Raphaël Visoz

Iterative decoding and channel estimation for space-time BICM over MIMO block fading multipath AWGN channel

In this paper, we consider a generic model of space-time bit-interleaved coded modulation (ST-BICM) on a multiple-input multiple-output (MIMO) Rayleigh fading multipath channel. A practical low-complexity receiver structure performing iteratively MIMO data detection, channel decoding and channel estimation, is presented. The MIMO data detection, employing a reduced-state list-type soft output Viterbi algorithm enables to cope with severe channel intersymbol interference (ISI) without MIMO prefiltering. Among other results, simulations show that our approach can dramatically improve the downlink performance of time-division multiple access (TDMA) systems with high order modulation, keeping a reasonable complexity at the receiver side.

Iterative decoding of convolutionally encoded signals over multipath Rayleigh fading channels

We analyze and compare several strategies for iteratively decoding trellis-encoded signals over channels with memory. Soft-in/soft-out extensions of reduced-complexity trellis search algorithms such as delayed decision-feedback sequence estimating (DDFSE) or parallel decision-feedback decoding (PDFD) algorithms are used instead of conventional BCJR and min-log-BCJR algorithms. It has been shown that for long channel impulse responses and/or high modulation orders where the BCJR algorithm becomes prohibitively complex, the proposed algorithms offer very good performance with low complexity. The problem of channel estimation in practical implementation of turbo detection schemes is studied in the second part. Two methods of channel reestimation are proposed: one based on the expectation-maximization (EM) algorithm and the second on a simple Bootstrap technique. Both algorithms are shown to dramatically improve the performance of the classical pseudo inverse channel estimation performed initially on a training sequence.

Frequency-Domain Block Turbo-Equalization for Single-Carrier Transmission Over MIMO Broadband Wireless Channel

This paper presents a new class of block turbo-equalizers for single-carrier transmission over multiple-input multiple-output (MIMO) broadband wireless channels. The key underlying idea consists in equalizing (nonoverlapping) groups of symbols and detecting their individual space-time components in a disjoint and iterative fashion. This functional split naturally induces new design options that have been accurately listed and described, i.e., choice of distinct criteria for intergroup interference (IGI) equalization and intragroup components detection, yielding hybrid structures, multiple iterative loops, and related scheduling variants. Selected algorithms in the proposed class are compared in terms of performance under various transmission scenarios. For all of them, minimum mean square error IGI equalization certainly occupies a central role (at least for the first iteration) and may be identified as the computational bottleneck. Fortunately, block spread transmission together with cyclic prefix operations make the channel matrix block circulant, thus allowing low-complexity inversion in the Fourier domain

A new class of iterative equalizers for space-time BICM over MIMO block fading multipath AWGN channel

This paper addresses the issue of advanced equalization methods for space-time communications over multiple-input multiple-output block fading channel with intersymbol interference. Instead of resorting to conventional multiuser detection techniques (based on the straightforward analogy between antennas and users), we adopt a different point of view, and separate time equalization from space equalization, thus introducing a higher degree of freedom in the overall space-time equalizer design. Time-domain equalization relies on minimum mean-square error criterion and operates on multidimensional modulation symbols, whose individual components can be detected in accordance with another criterion. In particular, when the optimum maximum a posteriori criterion is chosen, substantial performance gains over conventional space-time turbo equalization have been observed for different transmission scenarios, at the price of an increased, albeit manageable, computational complexity.

Matched filter bound for multichannel diversity over frequency-selective Rayleigh-fading mobile channels

We propose a theoretical evaluation of the matched filter bound (MFB) of L-branch antenna diversity with maximal-ratio-combining (MRC) in a frequency-selective Rayleigh-fading channel. The theoretical derivation is based on a Karhunen-Loeve expansion. Although many previous approaches of this kind have been presented in the past, we give a more general and straightforward one as our model accounts for the correlations between the L branches and between the channel taps within each branch, as well as the mismatch between the average received power per branch. The usefulness of our approach is shown through two different applications. First the benefits of a two branch receiver (L=2) using spatial or polarization diversity for the global system for mobile communication (GSM) and Interim Standard 95 (IS-95) downlink systems both using binary signaling, are compared. Second, with the use of our theoretical model, it is proved that the diversity gain is invariant by any unitary transformation. As a result, and contrary to widespread belief, polarization diversity gain is shown to be invariant by spatial rotation of the receiver antennas.

Sub-optimal turbo-detection for coded 8-PSK signals over ISI channels with application to EDGE advanced mobile system

A turbo-detector concept assuring a favorable tradeoff between performance and complexity is investigated for the novel radio access scheme at enhanced data rates for GSM evolution (EDGE). The high level modulation employed in EDGE (8-PSK) precludes MAP or MLSE turbo-detection, and consequently, computationally efficient sub-optimum solutions have to be considered. A turbo-detector based on a generic SISO delayed decision-feedback sequence estimator (DDFSE) is shown to be a very promising candidate, so long as we couple it with a discrete-time allpass pre-filter turning channel impulse response into minimum phase.

Calibration Issues of PHY Layer Abstractions for Wireless Broadband Systems

Today link-to-system (L2S) interfaces are more and more used in order to speed up complete system-level simulations. In this paper a simple extension to generic incremental redundancy (IR) hybrid automatic repeat request (HARQ) strategies is presented for two well-known L2S interfaces: the exponential effective SNR metric (EESM) and the mutual information effective SNR metric (MIESM). Then we focus on the problem of the L2S interface tuning, which is necessary to achieve the highest accuracy of the prediction models. The standard calibration procedure is compared with a new method, based on the average (over channel and noise) physical (PHY) layer performance. The latter, called average calibration procedure, is less time consuming than the standard procedure. Moreover, we show by simulation that the optimal calibration factors, calculated with the two methods, converge to close values thus obtaining equivalent prediction accuracy for the same L2S interface.

Iterative data detection and channel estimation for advanced TDMA systems

This letter presents a new receiver for Q-ary transmission, where all receiver blocks are embedded in an iterative structure. Packet data transmission in Global Systems for Mobile communications (GSM) and Enhanced Data rates for Global Evolution (EDGE) are considered as examples. A low-complexity soft-in-soft-out detector for EDGE is introduced and its modification suitable for iterative detection is derived. Application of iterative detection and channel estimation techniques in GSM/EDGE shows a significant performance enhancement. Additional improvement may be obtained if the iterative processing is applied to packet retransmission schemes.

Joint channel-network turbo coding for the non-orthogonal multiple access relay channel

In this paper, we investigate the benefit of joint network-channel distributed coding for the multiple-access half-duplex relay channel. In contrast with previous work, the medium access is not orthogonal and the source-to-relay links are not error-free, which render the reality of wireless environments and notably complicate the problem. Indeed, due to the broadcast nature of wireless media, transmitted signals interfere at the relay and at the destination and form multiple-access channels. This entails the need of more sophisticated joint multiuser detection and decoding procedures. Moreover, improper (erroneous) distributed codes may result from unsuccessful decoding at the relay. This second issue is handled with selection relaying. We describe in detail the structure of the encoders, when and how JNC coding is performed, and the structure of the joint multiuser detectors and decoders. For a given spectral efficiency, we show through simulations that our approach provides substantial performance gains compared to conventional joint network-channel distributed coding for the orthogonal multiple-access half-duplex relay channel. Equivalently, for a targeted performance, our approach allows to transmit at higher rates.

A Novel Fast Semi-Analytical Performance Prediction Method for Iterative MMSE-IC Multiuser MIMO Joint Decoding

This paper presents a novel fast performance prediction method for iterative minimum-mean square error interference cancellation based joint decoding in a very general multiuser MIMO setting considering a transmission over block-fading frequency-selective channels. A Gaussian approximation is made for all propagated messages between the outer codes and the space-time multiuser detector. The method relies on a signal-to-interference-plus-noise ratio compression, by means of the mutual information effective signal-to-noise metric (MIESM), which allows a one-dimensional AWGN look-up table to capture the soft decoding result of each distinct user. Simulations show that this method while being extremely simple and fast keeps very accurate and is thus adequate for radio resource management and link adaptation purposes.