenated Channel Codes for Wireless Communications by T. H. Liew and L. Hanzo
The emergence of third-generation (3G) mobile communications standards has been coupled with an associated demand for higher voice and data rates. One of the challenges faced by researchers in mobile communications is the task of maintaining the highest possible bit/symbol throughput for transmission over band-limited wireless channels. The 3G mobile communications standards are expected to provide users with a wide range of services and various data rates. In an effort to support high data rates, the bit/symbol throughput of band-limited wireless channels can be increased by employing multiple antennas. Classically, this involves multiple antennas at the receiver and the maximum ratio combining (MRC) of the received signals. In order to avoid the receiver-diversity-related complexity increase at the mobile station (MS), which has to have a low complexity and weight, receiver-diversity techniques typically have been applied at the base stations (BSs). However, there is a drawback: it only provides diversity gain for the BS’s receivers. Several transmit diversity techniques have been proposed for providing diversity gain for MSs by upgrading the BSs. These techniques fall into three main categories: information feedback-assisted schemes, feedforward or training assisted arrangements, and blind schemes. The space–time trellis (STT) coding scheme was proposed in 1998. By jointly designing the forward error correction coding, modulation, transmit diversity, and optional receiver diversity scheme, one can boost the throughput of band-limited wireless channels. STT coding operates under the assumption that the channel is fading slowly and that the fading is frequency nonselective. The employment of STT coding was then recently extended also to fast fading channels. Given a certain transmission channel, a joint coding and modulation scheme such as trellis-coded modulation (TCM) is capable of reducing the bit error rate (BER) at the cost of typically introducing a higher complexity and, hence, higher costs, as well as coding/interleaving delay and reduced effective throughput. STT codes do perform well, but at the cost of a high degree of complexity. The complexity issue was addressed by Alamouti, who discovered a scheme for transmission using two transmit antennas, which he referred to as a space–time block (STB) code. Later, it was shown by Tarokh that Alamouti’s simple scheme can be generalized to an arbitrary number of receiver antennas. When
Publisher Item Identifier S 0018-9219(02)01131-3.
a channel’s characteristics and the associated bit-error statistics change, different solutions may become more alluring because Gaussian channels, narrow-band and wide-band Rayleigh fading as well as various Nakagami fading channels, impose different impediments. Space–time codes are an especially apt solution to the problem of transmission over fading wireless channels, rather than conventional Gaussian channels. This paper details the encoding and decoding of STB codes and investigates their performance over perfectly interleaved nondispersive Rayleigh fading channels. A system is proposed that consists of STB codes and different channel coders. This kind of system forms the basis for a performance comparison of different systems. The goal of the authors’ performance study is to identify a STB code/channel code combination that offers a good engineering tradeoff with respect to effective throughput, BER performance, and estimated complexity. State-of-the-art transmission schemes based on multiple transmitters and receivers are reviewed and a brief historical perspective on channel coding is given, extending back half a century. In 1948, Shannon predicted that arbitrarily reliable communications would be achievable with the help of channel coding when redundant information was added to the transmitted messages. Latency or information delay is a byproduct of such schemes and researchers have striven to reduce the amount of latency in recent years. An overview of space–time codes and a rudimentary introduction to the MRC technique are provided, leading to the introduction and discussion of STB codes and channel coded space–time codes. The issue of bit-to-symbol mapping is addressed in the context of convolution codes and convolutional coding, as well as Bose–Chaudhuri–Hocquenghem coding-based turbo codes in conjunction with an attractive unity-rate space–time code and multilevel modulation. These schemes are benchmarked against a range of powerful TCM and turbo TCM schemes. The merits of concatenated channel-coded and STB-coded schemes are highlighted in the context of their coding gain versus estimated complexity tradeoffs. The discussion is extended by comparing channel coded STB codes and STT codes. Simulation results are divided into four categories. First, the performance results of space–time codes without using channel codecs are presented. Second, the effect of the binary channel codes’ data and parity bits mapped into different protection classes of multilevel modulation schemes is highlighted. Third, the performance of all
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PROCEEDINGS OF THE IEEE, VOL. 90, NO. 2, FEBRUARY 2002
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proposed channel codes in conjunction with Alamouti’s space–time code G2 is characterized. Finally, a performance comparison of the TC coded STB code G2 with STT codes is provided. The authors’ conclusion is that when communicating over the nondispersive or narrow-band fading channels, the best performance versus complexity tradeoff is constituted by Alamouti’s
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twin-antenna block space–time code concatenated with turbo convolutional codes. This result is inferred on the basis of attaining the highest coding gain in the scenarios considered at comparable complexity. —Jim Esch
PROCEEDINGS OF THE IEEE, VOL. 90, NO. 2, FEBRUARY 2002
