Xianbin Wang

A New Adaptive OFDM System with Precoded Cyclic Prefix for Cognitive Radio

Recent development in cognitive radio (CR) brings significant technical challenges in the design of robust and flexible transmission technique in hostile communication environment with varying channel condition. An adaptive orthogonal frequency division multiplexing (OFDM) system, with a precoded cyclic prefix (PCP), is proposed in this paper to address these challenges. Besides the basic function as a guard interval for the OFDM systems, the PCP provides an efficient way of sending the transmission system parameters of the cognitive radio simultaneously with the data carrying OFDM signal. These parameters may include the bandwidth, total number of OFDM subcarriers, modulation and coding schemes used. Overall spectrum efficiency can be improved due to the elimination of the preambles and handshaking signaling required when there is any change in the CR transmission parameters. The receiver design particularly a hybrid domain equalizer for the PCP-OFDM system is presented in this paper. Implementation related issues including exploitation of the PCP structure, interference cancellation and complexity reduction are investigated. The performance of the proposed OFDM systems and the channel estimators are analyzed and verified through numerical simulations.

Spectrum Sensing of Unsynchronized OFDM Signals for Cognitive Radio Communications

Sensing of orthogonal frequency division multiplex- ing (OFDM) signals at low signal-to-noise ratio (SNR) is of great importance for cognitive radio (CR) communications due to the wide applications of OFDM in many existing and evolving broadband wireless communications. In-band pilots, multiplexed with the data-carrying subcarriers, provides one distinct feature of OFDM signals. The objective of this paper is to investigate the application of in-band pilots in OFDM signal spectrum sensing, particularly when synchronization is not achievable at very low SNR. In this paper, the proposed sensing technique is to match the received OFDM signals with a self-defined pilot tone filter. Due to the extremely low requirement on sensing SNR for CR applications, synchronization of the sensing receiver and OFDM signals usually is not achievable. As a result, carrier frequency offset (CFO) and timing offset have to be dealt with during the sensing process. With the hybrid domain signal processing, the proposed sensing technique is very robust to both timing and frequency offset. We also apply the proposed technique to DVB-T to verify our proposal and subsequent analysis. Robust performance with very low false alarm probability and short sensing time were achieved under various channel conditions including Rician and Rayleigh channels, in the presence of random timing offset and CFO.

Effects of Side Information on Complexity Reduction in Superimposed Pilot Channel Estimation in OFDM Systems

A novel, reduced complexity iterative channel estimation algorithm for OFDM systems using superimposed pilots is proposed. It utilizes past channel estimations of double correlated channel as a side information to reduce number of iterations. Since pilots are available at all positions of the time-frequency OFDM grid in superimposed technique, the performance of the channel estimation does not degrade because of the variations of the fast fading channel between two pilots. On the other hand since no subcarrier is reserved for channel estimation purpose, superimposed pilot technique leads to improved spectral efficiency comparing to in-band OFDM pilots. However interference from data carrying signals made channel estimation more complex. In this paper, Least Square (LS) channel estimation followed by two dimensional Wiener filter for reducing OFDM symbol interference is done iteratively to achieve the Minimum Mean Square Error (MMSE). Small variations of the channel over each OFDM symbol duration are neglected due to a high data rate, but the values between different OFDM symbols are assumed correlated. The channel is modeled as a double selective, i.e. both frequency selectivity channel and Doppler shift are taken into consideration. Past channel estimates are used as side information for the present channel estimation to improve the forthcoming channel estimation at the first iteration and reduce the total number of iterations required.

Identification of PCP-OFDM Signals at Very Low SNR for Spectrum Efficient Communications

An adaptive Orthogonal Frequency Division Multiplexing (OFDM) system, with a preceded cyclic prefix (PCP), was proposed earlier to address the recent need of robust and flexible transmission technique in cognitive radio (CR) communications [1]. Identification of PCP-OFDM signals is therefore of great importance for the design of fair spectrum sharing mechanism, particularly at very low signal-to-noise ratio (SNR)when synchronization is not achievable. The preceded cyclic prefix, multiplexed with the data-carrying OFDM signals, provides one unique and recognizable feature of PCP-OFDM signals. In this paper, a robust PCP-OFDM signal identification technique is proposed under very low SNR in the presence of unknown timing and carrier frequency offset (CFO). Robust performance with very low false alarm probability and short sensing time was achieved under various channel conditions including Rician, Rayleigh and the additive white Gaussian noise (AWGN) channels.

An enhanced cross-layer authentication mechanism for wireless communications based on PER and RSSI

Recently physical layer attributes and statistics have been exploited in securing wireless communications. However, one major obstacle of physical layer security techniques is that not all of these attributes are accessible in practical wireless communication platforms. More precisely, once the hardware of a physical transceiver is implemented, most of the physical layer attributes are not accessible due to the highly integrated circuits. Consequently, it becomes essential to develop implementable security enhancement techniques by utilizing all available attributes and statistics at different layers of wireless communication networks. In this paper, we consider the packet error rate (PER) and the received signal strength indicator (RSSI) in IEEE 802.11 networks to improve the wireless communication security. These two unique user and environment dependent attributes are readily available in most of the currently deployed IEEE 802.11 platforms. To enhance the spoofing attack detection capability, we propose a practical authentication scheme by monitoring and analyzing the PER and RSSI at the same time. The hypothesis testing model for the proposed authentication using PER and RSSI as two testing variables is presented. In addition, a decision rule for authentication, which is able to differentiate between a legitimate transmitter and a potential attacker by combining both attributes together, is developed. To evaluate the feasibility of our proposed scheme, lab experiments have been conducted using an IEEE 802.11g Atheros platform. The proposed authentication technique is validated by the experimental and simulation data. Our final authentication results confirm the improved spoofing detecting capability of the proposed technique over the single-variable based authentication.

Misbehavior detection in amplify-and-forward cooperative OFDM systems

The success of cooperative communications hinges on the reliability of cooperating nodes between the communicating parties, which may not be guaranteed in realistic scenarios. To protect cooperative networks from selfish misbehaviors and malicious forwarding, a security mechanism monitoring various abnormal behaviors during cooperation is necessary because of the constantly changing network topology and variability of cooperating nodes. In this paper, based on the orthogonal time division protocol commonly used in cooperation, a misbehavior detection scheme is proposed for amplify-and-forward (AF) cooperative orthogonal frequency division multiplexing (OFDM) systems by introducing the time division duplexing (TDD) feature into the source node and exploiting the correlation properties between the transmitted and received signals. With the estimated amplification gain and noise power, two binary hypothesis tests are employed to detect power-reducing selfish behaviors and malicious jamming attacks respectively. Simulation results demonstrate the effectiveness of the proposed scheme in detecting different misbehaviors of cooperating nodes.

Analysis and Algorithm for Non-Pilot-Aided Channel Length Estimation in Wireless Communications

Channel estimation and equalization techniques are crucial for the ubiquitous wireless communication systems. Conventional receivers for most wireless standards preset the channel length to the maximal expected duration of the channel impulse response for the adopted channel estimation and equalization algorithms. The excessive channel length often significantly increases the implementational complexity of the wireless receivers and leads to the redundant information which would induce the additional estimation errors. Moreover, such a scheme does not allow the dynamic memory allocation for variable channel lengths. This could further increase the power consumption and reduce the battery life of a mobile device. The knowledge of the actual channel length would, in principle, help the system designers decrease the complexity of the channel estimators using maximum likelihood (ML) and minimum-mean-square-error (MMSE) algorithms. In this paper, we address this important channel length estimation problem and propose a novel algorithm to estimate the channel length without the need of pilots or training sequence. In addition, we provide the analysis on the effectiveness of the proposed non-pilot-aided channel length estimator through Monte Carlo simulations.

Secure OFDM transmission based on multiple relay selection and cooperation

In cooperative communications, spatial channel diversity introduced by scattered locations of cooperating nodes can be exploited to enhance transmission security. In this paper, we propose a secure cooperative orthogonal frequency division multiplexing (OFDM) transmission scheme with the help of multiple relay nodes. For each relay, the wireless channels related to the destination and the eavesdropper are independent of each other, leading to different achievable rates over each subcarrier. In the proposed scheme, relay nodes with high rate differences between the intended receiver and the eavesdropper are selected for transmission. Different cooperation strategies ranging from single-relay selection to per-subcarrier-based multiple-relay selection are studied and compared. As a result of relay selection, each cooperative relay node only forwards a subset of subcarriers that exhibits high capacity differences, thereby enhancing transmission security of the whole system. Secrecy outage probabilities for the proposed scheme with different relay selection strategies are formulated. Numerical simulation results demonstrate the validity of the scheme for security enhancement.

Radiation Footprint Minimization Using Encoded OFDM Pilots for Cognitive Radio Communications

The limited availability of spectrum and the inefficiency of its usage necessitate new research on spectral opportunistic communication technologies including cognitive radio (CR). Such new systems are characterized by the coexistence of the heterogeneous wireless systems. In this paper, a radiation footprint minimization technique is proposed through encoded in-band pilot tones for Orthogonal frequency division multiplexing (OFDM) system. A transmission power negotiation signaling between the transmitter and receiver is established through the encoded pilot tones. Electromagnetic interference to the primary and other cognitive radios can be minimized with an automatically reduced radiation footprint. In addition, system performance of the receiver can be guaranteed in the process of mutual interference minimization. The transceiver structure and the inband pilot tone detection algorithm are investigated. The principle and performance of the system are validated through numerical simulations.

Design and Performance Evaluation of Signaling Link Demodulator for PCP-OFDM System

An adaptive Orthogonal Frequency Division Mul- tiplexing (OFDM) system, with a precoded cyclic prefix (PCP) as a signaling link was proposed earlier to address the recent need of flexible transmission techniques in cognitive radio (CR) communications (1). The flexibility of the PCP-OFDM system relies on the concurrent transmission of OFDM signal and PCP signaling which represents OFDM system parameters including bandwidth, modulation, coding schemes etc. Efficient demodula- tion of PCP signaling is therefore of great importance for OFDM data recovery and reduce the delay from the system adaption. In this paper, we propose to use a three-stage demodulator to recover the signaling information carried by the PCPs. Design of this three-stage demodulator are analyzed and performance is evaluated through the simulations in different channel conditions. In addition, with proposed peak combining technique, impact of the multipath impairment is mitigated to a great extent. It is observed from the simulation results that this signaling demodulation scheme provides the same performance as the conventional optimal matched filter but with significantly reduced hardware and computational complexity. I. INTRODUCTION