In this work, a hardware testbed is proposed to validate predistortion schemes designed to compensate for imperfections in a satellite channel. Such imperfection arise due to the nonlinear power amplifier and output channel filter and it is crucial to verify predistortion methods using satellite hardware. The testbed can be used to research novel state-of-the-art algorithms or for acceptance testing of existing algorithms prior to satellite launch. Using the custom-build testbed, various predistortion schemes were compared for a realistic setup consisting of an X-band travelling-wave tube amplifier and output channel filter. An improvement in SNR of 8 dB at the receiver was demonstrated using a Wiener predistortion system.

In this paper, we present an analytical framework for detecting optical unipolar orthogonal frequency-division multiplexing (U-OFDM) signals. We derive analytical expressions for time-domain signal estimates based on different schemes including maximum likelihood, maximum a-posteriori and minimum mean-square error (MMSE) estimation. These schemes are compared with state-of-the-art U-OFDM detection schemes in terms of uncoded bit-error rate (BER). Numerical results show that the MMSE estimate-based scheme exhibits the least BER performance in the low signal-to-noise ratio (SNR) regime. Furthermore, we consider coded systems and reveal that the coded BER performance of the MMSE scheme is superior to previously proposed receivers. Finally, we propose an iterative-decoding scheme with soft-symbol estimation to further improve the performance of coded U-OFDM.

Narrowband satellite communications (SATCOM) in the ultra high frequency (UHF) band is an attractive option within the military SATCOM (MILSATCOM) community. The UHF user terminals (UTs) are typically low cost, easily deployable and have broad beamwidths so that precise pointing to the satellite is not a requirement for mobile users. At the same time, some inherent disadvantages of UHF SATCOM are limited bandwidth, the inability to apply frequency re-use in the geo-stationary arc and hence less capacity. The multiple antenna technique, popularly referred to as MIMO (multiple input multiple output) is known to increase the channel capacity using spatial multiplexing and offers the ability for frequency reuse. The intent of this paper is to analyse the applicability of MIMO to UHF SATCOM, based on some of the recent developments in MIMO SATCOM research. The SATCOM channel is principally dominated by the line of sight (LOS) path between the satellite and the ground user. To achieve spatial orthogonality in the LOS SATCOM channel, depending on the wavelength, antenna separation on the order of several kilometers is required either in space or on the ground. In this paper, we consider using multiple satellites for a large antenna separation in space to enable closer antenna separation for users on the ground. Channel modeling, capacity calculation, issues with the propagation time difference and receiver signal processing are addressed in this paper.

A novel transmission scheme for the forward link of a broadband satellite system that aims to avoid the need for feeder-link RF spectrum is described1. The hybrid scheme uses one or more optical uplinks from a hub station to geostationary satellite, plus one or more Ka band downlinks. The approach aims to retain the advantages of strong channel coding with high spectral efficiency, as used in recent high-speed digital video broadcast satellites, yet minimise on-board processing requirements. Promising simulation results are presented for a range of channel models, including channel measurements made with ESA's Artemis satellite.

We develop an estimator of time offset (or time-of-arrival) of a transmitted communications signal that contains both pilot symbols, known to the receiver, and data symbols, unknown to the receiver. We focus on signalling constellations that have symbols lying on the complex unit circle, such as M-ary phase shift keying (M-PSK). We describe an algorithm for computing the estimator that requires O(L log L) operations in the worst case, where L is the number of transmitted symbols. Our estimator integrates information from the pilot symbols more effectively than popular estimators from the literature that usually split estimation into two subproblems called symbol timing and frame synchronisation. Our estimator combines these subproblems into a single operation, that of estimating time offset. We hypothesise that our estimator will be statistically more accurate. Monte-Carlo simulations support this hypothesis.

We consider least squares estimators of carrier phase and amplitude from a noisy communications signal. We focus on signaling constellations that have symbols evenly distributed on the complex unit circle, i.e., M-ary phase shift keying. We show, under reasonably mild conditions on the distribution of the noise, that the least squares estimator of carrier phase is strongly consistent and asymptotically normally distributed. However, the amplitude estimator is not consistent, but converges to a positive real number that is a function of the true carrier amplitude, the noise distribution and the size of the constellation. The results of Monte Carlo simulations are provided and these corroborate the theoretical results.

This paper tackles the speech separation problem in a meeting room using a new acoustic beamforming method – adaptive blocking (AB) beamformer. The proposed method is an optimum beamforming with a structure similar to the generalized sidelobe canceller (GSC) structure, but simpler. Thus, it inherits the flexibility of GSC and functions well in dynamic environments. We investigate the performance of the proposed method through different experiments and compare the results with a GSC beamformer for minimum variance distortionless response (MVDR). The experimental setups include one wanted speaker, two interferers, air conditioner noise and uncorrelated sensor noise. AB provides improvement over MVDR-GSC.

We study hybrid free-space optical (FSO) and radio-frequency (RF) communications, whereby information is conveyed simultaneously using both optical and RF carriers. We consider the case where both carriers experience scintillation, which is a slow fading process compared to typical data rates. A parallel block-fading channel model is proposed, that incorporates differences in signalling rates, power scaling and scintillation models between the two carriers. Under this framework, we study the outage probability in the large signal-to-noise ratio (SNR) regime. First we consider the case when only the receiver has perfect channel state information (CSIR case) and obtain the SNR exponent for general scintillation distributions. Then we consider the case when perfect CSI is known at both the receiver and transmitter, and derive the optimal power allocation strategy that minimises the outage probability subject to peak and average power constraints. The optimal solution involves non-convex optimisation, which is intractable in practical systems. We therefore propose a suboptimal algorithm that achieves the same diversity as the optimal one and provides significant power savings (on the order of tens of dBs) over uniform allocation.

This paper considers the potential for communication within a group of independent low earth orbit satellites which are not members of a structured constellation. We assume inter-satellite links are used for all communications. Two models of "uncoordinated" satellite constellations are considered, with satellites locations defined by probability distributions and treated as random variables. Analytical results are given for the mean number of satellites which are visible to each other and so may establish inter-satellite links, plus the mean amount of data that may be transferred per satellite per day, for sample link budget parameters. Simulations are included to investigate some practical constraints. The results indicate that with 6 to 12 uncoordinated satellites, it will generally be possible to form at least one ISL per satellite and that mean data transfers of several GBytes per satellite per day could be achieved with modest ISL payloads.

In communication systems that use orthogonal frequency-division multiplexing and multiple transmit and receive antennas, beamforming (BF) is conventionally carried out on a subcarrier basis. The computational requirements are high as dedicated discrete Fourier transform (DFT) processors are needed for each antenna. Considerable complexity reductions can be achieved by symbol-wise BF, which performs the transmit and receive BF operations in the time domain, and therefore requires only one DFT processor per terminal. In this paper, we investigate symbol-wise BF for the mitigation of co-channel interference on spatially correlated channels, which are modelled with the Kronecker model. We show analytically that symbol-wise BF is the optimal BF scheme when the channel is frequency-flat or fully correlated, and present a novel iterative algorithm for the computation of the optimal antenna weights. Monte-Carlo simulations confirm that the relative performance loss of symbol-wise BF becomes negligible when approaching full correlation.

We study the hybrid free-space optical (FSO) and radio-frequency (RF) channel from an information theoretic perspective. Since both links operate at vastly different carrier frequencies, we model the hybrid channel as a pair of parallel channels. Moreover, since the FSO channel signals at a higher rate than the RF channel, we incorporate this key feature in the parallel channel model. Both channels experience fading due to scintillation, which is slow compared to typical signalling rates. Under this framework, we study the fundamental limits of the hybrid channel. In particular, we analyse the outage probability in the large signal-to-noise ratio (SNR) regime, and obtain the outage diversity or SNR exponent of the hybrid system. First we consider the case when only the receiver has perfect channel state information (CSIR case), and obtain the exponents for general scintillation distributions. These exponents relate key system design parameters to the asymptotic outage performance and illustrate the benefits of using hybrid systems with respect to independent FSO or RF links. We next consider the case when perfect CSI is known at both the receiver and transmitter, and derive the optimal power allocation strategy that minimises the outage probability subject to peak and average power constraints. The optimal solution involves non-convex optimisation, which is intractable in practical systems. We therefore propose a suboptimal algorithm that achieves significant power savings (on the order of tens of dBs) over uniform power allocation. We show that the suboptimal algorithm has the same diversity as the optimal power allocation strategy.

In MIMO-OFDM communications over channels subject to co-channel interference, beamforming (BF) is conventionally applied independently to all subcarriers. Whilst this approach maximises mutual information, it is highly computationally complex. Symbol-wise BF considerably reduces the complexity by carrying out BF in the time domain. In this paper, we generalise symbol-wise BF to take co-channel interference into account. Maximising the mutual information is infeasible in this case and instead, we propose a novel iterative algorithm that maximises the SINR before OFDM demodulation. Computer simulations show that the performance loss relative to subcarrier-wise BF reduces with decreasing frequency selectivity or increasing spatial correlation.

We present a novel LDPC code construction technique and an algorithm that assists phase estimation on sub- blocks to provide a near-coherent performance on channels with a large phase noise. Sub-blocks are very small observation intervals on the received vector over which phase estimation techniques are applied to provide ambiguous phase estimates. Iterative decoding on LDPC codes are affected by these phase ambiguities on sub- blocks. In our earlier work, we resolved this sub-block phase ambiguity at the start of the decoding based on a set of check nodes called 'local check nodes (LCN)' that are connected locally within the sub-blocks. Due to the poor signal-to-noise ratio at the start of decoding, a large number of LCNs were required to provide a reliable phase ambiguity. In this paper, we show that random LDPC codes can be constructed such that their decoding is not affected by the presence of sub-block phase ambiguities. Because of this, the signal quality is improved due to the code convergence and hence requires significantly less local check nodes to resolve phase ambiguity. We present performance results for a binary LDPC code with BPSK modulation. They show that with little loss, the receiver could tolerate a Wiener phase noise of up to 3deg standard deviation per symbol.

The main drawback in communicating via the free space optical (FSO) channel is the detrimental effect the atmosphere has on a propagating laser beam. Atmospheric turbulence causes random fluctuations in the irradiance of the received optical laser beam, commonly referred to as scintillation. This paper investigates the use of multiple lasers and multiple apertures to mitigate the effects of scintillation. In particular, the FSO multiple-input multiple-output (MIMO) channel with Q-ary pulse position modulation (QPPM) and transmit repetition under the assumption of non-ideal photodetection is analyzed in terms of its uncoded bit error rate (BER) and ergodic channel capacity. This analysis is based, in part, on the use of irradiance fluctuation samples that were obtained from a laser range experiment that was conducted in the presence of moderate turbulence conditions. Expressions are derived for the log-likelihood ratio (LLR) of a received bit, uncoded BER and ergodic capacity. Using these results it is shown that large gains are possible with the use of MIMO combined with strong coding techniques.

This paper presents the performance for turbo equalization of ISI caused by a nonlinear satellite channel. Mutual information transfer characteristics are used to highlight the performance of the proposed turbo equalization and confirm convergence of the iterative scheme. An equivalent SNR model for the nonlinear channel is presented which shows direct correspondence between the linear and nonlinear channels. The nonlinear channel equalizer is based on a soft interference cancellation approach which has linear complexity with modulation order and channel memory

The true Cramer-Rao lower bound (CRLB) is derived and evaluated for the estimation of carrier phase of Star 16-quadrature amplitude modulation (QAM) and can be simply applied to carrier frequency estimation. Different geometries are investigated by varying the ring ratio (RR). For signal-to-noise ratios (SNRs) between 6-15 dB, the CRLB with RR=3 is lower than that of Square 16-QAM. A modified phase estimator is presented, which closely follows the new CRLB. Investigation of symbol error performance in short packet length reveals Star 16-QAM to be superior to Square 16-QAM for SNR<13 dB, which is a reasonable operating range for a coded system. Although Square 16-QAM and Star RR=1.8 are optimum for a perfect receiver, when the effect of phase estimation is considered, we find Star RR=3 to be better for SNR below 10 dB.

In this paper, we present the true Cramer-Rao lower bounds (CRLBs) for the estimation of phase offset for common quadrature amplitude modulation (QAM), PSK, and PAM signals in AWGN channels. It is shown that the same analysis also applies to the QAM, FSK, and PAM CRLBs for frequency offset estimation. The ratio of the modulated to the unmodulated CRLBs is derived for all QAM, PSK, and PAM signals and calculated for specific cases of interest. This is useful to determine the limiting performance of synchronization circuits for coherent receivers without the need to simulate particular algorithms. The hounds are compared to the existing true CRLBs for an unmodulated carrier wave (CW), BPSK, and QPSK. We investigated new and existing QAM phase estimation algorithms in order to verify the new phase CRLB. This showed that new minimum distance estimator performs close to the QAM bound and provides a large improvement over the power law estimator at moderate to high signal-to-noise ratios.

A novel extension of multiple symbol detection (MSD) of non-coherent MPSK is presented in which the presence of an arbitrary and unknown frequency offset is considered. The resulting detector is insensitive to frequency offset present on the received signal. A near maximum-likelihood (ML), frequency insensitive multiple symbol detection (FI-MSD) metric is derived, and its performance in the detection of double differential MPSK (DD-MPSK) is analysed by simulation. It is shown that as the observation length is increased, the performance approaches that of coherent detection. The new detector removes the need for explicit estimation and removal of frequency offset prior to detection, and may be suited to use with random access communication channels that experience large frequency deviations (such as low Earth orbit (LEO) satellite channels).

Maximum a posteriori (MAP) sequence detection for channels with intersymbol interference (ISI) has previously required knowledge of the channel sampled impulse response (SIR). Generally the SIR coefficients are determined via least mean square (LMS) or recursive least squares (RLS) estimation algorithms. For many unguided media channels such as mobile radio and high frequency (HF) radio which exhibit a time-varying SIR, these estimators must be adaptive. Adaptive estimators often fail to track adequately and are a major source of detector deterioration. A novel, blind maximum likelihood sequence detection (BMLSD) formulation without the need for external channel SIR estimation is proposed. A new method for resolving ambiguity in the channel delay is also included. The BMLSD performance is evaluated via simulation.

New Cramer-Rao lower bounds (CRBs) are derived for the estimation of phase offset, and frequency offset, from a block of random BPSK or QPSK symbols at low signal-to-noise (SNR) ratios. These bounds are compared to existing bounds which apply to the unmodulated carrier (or to known PSK symbols), and to the performance of some estimation algorithms.

A statistical analysis of a decision-directed symbol timing algorithm for phase shift keyed modems is presented. The timing detector uses only one sample per symbol period and is suitable for high speed modems which employ discrete-time synchronization methods. Expressions for the timing detector’s mean value and variance, as a function of timing offset, are derived and compared to simulation results. The analysis includes the effects of decision errors which occur at low signal-to-noise ratios and eventually limit the useful operating range of the decisiondirected methods. A modification is described so that the need for prior phase recovery is avoided. These algorithms are compared to a popular two-sample-per-symbol nondecision-directed timing detector. A method of examining the relative performance of the various algorithms is presented and results are given for a range of signal-to-noise ratios and channel bandwidths.