Introduction - Introduction to Digital Signal Processing and Filter Design - page 25

different receivers or as it changes continuously between the transmitter
and the receiver. For example, when a telephone conversation is switched
from one point to another and the cable or the microwave link changes, or
when the mobile phone moves as the talker moves over a wide territory,
adaptive filters are absolutely necessary to compensate for the distortion of
the signal as it passes through the transmission link.
5. We have chosen filters only as an example to compare digital and analog
signal processors. There are many other types of digital signal processing
that are feasible and are being used, and these are not possible or very effi-
cient in analog filters. For example, error detection in transmitted signals
and correction to reduce the error rate is an advanced technique used in
many applications. Another example is our ability to compress the data by
a significant factor and receive the input signal at lower cost and very good
quality. To point out the power of digital signal processing theory and the
digital signal processors available, let us again consider the mobile phone.
Bateman and Patterson-Stephans state that “Within the phone, a single DSP
device may be performing real-time speech compression, video compres-
sion, echo cancellation, noise cancellation, voice recognition, waveform
coding, modulation/demodulation, interleaving, multipath equalization, soft
decision decoding, convolution, automatic frequency-, power- and gain-
control” [3], and all of them done in a triband phone with TDMA, CDMA,
and analog signal processing! The mobile phone is just an example to illus-
trate the large number of digital signal processing techniques that are built
into any of the applications described above. But an application such as
the mobile phone implements other functions also, and their features are
briefly described given below.
1.5.1 Operation of a Mobile Phone Network
Consider a geographic area in which a part of the mobile phone network operates.
It is divided into cells as indicated in Figure 1.18. The cells are not really equal
in area but could be as small as 300 m where the telephone traffic is high and as
large as 35 km in rural areas. The size and shape of each cell is determined by
the radiation pattern and the power output of the antenna (and is not hexagonal
of equal shape and size) serving the mobile phones. A base station controller
(BSC), usually installed on a tower, serves as many as 124 base transceiver
stations (BTSs). These stations communicate with all the cell phones that are
known to be located within the area covered by the cell. The BTSs operate on
different frequencies, also called “channels,” to transmit to and receive signals
from the cell phones. Global System for Mobile Communication (GSM) is one
of the most widely used mobile cellular phone network systems in the world,
and in that system, the frequencies for transmitting from the mobile phone and
receiving by the BTS lie in the band 890-915 MHz, and the frequencies for
transmitting from the BTS and receiving by the mobile phone lie in the band
935-960 MHz. But in order to utilize the frequency spectrum efficiently, cells