HANDOFF STRATEGIES

Outline..

Handoff Handoff strategies Hand off decision Dwell time Prioritizing Queuing

HAND OFF

“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”

Handoff depends on: Cell size Boundary length Signal strength Fading Reflection and refraction Man made noise

Handoff can be initiated either by the BS or the MS.

It could be due to:1. The radio link

2. Network management

3. Service issues

The radio link -primarily due to the mobility of MS and depends on : No. of MSs that are in the cell No. of MSs that have left the cell No. of calls generated in the cell No. of calls transferred to the cell from

neighbouring cell by the handoff No. and duration of calls terminated in the cell No. of calls handed off to the neighbouring cells Cell dwell time

Network management may cause handoff if there is a drastic imbalance of traffic over adjacent cells.

Service related handoff is due to degradation of quality of service (QoS) and handoff could be invoked when such a situation is detected.

Necessity for handoff is defined by: Signal strength Signal phase Combination of the above two Bit error rate (BER) Distance

Mobile Switching Center needs to: Identify the new base station Allocate new Voice and Control

channels associated with the new base station

Desired behavior: Prioritize Handoff requests over call initiation request Successful transfer of call Hand off should be as infrequent as possible Hand off should be imperceptible to the users

Hand off Threshold Power Level System designers must specify an optimum signal level at

which to initiate handoff There is a minimum power level of the signal at BS for

acceptable voice quality. (-90 to –100 dBm). A slightly stronger level is used as a threshold at which

handoff is made so that the system has time to process hand off.Margin is given by Δ = Pr handoff – Pr minimum usable

Margin Δ needs to be neither too small, nor too large Δ too small → too short time to perform handoff, The call may

be dropped due to weak signal condition

Δ too large → unnecessary handoff which burden the MSC, may occur.

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HANDOFF STRATEGIES

Dropped Call Events:

Dropped call event happen when there is an excessive delay by the MSC in assigning the handoff or when threshold is too small for handoff time in the system

Excessive delays may occur during high traffic conditions due to computational load on the MSC

Also if no channels available on any of the nearby stations (Forcing the MSC to wait until a channel in a nearby cell becomes free)

HANDOFF DECISION

Drop in the measured signal level is not due to the momentary fading and that the mobile is actually moving away from the serving base station.

Base station monitors the signal strength ensuring necessary handoffs to be completed before call is terminated due to poor signal level and also avoiding unnecessary handoffs

Length of time required for decision depend upon on speed at which the vehicle is moving

Handoff should be made quickly, for slope of short term avg. received signal level in a given time interval is steep.

DWELL TIMEThe time over which a call may be maintained within a cell,

without handoff

Vary greatly, depending upon the speed of the user Interference type of coverage

Cells providing coverage for vehicular highway users, having relatively constant speed travel along fixed path enjoy good radio coverage

The dwell time for a highway user is a random variable with distribution highly concentrated about the mean dwell time

For users in dense, cultured micro cell environments, there is typically a large variations in dwell time about the mean.

HANDOFF DECISION IN 1G 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

Locator receiver a spare receiver in base station is used to scan and determine signal strengths of mobile users in neighbouring cells requiring handoffs and reports all the RSSI values to the MSC

Based on locator receiver information MSC decides whether Handoff is necessary or not

HANDOFF DECISION IN 2G

In 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 staion

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

MAHO is much faster than a locator in 1G system, since measurements are made by mobile (MSC not monitoring signal strengths), suited for microcell environments

INTERSYSTEM HANDOFF

During the course of a call if a mobile moves from one cellular system to a different cellular system controlled by a different MSC an intersystem handoff is necessry.

Necessity : when a mobile signal becomes weak in a give cell MSC cannot find another cell within its system to which can it transfer the

call in progress

Issues: A local call may become a long distance call as

the mobile user moves out of home system and becomes a roamer in the neighbouring system.

compatibility between the two MSC must be determined before implementing intersystem handoff.

Different policies and methods for managing handoff requests

Some handle handoff requests in same way as they handle originating calls

From user point of view call abruptly terminated while in middle of conversation is more annoying than being blocked on call attempt

MANAGING HANDOFFS

1. Prioritizing Handoffs

2. Queuing of Handoffs

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Prioritizing Handoffs :To improve QoS various methods have been devised to

prioritize handoff.Guard Channel Concept

A fraction of the total available channel in a cell is reserved exclusively for handoff requests from ongoing calls which may be handed off into the cell

Disadvantage of reducing the total carried traffic, as fewer channels are allocated to originating calls

However it offers efficient spectrum utilization using dynamic channel assignment strategies

Queuing of Handoffs Decrease the probability of forced termination of a call due

to lack of available channels

Queuing is possible due to the fact that there is finite time interval between the time the received signal level drops below the handoff threshold and the time the call is terminated due to insufficient signal level.

Delay time and size of the queue is determined from the traffic pattern of particular service area

Does not guarantee a zero probability of forced termination, since large delays may cause the received signal level to drop below minimum level required.

PRACTICAL HANDOFF CONSIDERATIONS

1.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

2.Umbrella Cell Approach

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

Minimize handoff for high speed users and provides additional microcell channels for pedestrian users

The speed of each user may be estimated by the base station or MSC by evaluating how rapidly the short term average signal strength on RVC changes over time.

3.Cell Dragging (in microcell systems)

Results from pedestrian users that provide a very strong signal to the base station due to LOS radio path between the subscriber and the base station

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 a potential interference and traffic management problems since the user has meanwhile travelled deep into the neighbouring cell.

Handoff threshold and radio coverage parameters must be adjusted carefully.

4.Handoff decisions other than signal strength

The co-channel and adjacent channel interference levels may be measured at the base station or mobile station.

Information may be used with the conventional signal strength data to provide a multidimensional algorithm for determining when handoff is needed.

EXAMPLES• Handoff for first generation analog cellular systems

– 10 secs handoff time– ∆ is in the order of 6 dB to 12 dB

• Handoff for second generation digital cellular systems, e.g., GSM– 1 to 2 seconds handoff time– mobile assists handoff– ∆ is in the order of 0 dB to 6 dB– Handoff decisions based on signal strength, co-channel

interference, and adjacent channel interference.• IS-95 CDMA spread spectrum cellular system

– Mobiles share the channel in every cell.– No physical change of channel during handoff– MSC decides the base station with the best receiving signal as the

service station

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HANDOFF MANAGEMENT ISSUES

Three issues for handoff management

Handoff Detection Channel Assignment Radio Link Transfer

Metrics for Handoff Detection

Word Error Indicator (WEI) Received Signal Strength Indication

(RSSI) (dB) Quality Indicator (QI) {SNR or S/I}

STRATEGIES FOR HANDOFF DETECTION

Mobile Controlled Handoff (MCHO)

Network Controlled Handoff (NCHO)

Mobile Assisted Handoff (MAHO)

MCHO MS Quality maintenance processing

MeasurementProcess

Execute ALT or TST

Select new channel or time

slotLink Quality Acceptable?

yes

no

NCHO

BS supervise quality of current connections by making RSSI measurements

MSC commands surrounding BSs to make measurements of these links occasionally.

Based on these measurements MSC makes decision when and where to effect the handoff.

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MAHO

In 2G, handoff decisions are mobile assisted Each mobile measures RSSI of all surrounding BS Reports to serving BS Handoff is initiated if power of serving BS is lesser

than nearby BS by a certain level or for a certain period of time

Enables calls to be handed over between Base Stations at much faster rate than in 1G

MSC no longer constantly monitors RSSI. More suitable for microcellular where HO is frequent

Source:

1. Wireless Communications Principles and practice by T. S. Rappaport, Pearson Education