Impact of primary networks on the performance of energy harvesting cognitive radio networks
2016; Institution of Engineering and Technology; Volume: 10; Issue: 18 Linguagem: Inglês
10.1049/iet-com.2016.0400
ISSN1751-8636
AutoresJinghua Zhang, Nam‐Phong Nguyen, Junqing Zhang, Emiliano Garcia‐Palacios, Ngoc Phuc Le,
Tópico(s)Full-Duplex Wireless Communications
ResumoIET CommunicationsVolume 10, Issue 18 p. 2559-2566 Special Issue: Green Computing and Telecommunications SystemsFree Access Impact of primary networks on the performance of energy harvesting cognitive radio networks Jinghua Zhang, Jinghua Zhang School of Electronics, Electrical Engineering and Computer Science, Queen's University Belfast, Belfast, UKSearch for more papers by this authorNam-Phong Nguyen, Corresponding Author Nam-Phong Nguyen pnguyen04@qub.ac.uk orcid.org/0000-0002-9332-6282 School of Electronics, Electrical Engineering and Computer Science, Queen's University Belfast, Belfast, UKSearch for more papers by this authorJunqing Zhang, Junqing Zhang School of Electronics, Electrical Engineering and Computer Science, Queen's University Belfast, Belfast, UKSearch for more papers by this authorEmiliano Garcia-Palacios, Emiliano Garcia-Palacios School of Electronics, Electrical Engineering and Computer Science, Queen's University Belfast, Belfast, UKSearch for more papers by this authorNgoc Phuc Le, Ngoc Phuc Le Faculty of Electrical and Electronics Engineering, Duy Tan University, Danang, VietnamSearch for more papers by this author Jinghua Zhang, Jinghua Zhang School of Electronics, Electrical Engineering and Computer Science, Queen's University Belfast, Belfast, UKSearch for more papers by this authorNam-Phong Nguyen, Corresponding Author Nam-Phong Nguyen pnguyen04@qub.ac.uk orcid.org/0000-0002-9332-6282 School of Electronics, Electrical Engineering and Computer Science, Queen's University Belfast, Belfast, UKSearch for more papers by this authorJunqing Zhang, Junqing Zhang School of Electronics, Electrical Engineering and Computer Science, Queen's University Belfast, Belfast, UKSearch for more papers by this authorEmiliano Garcia-Palacios, Emiliano Garcia-Palacios School of Electronics, Electrical Engineering and Computer Science, Queen's University Belfast, Belfast, UKSearch for more papers by this authorNgoc Phuc Le, Ngoc Phuc Le Faculty of Electrical and Electronics Engineering, Duy Tan University, Danang, VietnamSearch for more papers by this author First published: 01 December 2016 https://doi.org/10.1049/iet-com.2016.0400Citations: 14AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract 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. 1 Introduction In some wireless communication networks (such as wireless sensor networks), energy restrains the performance of the networks. Prolonging the lifetime of these networks has many difficulties since replacing or recharging energy suppliers of the nodes is either inconvenient or undesirable. In these situations, energy harvesting (EH) has become a promising technique to power energy-constrained wireless networks and recently attracted a great deal of attention [1-3]. The main idea is that a wireless node is equipped with rectifying circuits that can convert the radio-frequency (RF) signal sent by power source nodes into DC current. This current is saved into batteries for serving signal processing and transmission. In practice, wireless EH has not been widely used because of the high propagation loss of RF signals. However, thanks to the latest developments in wireless communications, i.e. small cells [4], large-scale antenna arrays [5], millimetre-wave communications [6], and the transmission efficiency is significantly increased, which will significantly decrease the propagational loss and obtain much higher EH efficiencies [7]. Furthermore, users’ energy consumption will be continuously reduced by the advancements in low-power electronics [8]. Therefore, RF EH has a great potential to be widely implemented in the next-generation wireless communication systems. In [9, 10], Zhou et al. and Nasir et al. considered the scenarios where the destination simultaneously receives wireless information and harvests wireless power from the source. Motivated by these works, in [11-13], Lee et al. Ju and Zhang, and Nasir et al. studied performance of wireless systems that are applied EH. These studies have laid a solid foundation for understanding the role of EH in wireless communication networks. The booming in the growth of wireless devices and services has brought enormous request of spectrum resources while most of the licenced spectrum bands are occupied [14]. Under this circumstance, it is urgent to deploy new technologies that have the abilities of optimising the current spectrum usage and adapting to the present spectrum management policies. Fortunately, to tackle these challenges, cognitive radio (CR) was introduced in [15, 16]. In CR networks, the unlicenced users are allowed to transfer messages over the licenced spectrum under the constraint that the interference level at the primary users is kept below a harmless threshold [17]. Therefore, reliable communication can be established without considering the secondary networks’ operation [18]. The combination of CR and EH technologies can bring great advantages to wireless communication networks. This topic has been received wide attention recently. In [19], Pratibha et al. proposed a centralised channel access strategy for a multichannel low-power CR system, where the SUs can employ an unused spectrum band for either harvesting RF energy from PUs’ transmission or transmitting their information. In [20], an EH-CR system was investigated, in which the secondary users using the harvested energy for transmission after harvesting energy from ambient radio signal. Pratibha et al. in [21] considered finite batteries EH-CR systems where the SUs can be configured to improve PU detecting and PU spectrum opportunistically utilising process. However, the impact of primary network on the secondary network in the EH-CR context has not been well-investigated. In this paper, we propose an EH-CR network in the presence of multiple multi-antenna power beacons and multiple secondary transmitters. The contribution of this paper is summarised as follows: We propose an EH-CR network in which multiple multi-antenna power beacons are deployed to power secondary transmitters and a secondary transmitter selection scheme based on the channel state information (CSI) of the secondary network is investigated. In addition, we study the effect of primary transmitter on the secondary network by considering two scenarios of the primary transmitter's location, e.g. the primary transmitter is located near to the secondary network and the primary transmitter is located far from the secondary network. We develop the analytical expressions to investigate the effect of the primary network on the secondary network in the two scenarios of primary transmitter's location with respect to the primary transmitter's transmit power and the peak interference constraint at the primary receiver. We demonstrate that in the case of near primary transmitter, though secondary transmitters can benefit from the harvested energy from primary transmitter, the interference caused by the primary transmitter suppresses the secondary network performance. Meanwhile, the peak interference constraint at the primary receiver limits the advantage that powerful power beacons can bring to the secondary transmitter. Besides, increasing the number of power beacons and secondary transmitter can improve the performance of the secondary network. 2 System and channel models We consider an EH-CR network consisting of one primary transmitter PTX, one primary receiver PRX, N power beacons Bn for n = 1, …, N, M secondary transmitters Sm for m = 1, …, M, and one secondary receiver D as shown in Fig. 1. The power beacons are equipped with K antennas while the other nodes are equipped with one antenna. In this paper, we assume that all the nodes are located sufficiently far from each other so that all the channels experience independent and identically distributed Rayleigh fading. In this network, one secondary transmitter Ss will be selected from the M Sm to transmit information to D. The motivation of this scheme is that in wireless sensor networks, some of sensor nodes form a cluster that can exchange information among nodes. To save energy, one node in the cluster that has the best link to the sink will be chosen to transmit information to the sink. The selection is based on the CSI of the Sm → D link as follows (1)where and are the channel power gains from the chosen secondary transmitter and Sm to D, respectively. The selection process can be done by using feedback channels from D to Sm. After the secondary transmitter selection process, Ss harvests RF energy by implementing time-switching-based architecture as shown in Fig. 2 while other secondary transmitters enter the idle mode. In a transmission block time T, Ss uses τT to harvest energy (EH phase) and to transmit information to D (transmission phase), where 1 > τ > 0. In EH phase, to maximise the harvested energy at the selected transmitter, all the power beacons deploy beamforming technique to transmit power signal to Ss. In this paper, we consider two scenarios of PTX’s location, i.e. (i) PTX is near to secondary network-Near PTX (NP) and (ii) PTX is far from the secondary network - Far PTX (FP). Fig. 1Open in figure viewerPowerPoint System model Fig. 2Open in figure viewerPowerPoint Time-switching-based EH protocol 2.1 PTX is near to the secondary network In this scenario, PTX is located near the secondary network. Therefore, Ss can harvest energy from B and PTX. However, the primary network's signal transmitted from PTX can interfere D. The energy harvested at the Ss can be formulated as (2)where 0 < η < 1 is the conversion efficiency coefficient and depends on the AC–DC converter circuit, is the transmit power of Bn, is the transmit power of PTX, is the channel power gain of the kth antenna at the nth power beacon to Ss link, and is the channel power gain of PTX → Ss link. In the transmission phase, to protect the primary network, the transmit power of Sm must satisfy the maximal interference constraint at the primary receiver PRX. The transmit power of Ss is given as (3)where the coefficient indicates that Ss uses the harvested energy to transmit information to D in the transmission phase, and is the channel power gain of Ss → PRX link. At the secondary receiver, the information signal from Sm is interfered by the signal from the PTX and the additive white Gaussian noise (AWGN). The received signal at D is given as (4)where is the channel coefficient of Ss → D link, is the channel coefficient of PTX → D link, is the desired signal from Ss to D, is the interference signal from primary network, and σ is the AWGN at D with zero mean and N0 variance. The signal-to-interference plus noise ratio at D is given as (5)where and can be written as (6)where , , and . 2.2 PTX is far from the secondary network In this scenario, PTX is located far from the secondary network. Therefore, Ss cannot harvest energy from PTX and D is free from PTX’s interference. The energy harvested at the Ss can be formulated as (7)The transmit power of Ss is given as (8)The received signal at D is given as (9)The signal-to-noise ratio (SNR) at D is given as (10)where 3 Outage probability In this section, we analyse the outage probability (OP) of the considered system. The channel capacity of Ss → D link is given as (11)where the coefficient indicates that the transmission duration of the source node is of the total block time T. The OP of the considered system is the probability that channel capacity of Ss → D link is smaller than a target rate. The OP can be formulated as (12)where Rth is the target rate of the considered network, and is the cumulative distribution function (CDF) of ΨD. 3.1 PTX is near to the secondary network To facilitate finding the CDF of , we denote (13)The SNR at D can be rewritten as (14), , and are exponential random variables. Therefore, the CDF of , , and are given, respectively, as follows (15) (16) (17)where ΩSD, , and are the average power gains of Ss → D, PTX → D, and Ss → PRX links, respectively. From the CDF of and , the PDF of and are, respectively, given as (18) (19)The PDF of is given as (20)where Proof.The proof is given in Appendix 1. From (15) and (18)–(20), we have the following lemma. Lemma 1.The OP of the secondary network with neighbouring PTX is given as follows (21)where defined in [22, (8.407.1)] is the modified Bessel function of the second kind and where defined in [22, (8.221.1)] is the exponential integral function. Proof.The proof is given in Appendix 2.□ 3.2 PTX is far from the secondary network We denote The SNR at D can be rewritten as (22)The PDF of is given as (23) Proof.The PDF of can be calculated similarly to the PDF of by using moment generating function (MGF) and inverse Laplace transform (see Appendix 1).□ From (15), (19), and (23), we have the following lemma. Lemma 2.The OP of the secondary network without neighbouring PTX is given as follows (24)where Proof.The proof is given in Appendix 3.□ 4 Asymptotic analysis In this section, asymptotic expressions of OP in the two scenarios are derived to provide important insights of the considered system. To derive the asymptotic expressions, Bn is considered to have high transmit power to perform beamforming to the selected Ss. As a result, the asymptotic OP of the considered system in the far PTX and near PTX scenarios are, respectively, given as (25) (26) Proof.The proof is given in Appendix 4.□ From the asymptotic expressions, it is observed that in the near PTX scenario, when Bn has high transmit power, the secondary network does not take advantage of the primary network's interference. Therefore, the primary network's interference only has harmful effect on the secondary network. Meanwhile, in the far PTX scenario, the performance of the considered system is only restrained by the peak interference constraint of the primary network. 5 Numerical results In this section, the simulation results based on Monte Carlo method are provided to verify the accuracy of the above performance analysis. Without loss of generality, the following parameters are fixed throughout this section η = 0.8 and K = 3. Fig. 3 reveals the effect of PTX and the number of power beacons on the secondary network. In this figure, γP = 20 dB, , the number of power beacons is varied from N = 1 to 3, M = 3, τ = 0.6, and OP is the function of γB. As γB increases, the transmit power of Ss increases, followed by a reduction in the OP. Though Ss can create huge transmit power in the transmission phase when γB goes large, its transmit power is limited by . In near PTX scenario, D receives interference from PTX which results in a higher OP than that in the far PTX scenario. In addition, we also observe that increasing the number of power beacons N can provide higher amount of energy to Ss which leads to a decrease in the OP in both scenarios. Fig. 3Open in figure viewerPowerPoint OP of the considered system in the two scenarios with different numbers of power beacons In Fig. 4, the impact of the peak interference constraint at the primary receiver on the OP of secondary network in both near and far PTX scenarios is demonstrated. In this figure, γP = 20 dB, and 12 dB, N = 3, M = 3, τ = 0.6, and OP is the function of γB. As the peak interference constraint at the primary receiver is relaxed, the secondary transmitter can transmit with higher transmit power to improve the secondary network performance. Fig. 4Open in figure viewerPowerPoint OP of the considered system in the two scenarios with different values of In Fig. 5, the OP of the considered system in the near PTX scenario is plotted as a function of γP with a variation in the number of the secondary transmitters. γB is fixed at 20 dB, , τ = 0.6, and N = 3. In the near PTX scenario, Ss benefits from the energy harvested from PTX. However, D is impacted by the interference from the PTX. When the transmit power at Ss is limited by , increasing γP will result in high interference at D, followed by an increase in the OP. This figure also shows that increasing the number of secondary transmitters can reduce the OP of the secondary network. The benefit of increasing the number of secondary transmitters in the far PTX scenario can be witnessed in Fig. 6. Fig. 5Open in figure viewerPowerPoint OP against γP in the near PTX scenario with different numbers of S Fig. 6Open in figure viewerPowerPoint OP in the far PTX scenario with different numbers of S In Fig. 7, the effect of EH time on the OP of the considered system is demonstrated. On the one hand, if the EH time is short (small τ), Ss will not have enough energy to efficiently transmit its information to the destination, followed by an increase in the OP. On the other hand, if the EH time is long (large τ), the transmission time between Ss and the destination will be shorten which results in a reduction in the capacity of the considered system. Therefore, EH time at Ss should be carefully designed to enhance the performance of the considered system. As in Fig. 7, optimal value of τ for the considered system can be selected in the range of 0.6–0.7. Fig. 7Open in figure viewerPowerPoint OP against τ in the far PTX and near PTX scenarios 6 Conclusions In this paper, the impact of the primary network on the secondary network in EH-CR networks is investigated. In particular, a secondary transmitter is selected from multiple secondary transmitters based on the CSI of secondary network to transmit information to the secondary receiver. This secondary transmitter is powered by the energy harvested from beamformed power signals of multiple multi-antenna power beacons and power signal from the primary transmitter. To examine the influence of the primary network's interference on the secondary receiver, two scenarios of primary transmitter's locations are considered, i.e. near primary transmitter and far primary transmitter. The analytical and asymptotic expressions of the OP of the considered system in these two scenarios are derived. The results reveal that the appearance of the primary transmitter has negative effects on the secondary network performance. Despite the fact that the secondary transmitter can harvest energy from the primary transmitter, the secondary receiver suffers from the primary transmitter's interference, followed by a suppression in the secondary network transmission. In addition, though the secondary transmitters can be wireless powered by power sources, the peak interference constraint at the primary receiver limits this advantage. However, increasing the number of power beacons and primary transmitters can effectively improve the performance of the secondary network. Finally, the numerical results were provided to validate our correctness. 7 Acknowledgment This work was supported by the Newton Institutional Link under grant ID 172719890. 9 Appendices 9.1 Appendix 1: Proof of Recall from (13), is expressed as Since is an exponential random variable, follows gamma distribution. We denote and . Therefore, and . can be rewritten as follows (27)To find PDF of , we find the MGF of first. The MGF of and are obtained, respectively, as follows (28) (29)The MGF of is given as (30)From (30), after expanding in partial fractions and applying the inverse Laplace transform, we have the PDF of as in (20). 9.2 Appendix 2: Proof of Lemma 1 From (12) and (14), the OP of the secondary network with neighbouring PTX can be written as (31)To simplify the calculation process, we denote and calculate the OP conditioned on first (32) can be rewritten as (33)Plugging (33) into (32) and calculating the integral conditioned on , the OP is given as (34)(34) is obtained with the help of [22, (3.352.2)]. From (34), the OP is formulated as (35)where can be calculated as follows (36) and are given as (37) (38)(37) and (38) are obtained with the help of [22, (3.471.9)]. Q2 can be calculated as (39) and are given as (40) (41)(40) and (41) are obtained with the help of [22, (3.471.9)]. From (35)–(41), we obtain (21). 9.3 Appendix 3: Proof of Lemma 2 From (22), OP of the secondary network without neighbouring is given as (42)(42) is obtained with the help of [22, (3.471.9)]. 9.4 Appendix 4: Proof of asymptotic expressions When the transmit power at Bn is high, from (31), the OP of the considered system in the near PTX scenario is formulated as (43)(43) is obtained with the help of [22, (3.352.4)]. Similarly, from (42), the OP of the considered system in the far scenario is formulated as (44) References 1Hadzi-Velkov Z.Nikoloska I.Karagiannidis G.K. et al.: ‘Wireless networks with energy harvesting and power transfer: joint power and time allocation’, IEEE Signal Process. Lett., 2016, 23, (1), pp. 50– 54 (doi: 10.1109/LSP.2015.2500340) 2Hadzi-Velkov Z.Zlatanov N.Duong T.Q. et al.: ‘Rate maximization of decode-and-forward relaying systems with RF energy harvesting’, IEEE Commun. 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