Pilot contamination reduction can improve the achievable uplink and downlink rates in a multicell massive multiple-input–multiple-output system. To this end, this study first proposes two schemes to mitigate the effect of pilot contamination by implementing uplink training with time-aligned and time-shifted pilot transmissions. Accordingly, these techniques enable the system not only to exploit the advantages of time-shifted training but also to benefit from the use of conventional systems that improve the achievable rates. The authors introduce channel estimation in matrix form by estimating from a near minimum mean square error method. Furthermore, linear detector and precoding methods are used for a base station with a finite number of antennas in order to derive the closed-form lower bounds of the achievable rates with two uplink training schemes for both uplink and downlink transmissions. Finally, numerical results are presented to verify the analysis.

This paper studies the capacity and energy efficiency of non-coherent Rayleigh fading channels with Gaussian mixture noise where neither the transmitter nor the receiver has the knowledge of channel state information. The channel under consideration is suited for cellular networks having multi-tier heterogeneous architectures in which the channel conditions change rapidly. In the first part of the paper, we characterize the structure of a capacity-achieving input signal. Specifically, we establish an integrable upper bound on the integrand in the output entropy and demonstrate that there exists a unique optimal input. By formulating the Kuhn-Tucker condition and establishing a diverging lower bound on it, we show that the optimal input is discrete having a finite number of mass points. Using this result, we investigate the capacity and energy efficiency of the considered channel in the second part of the paper. In particular, we first develop a numerical method to evaluate the optimal input and compute the capacity. The energy efficiency, which is related to the capacity and optimal input in low-power regimes, is examined by calculating the minimum bit energy and wideband slope of the spectral-efficiency curve. We also analytically show the optimality of an on-off signal in this regime.

The downlink problem of a cloud-based central station (CCS) to multiple base stations (BSs) in a heterogeneous cloud radio access network (HCRAN) sharing the same time and frequency resources is studied. The authors adopt nonorthogonal multiple access (NOMA) and propose power allocation for the wireless downlink in the HCRAN. Taking into account practical channel modelling with power consumptions at BSs of different cell types (e.g. macrocell, micro-cell etc.) and backhauling power, they analyse the energy efficiency (EE) of the practical HCRAN utilising NOMA. Simulation results indicate that the proposed NOMA for the HCRAN outperforms the conventional orthogonal frequency division multiple access scheme in terms of providing higher EE of up to four times. Interestingly, the results reveal a fact that the EE of the NOMA varies as a quasi-concave function. An optimisation problem is accordingly introduced to find the optimal number of BSs that maximises the EE of the HCRAN. It is shown that, with a low power supply at the CCS, a double number of micro BSs can be served providing an improved EE of up to 1.6 times compared with the macro BSs and remote radio heads, while they achieve the same EE performance with high-power CCS.

This study devises for the first time a comprehensive analysis of decode-and-forward, space–time coded, fully cooperative orthogonal frequency division multiplexing (OFDM) systems from both error performance and energy efficiency perspectives, in either identically or non-identically distributed frequency selective Rayleigh fading channels. The authors’ analyses show the interesting result that the fully cooperative OFDM is better than direct OFDM transmission from both perspectives in many cases, especially at a low power and low signal-to-noise ratio regime, making it particularly useful for emerging low power applications, such as wireless body area networks.

Visible light communication (VLC) networks can provide high-rate transmissions with advantages such as high security, low power consumption and so on. Multiple users get access to the VLC network according to the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism defined by IEEE 802.15.7 media access control layer specifications. In this study, the authors propose an energy efficient medium access scheme to improve the performance of IEEE 802.15.7 CSMA/CA mechanism under unsaturated traffic conditions. Aiming at reducing the power consumed on unnecessary waiting and at improving the channel utilisation efficiency, they first apply a successive transmission scheme on the node, which has completed one successful transmission and gets the chance of transmitting the next packet without backoff delay period according to the network condition. Then, a dynamic contention window is exploited to mitigate the collisions, where they derive the optimal minimum contention window based on throughput analysis. The power consumption can be reduced with improved throughput, when both the traffic load and the number of nodes in the network are taken into account. Simulation results demonstrate that the authors’ proposed scheme outperforms the IEEE 802.15.7 CSMA/CA scheme with respect to the average power consumption, system throughput and packet dropping probability.

In wireless multimedia sensor systems (WMSSs), the devices are equipped with multiple energy-constrained camera sensors (CSs) distributed over bandwidth-constrained and lossy wireless channels, in catastrophe-prone areas. Meanwhile, multimedia applications, e.g. video streaming, require considerable energy and bandwidth resources to gain long lifetime and high streaming quality. This study proposes an energy, bandwidth, and quality (EBQ) optimisation framework for green two-tiered WMSSs. The first tier contains the CSs and the second tier includes cluster heads (CHs) selected from the CSs with higher available energy and processing capacity. In the EBQ optimisation framework, a rate allocation optimisation problem is formulated under given constraints of available backhaul bandwidth of the CHs and quality of received videos at base stations (BSs). This problem is solved for optimal encoding rates to packetise each video captured from different environments into multiple descriptions for transmission. Consequently, the average energy consumption per CS is minimised for long lifetime while conserving the bandwidth of the CHs and guaranteeing high quality of received videos for the purpose of monitoring at the BSs. Simulations demonstrate that the proposed EBQ optimisation framework can efficiently enhance the performance of green two-tiered WMSSs in terms of minimum energy consumption, bandwidth efficiency, and high quality.

In this study, the authors analyse the outage performance of multirelay decode-and-forward cooperative networks subject to two joint practical issues of wireless communications, namely, energy constraints and transceiver hardware impairment (HI). To deal with energy constraints at relay nodes, radio-frequency (RF) energy harvesting (EH) technique is adopted. Considering the joint impacts of RF EH technique and HI, two relay selection schemes, namely, harvested energy-based relay selection (HEbS) scheme and channel quality-base relay selection (CQbS) scheme are proposed. Tight closed-form approximate expressions for the outage probability of each scheme are derived and corroborated through Monte Carlo simulations. Some representative performance comparisons are carried out and show that the diversity order achieved by the HEbS scheme is 1 and is independent of the number of relays, K, whereas that achieved by the CQbS scheme is K, which is full diversity.

In this paper, we investigate the effect of of the primary network on the secondary network when harvesting energy in cognitive radio in the presence of multiple power beacons and multiple secondary transmitters. In particular, the influence of the primary transmitter's transmit power on the energy harvesting secondary network is examined by studying two scenarios of primary transmitter's location, i.e., the primary transmitter's location is near to the secondary network and the primary transmitter's location is far from the secondary network. In the scenario where the primary transmitter locates near to the secondary network, although secondary transmitter can be benefit from the harvested energy from the primary transmitter, the interference caused by the primary transmitter suppresses the secondary network performance. Meanwhile, in both scenarios, despite the fact that the transmit power of the secondary transmitter can be improved by the support of powerful power beacons, the peak interference constraint at the primary receiver limits this advantage. In addition, the deployment of multiple power beacons and multiple secondary transmitters can improve the performance of the secondary network. The analytical expressions of the outage probability of the secondary network in the two scenarios are also provided and verified by numerical simulations.

This study proposes a single frequency reconfigurable planar invert-F antenna (FRPIFA) using PIN diodes based on the changes of shorting pin positions. The single FRPIFA operates in four configurations which cover the bands of GSM 900/GSM 850, GPS 1575, GSM 1800/GSM 1900 and UMTS (Universal Mobile Telecommunication System) with peak gains of −7.24, 2.30, 2.98 and 3.53 dBi, respectively. Furthermore, a frequency reconfigurable multiple-input multiple-output (MIMO) antenna consisting of two single reconfigurable ones with a distance of half-wavelength is presented. By switching the PIN diodes, the reconfigurable MIMO antenna obtains four different configurations. For MIMO antenna, results of the operating bands, gains, radiation patterns are similar to those of the reconfigurable single one. The MIMO antenna achieves a high isolation with |S21| less than −20 dB in all operating bands. The simulated results of S-parameters of the antennas are in good agreement with measurements. The proposed compact single and MIMO antennas are suitable for green smart wireless communication systems.

This study discusses new frequency reversal orthogonal frequency-division multiplexing schemes for cooperative networks in order to minimise the effect of multiple carrier frequency offsets. Theorem 1 proposed and proved in this study tells that zero-forcing (ZF) decoding is equivalent to maximum-likelihood (ML) decoding if the equivalent channel matrix is complex orthogonal. Frequency reversal Alamouti code for two-relay scenario and new space-frequency codes for more relay nodes scenario are designed under the guidance of Theorem 1. The reversal operations of these codes make the equivalent channel matrix between the relays and the destination complex orthogonalised. Thus, ZF decoding achieves the advantages of ML decoding, and simulation results also confirm that. Benefiting from the simplified ZF decoding method in Theorem 1, the decoding complexity becomes really low.

In this study, the authors study ring-based linear network coding over erroneous cyclic networks over commutative rings such as a principal ideal domain or discrete valuation ring. In the first part, they study coherent field-based linear error-correcting network codes (LENCs) over cyclic networks. By changing alphabet symbols from fields to commutative rings, they extend Zhang's formulation for LENCs restricted on acyclic networks to cyclic networks, and generalise fundamental results and concepts such as the minimum rank distance and the refined Singleton bound, and show that this bound is tight. In the second part, they generalise some main results such as the free distance and the generalised Singleton bound from convolutional codes to ring-based LENCs over cyclic networks. In the third part, they propose an algebraic method for calculating sink bit error probability of ring-based linear network codes over all erroneous networks by using the authors’ formulations.

As more user applications emerge for wireless devices, the corresponding amount of traffic is rapidly expanding, with the corollary that ever-greater spectrum capacity is required. Service providers are experiencing deployment blockages due to insufficient bandwidth being available to accommodate such devices. TV White Space (TVWS) represents an opportunity to supplement existing licensed spectrum by exploiting unlicensed resources. TVWS spectrum has materialised from the unused TV channels in the switchover from analogue to digital platforms. The main obstacles to TVWS adoption are reliable detection of primary users (PUs) i.e. TV operators and consumers, allied with specifically, the hidden node problem. This study presents a new Generalised Enhanced Detection Algorithm (GEDA) that exploits the unique way Digital Terrestrial TV (DTT) channels are deployed in different geographical areas. GEDA effectively transforms an energy detector into a feature sensor to achieve significant improvements in detection probability of a DTT PU. Furthermore, by framing a novel margin strategy utilising a keep-out contour, the hidden node issue is resolved and a viable secondary user sensing solution formulated. Experimental results for a cognitive radio TVWS model have formalised both the bandwidth and throughput gains secured by TVWS users with this new paradigm.

This work is motivated by the following observation: modern wireless applications such as content prefetching, allow different extent of delay tolerance, thus in practice users may have different time-averaged data rates during a certain period of time. In this study, the authors consider resource allocation in small cell networks under time-varying channels while each user has an individual time-averaged rate requirement. Stochastic optimisation model is employed to minimise the time-averaged power consumption of small base stations subject to individual user's time-averaged rate constraint. They develop an online power optimal resource allocation (PORA) algorithm to achieve the optimal power allocation and subcarrier assignment decisions without prior knowledge of channel statistics. Furthermore, considering that the power allocation and subcarrier assignment problem is a nonconvex combinatorial problem, they further develop an iterative heuristic algorithm with polynomial complexity. Simulation results show the effectiveness of PORA and verify the theoretical analysis on the network performance.

In recent years, to efficiently deliver a specific function value is of great importance in sensing and monitoring wireless sensor networks. In this study, a priority-aware hybrid scheduling scheme is proposed to achieve fast decentralised energy-efficient max function computation in a single-hop network. In the authors’ proposed scheme, priority-aware time division multiple access (TDMA) scheme is used to first schedule the transmission. In contrast to traditional TDMA, where all the nodes in a network take turns to transmit, in the authors’ proposed scheme, the node with the maximum value is given the priority to transmit. However, the problem of collision exists and this still limits the performance of TDMA-based scheme. Therefore, in order to improve the performance with the influence of collision, code division multiple access (CDMA) is introduced to recover information whenever collision occurs. To solve this, an optimisation problem is formulated to minimise average computation time subjected to the constraints on failure probability. The synergy of CDMA and TDMA contributes to reduction of computation failure probability as well as average computation delay. Moreover, when compared with the traditional round-robin TDMA transmission, the scheduling rule in the proposed scheme helps to reduce number of transmissions for a single computation. Therefore, a reduction in energy consumption occurs as well. Numerical results show that our algorithm reduces average computation time and energy consumption for one-shot max function computation, and with the advantage of better scalability.

The common voice channels existing in cellular communication networks provide reliable, ubiquitously available and top priority communication mediums. These properties make voice dedicated channels an ideal choice for high priority, real time communication. However, such channels include voice codecs that hamper the data flow by compressing the waveforms prior to transmission. This study designs codebooks of speech-like symbols for reliable data transfer through the voice channel of cellular networks. An efficient algorithm is proposed to select proper codebook symbols from a database of natural speech to optimise a desired objective. Two variants of this codebook optimisation algorithm are presented: One variant minimises the symbol error rate and the other maximises the capacity achievable by the codebook. It is shown both analytically and by the simulation results that under certain circumstances, these two objective functions reach the same performance. Simulation results also show that the proposed codebook optimisation algorithm achieves higher data rates and lower symbol error rates compared with previously reported results while requiring lower computational complexity for codebook optimisation. The Gilbert–Elliot channel model is utilised to study the effects of adaptive compression rate adjustment of the vocoder on overall voice channel capacity. Finally, practical implementation issues are addressed.

Distributed switched diversity combining has drawn significant attention in the recent past due to its low-complexity nature in terms of channel state information requirement at the receiving end to achieve full diversity order. In this study, the authors propose a hybrid selection and switch-and-examine combining (HSSEC) scheme, which is a combination of selection combining and SEC schemes, for a multi-relay decode-and-forward system to improve the performance of distributed switched diversity combining schemes proposed in the literature. Furthermore, the performance of HSSEC scheme is investigated over non-identical Nakagami-m fading channels. Average end-to-end symbol error probability (SEP) expression is derived for M-ary phase-shift keying signalling, and in addition asymptotic SEP expression is also derived to analyse diversity order. From the derived asymptotic expression, it is inferred that the HSSEC scheme attains full diversity order over Nakagami-m fading channels, except for the case when threshold signal-to-noise ratio (SNR) is very much lesser than the average SNR. Furthermore, exact outage probability expression is derived for the HSSEC scheme. Finally, in the numerical results, the outage and SEP performances are compared with other schemes proposed in the literature.

In this study, power allocation over time-varying multi-user multi-relay amplify-and-forward networks is studied. Specifically, stochastic network sum-rate, max–min rate power allocation and total power minimisation problems are formulated. However, solving such stochastic problems relies on perfect global instantaneous channel state information (CSI), and thus entails complex computations and excessive communication overheads. To circumvent these issues, second-order statistics of the CSI (i.e. partial CSI) are utilised to transform the stochastic formulations into deterministic optimisation problems in terms of ergodic capacity while satisfying quality-of-service constraints via target outage probability. The obtained optimal deterministic problems are non-convex and thus are computationally prohibitive. However, at high enough signal-to-noise ratio, such problems can be transformed into asymptotically convex ones, and thus solved efficiently. Simulation results illustrate that the proposed approximate deterministic power allocation reformulations closely agree with their optimal exact deterministic and dynamic counterparts.

For bi-phase signals, an adaptive carrier tracking method based on sliding discrete Fourier transform (SDFT) frequency-locked loop (FLL) is proposed. Magnitude and phase spectra of closed-loop SDFT filter provide almost flat response around the centre frequency. However, the small magnitude and phase errors observed at the SDFT in-phase and quadrature components are corrected by adaptive sampling frequency control. The carrier frequency is estimated by computing the frequency error. Hence, the input frequency variation due to Doppler shift could be estimated for the bi-phase signal. Experimental investigations with digital implementation demonstrate the suitability of the FLL for bi-phase signals.

In the past few years, unmanned aerial vehicles (UAVs) have become a primary airborne platform for hyperspectral imager for studies on precision agriculture, defence, and the environment. The ‘push-broom’ type of hyperspectral sensors require moving vehicle, and transmission and analysis of hyperspectral data by means of a UAV's high-mobility channel is challenging. While high bandwidth of hyperspectral imaging justify using orthogonal frequency division multiplexing (OFDM) for data transmission, the high speed of UAVs imposes intercarrier interference (ICI) on the transmitted OFDM signal because of the Doppler shift. This study proposes a technique for channel estimation and equalisation in order to compensate the ICI. This technique uses a complete channel matrix estimation in the frequency domain in contrast to conventional methods that only use diagonal elements when recovering the data. In order to evaluate the received data using this technique, a classification framework was designed that took into consideration both spectral and spatial information. In order to verify the robustness of the proposed model, the system was analysed using a Pavia Center hyperspectral dataset, and evaluated against speeds of 50 and 500 m/s. By using this method, improvement in both data transmission and the analysis was achieved.

Capacity bounds are calculated for the differential chaos shift keying (DCSK) system with non-coherent detectors, including differentially coherent detector and energy detector, over additive white Gaussian noise (AWGN) and Rayleigh fading channels. Through theoretical analysis and numerical simulations, it is found that the capacity characteristics of the non-coherent DCSK system are significantly different from those of the coherent case. It is shown that there are different capacity bounds corresponding to different spreading factor values. Optimal code rates are found, which minimise the required bit signal-to-noise ratio for a reliable communication system, and they are increasing with the increase of the spreading factor value over an AWGN channel while decreasing over a Rayleigh fading channel. As compared with the AWGN channel, the performance over a Rayleigh fading channel is much more sensitive to the code rate. The new results are useful as benchmark for designing capacity-approaching codes for the non-coherent DCSK system.

Owing to the high cost of radio frequency (RF) chains and power consumption of broadband signals, it is suitable to include both analogue and digital processing for millimetre wave (mmWave) beamforming. In this study, hybrid beamforming (HBF) schemes are considered for multi-user downlink mmWave systems. RF beamforming schemes are proposed for HBF transmission with perfect channel state information at transmitter (CSIT) and limited feedback information in the multi-user mmWave channel. Simulation results show that proposed HBF schemes enhance sum rate performance than conventional HBF schemes in both cases of perfect CSIT and limited feedback.

The authors study the design of cooperative beamforming for an underlay cognitive relay network over frequency selective fading channels. The network under consideration consists of one primary link, one secondary link and relay nodes that assist the communication. It is assumed imperfect blind channel estimation, i.e. only the second order statistics – with uncertainty – of the channel gain between the primary transmitter-relay and relay-primary receiver, is available. To mitigate the channel's frequency selectivity, each relay is equipped with a finite impulse response filter, which performs filter and forward relaying. The authors consider two different scenarios: in the first, the objective is to minimise the total transmit power of the relays subject to a constraint on the interference induce on the primary receiver. Furthermore, a constraint is also considered on the received signal-to-interference plus noise ratio (SINR) in the secondary receiver. In the second scenario, the objective is to maximise SINR in the secondary receiver, subject to constraints on the sum power of the relays and also on the noise and interference power in the primary receiver. Simulation results show how total power of relay varies with the uncertainty in the blind channel estimation in the first scenario. In addition, the achievable SINR deteriorates in the second scenario when the uncertainty increases.

Non-orthogonal multiple access (NOMA), which is an effective solution to improve spectral efficiency, has been considered as an emerging candidate for the fifth generation multiple access. In addition, simultaneous wireless information and power transfer, which aims to maximise energy efficiency, has received significant attention. In this study, the authors design NOMA for downlink energy harvesting (EH) multiple-antenna relaying networks. The base station communicates with multiple mobile users simultaneously via an EH relay node. In the first slot, the relay node harvests energy from the received signals; and in the second slot, the relay node uses the harvested energy to broadcast the received signals to all mobile users. Antenna selection is adopted at the base station while maximal-ratio combining is applied at the mobile users. The outage performance for the NOMA-EH relaying networks is studied over Nakagami-m fading and closed-form expressions for the outage probability are obtained. In addition, the presented simulation results demonstrate the correctness of the authors’ analysis and the advantage of NOMA over conventional orthogonal multiple access.