BY: - NITISH JARYAL ECE 6th SEM HANDOFF BY: - NITISH JARYAL ECE 6th SEM BASE STATION MOBLIE UNIT

HANDOFF When a mobile moves into a different cell while a conversation is in progress, MSC transfers the call to a new channel belonging to the new base station.

Why handoff is needed? When mobile unit is at the boundary of cell site. When mobile unit is in hole (gaps) within the cell.

Types of handoff: Handoff is of two types: Based upon signal strength. Based upon C/I ratio.

Two-handoff-level algorithm:

Handoff Decision In 1G analog cellular system signal strength measurements are made by the base stations and supervised by the MSC. Each base station constantly monitors the signal strength of all of its reverse channel, determining relative position of the mobile user w.r.t base station tower

In today's 2G systems handoff decisions are mobile assisted Mobile Assisted Handoff (MAHO): Every mobile station measures the received power from the surrounding base stations and continually reports the result of these measurements to serving base station A handoff is initiated when the power received from the base station of neighbouring cell begins to exceed the power received from the current base station by certain level or for certain period of time Handoff Decision

Dwell Time The time over which a call may be maintained within a cell, without handoff It depends on the speed of the user and type of coverage ccc

Guard Channel Concept A fraction of the total available channel in a cell is reserved for handoff requests from ongoing calls which may be handed off into the cell Disadvantage of reducing the total carried traffic, as few channels are allocated to originating calls Prioritizing handoffs: Guard Channel Concept A fraction of the total available channel in a cell is reserved for handoff requests from ongoing calls which may be handed off into the cell Disadvantage of reducing the total carried traffic, as few channels are allocated to originating calls

Practical Handoff Problem Wide range of mobile velocities High speed vehicles pass through the coverage region of a cell within a matter of seconds, whereas pedestrians user may never need a handoff during a call The addition of microcells to provide capacity, the MSC will be quickly burdened if high speed users are constantly passed between very small cells

Umbrella Cell Approach Provide the large area coverage to high speed users while providing the small coverage area to users travelling at low speeds. Umbrella Cell Approach Provide the large area coverage to high speed users while providing the small coverage area to users travelling at low speeds.

Practical handoff problem: Cell Dragging (in microcell systems) Average signal strength does not decay rapidly, due to slow speed Even when the user has travelled well beyond the designed range of the cell, the received signal may be above handoff threshold, handoff may not be made Creates interference and traffic management problems Handoff threshold and radio coverage parameters must be adjusted carefully.

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