This paper considers large multiple-input multiple-output (MIMO) communication systems with linear precoding and linear minimum mean-squared error (LMMSE) equalization based on the iterative conjugate gradient (CG) algorithm. Convergence of the CG algorithm is fast when the eigenvalues of the received signal's covariance matrix are clustered, suggesting that mean-squared error and receiver complexity can be managed with judicious precoder design. In order to accelerate convergence of an iterative CG receiver, we incorporate constraints on two measures of eigenvalue clustering into the precoder design. Closed-form solutions to the optimal precoders are derived using majorization theory and convex optimization techniques. We show that if there are constraints on receiver complexity, the proposed precoders can improve performance for large MIMO systems operating over slowly time-varying fading channels.

This paper considers linear precoding for time-varying multiple input multiple-output (MIMO) channels. We show that linear minimum mean-squared error (LMMSE) equalization based on the conjugate gradient (CG) method can result in significantly reduced complexity compared with conventional approaches. This reduction is achieved by incorporating a condition number constraint into the precoder optimization framework, which leads to clustered eigen values of the measurement covariance matrix. The cost is a small increase in MSE compared to the optimal precoder.

This paper studies the design and analysis of large multiple-input multiple-output (MIMO) systems with linear precoding and Krylov subspace receivers. We design precoders that can improve performance with low-rank receivers. We then introduce a tool based on potential theory to analyze the convergence behavior of the mean-squared error (MSE). The effectiveness of the proposed precoder and the superexponential convergence of the MSE are demonstrated1.

In this paper, we present a new class of iteratively decoded error correction codes. These codes, which are a modification of irregular repeat-accumulate (IRA) codes, are termed generalized IRA (GIRA) codes, and are designed for improved error floor performance. GIRA codes are systematic, easily encodable, and are decoded with the sum-product algorithm. In this paper we present a density evolution algorithm to compute the threshold of GIRA codes, and find GIRA degree distributions which produce codes with good thresholds. We then propose inner code designs and show using simulation results that they improve upon the error floor performance of IRA codes.

In this paper we design practical interleavers for regular, systematic repeat-accumulate (RA) codes. The new interleavers, which we call L-type and modified L-type interleavers, are deterministic, described by a single parameter, and straightforward to implement. Despite their simple description, the new interleavers are shown to perform equally as well as, or better than, traditional interleavers over a wide range of code lengths and rates.

We present early results from a 4times4 2.4 GHz ISM band multiple-input multiple-output (MIMO) testbed developed at the University of Newcastle for over-the-air evaluation of MIMO algorithms. To provide maximum flexibility for development of 3 GPP Long Term Evolution (LTE) algorithms, the testbed is designed around a field-programmable gate array (FPGA). The testbed features a simple GUI interface for testbed control as well as a MATLAB interface allowing real channel measurements to be used in algorithm development and evaluation. These features are illustrated here with experimental results from the implementation of Alamouti Space-Time Coding with 64-QAM.

Markov-chain Monte-Carlo methods have been demonstrated to offer an attractive alternative to the design of approximate (near optimal) maximum a-posteriori (MAP) detectors for synchronous direct-sequence code-division multiple access (DS-CDMA) and multi-input, multi-output (MIMO) multiple antenna applications. Central to evaluating these method is understanding their convergence properties. In other works, this has been established via simulation, and the underlying theoretical basis has been identified. The contribution of this paper is to extend the theoretical understanding by rigorously establishing both convergence and convergence rate results for a wide class of Metropolis-Hastings methods.

This paper proposes novel interleaver and accumulator structures for systematic, regular repeat-accumulate (RA) codes. It is well known that such codes are amenable to iterative (sum-product) decoding on the Tanner graph of the code, yet are as readily encodable as turbo codes. In this paper, interleavers for RA codes are designed using combinatorial techniques as a basis for deterministic interleaver constructions, yielding RA codes whose Tanner graphs are free of 4-cycles. Further, a generalized RA code accumulator structure is proposed, leading to codes, termed w3RA codes, whose parity-check matrices have many fewer weight-2 columns than conventional RA codes. The w3RA codes retain the low-complexity encoding of conventional RA codes and exhibit improved error-floor performance.

In this paper we present an abstraction of the extrinsic information transfer (EXIT) chart as the interconnection of two nonlinear systems in feedback with each other. We present results on the stability of fixed points for such a dynamical system and use this framework to rederive the well-known stability condition, connecting this to the one-dimensional dynamical system describing the fractions of erasure for low-density parity-check (LDPC) codes on the binary erasure channel (BEC). We observe that the error threshold corresponds to a fixed point bifurcation for this one-dimensional system, and show that this information can be visualized using a well-known tool from control theory: the root locus plot. We further show that these bifurcations can be seen by examining the EXIT chart

In this paper we design deterministic interleavers for systematic repeat-accumulate (RA) codes. Despite their simple description the new interleavers are shown to outperform traditional interleavers over a wide range of code lengths and rates

Recent promising theoretical results for irregular repeat-accumulate (IRA) codes, together with their extremely simple encoding, motivates this investigation into the design and implementation of finite-length IRA codes. In this paper interleavers for RA codes are designed using combinatorial techniques to produce RA codes with Tanner graphs suitable for sum-product decoding. Further, a modified RA code accumulator is used to construct new IRA codes with columns of weight 3 in the accumulator. These new codes, called w3IRA codes, can be designed with flexible degree distributions and retain the simple encoding of traditional IRA codes

Multiuser detection (MUD) strategies have the potential to significantly increase the capacity of wireless communications systems, but for these to be useful they must also be practical for implementation in VLSI circuits. In particular, while the maximum-likelihood (ML) solution is optimal in bit error rate, it cannot be directly implemented due to its exponential computational complexity. Lattice decoders, such as the sphere search, exhibit near-optimal ML performance with reduced complexity, but their application is still limited by computational requirements. Here, a number of optimisations are presented, designed to reduce the computational cost of the sphere search, in the context of VLSI implementation. We also propose parallel implementation strategies for such a detector. This is then combined with a single-pass tree search approach that can be designed to not significantly impair error-rate performance. While the design is targeted towards a MUD application, the concepts may also be applied to a multiple-input multiple-output (MIMO) system, or similar applications.

This paper develops codes suitable for iterative decoding using the sum-product algorithm. By considering a large class of combinatorial structures, known as partial geometries, we are able to define classes of low-density parity-check (LDPC) codes, which include several previously known families of codes as special cases. The existing range of algebraic LDPC codes is limited, so the new families of codes obtained by generalizing to partial geometries significantly increase the range of choice of available code lengths and rates. We derive bounds on minimum distance, rank, and girth for all the codes from partial geometries, and present constructions and performance results for the classes of partial geometries which have not previously been proposed for use with iterative decoding. We show that these new codes can achieve improved error-correction performance over randomly constructed LDPC codes and, in some cases, achieve this with a significant decrease in decoding complexity.

The paper presents a construction of very high-rate low-density parity-check (LDPC) codes based on incidence matrices of unital designs. Like the projective geometry and oval designs, unital designs exist with incidence matrices which are significantly rank deficient. Thus high-rate LDPC codes with a large number of linearly dependent parity-check equations can be constructed. The LDPC codes from unitals have Tanner graphs free of 4-cycles and perform well with iterative decoding, offering new LDPC codes at rates and lengths not available with existing algebraic LDPC codes.

This paper extends the class of low-density parity-check (LDPC) codes that can be algebraically constructed. We present regular LDPC codes based on resolvable Steiner 2-designs which have Tanner graphs free of four-cycles. The resulting codes are (3, /spl rho/)-regular or (4, /spl rho/)-regular for any value of /spl rho/ and for a flexible choice of code lengths.

Gallager introduced low-density parity-check (LDPC) codes in 1962, presenting a construction method to randomly allocate bits in the parity-check matrix subject to certain structural constraints. Since then improvements have been made to Gallager's construction method and some analytic constructions for LDPC codes have been presented. However analytically constructed LDPC codes comprise only a very small subset of possible codes and as a result LDPC codes are still, for the most part, constructed randomly. This paper extends the class of LDPC codes that can be systematically generated by presenting a construction method for regular LDPC codes based on combinatorial designs known as Kirkman triple systems. That is, we construct (3, /spl rho/)-regular codes whose Tanner (1981) graph is free of 4-cycles for any integer /spl rho/.

Analytically constructed LDPC codes comprise only a very small subset of possible codes and as a result LDPC codes are still, for the most part, constructed randomly. This paper extends the class of LDPC codes that can be systematically generated by presenting a construction method for regular LDPC codes based on combinatorial designs known as Kirkman triple systems. We construct (3, /spl rho/)-regular codes whose Tanner graph is free of 4-cycles for any integer /spl rho/, and examine girth and minimum distance properties of several classes of LDPC codes obtained from combinatorial designs.

Previous analysis of linear multiuser receivers for DS-CDMA signals has considered the signal-to-interference (SIR) performance in large systems with each user employing random spreading sequences. Unfortunately, it is only in the special case of the received powers of all users being equal that a resulting integral equation for the SIR has an explicit solution with demonstrable approximation accuracy for finite-sized systems. This paper presents an alternative approach to calculating the SIR performance of MMSE receivers which relies primarily on the law of large numbers, and provides an estimate for the SIR that is valid for arbitrary distributions of the received interferer power.

This correspondence presents several corrections and an addendum to the original article by Feng-Wen Sun and H. Leib (see ibid., vol.43, no.4, p.1205-17, July 1997).