Cells

A cell is the coverage area of a single base station. As a mobile phone moves through a network, it accesses services via the base station of whichever cell it is in. The precise shape of a cell depends on the geography of the region; hills and tall buildings can block signals, while different types of soil or depths of water also have some influence. Ignoring these effects, radio waves form an arc radiating out from a transceiver, as shown in Figure 3.2. The signal gets weaker the further it is from the base station, so the cell boundary is the limit where the mobile terminal can no longer send and receive reliably. Unfortunately, the signal doesn't stop conveniently at this limit, resulting in interference for neighboring cells.

The simplest type of transmitter is omnidirectional, meaning it transmits equally in all directions, producing a circular cell. Circles don't tessellate very well, so mobile network architects usually approximate them to hexagons, shown in Figure 3.3. This is useful for planning a cellular system, but isn't strictly accurate—the overlap between cells also has to be taken into account, as in some parts a user may be able to communicate with two or even three base stations.

Microcells

Urban areas have a far greater density of mobile phone users than do rural areas. To serve these extra users, mobile operators deploy microcells, small cells that cover a particular street or even a particular building, such as a conference center. An overlay (or underlay) network has some areas covered by both large and small cells, usually because the larger cells were built first and then the microcells added as demand increased. Present-day technology limits cell diameters to a maximum of 100m (330 feet) because the internal circuitry of a transceiver generates so much radiation that it would interfere with very low power transmissions.

Some mobile vendors also produce private base stations, which allow a company to set up its own cell within an office or warehouse. This means that employees can use their mobile phones within the office, with internal calls routed via the switchboard. These picocells can also be installed in large public buildings, such as airports, railway stations, and even railway carriages.

Cellular operators occasionally use portable microcells to cover large, one-time events, such as major sporting fixtures. These rely on a low-power transceiver, either mounted in a vehicle or set up on a scaffold. The problem here is how to connect the temporary base station to the rest of the network. Such locations usually lack fiber-optic cables and the line of sight required by fixed wireless equipment, meaning that the operator has to rely on satellite links.

Obstacles can sometimes actually help microcells by reducing interference between transceivers in the different cells. A large building may block low-powered signals, allowing streets on either side of it to support different users on the same channel.

Handoff

One of the most important features of a mobile network is the ability for a user to move from one cell to another. This movement was originally known as roaming. Many operators charged higher rates to users moving outside their home cell, just as fixed line carriers charge more for long-distance than for local calls. But most operators now tend to bill calls at the same rate everywhere on their network, and the idea of a phone having one single home cell is quickly disappearing, at least as far as customers are aware. The term roaming is usually reserved for using a phone on a network owned by a different operator, usually one in another country.

The process of switching a user from one cell to another while a call is in progress is called a handoff, or handover. Handoffs are very complex procedures because the base stations have to calculate exactly when a user is crossing the cell boundary. This can take several seconds, so if users move too fast, their calls will be dropped.

The speed limit for analog systems is usually no more than about 100 kph, the same as freeway traffic. Some digital systems can function at speeds above 300 kph, meaning that they can be used on Japanese and European high-speed trains. No system can complete a handoff at the cruising speed of an airliner, which is one reason not to use a mobile phone on a plane. The others are that it's possibly dangerous, and illegal in many countries—in 1998, a man was sentenced to a year in jail for trying to send a text message while on board a British Airways flight.

There are three types of handoff systems in use.

• Soft handoff ensures that a link is set up to the base station in the new cell before the old one is torn down. Sometimes called make before break, this system is very reliable. It should result in dropped calls only if the user is moving extremely fast or actually passes outside the cellular network altogether.The trouble is that a connection with two different base stations is very difficult to achieve; in most types of network, adjacent base stations need to use different frequencies, while a phone can be tuned to only one at a time. Consequently, most existing systems cannot achieve a soft handoff. It is proposed for most third-generation systems, but the only one to use it so far is Qualcomm's cdmaOne.

• Hard handoff requires that a phone break its connection with one base station before connecting to another. This is less reliable than soft handoff because a phone is not always able to establish a new connection. The new cell could already be full, or there may not be another cell at all. A base station sometimes decides to make a handoff based on how far away a phone is, without considering whether another one can pick it up. This means that in areas of poor reception, a phone may be repeatedly handed off only to reconnect with the same cell.

  Hard handoffs cause a noticeable break in conversation, even on fairly advanced digital systems. This can be very annoying to a user moving rapidly between small cells, so networks with microcell overlays often try to detect which users are moving and connect them via the main, larger cell. In such a system, microcells are reserved for stationary users or those walking slowly.

• No handoff is very simple and relies on the mobile terminal actually making a new call once it has moved out of the range of one transmitter. It is very rare in traditional cellular networks because many mobile phone systems can take up to 30 seconds to set up a new call, an unacceptable delay. However, it is used by some newer systems aiming at the PMR market, which has traditionally demanded fast call setups for customers such as the police. The only real advantage of not having a handoff mechanism is this short setup time: fast connections are offered to new calls, as well as those already in progress.

Effect of Frequency

The frequencies used by cellphones all lie within the UHF microwave band, the same as used by TV transmissions. They range from 400 MHz to around 2000 MHz, with the precise frequency affecting the cell size. Higher frequencies are more easily blocked by droplets of cloud or mist in the atmosphere, and so have a shorter range. Networks based on these higher frequencies require smaller cells and more base stations.

Lower frequencies initially seemed preferable because fewer base stations mean lower costs. Governments licensed bands in the 450, 800 or 900 MHz regions, allowing operators to deploy networks quickly and cheaply. Competitors were later given frequencies around 1800 MHz, which disadvantaged them because it meant higher initial costs. The increased number of cells also led to bad publicity, both because of perceived health risks and because many people simply think base stations look ugly.

As mobile phone usage grew, the 1800 MHz operators found their investment paying off. The smaller cells meant that they could service a greater density of customers, while their lower frequency competitors had to build microcells. Mobile data typically requires higher capacity than voice, and so most third-generation systems will use even higher frequencies, around 2000 MHz.