Yi Gong

An efficient space-frequency coded OFDM system for broadband wireless communications

We propose an efficient space-frequency coded orthogonal frequency-division multiplexing (OFDM) system for high-speed transmission over wireless links. The analytical expression for the pairwise probability of the proposed space-frequency coded OFDM system is derived in slow, space- and frequency-selective fading channels. The design criteria of trellis codes used in the proposed system are then developed and discussed. It is shown that the proposed space-frequency coded OFDM can efficiently achieve the full diversity provided by the fading channel with low trellis complexity, while for traditional space-frequency coded OFDM systems, we need to design space-time trellis codes with high trellis complexity to exploit the maximum achievable diversity order. The capacity properties of space-frequency coded OFDM over multipath fading channels are also studied. Numerical results are provided to demonstrate the significant performance improvement obtained by the proposed space-frequency coded OFDM scheme, as well as the excellent outage capacity properties.

Cooperative OFDM Relaying for Opportunistic Spectrum Sharing: Protocol Design and Resource Allocation

In this paper, we propose an opportunistic spectrum sharing protocol that exploits the situation when the primary system is incapable of supporting its target transmission rate. Specifically, the secondary system tries to help the primary system to achieve its target rate via two-phase cooperative OFDM relaying, where the secondary system acts as an amplify-and-forward relay for the primary system by allocating a fraction of its subcarriers to forward the primary signal. At the same time, the secondary system uses the remaining subcarriers to transmit its own signal, and thus gaining opportunistic spectrum access. As a part of the protocol, if the primary system finds that outage will occur even when the secondary system serves as a pure relay, the primary system will cease transmission and the secondary system will be granted access to the primary spectrum. We study the joint optimization of the set of subcarriers used for cooperation, subcarrier pairing, and subcarrier power allocation such that the transmission rate of the secondary system is maximized, while helping the primary system, as a higher priority, to achieve its target rate. Simulation results demonstrate the performance of the proposed spectrum sharing protocol as well as the win-win solution for the primary and secondary systems.

Adaptive Power Allocation in Two-Way Amplify-and-Forward Relay Networks

Two-way amplify-and-forward relaying is considered in this paper. We propose adaptive power allocation (PA) algorithms to maximize the instantaneous achievable rate and minimize the system outage probability, respectively. Both single-and multi-relay systems are considered. It is shown that the proposed adaptive PA algorithms significantly outperform the uniform PA algorithms. Furthermore, exploiting multiple relays obtains higher diversity order and lower outage probability than using only one relay, at the price of a lower achievable information rate. The relay locations and relay selection are also taken into account to further improve the system performance.

On the diversity gain in cooperative relaying channels with imperfect CSIT

In this paper, we investigate the impact of imperfect channel state information at the transmitters (CSIT) on the achievable diversity gain in a cooperative relaying channel with multiple destination antennas, where the CSIT comes from channel estimation at the transmitters. Both decode-and-forward (DF) and amplify-and-forward (AF) relaying protocols are considered. We show that transmit power control based on the imperfect CSIT significantly improves the achievable diversity gain. The diversity and multiplexing tradeoff (DMT) as a function of the CSIT quality of the source-relay link, source-destination link and relay-destination link is derived for each of the considered relaying schemes. An upper bound on the DMT of the relaying channel is also provided.

Adaptive power allocation for regenerative relaying with multiple antennas at the destination

We consider a dual-hop regenerative relay system with multiple antennas at the destination. Subject to a total power constraint, we explore adaptive power allocation between the source and relay to minimize the system outage probability and the average symbol error rate. Simulation results show that the presented adaptive power allocation solutions significantly outperform the uniform power allocation. For a fixed relay location, we show that using two destination antennas brings significant power saving and performance improvement over using one destination antenna, and that using more than two destination antennas does not necessarily bring further performance improvement. It is also found that by using more destination antennas and/or choosing an appropriate relay location, less power will be needed at the relay.

An Energy-Ratio-Based Approach for Detecting Pilot Spoofing Attack in Multiple-Antenna Systems

The pilot spoofing attack is one kind of active eavesdropping conducted by a malicious user during the channel estimation phase of the legitimate transmission. In this attack, an intelligent adversary spoofs the transmitter on the estimation of channel state information (CSI) by sending the identical pilot signal as the legitimate receiver, in order to obtain a larger information rate in the data transmission phase. The pilot spoofing attack could also drastically weaken the strength of the received signal at the legitimate receiver if the adversary utilizes large enough power. Motivated by the serious problems the pilot spoofing attack could cause, we propose an efficient detector, named energy ratio detector (ERD), by exploring the asymmetry of received signal power levels at the transmitter and the legitimate receiver when there exists a pilot spoofing attack. Our analysis shows that by setting the ratio of received signal power levels at the transmitter and the legitimate receiver as the test statistic, the detecting threshold is derived without using the knowledge of the CSI of the legitimate channel as well as the illegitimate channel. Furthermore, we study the performance of the proposed ERD in various special cases in order to obtain useful insights. Numerical results are presented to further demonstrate the performance of our proposed ERD.

On the Error Probability of Orthogonal Space-Time Block Codes Over Keyhole MIMO Channels

It has been shown recently that multiple-input multiple-output (MIMO) fading channels could experience a keyhole phenomenon, under which the performance of MIMO systems will be degraded in terms of link quality as well as capacity. In this paper, the performance of orthogonal space-time block codes in MIMO fading channels under keyhole condition is analyzed. Closed-form expressions for the error probability of space-time block codes as well as diversity gains of keyhole channels are derived. We prove that the maximum spatial diversity gain of a keyhole channel with to transmit and n receive antennas is min(m, n) when m ne n. In the case of m = n, the achievable diversity gain is less than n but higher than n - 1. Accordingly, for space-time block codes in keyhole channels, there are two forms of error rate expressions for m ne n and m = n, respectively. Furthermore, coding gains of various space-time block codes in keyhole channels are obtained. Simulation results are also provided to demonstrate the accuracy of our analytical results.

Adaptive Power Allocation for Multihop Regenerative Relaying With Limited Feedback

A multihop regenerative relay system is considered. We explore adaptive power allocation (PA) under a total power constraint to minimize the system error probability when the perfect channel state information (CSI) is known at the transmitters. We propose adaptive PA schemes with limited feedback when the transmitters have no perfect knowledge of CSI. Simulation results demonstrate the good performance of the proposed PA scheme with perfect CSI. When no perfect CSI is available at the transmitters, the proposed PA schemes with a small number of feedback bits are able to achieve very close performance to the adaptive PA with perfect CSI.

Receiver design for multicarrier CDMA using frequency-domain oversampling

Based on the frequency-domain oversampling and minimum mean-square error (MMSE) principles, we propose three linear single-user detectors for downlink multicarrier codedivision multiple-access (MC-CDMA) systems. We begin with an optimal linear MMSE detector, which is computationally demanding. To reduce the complexity, a two-stage MMSE detector and a diagonal one-stage MMSE detector are developed subsequently. Simulation results show that the proposed detectors can efficiently suppress the multiple access interference (MAI) caused by frequency-selective fading, near-far effect, frequency offset, and nonlinear power amplification.

Cross-Channel Estimation Using Supervised Probing and Sensing in Cognitive Radio Networks

Prior work in implementing spectrum sharing scenarios for cognitive radio networks has relied on the unlikely assumption of full cross-channel knowledge made available to the cognitive transmitter (CT) and/or the target primary receiver (PR). However, estimation of this cross-channel knowledge is not only important from a practical stand-point, but also in limiting the real interference power felt at the PR due to concurrent CT transmissions. We propose a supervised probing and sensing model, which enables the CT to gain a decent estimate of the cross-channel, assess the spectrum opportunities in its region of interference, and according to its throughput needs safely share or access primary user spectrum. With the probing power increment playing an essential part in the probing model, its optimization with respect to cross-channel estimation success and its influence on mean square error of the cross-channel estimation are discussed.