Transcript Unit 7 - cecsrc

HANDOFFS AND DROPPED
CALLS
Harpinder Kang Khattra
Sub topics
• Why Handoff
• Types of Hand off and their characteristics
• Dropped call rates and their evaluation
Handoff
• Mobiles may move out of coverage area of a cell and into
coverage area of a different cell during a call
• MSC must identify new BS to handle call
– MSC must seamlessly transfer control of call to new BS
– MSC must assign call new forward and reverse channels within the
channels of new BS
• Some important performance metrics in handoff:
–
–
–
–
Seamless – user should not know handoff occurring
Minimum unnecessary Handoff due to short time fading
Low probability of blocking new calls in the new cell
Handoff to a good SNR channel so that an admitted call is not dropped
Handoff Main Steps
1.
2.
3.
4.
Initiation
Resource reservation
Execution
Completion
Important handoff parameter:
SNRold to initiate handoff based on minimum acceptable quality
SNRnew of the target channel (SNRnew > SNRold )
D = SNRnew - SNRold dB
 If D too small, unnecessary handoffs occur
 If D too large, may be insufficient time to complete handoff before
SNRold becomes too weak and signal is lost
Occurrence of handoff
Types of Handoff
A. Natures of handoff
1. Hard handoff
2. Soft handoff
3. Softer handoff
B. Purposes of handoff
1. Intracell handoff:
2. Intercell handoff:
3. Inter BSC/MSC handoff:
4. Intersystem handoff:
5. Intercarrier handoffs:
6. Intermode handoff:
C. Algorithms of handoff
1. MCHO (Mobile Control Handoff)
2. NCHO (Network Control Handoff)
3. NCHO/MAHO (Network Control Handoff/Mobile Assists Handoff)
Dropped Call Rates
• The dropped call is defined as an established
call which leaves the system before it is
normally terminated
• The Dropped Call Rate (DCR) parameter
represents what percentage of all established
calls is dropped during a specified time period
• The DCR and voice quality are inversely
proportional and high DCR may indicate
coverage, handoff, or channels accessibility
problems
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).
Relationship Among Capacity, Voice Quality,
Dropped Call Rate
Radio Capacity m is expressed as follows:
FORMULA OF DROPPED CALL RATE
• General Formula of Dropped Call Rate
The general formula of dropped call rate P in a
whole system can be expressed as:
Where
And