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

644 SPREAD SPECTRUM COMMUNICATIONS
Fi g
ure 11. 1 7
Cellular
telephone
system.
Telephone central
office
MTSO
to the mobile telephone switching office (MTSO), which in tum is wired to the telephone
central office. A caller communicates via radio channel to its base station, which sends the
signal to the MTSO. The MTSO connects to the receiver either via the land-based telephone
system or via another base station. As the caller moves from one cell to another, a handoff
process takes place. During handoff, the MTSO automatically switches the user to an available
channel in the new cell while the call is in progress. The handoff is so rapid that users usually
do not notice it.
The true ingenuity of the cellular network lies in its ability to reuse the same frequency
band in multiple cells. Without cells, high-powered transmitters can be used to cover an entire
city. But this would allow a frequency channel to be used only by one user in the city at any
moment. This posed serious limitations on the number of channels and simultaneous users.
The limitation is overcome in the cellular scheme by reusing the same frequencies in all the
cells except those immediately adjacent. This is possible because the transmitted powers are
kept small enough to prevent the signals from one cell from reaching beyond the immediately
adjacent cells. We can accommodate any number of users by increasing the number of cells as
we reduce the cell size and the power levels correspondingly.
The lG (first-generation) analog cellular schemes use audio signal to modulate an FM
signal with transmission bandwidth 30 kHz. This wideband FM signal results in a good SNR
but is highly inefficient in bandwidth usage and frequency reuse. The 2G (second-generation)
cellular systems are all digital. Among them, the GSM and cdmaOne are two of the most widely
deployed cellular systems. GSM adopts a TDMA technology through which eight users share
a 200 kHz channel. The competing technology of cdmaOne (known earlier as IS-95) is a DSSS
system.
Why CDMA in Cellular Systems
Although spread spectrum is inherently well suited against narrowband interferences and
affords a number of advantages in the areas of networking and handoff, the key characteristic
underlying the broad application of CDMA for wireless cellular systems is the potential
for improved spectral utilization. The capacity for improvement has two key sources. First,
the use of CDMA allows improved frequency reuse. Narrowband systems cannot use the same
transmission frequency in adjacent cells because of the potential for interference. CDMA has
inherent resistance to interference. Although users using different spreading codes from adjacent
cells will contribute to the total interference level, their contribution will be significantly
less than the interference from the same cell users. This leads to a much improved frequency