Lihai Liu

Interleave division multiple-access

This paper provides a comprehensive study of interleave-division multiple-access (IDMA) systems. The IDMA receiver principles for different modulation and channel conditions are outlined. A semi-analytical technique is developed based on the density evolution technique to estimate the bit-error-rate (BER) of the system. It provides a fast and relatively accurate method to predict the performance of the IDMA scheme. With simple convolutional/repetition codes, overall throughputs of 3 bits/chip with one receive antenna and 6 bits/chip with two receive antennas are observed for IDMA systems involving as many as about 100 users.

On interleave-division multiple-access

This work provides a comprehensive study of IDMA systems. We first outline the basic IDMA principles in single-path and multi-path environments. We then describe a simple semi-analytical technique to assess the performance of IDMA systems, based on which we develop a power allocation scheme for performance optimization. We also discuss the use of low-rate codes to further enhance the power efficiency of IDMA systems. Simulation results demonstrate the advantages of the IDMA scheme in terms of both bandwidth and power efficiencies. For example, with simple convolutional/repetition codes an overall throughput of 8 bits/chip is achieved in single antenna systems. With turbo-Hadamard codes, performance at 1.4 dB away from the theoretical limit is demonstrated in a Gaussian MAC.

A simple approach to near-optimal multiuser detection: interleave-division multiple-access

This paper presents an asynchronous interleave-division multiple-access (IDMA) scheme, in which users are distinguished by different chip-level interleaving methods instead of by different signatures as in a conventional code-division multiple -access (CDMA) scheme. A very low-cost iterative detection algorithm is derived for the IDMA scheme based on a chip-by-chip detection principle. The proposed scheme can achieve nearly optimal performance for systems with a large number of users. Furthermore, receiver simplicity as well as high performance can be maintained in multipath environments.

Analysis and optimization of CDMA systems with chip-level interleavers

In this paper, we present an unequal power allocation technique to increase the throughput of code-division multiple-access (CDMA) systems with chip-level interleavers. Performance is optimized, respectively, based on received and transmitted power allocation. Linear programming and power matching techniques are developed to provide solutions to systems with a very large number of users. Various numerical results are provided to demonstrate the efficiency of the proposed techniques and to examine the impact of system parameters, such as iteration number and interleaver length. We also show that with some very simple forward error correction codes, such as repetition codes or convolutional codes, the proposed scheme can achieve throughput reasonably close to that predicted by theoretical limit in multiple access channels.

Simple iterative chip-by-chip multiuser detection for CDMA systems

This paper examines several low-cost iterative chip-by-chip multiuser detection algorithms for chip-interleaved code-division multiple-access (CDMA) systems in multipath and multiple antenna environments. The complexities (per user) of these algorithms are very low and increase either linearly or quadratically with path number. It is shown that a system employing a rate-1/2 16-state convolutional code and a length-8 spreading sequence can support more than 100 users with two receive antennas.

Approaching the capacity of multiple access channels using interleaved low-rate codes

We show that the capacity of a Gaussian multiple access channel can be approached by interleaved low-rate codes together with a simple chip-by-chip iterative decoding strategy. Based on a rate /spl ap/ 1/69 code and with a total of 35 simultaneous users (the aggregate rate /spl ap/ 1/2), performance of BER=10/sup -5/ is observed at E/sub b//N/sub 0/ /spl ap/ 1.4 dB, which is close to the corresponding capacity limit (E/sub b//N/sub 0/ /spl ap/ 0 dB).

Analysis and design of IDMA systems based on SNR evolution and power allocation

We show that the performance of an interleave-division multiple-access (IDMA) system can be assessed by tracking the signal-to-noise-ratio (SNR) evolution of the iterative chip-by-chip (CBC) detection process. Based on this we develop a power allocation technique for performance optimization. Very high throughput (say 8 bits/chip) is observed for IDMA without sophisticated channel coding.

A simple, unified approach to nearly optimal multiuser detection and space-time coding

Techniques using interleaving as the basic means for signal separation are introduced for both multiple access systems and multiple transmit antenna systems. A very low-cost chip-by-chip iterative detection algorithm is presented. The proposed schemes can achieve nearly optimal performance for system with a large numbers of users or transmit antennas.

Power Allocation for Multiple Access Systems with Practical Coding and Iterative Multi-User Detection

This paper is concerned with the power allocation problem for practically coded interleave-division multiple-access (IDMA) systems with iterative multi-user detection (MUD) over multiple access channels (MACs). Both Additive White Gaussian Noise (AWGN) and fading channels are considered. For AWGN channels, two power allocation methods based on linear programming are discussed and compared with an interior-point method (IPM). These techniques are extended to power allocation over fading channels. Numerical results show that, with power optimization, IDMA can achieve significant performance improvements over TDMA in fading environments.

An extending window MMSE turbo equalization algorithm

We present a modified turbo minimum mean-squared error (MMSE) equalization algorithm using an extending window approach. We show that the new method can achieve nearly the same performance and considerable cost reduction compared with a recently proposed sliding window MMSE equalization technique.