Aditya Umbu Tana Amah

Non-regenerative multi-way relaying with linear beamforming

In this paper, we introduce non-regenerative multi-way relaying. A half-duplex multi-antenna relay station (RS) assists multiple nodes which want to communicate to each other. Each node has a message and wants to decode the messages from all other nodes. The number of communication phases is equal to the number of nodes, N, such that when N =2, we have the well known two-way relaying. In the first phase, all nodes transmit simultaneously to the RS and in the following (N-1) phases the RS applies transceive beamforming and transmits to all nodes. The achievable sum rate for asymmetric traffic and symmetric traffic cases are derived for N-phase multi-way relaying. Three low complexity linear transceive beamformers based on Zero Forcing (ZF), Minimum Mean Square Error (MMSE) and Maximisation of Signal to Noise Ratio (MSNR) criteria are designed for N-phase multi-way relaying. From sum rate analysis, MMSE outperforms the other beamformers at the expense of using feedback channel to obtain the noise variance of the nodes. If interference cancellation is performed at all nodes, MSNR achieves the highest performance gain.

Beamforming-based physical layer network coding for non-regenerative multi-way relaying

We propose non-regenerative multi-way relaying where a half-duplex multi-antenna relay station (RS) assists multiple single-antenna nodes to communicate with each other. The required number of communication phases is equal to the number of the nodes, N. There are only one multiple-access phase, where the nodes transmit simultaneously to the RS, and broadcast (BC) phases. Two transmission methods for the BC phases are proposed, namely, multiplexing transmission and analog network coded transmission. The latter is a cooperation method between the RS and the nodes to manage the interference in the network. Assuming that perfect channel state information is available, the RS performs transceive beamforming to the received signals and transmits simultaneously to all nodes in each BC phase. We address the optimum transceive beamforming maximising the sum rate of non-regenerative multi-way relaying. Due to the nonconvexity of the optimization problem, we propose suboptimum but practical signal processing schemes. For multiplexing transmission, we propose suboptimum schemes based on zero forcing, minimising the mean square error, and maximising the signal to noise ratio. For analog network coded transmission, we propose suboptimum schemes based on matched filtering and semidefinite relaxation of maximising the minimum signal to noise ratio. It is shown that analog network coded transmission outperforms multiplexing transmission.

Regenerative Multi-Group Multi-Way Relaying

We consider regenerative multi-group multi-way (MGMW) relaying. A half-duplex regenerative multi-antenna relay station (RS) assists multiple communication groups. In each group, multiple half-duplex nodes communicate with each other. The number of communication phases is defined by the maximum number of nodes among all groups. In the first phase, i.e., the multiple-access (MAC) phase, all nodes simultaneously transmit to the RS, and the RS decodes the data streams of all nodes. In the following broadcast (BC) phases, the RS transmits to the nodes. We propose three BC strategies, i.e., unicasting, multicasting, and hybrid uni/multicasting. For the multicasting strategy, either superposition or exclusive or (xor ) network coding is applied. On the one hand, we propose a transmit beamforming algorithm minimizing the RSs transmit power while guaranteeing that each node receives with a rate equal to the rate received at the RS for each particular data stream. The transmit beamforming algorithm is designed by coupling the MAC and BC phases. On the other hand, for the case where the RSs transmit power is fixed, we design low-complexity transmit beamforming algorithms: matched filter, zero forcing, minimization of mean square error, and MGMW-aware transmit beamforming. It is shown that the multicasting strategy with xor network coding improves the achievable sum rate and requires the least transmit power compared with other strategies.

Non-Regenerative Multi-Antenna Multi-Group Multi-Way Relaying

We consider non-regenerative multi-group multi-way (MGMW) relaying. A half-duplex non-regenerative multi-antenna relay station (RS) assists multiple communication groups. In each group, multiple half-duplex nodes exchange messages. In our proposal, the required number of communication phases is equal to the maximum number of nodes among the groups. In the first phase, all nodes transmit simultaneously to the RS. Assuming perfect channel state information is available at the RS, in the following broadcast (BC) phases the RS applies transceive beamforming to its received signal and transmits simultaneously to all nodes. We propose three BC strategies for the BC phases: unicasting, multicasting and hybrid uni/multicasting. For the multicasting strategy, network coding is applied to maintain the same number of communication phases as for the other strategies. We address transceive beamforming maximising the sum rate of non-regenerative MGMW relaying. Due to the high complexity of finding the optimum transceive beamforming maximising the sum rate, we design generalised low complexity transceive beamforming algorithms for all BC strategies: matched filter, zero forcing, minimisation of mean square error and BC-strategy-aware transceive beamforming. It is shown that the sum rate performance of non-regenerative MGMW relaying depends both on the chosen BC strategies and the applied transceive beamforming at the RS.

Non-Regenerative Multi-Way Relaying: Space-Time Analog Network Coding and Repetition

In this letter, we investigate non-regenerative multi-way relaying when channel state information is not available at the multi-antenna relay station (RS). For stationary channels, space-time analog network coding (STANC) transmission is applied at the RS. For non-stationary channels, the RS applies repetition transmission. For both transmission schemes, in order to obtain the data streams of the other nodes, each node has to perform three receive processing operations successively: Zero Forcing (ZF) detection, back-propagated self-interference cancellation and joint detection. We assess the achievable sum rate performance of non-regenerative multi-way relaying for the proposed transmission schemes. It is shown that STANC with joint detection outperforms both benchmark transmissions, namely, ZF and Maximisation of Signal to Noise Ratio (MSNR) transceive beamforming with single stream decoding performed at each node. When the channels are non-stationary, the repetition transmission enables the multi-way communication and in medium to high SNR region, it outperforms MSNR.

A transceive strategy for regenerative multi-antenna multi-way relaying

In this paper, firstly, we introduce regenerative multi-way relaying. A half-duplex multi-antenna relay station (RS) assists multiple nodes which want to communicate to each other. Each node has a message and wants to decode the messages from all other nodes. The number of communication phases is equal to the number of nodes, N, such that when N = 2, we have the well known two-way relaying. In the first phase, multiple access (MAC) phase, all nodes transmit simultaneously to the RS and the RS decodes the data streams of all nodes. In the following (N - 1) broadcast (BC) phases, the RS transmits to each node the intended (N - 1) data streams from the other (N -1) nodes. Secondly, we propose a transceive strategy at the RS which couples the MAC and the BC phases. The strategy exploits the information about the achievable MAC signal to interference and noise ratio at the RS which enables the RS to transmit with the achievable MAC rate in each BC phase. As the strategy involves transmit beamforming at the RS, we intend to minimise the power at the RS. Since the power minimisation problem is NP-hard, we rewrite the problem into a semidefinite optimisation to solve it in reasonable time.

Multi-Group Multi-Way Relaying: When Analog Network Coding Finds Its Transceive Beamforming

In this paper, we introduce non-regenerative multi-group multi-way relaying. A half-duplex non-regenerative multi-antenna relay station (RS) assists multiple communication groups. In each group, there are multiple nodes that want to communicate with each other. Each node has a message and wants to decode the messages only from other nodes in its group. The required number of communication phases for each group is equal to the number of nodes in the group. In the first phase, all nodes transmit simultaneously to the RS and in the following phases the RS applies transceive beamforming and transmits to all nodes. We derive the achievable sum rate of non-regenerative multi-group multi-way relaying for asymmetric and symmetric traffic and for two different cases, namely, with and without analog network coding (ANC). We specially design an ANC-aware transceive beamforming for non-regenerative multi-group multi-way relaying. The ANC-aware transceive beamforming is designed using two criteria, namely, matched filter (MF) and semidefinite relaxation (SDR) of a maximisation of minimum signal to noise ratio problem. From sum rate analysis, it is shown that applying ANC at the RS improves the performance. Regarding the ANC-aware transceive beamforming, the ANC-SDR outperforms ANC-MF at the cost of computational complexity and additional signaling from the nodes to the RS.

On the trade-off between transmit and leakage power for rate optimal MIMO precoding

In this paper, we study the interdependency between leakage reduction and transmit power control for rate maximization in leakage based precoding (LBP) for a multi user multiple-input multiple-output (MU-MIMO) scenario. We propose a LBP scheme which provides the rate optimal precoding for each MIMO link under a joint leakage and transmit power constraint and derive an iterative closed form solution for it. Depending on the constraints and the strength of the leakage power, the leakage power and/or the transmit power needs to be adjusted. Any leakage level between the egoistic case, where the leakage is not reduced at all, and the altruistic case, where the leakage is minimized, can be achieved. This allows to optimize the network performance in a MU-MIMO setup by optimally choosing the trade-off between leakage power and transmit power constraints. Using numerical simulations, we show the interdependency between the leakage power and the transmit power and demonstrate the gain we can achieve with the optimal trade-off.

Opportunistic relaying for wireless multicasting

In this paper, we consider relay-aided wireless multicasting where one source transmits the same information to multiple destinations. We assume that there is a half-duplex regenerative relay station (RS) which may cooperatively assist the source. Our underlying problems are two-fold: when and how should the source and the RS cooperate to improve the multicast rate? Firstly, we show the required conditions for using the RS for wireless multicasting and propose opportunistic relaying strategies where the RS, when in use, forwards only partial information of the source. Thus, the proposed opportunistic relaying strategies lead to an efficient use of the RS. Secondly, we explain how we can maximise the multicast rate by optimising both the source transmission rate and the time sharing between the source and the RS. It is shown that the proposed opportunistic relaying strategies improve the multicast rate of one-hop (direct links only) wireless multicasting and they outperform other benchmarked relaying strategies.

On combining SDMA and B-IFDMA: Multi-user detection and channel estimation

In this paper, we combine two different multiple access, namely, Space Division Multiple Access (SDMA) and Block-Interleaved Division Multiple Access (B-IFDMA), for uplink transmission, and we name the combined multiple access SD/B-IFDMA. Since SDMA suffers from multiple access interference (MAI) and B-IFDMA from intersymbol interference (ISI), a multi-user detector that can counteract both MAI and ISI is needed for SD/B-IFDMA. We propose three low complexity linear multi-user detectors for SD/B-IFDMA, namely, Zero Forcing (ZF), Minimum Mean Square Error (MMSE) and Non-Iterative Constrained Least Squares (NICLS). The NICLS is a heuristic multi-user detector that tries to improve the performance of ZF without the necessity of having to estimate the noise variance. Additionally, we also address the channel estimation procedure for SD/B-IFDMA by applying orthogonal time multiplexing training and using Chu sequence as the training sequence. Two estimators are considered for channel estimation for SD/B-IFDMA, namely, frequency domain Least Squares (LS) and time domain low complexity Maximum Likelihood (lcML). From bit error performance, it is shown that the MMSE multiuser detector performs best followed by NICLS and ZF. For performance assessment of the channel estimators, ZF multiuser detection is used. It is shown that the lcML outperforms LS with the penalty of having higher computational complexity.