Zhilan Xiong

A Cooperative MMSE Relay Strategy for Wireless Sensor Networks

We propose a minimum mean-square error (MMSE)-based signal forwarding technique for a cooperative relay network. Transmission of information between multiple source-destination pairs through a set of relays is considered. Cooperation between relays has been shown to improve substantially the bit-error rate (BER) performance as compared to noncooperative relays under a total power constraint. A general model for relay cooperation has been considered; however, for a single source-destination scenario, the proposed MMSE relaying strategy has been shown to be a product of receiver and transmitter beamformers at the relay layer.

A Novel Cooperative Relaying Strategy for Wireless Networks With Signal Quantization

We propose a cooperative signal-forwarding scheme for wireless sensor and mesh networks using semidefinite constraints. We consider a multiple source-destination scenario where a set of relays assists forwarding signals from sources to destinations. This paper assumes cooperation between relays so that the signals received from source nodes are passed between the relays. A semidefinite programming framework allows us to impose various quality-of-service (QoS) constraints for each source-destination pair. The proposed scheme outperforms a cooperative relaying strategy that is based on a minimum mean square error (MMSE). The design also considers quantization of signals that are passed between cooperating relays. The proposed scheme, even in the presence of signal-quantization noise, outperforms a noncooperative relaying scheme.

SINR Balancing Technique and its Comparison to Semidefinite Programming Based QoS Provision for Cognitive Radios

Cognitive radio networks opportunistically operate in frequency bands that have been licensed to other networks. Therefore, communication between unlicensed users should en- sure the interference leaked to the licensed users is kept below an acceptable level while achieving the required quality of services. In this paper, we extend SINR balancing technique to serve multiple cognitive users in the downlink while imposing constraints on interference temperature of primary users. We show that when the set interference temperatures is fixed, the proposed SINR balancing technique will always have a unique solution that is identical to semidefinite programming based optimal solution. The advantages and disadvantages of the SINR balancing technique and semidefinite programming based techniques are also discussed.

Cooperative relays for an underlay cognitive radio network

A cooperative relaying scheme is proposed for an underlay cognitive radio network where relays use their beam steering capability to impose target SINRs at the secondary user terminals whilst keeping the interference and noise leakage to the primary users below a threshold. The scheme is developed to incorporate per relay antenna power constraint that limits the power emitted by the relays. The scheme is shown to have a better performance, in terms of outage probability and transmission power as compared to a non cooperative relay scheme.

Performance Evaluation of 3D MIMO LTE-Advanced System

This paper demonstrates a simulation methodology based on a reasonable 3D channel model. System level simulations on various 3D antenna port configurations are performed, and results are compared with those of 2D MIMO system. Several interesting observations are made from the evaluation results: (1) the vertically distributed active antenna elements provide less MIMO gain than the horizontally distributed active antenna elements; (2) with the same number of antenna elements, the 3D active antenna configuration can provide better performance than the 2D one, and the performance improvement is much larger for MU-MIMO than SU-MIMO; (3) even when the 2D antenna configuration has more antenna elements (providing more antenna array gain) than the 3D active antenna configuration, the 3D configuration can still outperform the 2D one if there are enough transmitter ports.

Robust SINR Balancing Technique for a Cognitive Radio Network Using Probability Based Interference Constraints

A robust beamformer design is proposed for a spectrum sharing CR network consisting of multiple secondary users (SUs) in the presence of multiple primary users (PUs). The proposed algorithm aims to maximize the worst case signal-to-interference-plus-noise ratio (SINR) of the SUs while limiting the interference leakage to PUs. Since it may be difficult to obtain perfect information about the channels between the SU basestation and the PUs, we consider errors in the PU channel estimates and design beamformers to limit interference leakage to PU below a specific threshold with a certain probability. The simulation results demonstrate that the proposed beamformer design has the ability to maximize the worst case SINR (balanced SINR) while controlling the interference leakage to PUs.

Capacity Balancing for Multiuser MIMO Cognitive Radio Network

We propose a capacity balancing beamforming technique for a multiple inputs and multiple outputs (MIMO) based cognitive radio (CR) network. The proposed algorithm is based on mean square error (MSE) duality and it is aimed at maximizing the worst case user capacity of multiple secondary users (SUs) by jointly designing their transceiver beamformers while ensuring the interference leakage to multiple primary users (PUs) are below a specific set of thresholds. We use an iterative approach to determine the transceiver beamformers so that the capacities achieved by all SUs are balanced and maximized. The performance of the algorithm is demonstrated through Monte Carlo simulation results.

Optimization techniques for two-way relaying based multiuser multiplexing

We propose an optimization technique for the use of two way relays to forward signals and perform spatial multiplexing. The multiuser transmitter and receiver pairs employ single antennas, however, the two way relays can be used to forward signals to each other while performing spatial multiplexing by appropriately changing the phase and the amplitude of the signals at the relays. The problem has been formulated into a semidefinite programming (SDP) framework that can be solved using interior point methods. The simulation results demonstrate that the proposed technique has the ability to perform spatial multiplexing using two way relays for a wide range of target signal to interference and noise ratio requirements.

A GMD-based precoding scheme for downlink multiuser multistream MIMO channels

In order to obtain a good balance of bit error rate (BER) across channels, the geometric mean decomposition (GMD) is introduced to replace the singular value decomposition (SVD) for precoding in the downlink of a multiuser multistream multiple-input multiple-output (MIMO) system. By combining GMD with a block diagonalization method, we obtain two kinds of precoding schemes: iterative nullspace-directed GMD and non-iterative nullspace-directed GMD. Considering their respective advantages and disadvantages, a mixed nullspace-directed GMD is proposed to solve the convergence related problems of the iterative method. Furthermore, the computational complexity of the mixed scheme is similar to the iterative scheme under the same conditions. The simulation results show that the average BER performance of the block diagonalization method based on GMD is better than the same method based on SVD, and the mixed nullspace-directed GMD outperforms the iterative nullspace-directed GMD and the non-iterative nullspace-directed GMD.

A robust sinr balancing technique for a multiuser MIMO cognitive radio network

We propose a robust signal to interference and noise ratio (SINR) balancing beamforming technique for a multiple input and multiple output (MIMO) based cognitive radio (CR) network. The aim is to design transmitter and receiver beamformers for multiple secondary users (SUs) while ensuring interference leakage to multiple primary users (PUs) is below a prescribed threshold. We use an iterative approach to design the receiver beamformers and transmitter beamformers so that SINRs achieved by all SUs are balanced and maximized. Due to the limited cooperation between secondary basestation (SBS) and PUs, we assume the channel state information (CSI) between the SBS and PUs is imperfect. Hence a robust beamformer design has been used to ensure the interference leakage to PU is below the specific threshold for all possible errors in the CSI.