Once the Walsh code
is assigned, the digital data are first filtered digitally, then quadrature phase-shift keying (QPSK) modulated and analog summed in such a fashion as to combine other channels being downlinked to the mobile.
Specifications for the Zebra source include a 1.2288 Mbps chip rate, an IS-95 forward link pilot channel and 16 orthogonal channels with 0 to 63 selectable Walsh codes
. An internal temperature-compensated crystal oscillator (TCXO) is incorporated with a stability of 2.5 ppm from 10 [degrees] to 50 [degrees] C.
OVSF codes are orthogonal codes similar to the Walsh codes
used in 2G CDMA (IS-95) BTS.
Unique codes for each mobile are created on the forward channel by combining the voice data with 1 of 64 user Walsh codes
and then combining that with a sector unique offset of the PN (pseudo-noise) short code (see the Figure).
Besides all above three receivers with CQS, the result of traditional PIC-MMSE-WMRC (Walsh Codes) receiver is marked as the solid curve with squares.
We also notice that the PIC-MMSE-PMRC (CQS) outperforms over the PIC-MMSE-WMRC (Walsh Codes).
In all the cases of both Figures, the PIC-MMSE-WMRC (Walsh codes) has the worst performance, which shows that the CQS plays a key role to help the system improve the channel tracking ability.
This is a great improvement compared to the system applying Walsh Codes, which need to cancel N-1 users in the worst case.