Cellular network

From Academic Kids

A cellular radio network is a radio network made up of a number of radio cells (or just cells) each served by a fixed transmitter, normally known as a base station. These cells are used to cover different areas in order to provide radio coverage over a wider area than the area of one cell. Cellular networks are inherently asymmetric with a set of fixed main transceivers each serving a cell and a set of distributed (generally, but not always, mobile) transceivers which provide services to the network's users.

Cellular networks offer a number of advantages over alternative solutions,

  • increased capacity
  • reduced power usage
  • better coverage

A good (and simple) example of a cellular system is an old taxi driver's radio system where a city will have several transmitters based around a city. We'll use that as an example and assume that each transmitter is handled separately by a different operator.


General characteristics

The primary requirement for a cellular network is a way for the distributed stations to distinguish the signal from its own transmitter from the signal from other transmitters. There are two common solutions to this, frequency division multiple access (FDMA) and code division multiple access (CDMA). FDMA works by using a different frequency for each neighbouring cell. By tuning to the frequency of a chosen cell the distributed stations can avoid the signal from other neighbours. 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 Polarisation 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 case of our taxi company, each radio has a knob. The knob acts as a channel selector and allows the radio to tune to different frequencies. As the drivers move around, they change from channel to channel. The drivers know which frequency covers approximately what area, when they don't get a signal from the transmitter, they also try other channels until they find one which works. The taxi drivers only speak one at a time, as invited by the operator (in a sense TDMA).

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 system). Paging takes place by sending the broadcast message on all of those cells. In a few cases paging messages can be used for information transfer. This happens in pagers and also in the UMTS system where it allows for low downlink latency in packet based connections.

Our taxi network is a very good example here. The broadcast capability is often used to tell about road conditions and also to tell about work which is available to anybody. On the other hand, typically there is a list of taxis waiting for work. When a particular taxi comes up for work, the operator will call their number over the air. The taxi driver acknowledges that they are listening, then the operator reads out the address where the taxi driver has to go.

Frequency reuse

Frequency reuse in a cellular network
Frequency reuse in a cellular network

The increased capacity in a cellular network, as compared to 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/n where n is the number of cells which cannot use a frequency for transmission.

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. In other words, every cell uses the same frequency and the different systems are separated by codes rather than frequencies.

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.

Movement from cell to cell and handover

The use of multiple cells means that, if the distributed transceivers are mobile and moving from place to place, they also have to change from cell to cell. The mechanism for this differs depending on the type of network and the circumstances of the change. For example, if there is an ongoing continuous communication and we don't want to interrupt it, then great care must be taken to avoid interruption. In this case there must be clear coordination between the base station and the mobile station. Typically such systems use some kind of multiple access independently in each cell, so an early stage of such a handover is to reserve a new channel for the mobile station on the new base station which will serve it. The mobile then moves from the channel on its current base station to the new channel and from that point on communication takes place

The exact details of the mobile system's move from one base station to the other varies considerably from system to system. E.g. in all GSM handovers and WCDMA inter-frequency handovers the mobile station will measure the channel it is meant to start using before moving over. Once the channel is confirmed okay, the network will command the mobile station to move to the new channel and at the same time start bi-directional communication there, meaning there is no break in communication. In IS-95 and WCDMA same frequency handovers, both channels will actually be in use at the same time (this is called a soft handover). In IS-95 inter-frequency handovers and older analog systems such as NMT it will typically be impossible to measure the target channel directly whilst 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 whilst 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 station to spontaneously move from one cell to another and then notify the network if needed.

In the case of a the primitive taxi system that we are studying, handovers won't really be implemented. The taxi driver just moves from one frequency to another as needed. If a specific communication gets interrupted due to a loss of a signal then the taxi driver asks the controller to repeat the message. If one single taxi driver misses a particular broadcast message (e.g. a request for drivers in a particular area), the others will respond instead. If nobody responds, the operator keeps repeating the request.

Cell coverage area

The ideal cellular network, shown in textbooks, has hexagonal cells evenly spread out across the page. This might be approached in reality in a perfectly flat area with no buildings or other objects but it would be a rare system which would want to cover such an area. In practise, cell coverage varies considerably according to the terrain, the siting of the cell's antenna, intervening buildings, landmarks and barriers.

The other factor which influences cell coverage considerably is the frequency of the radio signal used. Simply put, lower frequencies tend to penetrate through obstacles well, whilst higher frequencies tend to be stopped by thin objects. For example, a five millimetre plaster board wall will completely stop light, but will have almost no noticeable effect on radio waves.

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 (900MHz) 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, 5GHz 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, such as the one we have been studying, 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.

To see real examples of cell coverage look at 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 located by working out the point of strongest coverage.

Cellular telephony

Missing image
Cellular transmitting tower in the United States

Another common example of a cellular network are mobile phone networks. A mobile phone is a portable telephone which receives or makes calls through a Cell site, 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) are split up into smaller cells to deal with line-of-sight signal loss and the large number of active phones in an area. Each cell site has a range of 3-15 miles and overlaps other cell sites. All of the cell sites are connected to one or more cellular switching exchanges which can detect the strength of the signal received from the telephone.

As the telephone user moves or from one cell area to another, the exchange automatically commands the handset and a cell site with a stronger signal (from the handset) to go to a new radio channel. When the handset responds through the new cell-site, the exchange switches the connection to the new cell-site.

With CDMA technology, the process is slightly different. Multiple CDMA handsets share a specific "channel"; the signals are separated by sending each bit using a pseudo-random code sequence specific to each phone. As the user moves from one cell to another, the handset actually connects to both sites simultaneously. This is known as a "soft handoff" because, unlike with traditional cellular technology, there is no one defined point where the phone switches to the new cell.

Modern mobile phones 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 reused by many callers with less interference. CDMA handsets, in particular, must have strict power controls to avoid interference with each other. An incidental benefit is that the batteries in the handsets need less power.

However, almost all mobile phones use cellular technology, including GSM, CDMA and the old analog mobile phone systems. Hence, many people use the term "cell phone" to mean any mobile telephone system. The exception to mobile phones using cellular technology are satellite phones.

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

There are a number of different digital cellular technologies; these include: GSM, GPRS (General Packet Radio Service), CDMA (Code Division Multiple Access), EDGE Enhanced Data for GSM Evolution, 3GSM, DECT, IS-136, and iDEN.

See also


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