Channel assignment strategies

Channel Assignment Process

1. Channel Assignment Strategies

2. Channel Assignment
Goals:
•achieve high spectrum utilization
•maintain a given service quality
•use a simple algorithm
•require a minimum number of database lookups
–Unfortunately it is hard to do all of these at once!
–If there is no available channel, then
•new calls are blocked
•existing calls that can’t be handed over - forced terminations

3. Channel Assignment Process
•Fixed Channel Assignment (FCA)
•Dynamic Channel Assignment (DCA)
•Quasi-static autonomous frequency assignment (QSAFA)

•Lots of schemes have been introduced to reduce the number of
forced terminations, at the cost of increased blocking or
decreased efficiency:

4. Channel Assignment Strategies
•Fixed channel assignment
–each cell is allocated a predetermined set of voice channel
–any new call attempt can only be served by the unused channels
–the call will be blocked if all channels in that cell are occupied

•Dynamic channel assignment
–channels are not allocated to cells permanently.
–BSC requests channel from MSC to allocate channel when call initiate.
–reduce the likelihood of blocking, increase capacity.
–MSC allocates channel to call based on algorithm that takes into account
•Probability of future blocking within cell
–MSC assigns channel that will not interfere with existing calls
–Reduces probability of blocking &Increases channel utilization

Frequency reuse scheme Channel assignment strategy
•increases capacity
•minimize interference
•fixed channel assignment
•dynamic channel assignment

5. Channel Assignment Process
•Nonprioritized scheme (NPS) - handoff call treated the same
as a new call.
•Reserved Channel scheme (RCS)- reserves some resources for
handoffs.
•Queuing Priority scheme (QPS) - exploit the over lap (handoff
area).
•Sub rating scheme (SRS) - switching CODECs of one or more
calls to free resources.

6. 6
Typical call
•Mobile unit initialization
–Scan and select strongest set up control channel
–Automatically selected BS antenna of cell
•Usually but not always nearest (propagation environment)
–Handshake to identify user and register location
–Scan repeated to allow for movement
•Change of cell
–Mobile unit monitors for pages (see below)

•Mobile originated call
–Check set up channel is free
•Monitor forward channel (from BS) and wait for idle
–Send number on pre-selected channel

7. 7
Typical call…
Paging
–MSC attempts to connect to mobile unit
–Paging message sent to BSs depending on called mobile number
–Paging signal transmitted on set up channel

Call accepted
–Mobile unit recognizes number on set up channel
–Responds to BS which sends response to MSC
–MSC sets up circuit between calling and called BSs
–MSC selects available traffic channel within cells and notifies BSs
–BSs notify mobile unit of channel

8. 8
Typical call…
•Ongoing call
–Voice/data exchanged through respective BSs and MSC

•Handoff
–Mobile unit moves out of range of cell into range of
another cell
–Traffic channel changes to one assigned to new BS
•Without interruption of service to user

9. 9
Power Control
•If MS is near the BS or in LOS situation, power to/from the MS can
be reduced
–Helps Reduce CCI
–Save battery power
–Alleviate health concerns
•Coarse power control is adequate
–Implemented through Open Loop Power Control
•In CDMA systems all MS use same frequency
•Fine power control is crucial to mitigate the near-far effect
–Both Open & Closed Loop Power Control

10. 10
Cellular System Channels
•Control channels
–Forward and Reverse
–Setting up and maintaining calls
–Exchange commands an/or messages between MS and BS
or MSC (as needed)
•Traffic channels
–Carry voice and/or data traffic