Distributed Multicell Beamforming Design Approaching Pareto Boundary with Max-Min Fairness

This paper addresses coordinated downlink beamforming optimization in multicell time-division duplex (TDD) systems where a small number of parameters are exchanged between cells but with no data sharing. With the goal to reach the point on the Pareto boundary with max-min rate fairness, we first develop a two-step centralized optimization algorithm to design the joint beamforming vectors. This algorithm can achieve a further sum-rate improvement over the max-min optimal performance, and is shown to guarantee max-min Pareto optimality for scenarios with two base stations (BSs) each serving a single user. To realize a distributed solution with limited intercell communication, we then propose an iterative algorithm by exploiting an approximate uplink-downlink duality, in which only a small number of positive scalars are shared between cells in each iteration. Simulation results show that the proposed distributed solution achieves a fairness rate performance close to the centralized algorithm while it has a better sum-rate performance, and demonstrates a better tradeoff between sum-rate and fairness than the Nash Bargaining solution especially at high signal-to-noise ratio.Comment: 8 figures. To Appear in IEEE Trans. Wireless Communications, 201

Inter-micro-operator interference protection in dynamic TDD system

Abstract. This thesis considers the problem of weighted sum-rate maximization (WSRM) for a system of micro-operators subject to inter-micro-operator interference constraints with dynamic time division duplexing. The WSRM problem is non-convex and non-deterministic polynomial hard. Furthermore, micro-operators require minimum coordination among themselves making the inter-micro-operator interference management very challenging. In this regard, we propose two decentralized precoder design algorithm based on over-the-air bi-directional signalling strategy. We first propose a precoder design algorithm by considering the equivalent weighted minimum mean-squared error minimization reformulation of the WSRM problem. Later we propose precoder design algorithm by considering the weighted sum mean-squared error reformulation. In both approaches, to reduce the huge signalling requirements in centralized design, we use alternating direction method of multipliers technique, wherein each downlink-operator base station and uplink-operator user determines only the relevant set of transmit precoders by exchanging minimal information among the coordinating base stations and user equipments. To minimize the coordination between the uplink-opeator users, we propose interference budget allocation scheme based on reference signal measurements from downlink-operator users. Numerical simulations are provided to compare the performance of proposed algorithms with and without the inter-micro-operator interference constraints

Precoder design for multi-antenna transmission in MU-MIMO with QoS requirements

Abstract. A multiple-input multiple-output (MIMO) interference broadcast channel (IBC) channel is considered. There are several base stations (BSs) transmitting useful information to their own users and unwanted interference to its neighboring BS users. Our main interest is to maximize the system throughput by designing transmit precoders with weighted sum rate maximization (WSRM) objective for a multi-user (MU)-MIMO transmission. In addition, we include the quality of service (QoS) requirement in terms of guaranteed minimum rate for the users in the system. Unfortunately, the problem considered is nonconvex and known to be non-deterministic polynomial (NP) hard. Therefore, to determine the transmit precoders, we first propose a centralized precoder design by considering two closely related approaches, namely, direct signal-to-interference-plus-noise-ratio (SINR) relaxation via sequential parametric convex approximation (SPCA), and mean squared error (MSE) reformulation. In both approaches, we adopt successive convex approximation (SCA) technique to solve the nonconvex optimization problem by solving a sequence of convex subproblems. Due to the huge signaling requirements in the centralized design, we propose two different distributed precoder designs, wherein each BS determines only the relevant set of transmit precoders by exchanging minimal information among the coordinating BSs. Initially, we consider designing precoders in a decentralized manner by using alternating directions method of multipliers (ADMM), wherein each BS relaxes inter-cell interference as an optimization variable by including it in the objective. Then, we also propose a distributed precoder design by solving the Karush-Kuhn-Tucker (KKT) expressions corresponding to the centralized problems. Numerical simulations are provided to compare different system configurations with QoS constraints for both centralized and distributed algorithms

Interference pricing mechanism for downlink multicell coordinated beamforming

We consider the downlink coordinated beamforming problem in a cellular network in which the base stations (BSs) are equipped with multiple antennas and each user is equipped with a single antenna. The BSs cooperate in sharing their local interference information, and they aim to maximize the sum-rate of the users in the network. A decentralized interference pricing beamforming (IPBF) algorithm is proposed to identify the coordinated beamformer, where a BS is penalized according to the interference it creates to its peers. We show that the decentralized pricing mechanism converges to an interference equilibrium, which is a KKT point of the sum-rate maximization problem. The proofs rely on the identification of rank-1 solutions of each BSs' interference-penalized rate maximization problem. Numerical results show that the proposed iterative mechanism reduces significantly the exchanged information with respect to other state-of-the-art beamforming algorithms with very little sum-rate loss. The version of the algorithm that limits the coordination to a cluster of base stations (IPBF-L) is shown to have very small sum-rate loss with respect to the full coordinated algorithm with much less backhaul information exchange.The work was partially supported by NSF grant CCF-1017982 and SICCNALS project (TEC2011-28219). The work of A. García was partially supported by NSF grant CCF-1017982. A. García-Armada’s work has been partially funded by research projects COMONSENS (CSD2008-00010) and GRE3N (TEC2011-29006-C03-02)Publicad

Analysis and Mitigation of Channel Non-Reciprocity in TDD MIMO Systems

The ever-growing demands for higher number of connected devices as well as higher data rates and more energy efficient wireless communications have necessitated the use of new technical solutions. One of the main enablers in this respect is Multiple-Input Multiple-Output (MIMO) systems in which transmitting and receiving sides are equipped with multiple antennas. Such systems need precise information of the MIMO radio channel available at the transmitter side to reach their full potential. Owing to the reciprocity of uplink and downlink channels in Time Division Duplexing (TDD) systems, Base Stations (BSs) may acquire the required channel state information for downlink transmission by processing the received uplink pilots. However, such reciprocity only applies to the physical propagation channels and does not take into consideration the so-called observable or effective uplink and downlink channels which also include the possible non-reciprocal behavior of the involved transceiver circuits and antenna systems. This thesis focuses on the channel non-reciprocity problem in TDD MIMO systems due to mismatches in Frequency Response (FR) and mutual coupling of transmitting and receiving chains of transceivers and associated antenna systems. The emphasis in the work and developments is placed on multi-user MIMO precoded downlink transmission. In this respect, the harmful impacts of channel non-reciprocity on the performance of such downlink transmission are analyzed. Additionally, non-reciprocity mitigation methods are developed seeking to reclaim TDD reciprocity and thus to avoid the involved performance degradations. Firstly, the focus is on the small-scale MIMO systems where BSs are equipped with relatively limited number of antennas, say in the order of 4 to 8. The provided analysis on Zero-Forcing (ZF) and eigen-based precoding schemes in single-cell scenario shows that both schemes experience considerable performance degradations in the presence of FR and mutual coupling mismatches. Whereas, in general, the system performance is more sensitive to i) non-reciprocity sources in the BS transceiver; and ii) mutual coupling mismatches. Then, assuming reasonably good antenna isolation, an Over-The-Air (OTA) pilot-based algorithm is proposed to efficiently mitigate the BS transceiver non-reciprocity. The numerical results indicate high accuracy in estimating the BS transceiver non- reciprocity parameters as well as considerable improvement in the performance of the system. In multi-cell scenario, both centralized and decentralized precoding approaches are covered while the focus is on the impacts of FR mismatches of UE transceivers. The how that there is severe degradation in the performance of decentralized precoding while centralized precoding is immune to such channel non-reciprocity impacts. Secondly, the so-called massive MIMO systems are considered in which the number of antennas in the BS side is increased with an order of magnitude or more. Based on the detailed developed signal models, closed-form analytical expressions are first provided for effective signal-to-interference-plus-noise ratios of both ZF and maximum ratio transmission precoding schemes. The analysis covers the joint impacts of channel non-reciprocity and imperfect uplink channel estimation and shows that while both precoding schemes suffer from channel non-reciprocity impacts, ZF is more sensitive to such non-idealities. Next, a concept and an algorithm are proposed, involving UE side measurements and processing, to be deployed in the UE side to efficiently estimate the level of BS transceiver non-reciprocity. This enables the UEs to inform the BS about the optimum time to perform channel non-reciprocity mitigation round and thus improves the spectral efficiency. Finally, in order to mitigate channel non-reciprocity in massive MIMO systems, an efficient iterative OTA pilot-based algorithm is proposed which estimates and mitigates transceiver non-reciprocity impacts in both BS and UE sides. Compared to the state-of-the-art methods, the simulation results indicate substantial improvements in system spectral efficiency when the proposed method is being used. Overall, the analyses provided in this thesis can be used as valuable tools to better understand practical TDD MIMO systems which can be very helpful in designing such systems. Furthermore, the channel non-reciprocity mitigation methods proposed in this thesis can be deployed in practical TDD MIMO syst channel reciprocity and thus significantly increase the spectral efficiency