Welcome to the World of CDMA
There are two different criteria that apply to the
reverse link spreading. When a mobile is engaged in user traffic, i.e.,
in a conversation, it is desirable that that mobile use a unique code
that is distinct from all others. A mobile-unique code, rather than
a base-station-associated code, facilitates handoff. With a mobile-unique
code, nothing needs to change about the mobile's modulation or coding
when handoff occurs.
Traffic Channel Spreading
Spreading is based on a single, universal 42-bit
Long Code LFSR sequence. In
a sense, the reverse spreading code borrows an idea from the
Forward Spreading. Rather than completely unique codes,
a single, universal 42-bit LFSR sequence is used.
This sequence is known as the Long
Code. Again, phase of the long code is used to
distinguish stations. All 242-1 possible phases
are available as logical addresses.
Short code is added to resolve ambiguities.Even though there are a large number of Long Code phases, without some modification, there is potential ambiguity between units that have similar LCMs. Timing errors in real systems will be of the order of a round trip delay to the farthest cell, in practice perhaps in excess of 100 microseconds. This would make groups of LCMs ambiguous if the Long Code were the only component to the spreading. A delay of X microseconds would be indistinguishable from a long code that was offset by X microseconds. For this reason the Long Code is modified by adding the Short Code. Each mobile, when it acquires a candidate base station, synchronizes its Short Code and its Long Code generators to System Time. The mobile applies its unique LCM to the long code generator, and modulo-2 adds the output, that is the unique-phase long code, to the universal short code. As in the Forward CDMA Channel, the spreading modulation is quadrature, so as to homogenize the phase of the interference. Again, both short code sequences are used. See Figure 1.
Astute readers might note that the addition of the Short
Code to the Long Code is not
without residual ambiguity. The combined "supersequence"
code is still periodic, but with a larger period.
The periods of the short and long codes are relatively prime,
so the period of the supersequence is the product of the original
periods: (242 -1)*215 = approximately
257, or about 3700 years. Any offset long code
sequence, added to the short code sequence, is some different
phase of the supersequence. Why is this not a problem? Although
the supersequence has period 257, we are only ever
using, at most, 242 -1 of the possible phases of
it. We have constructed the spreading sequence generator in
such a way that the smallest code phase separation between
users is one short code period. One is more than enough. The
mobile will have system time in error by at most a fraction
of one millisecond due to propagation delays. Although everyone
is using the same supersequence, their phases are so far apart
that they will never be confused by the base stations. This
is the contribution of the short code to the process: it changes
the time ambiguity from one chip displacement to 215
Each base station must support an access channel on at least one RF carrier of each sector. Each station is permitted, but not required, to support more than one access channel per sector. If more than one access channel is available then the one that a mobile uses is chosen through a Resource Hashing algorithm. The hashing ensures that the load on the available channels is statistically uniform.
A mobile attempting to gain access to a base station first identifies the base station by decoding the Sync Channel message and later acquires the information it needs to do system accesses by reading the Access Parameters Message on a paging channel. These message contains all information needed to construct the appropriate long code mask, including the identity of the base station and the number of access channels that it supports. See System Access for further details.
| Index | Topics | Glossary | Standards | Bibliography | Feedback |