Digital Signal Processing Reference
In-Depth Information
the proposed technique, the objective is to use the speech rate, i.e. the speech
packet size, most suitable for any given 802.11 channel condition.
The GSM-AMR speech coder is a multi-rate ACELP coder with 8 modes op-
erating at bit-rates from 12.2 kbps to 4.75 kbps. The coder modes are integrated
in a common structure, where the bit-rate scalability is obtained by adjusting
the quantization schemes for the different parameters. The frame size is 20 ms,
consisting of 4 sub-frames of 5 ms each.
3. ADAPTIVE REAL-TIME MULTIMEDIA
TRANSMISSION
The time-varying nature of wireless channels as well as network congestions
may significantly degrade the quality of speech communications. Adaptive
transmission techniques are designed to match the time-varying nature of the
wireless channel, thus typically delivering the desired level of QoS for real-time
multimedia more effectively than non-adaptive schemes (see, e.g., [12]).
Although non-adaptive schemes tend to be simpler and potentially more ro-
bust, they are optimal only for the operating point used for their design: their
performance quickly decreases when the scenario worsens and they also cannot
exploit better conditions when available. Marginal channel conditions are quite
common in real systems: these are encountered, for example, just prior to hand-
off or during deep shadowing, as when a mobile station suddenly goes behind a
building or a hill.
As the condition of the radio channel varies with both the location and mobil-
ity of the terminal, it is therefore desirable to employ an adaptive solution pro-
viding the user with maximum quality for any given channel condition. Adap-
tive solutions have been proposed in the literature for different layers of the
network infrastructure and the next section will present a brief survey to the
reader.
3.1 Adaptive Techniques for 802.11
Previous works presented several techniques —located at different network
layers— to increase the quality of service of wireless networks by means of
channel or traffic adaptation.
The IEEE 802.11b physical (PHY) layer provides four PHY rates from 1 to
11 Mbps at the 2.4 Ghz band. Link adaptation mechanisms have been discussed
so that the proper PHY rate can be adaptively selected to combat the variation
of the wireless medium condition, hence improving the goodput performance of
a WLAN [14]. In fact the higher the PHY rate, the shorter the transmission time
