A. Handoff in 3G system:
In 3G systems the majority of handoffs are intra frequency soft handoffs. A soft handoff performed between two sectors belonging to different base stations but not necessarily to the same BSC is called a 2-way soft handoff. A soft handoff may be more than 2-way if the number of sector involved in the handoff process is more than two.
In 3G system near and far terminal problem exists thus 3G systems implements soft handoff when MS moves from one location to another
During soft handoff a mobile station is in the overlapping cell coverage of two sectors belonging to two different base stations. The communication between mobile station and base station occur concurrently via two air interfacechannels from each base station separately. Both channels (signals) are receivedat the mobile station by maximal combining Rake processing. Soft handoff occurs in about 20–40% of calls.Soft handoffs are an integral part of CDMA design. The determination ofwhich pilots will be used in the soft handoff process has a direct impact on thequality of the call and the capacity of the system. Therefore, setting soft handoffparameters is a key element in the 3G system design.
When mobile ismoving away from base station it continuously measures the pilot signal coming from all nearby BS and select the best candidate BS for handoff. Thus the signal coming from pilot channel is used to make handoff decision. If the strength of pilot signal coming from old base station decreases then handoff decision is taken. Signal carried by pilot channel is used to measure power of signal to make handoff decision because this signal do not require power control and thus can be used to measure RSSI (Received Signal Strength Indicator) parameters of the signal which is very important in handoff.
Active set, candidate set, neighbour set and remaining set are different type of pilot set. Pilot signals which are continuously monitored by MS include in active set which are used in handoff.
Mobile Assisted Handoff (MAHO) is also used in 3G system.
B. Power control in 3G system:
In 3G system power control is necessary to ensure that each mobile user within a cell can receives and transmits the signal with enough power to correctly send the desired information while the signal is interfering with the transmission from other mobile users within the cells.
Power control is very important to avoid near far terminals problem in 3G system. Signal strength decreases a distance between sender and receiver increases. Near and far terminal create problem in 3G system as signals received by the receiver have different signal strength. The power of the signal coming from the nearer terminal is much more than that the signal from far terminal. Thus mobile is will not be able to receive the signal coming from the far terminal as it appear as noise. Thus due to signal coming from near terminal the receiver cannot be able to receive the signal coming from far terminal. Therefore there is need to control power of the signal coming from all senders such that all the signals received by the receiver should have the equal strength of the signal at the receiver.
If the mobile’s signal arrives at the base station with a lower power level than the required power level. Its error rate performance will be high while if the mobile’s signal is too high, it will interfere with other users with same radio channel. Power control minimizes interference to mobiles in same case station as well as mobiles in other base station by transmitting minimum required power.
Power control adjusts the target signal to interference ratio in the base station according to the needs of the individual radio links and aims at a constant quality which is defined as a certain target bit error rate (BER) or frame error ratio (FER).Power control also improves quality of signal at the receiver by reducing effect of fading and shadowing with increase in transmission power.