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Wireless Communications: System Design

Wireless Communications: System Design. Dr. Mustafa Shakir. Features of Fixed Channel Allocation Techniques. FCA is the optimum allocation strategy for uniform traffic across the cells.

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Wireless Communications: System Design

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  1. Wireless Communications:System Design Dr. Mustafa Shakir

  2. Features of Fixed Channel Allocation Techniques • FCA is the optimum allocation strategy for uniform traffic across the cells. • Impacts the performance of a system particularly as to managing calls when mobile user handed from one cell to another • A non uniform FCA strategy, when it is possible to evaluate GOS in real time and adjust the FCA accordingly. This requires a more complex algorithm.

  3. Channel Borrowing • Borrow frequencies from low traffic cells to high traffic cells. • Temporary channel borrowing: channel is returned after call is completed. • If channels from cell E are borrowed by cell A, then neighboring cells E cannot use those channels. • MSC supervises the borrowing procedures and ensures non disruption or interference with any of calls in progress

  4. Dynamic Channel Allocation • All channels are placed in a pool, and are assigned to new calls according to the reuse pattern. Signal is returned to the pool, when call is completed. • Issues related to channel allocation are still under research.

  5. Comparison of Channel Allocation Techniques Fixed Channel Allocation • Advantages: --- Less load on MSC --- Simple • Disadvantages: Blocking may happen Dynamic Channel Allocation • Advantages: Voice channels are not allocated permanently. That is, resource is shared on need-basis • Disadvantages: --- Requires MSC for processing---burden on MSC --- May be very complicated

  6. Handoff • Do we need hand off in old mobile telephony and why. • No because of single huge cell covering the whole service area. • What is handoff. • When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station • What are important considerations to design a handoff process. • Handoff must be performed successfully. • Handoff must occur infrequently. • Handoff should be smooth and the users must not be able to detect it.

  7. Handoff Contd. • Arrangements to fulfill the requirements of a successful handoff. • Handoff must be given priority over call initiation request . • Minimum usable signal for voice quality at the base station receiver is normally from –90 dbm to –100 dbm. • Optimum signal level at which to initiate a handoff should be taken in such away that  should not be too large or too small. Where  = Pr(handoff) – Pr(Min.usable) • If  is too large----- more handoffs-----Burden on MSC • If  is too small ----- more call drops.

  8. Handoff Contd. • When to handoff. • Signal level drop is not due to momentary fading and mobile is actually moving away from the BS. • For this the BS monitors the signal level for a certain period of time before a handoff is initiated.This period of time varies with the speed of MS. • Perfect relationship is required between speed of signal drop and required handoff time.

  9. Handoff Contd. • Who detects the need for handoff. • Network Initiated Handoff: • Signal measurements by BS & supervised by MSC. • BS monitors its all RVCs to determine the relative location of each mobile user with respect to the BS. • Mostly implemented in 1G systems. • This type of handoff takes almost 10 sec as in AMPS. • Two separate receivers on BS • One is used to measure RSSI(Radio Signal Strength Indication) of calls in progress with in the cell. • Second is used to scan and determine signal strengths of mobile users which are in neighboring cells. • Both the signals are monitored by the MSC which decides when to handoff.

  10. Handoff Contd • Who detects the need for handoff. Mobile Assisted Handoff( MAHO) • Every mobile station measures the received power from surrounding base stations and continually reports the results of these measurements to the serving base station. • A handoff is initiated when the power received from the base station of a neighboring cell begins to exceed the power received from the current base station (serving base station) by a certain level or for a certain period of time. • This handoff technique has been implemented in most of the 2G systems. • This type of handoff takes almost 1- 2sec as in GSM. • Preferred for microcellular system

  11. Handoff Contd. • What are the types of handoff. • Hard Handoff • Soft Handoff • What are different levels of handoff. • (1) Intra Cell (2) Inter cell (3) Inter system • Importance of handoff. • When no priority to handoff call blocking would be equal for call initiation and call handoff.

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