Wireless Communications: System Design

Dr. Mustafa Shakir

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.

Features of Fixed Channel Allocation Techniques

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

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.

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

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.

1) Handoff must be performed successfully.2) Handoff must occur infrequently.3) Handoff should be smooth and the users must not be able to

detect it.

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.

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.

Handoff Contd.

Who detects the need for handoff.1) 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(i) One is used to measure RSSI(Radio Signal Strength

Indication) of calls in progress with in the cell.(ii) Second is used to scan and determine signal strengths of

mobile users which are in neighboring cells. (iii) Both the signals are monitored by the MSC which decides

when to handoff.

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

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.