The next generation wireless ad hoc networks are evolving to accommodate a
variety of services, such as real-time or streaming video/audio, and the demand for high
data rates and quality of service is growing at a rapid pace. The bottleneck in
such networks is the wireless link, which is affected by the multipath, Doppler and
time-dispersive effects introduced by the wireless propagation. The
traditional approach of communication system design is based on isolated procedures,
while optimization is performed within the layers. However, such an isolated
approach usually results in suboptimal design for wireless systems because the wireless
channel is a time varying channel. Hence, in order to fully exploit the time varying
nature of the wireless channel and enhance the Quality of Service (QoS) performance
of wireless multimedia networks, adaptation techniques are needed not only at
each separate layer, but also across different layers, such as the combination of
Adaptive Modulation and Coding (AMC) at the physical layer and Automatic Repeat
Request (ARQ) at the data link layer.
Adapting transmission parameters to changing channel conditions bring
benefits. For example, a link adaptation technique compensates for the variation in
channel conditions. AMC is a typical link adaptation technique, widely adopted in
emerging broadband wireless access systems, for example, IEEE 802.16e Wireless Man,
WiMax and Third Generation Point-to-Point (3GPP) High Speed Downlink Packet
Access (HSDPA) (Alouini and Goldsmith, 2000; Biglieri et al., 2001; Doufexi et al., 2002;
and Liu, 2002). The common way of performing AMC is to dynamically change
the Modulation and Coding Set (MCS) transmission in subsequent frames based on
a channel quality report fed back by the receiver (Malkamaki and Leib, 2000; and
Liu, 2002) while maintaining a proper QoS requirement. AMC has been advocated at
the physical layer to enhance the channel utilization and throughput in future
high-speed wireless multimedia networks. |
