Mumbai university > Comp > SEM 8 > Mobile Communication

Marks: 10M

Year: Revised 2012

• With the demand for wireless service increases, the number of channels assigned to a cell becomes insufficient to support the required number of users.

• At this point cellular design techniques are needed to provide more channels per unit coverage area.

• Techniques discussed below are used in practice to expand the capacity of cellular system.

A Cell splitting:

• Cell splitting is the process of sub dividing a congested cell into smaller cells, each with its own base station and corresponding reduction in antenna height and transmitted power.

• Cell splitting increases capacity of a cellular system since it increases number of times that channels are reused.

• By defining new cells which have a smaller radius than the original cells and by installing these smaller cells between existing cells, capacity increases due to additional number of channels per unit cell area

• In this ( D)⁄R ratio is kept constant and entire system is rescaled.

B. Cell sectoring:

• In cell sectoring a single omnidirectional antenna at base station is replaced by several directional antennas, each radiating within a specified sector.

• By using directional antennas power is transmitted in single desired direction decreasing number of interfering co-channel cells and co-channel interference.

• The technique for decreasing co-channel interference and thus increasing system performance by using directional antennas is called sectoring.

• The factor by which the co-channel interference is reduced depends on the amount of sectoring used. A cell is normally partitioned into three sectors.

• When sectoring is employed, the channels used in a particular cell are broken down into sectored groups and are used only within a particular sector.

• For cluster size 7, sectoring reduces co-channel cells from 6 to 2 for $120^0$ sectoring and to 1 for $60^0$ sectoring.

• Advantages:

  • Improvement in S/I ratio.

  • Improvement in system capacity.

• Disadvantages:

  • Increased number of antennas at base station.

  • Decrease in trunking efficiency due to channel sectoring at the base station.

  • Increase in number of handoffs, since sectoring reduces the coverage area of the particular group of frequencies.

C. 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.

• 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.

• 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.