Top of a cellular radio tower
Indoor cell site in Germany
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 research or for large organizations and fleets, such as dispatch for local public safety agencies or a taxicab company.
Concept
Example of frequency reuse factor or pattern 1/4
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. Inevitably, there is 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 commercial 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.
Cell 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.
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
Cellular telephone frequency reuse pattern. See U.S. Patent 4,144,411
Cell towers frequently use a directional signal to improve reception in higher-traffic areas. In the United States, the Federal Communications Commission
(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
GSM network architecture
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
Small cells, which have a smaller coverage area than base stations, are categorised as follows:
Cellular handover in mobile phone networks
As the phone user moves from one cell area to another cell while a
call is in progress, the mobile station will search for a new channel to
attach to in order not to drop the call. Once a new channel is found,
the network will command the mobile unit to switch to the new channel
and at the same time switch the call onto the new channel.
With CDMA, multiple CDMA handsets share a specific radio channel. The signals are separated by using a pseudonoise
code (PN code) that is specific to each phone. As the user moves from
one cell to another, the handset sets up radio links with multiple cell
sites (or sectors of the same site) simultaneously. This is known as
"soft handoff" because, unlike with traditional cellular technology, there is no one defined point where the phone switches to the new cell.
In IS-95 inter-frequency handovers and older analog systems such as NMT
it will typically be impossible to test the target channel directly
while communicating. In this case other techniques have to be used
such as pilot beacons in IS-95. This means that there is almost always a
brief break in the communication while searching for the new channel
followed by the risk of an unexpected return to the old channel.
If there is no ongoing communication or the communication can be
interrupted, it is possible for the mobile unit to spontaneously move
from one cell to another and then notify the base station with the
strongest signal.
Cellular frequency choice in mobile phone networks
The effect of frequency on cell coverage means that different
frequencies serve better for different uses. Low frequencies, such as
450 MHz NMT, serve very well for countryside coverage. GSM 900
(900 MHz) is a suitable solution for light urban coverage. GSM 1800
(1.8 GHz) starts to be limited by structural walls. UMTS, at 2.1 GHz is quite similar in coverage to GSM 1800.
Higher frequencies are a disadvantage when it comes to coverage,
but it is a decided advantage when it comes to capacity. Pico cells,
covering e.g. one floor of a building, become possible, and the same
frequency can be used for cells which are practically neighbours.
Cell service area may also vary due to interference from
transmitting systems, both within and around that cell. This is true
especially in CDMA based systems. The receiver requires a certain signal-to-noise ratio,
and the transmitter should not send with too high transmission power in
view to not cause interference with other transmitters. As the
receiver moves away from the transmitter, the power received decreases,
so the power control
algorithm of the transmitter increases the power it transmits to
restore the level of received power. As the interference (noise) rises
above the received power from the transmitter, and the power of the
transmitter cannot be increased any more, the signal becomes corrupted
and eventually unusable. In CDMA-based systems, the effect of
interference from other mobile transmitters in the same cell on coverage
area is very marked and has a special name, cell breathing.
One can see examples of cell coverage by studying some of the
coverage maps provided by real operators on their web sites or by
looking at independently crowdsourced maps such as OpenSignal.
In certain cases they may mark the site of the transmitter, in others
it can be calculated by working out the point of strongest coverage.
A cellular repeater
is used to extend cell coverage into larger areas. They range from
wideband repeaters for consumer use in homes and offices to smart or
digital repeaters for industrial needs.
Coverage comparison of different frequencies
The following table shows the dependency of the coverage area of one cell on the frequency of a CDMA2000 network:
| Frequency (MHz) | Cell radius (km) | Cell area (km2) | Relative Cell Count |
|---|---|---|---|
| 450 | 48.9 | 7521 | 1 |
| 950 | 26.9 | 2269 | 3.3 |
| 1800 | 14.0 | 618 | 12.2 |
| 2100 | 12.0 | 449 | 16.2 |
