Advances in cognitive radio networks: A survey <s> C. Cognitive Relaying <s> We develop and analyze low-complexity cooperative diversity protocols that combat fading induced by multipath propagation in wireless networks. The underlying techniques exploit space diversity available through cooperating terminals' relaying signals for one another. We outline several strategies employed by the cooperating radios, including fixed relaying schemes such as amplify-and-forward and decode-and-forward, selection relaying schemes that adapt based upon channel measurements between the cooperating terminals, and incremental relaying schemes that adapt based upon limited feedback from the destination terminal. We develop performance characterizations in terms of outage events and associated outage probabilities, which measure robustness of the transmissions to fading, focusing on the high signal-to-noise ratio (SNR) regime. Except for fixed decode-and-forward, all of our cooperative diversity protocols are efficient in the sense that they achieve full diversity (i.e., second-order diversity in the case of two terminals), and, moreover, are close to optimum (within 1.5 dB) in certain regimes. Thus, using distributed antennas, we can provide the powerful benefits of space diversity without need for physical arrays, though at a loss of spectral efficiency due to half-duplex operation and possibly at the cost of additional receive hardware. Applicable to any wireless setting, including cellular or ad hoc networks-wherever space constraints preclude the use of physical arrays-the performance characterizations reveal that large power or energy savings result from the use of these protocols. <s> BIB001 </s> Advances in cognitive radio networks: A survey <s> C. Cognitive Relaying <s> In this paper, we focus on the cognitive radio, which reconfigures the radio transceiver according to the surrounding radio environment. We introduce a development to an ad-hoc cognitive radio that can realize a frequency sharing system under existing communication systems using a multi-hop small power communication. Here, we discuss the surrounding radio environment recognition methods in the distributed terminal cooperated with the media access protocols. In this paper, we present a multi-hop system that autonomously avoids the transmission in the interference area by recognizing the radio environment in each relay node. By using the proposed system, the frequency sharing system with adaptive route selection according to the surrounding radio environment can be realized <s> BIB002 </s> Advances in cognitive radio networks: A survey <s> C. Cognitive Relaying <s> In this paper, a novel cognitive multiple-access strategy in the presence of a cooperating relay is proposed. Exploiting an important phenomenon in wireless networks, source burstiness, the cognitive relay utilizes the periods of silence of the terminals to enable cooperation. Therefore, no extra channel resources are allocated for cooperation and the system encounters no bandwidth losses. Two protocols are developed to implement the proposed multiple-access strategy. The maximum stable throughput region and the delay performance of the proposed protocols are characterized. The results reveal that the proposed protocols provide significant performance gains over conventional relaying strategies such as selection and incremental relaying, specially at high spectral efficiency regimes. The rationale is that the lossless bandwidth property of the proposed protocols results in a graceful degradation in the maximum stable throughput with increasing the required rate of communication. On the other hand, conventional relaying strategies suffer from catastrophic performance degradation because of their inherent bandwidth inefficiency that results from allocating specific channel resources for cooperation at the relay. The analysis reveals that the throughput region of the proposed strategy is a subset of its maximum stable throughput region, which is different from random access, where both regions are conjectured to be identical. <s> BIB003 </s> Advances in cognitive radio networks: A survey <s> C. Cognitive Relaying <s> Cognitive radio has been proposed in recent years to promote the spectrum utilization by exploiting the existence of spectrum holes. The heterogeneity of both spectrum availability and traffic demand in secondary users has brought significant challenge for efficient spectrum allocation in cognitive radio networks. Observing that spectrum resource can be better matched to traffic demand of secondary users with the help of relay node that has rich spectrum resource, in this paper we exploit a new research direction for cognitive radio networks by utilizing cooperative relay to assist the transmission and improve spectrum efficiency. An infrastructure-based secondary network architecture has been proposed to leverage relay-assisted discontiguous OFDM (D-OFDM) for data transmission. In this architecture, relay node will be selected which can bridge the source and the destination using its common channels between those two nodes. With the introduction of cooperative relay, many unique problems should be considered, especially the issue for relay selection and spectrum allocation. We propose a central- ized heuristic solution to address the new resource allocation problem. To demonstrate the feasibility and performance of cooperative relay for cognitive radio, a new MAC protocol has been proposed and implemented in a Universal Software Radio Peripheral (USRP)-based testbed. Experimental results show that the throughput of the whole system is greatly increased by exploiting the benefit of cooperative relay. <s> BIB004
Utilizing the broadcasting nature of wireless networks, cooperative relaying is proposed in recent years BIB001 to improve the network performance through spatial and multi-user diversity. Combined with CR technology, cooperative relaying can offer more significant performance gain, because cognitive relay nodes can forward a source node's data by using the spectrum white space they have detected. A cognitive multiple access strategy in the presence of a cooperating relay is proposed in BIB003 . Since the cognitive relay only forwards data when the source is not transmitting, no extra channel resources are allocated for cooperation at the relay, and hence the proposed protocols provide significant performance gains over conventional relaying strategies. A frequency sharing multi-hop CR network is studied in BIB002 . By recognizing the radio environment in each relay node, the system can autonomously avoid the transmission in an interference area. In BIB004 , an infrastructure-based secondary network architecture is proposed to leverage relay-assisted discontiguous OFDM for data transmission. A relay nodes which can bridge the source and the destination using its common channels between the two nodes will be selected, and relay selection and spectrum allocation is jointly optimized.