Coherent receiver design and analysis for interleaved division multiple access (IDMA)

This thesis discusses a new multiuser detection technique for cellular wireless communications. Multiuser communications is critical in cellular systems as multiple terminals (users) transmit to base stations (or wireless infrastructure). Efficient receiver methods are needed to maximise the performance of these links and maximise overall throughput and coverage while minimising inter-cell interference. Recently a new technique, Interleave-Division Multiple Access (IDMA), was developed as a variant of direct-sequence code division multiple access (DS-CDMA). In this new scheme users are separated by user specific interleavers, and each user is allocated a low rate code. As a result, the bandwidth expansion is devoted to the low rate code and not weaker spreading codes. IDMA has shown to have significant performance gains over traditional DS-CDMA with a modest increase in complexity. The literature on IDMA primarily focuses on the design of low rate forward error correcting (FEC) codes, as well as channel estimation. However, the practical aspects of an IDMA receiver such as timing acquisition, tracking, block asynchronous detection, and cellular analysis are rarely studied. The objective of this thesis is to design and analyse practical synchronisation, detection and power optimisation techniques for IDMA systems. It also, for the first time, provides a novel analysis and design of a multi-cell system employing a general multiuser receiver. These tools can be used to optimise and evaluate the performance of an IDMA communication system. The techniques presented in this work can be easily employed for DS-CDMA or other multiuser receiver designs with slight modification. Acquisition and synchronisation are essential processes that a base-station is required to perform before user's data can be detected and decoded. For high capacity IDMA systems, which can be heavily loaded and operate close to the channel capacity, the performance of acquisition and tracking can be severely affected by multiple access interference as well as severe drift. This thesis develops acquisition and synchronisation algorithms which can cope with heavy multiple access interference as well as high levels of drift. Once the timing points have been estimated for an IDMA receiver the detection and decoding process can proceed. An important issue with uplink systems is the alignment of frame boundaries for efficient detection. This thesis demonstrates how a fully asynchronous system can be modelled for detection. This thesis presents a model for the frame asynchronous IDMA system, and then develops a maximum likelihood receiver for the proposed system. This thesis develops tools to analyse and optimise IDMA receivers. The tools developed are general enough to be applied to other multiuser receiver techniques. The conventional EXIT chart analysis of unequal power allocated multiuser systems use an averaged EXIT chart analysis for all users to reduce the complexity of the task. This thesis presents a multidimensional analysis for power allocated IDMA, and shows how it can be utilised in power optimisation. Finally, this work develops a novel power zoning technique for multicell multiuser receivers using the optimised power levels, and illustrates a particular example where there is a 50% capacity improvement using the proposed scheme. -- provided by Candidate

Analysis of the capacity enhancement of cellular systems using multiuser receivers and multiple power zones

In this paper we show analytically how power allocated multi-user receivers in conjunction with power zones can be used to increase the capacity of CDMA cellular systems. The proposed scheme has two mutual benefits firstly, the increased sum-rate capacity of unequal power allocated multi user detection, and secondly we show that power zones can be used to significantly reduce intercell interference. Our results show that as the loading increases the benefits of power allocation and zoning increases as well. Our results show that using power allocation along with soft handover we can significantly increase the achievable cell loading over the conventional multi-user receivers

Multi-Dimensional EXIT Analysis for Iterative Multi-User Detection with Unequal Power Allocation

A multi-dimensional extrinsic information transfer analysis of multi-power level multi-user detectors is proposed as a tool to better understand the convergence behavior of iterative decoding schemes. We develop a K dimensional EXIT chart in order to analyze a system with K power levels. We state a conjecture that predicts the convergence point of the system. We show through simulation our analysis closely matches the simulation results. The tools discussed here provides a new insight to receiver performance analysis and can be used to design near capacity achieving multi-user systems

Timing Acquisition for Multi-User IDMA

Acquisition of the timing in an IDMA system must take place before signal detection and decoding are performed. Acquisition in the presence of severe multiple-access interference with time varying codes makes the task even more difficult. Inefficient designs cause a large number of false alarms and/or missed detections. This paper applies a powerful acquisition technique to IDMA systems in the uplink. Under high multiple access interference conditions conventional acquisition techniques used for DS-CDMA systems simply fail. The proposed method utilises soft data from the iterative IDMA receiver to effectively cancel interference from the co-channel users. Analytical performance in terms of the number of users, spreading gain, performance of the IDMA receiver, and noise variance is developed, together with simulation results

An Optimal Asynchronous IDMA Receiver

In this paper we develop an optimal receiver for an asynchronous IDMA system. A super-block structure is used to formulate the band diagonal matrix model for asynchronous IDMA. We show that the cost function for the ML decoder can be viewed as a path metric through a structured trellis. We use a dynamic programming algorithm to find the maximum likelihood transmitted bit sequence. We compare the performance of the optimal receiver with the conventional iterative receiver. Our results show that the optimal asynchronous IDMA receiver outperforms the iterative IDMA receiver for the IDMA systems with small block sizes that we simulated

Iterative Timing Recovery for IDMA Receivers Operating Under Severe Timing Drift

Recently Interleaved Division Multiple Access (IDMA) has been proposed as an enhancement for 3GPP wireless broadband standards. Timing synchronisation in an IDMA system must take place in conjunction with signal detection and decoding. This paper compares an iterative timing recovery technique with a conventional timing recovery approach for IDMA systems in the uplink. Under high multiple access interference and very severe timing drift conventional timing recovery techniques have been shown to result in poor performance. In conventional multi-user DS/CDMA systems the pilot channel is used exclusively for single-shot timing synchronisation. In this paper we propose an iterative timing synchronisation algorithm which uses soft data channel to aid the synchronisation process. MMSE combining is used to determine the optimal timing error discriminator.Decoded information from the previous iteration is used to cancel interference from the co-channel users before timing recovery is performed on each iteration. Under severe drift scenarios, our results show that the new algorithm developed reduces the timing error variance by a factor of 15 times and, consequently results in a 2dB gain in bit-error rate at high SNR as compared with conventional techniques