Digital Signal Processing Reference
In-Depth Information
the Signal-to-Noise Ratio at the receiver, which is signaled to the transmitter using
the RTS/CTS mechanism [101]. Opportunistic Auto Rate (OAR) tries to exploit the
time when channel quality is high, by sending data packets back-to-back in these
conditions [102]. In recent years, rate adaptation has focused on cross-layer tech-
niques, loss-differentiation and more efficient feedback mechanisms [103-110].
7.2.1.2 Carrier Sense Threshold
Unlike the transmission rate, the flexibility of carrier sense threshold has been less
explored. However, even recent papers show analytically the importance of an opti-
mized carrier sense threshold for individual and system-wide throughput [46, 111,
112]. Here, we present some recent algorithms that try to optimize IEEE 802.11
networks by adapting the carrier sense threshold.
In [113], the authors show that the optimal choice of the CS threshold depends on
node density and propose a distributed algorithm for local CS threshold optimization
based on a local estimation of node density.
In [114], the authors use the carrier sense threshold to mitigate neighborhood
starvation effects in ad hoc networks. This is done by detecting neighborhood star-
vation and increasing the carrier sense threshold in these situations.
In [115], the authors present an adaptation algorithm based on loss differentia-
tion. Here, all stations measure the Packet-Error-Rate (PER). A linear adaptation
algorithm, based on the highest PER in the network, is used to converge to an opti-
mal value.
7.2.1.3 Transmit Power
Power control for IEEE 802.11 networks can target either throughput maximiza-
tion, full network connectivity or energy efficiency. As the algorithm presented in
this section targets throughput maximization, we will focus our overview on these
algorithms.
In [116], the authors propose a control channel in order to inform other nodes
about their interference tolerance. The RTS/CTS handshake is used to solve the
asymmetric starvation problem.
In [94] it is recommended that links should scale down the transmission power
until they see an increase of collisions. The goal is to decrease interference towards
neighbors, without significant impact on the individual throughput.
In [95], the hidden and exposed node problems are balanced out using power
control. The authors propose to decrease the transmission power of the link until
only the interference area is covered. We have shown that this approach need not be
optimal when considering neighbor location and neighborhood starvation [46].
