Stefan A. Fechtel

Optimum receiver design for wireless broad-band systems using OFDM. I

Orthogonal frequency-division multiplexing (OFDM) is the technique of choice in digital broad-band applications that must cope with highly dispersive transmission media at low receiver implementation cost. In this paper, we focus on the inner OFDM receiver and its functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. The effects of relevant nonideal transmission conditions are thoroughly analyzed: imperfect channel estimation, symbol frame offset, carrier and sampling clock frequency offset, time-selective fading, and critical analog components. Through an appropriate optimization criterion (signal-to-noise ratio loss), minimum requirements on each receiver synchronization function are systematically derived. An equivalent signal model encompassing the effects of all relevant imperfections is then formulated in a generalized framework. The paper concludes with an outline of synchronization strategies.

Optimum receiver design for OFDM-based broadband transmission .II. A case study

This paper details on the design of OFDM receivers. Special attention is paid to the OFDM-specific receiver functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. In part I of the paper, the effects of nonideal transmission conditions have been thoroughly analyzed. To show the impact of the synchronization algorithms-which are most critical in OFDM-on system performance and complexity we consider the design of a complete receiver consisting of symbol synchronization, carrier/sampling clock synchronization and channel estimation. The performance of the algorithms is analyzed and a qualitative estimate of the resulting complexity is given. This allows one to draw conclusions concerning the achievable system performance under realistic complexity assumptions.

A novel approach to modeling and efficient simulation of frequency-selective fading radio channels

A multipath radio channel concept based on channel orthogonalization techniques in two inner product spaces is presented and compared to the conventional approach of modeling each individual Rayleigh or Rice fading multipath ray individually. The proposed simulator can be applied to linear amplitude/phase modulation and linear fading channels including Nyquist filtering. It is shown to be a good approximation to the conventional model in the case of tight rolloff factors. Channels having quasi-or truly continuous delay profiles can now be properly represented at significantly reduced computational complexity. The proposed simulation concept leads to the lowest level of complexity being achieved for the prevailing channel and noise conditions on a particular channel. >

Optimal parametric feedforward estimation of frequency-selective fading radio channels

Estimation of time-varying dispersive radio channels is one of the most important tasks of receiver synchronisation. State-of-the-art adaptive estimators employing decision feedback suffer from error propagation and limited robustness against faster fading. In this paper, the optimal feedforward channel estimator using only known training symbols is systematically derived. Statistical channel information is assumed to be available. For moderately rapid fading channels (snapshot assumption), the optimal estimator is shown to be divided into the two subtasks of maximum-likelihood acquisition and subsequent Wiener filtering of the acquired quantities. If perfect training sequences resulting in optimal performance are used, the Wiener filtering operation collapses into independent filtering of the individual acquired tap estimates, and the resulting channel estimator becomes efficient and flexible. The performance of the optimal estimator is evaluated for important cases. A design example of a near-optimal HF channel estimator/receiver is discussed. Linear interpolation is used to reduce the computational complexity of receiver coefficient adjustment. Simulation results confirm the superiority and robustness of feedforward synchronization having near-optimal performance at reduced complexity. >

Digital Communication Receivers: Synchronization, Channel Estimation, and Signal Processing: Digital E-BK

From the Publisher: Digital Communication Receivers offers a complete treatment on the theoretical and practical aspects of synchronization and channel estimation from the standpoint of digital signal processing. The focus on these increasingly important topics, the systematic approach to algorithm development, and the linked algorithm-architecture methodology in digital receiver design are unique features of this book. The material is structured according to different classes of transmission channels. In Part C, baseband transmission over wire or optical fiber is addressed. Part D covers passband transmission over satellite or terrestrial wireless channels. Part E deals with transmission over fading channels. Designed for the practicing communication engineer and the graduate student, the book places considerable emphasis on helpful examples, summaries, illustrations, and bibliographies. Contents include basic material, baseband communications, passband transmission, receiver structure for PAM signals, synthesis of synchronization algorithms, performance analysis of synchronizers, bit error degradation caused by random tracking errors, frequency estimation, timing adjustment by interpolation, DSP system implementation, characterization, modeling, and simulation of linear fading channels, detection and parameter synchronization on fading channels, receiver structures for fading channels, parameter synchronization for flat fading channels, and parameter synchronization for selective fading channels.

Optimal feedforward estimation of frequency-selective fading, radio channels using statistical channel information

The optimal feedforward channel estimator relying only on known training symbols is systematically derived. Statistical channel information is assumed to be available. The resulting estimator is shown to decompose into the two subtasks of maximum likelihood (ML) acquisition and subsequent Wiener filtering. When using optimal training sequences, the corresponding channel estimator becomes very efficient and flexible. A design example of a near-optimal HF channel estimator is presented and its performance was verified by simulation. Bit error rate simulation results of an entire near-optimal maximum likelihood sequence estimation (MLSE) receiver employing feedforward synchronization demonstrate the superiority and robustness of Wiener channel estimation.<<ETX>>

A new method of evaluating the matched-filter bound for uncoded and trellis-coded transmission over frequency-selective fading diversity channels

The matched filter bound (MFB), which gives a lower bound, on the bit error rate (BER) under idealized conditions, is a useful means of assessing and comparing the average BER for various receiver and code designs. A method for evaluating the MFB based on an eigenvalue approach is presented for uncoded and trellis-coded transmission over selective diversity channels with arbitrary fading multipath profiles. Some examples (TCM with order of diversity 3 and 5; two-ray Rayleigh, GSM, and flat Rician channels) demonstrate the usefulness of the MFB in quantifying the potential benefits of time and multipath diversity transmission.<<ETX>>

Combined equalization, decoding and antenna diversity combining for mobile/personal digital radio transmission using feedforward synchronization

A digital radio transceiver concept employing interleaved trellis coded modulation and optional slow frequency hopping is presented. The receiver concept comprising combined equalization and decoding is extended to incorporate antenna diversity combining, as well as near-optimal receiver synchronization based on purely feedforward channel estimation. The resulting fully coherent and robust receiver has a low computational complexity. The average bit error performance of the novel transceiver operating over flat and selective Rayleigh fading channels is assessed via simulation. Emphasis is placed on the benefits of antenna diversity in combating the bit error rate (BER) degradation resulting from insufficient interleaving.

Matched Filter Bound for Trellis-Coded Transmission over Frequency-Selective Fading Channels with Diversity

Transmission of trellis-coded digital information over time-variant frequency-selective multipath radio channels leads to receiver design considerations and bit error characteristics fundamentally different from those of links using static channels. Making use of diversity is of prime importance in a fading environment. Explicit antenna diversity, implicit time diversity from interleaved trellis-coded modulation, and implicit multipath diversity via energy detection/equalization are three methods of transmission quality enhancement that can be jointly employed without loss of power and bandwidth. The matched filter bound (MFB) is a lower bound on the average bit error rate under ideal conditions, i.e. perfect channel estimation, interleaving, and equalization. In this paper, a new method of evaluating the MFB based on an eigenvalue approach is presented and the analysis extended to include transmission of trellis-coded linearly modulated signals over selective diversity channels having arbitrary fading multipath profiles. MFB results are generated for a number of illuminating examples (trellis codes with effective code lengths up to 5, selective two-ray and GSM Rayleigh as well as flat Rician channels) that demonstrate the potential benefits of implicit time and multipath diversity. Some simulation results are presented giving rise to concluding remarks on the tightness of the bound.

AN INVESTIGATION OF NEAR-OPTIMAL RECEIVER STRUCTURES USING THE M-ALGORITHM FOR EQUALIZATION OF DISPERSIVE FADING CHANNELS

The optimal digital receiver operating in a fading dispersive en vironment incorporates a Viterbi equalizer often being excessi vely complex Reduced complexity equalizers such as the M algorithm maintain near optimal performance yet drastically reduce the computational load In this paper we study the M algorithm as part of a near optimal receiver comprising a whitened matched lter Forney or a matched lter only Ungerboeck The choice of receiver structure a ects metric calculation and bit error performance By invoking the concept of minimal distance an irreducible bit error level is predicted for Ungerboeck s receiver A modi ca tion proposed by Brakemeier and mitigating this e ect is also investigated In order to implement all three receiver structures an algorithm for optimal adjustment of the parameters of the modi ed equa lizer and also an algorithm directly computing the whitening lter of Forney s receiver is outlined Finally simulated bit er ror curves for all three receivers corroborating the analytical forecasts are presented