Xiangwei Zhou

Simplified Relay Selection and Power Allocation in Cooperative Cognitive Radio Systems

In this paper, we investigate joint relay selection and power allocation to maximize system throughput with limited interference to licensed (primary) users in cognitive radio (CR) systems. As these two problems are coupled together, we first develop an optimal approach based on the dual method and then propose a suboptimal approach to reduce complexity while maintaining reasonable performance. From our simulation results, the proposed approaches can increase the system throughput by over 50%.

Detection Timing and Channel Selection for Periodic Spectrum Sensing in Cognitive Radio

In this paper, detection timing and channel selection for periodic spectrum sensing are investigated to improve the performance of cognitive radio (CR) users. Designed to maximize the channel efficiency of CR users under a given level of interference with licensed users, the detection timing scheme utilizes the statistics of the licensed channel occupancy to determine the optimal starting point of each sensing action. A channel selection scheme in the multichannel multiuser environment is also proposed to specify which channel to detect for the upcoming sensing action based on detection timing. Numerical results demonstrate that our schemes considerably improve the overall channel efficiency while protecting the communication among licensed users.

Energy-Efficient Transmission in Cognitive Radio Networks

Cognitive radio (CR) networks are designed to utilize the licensed spectrum when it is not llsed by the primary (licensed) users. In this paper, we investigate how a CR user senses multiple channels and determine the optimal transmission duration and power allocation. When performing optimization, we take energy efficiency, throughput, and interference with the primary users into consideration and find a closed-form solution for transmission duration for chosen channels. It is shown that the proposed optimization approach significantly improves energy efficiency and throughput of CR networks.

Probabilistic Resource Allocation for Opportunistic Spectrum Access

Opportunistic spectrum access (OSA) in cognitive radio (CR) networks significantly improves spectrum efficiency by allowing secondary usage of licensed spectrum. In this paper, we propose a probabilistic resource allocation approach to further exploit the flexibility of OSA. Based on the probabilities of channel availability obtained from spectrum sensing, the proposed approach optimizes channel and power allocation in a multi-channel environment. The given algorithm maximizes the overall utility of a CR network and ensures sufficient protection of licensed users from unacceptable interference, which also supports diverse quality-of-service requirements and enables a distributed implementation in multi-user networks. Both analytical and simulation results demonstrate the effectiveness of this approach as well as its advantage over conventional approaches that rely upon the hard decisions on channel availability.

Probability-based combination for cooperative spectrum sensing

This letter proposes a probability-based scheme for combination of spectrum sensing information collected from cooperative cognitive radio users. The proposed scheme enables combination of both synchronous and asynchronous sensing information by utilizing the statistics of licensed band occupancy and is superior to conventional schemes in terms of detection performance.

OFDMA Initial Ranging for IEEE 802.16e Based on Time-domain and Frequency-domain Approaches

In this paper, we propose and compare two ranging methods for the OFDMA uplink system prescribed by IEEE 802.16e, including multi-user ranging code detection, timing, frequency and power synchronization. In Method I, timing offset estimation is based on the auto-correlation and cross-correlation in time-domain, and an adaptive threshold is defined to detect transmitted ranging codes. The time-domain algorithm has a similar counterpart in the frequency-domain described in Method II, which is more accurate since it bears less interference by extracting the sub-carriers used for ranging from data sub- carriers. Moreover, Method II needs fewer computations since it shares the FFT calculations with the data demodulation process. However, Method I has the advantage of a flexible correlation window size. Simulations confirm that both methods perform well even in the condition of low SNR values and several simultaneous ranging users. To obtain the 90% probability of successful detection, Method II requires an SNR about 3.5 dB less than Method I. Simulations also show that both methods are better, compared with some existing methods.

Probability-based optimization of inter-sensing duration and power control in cognitive radio

Probability-based strategies are proposed in this letter to determine the optimal inter-sensing duration and power control for cognitive radio (CR). With utilization of the statistics of licensed band occupancy, appropriate inter-sensing duration is determined to capture the recurrence of spectrum opportunity in time when the licensed signal is detected, or to achieve the maximum spectrum efficiency under a certain level of interference with licensed communication when the licensed signal is declared absent. Transmit power is varied dynamically according to the non-interfering probability at each sample so as to increase the transmission rate and decrease the interference power.

Probability-based periodic spectrum sensing during secondary communication

Spectrum sensing in cognitive radio (CR) typically assumes that the primary user appears only at the beginning of the sensing block. In this paper, we first establish a probability model regarding the appearance of the primary user at any sample of a CR user frame by utilizing the statistical characteristics of the licensed channel occupancy. While conventional spectrum sensing schemes allocate the same weight to each sample, we vary the weight for each sample based on the probability of the presence of the primary user at the corresponding sample and show that such a probability-based spectrum sensing scheme has nearly optimal performance. Based on the assumption that the idle duration of the licensed channel is exponentially distributed, we further investigate how the probability model on the primary user appearance varies from frame to frame in periodic spectrum sensing and show that both the conventional fixed weight and the probability-based dynamic weight energy detection schemes converge to their respective stable detection probability.

Multiuser Spectral Precoding for OFDM-Based Cognitive Radio Systems

Orthogonal frequency-division multiplexing (OFDM) is an ideal transmission technique for cognitive radio (CR) systems because of its flexible nature to support dynamic spectrum access. However, the out-of-band (OOB) radiation of OFDM signals from different CR users must be strictly controlled to protect licensed users operating in the adjacent frequency bands. In this paper, we propose a spectral precoding approach for multiple OFDM-based CR users to reduce OOB leakage and enhance spectrum compactness. By constructing individual precoders to render selected spectrum nulls, our approach suppresses the overall OOB radiation without sacrificing bit-error rate performance of CR users. The proposed approach also ensures user independence thus with low encoding and decoding complexities. Furthermore, our approach can improve bandwidth efficiency by carefully selecting notched frequencies. As a comprehensive application of the proposed approach, two simplified multiuser spectral precoding schemes are provided to reduce the computational complexity. Simulation results demonstrate that our spectral precoding schemes effectively limit OOB radiation and enable efficient spectrum sharing.

Energy-Efficient Transmission for Protection of Incumbent Users

Cognitive radio (CR) technology can be used to provide flexible mobile multimedia communications. In this article, we investigate energy-efficient design of CR to protect incumbent users, such as broadcasters and telecommunications operators. Power allocation and transmission duration are determined for a CR user exploiting multiple channels. Both energy efficiency of the CR user and interference with the incumbent users are considered. Simulation results demonstrate that the proposed approaches achieve reliable protection and significant performance gain in terms of energy efficiency.