Processing of passive radar signals is hindered by the presence of clutter and sensor noise in the reference signal, and by strong direct and zero-Doppler clutter returns in the surveillance signal. Previous research has addressed methods of removing or mitigating unwanted components in either signal. However what is considered as interference in one signal may be considered as information in the other. In this study we investigate under what conditions both surveillance and reference information may be extracted from a single signal, thus enabling passive radar processing with a single receiver.

In this paper we investigate the interactions between antenna orientations, propagation link geometry and Faraday rotation for a line-of-sight VHF satellite communication link. A key contribution is development of a three dimensional (3D) equivalent baseband model that incorporates link geometry, antenna radiation patterns and the rotation of the polarization of the radio wave as it passes through the electron plasma of the ionosphere in the presence of Earth's magnetic field. Through simulations of an overhead pass for a low earth orbit (LEO) satellite in night and day ionospheric conditions, we characterize the combined effect of the link geometry and Faraday rotation. We show that Faraday rotation may in fact improve the channel quality. The power transferred from transmitter to receiver including Faraday rotation can be greater than that for a link with no Faraday effects. This occurs when the antenna orientation at the receiver is favourably aligned with the rotated polarization of the incoming radio wave.

In this paper, we consider a time-division duplex (TDD) multi-cell massive MIMO system. Channel estimation is hampered by pilot contamination from neighbouring cells. We propose a low-complexity distributed user selection scheme to improve the throughput performance and reduce pilot contamination. The scheduling decision is made among the users according to their virtual SINR, and only the self-selected users can be co- scheduled for data transmission. The advantages of our proposed scheme are twofold. Firstly, users experiencing a weak channel are less likely to be self-selected; this will increase the throughput performance of the system. Secondly, users that contribute to significant pilot contamination are also less likely to be selected; this will improve the accuracy of channel estimation. Analytical expressions for setting the user selection threshold and the average mean squared error (MSE) performance are derived. Finally, numerical results are also presented to provide further insights.

An adaptive waveform scheduling algorithm is presented for target tracking by a multistatic radar system with two key components: (i) a distributed multistatic tracking algorithm for target tracking in cluttered environments, and (ii) the next transmitted waveform selected to minimize the tracking mean squared error. The scheduling algorithm is developed based on the minimization of the trace of the expected tracking error covariance matrix. A simulation example is presented to demonstrate the superiority of the proposed waveform scheduling algorithm over conventional fixed waveforms.

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.

In Passive Bistatic Radar (PBR) a local communications transmitter is used as an illuminator. A Line Of Sight (LOS) signal is captured to use as a template for radar processing. This may be cleaned via demodulation and remodulation, removing short-delay copies and sensor noise. However differences between the transmitter clock and local clock cause the remodulated signal to be mismatched to the transmitted signal, causing degradation of delay-Doppler processing output. This study presents a model which captures the effect of clock disparity upon remodulated OFDM signals. The model is used in simulation to illustrate the specific effect of clock offsets on PBR by examination of delay-Doppler output. It is shown that modification of the template to account for time-varying clock offsets can restore the delay-Doppler output to be nearly ideal.

The use of discrete Hartley transform (DHT) pre-coding was recently shown to achieve peak to average power (PAPR) reduction for OFDM. In this paper, we show that DHT precoding also achieves full diversity. We consider the sparse matrix formed by the combination of DHT and the discrete Fourier transform (DFT) and propose a receiver configuration that minimizes complexity. Furthermore, we present a detailed account of the system complexity to show that it is slightly more than uncoded conventional OFDM. Also, since this system saves the complexity required for achieving diversity gain and reducing the peak to average power (PAPR), it reduces the system complexity when the PAPR reduction and diversity techniques are taken into account for conventional OFDM. We analytically show that this system achieves full diversity and then verify it using simulation results. Simulations results also show that the system has a substantial improvement in bit error performance over conventional OFDM.

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.

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.

Recently, a T-transform based multicarrier communication system with significant uncoded BER performance improvement over conventional OFDM in a Rayleigh fading channel was proposed. However, due to the involvement of Walsh Hadamard transform (WHT), the T-transform OFDM system does not satisfy the cyclic convolution property and hence fails to diagonalize the channel matrix, resulting in an enormous equalization complexity. In this paper, we propose a new receiver processing mechanism where the T-transform is replaced by two equivalent transforms and the equalization is carried out in between them. The new approach diagonalizes the channel matrix and single tap equalization is possible without any compromise in the performance. The detailed computational complexity of the proposed approach is also presented and compared with some other schemes.

In this paper, we propose a method for user selection and channel estimation using compressed sensing. In particular, we consider a multiple-input multiple-output (MIMO) downlink broadcast scenario. We establish that full channel state information (and not just channel quality) for each self-selecting user can be obtained at the base-station via compressed sensing with no increase in overhead for the uplink feedback channel. We demonstrate the new method as a medium access technique for MIMO downlink broadcast with transmitter precoding and linear receiver processing.

Space-time codes built with multiplexed Alamouti components provide multiplexing as well as diversity gain. Their orthogonal structure also leads to simpler decoding algorithms. We consider a 4 × 2 system with multiplexed Alamouti codes. An exhaustive search maximum likelihood (ML) decoding for a 4×2 system is of order M4, for a modulation scheme with constellation size M. For multiplexed orthogonal designs, an exact fast ML decoding algorithm has recently been reported whose complexity order is M2for a 4 × 2 system with QAM constellations. Nevertheless, the quadratic complexity of this fast ML algorithm may still be infeasible in practice for large constellations (e.g. M ≥ 64 QAM). In this paper, we present a method for designing low complexity sub-optimal decoders based on a combination of search based ML decoding and linear decoding. Our formulation facilitates a direct investigation of the trade-off between performance and complexity. The complexity of our hybrid decoder is flexible and it can be fixed based on the desired performance for a hardware implementation. Although extendable to more general multiplexed Alamouti systems, we focus here on a 4 × 2 space-time-polarization system comprising of two dual-polarized transmit antennas and one dual-polarized receive antenna.

This paper provides new insight into the classical problem of determining both the capacity of the discrete-time channel with uniform output quantization and the capacity achieving input distribution. It builds on earlier work by Gallager and Witsenhausen to provide a detailed analysis of two particular quantization schemes. The first is saturation quantization where overflows are mapped to the nearest quantization bin, and the second is modulo quantization where overflows are mapped to the nearest quantization bin after reduction by some modulus. Both the capacity of modulo quantization and the capacity achieving input distribution are determined. When the additive noise is gaussian and relatively small, the capacity of saturation quantization is shown to be bounded below by that of modulo quantization. In the limit of arbitrarily many uniform quantization levels, it is shown that the difference between the upper and lower bounds on capacity given by Ihara is only 0.26 bits.

We propose a novel access technique for cellular downlink resource sharing. In particular, a distributed self-selection procedure is combined with the technique of compressed sensing to identify a set of users who are getting simultaneous access to the downlink broadcast channel. The performance of the proposed method is analyzed, and its suitability as an alternate access mechanism is argued.

Multiple-input multiple-output (MIMO) functionality has been shown to dramatically increase the capacity of wireless communication systems when the environment provides rich multipath scattering. In a predominantly line-of-sight (LOS) environment, the loss of diversity reduces the potential gain considerably. This can be remedied in part by the use of dual-polarized antennas, which increases the rank of the wireless channel and introduces diversity, while minimizing the antenna's form factor. However the performance of a dualpolarized antenna is still degraded by antenna rotations that are typical of mobile terminal operation. This paper presents a solution which uses a triad antenna at the transmitter and a triad at the receiver, to provide a 8-10 dB gain over the baseline dual-polarized system. A triad is composed of three orthogonal dipoles oriented in perpendicular directions. A triad antenna can generate an arbitrary oscillating dipole moment at the transmitter and consequently an arbitrary polarized electric field at the receiver, subject only to the constraints imposed by the physics of the electromagnetic (EM) field. We show that, in LOS environments, the capacity of the channel is invariant under arbitrary rotations of the transmit and/or receive antennas about their centres. Simulation results show that the performance is stable as the propagation environment varies from rich scattering to pure LOS. A full rate 3 × 3 space-time block code (STBC) is proposed for the triad system that is designed for low complexity decoding.

Motivated by the need for the Cholesky factorization in implementing a spherical MIMO detector, this paper considers Cholesky and QR decompositions suitable for fixed-point implementation. In particular, we reformulate the decompositions to avoid the many square-root and division operations required in their natural form. This is achieved by decoupling the numerator and denominator calculations and applying scaling by powers of 2 (corresponding to bit shifts) to ensure numerical stability in the recursions. We consider the impact on the spherical detector formulation.

This letter compares pilot-aided phase-shift keyed (PSK) and differential PSK (DPSK) modulation when using new maximum a posteriori joint channel estimation and equalization receivers with frequency-selective fast-fading channels. We conclude that pilot-aided PSK has superior bit-error rate performance in this case. However, at low signal-to-noise ratio, performance is similar, and DPSK is competitive due to reduced receiver complexity.

This paper presents a new fractionally-spaced maximum a posteriori (MAP) equalizer for data transmission over frequency-selective fading channels. The technique is applicable to any standard modulation technique. The MAP equalizer uses an expanded hypothesis trellis for the purpose of joint channel estimation and equalization. The fading channel is estimated by coupling minimum mean square error techniques with the (fixed size) expanded trellis. The new MAP equalizer is also presented in an iterative (turbo) receiver structure. Both uncoded and conventionally coded systems (including iterative processing) are studied. Even on frequency-flat fading channels, the proposed receiver outperforms conventional techniques. Simulations demonstrate the performance of the proposed equalizer.

This paper considers new maximum a posteriori (MAP) receivers for phase-shift keyed (PSK) digital mobile communication systems. The focus is on the benefits of pilot-aided PSK over differential PSK (DPSK) when using these receivers in fast fading channel conditions. For fading channels, DPSK is typically employed in preference to PSK because it avoids the need for accurate channel tracking. However, for both the PSK and DPSK MAP receivers considered here, non-coherent quadrature analog front-ends are used, with channel estimation being performed digitally in conjunction with data estimation. Channel estimates are formed along each branch of an expanded MAP trellis. In the case of absolutely encoded PSK, phase ambiguity is resolved using pilot symbols. For the new MAP receivers, the choice between PSK and DPSK is no longer clear. This paper shows that pilot-aided PSK outperforms DPSK for these new receivers, despite the SNR penalty associated with pilot symbols. From a computational point of view however, the new MAP DPSK receiver developed in this paper is more efficient.

This letter presents a new method for identification of fast-fading mobile channels (for which combinations of vehicle speed and carrier frequency give rise to significant fading). Our new algorithm estimates both the channel statistics and the time-varying channel impulse response on-line. Simulation studies demonstrate the performance of the new estimator which couples an augmented Kalman filter with a recursive least squares algorithm.

We present a new method for on-line identification of time-varying FIR channels. Two conditionally coupled estimators are proposed. In both cases an augmented-state adaptive Kalman filter is employed for tracking the time-varying channel and estimating the mean channel response. Coupled to the Kalman filter is an algorithm for estimating the parameters of the underlying auto-regressive (AR) model which describes the time evolution of the channel. For the first coupled estimator, we propose a new recursive least squares algorithm for estimation of these AR parameters directly from the channel observations. An alternative algorithm based on estimation of the channel covariance is used in the second coupled estimator. A simulation example demonstrates the performance of the proposed estimators.