1,519 research outputs found
Multi-rate relaying for performance improvement in IEEE 802.11 WLANs
It is well known that the presence of nodes using a low data transmit rate has a disproportionate impact on the performance of an IEEE 802.11 WLAN. ORP is an opportunistic relay protocol that allows nodes to increase their effective transmit rate by replacing a low data rate transmission with a two-hop sequence of shorter range, higher data rate transmissions, using an intermediate node as a relay. ORP differs from existing protocols in discovering relays experimentally, by optimistically making frames available for relaying. Relays identify themselves as suitable relays by forwarding these frames. This approach has several advantages compared with previously proposed relay protocols: Most importantly, ORP does not rely on observations of received signal strength to infer the availability of relay nodes and transmit rates. We present analytic and simulation results showing that ORP improves the throughput by up to 40% in a saturated IEEE 802.11b network
Cooperative communication in wireless local area networks
The concept of cooperative communication has been proposed to improve link capacity, transmission reliability and network coverage in multiuser
wireless communication networks. Different from conventional point-to-point and point-to-multipoint communications, cooperative communication
allows multiple users or stations in a wireless network to coordinate
their packet transmissions and share each other’s resources, thus achieving
high performance gain and better service coverage.
According to the IEEE 802.11 standards, Wireless Local Area Networks
(WLANs) can support multiple transmission data rates, depending
on the instantaneous channel condition between a source station and
an Access Point (AP). In such a multi-rate WLAN, those low data-rate stations
will occupy the shared communication channel for a longer period
for transmitting a fixed-size packet to the AP, thus reducing the channel
efficiency and overall system performance.
This thesis addresses this challenging problem in multi-rate WLANs
by proposing two cooperative Medium Access Control (MAC) protocols,
namely Busy Tone based Cooperative MAC (BTAC) protocol and Cooperative
Access with Relay’s Data (CARD) protocol. Under BTAC, a low
data-rate sending station tries to identify and use a close-by intermediate
station as its relay to forward its data packets at higher data-rate to the AP through a two-hop path. In this way, BTAC can achieve cooperative
diversity gain in multi-rate WLANs. Furthermore, the proposed CARD
protocol enables a relay station to transmit its own data packets to the AP
immediately after forwarding its neighbour’s packets, thus minimising the
handshake procedure and overheads for sensing and reserving the common
channel. In doing so, CARD can achieve both cooperative diversity
gain and cooperative multiplexing gain. Both BTAC and CARD protocols
are backward compatible with the existing IEEE 802.11 standards.
New cross-layer mathematical models have been developed in this
thesis to study the performance of BTAC and CARD under different channel
conditions and for saturated and unsaturated traffic loads. Detailed simulation
platforms were developed and are discussed in this thesis. Extensive
simulation results validate the mathematical models developed and show
that BTAC and CARD protocols can significantly improve system throughput,
service delay, and energy efficiency for WLANs operating under realistic
communication scenarios
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