UNIT V

HANDOFFS & DROPPED CALLS

1. Value of Implementing Handoffs

1.1 Why Handoff is Necessary:

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; and 
2. when the mobile unit is reaching the signal-strength holes
(gaps) within the cell site as shown in Fig.1
Fig.1. Occurrence of Handoffs
1.2 Two types of Handoff:
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.
Type 1:
• It is easy to implement. The location receiver at each cell site
measures all the signal strengths of all receivers at the cell
site. However, the received signal strength (RSS) itself includes
interference. 
RSS = C + I
where C is the carrier signal power and I is the interference.
• Suppose that we set up a threshold level for RSS; then,
because of the I , which is sometimes very strong, the RSS
level is higher and far above the handoff threshold level. In
this situation handoff should theoretically take place.
• Another situation is when I is very low but RSS is also low. In
this situation, the voice quality usually is good even though
the RSS level is low, but since RSS is low, unnecessary handoff
takes place. Therefore, it is an easy but not very accurate
method of determining handoffs.
Type 2:
Handoffs can be controlled by using the carrier-to-
interference ratio C/I
(C+I)/I = C/I
• we can set a level based on C/I ,so C drops as a function of
distance but I is dependent on the location. If the handoff is
dependent on C/I , and if the C/I drops, it does so in response
to increase in (1) propagation distance or (2) interference.
• In both cases, handoff should take place. In today’s cellular
systems, it is hard to measure C/I during a call because of
analog modulation. Sometimes we measure the level I before
the call is connected, and the level C + I during the call. Thus
(C + I )/I can be obtained.
1.3 Number of Handoffs per Call:
• The smaller the cell size, the greater the number and the
value of implementing the handoffs.
• The number of handoffs per call is relative to cell size.
i. 0.2 handoff per call in a 16 – 24 Km cell
ii. 1-2 handoffs per call in 3.2 – 8 km cell
iii. 3-4 handoffs per call in 1.6 – 3.2 Km cell
2. Delaying a Handoff
• In many cases, a two-handoff-level algorithm is used. The
purpose of creating two request handoff levels is to provide
more opportunity for a successful handoff.
• A handoff could be delayed if no available cell could take the
call. A plot of signal strength with two request handoff levels
and a threshold level is shown in Fig.3.
• The plot of average signal strength is recorded on the channel
received Signal strength indicator (RSSI), which is installed at
each channel receiver at the cell site.
• When the signal strength drops below the first handoff level,
a handoff request is initiated. If for some reason the mobile
unit is in a hole (a weak spot in a cell) or a neighboring cell is
busy, the handoff will be requested periodically every 5 s.
Fig.3. A two level handoff scheme
• At the first handoff level, the handoff takes place if the new signal
is stronger. However, when the second handoff level is reached,
the call will be handed off with no condition.
• The MTSO always handles the handoff call first and the originating
calls second. If no neighboring calls are available after the second
handoff level is reached, the call continues until the signal
strength drops below the threshold level; then the call is dropped.
• In AMPS systems if the supervisory audio tone (SAT) is not sent
back to the cell site by the mobile unit within 5 s, the cell site
turns off the transmitter.
Advantages:
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 first-level 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.
3. FORCED HANDOFFS:
A forced handoff is defined as a handoff that would normally
occur but is prevented from happening, or a handoff that
should not occur but is forced to happen.
Controlling a Handoff: 
• The cell site can assign a low handoff threshold in a cell to
keep a mobile unit in a cell longer or assign a high handoff
threshold level to request a handoff earlier.
• The MTSO also can control a handoff by making either a
handoff earlier or later, after receiving a handoff request from
a cell site.
Creating a Handoff: 
• In this case, the cell site does not request a handoff but the
MTSO finds that some cells are too congested while others
are not. Then, the MTSO can request cell sites to create early
handoffs for those congested cells.
• In other words, a cell site has to follow the MSO’s order and
increase the handoff threshold to push the mobile units at the
new boundary and to handoff earlier.
5. Queuing of handoff:
• Queuing of handoffs is more effective than two- threshold-level
handoffs.
• The MTSO will queue the requests of handoff calls instead of
rejecting them if the new cell sites are busy.
• A queuing scheme becomes effective only when the requests
for handoffs arrive at the MTSO in batches or bundles.
• If handoff requests arrive at the MTSO uniformly, then the
queuing scheme is not needed.
• Before showing the equations, let us define the parameters as
follows.
1/µ average calling time in seconds, including new calls and
handoff calls in each cell
• λ1 arrival rate (λ1 calls per second) for originating calls
• λ2 arrival rate (λ2 handoff calls per second) for handoff calls
• M1 size of queue for originating calls
• M2 size of queue for handoff calls
• N number of voice channels
• a (λ1 + λ2)/µ
• b1 λ1/µ
• b2 λ2/µ
The following analysis can be used to see the improvement.
We are analyzing three cases.
1. No queuing on either the originating calls or the handoff
calls. The blocking for either an originating call or a handoff
call is
2. Queuing the originating calls but not the handoff calls.
The blocking probability for originating calls is
 The blocking probability for handoff calls is

3. Queuing the handoff calls but not the originating calls. The
blocking probability for handoff calls is

The blocking probability for originating calls is

POWER-DIFFERENCE HANDOFFS
• A better algorithm is based on the power difference ( Δ) of a
mobile signal received by two cell sites, home and handoff.
Intersystem handoff
• A handoff in which a call may be initiated in one cellular
system (controlled by one MTSO) is transferred to another
system (controlled by another MTSO) before terminating.

• This means that a call handoff can be transferred from one

system to a second system so that the call is continued while
the mobile unit enters the second system.
Intersystem handoff
 INTRODUCTION TO DROPPED CALL RATE

Definition of Dropped Call Rate:

• The definition of a dropped call is after the call is established
but before it is properly terminated.
• The definition of “the call is established” means that the call is
setup completely by the setup channel. If there is a possibility
of a call drop due to no available voice channels, this is
counted as a blocked call not a dropped call.
• If there is a possibility that a call will drop due to the poor
signal of the assigned voice channel, this is considered a
dropped call.
 The perception of dropped call rate by the subscribers can be
higher due to: 

1. The subscriber unit not functioning properly (needs repair).  

2. The user operating the portable unit in a vehicle (misused).

3. The user not knowing how to get the best reception from a
portable unit (needs education).

• The dropped call rate and the specified voice quality level are
inversely proportional.
 By maintaining a certain voice quality level, the dropped call
rate can be calculated by taking the following factors into
consideration:
1. Provide signal coverage based on the percentage (say 90
percent) that the entire received signal will be above a given
signal level.  
2. Maintain the specified co-channel and adjacent channel
interference levels in each cell during a busy hour (i.e., the
worst interference case).  
3. Because the performance of the call dropped rate is calculated
as possible call dropping in every stage from the radio link to
the PSTN connection, the response time of the handoff in the
network will be a factor when the cell becomes small, the
response time for a handoff request has to be shorter in order
to reduce the call dropped rate.
Relationship among Capacity, Voice Quality & Dropped
Call Rate:

Radio Capacity m is expressed as follows:

Where BT/BC is the total no of voice channels.

 General formula of dropped call rate
The general formula of dropped call rate P in a whole system can
be expressed as:

Pn is the probability of a dropped call when the call has gone through
n handoffs.