Graeme Woodward

Minimum mean-squared error multiuser decision-feedback detectors for DS-CDMA

Multiuser decision-feedback detectors (DFDs) for direct-sequence code-division multiple access, based on the minimum mean-squared error (MMSE) performance criterion, are described. Both successive and parallel feedback (interference cancellation) with hard decisions are considered. An iterative DFD is presented, which consists of cascaded DFDs, each performing successive cancellation. The two-stage DFD achieves the single-user bound in the absence of error propagation, and performs significantly better than an MMSE DFD with parallel feedback. The filter structures are generalized to include finite impulse response feedforward and feedback matrix filters, which account for asynchronous users and intersymbol interference. The effect of error propagation is illustrated through simulation. Both uncoded and coded performance results are presented. Although error propagation can significantly degrade performance, the DFDs still offer a significant performance gain relative to linear MMSE detection.

Multiuser decision-feedback detection: performance bounds and adaptive algorithms

Multiuser decision-feedback detection has been previously considered for DS-CDMA. Here we evaluate asymptotic MSE and capacity (averaged over the users) of the multiuser decision-feedback detector (DFD) in the absence of error propagation. Asymptotic means that the number of users K/spl rarr//spl infin/ and the processing gain N/spl rarr//spl infin/ with K/N fixed. The resulting capacity is the same as that achieved with optimal detection. (This is also true for finite K and N.) We also present and compare adaptive stochastic gradient and least squares algorithms for updating the DFD coefficients without explicit knowledge of channel state information.

Adaptive multiuser parallel-decision-feedback with iterative decoding

We combine the adaptive (least squares) parallel-multiuser decision feedback detector for CDMA with short spreading sequences, presented by Ratasuk et al. (see In Proc. Allerton Conf., Monticello, IL, 1999), with iterative (turbo) decoding and soft cancellation, presented by Alexander et al. (see European Trans. on Telecomm., vol.9, no.5, p.419-25, 1998). The resulting receiver requires only a training sequence and (coarse) timing for estimation of all filter coefficients, and performs close to the single-user bound with relatively low E/sub b//N/sub 0/. Prior knowledge of spreading codes and channels is unnecessary. For simplicity, we consider a synchronous CDMA system.

Linear transceivers for full duplex MIMO relays

Full duplex (FD) relays can provide throughput enhancements compared to half duplex (HD) operation. However, FD relays are known to suffer from loopback interference (LI) issues that degrade their performance. In this paper, we present several different precoder and weight vector designs using the principles of signal to leakage plus noise ratio, minimum mean square error and zero forcing to investigate the performance of a FD multiple input multiple output (MIMO) relay system. We derive a closed-form solution for the covariance matrix of the transmitted signal at the relay which enables a performance evaluation of a wide range of transceivers. Results show that performance improvements over HD can be achieved using only spatial processing (linear precoders and receive combiners) and that LI reduction at the relay is preferable to pre-cancellation at the relay transmitter.

Low complexity V-BLAST for massive MIMO

In an uplink massive multiple-input-multiple-output deployment with distributed single-antenna users and a large base-station array, we consider several combinations of receivers using linear combiners (maximum ratio combining (MRC) and zero forcing (ZF)) in conjunction with Vertical Bell Laboratories Layered Space Time (V-BLAST). We show that the performance loss of MRC relative to ZF can be removed in certain situations through the use of V-BLAST. Furthermore, we develop a low complexity ordering scheme which results in a V-BLAST scheme with MRC which has much less complexity than a single ZF linear combiner. An analysis of the signal-to-interference-and-noise-ratio at each stage of the V-BLAST approach is also given to support the findings of the proposed technique.

Performance of adaptive iterative multiuser parallel decision feedback with different code rates

An adaptive receiver which combines multiuser parallel decision feedback with MAP decoding in an iterative manner is presented for synchronous coded DS-CDMA. Performance with various convolutional code rates is considered. optimal filters are derived using both minimum mean squared error (MMSE) and least squares (LS) criteria. When used with short (repeated) spreading codes, the adaptive LS receiver requires only a training sequence and timing for estimation of all filter coefficients. The adaptive receiver performs significantly better than the standard (soft) interference canceller, since the adaptive algorithm attempts to measure and exploit the joint statistics of the output of the MAP decoder with the input symbols. Numerical results are presented which illustrate how performance depends on the rate of the convolutional code, assuming similar decoding complexity. These results show that low code rates perform best at moderate loads and low received power. Higher code rates can achieve very low packet error rates for an overloaded system (number of users greater than the bandwidth expansion).

Improving fountain codes for short message lengths by adding memory

We consider applications in which a larger chunk of data needs to be distributed from a centralized, high-powered base station to a set of distributed sensor nodes. Fountain codes offer attractive features for this kind of application, since they require no feedback from receivers and they work over a wide variety of channel characteristics. However, existing work has shown that fountain codes have a relatively high overhead especially in cases when the message length is only small to medium-sized, which is a typical setting in wireless sensor networks. In this paper we present a scheme that improves the efficiency of fountain codes and thus allows receiving sensor nodes to switch off their transceivers earlier. In contrast to earlier schemes that aim to improve the efficiency of fountain or LT codes, our scheme is the first one that introduces memory into the encoding process, i.e. the encoding of the current packet depends on the encoding of previous packets.

Multilayer LMS interference suppression algorithms for CDMA wireless networks

The general theory of adaptive self reconfigurable interference suppression schemes is applied to several specific practical problems mainly suppression of m-level m amplitude-shift keying, m phase-shift keying (PSK), and m quadrature amplitude modulation signals. This is a practical situation when a code-division multiple-access (CDMA) network is overlaid with standard microwave systems. Another example is a multirate CDMA network where a limited number of high bit rate CDMA signals are allowed to use much higher power level due to lower processing gain. The algorithm is well suited for a modular software radio concept, which we believe, will be more and more accepted in future wireless communications. Further modifications of the schemes necessary for these applications are described, and numerous results are presented to illustrate performance improvements. A general interpretation of these techniques based on so-called multilayer least mean squares (LMS) algorithm is introduced and discussed. The algorithm is based on estimating fast changing interfering signal parameters by using parallel structures, which are fast but complex. At the same time, estimation of slow-varying signal parameters over a large range is accomplished by using an LMS algorithm that is simple but slower. In this way, suppression of the interference occupying the same bandwidth as the CDMA signal is possible with reasonable implementation complexity. For this case, a BER 0 dB for binary PSK interference, J/S>17 dB for 8-PSK, J/S>27 dB for 32-PSK, and J/S>40 dB for 128-PSK.

A VLSI optimised parallel tree search for MIMO

Multiple input-multiple output (MIMO) systems are of great interest due to their ability to significantly increase the capacity of wireless communications systems, but for these to be useful they must also be practical for implementation in very large scale integrated (VLSI) circuits. A particularly difficult part of these systems is the detector, where the maximum-likelihood (ML) solution cannot be directly implemented due to its exponential complexity. Lattice decoders, such as the sphere search, exhibit near-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 for MIMO applications. We also propose parallel implementation strategies for such a detector, suitable for implementation in VLSI. This is then combined with a single-pass tree search approach and it is demonstrated that it can be designed so that the error-rate performance is not significantly impaired

Equalization for MIMO-OFDM Systems with Insufficient Cyclic Prefix

We investigate multiple input multiple output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems that operate with insufficient cyclic prefix (CP). Using a CP shorter than the channel delay spread can enable a significant improvement in bandwidth utilization or range extension for OFDM networks at the cost of increased intersymbol interference (ISI) and inter-carrier interference (ICI). We first analyze the effect of ICI and ISI on the received signal. A bi-directional M-algorithm (BDMA) is then proposed for high performance trellis-based equalization to construct an iterative interference mitigation and detection process. Simulations show that, after only 2 iterations, the bit error rate (BER) of the proposed equalization scheme can converge to that of a sufficient-CP system even when the channel delay spread is 6 times longer than the insufficient CP.