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CURRENTSTATE OFCELLULARSYSTEMS(FOCUS ON2G) 837digital transmission, the cellular systems gained tremen-
dous flexibility because of the inherent intelligence that
could now be built into the management and control of the
system. In addition, digital signals can be pre- and post-
processed to enhance the received signal quality. Third
generation systems, besides being all digital, promise to
offer higher bit rates for data services and to switch from
circuit- to packet-based transmission. To address the need
for higher bit rates for data services, some intermediate
technologies that fit into the 2G architecture were de-
fined. We discuss those in the section Higher Bit Rates
for Data—GPRS and HDR. A more detailed description
of 3G is given in the section 3G Cellular Systems and in
the final section of the chapter we very briefly discuss the
directions for 4G.
The advantage of creating cells is that after dividing
the service area into cells, the same frequency sets can
be used and reused systematically. Because the same set
of frequency bands can be reused many times, a larger
number of users could be supported (Jakes, 1993; Yacoub,
1993). Each cell has a base station, which contains an-
tenna and radio equipment, as well as a high-speed, high-
capacity connection to the network. Since the area of the
cell is typically much smaller than the carrier’s coverage
area, lower transmit power can be used to communicate
with the mobile station (MS). However, even with lower
transmit power, signals still propagate into neighboring
cells, causing interference. Thus, cells with the same fre-
quency sets are spaced many cells apart, and immediately
neighboring cells use different sets of frequency chan-
nels to reduce interference (one exception is CDMA sys-
tems, such as IS-95, in which the same frequencies can be
used in all cells due to its ability to work at low carrier-
to-interference ratios) (Viterbi, 1995). A group of nearest
cells that use disjoint sets of frequency bands is called
a cluster. The service area is composed of these clusters,
which reuse all frequency bands allocated to the service
provider.
Most current cellular networks utilize an additional
level of subdivision to further increase capacity beyond
that achieved using the cellular concept. Instead of mak-
ing cells even smaller (which introduces the problem of
frequent handoffs, which will be discussed later), each
cell is divided into 3 or more sectors (Yacoub, 1993). Pre-
viously, an omnidirectional antenna was used at the base
station to transmit signal equally in all directions, which
also distributes interference equally in all directions. The
omnidirectional antenna is replaced by several directional
antennas, each of which can direct the radio wave to a cer-
tain direction. For example, an omnidirectional antenna
may be replaced by three directional antennas, each with
a beam width of 120◦. Thus, the cell is effectively sector-
ized into three distinct areas, each with its own set of fre-
quencies. Since the signal is transmitted only in the sector
that contains the MS, sectorization reduces the interfer-
ence between cells in adjacent clusters. Reduced interfer-
ence means that clusters can be located closer together,
enabling more frequent reuse of the same frequencies and
subsequently increasing traffic capacity.
A major obstacle to the cellular concept working ef-
fectively involved a mobile user traveling from the cover-
age area of one cell to that of another during a call. Thisproblem did not exist, at least not to the same extent, in
noncellular systems where the service areas were much
larger. However, as the sizes of cells shrank (in order to
maximize frequency reuse), it became very likely for a
user to travel outside the coverage of a cell, and simply
dropping those calls was unacceptable from the point of
view of the mobile user. To address this concern, handoff
techniques were developed as a part of different wireless
standards. By using handoffs, it became possible to auto-
matically transfer a call from a radio channel in one cell
to that in another without disrupting the ongoing con-
nection. This ability made the adaptation of the cellular
concept practical and realizable.Mobility and Roaming
One of the primary benefits of wireless telephony is the
ability to move around without the concern of losing con-
nectivity, at least most of the time in most areas. However,
typical mobile calling plans specify a home calling area,
such as a particular metropolitan area, a state, or even the
entire United States. When a mobile user travels outside
this area, he is said to be “roaming.” Even if the home call-
ing area covers a large area, it is possible and quite likely
that in certain geographical locations, the user’s carrier
does not have coverage while another carrier may. Thus,
roaming capability is necessary in order to increase the
level of any time, anywhere access, to which mobile users
have become accustomed and expect. The latter type of
roaming is made possible through the business agree-
ments between carriers and service providers to grant
each other’s customers access to their networks, in ad-
dition to technologies and standards summarized below.
Two of the most popular technologies enabling the
roaming service are GSM Mobile Application Part (MAP)
(MAP Specification, 1997) and IS-41 (Telecommunica-
tions Industries Association, 1991). Both have become
standards adopted in their respective areas of application.
MAP is used in GSM networks (Mouly & Pautet, 1992),
while IS-41 is used in IS-136 (Telecommunications In-
dustries Association, 1996) and IS-95 networks (Telecom-
munications Industries Association, 1993). IS-41 was de-
veloped a few years after the development of GSM MAP,
and adopted terminology, network architecture, and some
protocol details from MAP. Three major components are
used to enable the roaming service: the home location reg-
ister (HLR), the visitor location register (VLR), and the
mobile switching center (MSC). The HLR is the database
that contains information about subscribers in the net-
work, including the current locations of the subscribers.
For roaming subscribers, the location is in the form of
the signaling address of the VLR associated with the sub-
scriber. The VLR temporarily stores a subset of informa-
tion available in the HLR for those mobile users whose
HLRs are located elsewhere. The MSC is a telephone ex-
change that is able to set up and route mobile calls. For
each user in its service area, the MSC utilizes either the
HLR or the VLR to setup and route calls. In both IS-41
and GSM MAP, common channel signaling system num-
ber 7 (SS7) (Black, 1997) is used to exchange call setup
and routing information, including HLR and VLR access,
over a digital signaling network.