A set of distributed non-cooperating relay antennas is known to be capable of orthogonalizing multiple source- destination pairs in space by coherent amplify-and-forward (AF) relaying techniques. Known relay gain allocation schemes for this setup require either global knowledge of the channel coefficients between all participating nodes or an excess number of relays. The required dissemination of the channel state information (CSI) introduces a major overhead, which in practice may well diminish the spatial multiplexing gain. We introduce a new distributed gradient based gain allocation scheme, which minimizes this overhead. Key to this is the proof, that the gradient of the destination signal-to-interference-plus-noise ratio (SINR) can be calculated in a distributed manner based on local CSI at the relays and very limited feedback from the destinations. In order to minimize the number of iterations in the distributed gradient algorithm we propose two distributed approaches to determine the optimal step size for each iteration. Finally we provide simulation results on the sum rate performance and identify the sequential opening of the spatial channels as a key factor that impacts speed of convergence.

In this paper, we propose multiuser multiple-input multiple-ouput (MIMO) one-and two-way relaying protocols. Several wireless MIMO node pairs are establishing either unidirectional or bidirectional transmission links via a single decode-and-forward MIMO relay without causing interference to each other. For both half-duplex two-phase relaying protocols, the relay separates messages of different node pairs spatially by using zero-forcing based methods, whereas, in order to cancel the interference within the bidirectional links, XOR preceding followed by self-interference cancellation is employed. Both sum rate and maxmin fairness optimizations, as well as quality-of-service assurance, are considered for the design of the precoders at the relay. The corresponding optimization algorithms for both one-and two-way relaying have been developed using semideflnite programming, which consider the intersection of multiple access and broadcast phases' rate regions.

Two-way relaying, which enables bidirectional simultaneous data transmission between two nodes, is an efficient means to reduce the spectral efficiency loss observed in conventional half-duplex relaying schemes. In this paper, we consider a multiuser cellular two-way relaying scenario with several mobile stations (MSs) at one end of the bidirectional link and a single base station (BS) serving all MSs at the other end. Both the BS and the MSs exchange private messages simultaneously via a single relay node, i.e., concurrent uplink and downlink, in only two time slots, independent of the number of MSs. In the downlink, while the relay separates different MSs spatially, e.g., using either zero-forcing beamforming or zero-forcing dirty paper coding, it benefits from XOR precoding followed by self-interference cancellation, in order to separate messages within a message pair to be exchanged between the BS and each MS. The corresponding sum rate optimization problem is solved with an iterative algorithm based on semidefinite programming.

A novel cluster-based coherent multi-user (MU) relaying system is proposed, where a number of source/destination pairs communicate concurrently over the same physical channel and a set of amplify-and-forward relays assist them. A secondary short-range wireless system with single/multi-hop unicast or broadcast ability is considered for the channel state information (CSI) dissemination. To reduce the CSI dissemination overhead, the relays are grouped in several independent clusters. Distributed diversity gain is attained at the destinations by applying phase rotations between clusters. The CSI dissemination is analyzed thoroughly, and compared with that of previous works on MU relaying. It is shown that for a given number of clusters and relays, the most uniform distribution of relays to clusters is optimum to minimize the CSI exchange traffic overhead and transmit energy consumption for CSI dissemination; besides it maximizes the average achievable rate for source/destination links. Furthermore, a new solution for a maxmin fairness based relay gain allocation is proposed and formulated as a semidefinite program.

We consider a wireless multiuser network where a number of source/destination pairs communicate concurrently over the same physical channel. A set of amplify-and-forward relay nodes assist the communication by multiplying their received signal with a complex gain factor before retransmission. We propose and compare different strategies for finding the gain factor for every relay terminal, motivated by the chosen figures of merit: average sum rate, diversity, and fairness. A new and practical definition of diversity is provided in the context of our system model. For each strategy we investigate the provided gains in terms of the figures of merit, and the trade-off between them. The existing schemes in the literature act as references to the proposed schemes.

Space-frequency coded (SFC) orthogonal frequency-division multiple access (OFDMA) is considered in the presence of multitone (MT) interference. Analytical expressions for the bit-error probability (BEP) are derived for OFDMA with SFC in a frequency-selective fading environment. It is shown that SFC increases the resistance of OFDMA against the MT interference and reduces BEP considerably