Welcome to the World of CDMA
Forward CDMA Channel
The FORWARD CDMA CHANNEL is the cell-to-mobile direction of communication. It carries traffic, a pilot signal, and overhead information. The pilot is a spread, but otherwise unmodulated DSSS signal. The pilot and overhead channels establish the system timing and station identity. The pilot channel also is used in the mobile-assisted handoff (MAHO) process as a signal strength reference.
The base station transmit frequency is 45 MHz above the mobile station transmit frequency in the cellular service (IS-95A), and 80 MHz above in the PCS service (ANSI J-STD-008). Permissible frequency assignments are on 30 kHz increments in cellular and 50 kHz in PCS. See Frequency Plans for further details.
Transmission ParametersThe IS-95A forward link currently supports a 9600 bps rate family in the three data-bearing channel types, as shown in the table. In all cases the FEC code rate is 1/2 and the PN rate is 1.2288 MHz. Note that 1.2288 MHz = 128*9600 bps.
The forward link consists of up to 64 logical channels (code channels). The channels are independent in that they carry different data streams, possibly at different rates, and are independently adjustable in amplitude (Cf. Reverse Channelization).
Coding and InterleavingThe figure shows the core processing that generates one forward code channel, rate set 1. Rate set 2 is identical except the coding rate is 3/4 rather than 1/2, yielding the same code symbol rate with 3/2 times the data rate.
The code channels, as transmitted, are mathematically orthogonal.
The orthogonality is established by covering the FEC code
symbols with one of a set of 64 so-called Walsh
functions. "Mutually orthogonal" means that
their cross correlations are small (ideally zero). Because
only whole periods of the Walsh functions occur in each code
symbol, the effect of the Walsh cover is to make the channels
completely separable in the receiver, at least in the absence
of multipath. The orthogonality not only means that there
is no co-mingling of channels, it means there is no interference
between users in the same cell, again in the absence of multipath.
This has a substantial beneficial effect on the forward link
SpreadingEach forward code channel is spread by the Short Code, which has I- and Q-components. The spreading is thus quadrature. That is, from a single binary-valued, covered, symbol stream, two binary sequences are generated by mod 2 addition of the short code PN sequences, as shown in the figure.
The two coded, covered, and spread streams are vector-modulated
on the RF carrier. The spreading modulation is thus QPSK,
superimposed on a BPSK code symbol stream.
There are three types of overhead channel in the forward link: pilot, sync, and paging. The pilot is required in every station.
The pilot channel is always code channel zero. It is both
a demodulation reference for the mobile receivers, and for
handoff level measurements, and thus must be present in every
station. It carries no information. It is pure short code,
with no additional cover or information content.
The sync channel carries a repeating message that identifies
the station, and the absolute phase of the pilot sequence.
The data rate is always 1200 bps. The interleaver period is
80/3 = 26.667 ms, equal to the period of the short code. This
simplifies finding frame boundaries, once the mobile has located
The paging channel is the vehicle for communicating with
mobile stations when they are not assigned to a traffic channel.
As the name implies, its primary purpose is to convey pages,
that is, notifications of incoming calls, to the mobile stations.
It carries the responses to mobile station accesses, both
page responses and unsolicited originations. Successful accesses
are normally followed by an assignment to a dedicated traffic
channel. Once on a traffic channel, signaling traffic between
base and mobile can continued interspersed with the user traffic.
Traffic channels are assigned dynamically, in response to
mobile station accesses, to specific mobile stations. The
mobile station is informed, via a paging channel message,
which code channel it is to receive (it is tempting, but inappropriate
to use the word "tune"!).
During soft handoffs each base station participating in the handoff transmits the same traffic over its assigned code channel. The code channel assignments are independent, and in general will be different in each cell. Whatever code channels are not in use for overhead channels are available, up to either a total of 64 or the available equipment limit, whichever is smaller.
Traffic channels carry variable rate traffic frames, either
1, 1/2, 1/4, or 1/8 of the maximum rate. In IS-95A only a
9600 bps rate family is currently available in the standard.
In J-STD-008 a second rate set, based on a maximum rate of
14,400 bps is available. The Rate Set 2 will be added in a
future revision of IS-95.
The 800 bps reverse link power control subchannel is carried on the traffic channel by puncturing 2 from every 24 symbols transmitted. The punctured symbols both carry the same power control bit, so they can be coherently combined by the receiver. Each base station participating in a soft handoff makes its own power control decision, independent of the others, unless they are different sectors of the same cell, in which case they all transmit a common decision. This special circumstance is made known to the mobile when the handoff is set up.
All base stations must be synchronized within a few microseconds for the station identification mechanisms to work reliably and without ambiguity. Any convenient mechanism can be used for this purpose, but the system was designed under the assumption that the Global Positioning System (GPS) would be used. This is a family of low-earth-orbit satellites that broadcast a spread-spectrum signal and ephemeris information from which a sophisticated Kalman filter algorithm in a receiver can derive both a very accurate position and a very accurate time.
Copyright © 1996-1999 Arthur H. M. Ross, Ph.D., Limited