3GPP2 Evolution

We have spent some time (some U.S. readers may say too much time) on the
IMT2000DS/W-CDMA air interface. It is time to review the parallel evolution of
CDMA2000/IS95. CDMA2000 is the term used to describe the air interface. IS2000
comprehends the air interface and network interfaces (interfaces to the IS41 network).
Table 3.9 shows how IS95A/B has evolved with the adoption of variable length Walsh
codes, use of QPSK on the downlink and HPSK on the uplink, more granular power
control, supplemental code channels (multiple per-user channels on the downlink and
uplink—one fundamental, up to seven supplemental), the option of multiple RF channels
within a 5 MHz channel spacing (3xRTT), and the option of higher-level modulation
(1xEV).
RC refers to radio configuration and specifies a set of data rates, spreading rates
(SR) and coding.
In practice, it seems unlikely that 3xRTT will be implemented, and most deployment
is presently focused on 1xEV using the present spreading rate of 1.288 Mcps as the
most logical forward-evolution path. It may also be that fixed-length Walsh codes are
used rather than variable length—variable data rates can be supported through adaptive
modulation. However, variable-length Walsh codes do remain as an option in the
standard.

The length of the Walsh code is varied—from 4 to 128 chips—to accommodate different
data rates. As the data rate increases, the symbol period gets shorter. The final
chip rate stays constant—that is, fewer Walsh code chips are accommodated within the
symbol period. If you re-order the code channels so that related code channels are adjacent
to each other, you will have reproduced the OVSF code tree used in W-CDMA! 103