The performance of mobile multiple-input–multiple-output (MIMO) systems is determined both by the instantaneous channel characteristics and by the rate of change of those characteristics. The local environment and operating frequency affect the spatial channel response, which impacts the ability of the system to sustain data throughput, while the dynamics of the channel response impact the accuracy of the available channel state information, limiting the supportable packet length. Measured MIMO data from two frequencies in the UHF band are characterised and used to investigate the dependence of system performance on the changing spatial channel responses. It is demonstrated that to maximise average throughput, taking into account retransmissions because of packet failures, a trade-off is required among the data rate, training overhead and packet length, which depends on both the operating frequency and the local environment. This work emphasises the importance both of adaptation in MIMO communication systems and of considering realistic channel conditions in their design and evaluation.

This study considers successive orthogonal beamforming (SOBF) for multicell downlink transmissions. A group of base stations (BSs) are in cooperation for effective downlink transmissions to cell-edge users, which is usually referred to as cooperative multi-point. As the number of cell-edge users is small and varying, conventional beamforming schemes, for example, zero-forcing beamforming (ZFBF), are not suitable for the multicell scenario. In the SOBF scheme considered here, the beamforming vectors are found in a successive manner when a new user comes in, so that the beamforming vectors of the existing users remain unchanged. With limited channel state information at the group of BSs, a new successive beam allocation (SBA) scheme is proposed for SOBF. In SBA, the candidate beams are broadcasted to all the users through downlink before the index of the best beam is sent back to the BS by each user. The sum rate performance of a system with two users is studied analytically. The numerical results are presented to show that the proposed SOBF with SBA outperforms ZFBF with conventional feedback strategy.

Let u and v be vertices of a graph G = (V, E) and d(u, v) be the distance between u and v in G. For positive integers k ₁, k ₂, …, k_n with k ₁>k ₂>⋯>k _n an L(k ₁, k ₂, …, k_n )-labelling of G is a function f: V(G)→{0, 1, …} such that for every u, v ∈ V(G) and for all 1 ≤ i ≤ n, |f(u)− f(v) | ≥ k_i if d(u, v) = i. The span of f is the difference between the largest and the smallest numbers in f(V(G)). The λ _{k 1} ,k ₂,…,k _n -number of G is the minimum span over all L(k ₁, k ₂, …, k_n )-labellings of G. In this study, an integer linear programming model and a satisfiability test reduction for an L(k ₁, k ₂, …, k_n )-labelling are proposed. Both approaches are used for studying the λ _3,2,1-numbers of strong, Cartesian and direct products of paths and cycles.

Frequency error, non-ideal channel estimation (CE) and inefficient seamless road side unit (RSU) service are critical issues occurring in conventional systems that are completely specified by current IEEE 802.11p standards. This study investigates novel techniques for achieving accurate frequency offset compensation and effective CE and RSU selection for handover in signal-overlapping areas. Recently, communications using data-aided orthogonal frequency-division multiplexing (DA-OFDM), such as pseudo-random-postfix OFDM and time-domain-synchronous OFDM (TDS-OFDM), have been actively studied because of their higher effectiveness, efficiency and better transmission quality. This study consists of three parts: (i) derivation of the maximum likelihood estimation for DA-OFDM, (ii) design of an accurate RSU selection scheme and (iii) implementation of a superior TD CE technique. Performance comparisons between the conventional and the proposed techniques are conducted through comprehensive computer simulations.

In this study, the authors consider the application of a system architecture called cognitive radio (CR) with fibre-connected distributed antennas in IEEE 802.22 wireless regional area networks (WRANs) as it could bring the benefits of much shorter wireless transmission distances, lower transmission power and the possibility of utilising multi-antenna transmission techniques. In this architecture, the authors study the secondary user (SU) access problem in uplink, where the SU to primary user (PU) link estimation is subject to random errors because PU could not assist link estimation of SU. This SU access problem is divided into two parts: antenna selection and access control. Thus, first antenna selection problem is modelled as a restless bandit problem, which is solved by the primal-dual index heuristic algorithm based on first order relaxation. In addition, the access control problem is modelled as a stochastic knapsack (SASK) problem with random weight, and then relaxed to be a deterministic second order cone programming problem. With the deduced upper bound, the access control problem is solved by the branch and bound algorithm, which yields the SU access based on SASK scheme. Simulation results illustrate the significant performance improvement of SASK scheme, compared with existing SU access methods.

In this study, the authors theoretically study the performance of direct-detection free-space optical communication systems using binary phase-shift keying subcarrier-intensity modulation and avalanche photodiode (APD). The system bit-error rate and channel capacity are theoretically derived in cases of log-normal and gamma-gamma channel models for weak-to-moderate and moderate-to-strong atmospheric turbulence conditions, respectively. The authors quantitatively discuss the optimal values of the APD average gain, required transmitted optical power, and operating bit-rate considering various turbulence conditions, APD shot noise and thermal noise. It is seen that, although the impact of turbulence is severe, a proper selection of APD average gain could significantly improve the system performance in both cases of turbulence channels. The optimal value of APD average gain remains almost the same for different levels of turbulence; nevertheless it varies significantly in accordance to the change of receiver noise temperature.

In this research work, the authors derive closed-form expressions for capacity per unit bandwidth (spectrum efficiency) of correlated Rayleigh fading channels under maximal ratio combining diversity at low signal-to-noise ratios (SNRs). Spectrum efficiency expressions are derived for M diversity branches under adaptation policies, such as: (i) optimal power and rate adaptation (OPRA) policy, (ii) optimal rate adaptation (ORA) policy and (iii) truncated channel inversion with fixed rate (TIFR) policy. Even at low SNRs, if space diversity is exercised using a single-input multiple-output system, spectrum efficiency achieved is higher compared with that achieved when signals are uncorrelated and have no diversity. This forms the focal point of this study. Analytical results show accurately that OPRA policy provides highest capacity over other adaptation policies. The spectrum efficiency for the three policies and outage probability for the low SNR case are derived, plotted and analysed in detail in this work.

Low implementation complexity, low delay and close-to-optimal performance over a wide variety of channels are some of the advantages of spatially-coupled low-density parity-check (LDPC) codes. However, the error performance of the sliding window decoding scheme that is used to decode these codes is considerably degraded over channels with memory, such as the correlated erasure channel. Employing a block interleaver to encounter this situation is not always a viable option, since it introduces a large amount of delay and cancels out the low-delay property of the sliding window decoder. Another way to reduce the effects of erasure bursts is to construct a more robust code ensemble by presenting additional code design rules. However, this approach results in additional constraints on the already complicated code construction process. The authors propose a novel communication system that combats the effects of the erasure bursts through the use of a convolutional interleaver. The proposed system combines the inherent convolutional nature of the spatially-coupled LDPC codes with that of a convolutional interleaver to achieve very low overall delay. The performance of the proposed approach is analysed using the density evolution technique and the performance improvement is demonstrated as a function of the interleaving delay via computer simulations.

In distributed query processing, good estimation algorithms of communication costs are critical for query processing, including distributed XML queries. There are techniques that estimate a communication cost for distributed SQL query processing, and some of techniques are adopted in numerous distributed SQL processors. Therefore adopting the processing techniques for SQL queries for the communication cost-based processing of the distributed XML queries seems natural. Unfortunately, however, the tree-structured XML document is different from the table-shaped relational data. These structural differences make adopting the techniques for SQL queries difficult. This study defines some of the considerations for estimating the communication cost of distributed XML queries, and proposes a method for communication cost-based query processing. The experiments show that the proposed algorithm is reasonable for estimating the communication cost for distributed XML queries.

In this study, a new peak-to-average power ratio (PAPR) reduction scheme for orthogonal frequency division multiplexing (OFDM) is proposed based on the selected mapping (SLM) scheme. The proposed SLM scheme generates alternative OFDM signal sequences by cyclically shifting the connections in each subblock at an intermediate stage of inverse fast Fourier transform (IFFT). Compared with the conventional SLM scheme, the proposed SLM scheme achieves similar PAPR reduction performance with much lower computational complexity and no bit error rate degradation. The performance of the proposed SLM scheme is analysed mathematically and verified through numerical analysis. Also, it is shown that the proposed SLM scheme has the lowest computational complexity among the existing low-complexity SLM schemes exploiting the signals at an intermediate stage of IFFT.