Primary user activity is a major bottleneck for existing routing protocols in cognitive radio networks. Typical routing protocols avoid areas that are highly congested with primary users, leaving only a small fragment of available links for secondary route construction. In addition, wireless links are prone to channel impairments such as multipath fading; which renders the quality of the available links highly fluctuating. In this paper, we investigate using cooperative communication mechanisms to reveal new routing opportunities, enhance route qualities, and enable true coexistence of primary and secondary networks. As a result, we propose Undercover: a cooperative routing protocol that utilizes the available location information to assist in the routing process. Specifically, our protocol revisits a fundamental assumption taken by the state of the art routing protocols designed for cognitive radio networks. Using Undercover, secondary users can transmit in the regions of primary users activity through utilizing cooperative communication techniques to null out transmission at primary receivers via beamforming. In addition, the secondary links qualities are enhanced using cooperative diversity. To account for the excessive levels of interference typically incurred due to cooperative transmissions, we allow our protocol to be interference-aware. Thus, cooperative transmissions are penalized in accordance to the amount of negatively affected secondary flows. We evaluate the performance of our proposed protocol via NS2 simulations which show that our protocol can enhance the network goodput by a ratio reaches up to 250 compared to other popular cognitive routing protocols with minimal added overhead.
One of the intensively studied cooperative communications techniques is cooperative beamforming, which relies on sending precoded versions of the same data to reshape the signal beam producing transmission nulls at certain spatial directions @cite_42 . A direct consequence of employing cooperative beamforming is to allow for spatial multiplexing of concurrent transmissions of multiple nodes @cite_14 . Fortunately, cooperative beamforming provides the means for hiding secondary user communication from primary users and avoiding interfering with primary user communication. This opportunity was considered by a small number of attempts like @cite_41 which utilizes beamforming by developing MAC layer protocols that maximize the received signal-to-noise ratio (SINR) among SUs with different power constraints and the QoS requirement of PUs. However, this protocol deploys beamforming among one relay node only between the source and the destination.
@cite_41 @cite_14 @cite_42
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In this paper we investigate the cooperative beamforming in cognitive radio network (CRN) composed of a primary network (PN) and a secondary network (SN). In the SN, the transmitter has to communicate with the receiver with the help of the hybrid relays which use cooperative beamforming to retransmit data. The relays will select amplify-and-forward (AF) or decode-and-forward (DF) according to the signal to interference pulse noise ratio (SINR). The aim of this paper is to maximize the received SINR in the SN with different power constraints and the QoS requirement of the primary user (PU). Numerical results compare the performance of the SN between relays with beamforming and relays without beamforming which indicates that the gap between them is related to the maximum interference power at primary destination, while is irrelevant to maximum the total transmitted power of the relays. Besides there is a tradeoff between the performance of the SN and the number of the relays. An overview of beamforming from a signal-processing perspective is provided, with an emphasis on recent research. Data-independent, statistically optimum, adaptive, and partially adaptive beamforming are discussed. Basic notation, terminology, and concepts are included. Several beamformer implementations are briefly described. >