Outage performance of relay-assisted primary and secondary transmissions in cognitive relay networks
© Zhao et al.; licensee Springer. 2014
Received: 11 March 2014
Accepted: 26 March 2014
Published: 16 April 2014
In this paper, a two-phase protocol for relay-assisted spectrum sharing in cognitive relay network is proposed. The primary system comprises a transmitter-receiver pair (PT-PR) and a decode-and-forward relay (Relay), while the secondary system comprises a transmitter-receiver pair (ST-SR). In the proposed protocol, the Relay assists the transmissions of the primary users as well as the secondary users where there is no direct link between the ST and the SR. Outage probabilities of the primary system and the secondary system are derived and verified through simulations. Compared with the protocol without cooperation, better outage performance of the primary system is achieved. Meanwhile, though the ST causes interference to the PR, the interference is compensated by the cooperation of the Relay. Moreover, the proposed protocol realizes the communications of the secondary users on the condition that the secondary transmission link is not ideal.
Traditional spectrum management policy, which is based on licensing of spectrum, allows the licensed users to access the frequency bands allocated to them. However, this policy cannot satisfy the increasing demand of users in wireless networks. The contradiction between the increasing demand and the limited spectrum resources compels us to improve the spectral efficiency.
For this purpose, some new technologies have been proposed. Cooperative diversity, as a new form of spatial diversity, was proposed to fight against channel fading and enhance throughput. The advantages of some cooperative diversity protocols have been studied in [1–3], and an adaptive cooperation diversity scheme has been studied in  which considered the mutual interference between the primary and secondary users. Cooperative relay, which uses intermediate entities to carry information from a source node to its destination node, is emerging as a promising technology to improve the reliability, throughput, and coverage areas of wireless networks. The most common relaying protocols are amplify-and-forward (AF) relaying and decode-and-forward (DF) relaying. The performance of DF relaying in spectrum sharing has been widely investigated in . With the purpose of improving spectral efficiency, the concept of cognitive radio  was proposed by J. Mitola. In cognitive radio networks, secondary users transmit signals by detecting spectrum holes, or share the same frequency bands with primary users as long as the induced interference from secondary users to primary users is below a threshold . The authors of  have studied a more general multi-user scenario which was extended from , and the authors of  have considered cooperative relaying for spectrum sensing in a two-user cognitive radio environment. Recently, cognitive relay network, which combines cognitive radio and cooperative relay, has attracted significant interests, and it is able to improve secondary users' transmission performance. The difference between cognitive relay networks and conventional relay networks has been studied in , and it showed that outage performance of cognitive relay networks is better than that of conventional relay networks. The authors of  have obtained a tight lower bound of the outage probability for underlay cognitive relay networks. A three-phase cognitive two-way relaying scenario has been analyzed in , and three different two-hop relaying protocols according to the amount of a prior knowledge have been investigated in . Cognitive radio networks with opportunistic DF and uncoded DF-relaying schemes were considered and analyzed in . When the achievements on the transmission performance of primary users increase, more and more attention has been paid to the transmission performance of secondary users. In , the authors have studied the capacity of the secondary users in cognitive relay networks, and proved that the reactive DF scheme is able to achieve the similar capacity of the proactive DF scheme. The performances of the secondary users in cognitive relay networks with the consideration of the primary users' interference have been analyzed in [16–18]. Especially, the authors of  have considered the outage performances of the primary and secondary systems on the condition that there exists a direct link between the secondary users. Power allocation has been applied to improve outage performance of the secondary system in .
Some other relevant researches have been done. Outage performances with relay selection in Rayleigh fading channels were studied in , and the full diversity order has been obtained for Rayleigh fading channels in [21, 22]. Moreover, the authors of  have analyzed the impact of cooperation communications on the outage probability of cognitive radio networks over the Nakagami-m fading channel and generalized the contributions in [21, 22]. While more and more achievements on outage performance with perfect channel state information have been obtained, a lot of attention has been put into the one with imperfect channel state information. The transmissions of the secondary users will cause more interference at the primary users with imperfect channel state information. The authors of [24–28] have studied the transmission performances of the primary and secondary systems under imperfect channel state information.
Most of the existing research works focused on the separate performance of the primary system or the secondary system, and little research work has considered the performances of both primary and secondary systems. In view of this situation, we propose a protocol which employs a relay to help the transmissions of the primary and secondary systems contemporarily. With the cooperation of the relay, we try to decrease outage probability of the primary system, and compensate for the interference produced by the secondary users. Meanwhile, the relay assists the transmissions of the secondary users on the condition that there is no direct link between the secondary transmitter and the secondary receiver.
The rest of this paper is organized as follows: Section 2 describes the system model. In Section 3, we express the transmission signals in every transmission phase, and calculate outage probabilities of the primary and secondary systems. Section 4 presents the simulation results and discussions, and Section 5 concludes the paper.
2 System model
The primary system comprises a transmitter (PT), a receiver (PR), and a decode-and-forward relay (Relay), while the secondary system comprises a transmitter (ST) and a receiver (SR). The Relay assists the transmissions of the primary users and secondary users simultaneously. In the proposed system model, outage performance of the primary system benefits from the assistance of the Relay, and communications of the secondary users are achieved under the assistance of the Relay when there is no direct link between the ST and the SR.
The whole transmission process is divided into two phases. In the first transmission phase, the PT and the ST transmit the primary and secondary signals, respectively. The PR, the Relay, and the SR receive the primary signal, and the Relay and the PR receive the secondary signal. Then, the primary signal and the secondary signal are decoded and superimposed at the Relay. In the second transmission phase, the PR and the SR receive a weighted linear composite signal which is transmitted by the Relay. At the PR, the secondary signal which is regarded as an interfering signal will be removed, and the primary signal will be restored if the PR is able to decode the secondary signal successfully. Then, the primary signal is retrieved by maximal-ratio combining (MRC) of the received signals from the two transmission phases at the PR. Similarly, the primary signal is regarded as an interfering signal and removed at the SR if the SR is successful to decode the primary signal, and the secondary signal is retrieved from the signal which is sent from the Relay.
All the channels are assumed to experience Rayleigh fading. The channel coefficients of the links PT → PR, PT → Relay, PT → SR, ST → Relay, ST → PR, Relay → PR, and Relay → SR are recorded by h1, h2, h3, h4, h5, h6, and h7, respectively. Moreover, we assume (i = 1, 2, 3, 4, 5, 6, 7), which means that hi is a circularly symmetric complex Gaussian random variable with variance . Here, d i represents the normalized distance between two nodes, and v represents the path loss exponent. That is to say d1, d2, d3, d4, d5, d6, and d7 denote the normalized distances between PT and PR, PT and Relay, PT and SR, ST and Relay, ST and PR, Relay and PR, and Relay and SR, respectively. This distance normalization is done with respect to the distance between PT and PR, i.e., d1 = 1.
3 Signal description and outage performance analysis
3.1 Outage performance of the proposed protocol (scheme A)
In this subsection, outage performances of the primary system and the secondary system will be analyzed.
3.1.1 Outage probability of the primary system
Here, the value is due to the fact that the whole transmission process is divided into two transmission phases, and for ease of presentation, we record γ j = |h j |2 (j = 1, 2, 3, 4, 5, 6, 7).
where d = 4, 5.
where h6 is the channel coefficient of the link between the Relay and PR, and n12 ~ N(0, σ2) is an additive white Gaussian noise.
For the primary system, there are two cases where the primary signal is transmitted successfully. The former is that x p is transmitted through the direct link from the PT to the PR on the condition that the Relay fails to decode either of x p or x s . In this case, the achievable rate between the PT and the PR is given by . The latter is that x p is transmitted from the relay-assisted link if the Relay decodes both x p and x s successfully. As mentioned, the former case includes three sub-cases, they are listed as follows: First, x p is transmitted through the direct link on the condition that the Relay succeeds to decode x p and fails to decode x s . Second, x p is transmitted through the direct link on the condition that the Relay fails to decode x p and succeeds to decode x s . Third, x p is transmitted through the direct link on the condition that the Relay fails to decode both x p and x s .
The latter case includes two sub-cases, and they are listed as follows: One case is that the Relay succeeds to decode both x p and x s , but the PR fails to decode x s . Then, the primary signal x p is transmitted through the relay-assisted link. The other case is that the Relay succeeds to decode both x p and x s , and the PR decodes x s successfully. Then, the primary signal x p is transmitted through the relay-assisted link.
3.1.2 Outage probability of the secondary system
3.2. Outage performance of the protocol without cooperation (scheme B)
where . This protocol will be compared with our proposed protocol in Section 4.
4 Simulation results and discussions
From Figure 2, we also find that the secondary users are able to communicate smoothly with the cooperation of the Relay when d2 = 0.5, 0.8, 1.2. Moreover, we can get that there exist two extreme cases, i.e., α = 0 and α = 1. The former (α = 0) corresponds to the protocol where the primary signal is only transmitted from the PT to the PR, and the whole transmission power of the Relay is used to transmit the secondary signal, which is regarded as an interference signal at the PR. It is easy to get that outage probability of the secondary system is satisfactory, but outage probability of the primary system is unsatisfactory. With the increasing of α, outage performance of the primary system becomes better, and there is no obvious effect on outage performance of the secondary system. That is to say, outage performance of the primary system in the proposed protocol is better than that in the protocol where the Relay assists the transmissions of the secondary users only. The latter (α = 1) means the protocol where the Relay helps the transmissions of the primary users only. In this protocol, though outage probability of the primary system is low than that in the proposed protocol, outage probability of the secondary system is equal to 1, which means that the ST cannot communicate with the SR. So, we are able to better balance outage performances of the primary system and the secondary system in the proposed protocol.
In Figure 5, we show the effect of P p on the outage performances. Through analysis, we know that , , and decrease with the increasing of P p from (16), (22), and (23), and they meet the simulation results in Figure 5. Because P p is the transmission power of the PT, the outage probability of the primary system will be better with the increasing of P p . For the secondary users, the decoding of the primary signal benefits from the increase of P p , so the outage probability of the secondary system also decreases with the increasing of P p .
The effects of P s on outage performances are shown in Figure 6. It can be seen that both and decrease with the increasing of P s . For the secondary system, decreases with the increasing of P s from (22). Even though, there is not an obvious diminution of from Figure 6. The reason is that there is not a direct link between the ST and the SR, which means that transmissions of the secondary users must be assisted by the Relay. So the effects of P s cannot be reflected obviously on . For the primary system, the interference caused by the secondary users will become larger, but the increase of P s also contributes to the decoding of secondary signal at the PR and Relay. Then, it helps the cooperative transmissions of the primary signal at the Relay. That is to say, the increase of P s will cause larger interference as well as better cooperation for the primary system, and the benefit of cooperation is far more than the interference when P s increases. So there is an obvious improvement on outage performance of the primary system with the increasing of P s .
In this paper, a relay-assisted spectrum sharing protocol in cognitive relay networks has been presented. In the proposed protocol, better outage performance of the primary users is achieved, and communications of the secondary users without a direct link are realized under the assistance of a decode-and-forward relay.
We would like to thank the National Natural Science Foundation of China (61162008, 61172055), the Guangxi Natural Science Foundation (2011GXNSFB018072, 2013GXNSFGA019004), the Key Project of Chinese Ministry of Education (212131), the Foundation of Department of Education of Guangxi Province (201202ZD045, 201202ZD046), the Open Research Fund of Guangxi Key Lab of Wireless Wideband Communication & Signal Processing (12103, 12106), and the Director Fund of Key Laboratory of Cognitive Radio and Information Processing (Guilin University of Electronic Technology), Ministry of Education, China (2013ZR02).
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