Walsh Codes

CDMA2000 introduces an increase in the number of Walsh codes, from 64
with IS-95 to a total of 256 with 3XRTT. As with IS-95, CDMA2000 utilizes
PN long codes for both the forward and reverse directions. However, in
CDMA2000, the introduction of variable-length Walsh codes is introduced
to accommodate fast-packet data rates.
The Walsh code chosen by the system is determined by the type of
reverse channel. The R-SCH also uses a reserve Walsh code. If only one
R-SCH is used, it utilizes a two- or four-chip Walsh code, but when the second
R-SCH is utilized, it uses a four- or eight-chip code. Therefore, in order to maintain or obtain the higher data rates on the F-SCH, the Walsh code
must be shorter in order to maintain the same spreading rate.
Table 7-10 shows the relationship between Walsh codes, the SR, the RC,
and, of course, the data rates. One very important issue or, rather, effect
with utilizing variable-length Walsh codes is that if a shorter Walsh code is
being used, then it precludes the use of the longer Walsh codes that are
derived from it.
Table 7-10 helps in establishing the relationship between which Walsh
code length, which is associated with a particular data rate.
Table 7-11, a simplified table, shows the maximum number of simultaneous
users for any data rate.
For an SR1 and RC1, a maximum number of users have individual
Walsh codes equating to 64, a familiar number from IS-95A.
Looking at Table 7-11, if we had a total of 12 RC1 and RC2 mobiles under
a sector, then one that would allow for three data users at 153.6K, 6 at 76.8
Kbps, 13 at 38.4 Kbps, 26 at 19.2 Kbps, or 104 at 9.6 Kbps. This relationship
between the number of simultaneous users for a cdma channel is depicted
in Table 7-12. Obviously, the negotiated mobile data rate complicates the
determination for the total throughput of traffic levels. The real issue behind this is the type of data that will be allowed to be transported over
the network, which has a direct impact on the available users.
It is important to note that the shorter Walsh codes inhibit the use of
longer Walsh codes because of the orthogonality required. Also, all channel
requests are allocated from the same Walsh code pool on a per-sector basis.
In addition, to achieve the higher data rate, not only is the Walsh code
implementation modified, but also the modulation scheme has been
changed.
Also, if there was a need for high-speed data for interactive video with
Phase 1 CDMA2000, the transport of 384 Kbps of data would not be feasible
with a SR1 as indicated in Table 7-11.