Digital Signal Processing Reference
In-Depth Information
Chapter 2
MODULATION-AWARE ERROR CODING
In contrast to wireline networks, wireless communications are strictly resource-
limited in every imaginable way. First of all, the wireless channel is a shared
medium, which implies that its resources have to be split among multiple
concurrent users (whether intended or not) at the same time and place. From a
receiver's point of view, several unwanted signal components will be present
at the antenna terminal, often several orders of magnitude stronger than the
actual signal-of-interest. Even after aggressive filtering in the front-end of
the receiver, a considerable amount of interferer power may still reside in the
polished signal that is offered to the digital back-end. Furthermore, the trans-
mitter is allowed to inject only a limited amount of power in the channel.
All this, combined with conservative restrictions on bandwidth usage, make
the wireless medium one of the most hostile environments for transporting
information.
In the urge to get data as quick and as reliable as possible from one place to
another, the efficiency of available resources is often pushed as far as possi-
ble towards the theoretical limit predicted by the Shannon theorem [Sha48].
Unfortunately, Shannon does not give any information on the way information
should be attached to the channel, nor does it give any clue about the way to
extract it again at receiver side. Consequently, chances are very likely that a
transmission method which has proven its effectiveness in one application area
will only make suboptimal use of the available resources if ported to another
domain without further ado. This can be clarified by some simple examples.
For a start, it is important to recognize that both the analog front-end and the
digital signal processor of a wireless transceiver are both located at the physical
layer (phy, layer 1) of the osi model [Zim80]. The user of this phy layer, in
this case the data link layer (dll, layer 2), expects a certain quality of the link.
