In wireless networks relay nodes can be used to assist the users' transmissions to reach their destination. Work on relay cooperation, from a physical layer perspective, has up to now yielded well-known results. This paper takes a different stance focusing on network-level cooperation. Extending previous results for a single relay, we investigate here the benefits from the deployment of a second one. We assume that the two relays do not generate packets of their own and the system employs random access to the medium; we further consider slotted time and that the users have saturated queues. We obtain analytical expressions for the arrival and service rates of the queues of the two relays and the stability conditions. We investigate a model of the system, in which the users are divided into clusters, each being served by one relay, and show its advantages in terms of aggregate and throughput per user. We quantify the above, analytically for the case of the collision channel and through simulations for the case of Multi-Packet Reception (MPR), and we provide insight on when the deployment of a second relay in the system can yield significant advantages.
We propose a buffer-aided relaying protocol for a three node relay network comprised of a source, a half-duplex relay with buffer, and a destination. We assume a direct source-destination link is available and all links undergo fading. The proposed protocol enables the half-duplex relay to choose its reception and transmission time slots adaptively and based on the quality of the involved links. We derive the achievable ergodic rate of the considered three-node network for the proposed protocol. Our results show that this achievable ergodic rate exceeds existing unachievable ergodic capacity upper bounds for the three-node half-duplex relay channel with the relay always alternating between reception and transmission in successive time slots. In this paper we consider a simple cooperative network consisting of a source, a destination and a cluster of decode-and-forward relays characterized by the half-duplex constraint. At each time-slot the source and (possibly) one of the relays transmit a packet to another relay and the destination, respectively. When the source and a relay transmit simultaneously, inter-relay interference is introduced at the receiving relay. In this work, with the aid of buffers at the relays, we mitigate the detrimental effect of inter-relay interference through either interference cancellation or mitigation. More specifically, we propose the min-power opportunistic relaying protocol that minimizes the total energy expenditure per time slot under an inter-relay interference cancellation scheme. The min-power relay-pair selection scheme, apart from minimizing the energy expenditure, also provides better throughput and lower outage probability than existing works in the literature. The performance of the proposed scheme is demonstrated via illustrative examples and simulations in terms of outage probability and average throughput. We consider a three-node relay network comprised of a source, a relay, and a destination, and compare the throughputs of ideal full-duplex (FD) and half-duplex (HD) relaying. For FD relaying, we assume that the relay uses half of its antennas for reception and the other half for transmission. In contrast, for HD relaying, the relay uses all of its antennas for either reception or transmission and a buffer for data storage. Our results show that if the numbers of antennas at source and destination are equal to or larger than the number of antennas at the relay, HD relaying may achieve a similar, and in some cases even higher, throughput compared to ideal FD relaying. Radio resource allocation for downlink transmissions in a cellular system based on orthogonal frequency-division multiple access (OFDMA) has been the subject of many research studies over the past few years. Nowadays, increasing attention has turned to cellular systems with relays, but only a few algorithms have been designed for OFDMA-based relay systems. In this paper, resource-allocation schemes are proposed for the cases of one relay station (RS) and multiple RSs in the cell. Due to the specific design of these algorithms, which operate in an RS-aided base station centralized manner, the amount of required channel-state information and algorithm complexity are minimized, making them suited for practical use. The idea of adaptive RS activation is introduced, where the frame structure is adapted depending on the active RS. Different numbers of relays are considered, and the corresponding algorithms are accordingly adapted. Simulation results show that our algorithms achieve a very good throughput outage tradeoff. In this paper, we study virtual full-duplex (FD) buffer-aided relaying to recover the multiplexing loss of half-duplex (HD) relaying in a network with multiple buffer-aided relays, each of which has multiple antennas, through opportunistic relay selection and beamforming. The main idea of virtual FD buffer-aided relaying is that a source and a relay simultaneously transmit their own information to another relay and a destination, respectively. In this network, inter-relay interference (IRI) is a crucial problem which has to be resolved like self-interference in the FD relaying. In contrast to previous work that neglected the IRI, we propose two buffer-aided relay selection and beam-forming schemes taking the IRI into consideration. Numerical results show that our proposed relay selection scheme with zero-forcing beamforming (ZFB)-based IRI cancellation approaches the average end-to-end capacity of IRI-free upper bound as the numbers of relays and antennas increase.