Per-survivor processing (PSP) provides a general framework for the approximation of maximum likelihood sequence estimation (MLSE) algorithms whenever the presence of unknown quantities prevents the precise use of the classical Viterbi algorithm. This principle stems from the idea that data-aided estimation of unknown parameters may be embedded into the structure of the Viterbi algorithm itself. Among the numerous possible applications, the authors concentrate on (a) adaptive MLSE, (b) simultaneous trellis coded modulation (TCM) decoding and phase synchronization, (c) adaptive reduced state sequence estimation (RSSE). As a matter of fact, PSP is interpretable as a generalization of decision feedback techniques of RSSE to decoding in the presence of unknown parameters. A number of algorithms for the simultaneous estimation of data sequence and unknown channel parameters are presented and compared with "conventional" techniques based on the use of tentative decisions. Results for uncoded modulations over interSymbol interference (ISI) fading channels and joint TCM decoding and carrier synchronization are presented. In all cases, it is found that PSP algorithms are clearly more robust than conventional techniques both in tracking a time-varying channel and acquiring its characteristics without training. >

Per-Survivor Processing: a general approach to MLSE in uncertain environments

A discrete multitone (DMT) transceiver design for high bit rate digital subscriber line (HDSL) access is presented and analyzed. The DMT transmitter and receiver structure and algorithms are detailed, and the computational requirements of DMT for HDSL are estimated. At a sampling rate of 640 kHz, using an appropriate combination of a short finite-impulse-response (FIR) equalizer and a length-512 DMT system, 1.6 Mb/s data transmission is possible within the carrier serving area (CSA) at an error rate of 10/sup -7/ on a single twisted pair. A significant performance margin can be achieved when two coordinated twisted pairs are used to deliver a total data rate of 1.6 Mb/s. In terms of a performance-per-computation figure of merit, the DMT system is an excellent candidate for HDSL implementation. >

A discrete multitone transceiver system for HDSL applications

The effect of spatial diversity on the throughput and reliability of wireless networks is examined. Spatial diversity is realized through multiple independently fading transmit/receive antenna paths in single-user communication and through independently fading links in multiuser communication. Adopting spatial diversity as a central theme, we start by studying its information-theoretic foundations, then we illustrate its benefits across the physical (signal transmission/coding and receiver signal processing) and networking (resource allocation, routing, and applications) layers. Throughout the paper, we discuss engineering intuition and tradeoffs, emphasizing the strong interactions between the various network functionalities.

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Great expectations: the value of spatial diversity in wireless networks

Driven primarily by cloud service and data-center applications, short-reach optical communication has become a key market segment and growing research area in recent years. Short-reach systems are characterized by direct detection-based receiver configurations and other low-cost and small form factor components that induce transmission impairments unforeseen in their coherent counterparts. Innovative signaling and digital signal processing (DSP) play a pivotal role in enabling these components to realize their ultimate potentials and meet data rate requirements in cost-effective manners. This paper presents an overview of recent DSP developments for short-reach communications systems and discusses future trends.

Digital Signal Processing for Short-Reach Optical Communications: A Review of Current Technologies and Future Trends

Recently developed techniques that achieve the joint objectives of increasing the signaling rate so as to use the maximum possible channel bandwidth, and then using powerful equalization techniques to cope with the resulting distortion are reviewed. These techniques generalize the Tomlinson-Harashima precoding techniques, which were proposed more than 20 years ago. The principal classes of classical equalization techniques are described, and Tomlinson-Harashima precoding is introduced. It is shown that precoding combines nicely with coded modulation and with a technique known as shaping. It is also shown that, in principle, optimized combinations of coding, shaping, and precoding can achieve nearly the channel capacity of any strictly band-limited, high-SNR Gaussian channel, and in practice can approach capacity within about 3 dB or less. >

Combined equalization and coding using precoding

The authors investigated the detection of trellis codes designed for channels that are intersymbol interference free when they operate in the presence of intersymbol interference. A well-structured reduced-state sequence estimation (RSSE) algorithm is described which can achieve the performance of maximum-likelihood sequence estimation (MLSE) with drastically reduced complexity. Well-defined reduced-state trellises are first constructed by merging the states of the ML supertrellis using set partitioning principles. Then the Viterbi algorithm is used to search these trellises. A special case of RSSE, called parallel decision-feedback decoding, uses the encoder trellis, yet on channels with large attenuation distortion it can provide a significantly better performance than linear equalization. The performance of RSSE is examined analytically and through simulation, and then compared to that of MLSE and ideal decision-feedback equalization. It is noted that the performance advantage of RSSE can be obtained without significantly increasing the decoding delay or complicating an adaptive implementation. >

Reduced-state sequence estimation for coded modulation of intersymbol interference channels

A system is described for high-speed digital subscriber line (HDSL) applications. It uses trellis coding combined with Tomlinson (1971) precoding. Theoretical performance results are provided for uncoded pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM) assuming an ideal decision feedback equalizer (DFE) that includes an optimum feedforward filter. The similarities and differences in behavior are explained. It is shown that combined coding and Tomlinson precoding can achieve the equalization performance of the DFE and full coding gain of the trellis code. Simulation results are given to show that about 3.4 dB additional margin is obtained by using a four-dimensional eight-state trellis code. The receiver complexity is increased by it is quite manageable. >

Combined coding and precoding for PAM and QAM HDSL systems

The two-channel problem is analyzed for the case in which equal power must be allocated to each transmitter. The performance improvement available using coordinated transmission is described. The scheme provides optimum theoretical performance when the SNR varies between channels and/or the interference is correlated, without varying the transmitted power for each channel. It is shown that a simplified scheme using only rotations in the transmitter and receiver achieves most of the improvement of the general scheme. Variations employing fixed rotations in the transmitter are shown to yield significant performance improvement over conventional schemes with very little cost in complexity. The high speed digital subscriber line using pulse amplitude modulation is chosen as an example. >

Coordinated transmission without power variations

In a two-channel transmission system, the signal-to-noise ratio (SNR) at the receiver can in general be different for the channels. Normally, performance will be dominated by the channel with the worst SNR, but it is possible to do much better. The general scheme proposed provides optimum performance when the SNR varies between the channels and/or when the interference is correlated, without varying the transmitted power for each channel. Simplified schemes attain nearly optimum performance under certain channel conditions. >

M.V. Eyuboglu

Papers

Reduced-state sequence estimation for coded modulation of intersymbol interference channels

Combined coding and precoding for PAM and QAM HDSL systems

Coordinated transmission without power variations

Equal-power coordinated transmission for HDSL