Opportunistic Relaying in Wireless Networks

Relay networks having $n$ source-to-destination pairs and $m$ half-duplex relays, all operating in the same frequency band in the presence of block fading, are analyzed. This setup has attracted significant attention and several relaying protocols have been reported in the literature. However, most of the proposed solutions require either centrally coordinated scheduling or detailed channel state information (CSI) at the transmitter side. Here, an opportunistic relaying scheme is proposed, which alleviates these limitations. The scheme entails a two-hop communication protocol, in which sources communicate with destinations only through half-duplex relays. The key idea is to schedule at each hop only a subset of nodes that can benefit from \emph{multiuser diversity}. To select the source and destination nodes for each hop, it requires only CSI at receivers (relays for the first hop, and destination nodes for the second hop) and an integer-value CSI feedback to the transmitters. For the case when $n$ is large and $m$ is fixed, it is shown that the proposed scheme achieves a system throughput of $m/2$ bits/s/Hz. In contrast, the information-theoretic upper bound of $(m/2)\log \log n$ bits/s/Hz is achievable only with more demanding CSI assumptions and cooperation between the relays. Furthermore, it is shown that, under the condition that the product of block duration and system bandwidth scales faster than $\log n$, the achievable throughput of the proposed scheme scales as $\Theta ({\log n})$. Notably, this is proven to be the optimal throughput scaling even if centralized scheduling is allowed, thus proving the optimality of the proposed scheme in the scaling law sense.Comment: 17 pages, 8 figures, To appear in IEEE Transactions on Information Theor

Throughput Scaling of Wireless Networks With Random Connections

This work studies the throughput scaling laws of ad hoc wireless networks in the limit of a large number of nodes. A random connections model is assumed in which the channel connections between the nodes are drawn independently from a common distribution. Transmitting nodes are subject to an on-off strategy, and receiving nodes employ conventional single-user decoding. The following results are proven: 1) For a class of connection models with finite mean and variance, the throughput scaling is upper-bounded by $O(n^{1/3})$ for single-hop schemes, and $O(n^{1/2})$ for two-hop (and multihop) schemes. 2) The $\Theta (n^{1/2})$ throughput scaling is achievable for a specific connection model by a two-hop opportunistic relaying scheme, which employs full, but only local channel state information (CSI) at the receivers, and partial CSI at the transmitters. 3) By relaxing the constraints of finite mean and variance of the connection model, linear throughput scaling $\Theta (n)$ is achievable with Pareto-type fading models.Comment: 13 pages, 4 figures, To appear in IEEE Transactions on Information Theor

DMT Optimal Cooperative Protocols with Destination-Based Selection of the Best Relay

We design a cooperative protocol in the context of wireless mesh networks in order to increase the reliability of wireless links. Destination terminals ask for cooperation when they fail in decoding data frames transmitted by source terminals. In that case, each destination terminal D calls a specific relay terminal B with a signaling frame to help its transmission with source terminal S. To select appropriate relays, destination terminals maintain tables of relay terminals, one for each possible source address. These tables are constituted by passively overhearing ongoing transmissions. Hence, when cooperation is needed between S and D, and when a relay B is found by terminal D in the relay table associated with terminal S, the destination terminal sends a negative acknowledgment frame that contains the address of B. When the best relay B has successfully decoded the source message, it sends a copy of the data frame to D using a selective decode-andforward transmission scheme. The on-demand approach allows maximization of the spatial multiplexing gain and the cooperation of the best relay allows maximization of the spatial diversity order. Hence, the proposed protocol achieves optimal diversitymultiplexing trade-off performance. Moreover, this performance is achieved through a collision-free selection process

Link Quality Control Mechanism for Selective and Opportunistic AF Relaying in Cooperative ARQs: A MLSD Perspective

Incorporating relaying techniques into Automatic Repeat reQuest (ARQ) mechanisms gives a general impression of diversity and throughput enhancements. Allowing overhearing among multiple relays is also a known approach to increase the number of participating relays in ARQs. However, when opportunistic amplify-and-forward (AF) relaying is applied to cooperative ARQs, the system design becomes nontrivial and even involved. Based on outage analysis, the spatial and temporal diversities are first found sensitive to the received signal qualities of relays, and a link quality control mechanism is then developed to prescreen candidate relays in order to explore the diversity of cooperative ARQs with a selective and opportunistic AF (SOAF) relaying method. According to the analysis, the temporal and spatial diversities can be fully exploited if proper thresholds are set for each hop along the relaying routes. The SOAF relaying method is further examined from a packet delivery viewpoint. By the principle of the maximum likelihood sequence detection (MLSD), sufficient conditions on the link quality are established for the proposed SOAF-relaying-based ARQ scheme to attain its potential diversity order in the packet error rates (PERs) of MLSD. The conditions depend on the minimum codeword distance and the average signal-to-noise ratio (SNR). Furthermore, from a heuristic viewpoint, we also develop a threshold searching algorithm for the proposed SOAF relaying and link quality method to exploit both the diversity and the SNR gains in PER. The effectiveness of the proposed thresholding mechanism is verified via simulations with trellis codes.Comment: This paper has been withdrawn by the authors due to an improper proof for Theorem 2. To avoid a misleading understanding, we thus decide to withdraw this pape