Handoff in Cellular Systems

By: Sireesha Vempati.

Venkata Uppuluri. Vijaya Petla.

What is a Handoff?

Handoff refers to a process of transferring an ongoing call or data session from one channel connected to the core network to another.

Process of transferring a MS from one base station to another.

Also called as ‘Handover’.

Reasons for a Handoff to be conducted To avoid call termination: call drops When the capacity for connecting new calls

of a given cell is used up. Interference in the channels. When the user behaviors change.

Speed and mobility.

Importance of handoff decision time

Different cell structures

Macro cells

Seven cell clusters in a macro cellular system.

Different cell structures

Micro cells

Half-Square Cell Plan Full-Square Cell Plan Rectangular Cell Plan

Different cell structures

Overlays:

Macrocell/Microcell Overlays

Types of Handoffs

Hard handoff: “break before make” connection Intra and inter-cell handoffs

Hard Handoff between the MS and BSs

Types of Handoffs

Soft handoff: “make-before-break” connection. Mobile directed handoff.

Multiways and softer handoffs

Soft Handoff between MS and BSTs

Handoff Prioritization:

Two basic methods of handoff prioritization are : Guard Channels Queuing of Handoff

Types of protocols

4 types of handoff protocols which help in providing continuous and QOS-guaranteed service. Network-controlled handoff (NCHO) Mobile-assisted handoff (MAHO) Soft handoff (SHO) and Mobile-controlled handoff (MCHO)

Performance evaluation approaches Three basic mechanisms used to evaluate

the performance of handoff algorithms include: Analytical approach, Simulation approach, and Emulation approaches

The Analytical approach

This approach is valid only under some specified constraints (considers RSSs criteria).

In real world situations this approach is complex and mathematically intractable.

The level crossings of the difference between the RSSs from two BSs were modeled as Poisson processes for stationary signal strength measurements.

An analytical model is developed for analyzing performance of handoff algorithms based on both absolute and relative signal strength measurements and compares analytical results with simulation results.

BS becomes a candidate only if its signal strength is strongest among all the BSs under consideration.

The Simulation approach

This simulation approach is the most commonly used

handoff evaluation mechanism. This approach provides a common testbed for

comparison of different handoff algorithms, and also provides insight into the behavior of the system.

Software simulation provides fast, easy, and cost-effective evaluation.

Simulation approach uses one or more of the available four basic components.

The Simulation approach

The Basic components of Simulation Model

The Emulation approach

The emulation approach uses a software simulator consisting of a handoff algorithm to process measured

variables. This approach has the advantage of giving better insight

into the behavior of the radio channels and more accurate data.

The main disadvantages are that this approach requires periodic measurement efforts and is not suitable for comparison of different handoff algorithms on the same platform.

Conclusion

Need handoffs in cellular systems, in order to reduce call terminations.

Provide QoS support.

Questions

Mobile stations are now expanding their types of gadgets. From traditional cell phones to PDAs, Laptops, GPS, Palm tops etc., many more gadgets can now be called as mobile stations which travel in between cells. Since the inputs, outputs and the identifying modules differ in each type of mobile stations, will the same Design for Handoff would work fine or how are these scenarios handled.

The main design strategies of handoff are Signal strength Cell structures

What are different system structures of a cell?

Macro, micro, macro/micro overlays

What happens if a user, after completion of his communication wants to disconnect and at the same time there is a handoff to other cell?(Will the purposeful termination detection be done by the first cell or the other cell to which the signal has been handed off?) The Serving BST handles it .Hand off is not a one time

process, so either of the BSTs handle the mobile stations call. At a particular time the station which is having more signal strength will handle the call. Even the termination is being handled by either of the base stations.

In types of handoff, there is “break before make” i.e. hard handoff & “make before break” that is soft handoff. So, which one of the two type is better and under which circumstances for each scenario?If possible, explain with example.

Soft handoff is advantageous over hard handoff because the mobile does not lose contact with the system during handoff execution, also unnecessary call terminations doesn’t occur. No one likes unnecessary call terminations.

Hard handoff is advantageous when system performance is not effected even if the mobile system has to reconnect to the BST.