Qimei Cui

An overview of cooperative communications

Cooperative communications have recently been migrated to one of state-of-the-art features of 3GPP LTE-Advanced (LTE-A). Cooperative communications fundamentally change the abstraction of a wireless link and offer significant potential advantages for wireless communication networks. This article visits cooperative techniques, such as relay, distributed antennas systems (DAS), multicell coordination, Group Cell, Coordinated Multiple Point transmission and reception (CoMP), those turn the traditional cellular system into a cooperative system. In particular, we propose several joint processing policies to demonstrate the Group Cell (CoMP) concept. Results of a trial network implementation and a performance evaluation of the concept are also reported.

Evolution of Limited-Feedback CoMP Systems from 4G to 5G: CoMP Features and Limited-Feedback Approaches

Coordinated multipoint (CoMP) transmission/reception has been adopted as a key technology to alleviate the intercell interference (ICI) in fourthgeneration (4G) mobile communication. This article reviews the state of the art in limited-feedback CoMP systems research. We first give an overview of recent related work in feedback-based coordinated transmission and then provide a performance analysis of typical feedback schemes and sensitivity to the channel state information (CSI) delay. To back up the discussion, we have validated the benefits of CoMP using field-test results from a CoMP trial network built at the Beijing University of Posts and Telecommunications (BUPT) campus. Furthermore, since mobile systems evolving to fifth generation (5G) have to bear a huge quantity of irregularly distributed wireless data traffic, cooperative communication will be more widely applied not only to address the ICI but also to improve the system resource utilization and data rate. We summarize the recent trend on 5G network architectures enabling the novel CoMP features, and we discuss the potential evolution directions of limited-feedback approaches motivated by CoMP development toward future 5G networks.

Energy-Efficient Distributed Data Storage for Wireless Sensor Networks Based on Compressed Sensing and Network Coding

Recently, distributed data storage (DDS) for Wireless Sensor Networks (WSNs) has attracted great attention, especially in catastrophic scenarios. Since power consumption is one of the most critical factors that affect the lifetime of WSNs, the energy efficiency of DDS in WSNs is investigated in this paper. Based on Compressed Sensing (CS) and network coding theories, we propose a Compressed Network Coding based Distributed data Storage (CNCDS) scheme by exploiting the correlation of sensor readings. The CNCDS scheme achieves high energy efficiency by reducing the total number of transmissions Nttot and receptions Nrtot during the data dissemination process. Theoretical analysis proves that the CNCDS scheme guarantees good CS recovery performance. In order to theoretically verify the efficiency of the CNCDS scheme, the expressions for Nttot and Nrtot are derived based on random geometric graphs (RGG) theory. Furthermore, based on the derived expressions, an adaptive CNCDS scheme is proposed to further reduce Nttot and Nrtot. Simulation results validate that, compared with the conventional ICStorage scheme, the proposed CNCDS scheme reduces Nttot, Nrtot, and the CS recovery mean squared error (MSE) by up to 55%, 74%, and 76% respectively. In addition, compared with the CNCDS scheme, the adaptive CNCDS scheme further reduces Nttot and Nrtot by up to 63% and 32% respectively.

Capacity analysis and optimal power allocation for coordinated transmission in MIMO-OFDM systems

Coordinated transmission in multicell coordination networks has attracted much attentions in recent years. In the paper, we investigate one open problem in the above area, which is concerned with how to solve the closed-form optimal power allocation scheme of coordinated transmission in downlink orthogonal frequencydivision multiplexing (OFDM) systems with multiple antenna configurations and individual power constraints. A frequency-selective fading multiple-input multiple-output (MIMO) channel is considered. We derive the closed-form solutions of sum-rate capacity and optimal power allocations for coordinated transmission under the diversity case and the multiplexing case by solving the constrained optimization problems. Numerical simulations are performed to verify the derived theoretical conclusions by means of comparing the proposed optimal power allocation (OPA) scheme with the other two typical power allocation schemes. One is the power allocation scheme combining traditional water-filling with equal power (WS-EF PA). The other is equal power allocation (EPA) scheme.

Optimal Energy-Efficient Relay Deployment for the Bidirectional Relay Transmission Schemes

Recently, the energy efficiency of a relay network has become a hot research topic in the wireless communication society. In this paper, we investigate the energy efficiency of three basic bidirectional relay transmission schemes [i.e., the four time-slot (4TS), three time-slot (3TS), and two time-slot (2TS) schemes] from the angle of relay deployment. Since a realistic power consumption model is very important in analyzing energy efficiency, and a power amplifier (PA) consumes up to 70% of the total power, we consider a realistic nonideal PA model. The derived closed-form expressions for the optimal relay deployment and the simulation results reveal the following important conclusions. First, it is possible to achieve the optimal energy efficiency and enlarge the cell coverage simultaneously in bad channel conditions, but it may be very challenging in good channel conditions. Second, under asymmetric traffic conditions, particularly when the downlink rate is larger than the uplink rate, all the aforementioned three schemes have almost the same optimal relay deployment, but the 2TS scheme has the highest energy efficiency when the spectral efficiency is large. Third, the relay node should be deployed closer to the base station with the nonideal PA than that with the ideal PA, and the optimal energy efficiency with the nonideal PA is much higher than that with the ideal PA. Moreover, the impact of small-scale fading depends on the value of path loss. To overcome the small-scale fading, the relay network needs to consume more energy.

Optimal Joint Water-Filling for Coordinated Transmission over Frequency-Selective Fading Channels

We consider a novel power allocation scheme for a frequency-selective fading channel with two coordinated transmission points (CTP) transmission. In order to maximize the channels throughput, closed form solutions are obtained by solving a non-convex constrained optimization problem. The solution which we name it joint water-filling (Jo-WF) turns out to take the form of traditional WF and also combined with regular cooperative feature. Numerical results are presented to verify the optimality of the derived scheme and to show throughput gains over traditional non-coordinated WF and equal power allocation (EPA).

Joint User Association and Spectrum Allocation for Small Cell Networks With Wireless Backhauls

Heterogenous networks and massive MIMO can significantly enhance spectral and energy efficiencies of wireless networks and have recently attracted lots of attention. In this letter, we investigate joint user association and spectrum allocation to further improve spectral efficiency for massive MIMO enabled heterogeneous networks with full-duplex wireless backhauls. The optimization problem is formulated as a mixed-integer nonlinear programming problem, and therefore, is nonconvex and combinational. To solve the problem efficiently, the original optimization problem is divided into two subproblems that can be solved with low-computational-complexity methods. The simulation results confirm our theoretical analysis and demonstrate the performance improvement of the proposed algorithm.

A unified protocol stack solution for LTE and WLAN in future mobile converged networks

The interworking of the LTE system and WLAN technologies has drawn much attention lately, due to the growing demands for various multimedia services and large data traffic in hotspot areas. Existing research studies have mostly investigated the coupling architectures for these two wireless communication standards at the network layer. However, in the current architectures, many important coordination functions and joint optimizations cannot be accomplished efficiently. To tackle this problem, a new CBS solution is proposed, which integrates different RATs at layer 2 in the true sense of convergence. We design a unified protocol stack that includes all the original functions of both LTE and WLAN systems. Then we propose a convergence architecture, the RMC sublayer, for joint management of these two RATs. The proposed CBS solution can support seamless offloading through soft handover, guaranteed QoS, forwarding management by a single IP address, and customized bandwidth aggregation service. Finally, our simulation and initial experiment results demonstrate the feasibility and efficiency of the CBS solution in future mobile converged networks.

Closed-form solution for minimizing power consumption in coordinated transmissions

The growth in the demand of energy, and its consequent contribution to the greenhouse effect, gives rise to new challenges in the design of future wireless networks. Keeping in mind these requirements, in this article we study the power allocation problem in the downlink of an orthogonal frequency division multiple access (OFDMA) system, where two (or more) coordinated transmission points (CTPs) should find the best way to allocate their transmit power through the multiple orthogonal sub-channels of the system. The ultimate goal of the power allocation scheme is to minimize the joint power consumption of the system, but verifying at the same time the target throughput and the individual power constraint per CTP. The power allocation problem is formulated as a constrained optimization problem, and a group of closed-form power allocation solutions are derived. Based on the derived solutions (that take the form of the traditional water-filling but demanding cooperation among CTPs), a novel power allocation algorithm with joint minimization power consumption (JMPC-PA) is proposed. Numerical results are presented to verify the optimality of the results that were obtained by the JMPC-PA scheme. It is important to note that, due to the flexibility that exist in the definition of CTPs in this article, the derived power allocation scheme is valid for any kind of network that incorporates the coordinated multipoint transmission feature in its design.

A Novel Location Model for 4G Mobile Communication Networks

A novel location model for 4G mobile communication networks is proposed in the paper. The model utilizes the more precise relative distance estimated by short-range signals between the destination mobile terminal to be located (abbr. DT) and the other mobile terminals near DT to be the reference mobile terminal assisting DT (abbr. RT) to improve the location accuracy of DT. Besides, Triangle Similarity Theorem is firstly introduced into the model to search the desired optimal /suboptimal triangle. It is shown that by analysis and simulation that more accurate estimated position for DT in stationary status can be achieved based on the model above.