Microcells

Date: 2015-10-07; view: 469.

Future developments

The technology of cellular radio systems continues to develop very rapidly. The early 1980s saw the introduction of the first commer­cial analogue systems and by the end of the decade trials of second generation digital systems were already under way. Systems such as GSM are now entering into service and work is already starting on the specification of a third generation world wide standard system. These developments are not introducing technology for its own sake, but are aimed at improving the quality, capacity, and availa­bility, and reducing the cost of mobile communications. In addition to these step changes in 'generations' of system there are technical advances which are applicable to current systems. These include techniques such as microcellular and intelligent networks.

As the capacity of cellular systems has increased, cell sizes have decreased, in some networks to as small as 0.5km radius, such that controlling co-channel interface becomes a major problem. The use of microcells, that is, very small cells, is a way of increasing capacity still further. In a microcellular layout, base station antennas are placed below the building height in urban areas, and low power is used such that the propagation characteristics between base sta­tion and mobile are dominated by the street layout. Interference from adjacent cells is blocked by buildings.

Microcellular techniques allow significantly higher traffic den­sities to be achieved, and also enable smaller, lower power mobiles to be used. The use of microcells requires improved handover techniques, which allow for fast and reliable handoff, for example when turning a street corner. One way of easing handover problems is to employ an 'umbrella cell' arrangement using conventional cells overlaying the microcells such that handover can be made into the umbrella cell where no suitable adjacent microcell can be identified. This also avoids the need to plan a contiguous coverage of micro-cells in an urban area.

New technology is now enabling the use of more compact and cheaper base stations. Conventional base sites have generally re­quired a purpose built building, or rented space within an existing building for installation of base station racks of equipment. Now, base stations can be housed in small roadside or roof top mounted cabinets, and further reductions in size can be expected. Small base station equipment, and antennas, are essential to enable microcells to be built cost effectively.