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Ugweje WL040/Bidgolio-Vol I WL040-Sample.cls June 19, 2003 17:10 Char Count= 0
188 RADIOFREQUENCY ANDWIRELESSCOMMUNICATIONShexagons. The coverage area of cells is called the footprint.
The BS simultaneously communicates with many mobiles
using one channel (pair of frequencies) per mobile. One
frequency is for the forward link (BS to the mobile), and
the other frequency is for the reverse link (mobile to the
BS). Each cell size varies depending on the landscape,
subscriber density, and demand within a particular re-
gion. Cells can be added to accommodate growth, e.g.,
creating new cells by overlaying, splitting, or sectoring ex-
isting cells. These techniques increase the capacity of the
system. Sectoring existing cells and then using directional
antennas can also increase capacity.Clusters
A cluster is a group of cells. No frequencies are reused
within a cluster. Figure 7 illustrates a 7-cell cluster, indi-
cated by the dotted circle. Frequency can be reused for all
cells numbered 7. Frequencies used in one cell cluster can
be reused in another cluster of cells. A larger number of
cells per cluster arrangement reduces interference to the
system.Frequency Reuse
Frequency reuse is a technique of allocating channels to
cellular systems. Because of the unavailability of spec-
trum at the cellular band, channel frequencies must be
reused. Cells are assigned groups of channels that are
completely different from those of neighboring cells. Cells
with the same number have the same set of frequencies.
If the number of available frequencies is 7, the frequency
reuse factor is 1/7, which implies that each cell is using
1 /7 of available frequencies (Rappaport, 2002). Frequency
reuse introduces interference into the system.Interference
In cellular mobile communications, there are two types
of intrinsic interference, namely, co-channel interference
(CCI) and adjacent channel interference (ACI). These in-
terferences are a result of frequency reuse. CCI is the
interference between signals having the same frequency
(i.e., the reuse frequencies), whereas ACI is the interfer-
ence between signals having frequencies close together.
For example suppose channel 1 has frequencies 825.030
MHz (mobile) and 870.030 MHz (BS) and channel 2 has
frequencies 825.060 MHz and 870.060 MHz. Channels 1
and 2 have frequencies close to one another, which will
result in ACI. Any other signals having the frequencies
of channel 1, 825.030 (mobile) and 870.030 MHz (BS),
are co-channel signals and will suffer from co-channel
interference. Note that the interference effect is related
to the ratio of the reuse distance Dand the cell ra-
diusR. This is known as the Q-factor Q=D/R and is
used to measure the level of CCI. A higher Q value im-
proves transmission quality due to smaller CCI. That is,
increasing D improves isolation of RF energy between
cells and hence minimizes interference. The ACI is mainly
due to imperfect filtering allowing nearby frequencies to
leak into the passband of the desired signal (out-of-band
interference).Cell Splitting
Cell splitting is the process of subdividing a congested cell
into smaller cells, each with its base station. As the traffic
load carried by a large cell reaches capacity, cell splitting is
used to increase system capacity. In this way, heavy-traffic
regions can be split into as many smaller areas as neces-
sary in order to provide acceptable service levels. Cell split-
ting decreasesR, while leavingQrelatively unchanged.
Notice that more cells imply that more cell boundaries will
be crossed more often, increasing trunking and handoff.
Only those cells that have traffic overloads are candidates
for splitting. However, if cells are split in only a part of a
system, serious channel assignment problems may result.
The difficulty encountered when all the cell sites are not
split can be resolved by implementing cell overlay.Cell Sectoring
Cell sectoring is the process of dividing cells into sectors
and replacing a single omni-directional antenna with a
directional antenna. Common sectors sizes are 120◦,90◦,
60 ◦, and 30◦. Cell sectors of 60◦and 120◦are illustrated
in Figure 7. When cells are sectored,Ris unchanged,D
is reduced, the amount of frequency reuse is increased,
and hence capacity is increased. It is observed that the
spectral efficiency of the system is enhanced because the
frequency can be reused more often.Handoff
Handoff is the process used to maintain a call in progress
when the mobile user moves between cells. Handoff is
generally needed in situations where a mobile is at a cell
boundary or reaches a gap in signal strength. Because
adjacent cells do not use the same frequency, a call must
either be dropped or transferred from one radio channel
to another when a mobile user crosses the line between
adjacent cells. Because dropping the call is unacceptable,
the process of handoff is necessary. As the user moves
between cells, the transmission is “handed off” between
cells in order to maintain seamless service.EMERGING RF WIRELESS
TECHNOLOGIES
The first generation (1G) and second generation (2G) of
cellular mobile telephony were intended primarily for
voice transmission. This will not be adequate for the
new generation of users. With the continued growth of
the Internet and World Wide Web, mobile users are con-
tinually looking for high-performance wireless Internet
technology to enhance their communication capabilities.
Although 3G wireless technology has not yet been real-
ized, it promises to enhance users’ communication ability,
ranging from receiving and sending e-mail to video tele-
conferencing. The services provided by the generations of
wireless technology are summarized in Table 3 (Evolution
to 3G/UMTS Service, 2002).
The 3G technologies use wideband code division mul-
tiple access (W-CDMA) technology to transfer data over
networks. W-CDMA sends data in a digital format over a
range of frequencies, which makes the data move faster,