Digital Signal Processing Reference
In-Depth Information
are very simple: mixing the spread sequence a second time with a replica of
the spreading code will yield the original information stream. The effect of
the despreader on a narrowband interferer is exactly the opposite: its energy is
spread out over a large bandwidth and can be removed by a simple low-pass
filter at the output of the despreader. In practice, things get a little bit more
complicated: the offset of the spreading sequence generated locally in the re-
ceiver must be aligned properly to the spreading sequence in the encoded data
stream. Failing to do so will result in a high-frequency noise signal, without
a single trace of the original information. Interestingly enough, this property
is at the same time the biggest disadvantage but also the biggest strength of
dsss-based systems.
This is best explained using the example of the gps system. A low-rate 50 bits/s
bit stream containing navigational information is spread using a 1,023-chip
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pseudo-random spreading code which is repeated every 1 ms. The carrier of
a gps signal
15
is bpsk modulated at a symbol rate of 1
.
023 Mbit/s which
results in a 2 MHz-wide null-to-null bandwidth of the gps spectrum. Such a
high symbol rate makes a gps signal extremely vulnerable to isi: the line-of-
sight (los) signal between the satellite and the gps receiver is easily disturbed
by indirect multipath reflections. Intersymbol interference emerges when the
reflected signal is delayed by one or more bit periods of the los signal. This
occurs when the surplus path length traversed by the reflected signal is in the
order of 300 m.
So how does the gps system defend itself against multipath interference? One
way or another, there must be a way for the receiver to
diversify
between the
signal-of-interest and delayed versions of the same los signal. The solution
lies in the characteristics of the spreading code used by the gps system. The
spreading code of a satellite is specially crafted so that it not only has a low
cross-correlation with the codes belonging to other satellites, but also a very
limited autocorrelation product. Only when the direct los signal of the satel-
lite is multiplied by a correctly aligned spreading code, the signal is despread
and the original 50 bits/s information stream becomes visible again. Delayed
versions of the los signal are misaligned with respect to the despreading code
in the receiver, as a result of which they are treated like any other unwanted
interferer: the energy remains distributed over the complete 2 MHz spectrum.
Dsss systems employ the time diversity of coded sequences to differentiate
between signals and multipath reflections of the same signal. In other words,
dsss gives each gps transmission some unique characteristics that allow it to
be identified among all other signals, including a time-delayed version of itself.
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Data bits of the high-rate code are called
chips
to distinguish them from the low-rate information bits.
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C/A-code, f
center
=
1
.
575 GHz.
