N. Seshadri

Joint data and channel estimation using blind trellis search techniques

A novel method of maximum likelihood sequence estimation is proposed for data that are transmitted over unknown linear channels. This procedure does not require a startup sequence for estimating the channel impulse response. Rather, the data and the channel are simultaneously estimated. It is implemented without any loss of optimality by a trellis search algorithm which searches for the best data sequence from among a number of hypothesized trellises which are constructed from the observed sequence. The number of states in each trellis and the number of trellises grow exponentially with the channel memory. A suboptimal trellis search algorithm is proposed whose complexity at best is slightly higher than that of the adaptive Viterbi algorithm operating with a known channel response. A simplified channel estimation algorithm when the number of data alphabets is greater than 2 is also proposed. Fast convergence of the algorithm in estimating the channel is demonstrated for binary pulse amplitude modulation over a variety of channels. Convergence over a wide range of SNR occurs within 100 symbols. The channel estimation algorithm for multi-level signals converges within 500-1000 symbols. We finally present an application of this algorithm to the problem of sequence estimation in the presence of rapidly time-varying intersymbol interference. The algorithm provides reliable zero-delay decisions which results in a better channel tracking algorithm when compared to previously proposed schemes. >

Multilevel trellis coded modulations for the Rayleigh fading channel

The authors present coded 8-phase-shift-keyed (8-PSK) modulations for the Rayleigh fading channel. The schemes are based on multilevel trellis-coded-modulation constructions and utilize maximum free Hamming distance binary convolutional codes as building blocks. A suboptimal multistage decoder that utilizes interstage interleaving and iterative decoding is proposed and evaluated. Examples are constructed to show that the proposed schemes outperform the best modified codes of the Ungerboeck type due to significantly higher implicit time diversity, yielding seven branches of built-in time diversity, whereas the Ungerboeck code yields four branches of time diversity for a 64-state system. The transmission delay is higher, however. The new schemes can provide three levels of unequal error protection when 8-PSK or 8-differential-phase-shift-keying (8-DPSK) modulations are used. They provide 10-14-dB channel signal-to-noise ratio gain over uncoded 4-DPSK at a bit error rate of 10/sup -3/ for a modest decoding complexity. >

Coded Modulations for Fading Channels: An Overview

It is by now well known that combined channel coding and digital modulation (in short coded modulation) yield power and bandwidth efficient digital transmission schemes for the Gaussian channel. These schemes perform closer to channel capacity than those where coding and modulation are treated separately. More recently, coded modulation methods have been investigated for transmission over Rayleigh fading channels. It has been discovered that systems optimized for the Gaussian channel are not optimum for fading channels. This is primarily due to the importance of built-in time diversity of the coded modulation scheme for fading channels. n n n nIn this paper we give a brief overview of recent developments in signal design for Rayleigh fading channels. Mainly schemes based on phase shift keying are presented. Theoretical guidance for design and identification of the most important system parameters are given. A couple of detailed examples of code designs and their error probability performance are also given. Optimum and suboptimum receivers are discussed. An overview of analytical performance analysis tools, as well as code designs for both trellis coded modulation and block coded modulation is included in the paper. Methods for unequal error protection are given. Potential application areas are identified. An extensive reference list is supplied.

On post-decision symbol-reliability generation

A post-processor for providing reliability information about every decoded data symbol at the output of an arbitrary decoder is proposed. This reliability information is generated by comparing the decoded sequence to a small list of alternative sequences which differ from the decoded sequence at least in the symbol for which the reliability is being evaluated. It is shown that this algorithm is a simplification of the optimal symbol-by-symbol detector (OSSD). The performance of the proposed algorithm is demonstrated by considering three concatenated coding systems.<<ETX>>

Shared time division duplexing: an approach to low-delay high-quality wireless digital speech communications

Various strategies to provide low-delay high-quality digital speech communications in a high-capacity wireless network are examined. Various multiple access schemes based on time-division and packet reservation are compared in terms of their statistical multiplexing capabilities, sensitivity to speech packet dropping, delay, robustness to lossy packet environments, and overhead efficiency. In particular, a low-delay multiple access scheme, called shared time-division duplexing (STDD) is proposed. This scheme allows both the uplink and downlink traffic to share a common channel, thereby achieving high statistical multiplexing gain even with a low population of simultaneous conversations. The authors also propose a choice of low delay, high quality speech coding and digital modulation systems based on adaptive DPCM, with QDPSK or pseudo-analog transmission (skewed DPSK), for use in conjunction with the STDD multiple access protocol. The choice of the alternative systems depends on required end-to-end delay, recovered speech quality and bandwidth efficiency. Typically, with a total capacity of 1 MBaud, 2 ms frame and 8 kBaud speech coding rate, low delay STDD is able to support 48 pairs of users compared to 38, 35, and 16 for TDMA with speech activity detection, basic TDMA and PRMA respectively. This corresponds to respective gains of 26%, 37% and 200%. >

Multi-Level Block Coded Modulations with Unequal Error Protection for the Rayleigh Fading Channel

Block coded 8-DPSK modulations are presented for the time selective Rayleigh fading channel. These coded modulations are based on multi-level code constructions and utilize short binary block codes of length 2 to 8 (binary symbols) as building blocks to construct 8-DPSK codes of length 2 to 8 symbols, respectively. All the constructed block coded modulations have a built in time diversity of at least 2 to combat Rayleigh fading. Unequal error protection (UEP) is obtained by providing higher time diversity for the important data. Further UEP can be obtained by using a non-uniform signal constellation that provides higher product distance (through increased component Euclidean distance) for the more important data. Low transmission delay is obtained by matching the code rate and interleaver to the channel conditions (Doppler spread). Block coded 8-DPSK schemes with rate 1.5-2.25 information bits per symbol and code length 2 to 8 symbols have been designed and simulated for various values of Doppler spread. Gains of more than 15 dB can be obtained at a bit error rate of 10−3 for a fraction of the data that is deemed to be most important. By using different binary codes in the multilevel scheme and by using different nonuniform signal constellations, the unequal error probability profile and the data rates can be controlled in a flexible manner.

Variable-rate sub-band speech coding and matched channel coding for mobile radio channels

The effect of digital transmission errors on a family of variable-rate embedded subband speech coders has been analyzed. It is shown that there is a difference in error sensitivity of four orders of magnitude between the most sensitive and the least sensitive bits of the speech coder. As a result, a family of rate-compatible punctured convolutional codes with flexible unequal error protection capabilities has been matched to the speech coder. On a Rayleigh fading channel with 4-DPSK modulation, as much as 5-dB gain in channel signal-to-noise ratio can be obtained by using four levels of error protection compared to only two levels in more conventional designs. The gain is achieved at no extra bandwidth requirement and at a negligible complexity increase. Among the results, analysis and informal listening tests show that with a four-level unequal error protection scheme, transmission of 12-kb/s speech is possible with little degradation in quality over speech is possible with little degradation in quality over a 16-kb/s channel with an average bit error rate of 2*10/sup -2/ at a vehicle speed of 60 mph.<<ETX>>

When can tentative decisions be used to cancel (linear or nonlinear) intersymbol interference? (With application to magnetic recording channels)

Adaptive cancellation of intersymbol interference using tentative decisions has been proposed by several researchers. These authors report satisfactory results in voiceband data transmission and satellite communications. However there are other applications where this technique is completely ineffective. One such case is cancellation of nonlinear distortion in magnetic recording channels. We formulate precise conditions under which tentative decisions can be effectively used to cancel linear or nonlinear intersymbol interference. In the case of magnetic recording channels, we show that these conditions are satisfied by the precursor interference generated by inductive read heads, resulting in an improvement of the noise margin, of 2 dB at the output of a symbol-by-symbol detector or 1 dB at the output of a Viterbi decoder at a bit error rate of 10/sup -9/. This technique has been incorporated in an experimental VLSI partial response maximum likelihood (PRML) receiver and the results confirmed by laboratory measurements. We also show that no improvement is obtained when this technique is used to compensate nonlinear distortion, which is another common impairment of magnetic recording channels, and we establish fundamental limits for the improvement achievable by any nonlinear equalization technique in magnetic recording.

Pseudo-analog speech transmission in mobile radio communication systems

A low-complexity pseudo-analog speech transmission scheme is proposed for portable communications. It uses a speech coder based on adaptive differential pulse code modulation (ADPCM) in combination with a multilevel digital modulation technique such as M-ary DPSK or M-ary FSK and features low quantization noise, bandwidth efficiency, and robustness to transmission errors. A nonsymmetric M-ary DPSK scheme called skewed M-ary DPSK is proposed to enhance the noisy channel performance. Comparison to conventional analog FM and a digital speech transmission scheme using adaptive predictive coding and forward error correction (FEC) based on convolutional coding shows that the pseudo-analog system has the best objective signal-to-noise ratio performance under most channel conditions. Informal subjective evaluations rate the digital system superior to the pseudo-analog scheme for bad channels and conversely for good channels. It is concluded that the pseudo-analog system can be designed with low delay and high speech quality for good channels with high spectral efficiency. >

Decoding of severely filtered modulation codes using the M-algorithm

The decoding of signals in infinite intersymbol interference caused by severe filtering is considered. A state-variable representation of a linear system is used to define the decoder tree. Tree decoding is then performed by the M-algorithm. It is shown through simulations that performance close to the optimal receiver can be obtained while realizing significant computational savings. Binary phase-shift keying with rectangular baseband pulse shaping and continuous phase/frequency-shift-keyed signals are considered. The channel is modeled as a narrowband filter with two and four-pole Butterworth spectra with symbol-rate normalized 3-dB bandwidths of 0.1 and 0.25.<<ETX>>