Answ*er:*

• Capacity of cellular system can be increased using additional cell sites but it is practically difficult to get new physical site in urban areas. Thus instead of new cell sites additional channels and base stations can be installed to increase the capacity.
• In practical cellular systems, several difficulties may be faced when trying to design a system to accomodate wide range of mobile devices with different velocities.
• With the addition of microcells in cellular systems to provide added capacity, the Mobile Switching Centre (MSC) can quickly become burdened if high speed mobile users are constantly being passed between very small cells.
• It is highly desirable to use different antenna heights and different power levels to design a good cellular system.
• It is possible to provide network to "large" and "small" cells colocated at the same location. This approach is called Umbrella approach.
• When a mobile user travels from one area of coverage or cell to another cell within a call’s duration the call should be transferred to the new cell’s base station. Otherwise, the call will be dropped because the link with the current base station becomes too weak as the mobile recedes. For fast moving mobile devices, the need of hand-off is increased even more rapidly. For such devices, a large cell will be desirable, along with a taller antenna, such that the need for frequent hand-off can be avoided.
• The advantage of this approach is that it provides large area coverage of high speed users, whereas small area coverage to low speed users.
• This approach helps to reduce number of hand-offs required by high speed users and provides additional microcell channels for low speed users.
• An umbrella cell or the larger cell covers several microcells. The power level inside an umbrella cell is increased comparing to the power levels used in the microcells that form the umbrella cell. This increased power level is due to the increased height of the antenna for the umbrella cell.
• The speed of hand-offs is estimated by MSC by evaluating how rapidly the signal strength on Reverse Voice Channel changes over a particular time period. More sophisticated algorithms may also be employed to evaluate and partition the users based upon their velocities.
• Thus, large umbrella cell supports the fast moving mobile users, while the smaller microcells support the slow moving mobile users or pedestrians.
• If high speed user in large umbrella cell is approaching the central base station, and its velocity is decreasing continuously, the base station may decide to hand user to the co-located microcell antenna, without the intervention of MSC.
• The main advantage of this umbrella approach is the reduction in number of hand-offs in case of fast moving mobile devices and increase in the traffic handling capacity of the whole cellular system.