Erhan Yilmaz

Bio: Erhan Yilmaz is an academic researcher from Institut Eurécom. The author has contributed to research in topics: Relay & Relay channel. The author has an hindex of 7, co-authored 21 publications receiving 215 citations.

Multi-Pair Two-Way Relay Channel with Multiple Antenna Relay Station

Abstract: We consider a multi-pair two-way relay channel (TWRC) where the single-antenna mobile terminals (MT) on each pair seek to communicate, and can do so, via a common multiple antenna relay station (RS). In the multi-pair TWRC, the main bottleneck on system performance is the interference seen by each MT due to the other communicating MT pairs. In this paper, we try to tackle this problem in the spatial domain by using multiple antennas at the RS. Considering Amplify-and-Forward (AF) and Quantize-and-Forward (QF) relaying strategies, different transmit/receive beamforming schemes at the RS are proposed. We compare our proposed schemes to each other and to the Decode-and-Forward (DF) relaying strategy with achievable sum rate taken as a performance metric and show that in a wide range of signal-to-noise ratio (SNR) our schemes outperform the DF relaying strategy.

Effects of Transmit Beamforming on the Capacity of Multi-Hop MIMO Relay Channels

Abstract: In this paper we investigate multi-hop fading relay channels where the source, the destination and the multiple relay nodes are all equipped with multiple antennas. We study the ergodic capacities of multiple relay networks based on Amplify-and- Forward (AF) and Decode-and-Forward (DF) relaying modes. We examine multi-user beamforming (MU-BF), where each data stream is assumed to be matched to a specified relay node, based on the conventional eigen-mode transmission for both modes, and derive ergodic capacity expressions. We also examine the impact of the number of selected relay nodes on the network capacity both for modes of relaying. We show that by using MU-BF at the source node and a maximum number of relay nodes selected for cooperation, the network gains from multiplexing, while beyond that number of cooperating relay nodes, it only gains from relay selection.

Amplify-and-Forward Capacity with Transmit Beamforming for MIMO Multiple-Relay Channels

Abstract: In this paper, we consider a MIMO wireless relay network where communication between a source and a destination node is assisted by multiple relay nodes using the amplify-and-forward (AF) strategy. We consider a system where the source node multiplexes the information into parallel streams and uses disjoint subsets of relays to assist the transmission of each stream. Furthermore, we assume that the source and the destination have the same number of antennas and that each transmit antenna is virtually paired to a different destination antenna. We consider source node channel side information (CSI) availability of just the source-relay and source-destination links, and propose a source beamforming method based on maximizing the sum of signal-to-leakage ratios (SSLR) for all of the data streams. This method maximizes the sum of signal strengths at the set of relay nodes assigned to that stream while suppressing the interference to other relay nodes. Maximizing the SSLR reduces the propagation of interference terms in the AF strategy and results in significant performance gains.

Error exponents for backhaul-constrained parallel relay networks

Abstract: In this paper, we assess the random coding error exponents (EEs) corresponding to decode-and-forward (DF), compress-and-forward (CF) and quantize-and-forward (QF) relaying strategies for a parallel relay network (PRN), consisting of a single source and two relays. Moreover, through numerical analysis we show that the EEs achieved by using QF relaying along with non-Gaussian signaling (coded modulation, M-QAM) at the source and symbol-by-symbol uniform scalar quantizers (uSQs) at the relays is better than that achieved by DF and CF relaying strategies when the system is in the low signal-to-noise ratio (SNR) regime and the backhaul capacity is sufficient. This behavior is due to the structure of coded modulation, as opposed to Gaussian signaling, which leads to better EEs for simple relaying strategies compared to its more complex counterparts.

On the gains of fixed relays in cellular networks with intercell interference

Abstract: In this paper, we consider a cellular network assisted by fixed relay stations (RS), which are used by mobile stations (MS) to access the base station (BS) via a relaying strategy, namely Amplify-and-forward (AF) and Compress-and-forward (CF). We analyze the achievable sum-of-rates for uplink communications. It is assumed that mobile signals and relay signals are emitted on orthogonal bands, with the possibility of having a larger bandwidth (BW) on the relay-to-base links. Our key result is that with a relay bandwidth just twice that of the mobile's bandwidth, the system capacity approaches that of an ideal distributed antenna system (DAS), while the ideal DAS requires new backhaul links with very high capacity. Moreover, with the successive interference cancellation (SIC) decoder at the BS, it is shown that under certain conditions the fairness performance in terms of minimum user rates achieved by relay-assisted cellular systems is the same as that of an ideal DAS.

Network Beamforming Using Relays with Perfect Channel Information

Abstract: This paper is on beamforming in wireless relay networks with perfect channel information at relays, the receiver, and the transmitter if there is a direct link between the transmitter and receiver. It is assumed that every node in the network has its own power constraint. A two-step amplify-and-forward protocol is used, in which the transmitter and relays not only use match filters to form a beam at the receiver but also adaptively adjust their transmit powers according to the channel strength information. For a network with any number of relays and no direct link, the optimal power control is solved analytically. The complexity of finding the exact solution is linear in the number of relays. Our results show that the transmitter should always use its maximal power and the optimal power used at a relay is not a binary function. It can take any value between zero and its maximum transmit power. Also, this value depends on the quality of all other channels in addition to the relay's own channels. Despite this coupling fact, distributive strategies are proposed in which, with the aid of a low-rate broadcast from the receiver, a relay needs only its own channel information to implement the optimal power control. Simulated performance shows that network beamforming achieves the maximal diversity and outperforms other existing schemes. Then, beamforming in networks with a direct link are considered. We show that when the direct link exists during the first step only, the optimal power control is the same as that of networks with no direct link. For networks with a direct link during the second step, recursive numerical algorithms are proposed to solve the power control problem. Simulation shows that by adjusting the transmitter and relays' powers adaptively, network performance is significantly improved.

Network Beamforming Using Relays With Perfect Channel Information

Abstract: This paper deals with beamforming in wireless relay networks with perfect channel information at the relays, receiver, and transmitter if there is a direct link between the transmitter and receiver. It is assumed that every node in the network has its own power constraint. A two-step amplify-and-forward protocol is used, in which the transmitter and relays not only use match filters to form a beam at the receiver but also adaptively adjust their transmit powers according to the channel strength information. For networks with no direct link, an algorithm is proposed to analytically find the exact solution with linear (in network size) complexity. It is shown that the transmitter should always use its maximal power while the optimal power of a relay ca.n take any value between zero and its maxima. Also, this value depends on the quality of all other channels in addition to the relay's own. Despite this coupling fact, distributive strategies are proposed in which, with the aid of a low-rate receiver broadcast, a relay needs only its own channel information to implement the optimal power control. Then, beamforming in networks with a direct link is considered. When the direct link exists during the first step only, the optimal power control is the same as that of networks with no direct link. For networks with a direct link during the second step only and both steps, recursive numerical algorithms are proposed. Simulation shows that network beamforming achieves the maximal diversity order and outperforms other existing schemes.

Cooperative dual-hop relaying systems with beamforming over nakagami-m fading channels

Abstract: In this paper, we investigate the end-to-end performance of dual-hop relaying systems with beamforming over Nakagami-m fading channels. Our analysis considers semi-blind (fixed-gain) relays with single antennas, and source and destination nodes equipped with multiple antennas. Closed-form expressions for the outage probability (OP), moment generating function (MGF), and generalized moments of the end-to-end signal-to-noise ratio (SNR) are derived. The proposed expressions apply to general operating scenarios with distinct Nakagami-m fading parameters and average SNRs between the hops. The influence of the power imbalance, fading parameters, and antenna configurations on the overall system performance are analyzed and discussed through representative numerical examples. Furthermore, the exactness of our formulations is validated by means of Monte Carlo simulations.