Week#3-4.ppt - EEE464 Wireless Communication Systems...

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EEE464 Wireless
Communication Systems
Lecture#5-8

Channel assignment strategies

Channel assignment strategies can be
classified as either fixed or dynamic.

The choice of channel assignment strategy
impacts the performance of the system,
particularly as to how calls are managed
when a mobile user is handed off from one
cell to another.
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Fixed Channel Assignment

Each cell is assigned a predetermined set of voice
channels

Any call attempt with in the cell can only be served
by the unused channels in that particular cell.

If all the channels in the cell are occupied or busy,
the call is blocked. The user does not get the
service.

In a variation of the fixed channel assignment, a
cell can borrow channels from its neighboring cells
if its own channels are already occupied.
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Dynamic Channel Assignment

Voice channels are not allocated to different cells
permanently.

Each time a call request is made, the BS requests
a channel from the MSC

MSC allocates channel to the requested cell using
an algorithm that takes into account

The likelihood of future blocking with in the cell

Frequency of use of the candidate channel

The reuse distance of the channel

Other cost functions.
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Dynamic Channel Assignment

MSC only allocates the given frequency if
that frequency is not presently in use in the
cell or any other cell which falls within the
minimum restricted distance of frequency
reuse to avoid co-channel interference.

DCA reduces the likelihood of blocking, thus
increasing the capacity of the system.

DCA strategies require the MSC to collect real
time data on channel occupancy and traffic
distribution on continuous basis.
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Handoff
o
Do we need hand off in old mobile telephony and
why?
What is handoff ?
o
What are important considerations to design a
handoff process ?
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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.
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Handoff Contd.
o
When to handoff ?