Digital Signal Processing Reference
In-Depth Information
DSSS - 1.023 Mchip/s
BPSK
GPS navigational data, 50bit/s
<< START HERE
17 dBW
Carrier frequency
L1 - 1.575 GHz
L1 c/a generator
GPS satellite 2.xx
path loss ~ 180dB
approx. 50Hz
low-pass
filter
recovered GPS signal
L1 c/a copy
GPS device 2.xx
receiv
e
d sign
al
despre
a
d sign
a
l
interferer power
is spread out
GPS signal buried below noise floor
narrowband interferer
channel noise
2 MHz
f
c
= IF frequency
Figure 2.9.
The gps system uses direct sequence spread spectrum (dsss)to
extract the satellite signal from below the noise floor. The original
information can be recovered thanks to the large minimum free
distance in the subset of allowed code sequences.
Dsss is a channel coding technique where a low speed information bit stream
is mixed (multiplied/xor-ed) with a high-rate
spreading code
. This technique
is known as frequency spreading, since the resulting signal at the output of
the modulator has a much higher spectral footprint than the original infor-
mation stream. In the receiver, a 'despreader' is used to extract the original
low-frequency information from the spread data sequence (Figure 2.9).
In terms of signal-to-noise performance, the frequency spreading technique
performs no better than a simple bpsk modulated system: the probability of
error is only dependent on the integrated received bit-energy to awgn spec-
tral density (E
b
/N
0
) in the transmission band. However, dsss offers a limited
protection against accidental in-band spurious signals, because they are easily
removed by the despreader in the receiver. The basic principles of despreading
