Kwok Hung Li

Energy-Efficient Design of Sequential Channel Sensing in Cognitive Radio Networks: Optimal Sensing Strategy, Power Allocation, and Sensing Order

Energy-efficient design has become increasingly important to battery-powered wireless devices. In this paper, we focus on the energy efficiency of a cognitive radio network, in which a secondary user senses the channels licensed to some primary users sequentially before it decides to transmit. Energy is consumed in both the channel sensing and transmission processes. The energy-efficient design calls for a careful design in the sensing-access strategies and the sensing order, with the sensing strategy specifying when to stop sensing and start transmission, the access strategy specifying the power level to be used upon transmission, and the sensing order specifying the sequence of channel sensing. Hence, the objective of this paper is to identify the sensing-access strategies and the sensing order that achieve the maximum energy efficiency. We first investigate the design when the channel sensing order is given and formulate the above design problem as a stochastic sequential decision-making problem. To solve it, we study another parametric formulation of the original problem, which rewards transmission throughput and penalizes energy consumption. Dynamic programming can be applied to identify the optimal strategy for the parametric problem. Then, by exploring the relationship between the two formulations and making use of the monotonicity property of the parametric formulation, we develop an algorithm to find the optimal sensing-access strategies for the original problem. Furthermore, we study the joint design of the channel sensing order and the sensing-access strategies. Lastly, the performance of the proposed designs is evaluated through numerical results.

Secure communication over MISO cognitive radio channels

In this paper, we address the physical-layer security issue of a secondary user (SU) in a spectrum-sharing cognitive radio network (CRN) from an information-theoretic perspective. Specially, we consider a secure multiple-input single-output (MISO) cognitive radio channel, where a multi-antenna SU transmitter (SU-Tx) sends confidential information to a legitimate SU receiver (SU-Rx) in the presence of an eavesdropper and on the licensed band of a primary user (PU). The secrecy capacity of the channel is characterized, which is a quasiconvex optimization problem of finding the capacity-achieving transmit covariance matrix under the joint transmit power and interference power constraints. Two numerical approaches are proposed to derive the optimal transmit covariance matrix. The first approach recasts the original quasiconvex problem into a single convex semidefinite program (SDP) by exploring its inherent convexity; while the second one explores the relationship between the secure CRN and the conventional CRN and transforms the original problem into a sequence of optimization problems associated with the conventional CRN, which helps to prove that beamforming is the optimal strategy for the secure MISO CR channel. In addition, to reduce the computational complexity, three suboptimal schemes are presented, namely, scaled secret beamforming (SSB), projected secret beamforming (PSB) and projected cognitive beamforming (PCB). Lastly, computer simulation results show that the three suboptimal schemes can approach the secrecy capacity well under certain conditions.

How much time is needed for wideband spectrum sensing

In this paper, we consider a wideband cognitive radio network (CRN) which can simultaneously sense multiple narrowband channels and thus aggregate the perceived available channels for transmission. We study the problem of designing the optimal spectrum sensing time and power allocation schemes so as to maximize the average achievable throughput of the CRN subject to the constraints of probability of detection and the total transmit power. The optimal sensing time and power allocation strategies are developed under two different total power constraints, namely, instantaneous power constraint and average power constraint. Finally, numerical results show that, under both cases, for a CRN with three 6 MHz channels, if the frame duration is 100 ms and the target probability of detection is 90% for the worst case signal-to-noise ratio of primary users being -12 dB, -15 dB and -20 dB, respectively, the optimal sensing time is around 6 ms and it is almost insensitive to the total transmit power.

Secure Communication in Multiantenna Cognitive Radio Networks With Imperfect Channel State Information

In this paper, we address the issue of optimal transmitter design to achieve physical layer security for a multiple-input single-output (MISO) cognitive radio network (CRN), in which a secondary user transmitter (SU-Tx) sends confidential information to a SU receiver (SU-Rx) on the same frequency band with a primary user (PU) in the presence of an eavesdropper receiver (ED-Rx). It is assumed that all the channel state information (CSI) of the secondary, primary and eavesdropper channels is not perfectly known at the SU-Tx. The optimal transmitter design, under the restriction of Gaussian signaling without preprocessing of information, involves a nonconvex semiinfinite optimization problem which maximizes the rate of the secondary link while avoiding harmful interference to the PU and keeping the eavesdropper totally ignorant of the messages sent regardless of the uncertainties in the CSI. We propose two approaches to solve this challenging optimization problem. The first one relates the original problem to a sequence of semiinfinite capacity-achieving transmitter design problems in an auxiliary CRN without any eavesdropper, which can then be solved through transformations and using convex semidefinite programs (SDPs). The second approach explores the hidden convexity of the problem and hence transforms it into a single SDP, which significantly reduces the computational complexity. Furthermore, a few heuristic beamforming solutions for the ease of implementation are also introduced. Finally, simulation results are presented to evaluate the performance of the proposed optimal and suboptimal solutions.

Energy-Efficient Joint Design of Sensing and Transmission Durations for Protection of Primary User in Cognitive Radio Systems

In this paper, we consider a joint design of energy-efficient sensing and transmission durations for a cognitive radio (CR) system in which the primary user (PU) is protected. The design problem is formulated as a function of two variables, which are sensing and transmission durations, subject to the interference to the PU. The optimal solutions are analyzed and a sub-optimal algorithm that maximizes the energy efficiency for the CR system is presented. The performance of the proposed energy-efficient design for the CR system is also evaluated.

BER performance of BPSK receivers over two-wave with diffuse power fading channels

This letter presents the bit error rate (BER) performance analysis for an uncoded binary phase-shift keying (BPSK) system in two-wave with diffused power (TWDP) fading channels. Simulation results of receiver performance, with or without maximal-ratio combining (MRC), are also presented. These performance studies quantitatively reveal that TWDP fading can produce a link worse than Rayleigh fading when the two direct waves are equal in strength and have a combined power of /spl ges/ 6 dB higher than the diffused power. These results point to using TWDP fading rather than Rayleigh fading as a worst case scenario in designing communication systems, especially in situations where TWDP fading is likely, such as when directional antennas or arrays and when wideband signals are used.

Performance analysis of an FFH/BFSK product-combining receiver under multitone jamming

The performance of a fast frequency-hopped binary frequency-shift keying (FFH/BFSK) spread-spectrum (SS) communication system is studied in this paper. The FFH system employs a product-combining receiver against the worst case multitone jamming (MTJ) and additive white Gaussian noise (AWGN). The compact characteristic functions of the natural logarithm of the product-combiner outputs are obtained based on the Taylor series expansion. The characteristic functions are then used to derive the bit error rate (BER) expressions that are applicable to higher diversity levels. Our analysis, validated by the simulation results, shows that the band MTJ is generally more harmful than the corresponding independent MTJ. The BER performance of the product-combining receiver is shown to possess good MTJ rejection capability compared to that of the soft-decision linear-combining receiver, especially under both moderate and strong MTJ power conditions.

Performance of BPSK Pre-detection MRC Systems over Two-Wave with Diffuse Power Fading Channels

The bit-error rate (BER) analysis for BPSK pre-detection maximal ratio combining (MRC) systems in two-wave with diffuse power (TWDP) fading is presented. In TWDP fading, the received signal is composed of two specular components in addition to the diffuse signal. The analysis shows that in TWDP fading, the BER of the MRC system is given by the weighted sum of the BERs of a number of MRC systems in Rician fading. The methodology used in the analysis is verified through simulation.

Multitone jamming rejection of FFH/BFSK spread-spectrum system over fading channels

Analytical expressions for bit-error probability are derived for a fast frequency-hopping binary frequency-shift keying (FFH/BFSK) spread-spectrum communication system over a fading channel with worst-case band multitone jamming (MTJ) and additive white Gaussian noise (AWGN). An FFH system employing either a linear-combining receiver or a clipper receiver is investigated. The desired signal and MTJ are assumed to undergo independent fading, and our analysis, validated with simulation results, shows that the performance of the system is slightly improved as the severity of the MTJ fading is increased. The clipper receiver is found to be superior to the linear-combining receiver when the jamming power is strong. The worst-case MTJ is shown to be more harmful than the corresponding worst-case partial-band noise jamming over a fading channel with AWGN.

Sensing-throughput tradeoff for cognitive radio networks: A multiple-channel scenario

In this paper, we study the sensing-throughput tradeoff problem for a multiple-channel cognitive radio (CR) network. In particular, using the sensing-throughput tradeoff metric, we investigate the design of the optimal spectrum sensing time and power allocation schemes so as to maximize the aggregate ergodic throughput of the cognitive radio network to guarantee the quality of service (QoS) of the primary users (PUs) without exceeding the power limit of the secondary transmitter. The optimal sensing time and power allocation strategies are developed under the average power constraint. Finally, numerical results show that, for a CR network with 3 channels, whose signal-to-noise ratio of PUs are −12dB, −15dB and −20dB, respectively, there is an optimal sensing time, and the optimal sensing time is almost insensitive to the total transmit power.