Abstract Simultaneous wireless information power transfer with power splitting is a new technology for energy harvesting that splits the received signal into two streams with adjustable power levels for information decoding and energy saving. Traditionally, decode and forward (DF) and amplify and forward (AF) schemes are used for energy harvesting in three‐step two‐way relay networks (TWRNs). In this paper, piece‐wise and forward (PF) scheme are applied with power splitting for energy harvesting in TWRNs. In PF, the received signal from each source at relay is compared to an adaptive threshold, if the signal strength is above the threshold, the relay functions similar to DF scheme, otherwise, the relay transmits a particular combination of its received signals from the sources. Next, the outage performance of PF protocol in a three‐step TWRN is studied and optimal lossless energy saving (OLES) method are presented to minimise the outage probability by computing the optimum power splitting ratio. The simulation and analytical results indicate that the outage probability of PF can be significantly improved using OLES technique in comparison with other related relaying schemes. The results show that the outage probability of PF‐OLES is 13, 3, and lower than AF‐OLES, DF‐OLES, and DAPS [1], respectively.

Multiple antennas can be used for increasing the amount of diversity or number of degrees of freedom in wireless communication systems. In this study, the authors consider a wireless half‐duplex relay network consisting of one source–destination pair each with a single antenna and one relay with multiple antennas. They assume an approach where cooperation of the relay in the transmission depends on the decoding status at the relay. If the relay participates in transmission, the source and the relay use the distributed space‐time code simultaneously. They derive the outage probability and the diversity‐multiplexing tradeoff for the proposed protocol at high SNR. They have shown that the use of the centralised antennas with simultaneously decoding compared to the distributed antennas with independently decoding can improve the system performance.

In this study, the authors address the spreading sequence estimation in direct‐sequence spread‐spectrum signals. At first, the maximum‐likelihood (ML) estimator is derived. In order to alleviate the greater computational complexity of the ML estimator, an innovative algorithm based on the ML method is proposed. The authors’ proposed algorithm uses an initial estimation with low complexity and low estimation accuracy as a result. In the second step, the estimation accuracy increases using the ML decision rule. They analyse their proposed algorithm and they derive an analytical approximation for the error probability of the proposed suboptimal algorithm. Simulation and analytical results show great performance and acceptable complexity of the proposed method.

Cognitive radio (CR) is proposed to solve the spectrum scarcity problem. One of the significant issues which arises recently in CR networks is security threats. Most of the proposed methods to defend against the attacks need the previous knowledge about the system. In this study, the authors use the statistical characteristics of the users to address one of the important kinds of attacks called primary user emulation attack (PUEA). In the proposed scheme, the received power statistics of the CR users is used to detect the PUEA. The results show that their scheme has higher performance in detecting the primary transmission comparing with the previous works.

In this paper, the authors show that using physical layer network coding (PNC) in wireless relay systems does not always increase throughput performance, and to achieve maximum throughput, PNC should be exploited intelligently regarding to the channel conditions. In this study, they propose a selective PNC (SPNC) scheme in bidirectional relay systems for binary phase shift keying. In the proposed scheme, the relay chooses either to detect network‐coded data from superimposed signal by PNC scheme or single source data by a so‐called single node detection (SND) scheme, based on which offers more end‐to‐end throughput for current channel realisation. They analytically determine the region of channel state plane in which SPNC selects SND scheme. Also, they obtain expressions for instantaneous bit error rates (BERs) of PNC and SND protocols. Moreover, they derive upper bounds in closed‐form for average BERs of PNC and SND schemes during multiple access phase at the relay. Then, they investigate SPNC with high‐order modulations. Simulations and analytical results confirm that SPNC achieves considerable throughput gain over PNC in Rayleigh fading channels.

In cognitive radio (CR) networks, the incorrect detection of primary users causes inefficient spectrum utilisation and harmful interference to the primary users, so spectrum sensing is an important problem that must be well done. However, multipath fading and path loss effects always reduce the performance of spectrum sensing. Cooperative spectrum sensing improves the sensing performance of cognitive users. However, when the number of cognitive users cooperating with each other is very large, the reporting bits increase and therefore more bandwidth will be needed. In this study, the authors try to reduce the average number of control bits that is exchanged between CR users. The authors consider two different systems: single band detection system and wideband system. The analysis are done for perfect and imperfect control channels. By using a ring‐based distributed spectrum sensing model, they propose a method with little required bandwidth to inform the CRs from the final decision about the activity of the primary user. Analytical and numerical results show that this method reduces the average number of control bits and therefore the required control bandwidth. Moreover, the analytical results represent that the imperfect control channels do not affect the performance of the proposed single band system and has a little effect on the performance of our wideband scheme.

In cognitive radio (CR) networks, the channel sensing performance has significant impact on the throughput of the system. In this study, the authors formulate the spectrum sensing to maximise the total throughput of the system under some constraints on the interference to the primary users (Pus). The total throughput defined in this study accounts not only for the throughput achieved by the CR user but also by the PU over each subband. In the numerical results, the authors compare their proposed idea with the previous work which finds the threshold of the detector to maximise CR throughput.

Cognitive radio is a technology with tremendous potential for more efficient utilisation of the radio spectrum. Based on the available network side information to the secondary user, three transmission models are proposed in the literature: the ‘underlay’ method, the ‘overlay’ method and the ‘hybrid’ method. The authors study here the capacity and power allocation for the hybrid method. In particular, the authors investigate the ergodic capacity of the secondary user under joint average transmission power and average interference to the primary user receiver constraints when no channel state information is available to the secondary user except its probability density function. An algorithm based on the barrier method is proposed to achieve this. Finally, simulation results are presented and analysed to validate the authors’ theoretical results.

A cognitive radio (CR) network is formed by allowing the secondary users to opportunistically access the frequency bands that are idle in time or frequency whenever the primary users (PUs) are not using these bands. In fact, this new technology tries to solve the spectrum scarcity. In this study, with the assumption that the CR users operate in a wideband communication system, first the bandwidth of the system is divided into some equal parts that are larger than the coherent bandwidth of the channel, and then the capacity of the channel by considering the CR user's outage capacity criteria is compared in two structures: one with the use of frequency diversity technique and the other without the use of frequency diversity. After that, the authors will evaluate the total interference power in the primary system and its statistical characterisation. Finally, the permissible ranges of the CRs transmit power by considering the outage probability and interference probability constraints will be obtained in these two structures.

Owing to multiple access capability, low probability of intercept and immunity against inter symbol interference (ISI) caused by multipath propagation, direct sequence spread spectrum (DSSS) techniques have been utilised in both underwater acoustic (UWA) communication networks and reliable point-to-point communications. To overcome the severe impact of ISI in the UWA channels, we propose a novel multiple pseudo noise (PN) sequence DSSS scheme and analyse its performance in sparse time varying UWA channels. Analytical and simulation results show that using a few PN sequences can remove most of the multipath induced ISI. To further alleviate the ISI effect, we propose a simple ISI reduction algorithm. Numerical results show that our proposed algorithm along with the proposed DSSS scheme can effectively mitigates the effect of remaining ISI, especially in sparse multipath channels.

Collaborative spectrum sensing is regarded as a key technology for tackling the challenges of the practical implementation of cognitive radio (CR). However, most of the proposed solutions require infinite bits or several bits exchanging per each CR to make the final decision to indicate the absence or presence of a primary user. It is well known that if N CRs employ energy detector with the same threshold and then send individual decision bits to a fusion centre, the OR-rule (1-out-of-N rule) will be optimal under Neyman-Pearson criterion. To decrease the average number of exchanging bits, we propose novel schemes while they perform as well as the OR-rule, our analytical and simulation results show that the proposed schemes has two important advantages over previously proposed schemes: (1) the average number of exchanging bits for N CRs always is 1/N bits per CR in perfect control channels; however, it is much higher than 1/N for the other proposed methods, e.g. it is 1 bit for the OR-rule and (2) the proposed method is very robust against the errors of imperfect control channels, specially, in low signal-to-noise ratio (SNR) regimes. Furthermore, the average number of reporting bits do not change considerably under the imperfect control channels.

This paper considers a cooperative space-frequency (SF) coded OFDM system and evaluates its performance over quasi-static frequency selective Rayleigh fading channels. An expression for exact outage error probability is derived and its tight closed form lower bound is presented. The tightness of the lower bound is demonstrated through the Monte Carlo simulations. The asymptotic analysis indicates that the proposed protocol achieves full spatial and frequency diversities available in the cooperative communication system. The theoretical analysis of the proposed space-frequency coded cooperation protocol is further confirmed by computer simulations.

Recent measurements suggest the possibility of sharing spectrum among different parties subject to interference-protection constraints. In order to enable access to an unused licensed spectrum, a secondary user has to monitor licensed bands and opportunistically transmit whenever no primary signal is detected. In this paper, we study spectrum-sharing between a primary licensee and a group of secondary users. The structure of an asymptotically optimum detector based on the measurements of all secondary users is derived and the effect of the quantization error in such a system is evaluated. The results show the superiority of the proposed detector to other schemes.

A cooperative space–frequency (SF) coded orthogonal frequency division multiplexing system is considered and its performance over quasi-static frequency-selective Rayleigh fading channels is evaluated. An expression for exact outage error probability is derived and its tight closed-form lower bound is presented. The tightness of the lower bound is demonstrated through Monte Carlo simulation. Asymptotic analysis indicates that the proposed protocol achieves full spatial and frequency diversities available in the cooperative communication system. The theoretical analysis of the proposed SF coded cooperation protocol is further confirmed by computer simulation using a previously introduced SF block code that is capable of achieving full spatial and frequency diversities.

Traditionally, the frequency spectrum is licensed to users by government agencies in a fixed manner where the licensee has exclusive right to access the allocated band. However, with increasing demand for the spectrum and scarcity of vacant bands, a spectrum policy reform seems inevitable. Meanwhile, recent measurements suggest the possibility of sharing spectrum among different parties subject to interference-protection constraints. In order to enable access to an unused licensed spectrum, a secondary user has to monitor licensed bands and opportunistically transmit whenever no primary signal is detected. Spectrum-sharing between a primary licensee and a group of secondary users has been studied. The structure of an asymptotically optimum detector based on the measurements of all secondary users is derived and the effect of the quantisation error in such a system is evaluated. Also, it is shown that by using the proposed detector in a sequential detection structure, it is possible to shorten the decision time needed by the detector. The results show the superiority of the proposed detector to other schemes.