Spread Spectrum Modulation
Application:
IS-95 uses Walsh words and PN sequences to spread the spectrum of its transmitted signals. Since spreading the signal power with Walsh codes is typically far from uniform over the bandwidth of the Walsh code words, PN spreading is used in conjunction with Walsh code spreading. The primary use of Walsh code words on the forward link is to uniquely identify the mobile user. The primary use of Walsh words on the reverse link is to implement the 64-ary orthogonal modulation scheme. The PN sequences spread the power of the signal more uniformly or more evenly over the available bandwidth. In this way, the available bandwidth is used more efficiently since each portion of the bandwidth is equally to transmit the signal.
Spread Spectrum Modulation
Example:
A bipolar ±1 volt symbol signal s(t) with a symbol rate of Rs symbols/sec has a bandwidth of about Rs Hz. Likewise a bipolar PN sequence PN(t) having a chip rate of Rc chips/sec has a bandwidth of about Rc Hz. Multiplying the symbol stream s(t) by PN(t) produces a signal which looks very similar to PN(t) and, in fact, has a bandwidth nearly equal to PN(t). However, the signals s(t), PN(t), and PN(t)s(t) all have unit power since the square of these signals is always one. Since PN(t)s(t) has the same power as s(t), but a bandwidth Rc/Rs times greater than the signal, the power spectral density of the transmitted signal PN(t)s(t) averages Rs/Rc times lower than the power spectral density of s(t). The power of s(t) is thinly spread over the large bandwidth of PN(t) and it is from this spreading that the modulation technique gets its name.
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