Handoff / Handover Strategies bitwali.com
@ Olympics If you’re a fan of the summer Olympics, I am sure you have watched plenty of track events – especially the exciting 400 and 800 meter relays. Four athletes run in each event and hand a baton off to the next athlete until the race is complete. Although the handoffs are usually done without mishap. bitwali.com 2
Handoff Strategies Handoff: a mobile user moves to a different cell while conversation is in progress, MSC transfers the call to a new BS. Identifying new BS New voice and control channels to be allocated Handoff must be performed Successfully Infrequently Imperceptible (غیر محسوس ). To achieve this, designer must specify optimum (زیادہ سے زیادہ . بہترین ) signal level at which handoff initiates bitwali.com
System Model for the Cellular Handoff bitwali.com
Definition of Handoff Horizontal Handoff Vertical Handoff Occurs when the user switches between different network access points of the same kind. e.g. Handoff among 802.11 APs. Vertical Handoff Involves two different network interfaces which usually represent different technologies. e.g. Handoff from 802.11 to 1xRTT (CDMA 2000). bitwali.com
Handoff Illustration bitwali.com
Need of Handoff? Power Consideration Traffic Consideration Channel Quality Consideration Distance Consideration Administrative Consideration bitwali.com
This margin ∆ = Pr_handoff – Pr_min, can not be too large or too small Once, a signal level is specified as min usable level for acceptable voice quality A slightly stronger signal level is used as threshold(حد.چوکھٹ ) Normally taken between -90 dBm and -100 dBm. This margin ∆ = Pr_handoff – Pr_min, can not be too large or too small If ∆ is too large, unnecessary handoffs, burden on MSC If ∆ is too small, insufficient time to complete a handoff before a call is lost due to weak signal ∆ should be chosen carefully to meet conflicting requirements bitwali.com
Handover decision receive level BTSold receive level BTSnew HO_MARGIN MS MS BTSold BTSnew bitwali.com
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CASE 1: Handoff - Successful Value of delta is large enough. When the Phandoff is reached, the MSC initiates the handoff. Value of delta is large enough. When the PHandoff is reached, the MSC initiates the handoff. bitwali.com 11
CASE:2 Handoff - Unsuccessful In this case, the MSC was unable to perform the handoff before the signal level dropped below the minimum usable level, and so the call was lost. In this case, the MSC was unable to perform the handoff before the signal level dropped below the minimum usable level, and so the call was lost. bitwali.com 12
Handoff – Unsuccessful (cont’d) Reasons for failed handoff: ∆ too small (i.e. PHANDOFF too low) high mobile speeds Excessive (تجاوُز ۔ بُہتات ۔ کَثرَت ) delay at MSC High traffic level Un-availability of channels bitwali.com
Handoff (cont’d) Optimize ∆ Ways to improve handoff performance Optimize ∆ Account (لحاظ کرنا ۔ خیال کرنا )for varying mobile speeds Minimize delay at MSC Prioritize handoffs Mobile assisted handoffs, mobile locators, minimize number of BS’s to search, ... We have already discussed why optimizing D leads to better handoff performance. bitwali.com 14
cannot be too large or too small. But, the margin, given by cannot be too large or too small. If Is too large, unnecessary handoffs which burden the MSC may occur. If is too small, there may be insufficient time to complete a handoff before a call is lost due to weak signal conditions. bitwali.com
Of course, the dropped call can happen when there is an excessive delay by the MSC in assigning a channel (during high traffic conditions) as well. On the other hand, in deciding when to handoff, it is important to ensure that the mobile is actually moving away from the serving base station, (not that the drop in the measured signal level is due to momentary( پل بھر کے لیے ) fading.) PRECAUTION. In order to ensure this, the base station monitors the signal level for a certain period of time before a handoff is initiated. bitwali.com
Dwell time The length of needed monitoring largely depends on the speed of mobile units. Dwell(بسنا) time: the amount of time over which a call may be maintained within a cell without handoff. The statistics(اعداد و شمار) 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. bitwali.com
Types of Handoffs Hard Handoff: Break-before-make Soft Handoff: Make-before-Break. Need to use two radios Make-before-Break Horizontal Handoff: Same radio access technology (RAT) Vertical Handoff: Different Technologies Terminal Controlled Terminal Initiated, Network Assisted Network Initiated, Network controlled Old Old New New Old New bitwali.com
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@ 1G In 1G, signal level was measured by BS and supervised by MSC Each BS constantly monitors the signal strength of all its reverse channels to determine relative location of each mobile user In addition, the locator receiver (a spare receiver) is used to scan and measure RSSI( Received Signal Strength Indicator ) of mobile users in neighboring cells and reports to MSC Based on these measurements, MSC decides if handoff is necessary 20 bitwali.com
@ 2G Mobile assisted handoff (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 21 bitwali.com
Detection of the need of Handoff Three strategies have been proposed to detect the need for handoff: mobile‑controlled handoff (MCHO) network‑controlled handoff (NCHO) mobile‑assisted handoff (MAHO) bitwali.com
Mobile‑Controlled Handoff (MCHO) The MS continuously monitors the signals of the surrounding BSs and initiates the handoff process when some handoff criteria are met. MCHO is used in DECT and PACS. bitwali.com
Network‑Controlled Handoff (NCHO) The surrounding BSs measure the signal from the MS, and the network initiates the handoff process when some handoff criteria are met. NCHO is used in CT‑2 Plus and AMPS. bitwali.com
Mobile‑Assisted Handoff (MAHO) The network asks the MS to measure the signal from the surrounding BSs. The network makes the handoff decision based on reports from the MS. MAHO is used in GSM and IS‑95 CDMA. bitwali.com
Two types of handoff inter‑cell handoff or inter‑BS handoff intersystem handoff or inter‑MSC handoff bitwali.com
Two types of handoff The BSs involved in the handoff may be connected to the same MSC (inter‑cell handoff or inter‑BS handoff) The BSs involved in the handoff may be connected to two different MSCs (intersystem handoff or inter‑MSC handoff ). bitwali.com
Handover Performance Metrics (مِقدار یا پیمائِش کا ایک طریقہ) Cell blocking probability – probability of a new call being blocked Call dropping probability – probability that a call is terminated due to a handover Call completion probability – probability that an admitted call is not dropped before it terminates Probability of unsuccessful handover – probability that a handover is executed while the reception conditions are inadequate bitwali.com
Handover Performance Metrics Handoff blocking probability – probability that a handoff cannot be successfully completed Handoff probability – probability that a handoff occurs before call termination Rate of handoff – number of handoffs per unit time Interruption duration – duration of time during a handoff in which a mobile is not connected to either base station Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur bitwali.com
Practical Handoff Consideration Different type of users High speed users need frequent handoff during a call. Low speed users may never need a handoff during a call. Micro cells to provide capacity, the MSC can become burdened if high speed users are constantly being passed between very small cells. Minimize handoff intervention(مداخلت) handle the simultaneous traffic of high speed and low speed users. Large and small cells can be located at a single location (umbrella cell) different antenna height different power level Cell dragging problem: pedestrian (پیدل چلنے والا)users provide a very strong signal to the base station The user may travel deep within a neighboring cell bitwali.com