09-10-2012, 04:28 PM
CELLULAR AND MOBILE COMMUNICATIONS
[attachment=35825]
Why hand off is necessary ?
In an analog system, once a call is established, the set-up channel is not used again during the
call period. Therefore, handoff is always implemented on the voice channel. In the digital
systems, the handoff is carried out through paging or common control channel. The value of
implementing handoffs is dependent on the size of the cell. For example, if the radius of the cell
is 32 km (20 mi), the area is 3217 km^2(1256 mi^2). After a call is initiated in this area, there is
little chance that it will be dropped before the call is terminated as a result of a weak signal at the
coverage boundary. Then why bother to implement the handoff feature? Even for a 16-km
radius, cell handoff may not be needed. If a call is dropped in a fringe area, the customer simply
redials and reconnects the call. Today the size of cells becomes smaller in order to increase
capacity. Also people talk longer. The handoffs are very essential. Handoff is needed in two
situations where the cell site receives weak signals from the mobile unit: (1) at the cell boundary,
say, −100 dBm, which is the level for requesting a handoff in a noise-limited environment;
What are the two decision making parameters of handoff explain.
There are two decision-making parameters of handoff: (1) that based on signal strength and (2)
that based on carrier-to-interference ratio. The handoff criteria are different for these two types.
In type 1, the signal-strength threshold level for handoff is −100 dBm in noise-limited systems
and −95 dBm in interference-limited systems. In type 2, the value of C/I at the cell boundary for
handoff should be at a level, 18 dB for AMPS in order to have toll quality voice. Sometimes, a
low value of C/I may be used for capacity reasons.
What are the advantages of delayed handoff
Consider the following example. The mobile units are moving randomly and the terrain contour
is uneven. The received signal strength at the mobile unit fluctuates up and down. If the mobile
unit is in a hole for less than 5 s (a driven distance of 140 m for 5 s, assuming a vehicle speed of
100 km/h), the delay (in handoff) can even circumvent the need for a handoff. If the neighboring
cells are busy, delayed handoff may take place. In principle, when call traffic is heavy, the
switching processor is loaded, and thus a lower number of handoffs would help the processor
handle call processing more adequately. Of course, it is very likely that after the second handoff
level is reached, the call may be dropped with great probability. The other advantage of having a
two-handoff-level algorithm is that it makes the handoff occur at the proper location and
eliminates possible interference in the system. Figure 3, case I, shows the area where the firstlevel
handoff occurs between cell A and cell B. If we only use the second-level handoff
boundary of cell A, the area of handoff is too close to cell B. Figure 3, case II, also shows where
the second-level handoff occurs between cell A and cell C. This is because the first-level handoff
cannot be implemented.
[attachment=35825]
Why hand off is necessary ?
In an analog system, once a call is established, the set-up channel is not used again during the
call period. Therefore, handoff is always implemented on the voice channel. In the digital
systems, the handoff is carried out through paging or common control channel. The value of
implementing handoffs is dependent on the size of the cell. For example, if the radius of the cell
is 32 km (20 mi), the area is 3217 km^2(1256 mi^2). After a call is initiated in this area, there is
little chance that it will be dropped before the call is terminated as a result of a weak signal at the
coverage boundary. Then why bother to implement the handoff feature? Even for a 16-km
radius, cell handoff may not be needed. If a call is dropped in a fringe area, the customer simply
redials and reconnects the call. Today the size of cells becomes smaller in order to increase
capacity. Also people talk longer. The handoffs are very essential. Handoff is needed in two
situations where the cell site receives weak signals from the mobile unit: (1) at the cell boundary,
say, −100 dBm, which is the level for requesting a handoff in a noise-limited environment;
What are the two decision making parameters of handoff explain.
There are two decision-making parameters of handoff: (1) that based on signal strength and (2)
that based on carrier-to-interference ratio. The handoff criteria are different for these two types.
In type 1, the signal-strength threshold level for handoff is −100 dBm in noise-limited systems
and −95 dBm in interference-limited systems. In type 2, the value of C/I at the cell boundary for
handoff should be at a level, 18 dB for AMPS in order to have toll quality voice. Sometimes, a
low value of C/I may be used for capacity reasons.
What are the advantages of delayed handoff
Consider the following example. The mobile units are moving randomly and the terrain contour
is uneven. The received signal strength at the mobile unit fluctuates up and down. If the mobile
unit is in a hole for less than 5 s (a driven distance of 140 m for 5 s, assuming a vehicle speed of
100 km/h), the delay (in handoff) can even circumvent the need for a handoff. If the neighboring
cells are busy, delayed handoff may take place. In principle, when call traffic is heavy, the
switching processor is loaded, and thus a lower number of handoffs would help the processor
handle call processing more adequately. Of course, it is very likely that after the second handoff
level is reached, the call may be dropped with great probability. The other advantage of having a
two-handoff-level algorithm is that it makes the handoff occur at the proper location and
eliminates possible interference in the system. Figure 3, case I, shows the area where the firstlevel
handoff occurs between cell A and cell B. If we only use the second-level handoff
boundary of cell A, the area of handoff is too close to cell B. Figure 3, case II, also shows where
the second-level handoff occurs between cell A and cell C. This is because the first-level handoff
cannot be implemented.