Yuan Liu

Shanghai Jiao Tong University, Shanghai, Shanghai Shi, China

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Publications (21)22.08 Total impact

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    ABSTRACT: This paper studies the secure beamforming design in a multiple-antenna three-node system where two source nodes exchange messages with the help of an untrusted relay node. The relay acts as both an essential signal forwarder and a potential eavesdropper. Both two-phase and three-phase two-way relay strategies are considered. Our goal is to jointly optimize the source and relay beamformers for maximizing the secrecy sum rate of the two-way communications. We first derive the optimal relay beamformer structures. Then, iterative algorithms are proposed to find source and relay beamformers jointly based on alternating optimization. Furthermore, we conduct asymptotic analysis on the maximum secrecy sum-rate. Our analysis shows that when all transmit powers approach infinity, the two-phase two-way relay scheme achieves the maximum secrecy sum rate if the source beamformers are designed such that the received signals at the relay align in the same direction. This reveals an important advantage of signal alignment technique in against eavesdropping. It is also shown that if the source powers approach zero the three-phase scheme performs the best while the two-phase scheme is even worse than direct transmission. Simulation results have verified the efficiency of the secure beamforming algorithms as well as the analytical findings.
    IEEE Transactions on Signal Processing 12/2013; 62(9). · 2.81 Impact Factor
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    Yanbo Ma, Yuan Liu, Meixia Tao
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    ABSTRACT: The concept of device-to-Device (D2D) communication as an underlay coexistence with cellular networks gains many advantages of improving system performance. In this paper, we model such a two-layer heterogenous network based on stochastic geometry approach. We aim at minimizing the expected power consumption of the D2D layer while satisfying the outage performance of both D2D layer and cellular layer. We consider two kinds of power control schemes. The first one is referred as to independent power control where the transmit powers are statistically independent of the networks and all channel conditions. The second is named as dependent power control where the transmit power of each user is dependent on its own channel condition. A closed-form expression of optimal independent power control is derived, and we point out that the optimal power control for this case is fixed and not relevant to the randomness of the network. For the dependent power control case, we propose an efficient way to find the close-to-optimal solution for the power-efficiency optimization problem. Numerical results show that dependent power control scheme saves about half of power that the independent power control scheme demands.
    07/2013;
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    Yuan Liu, Jianhua Mo, Meixia Tao
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    ABSTRACT: Two-way relaying can considerably improve spectral efficiency in relay-assisted bidirectional communications. However, the benefits and flexible structure of orthogonal frequency division multiplexing (OFDM)-based two-way decode-and-forward (DF) relay systems is much less exploited. Moreover, most of existing works have not considered quality-of-service (QoS) provisioning for two-way relaying. In this paper, we consider the OFDM-based bidirectional transmission where a pair of users exchange information with or without the assistance of a single DF relay. Each user can communicate with the other via three transmission modes: direct transmission, one-way relaying, and two-way relaying. We jointly optimize the transmission policies, including power allocation, transmission mode selection, and subcarrier assignment for maximizing the weighted sum rates of the two users with diverse quality-of-service (QoS) guarantees. We formulate the joint optimization problem as a mixed integer programming problem. By using the dual method, we efficiently solve the problem in an asymptotically optimal manner. Moreover, we derive the capacity region of two-way DF relaying in parallel channels. Simulation results show that the proposed resource-allocation scheme can substantially improve system performance compared with the conventional schemes. A number of interesting insights are also provided via comprehensive simulations.
    IEEE Transactions on Wireless Communications 01/2013; · 2.42 Impact Factor
  • Bo Zhou, Yuan Liu, Meixia Tao
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    ABSTRACT: Most existing works about scheduling and resource allocation for orthogonal frequency division multiplexing (OFDM) based two-way relay networks have focused on immediate relay forwarding. In this paper, we consider relay buffering in delay-tolerant networks. The relay node is aided by two buffers and one for each user, so that it can adaptively decide when to buffer the received packets or to forward them according to the instantaneous channel and queue conditions. We formulate the joint optimization of subcarrier assignment, transmission mode selection (direct or relay mode), and relay strategy selection (buffering or forwarding), for maximizing the long-term average throughput. An efficient dual-based algorithm is proposed to characterize the optimal policy. Simulation results show that relay buffering can significantly enhance the long-term throughput in OFDM bidirectional transmission systems.
    Wireless Communications and Networking Conference (WCNC), 2013 IEEE; 01/2013
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    ABSTRACT: From security perspective, a friendly relay may help to keep the confidential messages from being eavesdropped, while an untrusted relay may intentionally eavesdrop the messages when relaying. This paper studies the secure beamforming for multiple-input multiple-output (MIMO) two-way communications, where two source nodes exchange information with the help of an untrusted relay node. The relay adopts amplify-and-forward (AF) strategy and acts as both an essential helper and a potential eavesdropper. Our goal is to maximize the secrecy sum rate of the bidirectional links by jointly optimizing the source and relay beamformers. For the two-phase two-way relay scheme, we first derive the optimal structure of the relay beamformer and then propose an iterative algorithm to jointly optimize the source and relay beamformers. Then, a comprehensive study on the asymptotical performance is conducted by letting the source and relay powers approach zero or infinity. In particular, we show that when all powers approach infinity, the two-way relay scheme achieves the maximum secrecy rate if the transceiver beamformers are designed such that the received signals at the relay can be aligned to be parallel.
    Wireless Communications and Networking Conference (WCNC), 2013 IEEE; 01/2013
  • Meixia Tao, Yuan Liu
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    ABSTRACT: In wireless cooperative orthogonal frequency-division multiple-access (OFDMA) networks, it is important to adapt the transmission strategies for each user according to the network channel dynamics in order to optimize the overall system performance. The adaption involves transmission mode selection (a user can choose from direct or cooperative transmission), subcarrier assignment, subcarrier pairing (the incoming and outgoing subcarriers at the relay for cooperative transmission need to be matched), relay selection, as well as power allocation and hence is highly challenging. Many previous works only consider a subset of the adaptation. In this paper, we tackle the joint optimization problem using a network flow approach. Specifically, we first show that for given power allocation, the combinatorial optimization problem of transmission mode selection, subcarrier assignment, relay selection and subcarrier pairing for the system total throughput maximization can be transformed into a minimum cost network flow (MCNF) problem with integer solutions. The linear optimal distribution (LOD) algorithm is applied to find the optimal solution in polynomial time. We then solve the mixed integer programming problem of the joint assignment and power allocation in an asymptotically optimal way in the dual domain. Simulation results show that the proposed algorithms can significantly enhance the overall system throughput.
    IEEE Transactions on Wireless Communications 01/2013; 12(3):1138-1148. · 2.42 Impact Factor
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    ABSTRACT: As an effective solution for indoor coverage and service offloading from the conventional cellular networks, femtocells have attracted a lot of attention in recent years. From the perspective of spectral efficiency, the macrocell base station (MBS) and femtocell base stations (FBSs) are usually deployed in the same spectrum. Then the interference problem has become a key obstruction that limits the network performance. In this paper, we study the spectrum reuse in the two-tier LTE femtocell network. In order to improve the network performance, the FBSs are encouraged to provide services to nearby macrocell users, and the MBS releases a fractional spectrum to the FBSs for avoiding cross-tier interference in return. We model this problem as a Stackelberg game where the MBS acts as a leader and the FBSs as the followers. We define the utilities for the MBS and FBSs as the average throughput and the distortion-rate function, respectively. It is worth noting that in our Stackelberg game model, there is no monetary price for the interaction between the leader and followers, which is the significant distinction from previous works. The optimal strategies of spectrum reuse for both MBS and FBSs are proposed by analyzing the Stackelberg game model. The simulation results show that the proposed spectrum reuse scheme can significantly improve the network performance.
    Wireless Communications and Networking Conference (WCNC), 2013 IEEE; 01/2013
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    ABSTRACT: In this paper, we study the cross layer design and optimization for delay quality-of-service (QoS) provisioning in spectrum sharing femtocell networks. Our goal is to find the optimal resource allocation policy to maximize the throughput for each femtocell user, addressing the co-channel interference problem in the physical layer and the individual statistical delay-QoS guarantee problem from the upper layers. The statistical delay-QoS requirement is characterized by the QoS exponent. By integrating the concept of effective capacity, the cross-layer optimization problem is formulated as an effective capacity maximization game. With partial dual decomposition, this game is solved through a hierarchical structure. Specifically, we derive the optimal power allocation policy for femtocell users and design a distributed algorithm to obtain the Nash Equilibrium (N.E.). Numerical results show that the proposed policy can efficiently improve the performance of the networks.
    Communications (ICC), 2013 IEEE International Conference on; 01/2013
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    Rui Wang, Meixia Tao, Yuan Liu
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    ABSTRACT: This paper studies a cooperative cognitive radio network where two primary users (PUs) exchange information with the help of a secondary user (SU) that is equipped with multiple antennas and in return, the SU superimposes its own messages along with the primary transmission. The fundamental problem in the considered network is the design of transmission strategies at the secondary node. It involves three basic elements: first, how to split the power for relaying the primary signals and for transmitting the secondary signals; second, what two-way relay strategy should be used to assist the bidirectional communication between the two PUs; third, how to jointly design the primary and secondary transmit precoders. This work aims to address this problem by proposing a transmission framework of maximizing the achievable rate of the SU while maintaining the rate requirements of the two PUs. Three well-known and practical two-way relay strategies are considered: amplify-and-forward (AF), bit level XOR based decode-and-forward (DF-XOR) and symbol level superposition coding based DF (DF-SUP). For each relay strategy, although the design problem is non-convex, we find the optimal solution by using certain transformation techniques and optimization tools such as semidefinite programming (SDP) and second-order cone programming (SOCP). Closed-form solutions are also obtained under certain conditions. Simulation results show that when the rate requirements of the two PUs are symmetric, by using the DF-XOR strategy and applying the proposed optimal precoding, the SU requires the least power for relaying and thus reserves the most power to transmit its own signal. In the asymmetric scenario, on the other hand, the DF-SUP strategy with the corresponding optimal precoding is the best.
    IEEE Transactions on Signal Processing 10/2012; 61(4). · 2.81 Impact Factor
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    ABSTRACT: This paper studies a wireless network where multiple users cooperate with each other to improve the overall network performance. Our goal is to design an optimal distributed power allocation algorithm that enables user cooperation, in particular, to guide each user on the decision of transmission mode selection and relay selection. Our algorithm has the nice interpretation of an auction mechanism with multiple auctioneers and multiple bidders. Specifically, in our proposed framework, each user acts as both an auctioneer (seller) and a bidder (buyer). Each auctioneer determines its trading price and allocates power to bidders, and each bidder chooses the demand from each auctioneer. By following the proposed distributed algorithm, each user determines how much power to reserve for its own transmission, how much power to purchase from other users, and how much power to contribute for relaying the signals of others. We derive the optimal bidding and pricing strategies that maximize the weighted sum rates of the users. Extensive simulations are carried out to verify our proposed approach.
    IEEE Transactions on Wireless Communications 10/2012; · 2.42 Impact Factor
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    Meixia Tao, Yuan Liu
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    ABSTRACT: This paper considers a cooperative OFDMA-based cognitive radio network where the primary system leases some of its subchannels to the secondary system for a fraction of time in exchange for the secondary users (SUs) assisting the transmission of primary users (PUs) as relays. Our aim is to determine the cooperation strategies among the primary and secondary systems so as to maximize the sum-rate of SUs while maintaining quality-of-service (QoS) requirements of PUs. We formulate a joint optimization problem of PU transmission mode selection, SU (or relay) selection, subcarrier assignment, power control, and time allocation. By applying dual method, this mixed integer programming problem is decomposed into parallel per-subcarrier subproblems, with each determining the cooperation strategy between one PU and one SU. We show that, on each leased subcarrier, the optimal strategy is to let a SU exclusively act as a relay or transmit for itself. This result is fundamentally different from the conventional spectrum leasing in single-channel systems where a SU must transmit a fraction of time for itself if it helps the PU's transmission. We then propose a subgradient-based algorithm to find the asymptotically optimal solution to the primal problem in polynomial time. Simulation results demonstrate that the proposed algorithm can significantly enhance the network performance.
    Journal of Communications and Networks 09/2012; · 0.75 Impact Factor
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    Cen lin, Yuan Liu, Meixia Tao
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    ABSTRACT: Two-way relaying promises considerable improvements on spectral efficiency in wireless relay networks. While most existing works focus on physical layer approaches to exploit its capacity gain, the benefits of two-way relaying on upper layers are much less investigated. In this paper, we study the cross-layer design and optimization for delay quality-of-service (QoS) provisioning in two-way relay systems. Our goal is to find the optimal transmission policy to maximize the weighted sum throughput of the two users in the physical layer while guaranteeing the individual statistical delay-QoS requirement for each user in the datalink layer. This statistical delay-QoS requirement is characterized by the QoS exponent. By integrating the concept of effective capacity, the cross-layer optimization problem is equivalent to a weighted sum effective capacity maximization problem. We derive the jointly optimal power and rate adaptation policies for both three-phase and two-phase two-way relay protocols. Numerical results show that the proposed adaptive transmission policies can efficiently provide QoS guarantees and improve the performance. In addition, the throughput gain obtained by the considered three-phase and two-phase protocols over direct transmission is significant when the delay-QoS requirements are loose, but the gain diminishes at tight delay requirements. It is also found that, in the two-phase protocol, the relay node should be placed closer to the source with more stringent delay requirement.
    IEEE Journal on Selected Areas in Communications 05/2012; · 3.12 Impact Factor
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    Jianhua Mo, Meixia Tao, Yuan Liu
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    ABSTRACT: This work studies the problem of secure connection in cooperative wireless communication with two relay strategies, decode-and-forward (DF) and randomize-and-forward (RF). The four-node scenario and cellular scenario are considered. For the typical four-node (source, destination, relay, and eavesdropper) scenario, we derive the optimal power allocation for the DF strategy and find that the RF strategy is always better than the DF to enhance secure connection. In cellular networks, we show that without relay, it is difficult to establish secure connections from the base station to the cell edge users. The effect of relay placement for the cell edge users is demonstrated by simulation. For both scenarios, we find that the benefit of relay transmission increases when path loss becomes severer.
    IEEE Communications Letters 04/2012; · 1.16 Impact Factor
  • Cen Lin, Yuan Liu, Meixia Tao
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    ABSTRACT: In this paper, we consider the cross-layer design for delay quality-of-service (QoS) provisioning in two-way relay systems. We aim to find the optimal resource allocation policy to maximize the weighted sum-rate while guaranteeing the statistical delay-QoS requirements for both users. The delay requirement is characterized as the QoS exponent. With the integration of the concept of effective capacity, the cross-layer optimization problem is equivalent to a weighted sum effective capacity maximization problem. We derive the optimal joint power and rate adaptation policy for the two-phase two-way relaying. Numerical results show that the proposed policy can efficiently support diverse QoS requirements and significantly improve the performance compared with both the fixed power scheme and the weight-based method.
    Communications (ICC), 2012 IEEE International Conference on; 01/2012
  • Yuan Liu, Meixia Tao
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    ABSTRACT: This paper considers a relay-assisted cooperative network where multiple relays assist the communication of multiple users using orthogonal frequency-division multiple-access (OFDMA). Our goal is to improve system performance by exploring full potential of the network in various dimensions, including user, relay, channel, and transmission mode. We formulate the joint optimization of subcarrier pairing, subcarrier assignment, relay selection, and transmission mode selection. We show that this combinatorial optimization problem can be transformed into a minimum cost network flow (MCNF) problem with integer solutions in graph theory. Then the linear optimal distribution (LOD) algorithm is applied to find the optimal solution in polynomial time. Simulations show that the proposed algorithm can significantly enhance the overall system throughput.
    Communications (ICC), 2012 IEEE International Conference on; 01/2012
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    Yuan Liu, Meixia Tao
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    ABSTRACT: Efficient utilization of radio resources in wireless networks is crucial and has been investigated extensively. This letter considers a wireless relay network where multiple user pairs conduct bidirectional communications via multiple relays based on orthogonal frequency-division multiplexing (OFDM) transmission. The joint optimization of channel and relay assignment, including subcarrier pairing, subcarrier allocation as well as relay selection, for total throughput maximization is formulated as a combinatorial optimization problem. Using a graph theoretical approach, we solve the problem optimally in polynomial time by transforming it into a maximum weighted bipartite matching (MWBM) problem. Simulation studies are carried out to evaluate the network total throughput versus transmit power per node and the number of relay nodes.
    IEEE Transactions on Communications 01/2012; 60(2):317-321. · 1.75 Impact Factor
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    Hao Zhang, Yuan Liu, Meixia Tao
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    ABSTRACT: This study considers an orthogonal frequency-division multiple-access (OFDMA)-based multi-user two-way relay network where multiple mobile stations (MSs) communicate with a common base station (BS) via multiple relay stations (RSs). We study the joint optimization problem of subcarrier-pairing based relay-power allocation, relay selection, and subcarrier assignment. The problem is formulated as a mixed integer programming problem. By using the dual method, we propose an efficient algorithm to solve the problem in an asymptotically optimal manner. Simulation results show that the proposed method can improve system performance significantly over the conventional methods.
    12/2011;
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    Yuan Liu, Meixia Tao
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    ABSTRACT: This paper considers a relay-assisted bidirectional cellular network where the base station (BS) communicates with each mobile station (MS) using orthogonal frequency-division multiple-access (OFDMA) for both uplink and downlink. We first introduce a novel three-time-slot time-division duplexing (TDD) transmission protocol. This protocol unifies the direct transmission, one-way relaying and network-coded two-way relaying between the BS and each MS. Using the proposed TDD protocol, we then propose an optimization framework for resource allocation to achieve the following gains: cooperative diversity gain (via relay selection), network coding gain (via bidirectional transmission mode selection), and multiuser diversity gain (via subcarrier assignment). We formulate the problem as an integer programming problem. By establishing its equivalence with a maximum weighted clique problem (MWCP) in graph theory, we show that the problem can be solved using an ant colony optimization (ACO) based metaheuristic algorithm in polynomial time. Simulation results demonstrate that the proposed protocol together with the ACO algorithm significantly enhances the system total throughput compared with conventional schemes.
    Global Telecommunications Conference (GLOBECOM 2010), 2010 IEEE; 01/2011
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    Proceedings of the Global Communications Conference, GLOBECOM 2011, 5-9 December 2011, Houston, Texas, USA; 01/2011
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    ABSTRACT: Radio resource allocation in multi-cell OFDMA (orthogonal frequency-division multiple access) networks is a challenging and practically important problem. In this work we propose a new graph-based sub-channel assignment scheme for downlink multi-cell OFDMA networks. Our scheme consists of two phases, both of which are formulated as graph problems. The first phase is location-aware interference management, including inter-cell interference coordination (ICIC) as well as intra-cell interference avoidance. The second phase is channel-aware sub-channel assignment. Simulation results demonstrate that the proposed scheme significantly enhances the system throughput while maintaining user-Quality of Service (QoS) compared with existing subchannel assignment schemes.
    Communication Technology and Application (ICCTA 2011), IET International Conference on; 01/2011