Xuewen Liao

Blind Channel Estimation for OFDM Modulated Two-Way Relay Network

In this paper, we develop a simple blind channel estimation algorithm for two-way relay network (TWRN) that consists of two terminal nodes and one relay node. We consider the frequency selective channels and adopt the orthogonal frequency-division multiplexing (OFDM) modulation to compensate the inter-symbol interference (ISI). By applying a non-redundant linear precoding at both terminals, we propose an algorithm that is effective of estimating two cascaded channels and is consistent with the two-phase two-way transmission protocols. The method to remove the inherent ambiguity of the blind channel estimation is discussed. Finally, the numerical results are provided to corroborate the proposed studies.

Achievable Degrees of Freedom of the Four-User MIMO Y Channel

In this letter, we investigate the achievable degrees of freedom (DoF) of four-user MIMO Y channel where four users each with M antennas intend to exchange messages via a relay of N antennas. By adopting the signal space alignment and a new transmission scheme which jointly align the subspace of unpaired signals from three different users among all the four users, we derive that the DoF per user of max{min(M,3/8N), min(6/7M, N/2)} can be achieved for this channel. Since signal space alignment relies on the dimensions of intersection space of two users, we show that the signal space is utilized efficiently without the restriction of two-user intersection space in the proposed scheme.

Impact of Limited Feedback on the Performance of MIMO Macrodiversity Transmission

In this paper, we study the performance of the multiple-input multiple-output macrodiversity transmission with limited feedback. The downlink scenario, where multiple transmitters serve the user simultaneously, is considered. We find it necessary to modify the model of the quantized channel by Jinal (2006) such that the phase ambiguity in the vector quantization procedure can be characterized. Using the modified model, we show that the conventional limited feedback methods cannot obtain the macrodiversity gain even when the size of the codebook is asymptotically large, and that the macrodiversity gain can be attained by adding only one bit of phase feedback. Our analysis can apply to both random vector quantization and any fixed codebook setup, and is verified by the simulation results.

Price Discount Strategy for WSP to Promote Hybrid Access in Femtocell Networks

Femtocell technology, which aims at improving indoor signal coverage and offloading traffic from macro base stations (MBSs), has attracted interest in wireless industry. Among all the access methods, hybrid access proves to be the most promising one. However, it is difficult to promote the hybrid access mode since all the femto holders (FHs) merely care about their own benefits. In this paper, we propose a price discount strategy for wireless service provider (WSP) to promote the hybrid access mode of femtocell in which WSP provides a price discount in exchange for the femto holders to share part of their resource to macro users. The problem is formulated and analyzed as a Stackelberg game where WSP acts as the leader and femto holders as the followers. The optimal resource allocation ratio for each FH is decided independently and the optimal price discount factor for WSP is also obtained. Furthermore, we analyze how the bandwidth allocation strategy will affect the utility of WSP and an optimal bandwidth allocation ratio is decided. Numerical results show that both WSP and femto holders can benefit from the price discount strategy.

Joint power allocation and subcarrier assignment for two-way OFDM multi-relay system

In this paper, we investigate a joint resource allocation problem for two-way OFDM multi-relay network, where two transceivers exchange OFDM signal with the help of multiple relays. Considering a constraint on the total transmit power consumed in the whole network, the assignment of subcarriers to multiple relays and power allocation at both transceivers as well as relays are jointly optimized to maximize the sum-rate. The maximization problem turns out a mixed integer programming problem, which can be asymptotically solved through Lagrange dual decomposition approach. Further, we propose a suboptimal algorithm to trade off performance for complexity. Finally, simulation results are provided to demonstrate the performance gain of the proposed algorithms.

Quality-Optimized Energy Neutrality with Link Layer Resource Allocation for Zero-Power Harvesting Wireless Communications

There is a strong need to explore green and harvestable energy in computer communications. However, adapting wireless network performance to harvested energy has largely been ignored in literature. In this paper, we propose a new resource allocation scheme to improve data delivery quality in energy harvesting enabled wireless networks. In the proposed approach, packet Automatic Repeat reQuest (ARQ) limit of each wireless node is adaptively adjusted according to harvested energy. To achieve such optimal retry adaptation, energy neutrality constraint is considered in the overall optimization process. Simulation results show that the proposed retry adaptation approach significantly improves packet delivery ratio by exploring the harvested energy.

Multicell Downlink Beamforming with Imperfect Channel Knowledge at Both Transceiver Sides

In this letter, we design downlink beamforming vectors for a multicell network when only imperfect channel state information (CSI) is available at both base stations (BS) and mobile users (MU). We consider two different optimization criteria, i.e., minimizing the total downlink transmit power subject to quality of service (QoS) constraints at each MU and maximizing the worst-case effective signal-to-interference-plus-noise ratio (SINR) subject to BS power constraints. To solve the optimization problems, the imperfect CSI embedded QoS constraints are converted to linear matrix inequalities (LMI) with the aid of S-procedure, and then a semidefinite relaxation (SDR) approach is applied. Numerical results are provided to corroborate the proposed studies.

Lifetime maximization in multihop mixed AF-DF relay networks

In this paper, we study the problem of the lifetime maximization in a new type of multi-hop relay network where the amplify-and-forward (AF) and the decode-and-forward (DF) relay nodes are alternatingly cascaded. In order to cope with the general frequency selective fading channel, the orthogonal frequency division multiplexing (OFDM) modulation is adopted over each hop. We formulate the optimization problem as maximizing the network lifetime subject to the constraint of minimum end-to-end rate requirement, while each relay node is constrained to its limited battery energy as well as the maximum power supply. Under certain assumptions, we transform the original problem into the standard convex optimization problem and solve it from the dual decomposition technique. Simulation results are provided to corroborate the proposed strategy.

Circular polarization benefits in outdoor to indoor scenarios for MIMO cellular networks

MIMO transmission scheme equipped with cross linear polarization antennas has been widely used in mobile radio networks. However, some realistic measurement results show that MIMO scheme equipped with circular polar antennas also has some benefits in live mobile radio networks. In this paper, the penetration capability for linear and circular polarization is studied. Our theoretical analysis shows that circular polar antenna has higher penetration capability than linear polar antenna. Moreover, the measurement campaign is conducted to verify our analysis. It means that the application of circular polarization for MIMO transmission will be a promising method in outdoor to indoor environments.

A general framework for optimizing AF based multi-relay OFDM systems

In this paper, we study the joint resource allocation problem in multi-carrier multi-relay aided dual hop single-user communication. We adopt orthogonal frequency division multiplexing (OFDM) as the transmission modulation and consider amplify-and-forward (AF) relaying scheme. The optimization is performed over power allocation at source node, beamforming at relay nodes and subcarrier pairing at two hops, such that the overall system throughput is maximized under limited power budget at the source and relay nodes. The optimization is a mixed integer programming problem which is solved through dual decomposition approach. To further reduce the complexity, we propose a suboptimal algorithm which sacrifices very little on the performance. Finally, simulation results are provided to corroborate the proposed studies.