A
cellular network or
mobile network is a communication
network where the last link is wireless. The network is distributed
over land areas called cells, each served by at least one fixed-location
transceiver, but more normally three cell sites or base transceiver stations.
These base stations provide the cell with the network coverage which
can be used for transmission of voice, data, and other types of content.
A cell typically uses a different set of frequencies from neighboring
cells, to avoid interference and provide guaranteed service quality
within each cell.
When joined together, these cells provide radio coverage over a wide
geographic area. This enables a large number of portable transceivers
(e.g., mobile phones, tablets and laptops equipped with mobile broadband modems, pagers,
etc.) to communicate with each other and with fixed transceivers and
telephones anywhere in the network, via base stations, even if some of
the transceivers are moving through more than one cell during
transmission.
Cellular networks offer a number of desirable features:
- More capacity than a single large transmitter, since the same
frequency can be used for multiple links as long as they are in
different cells
- Mobile devices use less power than with a single transmitter or satellite since the cell towers are closer
- Larger coverage area than a single terrestrial transmitter, since
additional cell towers can be added indefinitely and are not limited by
the horizon
Major telecommunications providers have deployed voice and data cellular
networks over most of the inhabited land area of Earth. This allows
mobile phones and mobile computing devices to be connected to the public switched telephone network and public Internet. Private cellular networks can be used for researchor for large organizations and fleets, such as dispatch for local public safety agencies or a taxicab company.
Cellular network Concept
In a cellular radio
system, a land area to be supplied with radio service is divided into
cells, in a pattern which depends on terrain and reception
characteristics but which can consist of roughly hexagonal, square,
circular or some other regular shapes, although hexagonal cells are
conventional. Each of these cells is assigned with multiple frequencies (f1 – f6) which have corresponding radio base stations.
The group of frequencies can be reused in other cells, provided that
the same frequencies are not reused in adjacent neighboring cells as
that would cause co-channel interference.
The increased capacity
in a cellular network, compared with a network with a single
transmitter, comes from the mobile communication switching system
developed by Amos Joel of Bell Labs
that permitted multiple callers in the same area to use the same
frequency by switching calls made using the same frequency to the
nearest available cellular tower having that frequency available and
from the fact that the same radio frequency can be reused in a different
area for a completely different transmission. If there is a single
plain transmitter, only one transmission can be used on any given
frequency. Unfortunately, there is inevitably some level of interference from the signal from the other cells which use the same frequency. This means that, in a standard FDMA system, there must be at least a one cell gap between cells which reuse the same frequency.
In the simple case of the taxi company, each radio had a manually
operated channel selector knob to tune to different frequencies. As the
drivers moved around, they would change from channel to channel. The
drivers knew which frequency
covered approximately what area. When they did not receive a signal
from the transmitter, they would try other channels until they found one
that worked. The taxi drivers would only speak one at a time, when
invited by the base station operator. This is, in a sense, time-division multiple access (TDMA).
The first commercially cellular network, the 1G generation, was launched in Japan by Nippon Telegraph and Telephone (NTT) in 1979, initially in the metropolitan area of Tokyo.
Within five years, the NTT network had been expanded to cover the whole
population of Japan and became the first nationwide 1G network.
Network signal encoding
To distinguish signals from several different transmitters, time-division multiple access (TDMA), frequency-division multiple access (FDMA), code-division multiple access (CDMA), and orthogonal frequency-division multiple access (OFDMA) were developed.
With TDMA, the transmitting and receiving time slots used by different users in each cell are different from each other.
With FDMA, the transmitting and receiving frequencies used by
different users in each cell are different from each other. In a simple
taxi system, the taxi driver manually tuned to a frequency of a chosen
cell to obtain a strong signal and to avoid interference from signals
from other cells.
The principle of CDMA is more complex, but achieves the same result; the distributed transceivers can select one cell and listen to it.
Other available methods of multiplexing such as polarization-division multiple access
(PDMA) cannot be used to separate signals from one cell to the next
since the effects of both vary with position and this would make signal
separation practically impossible. TDMA is used in combination with
either FDMA or CDMA in a number of systems to give multiple channels
within the coverage area of a single cell.
Network Frequency reuse
The key characteristic of a cellular network is the ability to re-use
frequencies to increase both coverage and capacity. As described above,
adjacent cells must use different frequencies, however there is no
problem with two cells sufficiently far apart operating on the same
frequency, provided the masts and cellular network users' equipment do
not transmit with too much power.
The elements that determine frequency reuse are the reuse distance and the reuse factor. The reuse distance,
D is calculated as
,
where
R is the cell radius and
N is the number of cells
per cluster. Cells may vary in radius from 1 to 30 kilometres (0.62 to
18.64 mi). The boundaries of the cells can also overlap between adjacent
cells and large cells can be divided into smaller cells.
The frequency reuse factor is the rate at which the same frequency can be used in the network. It is
1/K (or
K according to some books) where
K
is the number of cells which cannot use the same frequencies for
transmission. Common values for the frequency reuse factor are 1/3, 1/4,
1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation).
In case of
N sector antennas on the same base station site,
each with different direction, the base station site can serve N
different sectors.
N is typically 3. A
reuse pattern of
N/K denotes a further division in frequency among
N
sector antennas per site. Some current and historical reuse patterns
are 3/7 (North American AMPS), 6/4 (Motorola NAMPS), and 3/4 (GSM).
If the total available bandwidth is
B, each cell can only use a number of frequency channels corresponding to a bandwidth of
B/K, and each sector can use a bandwidth of
B/NK.
Code-division multiple access-based
systems use a wider frequency band to achieve the same rate of
transmission as FDMA, but this is compensated for by the ability to use a
frequency reuse factor of 1, for example using a reuse pattern of 1/1.
In other words, adjacent base station sites use the same frequencies,
and the different base stations and users are separated by codes rather
than frequencies. While
N is shown as 1 in this example, that
does not mean the CDMA cell has only one sector, but rather that the
entire cell bandwidth is also available to each sector individually.
Depending on the size of the city, a taxi system may not have any
frequency-reuse in its own city, but certainly in other nearby cities,
the same frequency can be used. In a large city, on the other hand,
frequency-reuse could certainly be in use.
Recently also orthogonal frequency-division multiple access based systems such as LTE
are being deployed with a frequency reuse of 1. Since such systems do
not spread the signal across the frequency band, inter-cell radio
resource management is important to coordinate resource allocation
between different cell sites and to limit the inter-cell interference.
There are various means of Inter-Cell Interference Coordination (ICIC) already defined in the standard.Coordinated scheduling, multi-site MIMO or multi-site beam forming are
other examples for inter-cell radio resource management that might be
standardized in the future.
Directional antennas
Cell towers frequently use a directional signal
to improve reception in higher-traffic areas. In the United States, the
FCC limits omnidirectional cell tower signals to 100 watts of power. If
the tower has directional antennas, the FCC allows the cell operator to
broadcast up to 500 watts of effective radiated power (ERP).
Although the original cell towers created an even, omnidirectional
signal, were at the centers of the cells and were omnidirectional, a
cellular map can be redrawn with the cellular telephone towers located
at the corners of the hexagons where three cells converge.Each tower has three sets of directional antennas aimed in three
different directions with 120 degrees for each cell (totaling 360
degrees) and receiving/transmitting into three different cells at
different frequencies. This provides a minimum of three channels, and
three towers for each cell and greatly increases the chances of
receiving a usable signal from at least one direction.
The numbers in the illustration are channel numbers, which repeat
every 3 cells. Large cells can be subdivided into smaller cells for high
volume areas.
Cell phone companies also use this directional signal to improve
reception along highways and inside buildings like stadiums and arenas.
Broadcast messages and paging
Practically every cellular system has some kind of broadcast
mechanism. This can be used directly for distributing information to
multiple mobiles. Commonly, for example in mobile telephony
systems, the most important use of broadcast information is to set up
channels for one-to-one communication between the mobile transceiver and
the base station. This is called paging. The three different paging procedures generally adopted are sequential, parallel and selective paging.
The details of the process of paging vary somewhat from network to
network, but normally we know a limited number of cells where the phone
is located (this group of cells is called a Location Area in the GSM or UMTS system, or Routing Area if a data packet session is involved; in LTE,
cells are grouped into Tracking Areas). Paging takes place by sending
the broadcast message to all of those cells. Paging messages can be used
for information transfer. This happens in pagers, in CDMA systems for sending SMS messages, and in the UMTS system where it allows for low downlink latency in packet-based connections.
Movement from cell to cell and handing over
In a primitive taxi system, when the taxi moved away from a first
tower and closer to a second tower, the taxi driver manually switched
from one frequency to another as needed. If a communication was
interrupted due to a loss of a signal, the taxi driver asked the base
station operator to repeat the message on a different frequency.
In a cellular system, as the distributed mobile transceivers move
from cell to cell during an ongoing continuous communication, switching
from one cell frequency to a different cell frequency is done
electronically without interruption and without a base station operator
or manual switching. This is
called the handover
or handoff. Typically, a new channel is automatically selected for the
mobile unit on the new base station which will serve it. The mobile unit
then automatically switches from the current channel to the new channel
and communication continues
The exact details of the mobile system's move from one base station
to the other varies considerably from system to system (see the example
below for how a mobile phone network manages handover)
Mobile phone network
The most common example of a cellular network is a mobile phone (cell phone) network. A mobile phone is a portable telephone which receives or makes calls through a cell site (base station), or transmitting tower. Radio waves are used to transfer signals to and from the cell phone.
Modern mobile phone networks use cells because radio frequencies are a
limited, shared resource. Cell-sites and handsets change frequency
under computer control and use low power transmitters so that the
usually limited number of radio frequencies can be simultaneously used
by many callers with less interference.
A cellular network is used by the mobile phone operator
to achieve both coverage and capacity for their subscribers. Large
geographic areas are split into smaller cells to avoid line-of-sight
signal loss and to support a large number of active phones in that area.
All of the cell sites are connected to telephone exchanges (or switches), which in turn connect to the public telephone network.
In cities, each cell site may have a range of up to approximately
1⁄2
mile (0.80 km), while in rural areas, the range could be as much as 5
miles (8.0 km). It is possible that in clear open areas, a user may
receive signals from a cell site 25 miles (40 km) away.
Since almost all mobile phones use cellular technology, including GSM, CDMA, and AMPS (analog), the term "cell phone" is in some regions, notably the US, used interchangeably with "mobile phone". However, satellite phones
are mobile phones that do not communicate directly with a ground-based
cellular tower, but may do so indirectly by way of a satellite.
There are a number of different digital cellular technologies, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), cdmaOne, CDMA2000, Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN). The transition from existing analog to the digital standard followed a very different path in Europe and the US.As a consequence, multiple digital standards surfaced in the US, while
Europe and many countries converged towards the GSM standard.
Structure of the mobile phone cellular network
A simple view of the cellular mobile-radio network consists of the following:
- A network of radio base stations forming the base station subsystem.
- The core circuit switched network for handling voice calls and text
- A packet switched network for handling mobile data
- The public switched telephone network to connect subscribers to the wider telephony network
This network is the foundation of the GSM
system network. There are many functions that are performed by this
network in order to make sure customers get the desired service
including mobility management, registration, call set-up, and handover.
Any phone connects to the network via an RBS (Radio Base Station) at a corner of the corresponding cell which in turn connects to the Mobile switching center (MSC). The MSC provides a connection to the public switched telephone network (PSTN). The link from a phone to the RBS is called an
uplink while the other way is termed
downlink.
Radio channels effectively use the transmission medium through the use of the following multiplexing and access schemes: frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and space division multiple access (SDMA).
Small cells
Main article: Small cell
Small cells, which have a smaller coverage area than base stations, are categorised as follows:
- Microcell, less than 2 kilometres
- Picocell, less than 200 metres
- Femtocell, around 10 metres
