narrow guard band
The fundamental challenge of using adjacent channels is squeezing a
multitude of signals into a limited amount of spectrum. In order to
compensate for a very small band gap between channels, adjacent channel
combining centres around the concept of partially band limiting signals
and then applying pre-correction to the transmitters to compensate.
This situation is particularly severe for cases where the lower channel
is analogue. Figure
2 illustrates that there is only a very narrow guard band between
the second sound carrier of a lower analogue channel and the lower edge
of digital spectrum for the upper channel. The size of this guard band
depends upon system types, but is typically of the order of a couple
of hundred kHz; for example, between the PAL B stereo sound carrier
and the upper adjacent channel lower extreme carrier in a Coded Orthogonal
Frequency Division Multiplex (COFDM) system, the frequency separation
is just 170 kHz.
In order to overcome the potential system degradation resulting from
this narrow guard band, a practical solution widely implemented with
COFDM systems is to offset the digital channel by a positive frequency
of 100-150 kHz. The resulting encroachment into the vestigial sideband
of the upper analogue channel has been shown to have little effect,
provided other signal corrections are applied.