The effect of user antenna pointing error in multiple high-altitude platform (HAP) systems is examined. A truncated Gaussian distribution is applied to the pointing error. Ranges of different standard deviations and user antenna beamwidth are combined to show different aspects of the system capacity degradation. A concept of virtual points of HAP locations is introduced to give a direct indication of the severity of the pointing error. Optimum user antenna beamwidths are determined as a function of standard deviation of pointing error, for various percentiles of the spectral efficiency, to aid the system designer. The multiple HAP system itself will not jeopardise the system capacity by introducing the user antenna pointing error. It is shown that pointing error reduces the received carrier power with the interference little affected and hence multiple HAP systems are no more susceptible than a single HAP system.

Spectral domain B-spline identification is proposed for acoustic echo cancellation. Two approaches are considered. The first is based on the solution of normal equations; we describe an efficient technique for such a solution, which benefits from the sparseness of the system matrix due to B-splines. The second approach is based on using local splines, enabling further simplification. We also show how the proposed techniques can be used for efficient double-talk detection. The echo cancellation performance and complexity of the proposed techniques are compared with that of a low-complexity cross-spectral technique and the affine projection (AP) algorithm possessing high cancellation performance. The B-spline identification allows cancellation performance comparable with that of the AP algorithm and complexity close to that of the cross-spectral algorithm.

A method of significantly improving the capacity of high-altitude platform (HAP) communications networks operating in the millimeter-wave bands is presented. It is shown how constellations of HAPs can share a common frequency allocation by exploiting the directionality of the user antenna. The system capacity of such constellations is critically affected by the minimum angular separation of the HAPs and the sidelobe level of the user antenna. For typical antenna beamwidths of approximately 5/spl deg/ an inter-HAP spacing of 4 km is sufficient to deliver optimum performance. The aggregate bandwidth efficiency is evaluated, both theoretically using the Shannon equation, and using practical modulation and coding schemes, for multiple HAP configurations delivering either single or multiple cells. For the user antenna beamwidths used, it is shown that capacity increases are commensurate with the increase in the number of platforms, up to 10 HAPs. For increases beyond this the choice of constellation strategy becomes increasingly important.

The burst targeted demand assignment multiple access (BTDAMA) protocols incorporate an original approach to DAMA, which eliminates the fundamental end-to-end delay bound associated with the traditional DAMA techniques. Whilst they offer significant improvements in the achievable delay/utilisation performance for bursty packet transmission over satellite, they are constrained by the initial build up of packets at the start of traffic bursts. In this paper, we introduce the adaptivity to the uplink frame that enables the proportion of alternative types of capacity allocation to be varied on a dynamic basis. Sufficient demand assigned capacity is assigned to meet the instantaneous user requirements, with spare capacity allocated by an alternative strategy to assist with initial throughput at the start of bursts. Results show that this approach enhances the end-to-end delay distribution and delay jitter performance compared with the conventional static frame BTDAMA schemes.

We investigate how pilot signals affect complexity and accuracy of maximum likelihood (ML) frequency estimation in frequency-flat channels with transmit diversity. We show that for arbitrary pilot signals, the complexity can be as low as O(N/sub t/), where N/sub t/ is the number of transmit antennas. For linearly dependent pilot signals this can be further reduced down to O(1) with even better estimation accuracy. Lower and upper bounds of the Cramer-Rao lower bound (CRLB) over possible channel gains are derived. We define locally optimal pilot signals as those minimising the upper bound and show how to find such signals. The CRLB characterises local properties of ML estimates, while the ambiguity function allows analysis of global properties of estimates in the whole acquisition range. We analyse the ambiguity function for binary pilot signals, in particular Hadamard sequences and show how it relates to behaviour of frequency estimates over the signal-to-noise ratio and frequency acquisition range.

Hierarchical mobile IPv4 provides micro-mobility enhancements for the original mobile IPv4 standard. However, as the size of wireless cells decreases, the macro-mobility (or inter-domain handoff) case becomes more common. We propose a fast inter-domain handoff algorithm to provide efficient macro-mobility management and present the simulation results. Our scheme reduces the home registration latency and hence minimizes the disruption caused by macro-mobility handoffs for both downlink and uplink packet streams.

This paper presents a detailed performance evaluation of the burst targeted demand assignment multiple-access (BTDAMA) schemes, which employ an original approach to implementing demand assignment multiple-access (DAMA), suited to supporting ON/OFF type traffic over a broadband geostationary satellite link. A novel analytical model is introduced which can be used to predict and evaluate the delay performance of rate-based medium access control schemes, and is applied to BTDAMA to investigate the potential performance enhancements offered by introducing free assigned capacity to the scheme. Results show that BTDAMA eliminates the end-to-end delay bound of traditional DAMA techniques, offering significantly superior performance compared with conventional schemes, with free assignment effective in improving the delay performance for a wide range of traffic burst durations.

Maximum-likelihood (ML) data-aided frequency estimation in multipath Rayleigh-fading channels with sparse impulse responses is investigated. We solve this problem under the assumption that the autocorrelation matrix of the pilot signal can be approximated by a diagonal matrix, the fading of different path amplitudes are independent from each other, and the additive noise is white and Gaussian. The ML frequency estimator is shown to be based on combining nonlinearly transformed path periodograms. We have derived the nonlinear function for the two cases: known and unknown fading variances. The new frequency estimators lead, in particular cases, to known ML frequency estimators for nonsparse multipath fading channels. The use of a priori information about the mean number of paths in the channel allows a significant improvement of the accuracy performance. Exploiting the sparseness of the channel impulse response is shown to significantly reduce the threshold signal-to-noise ratio at which the frequency error departs from the Cramer-Rao lower bound. However, precise knowledge of the channel sparseness is not required in order to realize this improvement.

An investigation is performed into the capacity enhancements achievable from deploying multiple high altitude platforms (HAPs) arranged in different constellations but sharing common frequency bands and serving the same coverage area. Single and multiple cell configurations per HAP are examined. It is shown how overall system capacity may be increased, largely commensurate with the number of extra platforms deployed. This enhanced spectrum utilisation relies upon the fixed users employing a sufficiently directive antenna, pointing to HAPs located in different parts of the sky. Such deployments allow services to be offered to a significantly increased user population density, and facilitate incremental service roll-out.

In a wireless communications network served by a high altitude platform (HAP) the cochannel interference is a function of the antenna beamwidth, angular separation and sidelobe level. At the millimeter wave frequencies proposed for HAPs, an array of aperture type antennas on the platform is a practicable solution for serving the cells. We present a method for predicting cochannel interference based on curve-fit approximations for radiation patterns of elliptic beams which illuminate cell edges with optimum power, and a means of estimating optimum beamwidths for each cell of a regular hexagonal layout. The method is then applied to a 121 cell architecture. Where sidelobes are modeled as a flat floor at 40-dB below peak directivity, a cell cluster size of four yields carrier-to-interference ratios (CIRs), which vary from 15 dB at cell edges to 27 dB at cell centers. On adopting a cluster size of seven, these figures increase, respectively, to 19 and 30 dB. On reducing the sidelobe level, the improvement in CIR can be quantified. The method also readily allows for regions of overlapping channel coverage to be shown.

Maximum likelihood (ML) data-aided frequency estimation in multipath Rayleigh fading channels with sparse impulse responses is investigated. We solve this problem under the assumption that the autocorrelation matrix of the pilot signal can be approximated by a diagonal matrix, the fading of different path amplitudes are independent from each other, and the additive noise is white and Gaussian. The ML frequency estimator is shown to be based on combining nonlinear transformed path periodograms. We have found the nonlinear function for the two cases: known and unknown fading variances. The new frequency estimators lead, in particular cases, to known ML frequency estimators for non-sparse multipath fading channels. Exploiting the sparseness of the channel impulse response is shown to significantly reduce the threshold signal-to-noise ratio (SNR) at which the frequency error departs from the Cramer-Rao lower bound. However, precise knowledge of the channel sparseness is not required in order to realise this improvement. The assumption of a diagonal autocorrelation matrix is shown to affect the accuracy performance only at high SNRs which however can often be outside the SNRs of interest.

Channel assignment strategies for use with a high altitude platform spot beam architecture are examined. A novel power roll-off approximation is derived to allow terrestrial simulation tools to be used, and simulation performance is compared with a theoretical lower bound derived from the Engset distribution. It is shown that the use of cell overlap considerably enhances the blocking probability performance, in addition to boosting flexibility.

This paper explores the relative performance of the combined free/demand assignment multiple access (CFDAMA) protocol with random access and with packet accompanied request strategies in supporting self-similar traffic over a geostationary satellite link. It Is shown that both request strategies have certain advantages and are suited to different levels of channel loading. A new scheme is proposed, CFDAMA with combined requests (CFDAMA-CR), which combines the two request strategies to gain the benefits of both. Simulation results show that the CFDAMA-CR scheme can achieve better delay/throughput performance for a satellite channel than either of the standard schemes.

This paper introduces the burst targeted demand assignment multiple access scheme with pure demand assigned slots (BTDAMA-PD), which employs a different approach to implementing DAMA, suited to handling long bursts from on-off type data traffic sources. Its delay/throughput performance is compared with a variant of the combined free/demand assignment multiple access (CFDAMA) schemes. Simulation results show that BTDAMA-PD can achieve better delay/throughput performance of geo-stationary satellite channels carrying data traffic, with an excellent distribution of end-to-end delay values and low variation in the end-to-end delay of consecutive packet transmissions.

This paper investigates three techniques for optimising the capacity of cellular communication schemes: power control, dynamic channel assignment and adaptive modulation and coding. Each scheme, and combination of schemes, is investigated under the same assumptions for channel model, number of cells and quality of service provision; and simulation is used to derive the number of simultaneous users that can be supported in each case. It is shown that the additional complexity of adaptive modulation and coding is rarely justified, and that the facility for intracell handoffs is the most important technique to include in an efficient resource allocation algorithm.

Provides an abstract of articles to be presented in a forthcoming issue.

This paper concerns the estimation of a frequency offset of a known (pilot) signal propagated through a slowly fading multipath channel, such that channel parameters are considered to he constant over the observation interval. We derive a maximum-likelihood (ML) frequency estimation algorithm for additive Gaussian noise and path amplitudes having complex Gaussian distribution when covariance matrices of the fading and noise are known; we consider in detail the algorithm for the white noise and Rayleigh fading, in particular, for independent fading of path amplitudes and pilot signals with diagonal autocorrelation matrices. For the latter scenario, we also derive an ML frequency estimator when the power delay profile is unknown, but the noise variance and bounds for the path amplitude variances are specified; in particular, this algorithm can be used when path delays and amplitude variances are unknown. Finally, we consider frequency estimators which do not use a priori information about the noise variance; these algorithms are also operable without timing synchronization. All the frequency estimators exploit the multipath diversity by combining periodograms of multipath signal components and searching for the maximum of the combined statistic. For implementation of the algorithms, we use a fast Fourier transform-based coarse search and fine dichotomous search. We perform simulations to compare the algorithms. The simulation results demonstrate high accuracy performance of the proposed frequency estimators in wide signal-to-noise ratio and frequency acquisition range.

This paper examines the relative performance of the Combined Free/Demand Assignment Multiple Access (CFDAMA) protocol with different terminal request strategies. A new scheme is proposed employing round robin assignment of request slots (CFDAMA-RR), which is compared with the standard schemes incorporating random, packet accompanied, and Combined Requests (CFDAMA-CR). It is shown that the CFDAMA-RR scheme can offer improved delay/utilisation performance of a satellite channel due to its inherent fairness and contentionless nature. Simulation results are presented with three different source traffic models, Poisson, self-similar (Pareto ON-OFF) and exponential ON-OFF. The results indicate that the ON-OFF nature of a traffic model has a more significant impact on Medium Access Control (MAC) protocol performance than the specific distribution of ON and OFF times.

This paper concerns the estimation of the frequency offset of a known (pilot) signal propagated through a slowly fading multipath channel, such that channel parameters are considered to be constant over the observation interval. We derive a Cramer-Rao lower bound (CRLB) and maximum likelihood (ML) frequency estimation algorithm for additive Gaussian noise and path amplitudes having complex zero-mean Gaussian distribution when covariance matrices of the fading and noise are known. In particular, we consider the scenarios with white noise, independent fading of path amplitudes and pilot signals with a diagonal correlation matrix. We compare simulation results for the ML estimator with the CRLB. We also show that the results obtained can be extended to scenarios with fast fading channels.

This paper shows the comparable performance of Tomlinson-Harashima precoding (1971, 1972) and MMSE-DFE (minimum mean square error-decision feedback equaliser) over a broadband wireless access (BWA) channel, for several Doppler frequencies. The system is operating in the 2.6 GHz MMDS (multichannel multipoint distribution service) band, which requires multipath equalisation techniques. Precoding is an equalisation technique requiring accurate information of the channel characteristics. The accuracy of the channel estimate, which ultimately determines the convergence and performance of the equalisation scheme, can be affected either by the length of the training or pilot sequence, or the duration between channel estimations. This paper examines both the minimum required training sequence length and the effects of increasing the equaliser up-date period as a function of the number of data packets. It is shown through simulation, that precoding can be used as an alternative equalisation strategy for multipath channels with slow fading characteristics.

We propose an algorithm for joint channel estimation and detection in application to a practical scenario of detection of random access bursts in the W-CDMA UMTS system. The proposed algorithm is based on using the FFT algorithm for matched filtration and further signal processing with symbol rate sampling; as a result, it is well suited to implementation with modest DSP resources. We have performed simulation of the algorithm in different multiuser, multipath and frequency uncertainty environments; the simulation has shown high detection performance of the proposed algorithm.

Methods of substantially reducing call dropping in networks which use distributed dynamic channel assignment (DDCA) schemes are discussed. Interference and received power thresholds coupled with power control are used to maintain performance, without the need for intra-cell handoffs. It is shown that the schemes reduce call dropping and increase capacity compared to those using fixed transmitter power. The schemes are developed with the aid of mathematical analysis and a pictorial model. Results are presented which show that call dropping may be virtually eliminated in shadowing environments with the median transmitter power being reduced by 15 dB. The various call dropping mechanisms are discussed, and it is suggested that the residual level of call dropping is principally a result of multiple additional call arrivals close to an active link. Methods to make further reductions in the call dropping probability are also proposed.

This paper compares the performance of three techniques for providing tolerance to slow Rayleigh fading channels in cellular communication schemes: regular cyclic frequency hopping, adaptive modulation and coding, and dynamic carrier assignment (frequency jumping). It is assumed for the channels considered that the fading rates are slow enough that simple feedback adaptive techniques are applicable. The theoretical maximum performance of each scheme is evaluated as a function of the number of conversations supportable and the average received power required, and in terms of the effects on overall system capacity. Various combinations of these schemes are also evaluated, and found to provide greater advantages.

The effects of building geometric placement error have been considered both theoretically and using a two dimensional ray based model. The ray based model implements both reflection and diffraction and operates on a uniform, 1 km square grid of building data. Placement error is modelled by randomly displacing the building coordinates using a truncated Gaussian distribution. It has been found both theoretically and verified with results from a computer testbed that, when reflection predominates, the effect of angular error (rather than spatial error) is most significant as this causes rays to deviate away from their true route. It has been found that close to the transmitter, with an RMS deviation of 2.0 m in the building vertex position, that path loss peaks, caused by reflection, and corresponding path loss "nulls" can be moved up to 50 m.

This paper reports work carried out, largely within the COST 231 programme, on the uplink capacity of cellular multiple access schemes, and in particular CDMA and TDMA. It determines the probability density function of the interference in the various types of system, and hence estimates the capacities attainable with "industry standard" FEC codes of various rates. This shows that under the assumptions made, conventional CDMA can give a capacity about 65% greater than TDMA. The paper then introduces a new multiple access scheme called adaptive TDMA (A-TDMA) which adapts the code rate to the interference conditions encountered. A proposed A-TDMA system is outlined, using a packet reservation multiple access (PRMA) protocol, and an initial capacity estimate is given, and compared with the conventional schemes. The estimated spectral efficiency obtained is more than twice that of conventional CDMA.

A technique is described to reduce the probability of false-lock in Costas loops. This is where a frequency offset arises at a multiple of half the incoming data rate due to finite bandwidth of the arm filters. A modification is described to process components of this offset so as to augment the feedback control voltage. The performance is analyzed and it is shown how certain false-lock positions may be avoided.