Handoffs for Cellular Systems : Handoffs for Cellular Systems
Itinerary : Itinerary Cellular Concept
Frequency Reuse
Handoff
Basic theory
Performance
Design of cellular systems
Prioritization of handoffs
Wireless Comm. Systems : Wireless Comm. Systems Goals
provide coverage over a large area
provide coverage to a large number of users
maintain an acceptable level of quality
Wireless Systems (cont’d) : Wireless Systems (cont’d) A single tower at the center called a base station.
Powerful transmitter attached to top of base station.
Coverage area increased by making tower taller and/or increasing transmission power.
Wireless Systems (cont’d) : Wireless Systems (cont’d) Does this model satisfy our goals?
Large coverage area
Wireless Systems (cont’d) : Wireless Systems (cont’d) Does this model satisfy our goals?
Large coverage area
High call quality
Wireless Systems (cont’d) : Wireless Systems (cont’d) Does this model satisfy our goals?
Large coverage area
High call quality
Limited number of users
Problem! : Problem! Limit on number of users due to frequency limitations (only so much spectrum)
Some way to reuse frequencies?
Solution! : Solution! Cellular Systems to the rescue!
Cellular System : Cellular System Cellular concept:
Many small base stations
Each BS communicates with mobiles in its respective cell.
MSC is responsible for coordinating communications within the system
Cellular System (cont’d) : Cellular System (cont’d) Frequency reuse
The same frequencies are reused in multiple cells.
Cellular Systems (cont’d) : Cellular Systems (cont’d) Does this model satisfy our goals?
Large coverage area
High call quality
Cellular Systems (cont’d) : Cellular Systems (cont’d) Does this model satisfy our goals?
Large coverage area
High call quality
High capacity
Question : Question What happens when mobile moves from one cell to another?
Handoff!!
The process of transferring a mobile user from one channel or base station to another.
Handoff : Handoff
Handoff (cont’d) : Handoff (cont’d) Handoffs must be:
performed quickly
performed infrequently
imperceptible to users
performed successfully
Handoff (cont’d) : Handoff (cont’d) When will a handoff occur?
Initiated when received signal level drops below a certain threshold value.
Not as simple as it seems
Actually consider a time average of the received signal instead of the instantaneous level.
Handoff (cont’d) : Handoff (cont’d) Define
PMIN_USABLE as the minimum usable signal level
PHANDOFF as the threshold received signal level at which a handoff will be initiated
∆ = PHANDOFF – PMIN_USABLE
Handoff (cont’d) : Handoff (cont’d) Must optimize ∆.
Too large => too many handoffs
Too small => too many lost calls
Value of ∆ depends on
Environment
Expected mobile speeds
Time required to perform a handoff
Handoff - Successful : Handoff - Successful
Handoff - Unsuccessful : Handoff - Unsuccessful
Handoff – Unsuccessful (cont’d) : Handoff – Unsuccessful (cont’d) Reasons for failed handoff:
∆ too small (i.e. PHANDOFF too low)
high mobile speeds
excessive delay at MSC
High traffic level
Unavailability of channels
Handoff (cont’d) : Handoff (cont’d) Ways to improve handoff performance
Optimize ∆
Account for varying mobile speeds
Minimize delay at MSC
Prioritize handoffs
Mobile assisted handoffs, mobile locators, minimize number of BS’s to search, ...
Performance of Handoffs : Performance of Handoffs Accounting for varying mobile speeds.
Affects design phase of cellular system
What size of cells should be used?
Large Cells (Macrocellular)
Small Cells (Microcellular)
Large and Small Cells (Overlay/Underlay)
Performance (cont’d) : Performance (cont’d) Macrocellular Systems
cell radii several km
high speed mobiles
Problem:
Cells too large to provide required capacity for dense user bases.
Performance (cont’d) : Performance (cont’d) Microcellular Systems
Cell radii only 100-200 m
Low-speed mobiles
Problem:
Excessive handoffs occur when high-speed mobiles are present.
Performance (cont’d) : Performance (cont’d) Overlay/Underlay Systems (Umbrella cells)
Combine large and small cells
Performance (cont’d) : Performance (cont’d) Overlay/Underlay Systems (Umbrella cells)
Microcells increase system capacity
Macrocells reduce load on system due to excessive handoffs
Allows for increased capacity while reducing number of handoffs.
Performance (cont’d) : Performance (cont’d) Priority Schemes for Handoffs
Goal is to minimize rate of handoff failure
Guard Channel Concept
Queuing of Handoffs
Giving handoffs priority over call requests
Performance (cont’d) : Performance (cont’d) Guard Channel Concept
reserve set of channels for handoffs only
reduces number of blocked handoffs
reduces system capacity
Performance (cont’d) : Performance (cont’d) Queuing of Handoffs
Put handoff requests in a queue
Serves handoffs on a FCFS basis
reduces number of failed handoffs
reduces system capacity
Performance (cont’d) : Performance (cont’d) Giving handoffs priority over call requests
reduces rate of handoff failure
desirable from user’s point of view
Concluding Remarks : Concluding Remarks Cellular concept is very popular
Handoffs are essential in cellular systems
System design must take handoff-related considerations into account.
Performance of handoffs is complex and depends on many factors.
Slide 34 : The End