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In systems with parallel analogue-to-digital converters (ADC), diversity combining can improve the signal to noise ratio (SNR) of the digitised signal. We consider a novel architecture, where the parallel ADCs operate on differently attenuated signals, in conjunction with maximal ratio combining and gain weighted combining. Both schemes reduce the effective quantization noise and provide significant SNR improvements over a single ADC and signal averaging, which takes the unweighted average of the ADC output signals. The key to achieving these SNR gains is to carefully select branch attenuations that lead to negative correlation of the quantization noise across branches. Refereed/Peer-reviewed

To alleviate the high peak-to-average power ratio (PAPR), high complexity in user terminal and sensitivity to carrier frequency offset (CFO) problems in current orthogonal frequency division multiple access (OFDMA) systems, a Quadrature OFDM (Q-OFDMA) system has been recently proposed in the single-input single-output environment. In this paper we study the realization of multi-input multi-output (MIMO) diversity- and multiplexing- oriented methods for Q-OFDMA systems. An Alamouti-like space-time block code (STBC) and simple detection for spatial multiplexing (SM) for Q-OFDMA systems are constructed, both zero forcing (ZF) and minimum mean square error (MMSE) equalizers are investigated. The proposed STBC is a full diversity scheme, which encodes in intermediate domain and decodes in frequency domain. Analytical and empirical results demonstrate that the Q-OFDMA systems can be implemented flexibly and efficiently in a MIMO framework, and the proposed scheme can be easily applied in OFDMA and Single-Carrier Frequency Division Multiple Access (SC-FDMA) by adjusting the parameters of Q-OFDMA. Refereed/Peer-reviewed

Low complexity optimal (or nearly optimal) decoders for space-time codes have recently been under intensive investigation. For example, recent works by Sirianunpiboon and others show that the Silver code and the Golden code can be decoded optimally (or nearly optimally) with quadratic decoding complexity. Fast decodability makes them very attractive in practice. In implementing these decoders, floating-point to fixed-point conversion (FFC) needs to be carefully undertaken to minimize hardware cost while retaining decoding performance. The process of quantization for fixed-point representations is often ignored by research community and lacks investigation, and so FFC is often conducted heuristically based on simulations. This paper studies the effects of quantization to space-time coded systems from an information theoretic perspective. It shows the analytical relationship between quantization error and decoding performance deterioration. This paper also proposes a general finite precision implementation methodology including two FFC criteria for space-time coded systems within an integer optimization framework. As a particular example, this paper examines the finite precision implementation of the quadratic optimal decoding algorithm of the Silver code. However, our methodology and techniques can be applied to general space-time codes.

In this paper we present a new multiple access architecture for the uplink in a multi-carrier communications system. We are motivated by the case where the transmitters are low-power, low-complexity devices in a machine-to-machine(M2M) transmission scenario, such as is the case for a widespread sensor network with terrestrial base station or device-to-device relay. Each user transmits using single carrier (SC) modulation on separate pre-arranged contiguous carriers. The receiver employs orthogonal frequency division multiple access (OFDMA)processing. No frequency domain to time domain conversion is required in the transmitters, yet the receiver has the advantage of frequency domain equalization as well as a mechanism to separate each of the users. We highlight the savings in complexity at the transmitter and investigate the peak to average power ratio of the time domain signal. We demonstrate through simulation that our multiple access architecture has similar performance to conventional OFDMA in typical use cases and will out perform OFDMA when the dynamic range of transmitted signals is limited, such as by low resolution digital to analog converters. Refereed/Peer-reviewed

There are several approaches to the Monte Carlo simulation of a time-varying multipath radio channel with a given Doppler spectrum. This paper concentrates on a particular method in which the channel, having a discrete tapped-delayline structure, is simulated by generating a time-varying Doppler spread channel tap for each resolvable delay path. Each channel tap is created by the addition of multiple sinusoidal echoes, where the frequency of an echo is assigned randomly based on the channel Doppler spectrum. A model of this type has three key parameters: the sampling rate, the number of sinusoidal echoes and the total number of samples used in the channel realisation. Quantitative criteria are developed which provide guidelines for selecting values of parameters in the simulator design. These criteria are illustrated by examining the performance of two simulated channels, one having the Jakes Doppler spectrum the other a Gaussian Doppler spectrum. The performance is measured in terms of a normalised root-mean-squared-error between the specified and the achieved Doppler spectra. We conclude that to keep the NRMSE of the achieved Doppler spectrum to within 10%, 200 sinusoids are sufficient, provided that a suitable sampling frequency is selected. Refereed/Peer-reviewed

In this paper we investigate the dynamic range requirements of a passive radar system and show the impact of analog-to-digital converter (ADC) quantization on system performance. This analysis is carried out with a view towards understanding how many ADC bits are required in each channel of the passive radar system. To give practical insight, we consider the parameters of a passive radar system which uses terrestrial digital video broadcast (DVB-T) signals, and the detection of a hypothetical 10 m2 target at 150 km using well documented passive radar signal processing techniques. Refereed/Peer-reviewed

We present a low complexity iterative decoding method for a multiplexed Alamouti space-time code serially concatenated by a convolutional code. The space-time signalling code consists of a Gray coded 16-QAM mapper and two spatially overlaid Alamouti signalling blocks. A linear complexity decoder is used for decoding the inner space-time code. Ordinarily, an iterative decoding between the inner and outer decoders at the receiver does not improve performance when using a Gray coded modulation scheme. However we show that turbo gains are possible when a multiplexed Alamouti code is used. The low complexity inner decoder is based on Bayesian interference cancelling approach. We present a method of using both the extrinsic and full a posteriori values from the outer decoder to facilitate gains from iterations. The modified iterative decoder presented here favourably compares to a space-time bit interleaved coded modulation (ST-BICM) system providing the same spectral efficiency in terms of complexity-performance tradeoff. Refereed/Peer-reviewed

In this paper, we consider a simple two-relay wireless sensor network with packet retransmission where transmission energy and network geometry can be adjusted to improve the energy efficiency of the whole network. We model the energy states of the system as a Markov chain. With both analytical and numerical methods, we investigate how total energy consumption per packet and system throughput are affected by transmit energy allocation and network geometry. The results indicate that the maximum system throughput and minimum system energy consumption per packet can not be optimized simultaneously. We show there are three operating regions representing different throughput and total energy consumption tradeoffs. The approach taken in this paper could be applied to the sensor networks that have no hard requirement on the throughput and delay. Refereed/Peer-reviewed