Today’s Cellular Telephone Systems
• Cell concept
• Frequency Reuse
• In 1895, the telephone was invented by MARCONI
demonstrated the first radio transmission from the
Isle of Wight to a tugboat 18 miles away, and radio
communications was born.
• Today most radio systems transmit digital signals
composed of binary bits, where the bits are obtained
directly from a data signal or by digitizing an analog
voice or music signal.
Cellular Telephone Systems
• Cellular telephone systems are designed to provide
• Cellular systems were initially designed for mobile
terminals inside vehicles with antennas mounted on
the vehicle roof.
• The basic feature of the cellular system is frequency
• In a cellular system, the signal from a mobile unit
(cell phone) to a base station is transmitted by radio
waves through the air, instead of through metallic
wires, However, the signal from the base station is
sent to a mobile switching center and possibly to a
telephone central office through electrical wires.
• The antenna at the base station converts the radio
waves to electrical signals and circuits in the base
station send the signal to the appropriate mobile
• Initial cellular system designs were mainly driven by
the high cost of base stations, about one million
dollars each. For this reason early cellular systems
used a relatively small number of cells to cover an
entire city or region. The cell base stations were
placed on tall buildings or mountains and
transmitted at very high power with cell coverage
areas of several square miles.
• These large cells are called macrocells.
• Signals propagated out from base stations uniformly
in all directions, so a mobile moving in a circle
around the base station would have approximately
constant received power.
• Cellular telephone systems are now evolving to
smaller cells with base stations close to street level
or inside buildings transmitting at much lower power.
These smaller cells are called microcells or
picocells, depending on their size.
•In a cellular radio system, a land area to be supplied
with radio service is divided into regular shaped
cells, which can be hexagonal, square, circular or some
other regular shapes. Each of these cells is assigned
multiple frequencies (f1 - f6) which have corresponding
radio base stations. The group of frequencies can be
reused in other cells Cellular Concept to increase both
coverage and capacity.
• The term handover or handoff refers to the process
of transferring an ongoing call or data session from
one channel connected to the core network to
• When the phone is moving away from the area
covered by one cell and entering the area covered by
another cell the call is transferred to the second cell
in order to avoid call termination when the phone
gets outside the range of the first cell.
Types of handoff
• Hard handoff is one in which the channel in the
source cell is released and only then the channel
in the target cell is engaged. Thus the connection
to the source is broken before the connection to
the target is made.
• Soft handoff is one in which the channel in the
source cell is retained and used for a while in
parallel with the channel in the target cell. In this
case the connection to the target is established
before the connection to the source is broken.
• The area (a city, or a part of town) is divided into a
number of cells and a base station within each cell.
• If a user (mobile phone) is within a particular cell, the
call is handled by the corresponding base station
within that cell.
• The base station transmits the signal to the mobile
switching center (MTSO) which switches the signal to
another base station, or to a Public Switched
Telephone Network (PSTN) depending on the
destination of the call.
• As a user moves from one cell to another, the call is
“handed over” to the base station of the other cell.
• All base stations in a city are connected via a
high-speed link to a mobile telephone
switching office (MTSO).
• The MTSO acts as a central controller for the
network, allocating channels within each
cell, coordinating handoffs between cells
when a mobile traverses a cell boundary, and
routing calls to and from mobile users in
conjunction with the public switched
telephone network (PSTN).
• A new user located in a given cell requests a
channel by sending a call request to the cell’s
base station over a separate control channel.
The request is relayed to the MTSO, which
accepts the call request if a channel is
available in that cell. If no channel is
available, the call request is rejected.
• All cellular systems being deployed today are
digital, and these systems provide voice
mail, paging, and email services in addition to voice.
Due to their lower cost and higher efficiency.
• Digital cellular systems can use any of the multiple
access techniques TDMA ,FDMA or CDMA.
• There are two standards in the 900 MHz (cellular)
frequency band: IS-54 (Interim Standard), which uses
a combination of TDMA and FDMA, and IS-95, which
uses semi-orthogonal CDMA.
• IS-95 is a digital cellular phone system using CDMA
• GSM is a digital cellular phone system using TDMA