11,361 research outputs found
Opportunistic Relaying in Wireless Networks
Relay networks having source-to-destination pairs and 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 is large and is fixed, it is shown that
the proposed scheme achieves a system throughput of bits/s/Hz. In
contrast, the information-theoretic upper bound of 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 , the
achievable throughput of the proposed scheme scales as .
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
Network-Level Performance Evaluation of a Two-Relay Cooperative Random Access Wireless System
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.Comment: Submitted for journal publicatio
STiCMAC: A MAC Protocol for Robust Space-Time Coding in Cooperative Wireless LANs
Relay-assisted cooperative wireless communication has been shown to have
significant performance gains over the legacy direct transmission scheme.
Compared with single relay based cooperation schemes, utilizing multiple relays
further improves the reliability and rate of transmissions. Distributed
space-time coding (DSTC), as one of the schemes to utilize multiple relays,
requires tight coordination between relays and does not perform well in a
distributed environment with mobility. In this paper, a cooperative medium
access control (MAC) layer protocol, called \emph{STiCMAC}, is designed to
allow multiple relays to transmit at the same time in an IEEE 802.11 network.
The transmission is based on a novel DSTC scheme called \emph{randomized
distributed space-time coding} (\emph{R-DSTC}), which requires minimum
coordination. Unlike conventional cooperation schemes that pick nodes with good
links, \emph{STiCMAC} picks a \emph{transmission mode} that could most improve
the end-to-end data rate. Any station that correctly receives from the source
can act as a relay and participate in forwarding. The MAC protocol is
implemented in a fully decentralized manner and is able to opportunistically
recruit relays on the fly, thus making it \emph{robust} to channel variations
and user mobility. Simulation results show that the network capacity and delay
performance are greatly improved, especially in a mobile environment.Comment: This paper is a revised version of a paper with the same name
submitted to IEEE Transaction on Wireless Communications. STiCMAC protocol
with RTS/CTS turned off is presented in the appendix of this draf
Coalition Formation Game for Cooperative Cognitive Radio Using Gibbs Sampling
This paper considers a cognitive radio network in which each secondary user
selects a primary user to assist in order to get a chance of accessing the
primary user channel. Thus, each group of secondary users assisting the same
primary user forms a coaltion. Within each coalition, sequential relaying is
employed, and a relay ordering algorithm is used to make use of the relays in
an efficient manner. It is required then to find the optimal sets of secondary
users assisting each primary user such that the sum of their rates is
maximized. The problem is formulated as a coalition formation game, and a Gibbs
Sampling based algorithm is used to find the optimal coalition structure.Comment: 7 pages, 2 figure
- …