ABSTRACT We propose the use of low‐rate layered serially concatenated low‐density generator matrix (SCLDGM) codes in interleave‐division multiple access (IDMA) systems to approach the multiuser capacity. We study the behavior of the soft interference cancellation (SIC) detector employed in IDMA systems and design the channel codes using EXtrinsic Information Transfer (EXIT) evolution, aiming at optimizing the system performance. Simulation results show that the designed codes approach the theoretical limits and outperform previous IDMA schemes based on Turbo‐Hadamard codes. Copyright © 2010 John Wiley & Sons, Ltd.

Recently, the use of analog joint source-channel coding for the transmission of data samples at high rates over AWGN channels has been proposed in the literature. Simulation results have shown that this analog scheme performs close to the theoretical limits for several source distributions and transmission rates. In this paper we study the performance of such a system in comparison with optimized capacity approaching digital Bit Interleaved Coded Modulation (BICM) schemes. We show that in practical situations the analog transmission performs better than the digital scheme with a much lower encoding and decoding complexity when considering Gaussian and Laplacian source distributions.

Recently, analog joint source-channel coding has been proposed as a means of achieving near-optimum performance for high data rates with a very low complexity. However, no experimental evaluation showing the practical feasibility of this scheme has been performed to date. In this paper, we describe a software-defined radio implementation of an analog joint source-channel coded wireless transmission system. Experimental evaluation carried out in an indoor environment making use of a wireless testbed show that the performance perfectly matches that originally reported by simulations in additive white Gaussian noise channels for signal-to-noise ratio values below 20 dB.

We study the ability of low-rate Serially-Concatenated Low Density Generator Matrix (SCLDGM) codes to approach theoretical limits. Although two layer SCLDGM codes approach capacity for medium rates (0.25 <; R <; 0.9), they do not perform adequately for lower rates. In this case, substantial performance improvements are obtained using three layers.

Abstract We analyze Multiple‐Input Multiple‐Output (MIMO) coded modulation systems where either Bit‐Interleaved Coded Modulation (BICM) with spatial multiplexing or concatenation of channel coding and Space‐Time Block Codes (STBCs) is used at transmission, assuming iterative Turbo‐like decoding at reception. We optimize Serially‐Concatenated Low‐Density Generator Matrix (SCLDGM) codes (a subclass of LDPC codes) for each system configuration, with the goal of assessing its ability to approach the capacity limits in either ergodic or quasi‐static channels. Our focus is on three relevant STBCs: the Orthogonal Space‐Time Block Codes (OSTBCs) for two transmit antennas (i.e., the Alamouti code), which enables optimum detection with low complexity; the Golden code, which provides a capacity increase with respect to the input constellation; and Linear Dispersion (LD) codes, which enable practical detection in asymmetrical antenna configurations (i.e., more transmit than receive antennas) for cases in which optimum detection is infeasible. We conclude that BICM without concatenation with STBCs is in general the best option, except for Alamouti‐coded 2×1 and Golden‐coded 2×2 MIMO systems. Copyright © 2009 John Wiley & Sons, Ltd.

We propose a distributed multilayer architecture that provides user access to a MIMO testbed at the convenient abstraction level for researchers. This novel architecture releases the user from the need of low-level programming, making the MIMO testbed accessible through high level software (i.e. Matlab or similar). The layers of this architecture are highly decoupled among them, allowing us to run each layer on a different PC. Also, each layer permits independent extension or customisation as needed, for example to implement new testbed features such as a feedback channel or to deploy multiuser MIMO scenarios.

Abstract In this paper we employ a 2 × 2 Multiple‐Input Multiple‐Output (MIMO) hardware platform to evaluate, in realistic indoor scenarios, the performance of different space‐time block coded (STBC) transmissions at 2.4 GHz. In particular, we focus on the Alamouti orthogonal scheme considering two types of channel state information (CSI) estimation: a conventional pilot‐aided supervised technique and a recently proposed blind method based on second‐order statistics (SOS). For comparison purposes, we also evaluate the performance of a Differential (non‐coherent) space‐time block coding (DSTBC). DSTBC schemes have the advantage of not requiring CSI estimation but they incur in a 3 dB loss in performance. The hardware MIMO platform is based on high‐performance signal acquisition and generation boards, each one equipped with a 1 GB memory module that allows the transmission of extremely large data frames. Upconversion to RF is performed by two RF vector signal generators whereas downconversion is carried out with two custom circuits designed from commercial components. All the baseband signal processing is implemented off‐line in MATLAB®, making the MIMO testbed very flexible and easily reconfigurable. Using this platform we compare the performance of the described methods in line‐of‐sight (LOS) and non‐line‐of‐sight (NLOS) indoor scenarios. Copyright © 2007 John Wiley & Sons, Ltd.

Design of MIMO capacity-approaching codes relies on matching the extrinsic information transfer (EXIT) functions of the code and the detector. We focus on list sphere detection (LSD) because it presents the best performance/complexity tradeoff. We obtain the EXIT functions of the most representative LSDs: maximum likelihood (ML) and maximum a posteriori (MAP). We then utilize these EXIT functions to design good IRA and SCLDGM codes that perform close to the capacity limits.

Low density generator matrix (LDGM) codes are a particular class of low density parity check (LDPC) codes with very low encoding complexity. Single LDGM codes present high error-floors, which can be substantially reduced with the serial concatenation of two LDGM (SCLDGM) codes. We propose a technique to obtain good SCLDGM codes using extrinsic information transfer (EXIT) functions in a novel way. Although the optimization is performed for AWGN channels with binary signaling, the resulting codes are also optimal for AWGN and perfectly-interleaved Rayleigh fading channels with non-binary signaling and perfect CSI at reception, provided that Gray mapping is utilized. Optimized regular and irregular SCLDGM codes outperform heuristically-designed LDGM codes existing in the literature, and have a performance similar to or better than that of irregular repeat accumulate (IRA) codes.

We investigate the ability of serially-concatenated low density generator matrix (SCLDGM) codes to approach the capacity of ergodic multiple-input multiple-output (MIMO) channels. First, we explain how to design good SCLDGM codes using extrinsic information transfer (EXIT) analysis and optimum detection. However, optimum detection becomes unfeasible in many cases of practical interest where the number of transmitting antennas and/or the number of bits per constellation symbol is large. In this case, we resort to list sphere detection (LSD), considering two versions: maximum likelihood (ML) and maximum a posteriori (MAP). Again, we use EXIT analysis to obtain good SCLDGM codes for these specific LSD versions. We provide examples of optimized rate 1/2 SCLDGM codes and show that they perform within 1 dB from the theoretical capacity limit for a large number of antenna configurations and modulation formats. Finally, we discuss how to iteratively improve channel estimation using the soft information computed during the decoding process. We also show how the complexity of channel estimation is reduced by utilizing the candidate list calculated by the LSD.

In this work we study the ability of regular serially concatenated low-density generator matrix (SCLDGM) codes to approach the capacity of MIMO channels with two transmit antennas. We focus on two schemes: spatial multiplexing in a BICM fashion and concatenation with the Alamouti code. We show that good SCLDGM codes designed for SISO channels also approach MIMO capacity in the Alamouti concatenated scheme. These codes, however, do not perform adequately in spatial multiplexing and, in this case, have to be specifically designed for each antenna configuration and modulation format. Decoding complexity in spatial multiplexing is larger but the performance is in general better because the Alamouti code reduces the MIMO channel capacity when there is more than one receiving antenna

We propose a bit interleaved coded modulation with iterative decoding (BICM-ID) scheme for MIMO systems using systematic linear codes with low-density generator matrix (LDGM). We compare the performance of the proposed scheme with the (constrained input) channel capacity and show its ability to perform close to the theoretical limit. The main advantage of the proposed structure relies on its low encoding and decoding complexity.

We present a novel turbo equalization structure for space-time coded systems. It is based on performing channel equalization via a decision feedback equalizer (DFE) designed using a novel maximum likelihood criterion. The resulting optimization problem is solved using the space-alternating generalized EM (SAGE) algorithm. This results in expressions for the forward and backward filters of the DFE that take large benefit from the more and more refined estimations of the transmitted symbols iteratively obtained by the outer decoder.

In this paper we investigate how to increase the capacity of GSM radio interfaces in subway tunnel environments by means of antenna arrays and Space-Time Coding (STC) techniques. We first consider space-time equalization techniques that only account for the noise and the Intersymbol Interference (ISI). Then, we show how to improve the performance by considering of a ST encoder at transmission and an iterative ST decoding algorithm at reception.

We propose a novel approach to estimating multiple-input-multiple-output (MIMO) channels in space-time coded systems. The channel is assumed to introduce dispersion in both the spatial and temporal dimensions. The channel estimator is obtained by applying the maximum likelihood principle, not only over a known pilot sequence, as in classical least-squares approaches, but also over all the symbols (both known and unknown) in a data frame. Since this results in an optimization problem without closed-form solution, we utilize an iterative method, the expectation-maximization (EM) algorithm, to calculate the solution. The resulting channel estimator is particularly suitable to be used in a turbo equalizing structure because it benefits from the a posteriori probabilities about the transmitted symbols computed at each decoding iteration of the turbo process.

Abstract In this paper, we investigate how to increase the capacity of GSM (Global System for Mobile communications) radio interfaces in subway tunnel environments by means of antenna arrays and Space–Time (ST) receivers. We address the modeling, both theoretically and experimentally, of the multiple‐antenna wireless channels encountered in subway tunnels. We demonstrate that propagation is conveniently modeled by a flat‐fading channel, but there exist strong spatial correlations among channel components that decrease capacity and affect receivers' performance. Different space–time GSM receiving strategies have also been investigated. We first consider ST equalization techniques that only account for the noise and the controlled Intersymbol Interference (ISI) introduced by the modulation format employed in GSM. Then, we analyze the performance of receivers that incorporate ST coding capabilities and show the superior performance of iterative MAP (Maximum a Posteriori) receivers that interchange soft information among the equalizer and the ST decoder. All receivers are evaluated with experimental channels measured in the subway of Paris. Copyright © 2002 John Wiley & Sons, Ltd.