Performance Analysis And Optimization For The Cognitive Radio Network Based On The Cooperative Communications

With the unprecedented growth in the number of wireless users and growing demand for high-throughput services,the large bandwidth requirement and energy consumption have become the main concerns in the wireless networks.Besides,the current fixed spectrum allocation approach has severely under-utilized the available spectrum.Under this background,cognitive radio(CR)has been proposed,which is an effecitive solution to the spectrum scarcity and under-utilization problem.Generally,CR has three access strategies,i.e.,opportunity access where secondary users firstly sense the channel and then transmit the date only when the channel is idle;sensing-enhanced access where secondary users firstly perform channel sensing and then initiate data transmission with two power levels based on the sensing decisions(e.g.,idle or busy);spectrum sharing access where secondary users(SU)share the same spectrums with the primary users(PU).Among these access schemes,the underlay CR(spectrum sharing)is widely used due to its lower complexity compared to the other two schemes.However,the transmit power of the cognitive radio system in underlay cognitive radio scenarios is strictly limited in order to restrict the interference to the primary receiver.This significantly affects the performance of cognitive transmission.Hence,relay based cooperative communication has been proposed into the CR network as a new paradigm towards improving coverage and reliability.Also,the primary user is severely influenced by the interference from the SU.To address these problems,in this paper,we adopt the multi-antenna technology at the secondary transmitter and receiver.We firstly design the beamforming at the secondary transmitter to null the interference on PU and then use the maximal-ratio combing at the secondary destination to enhance the performance.due to the rapidly varying channels,channel state information required for beamforming design can be outdated.Thereby,we design the beamforming based on the outdated CSI and investigate the effects of outdated CSI on the system performance.Additionally,we employ the more general Nth best relay selection scheme to provide the diversity order for the proposed multi-antenna multi-relay CR network.To save the bandwidth,coded cooperation has been proposed due to its improved bandwidth efficiency by integrating channel coding with cooperative signaling.It has been proved that coded-cooperation can improve the outage probability(OP)and biterror rate over the conventional cooperative communication systems [5].Inspired by the advantages of these technologies,researchers proposed the coded-cooperation cognitive multi-relay network.In this network,the spectrum utilization,diversity gain and bandwidth efficiency can all be improved.However,few works have studied the performance of such a system.A lot of works studied the performance of the cognitive relay networks.To make up for this hole,we study the outage performance of the code-cooperation CR network.We use both the Nth best relay selection scheme and sum relays scheme to compare their performance and also examine their performance in the high signal-tonoise ratio(SNR)region to get further insights into the system behaviors,i.e.,diversity and coding gains.FSO communication has recently attracted a significant research interest due to its license free spectrum for operation,high security,higher date-rates,low cost and so on.These advantages have made them appealing for emergency situation recovery and military applications.However,FSO is limited by the atmospheric turbulence condition,resulting in coverage distance to few hundreds of meters.The mixed RF/FSO relaying scheme has been recently presented as an efficient means to deal with these difficulties.Mixed radio frequency/free space optical(RF/FSO)relaying systems can connect a large number of RF devices simultaneously to the last-mile access network using a single broadband FSO link.To alleviate radio frequency(RF)spectrum under-utilization and performance limitation of the secondary system,a CR mixed RF/FSO network has been proposed towards enhancing the coverage and data rate of the downlink through a cooperative FSO link.In a cognitive mixed RF/FSO network,FSO link doesn't share the spectrum with the first-hop and primary system and thereby,can improve the total throughput and reduce the interference.In addition,due to the broadcasting nature of the wireless RF network and the line of sight nature of FSO channels,the RF link is the only weak link from the secrecy performance viewpoint.In this paper,we propose the three transmission schemes to improve the security performance of the CR mixed RF/FSO network.Moreover,we optimize the power allocation among users to maximize the security throughput.The security reliability trade-off(SRT)is clearly illustrated by the relationship between the outage probability as the reliability measure and the intercept probability as the security measure.To prolong the lifetime of energy-constrained wireless nodes,energy-harvesting has been proposed as a promising approach.The key idea of energy harvesting is that a wireless node can capture RF signal sent by a source node,charge its battery,and then use it for signal processing or transmission.This technology is important for sensing spectrum CR because the secondary user is a battery powered device,and the spectrum sensing and data transmission procedures usually accompany energy consumption.Therefore,the CRNs with energy harvesters have been proposed.However,to the best of the authors' knowledge,the related works mainly focused on the throughput or energyefficiency(EE)of such a system,but did not consider the energy-harvesting problems.So,in this paper,we investigate the sum energy-harvesting and max-min fairness energyharvesting problems fo ra Wideband Cognitive Radio Network with energy harvesters(EHRs).Our formulated problems are subject to the constraints on the CBS transmit power,the SU rates and the interference temperature.We derive a new trade-off between the optimal harvested energy and sensing time.