In this study, the design of a superimposed training (ST) for an interference-limited spatially correlated multiple-input–multiple-output (MIMO) system is addressed and a closed-form solution for designing the training signal is proposed in a sub-optimal way. Earlier papers have considered noise limited MIMO systems. The authors also propose random/orthogonal variable spread factor (OVSF) codes as a choice for the ST signal. The mean-squared error of the channel estimate and symbol error rate performances are obtained through simulation. Considering the power allocation issue between the data and training symbols, a sub-optimal average training power that maximises the lower bound on the effective signal-to-interference ratio is also proposed. Simulation results show that the bit error rate performance of the ST is indistinguishable from the OVSF/random ST sequence.

The authors consider the effect of caching in interference-limited wireless networks where fading is the dominant channel effect. First, the authors propose a one-hop transmission strategy for cache-enabled wireless networks, which is based on exploiting multi-user diversity gain. Then, they derive a closed-form result for throughput scaling of the proposed scheme in large networks, which reveals the inherent trade-off between cache memory size and network throughput. Their results show that substantial throughput improvements are achievable in networks with sources equipped with large cache size, compared with previous works where each transmitter could just store one file. Also, the authors provide extensive simulations through which they analyse network throughput and verify their analytical results. Their simulations suggest that the scaling results also hold true even by introducing correlation between channels, making the results more general.

An interference alignment (IA) scheme for interference channels was recently proposed to achieve the maximum degrees of freedom (DoF). Although most studies of IA focus on network throughput, that is, DoF, reliability in terms of diversity order is also an important performance measure. In this study, interference cancellation (IC) scheme based on Alamouti code in the multi-access scenario is applied to the K-user, multi-input–multi-output (MIMO) interference channel. This IC scheme gives the benefit of diversity order and requires no channel-state-information at the transmitters (CSIT). However, it requires more receive antennas than the IA scheme to achieve the same DoF. In order to reduce the number of receive antennas, especially for the three-user MIMO interference channel, an IA-and-cancellation (IAC) scheme based on Alamouti code is proposed. It keeps the same DoF as the IC scheme, but requires partial CSIT. It is analytically shown that the IC and IAC schemes enable symbol-by-symbol decoding and achieve diversity order of two, while the conventional IA scheme achieves diversity order of one.

Spectrum auctions are one of the best-known solutions to improve the efficiency of spectrum use. However, there can be many challenges in the design of a practical spectrum auction. Heterogeneity is one of the most major challenges. Unfortunately, most of the existing auction designs either do not take into account the various aspects of heterogeneity or assume only the scenario where each seller supplies one distinct channel and each buyer wishes to buy merely one channel. The authors propose a spectrum auction mechanism which considers the various aspects of heterogeneity as well as multi-channel purchasing. They prove that the auction design preserves three important economic aspects including truthfulness, budget balance and individual-rationality. Moreover, most of the existing works only provide the bidders a simple demand format. Their auction mechanism enables bidders to use diverse demand formats. Furthermore, they propose some novel adaptive grouping algorithms to improve the auction's performance. The simulation results demonstrate good performance of the proposed algorithms on various auction metrics.

A new approach for performance analysis of orthogonal space–time block coding systems with antenna selection is presented. Antenna selection has been performed at the transmitter and/or at the receiver sides. Closed-form expressions are derived for the approximate average bit error rate (BER) of the considered system for M-ary quadrature amplitude modulation and phase-shift keying schemes. Different from other approaches presented in the literature, analytical expressions are derived for the average signal-to-noise ratio (SNR) gain obtained from each antenna selection scheme, and then using those SNR gains closed-form expressions are obtained for the approximate average BER performance. The system performances of several forms of the presented scheme are evaluated and compared. It is shown that the results obtained from the mathematical expressions match closely with simulation results.

In order to effectively utilise the spectrum of wireless network, secondary users (SUs) are often authorised to share spectrum with primary users (PUs) in cognitive networks. To guarantee effective transmission of PUs and SUs, the interference from the SUs to the PUs is limited and only the SUs that have the information rates (rates) than the corresponding thresholds are allowed to transmit their data. In order to select proper SUs to transmit and to maximise their achievable weighted sum rate (WSR) under rate requirement and interference constraint, the authors propose a joint admission control and beamforming design algorithm for partial channel state information (CSI) case. The WSR maximisation problem is first constructed by using approximation and convex relaxation, according to the statistical property of the CSI. Then a penalty factor is introduced to check the feasibility of such problem and instruct the admission of the SUs. Beyond the feasible SUs set, the WSR is maximised by updating the achievable rate of each user with rate profile technique and polyblock outer approximation method. Simulation results show that the proposed beamforming can achieve higher WSR than the beamforming of the existing baseline method.

In this study, the authors investigate the benefits of phase-rotation-assisted precoding (PRP) technique in spatial modulation (SM) systems, which are based on maximum free distance d _min. First, a closed-form solution of the maximum-d _min PRP matrix is derived for the scenario of having two transmit antennas (N _t = 2). Moreover, two numerical methods are proposed for dealing with the case of N _t > 2. The complexity of the proposed algorithms is presented. The authors simulation results show that the proposed PRP algorithms provide beneficial bit error ratio performance improvements compared with both the conventional SM and with the existing adaptive SM.

In this work, the authors derive the optimal signal-to-interference-ratio (SIR) thresholds S _t which maximise coverage probability for both fractional frequency reuse (FFR) and soft frequency reuse (SFR) networks with base station locations modelled using Poisson point process. It is analytically shown that for both FFR and SFR, the optimal SIR threshold is equal to the target SIR T, i.e, S _t = T. The authors also show that at the optimal SIR threshold, FFR achieves a higher coverage than frequency reuse (1/Δ). Furthermore, for the cases when S _t > T and S _t < T, FFR achieves a higher coverage than reuse (1/Δ) and reuse 1, respectively. On the other hand, SFR coverage can be higher or lower than reuse (1/Δ) coverage, even when S _t = T. However, when S _t > T, SFR achieves a higher coverage than reuse 1, and when S _t < T, the SFR coverage can be lower than reuse 1 coverage. The FFR and SFR coverages are also compared for a given Δ, and it is shown that FFR achieves a higher coverage than SFR at the optimal value of S _t.

By introducing the imitate infrared encoding technique and employing a white light emitting diode (WLED), a pair of standalone visible light communication (VLC) terminals, including a transmitter and a receiver, were experimentally demonstrated. The mathematic modelling and formulation were proposed on the WLED's luminous characteristics and the VLC channel response. The terminals were developed using ARM7-based embedded system, and experiments were carried out to derive their performances. A suitable illuminance of 200 lx (measured before the WLED) is preferred for obtaining a uniform bit error ratio (BER) over the distance of 0–2 m, and the measured BER is <10⁻⁴ with a communication data rate of 0.1–10 Mbps. High stability was also confirmed through measuring the BER results over 5 h at the two communication distances of 0.2 and 0.3 m and at 1 Mbps data transmission rate. Owing to the advantages of small size, low cost, and 10 Mbps data rate, this kind of terminals can be adopted in indoor short-distance wireless communication.

The opportunistic spectrum access technology is one of the most promising methods for alleviating the spectrum scarcity problem, which enables a secondary user (SU) to utilise a primary user spectrum band that is detected idle. However, the throughput achieved by cognitive radios is limited by the interference constraint imposing on the SU. Multiple input single output antenna techniques and antenna selection (AS) techniques are exploited to combat the interference constraint and improve the achievable average throughput of the SU. The optimal power allocation strategy is proposed to maximise the achievable average throughput. Performance analyses for the achievable average throughputs are performed under the maximum channel gain AS strategy, the minimum interference channel gain AS strategy and the ratio AS strategy. It is proved that the optimal transmitted AS strategy is the ratio AS strategy when the optimal power allocation strategy is used. The optimal sensing parameters are designed to further improve the maximum average throughput. Extensive simulation results are conducted to verify this analysis.