Forward Link

·
**Walsh Codes**

In the forward link (the
leg from the tower to the handset), CDMA uses Walsh codes which are generated
by applying the Haddamard transform on 0 repeatedly.

H_{1} = [0]

H_{2n} = [H_{n}
H_{n} ; H_{n} 1-H_{n}]

The matrix will grow by powers
of 2. The first row is discarded because it will usually be a constant but all
the rows below it will be mutually orthogonal. That is why you only get N-1
different codes for N length codes.

For H_{4} the first
row (W_{0}) is [ 0 0 0 0]

W_{1} = [ 0 1 0 1 ]

W_{2} = [ 0 0 1 1 ]

W_{3} = [ 0 1 1 0 ]

Notice first that W_{1},
W_{2} and W_{3} are all orthogonal to each other. Second, note
that a shifted version of W_{2} is W_{3}. That is why Walsh
codes can only be used for the forward link. Although Walsh codes are perfectly
orthogonal to each other, shifted Walsh codes are not. That means that during
transmission, all the signals sent using a particular set of Walsh codes must
be in perfect sync. This is only possible from the tower to the handset since
the tower is sending all the outgoing signals and thus has no problem with
being in synchronization with itself. However, there is no effective way to
synchronize all of the handsets to the degree necessary, so Walsh codes can't
be used for the reverse link.

©2001 Kyle Bryson, Alison Chen, and Allen Wan