Adaptive Radio Bandwidth

At this point, it is worth summarizing what we mean by adaptive bandwidth or, more
specifically, adaptive radio bandwidth. We cover adaptive network bandwidth later in
this book. There are five stages at which we can influence bit rate and bit quality—and
hence application quality—are as follows:
 We can change the source coding rate and use processor overhead to preprocess
images and video content to make the content more robust and
resilient to channel errors. The source coding can be adaptive—responding to
the dynamic range of the information stream.
 We can adaptively change the channel coding that we add to the source coded
bit stream. For example, we can increase or decrease the interleaving depth, we
can choose half rate (2/1) or third rate (3/2) convolutional encoding—two bits
out for one bit in, or three bits out for every two bits in—or we can use turbo
coding.
 We can change modulation, going from GMSK to 8 PSK (in GSM EDGE) or
from QPSK to 8 PSK to 16 level QAM in CDMA2000/1XEV.
 We can provide adaptive bandwidth on demand by using CDMA multiplexing
(moving up or down, left or right on the OVSF code tree, or adding or subtracting
additional OVSF code streams).
 We can make our RF bandwidth adaptive by varying the power allocation to
each user or to each user’s channel stream/channel streams.
Even analog (1G) cellular handsets had adaptive bandwidth, in that fairly simple
power control was supported together with DTX (discontinuous reception). When you
didn’t speak, the RF power dropped out. In 2G, DTX is also available and used for
voice. For data, variable power is delivered by adding additional slots in addition to
the existing power control.
3G effectively brings together adaptive source coding, adaptive channel coding,
adaptive modulation, and adaptive multiplexing—in all of which, the RF channel
spacing stays constant:
 25 or 30 kHz for first-generation cellular
 30 kHz, 200 kHz, or 1.25 MHz for second-generation cellular
 1.25 or 5 MHz for third-generation cellular

In 4G cellular, we may also adaptively change the occupied RF bandwidth, as
shown in Table 4.13. If we use Orthogonal Frequency-Division Multiplexing (OFDM),
for example, we can increase the number of frequency carriers used. In digital TV systems
already in place, there is a choice of 2000 or 8000 carriers (2k or 8k systems). It is
possible that a similar approach will be taken for fourth-generation cellular. We can
thus show the progression over time, that is, how bandwidth has become more adaptive
over time.
Analog cellular handsets effectively had adaptive variable-rate encoding and adaptive
variable-rate modulation. Some would argue it has taken digital processing 20
years to catch up with analog processing!