B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

646 SPREAD SPECTRUM COMMUNICATIONS
spreading codes to generate synchronous signals with zero cross-correlation. Reverse-link does
not enjoy this luxury. For this reason, more powerful error correction (rate 1/3) is employed
on the reverse link. Still, like the forward link, the raw QCELP vocoder bit rate is 9.6 kbit/s,
which is eventually spread to 1.2288 Mchip/s over a 1.25 MHz bandwidth.
As mentioned earlier, the near-far problem needs to be addressed when spread spectrum is
utilized in mobile communications. To combat this problem, IS-95 uses power control. On the
forward link there is a subchannel for power control purposes. Every 1.25 ms, the base station
receiver estimates the signal strength of the mobile unit. If it is too high, the base transmits a
1 on the subchannel. If it is too low, it transmits a 0. In this way, the mobile station adjusts its
power based on the 800 bit/s power control signal to reduce interference to other users.
3G Cellular Services 16 - 19
In the new millennium, wireless service providers are shifting their voice-centric 2G cellular
systems to the next-generation (3G) wireless systems, which are capable of supporting highspeed
data transmission and internet connection. For this reason, the International Mobile
Telecommunications-2000 standard (IMT-2000) is the global standard for third-generation
wireless communications. IMT-2000 provides a framework for worldwide wireless access of
fixed and mobile wireless access systems. The goal is to provide wireless cellular coverage up
to 144 kbit/s for high-speed mobile, 384 kbit/s for pedestrian, and 2.048 Mbit/s for indoor users.
Among the 3G standards, there are three major wireless technologies based on CDMA DSSS,
namely, the two competing versions of wideband CDMA from the 3rd Generation Partnership
Project (3GPP) and the 3rd Generation Partnership Project 2 (3GPP2), plus the TD-SCDMA
from the 3GPP for China.
Because 3G cellular systems continue to use the existing cellular band, a high data rate
for one user means a reduction of service for other active CDMA users within the same cell.
Otherwise, given the limited bandwidth, it is impossible to serve the same number of active
users as in cdmaOne while supporting data rate as high as 2.048 Mbit/s. Thus, the data rate
to and from the mobile unit must be variable according to the data traffic intensity within the
cell. Since most data traffic patterns (including internet usage) tend to be bursty, variable rate
data service offered by 3G cellular is suitable for such applications.
Unlike FDMA and TDMA, CDMA provides a perfect environment for variable data rate
and requires very simple modifications. While FDMA and TDMA would require grouping
multiple frequency bands or time slots dynamically to support variable rate, CDMA needs to
change only the spreading gain. In other words, at higher data rates, a CDMA transmitter can
use a lower spreading factor. In this mode, its MAI to other users is high, and fewer such users
can be accommodated. At lower data rates, the transmitter uses a larger spreading factor, thus
allowing more users to transmit.
In 3GPP2's CDMA2000 standard, there are two ratio transmission modes: lxRTT utilizing
one 1.25 MHz band and 3xRTT that aggregates three 1.25 MHz bands. On lxRTT forward
link, the maximum data rate is 307.2 kbit/s with a spreading gain of 4. Thus, the chip rate
is still 1.2288 Mchip/s. A more recent 3GPP2 release is called CDMA 2000 lx EV-DO revision
A, where EV-DO stands for "evolution data-optimized." It can support a peak data rate
of 3.1 Mbit/s on the forward link of 1.25 MHz bandwidth. It does so by applying adaptive
coding and adaptive modulations, including QPSK, 8-PSK, and 16-QAM. At the peak rate,
the spreading gain is 1 (i.e., no spreading).
At the same time, the WCDMA by 3GPP applies similar ideas. Unlike CDMA2000,
WCDMA has a standard bandwidth of 5 MHz. When spreading is used, the chip rate is 4.096
Mchip/s. On downlink, the variable spreading factor of 3GPP WCDMA ranges from 512 to 4.
With QPSK modulation, this provides a variable data rate from 16 kbit/s to 2.048 Mchip/s.