Design and Optimization of Distributed Multiuser Cooperative Wireless Networks

In this project, we focus on wireless multiuser networks and aim at designing novel cooperative communication protocols, developing corresponding transmission and signal processing techniques, and optimizing the network performance. In the first part, we consider coherent multiuser amplify-and-forward (AF) relaying, where a set of source-destination (S-D) terminal pairs communicate concurrently over the same physical channel and a set of AF relay nodes assist the communication in a half-duplex scheme. It is known that multiuser interference can be cancelled with sufficient spatial degrees of freedom, i.e. relays. When additional relays are introduced to the network, we show that further multiple- input multiple-output (MIMO) gains can be achieved in a distributed manner through efficient (complex) relay gain optimizations. We choose two objective functions to maximize: sum rate and minimum link rate (max-min fairness). Moreover, it is shown that distributed diversity can be attained at the destinations through either relay selection or max-min type relay gain optimizations. Such gain allocation schemes require global knowledge of the channel coefficients between all participating nodes, which in practice may well diminish the spatial multiplexing gain. Addressing this issue, we introduce a new distributed gradient based gain allocation scheme, which substantially reduces this overhead. Key to this is the proof, that the gradient of the destination signal-to- interference-plus-noise ratio can be calculated in a distributed manner based on local channel state information (CSI) at the relays and limited feedback from the destinations.

The two sine qua non assumptions for the efficiency of coherent multiuser relaying are perfect CSI knowledge per relay and globally phase-synchronous relays. We take a practical look on these two assumptions and assume that there are imperfections with both. After modeling the corresponding data mismatches within given uncertainty sets, we follow a worst-case framework and design semidefinite programming (SDP) based robust counterparts for max-min beamforming.

Lastly in order to have a bounded CSI dissemination overhead independent from the number of relays, we propose to partition the relays into multiple independent clusters. We consider either homogeneous relay clusters where each cluster independently manages the multiuser interference, or heterogeneous clusters where we establish a hierarchy in between different clusters each with a specific gain (array or spatial multiplexing) to achieve. We obtain effective diversity gain through cluster and time specific phase rotations in the former case, and through relay selection for clusters in the latter.

We shift our focus to decode-and-forward (DF) relaying in the second part of the project, and address both multiuser one- and two-way MIMO relaying. First we consider two MIMO terminals exchanging information via a single MIMO relay node. We extend the two-way protocol to multiple antenna equipped nodes with a primary focus on network coding based signal combination at the relay. In order to support unbalanced relay-to- terminal rates, we propose a modified network coding based approach, where we apply zero padding per symbol on the information sequence to be transmitted to the weak terminal, prior to XOR addition, and further we provide a priori decoding information to the corresponding terminal. Afterwards, we assume transmit CSI at the terminals/relay and design the transmit covariances that characterize the capacity boundaries of two- way relaying. We design SDP based covariance optimization schemes, each of which maximizes sum or minimum of the two terminal-to-terminal information rates. Moreover, we consider imperfect transmit CSI for the special case of single antenna terminals. Therein, we derive the worst-case broadcast capacity region, and further propose robust counterparts for the two optimization problems designed for perfect transmit CSI. Finally, we extend one- and two-way DF relaying to the case of simultaneous MIMO communication of multiple S-D pairs. The relay is equipped with sufficient multiple antennas to resolve the MIMO multiple access channel spatially in the uplink and also to manage the interference through the downlink. Through the broadcast phase of multiuser one-way relaying, we apply conventional zero-forcing beamforming based MIMO broadcasting techniques. However, for the two-way case, we propose a novel two-level terminal separation for broadcasting: bit-level and spatial-level. Consequently, we design a modular relay transmit covariance optimization scheme that aims at maximizing the sum rate of all terminal-to-terminal rates optimally over the two relaying phases.

Finally, we propose a novel method to exploit the bit-level SI so that asymmetric data rates can be transmitted in the BRC phase of two-way DF relaying systems when the network coding scheme is applied. Since the network coding scheme combines the data on the bit level, the major problem faced by the network coding scheme is how to transmit with asymmetric data rates to the user stations according to their individual link qualities in the BRC phase. In the proposed scheme, the weaker link receiver exploits the a priori bit information in each received data symbol, so that it only needs to decode on a subset of the signal constellation. Subject to the same bit error rate constraint, the weaker link receiver can decode at lower signal-to-noise ratio (SNR) compared to the stronger link.