Erhan Yilmaz

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

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

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

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.

Distributed and Centralized Architectures for Relay-Aided Cellular Systems

In this paper we propose a two-step distributed scheduling algorithm for relay-assisted cellular networks where a given user can be either served by the base station or by a relay, in a opportunistic way. Such a distributed approach allows a reduced feedback signaling with respect to the centralized case, especially when a simple scalar feedback is not sufficient for estimating the channel quality. As a result of the reduced feedback signaling requirements the system becomes more scalable, as new relays can be deployed where required without need of a careful network planning. We study the effectiveness of the proposal by means of a multicell simulator.

Error exponents for backhaul-constrained parallel relay networks

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

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 mobiles 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.

Some Systems Aspects Regarding Compressive Relaying with Wireless Infrastructure Links

In this paper, we consider single-cell cellular networks assisted with fixed relay station (RS), used by mobile stations (MS) to access the base station (BTS) via a relaying strategy. The RSs are positioned around the BTS, in such a way that wireless channels on the relay link (from RSs to the BTS) are line-of-sight, we analyze the achievable sum-of-rates for up-link communications. We compare two relaying strategies at the RSs, namely amplify-and-forward (AF) and compress-and-forward (CF). It is assumed that mobile signals and relay signals are emitted on orthogonal bands (FDD), with the possibility of having a larger bandwidth (BW) on the relay-to-base links. We predict the system gains bought by relays, in comparison with two other reference systems. One reference is an ideal relay-based system where the relays enjoy noiseless communications to the BTSs, i.e. a so-called distributed antenna system (DAS). The second reference is offered by a conventional cellular systems without relays, but same number of overall infrastructure antennas. In this paper, it is demonstrated the surprising result that with a relay bandwidth just twice that of the mobiles bandwidth, the system capacity approaches that of an ideal distributed antenna system, (while probably being much superior in practice in terms of ease of deployment and cost). The capacity gains of the relay-assisted network over a conventional network are also analyzed.

Parallel Relay Networks with Phase Fading

In this paper, we consider Gaussian parallel relay networks with phase fading where a source node wants to communicate with a destination node with the assistance of two intermediate relay nodes. For this scenario, outer bounds are derived and three achievable schemes are considered. As well as amplify-and-forward (AF) and decode-and-forward (DF) schemes, we also consider a scheme where the relays exploit block quantization and random binning, which we call BQRB relaying. We show that in the broadcast channel limited regime, where received powers at the relay nodes are very small, BQRB outperforms the other schemes with increasing multiple access channel quality. Moreover, it is seen that BQRB achievable rate performance tends to the rate achievable by a point-to-point single-input multiple-output system.

Hash-and-forward relaying for two-way relay channel

This paper considers a communication network comprised of two nodes, which have no mutual direct communication links, communicating two-way with the aid of a common relay node (RN), also known as separated two-way relay (TWR) channel. We first recall a cut-set outer bound for the set of rates in the context of this network topology assuming full-duplex transmission capabilities. Then, we derive a new achievable rate region based on hash-and-forward (HF) relaying where the RN does not attempt to decode but instead hashes its received signal, and show that under certain channel conditions it coincides with Shannons inner-bound for the two-way channel [1]. Moreover, for binary adder TWR channel with additive noise at the nodes and the RN we provide a detailed capacity achieving coding scheme based on structure codes.

Low-complexity multiple-relay strategies for improving uplink coverage in 4G wireless networks

In this work we present low-complexity coded-modulation strategies for distributed relaying in 4G wireless networks. The primary goal of these strategies is to improve coverage on the uplink while retaining high spectral efficiency through multiuser spatial-multiplexing using two or more relays between the users and the base station. We contrast layer 2 techniques based on full decoding at relay stations and simple compression-based (quantization) techniques with QAM alphabets. Mutual-information and error-exponent analysis clearly show the benefits of distributed quantization both in the high and medium spectral efficiency regions. We further present these results in the context of evolving LTE-Advanced standardization activities, primarily by suggesting adaptations to standardized coding and retransmission mechanisms for a multiple-relay system. 1