HANDOFF STRATEGIES
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Transcript of HANDOFF STRATEGIES
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.
…
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
.
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
•
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