Radio Bandwidth Quality/Time Domain Issues
We mentioned channel coding briefly in Chapter 1. 3G cellular handsets and Node Bs
use many of the same channel coding techniques as 2G cellular—for example, block
coding and convolutional coding. We showed how additional coding gain could be
achieved by increasing the constraint length of a convolutional decoder. This was
demonstrated to yield typically a 1/2 dB or 1 dB gain, but at the expense of additional
decoder complexity, including processor overhead and processor delay.
In GPRS, adaptive coding has been, and is being, implemented to respond to
changes in signal strength as a user moves away from a base station. This has a rather
unfortunate side effect of increasing a user’s file size as he or she moves away from the
base station. At time of writing only CS1 and CS2 are implemented.
We also described interleaving in Chapter 1 and pointed out that increasing the
interleaving depth increased the coding gain but at the cost of additional fixed delay
(between 10 and 80 ms). Interleaving has the benefit of distributing bit errors, which
means that convolutional decoders produce cleaner coding gain and do not cause error
extension. If interleaving delay is allowable, additional coding gain can be achieved by
using turbo coding.
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