Rakash SivaSiva Ganesan

Interference Alignment in Multi-User Two Way Relay Networks

In this paper, we consider a bidirectional communication between K node pairs, where each node is equipped with multiple antennas and has a message to be transmitted to its communication partner. There is no direct link between the 2K nodes and a relay is employed to enable the communication. It is assumed that the relay does not have enough antennas to perform receive and transmit zero forcing. As interferences are unavoidable at each receiver, we call this channel K user pair symmetric relay interference channel. We show that the Degrees of Freedom (DoF) achievable by the multiple input multiple output (MIMO) Interference Alignment in such a K user pair symmetric relay interference channel are the same as in a K user symmetric interference channel [1] without a relay. Besides this, the presence of a relay simplifies the algorithm for computing the alignment solution. We propose a two step closed form solution for the alignment problem based on the two way relaying protocol.

Pair-Aware Interference Alignment in Multi-User Two-Way Relay Networks

In this paper, K bidirectionally communicating node pairs with each node having N antennas and one amplify and forward relay having R antennas are considered. Each node wants to transmit d data streams to its communication partner. Taking into account that each node can perform self interference cancellation, a new scheme called Pair-Aware Interference Alignment is proposed. In this scheme, the transmit precoding matrices and the relay processing matrix are chosen in such a way that at any given receiver all the interfering signals except the self interference are within the interference subspace and the useful signal is in a subspace linearly independent of the interference subspace. If the number of variables is larger than or equal to the number of constraints in the system, the system is classified as proper, else as improper. Through simulations it is shown that for a proper system (2Kd≤2N+R-d), interferences can be perfectly aligned and the useful signals can be decoded interference-free at the receivers. An iterative algorithm to achieve the interference alignment solution is proposed. Also for the proper system fulfilling a certain additional condition, which will be derived in this paper, a closed form solution is proposed.

Interference alignment using a MIMO relay and partially-adapted transmit/receive filters

Interference alignment is proposed for achieving the maximum degrees of freedom in interference channels. In the present paper, a scenario consisting of several pairs of multiple antenna nodes and a single MIMO relay helping the interference alignment is considered. The interference alignment is performed in two subsequent transmission phases assuming a time-invariant channel during the two transmission phases. Firstly, the relay and the destination nodes receive the signals from all the source nodes, i.e., also the direct links between the communicating node pairs are exploited. In the second transmission phase, both the source nodes and the relay retransmit the signals to the destination nodes. By adapting the relays linear signal processing and partially adapting both the transmit and receive filters to the channel, a closed form solution for interference alignment is obtained. The performance of the proposed scheme is investigated by simulations. The results show that the proposed scheme achieves the maximum degrees of freedom and outperforms conventional relaying schemes at high SNRs.

Cooperative zero forcing in multi-pair multi-relay networks

In this paper, unidirectional communication between K half-duplex node pairs is considered. The source nodes have N antennas each and the destination nodes have M antennas each. There is no direct link between the source and the destination nodes. Q half-duplex relays, each with R antennas, assist in the communication. It is assumed that the relays do not have enough antennas to spatially separate the data streams and hence, transceive zero forcing cannot be performed at the relays. In this paper, we propose a scheme in which the source nodes and the relays cooperate in choosing their precoding matrices and the filter coefficients, respectively, to perform a cooperative zero forcing. A closed form solution is proposed and the feasibility condition is derived. Simulation results show that the proposed cooperative zero forcing scheme achieves more degrees of freedom and hence, achieves higher sum rate as compared to reference schemes.

Projection based space-frequency interference alignment in a multi-carrier multi-user two-way relay network

We consider a multi-carrier multi-user two-way relay network with multiple single antenna node pairs and a single non-regenerative relay with two antennas. In a conventional multi-user two-way relaying scheme where different node pairs transmit their data streams on different subcarriers, only one data stream per channel use can be achieved. Taking into account that the nodes can perform self interference cancellation, one antenna at the relay is sufficient to achieve one data stream per channel use. Hence, the two antennas at the relay are not fully utilized. In this paper, using the interference alignment technique in space-frequency dimensions, it is shown that when the number of users in the system is large, almost two data streams per channel use are achievable. The gain comes from the joint consideration of the spatial and frequency dimensions at the relay. A new projection based interference alignment algorithm is proposed. The ideas of signal alignment and channel alignment [1] are used to achieve the interference alignment solution. Furthermore, for the performance improvement of the proposed interference alignment scheme at moderate Signal to Noise Ratios (SNRs), a new gradient based method for optimization of the projection space at the relay is proposed.

Integrating 3D Channel Model and Grid of Beams for 5G mMIMO System Level Simulations

Massive MIMO (mMIMO) antenna array and Grid of Beams (GoB) are one of the key components for 5G to achieve high spectral efficiency and coverage. System level simulations are necessary to optimally intergrate the components of 5G system. Simulations with mMIMO antenna array involve handling large matrices, which results in increased simulation time typically upto several days depending on the size of the antenna array. However, GoB are fixed wideband beams and the receiver sees only the effective channel which is a combination of the channel and the GoB. Therefore, for system level simulations it is sufficient to general only the effective channel. In this paper, we propose a simulation model to directly generate the effective channel based on the 3GPP 3D channel model. It is shown that for the simulation of urban macro cells scenario specified in TR 38.913, the computational effort can be reduced by a factor of 1000. Furthermore, simulations show that very high SINR and user throughput values are achievable through GoB.

Maximizing the Sum Rate in Cellular Networks Using Multiconvex Optimization

In this paper, we propose a novel algorithm to maximize the sum-rate in interference-limited scenarios where each user decodes its own message with the presence of unknown interferences and noise. The problem of adapting the transmit and receive filters of the users to maximize the sum-rate with a transmit power constraint is nonconvex. Our novel approach is to formulate the sum-rate maximization problem as an equivalent multiconvex optimization problem by adding two sets of auxiliary variables. An iterative algorithm, which alternatingly adjusts the system variables and the auxiliary variables is proposed to solve the multiconvex optimization problem and we show that the algorithm converges to a stationary point. The proposed algorithm is applied to a downlink cellular scenario consisting of several cells each of which contains a base station serving several mobile stations. We examine the two cases, with or without several half-duplex amplify-and-forward relays assisting the transmission. A sum power constraint at the base stations and at the relays are assumed. The applicability of our approach to the individual power constraints case is also shown. Finally, we show that the proposed multiconvex formulation of the sum-rate maximization problem is applicable to many other wireless systems in which the estimated data symbols are multiaffine functions of the system variables.

Relay-aided interference alignment for multiple partially connected subnetworks

In this paper, large partially connected wireless relay interference networks are considered. The entire network is made up of multiple disjoint groups of source-destination node pairs and relays called subnetworks. Each subnetwork is assumed to be fully connected. The different subnetworks are mutually connected by a few inter-subnetwork links. A two-hop relay-aided interference alignment scheme is applied to achieve interference-free communication in the whole network. The feasibility conditions for interference alignment in the considered networks are investigated. To this end, we extend the concept of external constraints, which was proposed in [1] for the two-subnetwork case, and characterize the proper relay antenna configurations in large networks with an arbitrary number of subnetworks.

Combining interference alignment and two-way relaying in partially connected networks with only local CSI

In this paper, a network consisting of several partially connected subnetworks where each subnetwork includes one relay is considered. “Partially connected” means that not all nodes are connected to all relays. Some nodes may be connected to multiple relays. The entire bidirectional pair-wise communication between the nodes takes place via the intermediate half-duplex amplify-and-forward relays, considering two-way relaying. The algorithm proposed in this paper is a closed-form solution which requires only local channel state information (CSI) to achieve interference alignment. The properness condition for the proposed algorithm is derived. The process of interference alignment is decomposed into signal alignment, channel alignment and transceive zero forcing. It is shown that all subnetworks can be investigated separately. This means the whole problem can be divided into subproblems. The simulation results show that the degrees of freedom increase for the considered partially connected network in comparison with the fully connected version of the considered network.

Multi-group multi-way relaying with reduced number of relay antennas

In this paper, multi-group multi-way relaying is considered. There are L groups with K nodes in each group. Each node wants to share d data streams with all the other nodes in its group. A single MIMO relay assists the communications. The relay does not have enough antennas to spatially separate the data streams. However, the relay assists in performing interference alignment at the receivers. In order to find the interference alignment solution, we generalize the concept of signal and channel alignment developed for the MIMO Y channel and the two-way relay channel to group signal alignment and group channel alignment. In comparison to conventional multi-group multi-way relaying schemes [1, 2], where at least R ≥ LKd - d antennas are required, in our proposed scheme, exploiting the multiple antennas at the nodes, only R ≥ LKd - Ld antennas are needed. The number of antennas required at the nodes to achieve this is also derived. It is shown that the proposed interference alignment based scheme achieves more degrees of freedom than the reference schemes without interference alignment.