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Wireless transmission 59
7 to a binary 1. However, it is important to stay synchronized with the transmitter
of a signal. But what happens in case of multi-path propagation? Then
several paths with different delays exist between a transmitter and a receiver.
Additionally, the different paths may have different path losses. In this case,
using so-called rake receivers provides a possible solution. A rake receiver uses
n correlators for the n strongest paths. Each correlator is synchronized to the
transmitter plus the delay on that specific path. As soon as the receiver detects a
new path which is stronger than the currently weakest path, it assigns this new
path to the correlator with the weakest path. The output of the correlators are
then combined and fed into the decision unit. Rake receivers can even take
advantage of the multi-path propagation by combining the different paths in a
constructive way (Viterbi, 1995).
2.7.2 Frequency hopping spread spectrum
For frequency hopping spread spectrum (FHSS) systems, the total available
bandwidth is split into many channels of smaller bandwidth plus guard spaces
between the channels. Transmitter and receiver stay on one of these channels
for a certain time and then hop to another channel. This system implements
FDM and TDM. The pattern of channel usage is called the hopping sequence,
the time spend on a channel with a certain frequency is called the dwell time.
FHSS comes in two variants, slow and fast hopping (see Figure 2.38).
f
f3 f2 f1 f
f 3
f 2
f 1
0 1 0 1 1
t d
t b
t d
t
t
t
User data
Slow
hopping
(3 bits/hop)
Fast
hopping
(3 hops/bit)
Figure 2.38
Slow and fast
frequency hopping