Guodong Zhao

Soft Combination and Detection for Cooperative Spectrum Sensing in Cognitive Radio Networks

In this paper, we consider cooperative spectrum sensing based on energy detection in cognitive radio networks. Soft combination of the observed energy values from different cognitive radio users is investigated. Maximal ratio combination (MRC) is theoretically proved to be nearly optimal in low signal- to-noise ratio (SNR) region, an usual scenario in the context of cognitive radio. Both MRC and equal gain combination (EGC) exhibit significant performance improvement over conventional hard combination. Encouraged by the performance gain of soft combination, we propose a new softened hard combination scheme with two-bit overhead for each user and achieve a good tradeoff between detection performance and complexity. While traditionally energy detection suffers from an SNR wall caused by noise power uncertainty, it is shown in this paper that an SNR wall reduction can be achieved by employing cooperation among independent cognitive radio users.

Power and Channel Allocation for Cooperative Relay in Cognitive Radio Networks

In this paper, we investigate power and channel allocation for cooperative relay in a three-node cognitive radio network. Different from conventional cooperative relay channels, cognitive radio relay channels can be divided into three categories: direct, dual-hop, and relay channels, which provide three types of parallel end-to-end transmission. In the context, those spectrum bands available at all three nodes may either perform relay diversity transmission or assist the transmission in direct or dual-hop channels. On the other hand, the relay node involves both dual-hop and relay diversity transmission. In this paper, we develop power and channel allocation approaches for cooperative relay in cognitive radio networks that can significantly improve the overall end-to-end throughput. We further develop a low complexity approach that can obtain most of the benefits from power and channel allocation with minor performance loss.

Spatial spectrum holes for cognitive radio with relay-assisted directional transmission

Spectrum hole (SH) is defined as a spectrum band that can be utilized by unlicensed users, which is a basic resource for cognitive radio (CR) systems. Most of existing contributions detect SHs by sensing whether a primary signal is present or absent and then try to access them so that the CR and primary users use the spectrum band either at different time slots or in different geographic regions. In this paper, we propose a novel scheme with relays or directional relays for CR users to exploit new spectrum opportunity, called spatial SH. It can provide higher spectrum efficiency by coexistence of primary and CR users at the same region, time, and spectrum band. In particular, when the spectrum opportunity of a direct link from a CR transmitter to a CR receiver does not appear, our scheme may still establish the communication through indirect links, i.e., other CR users act as relay stations to assist the communication by using other spatial domains. Furthermore, we analyze the successful communication probabilities of CR users and demonstrate that the spectrum efficiency can be considerably improved by our scheme.

Proactive detection of spectrum opportunities in primary systems with power control

Spectrum sensing finds spectrum opportunities for cognitive radio (CR) and enables CR users to work without harmful interference to primary users. Most of existing contributions on spectrum sensing detect whether a primary signal is present or absent. Since the ultimate goal of spectrum sensing is to avoid interfering with primary receivers (PRs), it is more efficient to detect PRs directly. In this paper, we propose a proactive spectrum sensing scheme to detect whether a PR is within the coverage or the interference range of a CR transmitter by exploiting the close-loop power control that has been widely used in wireless systems. With the proposed scheme, the CR user may still be able to access the licensed spectrum band even though a primary signal is detected as long as its transmission does not interfere with the PR. As a result, more spectrum opportunities can be exploited compared to conventional spectrum sensing methods.

Proactive Detection of Spectrum Holes in Cognitive Radio

Most of existing works on spectrum sensing detect primary transmitters while the purpose of spectrum sensing is to avoid interfering with primary receivers (PRs). Therefore, it is more important to detect PRs. In this paper, we propose a proactive spectrum sensing method that detects whether a PR is within the coverage or the interference range of a CR transmitter by exploiting the close-loop power control policy in primary systems. With the proposed scheme, the CR user may still access the spectrum band even though a primary signal is present as long as its transmission does not interfere with the PR. Simulation results show the advantages of the proposed method.

Spatial Spectrum Holes for Cognitive Radio with Directional Transmission

In this paper, we propose a cognitive radio (CR) transmission scheme, which enables secondary users to coexist with primary users by exploiting spatial spectrum holes (SSHs) through directional antennas or antenna arrays with beamforming. To ensure reliable CR links and avoid interference to primary users, some CR users may act as relays . We investigate successful communication probability of CR users when this scheme is applied. We further demonstrate that the spectrum efficiency can be greatly improved by multiplexing CR links with directional transmission.

Probability-Based Transmit Power Control for Dynamic Spectrum Access

To take advantage of time-varying spectrum opportunities, a cognitive radio (CR) user continuously monitors the dynamic usage of the licensed frequency band and is allowed to utilize the spectrum resource when it does not cause unacceptable interference with licensed users. In this paper, the transmit power control scheme for each data transmission between periodic spectrum sensing activities is proposed to improve the performance of the CR user with utilization of the statistics of the licensed band occupancy. While the conventional CR transmission scheme allocates the same transmit power to each data sample, our scheme varies the transmit power dynamically according to the non-interfering probability at each sample so that the effective transmission rate of the CR user increases and the expected interference level with the licensed communication decreases. Detection errors are also incorporated into the analysis. It is demonstrated by numerical results that our scheme considerably improves the overall bandwidth efficiency while ensuring the priority of licensed users.

Channel allocation for cooperative relays in cognitive radio networks

In this paper, we investigate channel allocation for cooperative relays in cognitive radio networks. Different from conventional cooperative relay channels, cognitive radio relay channels are actually a combination of three kinds of channels: direct, dual-hop, and relay channels, which belong to different spectrum bands and provide parallel end-to-end transmission. In order to maximize the achievable end-to-end throughput, we propose two channel allocation approaches with different complexities to assign all the channels cooperatively. Numerical results illustrate the performance improvement in different number of available channels. In particular, it has about 40% improvement in throughput when the average SNR is 15 dB and eight available channels are used.

Spatial Spectrum Holes in Cognitive Radio with Relay Transmission

In this paper, we propose a relay-assisted transmission scheme in cognitive radio (CR) to exploit spatial spectrum holes, which are generated by relay techniques. The proposed scheme enables CR users to coexist with primary users at the same time in the same geographic area and spectrum band. Compared to conventional schemes, a higher spectrum efficiency is achieved by our method. We further analyze the successful communication probability and present numerical results to show advantages of our method.

Communication-oriented cooperative spectrum sensing in cognitive radio

Cooperative spectrum sensing improves detection performance over fading and shadowing channels by using multiple cognitive radio (CR) users. Compared with local spectrum sensing with individual CR user, it can either reduce sensitivity requirements for each CR user or shorten the overall sensing duration. These advantages are from spatial diversity when all cooperative users sense a common primary user. However, in practice, CR users at different locations may receive signals from different primary users, which may confuse the fusion user and result in the loss of communication opportunities. In this paper, we propose a cooperative sensing approach by taking the reliability of each partner user into account. Simulation results illustrate significant performance improvement of the proposed approach.