In radio network the number of simultaneous cells may occur is governed largely by the available frequency spectrum and the number of channels that can be supported using the available bandwidth. Such is the pressure for space in the radio spectrum that few users will have more than a harmful of radio channels available to them and therefore should adopt whatever technology makes the most efficient use of the available bandwidth.
In many conventional radio systems for example, groups or departments are allocated dedicated radio frequencies, In order to ensure that those channels are not affected by transmissions from other user operating on the same frequency, sufficient separation between the transmitters must be allowed for when allocating the frequencies. This proactive results in a poor utilisation of radio spectrum supporting comparatively few channels
Whatever of frequency re-use is employed. By assigning only a subset of the total number of channels available to each transmitter site (known as a base station) and applying strict control over the radiated power output, the same frequency can be re-used by other base stations. The coverage area of each base station transmitter forms a ‘cell’ and by laying down a pattern of cells (often represented as an interlocking honeycomb structure, each assigned a different set of frequencies from its neighbours), the structure can be extended to provide regional or national coverage.
In addition, frequency re-use greatly increases the channel capacity, which in turn allows a superior service to be provided.
TYPICAL CELL OPERATION
Each cell has assigned to it a number of channels which it can use on demand for the purpose of voice communication or control. When a mobile is active, it ‘registers’ at an appropriate base station, and information regarding its validity and cell position is stored in the responsible Mobile Switching Centre. When a call is set up, either from, or to , the mobile, the control system will assign a voice channel from the available subset and via the control channel for that cell (which is monitored by all mobiles within it), instructs the mobile to tune to it.
A connection has now been established between the mobile (via the radio link), the base station and the carrier network exchange. For the duration of the call, the quality of the voice channel will be monitored by the base station and reported to the controlling Mobile Switching Centre. This will make decisions concerning the quality and instruct the mobile and base station accordingly.
One of the most significant decisions is the ‘hand off’ of the mobile from one base station area to another. As a vehicle moves about an area, its signal-to-noise is constantly monitored. If this falls below a minimum, perhaps as it is leaving a cell, or if three is excessive co-channel interference-a side effect of frequency re-use in geographically small cell-the signal from adjacent base station is compared, and the cell with the best signal is marked to accept a ‘hand off’. This decision is complicated by several considerations, namely whether the deterioration of the original cell is spurious or temporary, whether the quality of the original could be improved, by increasing power or receiver sensitivity, whether the adjoining base station
comparisons valid, and not itself spurious, and finally whiter the newly selected base station has spare capacity. The algorithm governing selection is complicated and does not even rule out assigning to base station a mobile outside of its nominal geographical area.
In order to allow cells to be adjacent, without causing interference, frequency re-use is employed. This required the mobile to change frequency when it leaves on cell to enter another; during this period calls are interrupted and the voice channel muted. This interruption is usually short and largely transparent to voice uses. Upon call completion, the mobile relinquishes the voice channel, which can then be re-assigned by the base station to another mobile.
Several of the countries operating cellular radio have licensed more than on organisation to provide cellular radio services-particularly those like the UK with liberalised telecommunications markets. This has implications for the manufacturers of handsets such that to ensure the widest possible market, their products must be designed to operate on several different networks; in the UK this is a mandatory requirement.
On most handsets switching between the different networks requires a change of the handset’s operating software; this involves changing or possibly reprogramming a memory device (internal to the telephone) which holds details of channel frequency allocations, serial number and subscriber number. In some units, this can be accomplished simply by a switch on the handset. Mobiles wishing to use this facility must be registered on both systems, pay standing changes for both systems and be billed separately by each system.
The advantage this brings is that should one system deteriorate significantly (as in the case of strike action), then full service can be maintained. This would be of particular interest to emergency or security services, for instance. A lesser advantage would be the ability to change systems in times of congestion to find a spare channel, or in areas of fringe reception of on system.
Roaming is the capacity of a cellular phone, registered on one system, to be able to enter and use other systems. This access should be transparent to the user and should not be confused with the ability to switch the phone between systems.
Call costing would be done by registering network. This would not be used for problems of capacity (or for system failures resulting from industrial action, one suspect). In the UK it is likely to be used to allow coverage in low user base areas, such as the mountain regains of Scotland and Wales where it would be impractical for both operators to provide service. Another use would be for periods of maintenance of base station or mobile control switch.
The term ‘roaming’ is often used loosely when describing the ability of mobiles of one system to be able to move throughout the geographical area of that system, crossing the control areas of the mobile control switch. This is a different requirement from merely being able to move between cells within the area of a single mobile control switch. To be fully mobile over an entire system requires the following abilities:
– To be switched on anywhere in the system, even outside its own home base, and to be able to register onto the system.
– Having registered, to be able to move out of the control area of the mobile control switch it registered with, and still gain access to the system.
– While a call is in progress, to be given automatic ‘hand off’
at the boundary of two cells, both of which are controlled by the same mobile control switch. Nearly all cellular systems have this ability, and indeed this might stand as a definition of a cellular system.
– While a call is in progress, to be given automatic ‘hand off’
between mobile switches. Both networks now offer this feature so that a call in progress is not dropped.
– To have incoming calls routed to it, irrespective of where it is in relation to its home base.
CELL SYSTEM CHARACTERISTICS.
The capacity of the system is usually described in on of two ways. The total available channels, or the subscriber base these channels will allow. The capacity has obvious implications for commercial viability. The greater the user bases the cheaper the cost. Additional facilities
then become more viable. The capacity will depend on several factors:
– the total number of radio channels;
– size of each cell;
– the frequency re-use or repeat factor.
The total number of voice channels that can be made available to any system depends on the frequency spectrum available, the channel bandwidth requirement and the spectrum efficiency of each channel. Spectrum efficiency depends largely on the quality that is required of the channel. Once the total number of channels has been settled, optimum use of them can be made by frequency re-use, and this is closely coupled with the coverage of each cell.
Fading is the reduction of signal strength at the receiver, due to propagation effects. It has the obvious direct effect on the radio signal and, depending upon circumstances, can be severe enough to prevent operation. In any radio communication system, it plays a significant part in the overall performance. It has particularly marked effect for a cellular network. The International Radio Consultative Committee (CCIR) grades propagation path profiles into a number of categories, depending on the severity of fads that need to be allowed for Areas of hilly country, with a turbulent atmosphere and propagation align line of sight paths, are accorded favourable fading characteristics. Unfavourable areas are over humid, flat terrain, especially over water, where the radio signal is diffused by differing propagation qualities of the surrounding environment (in a similar way that light is diffused as it passes through water)
Multipath fading is caused by the transmission of signals along several paths, resulting in simultaneous reception. Depending on their relative phase and amplitude, the resultant signal strength can be more than a normal (single) path, or can result in complete cancellation or loss of signal. The phenomenon is the result of the radio signal being refracted as it propagates deferring environments, hills, water, cloud, even different layer of the atmosphere, each conferring differing degrees of refraction, resulting in signal propagation paths of differing lengths.
A very similar effect is obtained from reflective multipath. This is common over water, or marsh and bog, and also over desert plains. This effect places serious limitations on microwave transmission over water especially for tidal areas where the reflecting surface rises and falls. These two forms of fading are usually slow to develop, and can exist for a period of hours.
Re-radiation can also be considered in the light of multipath. Especially in the urban environment, a large number of structures can be considered as radiating antennas, for example buildings, bridges, pylons, even aircraft and other vehicles. A complicated coverage patter will result from the multiple re-radiations from these, and produce a spatial field of peaks and troughs of field strength. In many cases these will be static, time in invariant spatial troughs, or nulls. The patter produced will depend on the frequency used. The wavelength of a signal is inversely proportional to the frequency; hence the lover the wavelength the wider will be the interference patterns. The distance between peaks and troughs will increase, as will the extent of the null. In general, for this and other reasons, higher frequencies give better coverage in urban environments.
Multipath fading is a very localised effect. The distance at any time between nulls and peaks of field strength need be more than a few wavelengths. At 900 MHz this is of the order of 30 centimetres.
Shielding is the absence of field strength. Most common causes are tunnels, hills, inside certain buildings, etc. It can also mean the complete loss of signal. In contrast to fading, it is usually time in variant and exists over the whole area affected. To escape from a shielded area means moving completely out of the area, as fads can last a considerable time. There are methods of improving the coverage in a shielded area. One such method is the use of ‘leaky’ feeders. This is being discussed for use in tunnels particularly, and in ships or oil platforms. A transmitter/receiver is used in an unshielded position to make a normal radio link, and the signal is re-transmitted along refection-free cables which ‘leak’ a proportion of the power along the shielded route.