Subject: Mobile Communication

Topic: Cellular Communication System

Difficulty: High

Soft Handoff:

• In soft handoff mobile phone may be actively connected to multiple base stations simultaneously, possibly for a considerable length of time.

• Which is like suppose a mobile station (MS) is up on a call and moves from one base station (BS) to another, but the MS starts communicating with a new BS before terminating communications with the old base station.

• In Soft Handoff, at least one of the links is kept when radio signals are added or removed to the Base Station. Soft Handoff adopted the ‘make before break’ policy.

• Soft handoffs can only be used between base stations on the same frequency. The technique improves reception as MSs move between cells.

Fig: Soft Handoff:

• During soft handoff the mobile station actually communicates with more than one BS at a time, so that when it's time to move from the weaker BS to the stronger one, the MS is already in communication with the stronger one.

• In soft handoff the mobile station receives independent closed loop power control bits from the two BSs and perform "Or of Downs" logic to determine how to adjust its power. Which means the mobile station will increase its power level if and only if both power control bits from the two BSs are 0 (indicating up). If the power control bit from any base station equals to '1' (indicating down), the mobile station shall decrease its power.