Shunqing Zhang

A low-overhead energy detection based cooperative sensing protocol for cognitive radio systems

Cognitive radio and dynamic spectrum access represent a new paradigm shift in more effective use of limited radio spectrum. One core component behind dynamic spectrum access is the sensing of primary user activity in the shared spectrum. Conventional distributed sensing and centralized decision framework involving multiple sensor nodes is proposed to enhance the sensing performance. However, it is difficult to apply the conventional schemes in reality since the overhead in sensing measurement and sensing reporting as well as in sensing report combining limit the number of sensor nodes that can participate in distributive sensing. In this paper, we shall propose a novel, low overhead and low complexity energy detection based cooperative sensing framework for the cognitive radio systems which addresses the above two issues. The energy detection based cooperative sensing scheme greatly reduces the quiet period overhead (for sensing measurement) as well as sensing reporting overhead of the secondary systems and the power scheduling algorithm dynamically allocate the transmission power of the cooperative sensor nodes based on the channel statistics of the links to the BS as well as the quality of the sensing measurement. In order to obtain design insights, we also derive the asymptotic sensing performance of the proposed cooperative sensing framework based on the mobility model. We show that the false alarm and mis-detection performance of the proposed cooperative sensing framework improve as we increase the number of cooperative sensor nodes.

Resource Allocation for OFDMA System with Orthogonal Relay using Rateless Code

This paper considers the resource allocation problem in the wireless OFDMA system with a relay node. We consider orthogonal relay in which the relay node cannot transmit and receive simultaneously on the same frequency. As a result, the use of relay node may enhance or degrade the achievable transmission rate depending on the instantaneous channel states between the source and the relay, the relay and the destination as well as the source and the destination. Hence, it is very important to dynamically adjust the resources (subbands) allocated to the relay node so that the relay is used only at the right time according to the instantaneous channel states. Conventional approaches dynamically schedule the usage of the relay node in a centralized manner in which full knowledge of the channel states between any two nodes in the network is required. However, perfect knowledge of the channel states at various nodes is very difficult to obtain. In this paper, we shall propose a resource allocation algorithm, which iteratively allocates resource to the source and relay, and converge to the close-to-optimal allocation. Asymptotic achievable rate of the proposed algorithm is derived. We show that the system achieves significant improvement of the achievable rate compared to the point-to-point baseline system without relay as well as the baseline system with random subband allocation.

Design and Analysis of Multi-Relay Selection for Cooperative Spatial Multiplexing

This paper considers the relay selection problem for the cooperative spatial multiplexing scheme in wireless systems with M relay nodes. We consider half-duplex relays in which the relay nodes cannot transmit and receive simultaneously on the same frequency. To improve the transmission reliability, traditional approaches employ cooperative spatial diversity (such as distributed space-time coding) during the cooperative phase when relay and source both transmit to the destination. Most of the relay selection schemes considered in the literature deal with selecting one relay only. However, when we consider wireless systems with multi-antenna destination, we may have opportunities to transmit extra data through cooperative spatial multiplexing by selecting multiple relays to participate in the cooperative phase. In this paper, we shall propose a low complexity multi- relay selection algorithm for the cooperative spatial multiplexing using decode-and-forward (DF) protocol. We have also analyzed the asymptotic performance as well as the diversity-multiplexing tradeoff (DMT) of the proposed system. We found that the proposed cooperative spatial multiplexing scheme with multi- relay selection achieves a much better DMT tradeoff than the traditional cooperative diversity schemes.

Multi-Relay Selection Design and Analysis for Multi-Stream Cooperative Communications

In this paper, we consider the problem of multi-relay selection for multi-stream cooperative MIMO systems with M relay nodes. Traditionally, relay selection approaches are primarily focused on selecting one relay node to improve the transmission reliability given a single-antenna destination node. As such, in the cooperative phase whereby both the source and the selected relay nodes transmit to the destination node, it is only feasible to exploit cooperative spatial diversity (for example by means of distributed space time coding). For wireless systems with a multi-antenna destination node, in the cooperative phase it is possible to opportunistically transmit multiple data streams to the destination node by utilizing multiple relay nodes. Therefore, we propose a low overhead multi-relay selection protocol to support multi-stream cooperative communications. In addition, we derive the asymptotic performance results at high SNR for the proposed scheme and discuss the diversity-multiplexing tradeoff as well as the throughput-reliability tradeoff. From these results, we show that the proposed multi-stream cooperative communication scheme achieves lower outage probability compared to existing baseline schemes.

Protocol design and delay analysis of half-duplex buffered cognitive relay systems

In this paper, we quantify the benefits of employing relay station in large-coverage cognitive radio systems which opportunistically access the licensed spectrum of some small-coverage primary systems scattered inside. Through analytical study, we show that even a simple decode-and-forward (SDF) relay, which can hold only one packet, offers significant pathloss gain in terms of the spatial transmission opportunities and link reliability. However, such scheme fails to capture the spatial-temporal burstiness of the primary activities, that is, when either the source-relay (SR) link or relay-destination (RD) link is blocked by the primary activities, the cognitive spectrum access has to stop. To overcome this obstacle, we further propose buffered decode-and-forward (BDF) protocol. By exploiting the infinitely long buffer at the relay, the blockage time on either SR or RD link is saved for cognitive spectrum access. The buffer gain is shown analytically to improve the stability region and average end-to-end delay performance of the cognitive relay system.

Precoder Design for Multi-Antenna Partial Decode-and-Forward (PDF) Cooperative Systems with Statistical CSIT and MMSE-SIC Receivers

Cooperative communication is an important technology in next generation wireless networks. Aside from conventional amplify-and-forward (AF) and decode-and-forward (DF) protocols, the partial decode-and-forward (PDF) protocol is an alternative relaying scheme that is especially promising for scenarios in which the relay node cannot reliably decode the complete source message. However, there are several important issues to be addressed regarding the application of PDF protocols. In this paper, we propose a PDF protocol and MIMO precoder designs at the source and relay nodes. The precoder designs are adapted based on statistical channel state information for correlated MIMO channels, and matched to practical minimum mean-square-error successive interference cancelation (MMSE-SIC) receivers at the relay and destination nodes. We show that under similar settings, the proposed MIMO precoder design with PDF protocol and MMSE-SIC receivers achieves substantial performance enhancement compared with conventional baselines.

Exploiting buffers in cognitive multi-relay systems for delay-sensitive applications

Cognitive and cooperative technologies are two core components in the design of next generation wireless networks. One key issue associated with cognitive transmission is the inefficient spectrum sharing of the secondary system, especially for secondary communications separated by long distance. To boost the spectrum sharing efficiency, cognitive multi-relay system appears to be an attractive solution for the cognitive transmission systems. In this paper, we consider a cognitive multi-relay (CMR) system and propose a novel CMR buffered decode-and-forward protocol that exploit the buffers in the source and each relay node. Moreover, we derive the closed-form average end-to-end delay and the stability region by exploiting the birth-death nature of the queue dynamics and the methods of state aggregation and queue dominance. Comparing with the baseline protocols through analytical and numerical results, the proposed CMR-BDF scheme can dynamically adjust the cognitive transmission to exploit the spatial PU burstiness while simultaneously benefits from the advantage of double-sided selection diversity in both the source-relay and relay-destination interfaces.

Game Theoretical Power Control for Open-Loop Overlaid Network MIMO Systems with Partial Cooperation

In this paper, we consider an open-loop network MIMO system with K BSs serving K private MSs and Mc common MS based on a novel partial cooperation overlaying scheme. Exploiting the heterogeneous path gains between the private MSs and the common MSs, each of the K BSs serves a private MS non-cooperatively and the K BSs also serve the Mc common MSs cooperatively. The proposed scheme does not require closed loop instantaneous channel state information feedback, which is highly desirable for high mobility users. Furthermore, we formulate the long-term distributive power allocation problem between the private MSs and the common MSs at each of the K BSs using a partial cooperative game. We show that the long-term power allocation game has a unique Nash Equilibrium (NE) but standard best response update may not always converge to the NE. As a result, we propose a low-complexity distributive long-term power allocation algorithm which only relies on the local long-term channel statistics and has provable convergence property.

Game theoretical power control for open-loop network MIMO systems with partial cooperation

Network MIMO is considered to be a key solution for the next generation wireless systems in breaking the interference bottleneck in cellular systems. In the network MIMO systems, open-loop transmission scheme is used to support mobiles with high mobilities because the BSs do not need to track the fast varying channel fading. In this paper, we consider an open-loop network MIMO system with L BSs serving L+1 mobiles based on partial cooperation where all the L BSs cooperatively serve a cell-edge MS. Exploiting the heterogeneous path loss between cellcenter MS and cell-edge MS, each of the L BS also serves a cellcenter MS non-cooperatively. We first propose a flexible open-loop SDMA scheme to address potential issues associated with dynamic and heterogeneous MIMO configurations and to adjust the long-term power allocation between the cell-edge MS (cooperatively served by L BSs) and the cell-center MS at each of the L BSs, we formulate the long-term distributive power control problem using game theoretic approach. Through numerical simulations, we show that the proposed open-loop SDMA scheme with distributed long-term power allocation can achieve significant performance advantages over the other reference baseline schemes.

A novel unequal error protection (UEP) scheme using D-STTD for multicast service

To significantly enhance the spectral efficiency of the MBS service, UEP and MIMO are important core technologies to realize such challenging goal. Taking into account of the implementation constraints, 4 times 2 is widely accepted as a reasonable MIMO setup and is a mandatory configuration of the next generation wireless systems. Within the 4 times 2 MIMO configuration, D-STTD is an important transmission scheme which strikes a balance between spatial diversity and spatial multiplexing and it can achieve better performance than VBLAST. Traditional approaches focus on different ways to introduce UEP in single antenna systems. However, it is difficult to apply the conventional schemes in the D-STTD systems since the decoding error is mainly contributed by the spatial interference between different streams. In this paper, we shall propose a novel UEP scheme for MBS services using D-STTD in 4times2 MIMO systems and formulate a general design framework to determine various parameters of the proposed UEP scheme. For illustration, we give two examples on the determination of the UEP parameters for the uncoded UEP design and the coded UEP design. From the results, we found that the proposed UEP scheme can achieve significant MBS system performance gain compared to those baseline systems.