Xueying Hou

How do we design CoMP to achieve its promised potential

Coordinated multipoint, or CoMP, transmission has been recognized as a spectrally efficient technique for full frequency reuse cellular systems, in which base stations cooperate to reduce or eliminate intercell interference. However, there are still many obstacles before it can be put into practical use. In this article, we first discuss the features of CoMP systems and channels that are distinct from single-cell multi-antenna systems. We then give an overview of state-of-the-art approaches for coping with the factors that limit the potential of CoMP. A major issue is the acquisition of channel state information, which creates different challenges for TDD and FDD systems. Another set of challenges arises from the limited capacity available on the backhaul connections between the cooperating base stations. Both the fundamentals of possible solutions and their relations to cellular standards are discussed.

Quantization based on per-cell codebook in cooperative multi-cell systems

Cooperative base station transmission, which is also called coordinated multi-point (CoMP) transmission, can significantly enhance spectrum efficiency of future cellular systems. To gain the promised benefits of coherent CoMP transmission, limited feedback is necessary for conveying channel state information in frequency division duplex systems. In this paper, we address per-cell codebook based quantization for CoMP channels, which is flexible and scalable in practice. We first analyze the structure of channel direction information (CDI) of CoMP channel and its connection with per-cell channels, then provide an approximate method to re-construct the CoMP CDI. Exploiting the degrees of freedom provided by using the per-cell codebook, we present and compare several ways of generating global codebook and selecting per-cell codewords for CoMP channel quantization. Simulation results are given that validate our analysis.

Codebook Design and Selection for Multi-Cell Cooperative Transmission Limited Feedback Systems

Coherent multi-cell cooperative transmission, also referred to as coordinated multi-point transmission (CoMP), is a promising way to provide high spectral efficiency for universal frequency reuse cellular systems. To report the required channel information to the transmitter in frequency division duplexing systems, limited feedback techniques are often applied. Considering that the large scale fading gains of channels from multiple base stations (BSs) to one mobile station are different and the number of cooperative BSs may be dynamic, it is not flexible nor compatible to employ a large codebook for directly quantizing the CoMP channel. In this paper, per-cell codebook for separately quantizing local and cross channels are studied. We first optimize the bit allocation among per-cell codebooks, aiming at minimizing the average quantization error of the aggregated CoMP channel. A closed-form codebook size allocation method is proposed, which only depends on the large scale fading gains of per-cell channels. Considering that the optimal per-cell codeword selection for CoMP channel is of high complexity, we propose a serial codeword selection method, whose complexity is quite low but the performance approaches that of the optimal codeword selection. Simulation results validate our analysis and demonstrate an evident performance gain of our methods.

How much feedback overhead is required for base station cooperative transmission to outperform non-cooperative transmission?

Coherent base station (BS) cooperative transmission provides high spectrum efficiency for cellular systems when channel state information (CSI) is perfectly known at the BSs. For frequency division duplexing systems, the CSI is fed back with limited number of bits, which linearly increases with the number of cooperating BSs. In this paper, we study the impact of the quantized CSI on cooperative transmission when zero-forcing beamforming is used. By deriving an optimal bit allocation among local and cross channels according to the locations of users, we show the minimal feedback bits required by cooperative transmission for providing performance gain over non-cooperative transmission.

Impact of Nonorthogonal Training on Performance of Downlink Base Station Cooperative Transmission

Base station (BS) cooperative transmission is a promising technique to improve the spectral efficiency of cellular systems, and by using it, the channels become asymmetric in average gain. In this paper, we study the impact of the asymmetric channel gains on the performance of coherent cooperative transmission systems, when minimum mean square error (MMSE) and least square (LS) channel estimators are applied to jointly estimate the channel state information (CSI) under nonorthogonal training. We first derive an upper bound of rate loss caused by both channel estimation errors and CSI delay. We then analyze the mean square errors of the MMSE and LS estimators under both orthogonal and nonorthogonal training, which finally reveals the impact of different kinds of training on the precoding performance. It is shown that nonorthogonal training for the users in different cells leads to minor performance degradation for the MMSE channel-estimator-assisted downlink precoding. The performance degradation induced by channel estimation errors is almost independent of the users location. By contrast, the performance loss caused by the CSI delay is more severe for users located at the cell center than that for users located at the cell edge. Our analysis is verified via simulation results.

Training Resource Allocation for User-Centric Base Station Cooperation Networks

User-centric base station (BS) cooperative transmission strives to satisfy the quality of service of each user no matter where the user is located. The resulting user-dependent cooperative clusters are inevitably overlapped. To minimize the mean square error of channel estimation assisting user-centric downlink cooperative transmission, the training signals sent from the BSs in each cluster or from the users selecting the same BS in their clusters should be mutually orthogonal. In this paper, we study the orthogonal training resource-allocation problem for user-centric cooperative network aiming at minimizing the overall training overhead. We find the optimal solution through a graph-theoretic approach. To provide a feasible solution for large-scale networks, a low-complexity algorithm is then proposed. Simulation results show that the algorithm performs closely to the optimal solution, and both provide remarkably higher net throughput than the system with fixed clustering.

Adaptive interference alignment for femtocell networks

In this paper, we propose a spectral efficient transmission scheme for femtocell networks, which includes an adaptive subband partition method and an adaptive interference alignment transceiver. By introducing random frequency hopping to achieve the subband partition, we do not need a central coordinator to control the interference among the femtocells, and we can adjust the number of the interference in each subband. By employing adaptive interference alignment transceiver, the interference led by the subband collision can be suppressed. Simulation results show that the transmission scheme improves the system throughput significantly.

Cell-grouping based distributed beamforming and scheduling for multi-cell cooperative transmission

Base station cooperative transmission is an effective strategy to mitigate inter-cell interference. Centralized multi-cell transmission provides considerable performance gains but is impractical in large cellular systems, due to its prohibitive complexity and large amount of overhead. Dividing cells into small clusters enables practical channel acquisition and coordination within each cluster but still suffers from out-of-cluster interference. In this paper, we propose a dynamic cooperative framework for large cellular systems, which divides cells into groups such that neighboring cells belong to different groups. Based on the cell-grouping, a distributed scheduling strategy is proposed which can effectively coordinate the interference between cell-groups. With limited signalling among BSs and lower complexity, the cell-grouping based distributed scheduling and beamforming shows performance advantages over the fixed clustering based centralized scheduling and beamforming.

Low complexity channel estimation in TDD coordinated multi-point transmission systems

Coordinated multi-point transmission (CoMP) is a promising strategy to provide high spectral efficiency for cellular systems. To facilitate multicell precoding, downlink channel is estimated via uplink training in time division duplexing systems by exploiting channel reciprocity. Virtual subcarriers in practical orthogonal frequency division multiplexing (OFDM) systems degrade the channel estimation performance severely when discrete Fourier transform (DFT) based channel estimator is applied. Minimum mean square error (MMSE) channel estimator is able to provide superior performance, but at the cost of high complexity and more a priori information. In this paper, we propose a low complexity channel estimator for CoMP multi-antenna OFDM systems. We employ series expansion to approximate the matrix inversion in MMSE estimator as matrix multiplications. By exploiting the feature of frequency domain training sequences in prevalent systems, we show that the proposed estimator can be implemented by DFT. To reduce the required channel statistical information, we use the average channel gains instead of channel correlation matrix, which leads to minor performance loss in CoMP systems. Simulation results show that the proposed channel estimator performs closely to the MMSE estimator.

Feedback Overhead Analysis for Base Station Cooperative Transmission

In this paper, we analyze the feedback overhead of channel direction information for downlink coherent base station (BS) cooperative transmission. The per-cell codebooks are considered, which are of practice importance. Instead of analyzing the required number of bits for feedback to keep a constant rate loss, we analyze the required overhead to ensure a target average signal-to-interference-plus-noise ratio (SINR) of each user. To this end, we formulate an optimization problem of bit allocation among the codebooks for local and cross channels that minimizes the total number of bits under the constraint of the average SINR, and find the explicit expression of the solution. We proceed to study the impact of various system parameters and channel features on the overall feedback overhead. Analytical and simulation results reveal that the overhead scales linearly with the overall number of transmit antennas, but decreases with the grow of the cell-edge signal-to-noise ratio. Moreover, the overhead can be significantly reduced by exploiting the diverse performance requirements of the users and the heterogeneous channel features through allocating the number of bits for feedback among multiple users and multiple per-cell links. This provides useful insight for the feedback strategy design of BS cooperation systems.