Subject : Mobile Communication

Topic : Cellular Communication System

Difficulty : Medium

Umbrella Cell Approach

• 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.

• Different antenna heights and different power levels can be used to provide small and large cells located at single site. This technique is called as umbrella cell approach.

• This approach is used to provide large area coverage to high speed users while small area coverage to low speed users. It also ensures additional microcell channels for pedestrian users.

• The speed of each user is estimated by base station or MSC by evaluating how rapidly the short term average signal strength on RVC changes over time or with sophisticated algorithms.

• If high speed user in the large umbrella cell is approaching the base station, and its velocity is rapidly decreasing, the base station may decide to hand the user into the co-located microcell without MSC permission.

• This approach is basically used to reduce number of hand off for high speed users.

Micro- cell in cellular system

Microcell zone concept:.

• The increased number of hand off, increase load on the switching and control link because of sectoring. A solution to this problem is given by microcell zone concept
• Large control base station is replaced by several lower power transmitters on the age of cell.
• The mobile retains the same channel and the base station simply switches the channel to a different zone site and the mobile moves from zone to zone.
• Since a given channel is active only in a particular zone in which mobile is travelling, base station radiation is localized and interference is reduced.
• The advantage of zone cell technique is that while the cell maintains a particular coverage radius, co-channel interference in the cellular system is reduced. As the large central base station is replaced by several lower power transmitters on ages of cell. Decreased co-channel interference improves signal quality leads to increase in capacity without degradation in trunking efficiency caused by sectoring.

1. Advantages:

   • Decrease co-channel interference which leads to an improvement in signal quality and also leads to an increase in capacity. No degradation of trunking efficiency.

2. Disadvantages:

   • More antennas are required.

   • Base station need to be more sophisticated to handle transfer of call from one zone to another zone within the cell.