28-05-2013, 03:37 PM
Fundamentals of Cellular Communications
Introduction
In this chapter, we present the concept of a cellular system and discuss the
fundamentals of cellular communications. We develop a relationship between
reuse ratio (q) and cluster size or reuse factor (N) for hexagonal cell geometry, as
well as study cochannel interference for omnidirectional and sectorized cells. We
also discuss cell splitting and segmentation procedures used in cellular systems.
Cellular Systems
Most commercial radio and television systems are designed to cover as much
area as possible. These systems typically operate at maximum power and with
the tallest antennas allowed by the Federal Communications Commission (FCC).
The frequency used by the transmitter cannot be reused again until there is
enough geographical separation so that one station does not interfere signifi -
cantly with another station assigned to that frequency. There may even be a large
region between two transmitters using the same frequency where neither signal is
received
The cellular system takes the opposite approach [1,3,4,5,9,11–14]. It seeks to
make an effi cient use of available channels by employing low-power transmitters
to allow frequency reuse at much smaller distances (see Figure 5.1). Maximizing
the number of times each channel may be reused in a given geographic area is the
key to an effi cient cellular system design.
Adjacent Channel Interference (ACI)
Signals which are adjacent in frequency to the desired signal cause adjacent channel
interference [7,8]. ACI is brought about primarily because of imperfect receiver
fi lters which allow nearby frequencies to move into the pass band, and nonlinearity
of the amplifi ers. The ACI can be reduced by: (1) using modulation schemes
which have low out-of-band radiation; (2) carefully designing the bandpass fi lter
at the receiver front end; and © assigning adjacent channels to different cells in
order to keep the frequency separation between each channel in a given cell as
large as possible. The effects of ACI can also be reduced using advanced signal
processing techniques that employ equalizers.
Segmentation
Sometimes engineers have to add an additional cell at less than the reuse distance
without using a complete cell splitting process. This method might be used to fi ll
in a coverage gap in the system. This can result in cochannel interference. The
most straightforward method to avoid an increase in cochannel interference is
simply not to reuse them.
Segmentation divides a channel group into segments of mutually exclusive channel frequencies. Then, by assigning different segments to particular cell sites, cochannel interference between these cell sites can be avoided. The disadvantage of segmentation is that the capacity of the segmented cells is lower than the unsegmented cell