In this paper we review the most peculiar and interesting information-theoretic and communications features of fading channels. We first describe the statistical models of fading channels which are frequently used in the analysis and design of communication systems. Next, we focus on the information theory of fading channels, by emphasizing capacity as the most important performance measure. Both single-user and multiuser transmission are examined. Further, we describe how the structure of fading channels impacts code design, and finally overview equalization of fading multipath channels.

We apply coset codes to adaptive modulation in fading channels. Adaptive modulation is a powerful technique to improve the energy efficiency and increase the data rate over a fading channel. Coset codes are a natural choice to use with adaptive modulation since the channel coding and modulation designs are separable. Therefore, trellis and lattice codes designed for additive white Gaussian noise (AWGN) channels can be superimposed on adaptive modulation for fading channels, with the same approximate coding gains. We first describe the methodology for combining coset codes with a general class of adaptive modulation techniques. We then apply this methodology to a spectrally efficient adaptive M-ary quadrature amplitude modulation (MQAM) to obtain trellis-coded adaptive MQAM. We present analytical and simulation results for this design which show an effective coding gain of 3 dB relative to uncoded adaptive MQAM for a simple four-state trellis code, and an effective 3.6-dB coding gain for an eight-state trellis code. More complex trellis codes are shown to achieve higher gains. We also compare the performance of trellis-coded adaptive MQAM to that of coded modulation with built-in time diversity and fixed-rate modulation. The adaptive method exhibits a power savings of up to 20 dB.

This paper proposes a bandwidth-efficient fading-resistant transmission scheme which implements transmitter diversity using L antennas at the base station. When the antennas are spaced sufficiently far apart, the transmission from each antenna undergoes a different degree of fading. These transmissions are coordinated to mitigate the effects of Rayleigh fading and the mobile receiver can recover the entire L-dimensional transmitted vector signal as long as the signal energy of at least one coordinate is large enough. L-dimensional fading-resistant signal constellations are generated by maximizing a figure of merit for the Rayleigh fading channel. This scheme offers a significant performance improvement over a conventional single-antenna BPSK scheme when coding is ineffective due to slow fading. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada, the Canadian Space Agency, and the Communications Research Centre in the form of scholarshi...

A power and bandwidth efficient concatenated coding system for reliable data transmission on time- and frequency--selective mobile fading channels is considered. Multi--tone signaling with spectrally overlapping pulses is achieved by orthogonal frequency--division multiplexing (OFDM). The inner coding/modulation system is trellis--coded modulation (TCM) consisting of two binary convolutional codes, outputting 16QAM symbols. This class of codes was found to be optimum in sense of complexity and performance on Rayleigh/Rician fading channels. A soft--output (Viterbi) decoding algorithm accepts channel state information, and even delivers reliability information to an outer decoder. The outer code is a rate--compatible punctured convolutional (RCPC) code providing unequal error protection (UEP). Concatenated coding in conjunction with double interleaving and slow frequency hopping (SFH) provides efficient dual diversity. A two-- dimensional pilot sequence multiplexed into the time and frequency sequence according to the sampling theorem allows a robust, truly coherent carrier synchronization, which can be realised, e.g., by a two--dimensional Wiener filter. Emphasis here is on sophisticated, yet reduced complexity channel estimation (CE) making use of the multi--tone structure and a--priori channel knowledge. The use of a guard interval circumvents the implementation of explicit equalization even under severe multipath distortions. Results (BER und power/bandwidth diagram) are presented on basis of a hilly--terrain land--mobile fading channel with f Dmax =200 Hz one--sided maximum Doppler frequency, max 64 s maximum echo delay, and system parameters (2 \Delta 128 kbit/s data rate, bit sensitivity classes ranging from 5 \Delta 10

Adaptive coded modulation is a powerful method for achieving a high spectral efficiency over fading channels. Recently proposed adaptive schemes have employed set-partitioned trellis-coded modulation (TCM) and have adapted the number of uncoded bits on a given symbol based on the corresponding channel estimate. However, these adaptive TCM schemes will not perform well in systems where channel estimates are unreliable, since uncoded bits are not protected from unexpected fading. In this paper, adaptive bit-interleaved coded modulation (BICM) is introduced. Adaptive bit-interleaved coded modulation schemes remove the need for parallel branches in the trellis - even when adapting the constellation size, thus making these schemes robust to errors made in the estimation of the current channel fading value. This motivates the design of adaptive BICM schemes, which will lead to adaptive systems that can support users with higher mobility than those considered in previous work. In such systems...

The performance of uncoded PSK schemes over fading channels is only inversely proportional to the signal-tonoise ratio, which is not very reliable for digital communication applications. Improving their performance has been approached by means of coded modulation schemes where code redundancy combined with interleaving introduces some degree of diversity that depends on the complexity of the code. This paper proposes an alternative way in improving the performance of PSK schemes over fading channels by looking at the reference scheme first. It is shown that by using interleaving combined with a proper signal constellations and still using symbol-by-symbol detection, a higher diversity is obtained and the performance of uncoded PSK schemes over fading channels is considerably improved. With this simple modification, the obtained performance is comparable to that of the best 4-state Trellis Coded Modulation (TCM) 8PSK schemes reported in the literature. By optimizing the reference scheme...

Abstract—This paper considers the use of sequence maximum ��������� � ��������� � (MAP) decoding of trellis codes. A MAP receiver can exploit any “residual redundancy ” that may exist in the channelencoded signal in the form of memory and/or a nonuniform distribution, thereby providing enhanced performance over very noisy channels, relative to maximum likelihood (ML) decoding. The paper begins with a first-order two-state Markov model for the channel encoder input. A variety of different systems with different source parameters, different modulation schemes, and different encoder complexities are simulated. Sequence MAP decoding is shown to substantially improve performance under very noisy channel conditions for systems with low-to-moderate redundancy, with relative gain increasing as the rate increases. As a result, coding schemes with multidimensional constellations are shown to have higher MAP gains than comparable schemes with two-dimensional (2-D) constellations. The second part of the paper considers trellis encoding of the code-excited linear predictive (CELP) speech coder’s line spectral parameters (LSP’s) with four-dimensional (4-D) QPSK modulation. Two source LSP models are used. One assumes only intraframe correlation of LSP’s while the second one models both intraframe and interframe correlation. MAP decoding gains (over ML decoding) as much as 4 dB are achieved. Also, a comparison between the conventionally designed codes and an I-Q QPSK scheme shows that the I-Q scheme achieves better performance even though the first (simpler) LSP model is used. Index Terms — AWGN and Rayleigh channels, CELP speech coding, Markov sources, sequence MAP decoding, source redundancy, trellis codes. I.

This thesis addresses low-complexity algorithms in digital receivers. This includes algorithms for estimation, detection, and source coding.

Abstract: The system spectral efficiency is considered for the single-cell DS-CDMA uplink with random spreading and flat Rayleigh fading channels. Based on the asymptotic analysis for large systems, the signal-to-interferenceand-noise ratio after the linear minimum mean-squarederror detector is determined analytically for multi-code transmission. This measure is used to obtain the maximum rate and thus efficiency by applying the concepts of coding at capacity and at cut-off rate. The efficiency of multiple codes is compared to multi-level modulation for different signal-to-noise ratios and system loads. It is shown that multi-code transmission improves efficiency for lower loads, but cannot increase its maximum value. In contrast, with multi-level modulation the maximum efficiency can be increased in the low-noise region. The accuracy of these results is investigated by means of simulations. Both simulation and asymptotic theoretical results are very close. Finally, the optimal parameters are compared to the UMTS parameter set. The study suggests increasing both spreading factor and modulation order to achieve better spectral efficiency when using an linear minimum mean-squarederror detector. 1.

Abstract—We present an efficient algorithm to compute the distance spectrum of a general finite intersymbol interference (ISI) channel, whose complexity is lower than those of existing methods. Closed-form expressions are derived for both input–output Euclidean distance enumerators and asymptotic distance spectrum shapes for 2-tap and 3-tap ISI channels. Coded and/or precoded ISI channels are also discussed. Index Terms—Distance spectrum, input–output Euclidean distance enumerator (IOEDE), input–output weight enumerator, intersymbol interference (ISI) channel, precoding. I.