Modulation and Demodulation
Application: IS-95 uses modulation as defined above to convey information in the form of Binary Phase Shift Keying (BPSK) on the forward link. On the reverse link, 64-ary orthogonal modulation using Walsh words are employed which convey 6 coded symbols per Walsh word. In both cases, the changes or perturbations to the carrier are in the form of phase modulation. That is a Walsh word w(t) whose binary chips have a value of +1 or -1 in effect multiply a sinusoidal carrier of the form Acos2pfct yielding a signal of the form Aw(t)cos2pfct.
The -1 values of the Walsh word in effect shift the phase of the sinusoid by 180 degrees or p radians since -Acos2pfct equals Acos(2pfct+p) while the Walsh chips with value +1 do not shift the phase.
On the forward link, the mobile determines the data bit by determining if all the Walsh chips in the received Walsh word have either all been inverted (-1 bit) or all not inverted (+1 bit) relative to the chip values of the Walsh word assigned to it. On the reverse link, the pattern of the ±1 values of the transmitted Walsh word is recognized by the base station Fast Walsh Trans-form (FWT), which compares the received ±1 pattern with the pattern of each of the possible 64 Walsh words .
Modulation and Demodulation
Example: As an illustration, assume the Walsh words used in a communication system have only 8 Walsh chips. For example, +1-1+1-1+1-1+1-1 equals w1(t). Then when the coded bit or symbol in the message transmitted to the mobile which has been assigned Walsh word w1(t) is +1, the transmitted signal before PN spreading equals +w1(t) or +1-1+1-1+1-1+1-1. When the coded bit or symbol is -1, the transmitted signal before PN spreading equals -w1(t) or -1+1-1+1-1+1-1+1. All the Walsh chips of the assigned Walsh word are either inverted or not inverted according to the coded bit or symbol value.
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