Sungyoon Cho

Feedback-Topology Designs for Interference Alignment in MIMO Interference Channels

Interference alignment (IA) is a joint-transmission technique for the interference channel that achieves the maximum degrees-of-freedom and provides linear scaling of the capacity with the number of users for high signal-to-noise ratios (SNRs). Most prior work on IA is based on the impractical assumption that perfect and global channel-state information (CSI) is available at all transmitters. However, to implement IA, each receiver has to feed back CSI to all interferers, resulting in overwhelming feedback overhead. In particular, the sum feedback rate of each receiver scales quadratically with the number of users even if the feedback CSI is quantized. To substantially suppress feedback overhead, this paper focuses on designing efficient arrangements of feedback links, called feedback topologies, under the IA constraint. For the multiple-input multiple-output (MIMO) K-user interference channel, we propose the feedback topology that supports sequential CSI exchange (feedback and feedforward) between transmitters and receivers so as to achieve IA progressively. This feedback topology is shown to reduce the network feedback overhead from a quadratic function of K to a linear one. To reduce the delay in the sequential CSI exchange, an alternative feedback topology is designed for supporting two-hop feedback via a control station, which also achieves the linear feedback scaling with K. Next, given the proposed feedback topologies, the feedback-bit allocation algorithm is designed for allocating feedback bits by each receiver to different feedback links so as to regulate the residual interference caused by finite-rate feedback. Simulation results demonstrate that the proposed bit allocation leads to significant throughput gains especially in strong interference environments.

Interference Alignment for Uplink Cellular Systems with Limited Feedback

Assuming perfect channel state information, the existing interference alignment (IA) algorithm proposed in [2] suppresses inter-cell interference (ICI) by aligning ICI to a randomly selected reference vector. However, IA in practice relies on limited feedback, resulting in residual ICI. In this letter, we propose the optimization of the reference vector for regulating the residual ICI. Specifically, it is shown that the reference vector that minimizes an upper bound on the residual ICI power is the eigenvector corresponding to the largest eigenvalue of the sum of the interference-channel matrices multiplied by their corresponding Hermitian matrices. Moreover, the performance gain of the proposed IA algorithm compared with the existing one in [2] is analyzed and demonstrated by simulation to be significant.

Efficient Feedback Design for Interference Alignment in MIMO Interference Channel

Interference alignment (IA) is a joint-transmission technique that achieves the capacity of the interference channel for high signal-to-noise ratios (SNRs). However, most prior works on IA are based on the impractical assumption that perfect and global channel-state information(CSI) is available at all transmitters, resulting in overwhelming feedback overhead. To substantially suppress the feedback overhead, this paper proposes an efficient design of the feedback framework for IA in the K-user multiple-input multiple-output (MIMO) interference channel. The proposed feedback topology supports sequential CSI exchange (feedback and feedforward) between transmitters and receivers and reduces the feedback overhead from a cubic function of K to a linear one, compared to conventional feedback approaches. Given the proposed feedback topology, we consider the limited feedback channel from the receivers to corresponding interferers and analyze the effect of quantization error which generates the residual interference. Also, an efficient feedback-bit allocation algorithm that minimizes the upper-bound of sum residual interference is proposed.

Interference alignment with range expansion in a heterogeneous MIMO network

Managing smaller cells such as micro and picocells in conventional macro cellular networks is expected to provide better quality of data service. However, frequency reuse-1 operation of smaller cells with the macrocells can create additional “cell edge” area that can degrade the system performance significantly. This paper proposes an interference avoidance scheme based on interference alignment and region based zero-forcing in a heterogeneous cellular system consisting of two overlaying macro and picocells with multiple antennas. The proposed transmission strategy effectively suppresses the cross-tier interference at both macro and pico users who are in severe interference limited region, providing the benefits of coverage expansion of picoocells. Simulation results show that the proposed interference avoidance scheme provides a significant performance gain over the non-coordinated scheme in the two macro/picocells network.

How many degrees of freedom can be achieved for mutually interfering MIMO broadcast channels

In this paper, we consider homogeneous mutually interfering multiple-input multiple-output broadcast channels(MI-MIMO-BC). It is investigated that maximally achievable spatial degrees of freedom(DoF) for MI-MIMO-BC with constant channel coefficients. Assuming that there are two base station(BS) M transmit antennas, and each mobile stations(MS) has N receive antennas, it is shown that maximally min(M + N − 1, 2M) DoF can be achieved. We formulate zero intra- and inter-cell interference equations as function of the precoders and decoders. By checking solvability of these equations as multivariate polynomial equations, an achievable DoF upper bound can be obtained. To achieve the DoF upper bound, at least M + N − 1 MSs in each cell when M + N is an even number or equation MSs when M + N is an odd number are required. Our analysis is verified by numerical results.

Mobility-aware spatial interference cancellation for mobile ad hoc networks

Interference limits the throughput of a mobile ad hoc network (MANET). Multi-antennas can be employed at a node for interference cancellation besides attaining array gain. Spatial interference cancellation requires each node to estimate the interference channels, which is potentially inaccurate due to node mobility. As a result, maximizing the network capacity requires optimally allocating spatial degrees of freedom (DoF) to cancel interference and enhance link reliability based on the node mobility. This paper addresses this issue for a MANET with Poisson distributed transmitters and employing zero-forcing beam for spatial interference cancellation. Specifically, the residual interference power for each node from partially canceled interferers is characterized as a function of Doppler frequency and the number of DoF for interference cancellation is shown to decrease with the Doppler frequency. The adaption of spatial interference cancellation to mobility is observed to significantly improve the network performance in terms of both outage probability and capacity compared with the case without adaptation.

An efficient feedback architecture design for three-user MIMO interference channels

In this paper, an efficient feedback structure is proposed to reduce the feedback overhead of three-user multiple input multiple output interference channels (MIMO-IFC). We present a sequential and iterative feedback structure based on interference cancellation condition. Compared to the previous feedback structure for interference alignment (IA) in K-user MIMO-IFC [1], the proposed method does not need a cooperation between receivers. Even though the proposed scheme inherently requires the infinite number of iterations to remove the residual interference clearly, almost all of the residual interference is eliminated by the low number of iterations enough that its total feedback overhead is dramatically reduced compared to the conventional approach [2]. Furthermore, in order to improve throughput performance, we propose a concatenated receive filter for maximizing the effective SINR.

Precoder design with non-uniform power allocation for multimode precoded MIMO schemes with limited feedback

This paper investigates the problem of designing a codebook with non-uniform power allocation for multimode precoded MIMO schemes (e.g., spatial multiplexing and STBC) with limited feedback in fading channels. The generalized Lloyd algorithm is employed for the design, and two methods with different complexities are addressed for the computation of the centroid which is formulated as an optimization problem. Numerical results show that the proposed design outperforms comparable algorithms which equally allocate the total transmit power to each data stream.

A Low Complexity scheduling of Antenna and User in Multiuser MIMO/FDD Uplink System

This paper investigates the problem of scheduling of antenna and user for the uplink MIMO/FDD system. Upper bound for sum capacity is found via considering all combinations of antenna and user. We suggest two different scheduling algorithms such as max-SNR and max-SINR that schedule user and antenna based on the instantaneous SNR and SINR of each antenna stream. The performance of sum capacity with those proposed schedulers approaches upper bound while providing the reduction of scheduling complexity.