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Channel Assignment Strategies

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Handoff (Handover) Process

• Handoff: Changing physical radio channels of network connections involved in a call, while maintaining the call

• Basic reasons for a handoff– MS moves out of the range of a BS (signal level

becomes too low or error rate becomes too high)– Load balancing (traffic in one cell is too high, and shift

some MSs to other cells with a lower load)– GSM standard identifies about 40 reasons for a

handoff!

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Handoff

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Design Tradeoff

• Smaller cell means higher capacity (frequency reused more)

• However, smaller cell also results in higher handoff probability, which also means higher overhead

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Phases of Handoff

• MONITORING PHASE

- measurement of the quality of the current and possible candidate radio links

- initiation of a handover when necessary• HANDOVER HANDLING PHASE

- determination of a new point of attachment

- setting up of new links, release of old links

- initiation of a possible re-routing procedure

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• The length of monitoring needed depends largely on the speed of mobile units.

• Dwell time: the time over which a call may be maintained within a cell without handoff.

• The statistics of dwell time, vary greatly depending on the type of radio coverage and user profiles within a cell, are important in the practical design of handoff algorithms.

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Handover Scenario at Cell Boundary

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Handoff Based on Receive Level

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Handoff – 1G (Analog) systems

• Signal strength measurements made by the BSs and supervised by the MSC

• BS constantly monitors the signal strengths of all the voice channels

• Locator receiver measures signal strength of MSs in neighboring cells

• MSC decides if a handover is necessary

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Handoff – 2G (Digital) TDMA

• Handoff decisions are mobile assisted• Every MS measures the received power from

surrounding BSs and sends reports to its own BS

• Handoff is initiated when the power received from a neighbor BS begins to exceed the power from the current BS (by a certain level and/or for a certain period)

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Practical Handoff Considerations

• The umbrella cell approach

• Cell dragging

• Soft handoff

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Avoiding handoff: Umbrella cells

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Cell dragging

• If there is line of sight path between MS and BS, even when the user has travelled well beyond the designed range of the cell, the received signal at the base station may be above the handoff threshold, thus a handoff may not be made. This creates a potential interference and traffic management problem, since the user has meanwhile travelled deep within a neighboring cell.

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Soft Handoff – CDMA

• CDMA uses code to differentiate users• Soft handoff: a user keeps records of several

neighboring BSs

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• Soft Handoff in CDMA– CDMA uses the same channel (same carrier

frequency) in every cell.– Consequently, handoff does not mean a

physical change in the assigned channel, rather that a different base station handles the radio communication task.

– By simultaneously evaluating the receiver signals from a single subscriber at several neighboring base stations, the MSC may actually decide which version of the user’s signal is best at any moment in time.