IS-95 CDMA

Although they have significant differences, both IS-
136 and GSM use Time Division Multiple Access (TDMA). This means that
individual radio channels are divided into timeslots, enabling a number of
users to share a single RF channel on a time-sharing basis. For several reasons,
this technique offers an increase in capacity compared to an analog
system where each radio channel is dedicated to a single conversation.
TDMA is not the only system that enables multiple users to share a given
radio frequency, however. A number of other options exist—most notably
Code Division Multiple Access (CDMA).
CDMA is a technique whereby all users share the same frequency at the
same time. Obviously, since all users share the same frequency simultaneously,
they all interfere with each other. The challenge is to pick out the signal
of one user from all of the other signals on the same frequency. This can
be done if the signal from each user is modulated with a unique code
sequence, where the code bit rate is far higher than the bit rate of the information
being sent. At the receiving end, knowledge of the code sequence
being used for a given signal allows the signal to be extracted.
Although CDMA had been considered for commercial mobile communications
services by several bodies, it was never considered a viable technology
until 1989 when a CDMA system was demonstrated by Qualcomm in
San Diego, California. At the time, great claims were made about the potential
capacity improvement compared to AMPS, as well as the potential
improved voice quality and simplified system planning. Many people were
impressed with these claims and the Qualcomm CDMA system was standardized
as IS-95 in 1993 by the U.S. Telecommunications Industry Association (TIA). Since then, many IS-95 CDMA systems have been deployed,
particularly in North America and Korea. Although some of the initial
claims regarding capacity improvements were perhaps a little overstated,
IS-95 CDMA is certainly a significant improvement over AMPS and has
had significant success. In North America, IS-95 CDMA has been deployed
in the 800-MHz band and a variation known as J-STD-008 has been
deployed in the 1900-MHz band.
CDMA is unique to wireless mobility in that it spreads the energy of the
RF carrier as a direct function of the chip rate that the system operates at.
The CDMA system utilizing the Qualcomm technology utilizes a chip rate
of 1.228 MHz. The chip rate is the rate at which the initial data stream, the
original information, is encoded and then modulated. The chip rate is the
data rate output of the PN generator of the CDMA system. A chip is simply
a portion of the initial data or message that is encoded through use of a
XOR process.
The receiving system also must despread the signal utilizing the exact
same PN code sent through an XOR gate that the transmitter utilized in
order to properly decode the initial signal. If the PN generator utilized by
the receiver is different or is not in synchronization with the transmitter’s
PN generator, then the information being transmitted will never be properly
received and will be unintelligible. Figure 1-1 represents a series of
data that is encoded, transmitted, and then decoded back to the original
data stream for the receiver to utilize.
The chip rate also has a direct effect on the spreading of the CDMA signal.
Figure 1-2 shows a brief summary of the effects on spreading the original
signal that the chosen chip rate has on the original signal. The heart of
CDMA lies in the point that the spreading of the initial information distributes
the initial energy over a wide bandwidth. At the receiver, the signal
is despread through reversing the initial spreading process where the
original signal is reconstructed for utilization. When the CDMA signal
experiences interference in the band, the despreading process despreads
the initial signal for use but at the same time spreads the interference so it
minimizes its negative impact on the received information.
The number of PN chips per data bit is referred to as the processing gain
and is best represented by the following equation. Another way of referencing
processing gain is the amount of jamming, or interference, power that is
reduced going through the despreading process. Processor gain is the
improvement in the signal-to-noise ratio of a spread spectrum system and
is depicted in Figure 1-3.