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Top of a cellular radio tower
Structure of a 2G cellular network

A cellular network is a radio network made up of a number of radio cells (or just cells) each served by at least one fixed-location transceiver known as a cell site or base station. These cells cover different land areas to provide radio coverage over a wider area than the area of one cell, so that a variable number of portable transceivers can be used in any one cell and moved through more than one cell during transmission.

Cellular networks offer a number of advantages over alternative solutions:

  • increased capacity
  • reduced power usage
  • larger coverage area
  • reduced interference from other signals

An example of a simple non-telephone cellular system is an old taxi driver's radio system where the taxi company has several transmitters based around a city that can communicate directly with each taxi.

Contents

General characteristics

GSM network architecture

To distinguish signals from several different transmitters, frequency division multiple access (FDMA) and code division multiple access (CDMA) were developed.

With FDMA, the transmitting and receiving frequencies used in each cell are different from the frequencies used in each neighbouring cell. 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) and time division multiple access (TDMA) 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. Time division multiple access, however, 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.

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 know which frequency covers approximately what area. When they do not receive a signal from the transmitter, they will try other channels until they find one that works. The taxi drivers only speak one at a time, when invited by the base station operator (in a sense TDMA).

There are multiple concepts behind mobile phones, including the physics behind the network and towers that connect the cellular phones.

The concept

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 irregular shapes, although hexagonal cells are conventional. 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, provided that the same frequencies are not reused in adjacent neighboring cells as that would cause co-channel interference. A reuse distance, D is calculated as

D=R\sqrt{3N},\,

where R is the cell radius and N is the number of cells per cluster. Cells may vary in radius in the ranges (1 km to 30 km). The boundaries of the cells can also overlap between adjacent cells and large cells can be divided into smaller cells [1]

Structure of the cellular network

A simple structure of the cellular mobile-radio network consists of the following:

The concept of cells consists of mobile network equipment providing a cellular network for mobile station (MS) and PSTN users. 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 telephone customers get the desired service; some of them include mobility management, registration, call set up, and handover. Any MS connects to the network via an RBS in the corresponding cell which in turn connects to the MSC. The MSC allows the connection of other networks apart from mobile networks. The link from an MS to the RBS is called an uplink while that from an RBS to the MS is termed downlink; for more information, see GSM.

Radio channels effectively use the transmission medium through the use of the following multiplexing schemes: frequency division multiplex (FDM), time division multiplex (TDM), code division multiplex (CDM), and space division multiplex (SDM). Corresponding to these multiplexing schemes are the following access techniques: frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and space division multiple access (SDMA).[2]

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 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). 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.

Frequency reuse

Example of frequency reuse factor or pattern 1/4

The increased capacity in a cellular network, compared with a network with a single transmitter, comes 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.

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 utilize 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 big city, on the other hand, frequency-reuse could certainly be in use.

Directional antennas

Cellular telephone frequency reuse pattern. See U.S. Patent 4,144,411

Although the original 2-way-radio cell towers were at the centers of the cells and were omni-directional, a cellular map can be redrawn with the cellular telephone towers located at the corners of the hexagons where three cells converge.[3] Each tower has three sets of directional antennas aimed in three different directions and receiving/transmitting into three different cells at different frequencies. This provides a minimum of three channels for each cell. 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.[4]

Movement from cell to cell and handover

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. For example, in all GSM handovers and W-CDMA inter-frequency handovers the mobile station will test the reserved channel before changing channels. Once the channel is confirmed okay, the network will command the mobile unit to switch to the new channel and at the same time start bi-directional communication on the new channel. In CDMA2000 and W-CDMA same-frequency handovers, both channels will actually be in use at the same time (this is called a soft handover or soft handoff). 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.

Frequency choice

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. This is 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. UMTS, at 2.1 GHz is quite similar in coverage to GSM 1800. At 5 GHz, 802.11a Wireless LANs already have very limited ability to penetrate walls and may be limited to a single room in some buildings. At the same time, 5 GHz can easily penetrate windows and goes through thin walls so corporate WLAN systems often give coverage to areas well beyond that which is intended.

Moving beyond these ranges, network capacity generally increases (more bandwidth is available) but the coverage becomes limited to line of sight. Infra-red links have been considered for cellular network usage, but as of 2004 they remain restricted to limited point-to-point applications.

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. As the receiver moves away from the transmitter, the power transmitted is reduced. 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.

Old fashioned taxi radio systems generally use low frequencies and high sited transmitters, probably based where the local radio station has its mast. This gives a very wide area coverage in a roughly circular area surrounding each mast. Since only one user can talk at any given time, coverage area doesn't change with number of users. The reduced signal to noise ratio at the edge of the cell is heard by the user as crackling and hissing on the radio.

One can see examples of cell coverage by studying some of the coverage maps provided by real operators on their web sites. 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.

Coverage comparison

Following table shows the dependency of frequency on coverage area of one cell of a CDMA2000 network:[5]

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

Mobile phone networks

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. Large geographic areas (representing the coverage range of a service provider) may be split into smaller cells to avoid line-of-sight signal loss and the large number of active phones in an area. In cities, each cell site has a range of up to approximately ½ mile, while in rural areas, the range is approximately 5 miles. Many times in clear open areas, a user may receive signals from a cellsite 25 miles away. All of the cell sites are connected to cellular telephone exchanges "switches", which connect to a public telephone network or to another switch of the cellular company.

As the phone user moves from one cell area to another cell, the switch automatically commands the handset and a cell site with a stronger signal (reported by each handset) to switch to a new radio channel (frequency). When the handset responds through the new cell site, the exchange switches the connection to the new cell site.

With CDMA, multiple CDMA handsets share a specific radio channel. The signals are separated by using a pseudonoise code (PN code) 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.

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 a limited number of radio frequencies can be simultaneously used by many callers with less interference.

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.

Old systems predating the cellular principle may still be in use in places. The most notable real hold-out is used by many amateur radio operators who maintain phone patches in their clubs' VHF repeaters.

There are a number of different digital cellular technologies, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), Code Division Multiple Access (CDMA), Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), 3GSM, Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN).

See also

References

  1. ^ J. E. Flood. Telecommunicattions Networks. Institution of Electrical Engineers, London, UK, 1997. chapter 12.
  2. ^ Bernhard H. Walke. Mobile Radio Networks: Networking, protocols and traffic performance. John Wiley and Sons, LTD West Sussex England, 2002. Chapter 2.
  3. ^ Cell towers at corners of hexagon cells
  4. ^ U.S. Patent 4,144,411 -- Cellular Radiotelephone System for Different Cell Sizes -- Richard H. Frenkiel (Bell Labs), filed Sep 22, 1976, issued March 13, 1979
  5. ^ http://www.itu.int/ITU-D/imt-2000/Meetings/Slovenia/Presentations/Day%203/3.3.1_Chandler.pdf page 17
  • P. Key, D. Smith. Teletraffic Engineering in a competitive world. Elsevier Science B.V., Amsterdam Netherlands, 1999. Chapter 1 (Plenary) and 3 (mobile).







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