Qiyue Zou

Cooperative Sensing via Sequential Detection

Efficient and reliable spectrum sensing plays a critical role in cognitive radio networks. This paper presents a cooperative sequential detection scheme to reduce the average sensing time that is required to reach a detection decision. In the scheme, each cognitive radio computes the log-likelihood ratio for its every measurement, and the base station sequentially accumulates these log-likelihood statistics and determines whether to stop making measurement. The paper studies how to implement the scheme in a robust manner when the assumed signal models have unknown parameters, such as signal strength and noise variance. These ideas are illustrated through two examples in spectrum sensing. One assumes both the signal and noise are Gaussian distributed, while the other assumes the target signal is deterministic.

Compensation of Phase Noise in OFDM Wireless Systems

Phase noise causes significant degradation in the performance of orthogonal frequency division multiplexing (OFDM)-based wireless communication systems. The presence of phase noise can reduce the effective signal-to-noise ratio (SNR) at the receiver, and consequently, limit the bit error rate (BER) and data rate. In this paper, the effect of phase noise on OFDM wireless systems is studied, and a compensation scheme is proposed to mitigate the common phase error and intercarrier interference (ICI) caused by phase noise. In the proposed scheme, the communication between the transmitter and receiver blocks consists of two stages. In the first stage, block-type pilot symbols are transmitted and the channel coefficients are jointly estimated with the phase noise in the time domain. In the second stage, comb-type OFDM symbols are transmitted such that the receiver can jointly estimate the data symbols and the phase noise. It is shown both by theory and computer simulations that the proposed scheme can effectively mitigate the ICI caused by phase noise and improve the BER of OFDM systems. Another benefit of the proposed scheme is that the sensitivity of OFDM receivers to phase noise can be significantly lowered, which helps simplify the oscillator and circuitry design in terms of implementation cost and power consumption.

Joint compensation of IQ imbalance and phase noise in OFDM wireless systems

Physical impairments like IQ imbalance and phase noise can cause significant degradation in the performance of wireless communication systems. In this paper, the joint effects of IQ imbalance and phase noise on OFDM systems are analyzed, and a compensation scheme is proposed to improve the system performance in the presence of IQ imbalance and phase noise. The scheme consists of a joint estimation of channel and impairment parameters and a joint data symbol estimation algorithm. It is shown both by theory and computer simulations that the proposed scheme can effectively improve the signal-to-noise ratio at the receiver. As a result, the sensitivity of OFDM receivers to the physical impairments can be significantly lowered, simplifying the RF and analog circuitry design in terms of implementation cost, power consumption, and silicon fabrication yield.

Digital Compensation of Cross-Modulation Distortion in Software-Defined Radios

The wideband RF receiver in a software-defined radio (SDR) system suffers from the nonlinear effects caused by the front-end analog processing. In the presence of strong blocker (interference) signals, such nonlinearities introduce severe cross modulation over the desired signals. This paper investigates how the cross-modulation distortion can be compensated for by using digital signal processing techniques. In the proposed solution, the SDR scans the wide spectrum and locates the desired signal and strong blocker signals. After down-converting these signals separately to the baseband, the baseband processor processes them jointly to mitigate the cross-modulation interferences. As a result, the sensitivity of the wideband RF receiver to the nonlinearity impairment can be significantly lowered, simplifying the RF and analog circuitry design in terms of implementation cost and power consumption. The proposed approach also demonstrates how mixed-signal, i.e., joint analog and digital, processing techniques play a critical role in the emerging SDR and cognitive radio technologies.

Cooperative spectrum sensing via sequential detection for cognitive radio networks

Efficient and reliable spectrum sensing plays a critical role in cognitive radio networks. This paper presents a cooperative sequential detection scheme tominimize the average sensing time that is required to reach a detection decision. In the scheme, each cognitive radio computes the Log-Likelihood ratio for its every measurement, and the base station sequentially accumulates these Log-Likelihood statistics and determines whether to stop making measurement. The average number of required samples depends on the Kullback-Leibler distance between the distributions of the two hypotheses under test. This suggests a criterion for selecting the most efficient radios to facilitate spectrum sensing. The paper also studies how to implement the scheme in a robust manner when the assumed statistical models have uncertainties. These ideas are illustrated through an example that assumes both the signal and noise are Gaussian distributed.

Performance Analysis and Range Improvement in Multiband-OFDM UWB Communications

The performance of multiband-OFDM UWB systems is analyzed using the S-V indoor channel model. Based on the analysis, two methods are proposed to improve the reliable transmission range of UWB devices by exploiting the rich spectral and spatial diversity available in the system. It is shown both by theory and computer simulations that the proposed methods can effectively enlarge the transmission range of UWB devices

Cooperative spectrum sensing via coherence detection

Efficient and reliable spectrum sensing plays a critical role in cognitive radio networks. This paper proposes a cooperative sensing scheme that detects the existence of a common signal component in the signals received by multiple geographically distributed radios. The scheme assumes that signals received by different radios display strong coherence if they have a common source. Detection of this coherence in a wireless environment is studied, especially when the transmitted signal is distorted by multipath channels.

An Efficient Carrier Phase Synchronization Technique for High-Order M-QAM–OFDM

In this correspondence, we propose a blind carrier phase synchronization algorithm for high-order M-ary quadrature amplitude modulation-orthogonal frequency division multiplexing (M-QAM-OFDM) systems, which can effectively recover residual frequency offset (RFO) in the presence of intercarrier interference (ICI). The proposed algorithm performs frequency and phase synchronization by using post-fast Fourier transform (FFT) demodulated signals without the aid of reference signals (e.g., pilots, guard intervals, and virtual carriers), and is simple to implement. By analyzing its open-loop characteristics, we show that the proposed algorithm is superior to the conventional decision-directed (DD) scheme for high-order M-QAM-OFDM systems.

OFDM Channel Estimation in the Presence of Frequency Offset, IQ Imbalance and Phase Noise

OFDM systems are susceptible to receiver impairments like frequency offset, IQ imbalance and phase noise. In this paper, OFDM channel estimation in the presence of these impairments is studied, and an iterative algorithm is proposed to jointly estimate the channel coefficients and the impairment parameters. It is shown by computer simulations that the algorithm performs close to its associated Cramer-Rao lower bound.

Joint Compensation of IQ Imbalance and Phase Noise in OFDM Systems

The joint effects of IQ imbalance and phase noise on OFDM systems are analyzed, and a compensation scheme is proposed to improve the system performance. The scheme consists of a joint channel estimation algorithm and a joint data symbol estimation algorithm. In the proposed channel estimation algorithm, the channel coefficients are jointly estimated with the IQ imbalance parameters and the phase noise components. Its performance is demonstrated to be close to the associated Cramer-Rao lower bound. In the proposed data symbol estimation algorithm, the joint compensation is decomposed into IQ imbalance compensation and phase noise compensation. It is shown both by theory and computer simulations that the proposed scheme can effectively improve the signal-to-noise ratio at the receiver. As a result, the sensitivity of OFDM receivers to the physical impairments can be significantly lowered, simplifying the RF and analog circuitry design in terms of implementation cost, power consumption, and silicon fabrication yield.