1. Wireless & Mobile Networking: Channel Allocation
Azizol Bin Abdullah
(A2.04)
Reference: Text Book Chapter 8

2. Introduction
Allocation of channels to a cellular system is important from a performance point of view.
Why????
Whenever a MS want to make a new call, it must submit a request for a channel to a BS.
BS can grant such an access provided that channel is readily available.
The probability of a new call to be dropped or blocking probability for calls originated in a cell can be minimized.

3. Introduction (cont.)
How to solve the dropped or blocking probability for calls problems????
Increase the number of channels per cell. But????

4. Introduction (cont.)
We have a limited frequency to allocate for wireless communication – there is a limit to maximum number of channels – restricting availability of channel in each cell.
Channel Allocation – a given spectrum is to be divided into a set of disjoint channels, which can be used simultaneously while minimizing interference in adjacent channels.
Interference can be avoid using appropriate reuse distance.
One simplistic approach – divide the channels equally among the cells.

5. Introduction (cont.)
If Stotal is the total number of channels and N is the size of reuse cluster, then
S = the number of channels per cell = Stotal /N
If we reducing N, we can get more channels…but
It will against the philosophy of frequency reuse.
Why????

6. Introduction (cont.)
Any reduction in the reuse distance increase the interference.
Any other option?
Allocate channel to different cell as per their load.
However, it hard to predict the instantaneous traffic.

7. Introduction (cont.) An ideal situation:
All parameters are assumed to be the same:
Traffic
External condition
Etc.

8. Static vs Dynamic Allocation
Two ways the traffic channels can be allocated to different cells in cellular system: static and Dynamic.
In static allocation: a fixed number of channels is allocated to each cells.
In dynamic allocation: channels are allocated as needed dynamically, possibly from a central pool.
In brief, channel allocation schemes can be divided as:
Fixed Channel Allocation (FCA) schemes.
Dynamic Channel Allocation (DCA) schemes.
Hybrid Channel Allocation (HCA) schemes.

9. Fixed Channel Allocation Schemes
A set of channel is permanently allocated to each cell of the system.
If the total number of available channels in the system is divided into sets, the minimum number of channel sets N required to serve the entire coverage area:
Where, D - frequency Reuse distance
R - radius

10. Fixed Channel Allocation Schemes(cont.)
How to address increased traffic of new originating and handoff call in a cell in FCAS ???
Borrow free channels from neighboring cells.
The borrowing process must make sure that there is no interference.
There are many possible channel borrowing schemes:
Simple Borrowing Schemes
Complex Borrowing Schemes

11. Fixed Channel Allocation Schemes(cont.)
Simple Borrowing Schemes
If all channels allocated to a cell are being used, then additional channels can be borrowed from any cell that has some free unused channels.
The selection process:
Select a donor cell from among adjacent cells that has the largest number of free channels.
Select the first free channel found for borrowing when the search follows a predefined sequence.

12. Fixed Channel Allocation Schemes(cont.)
Complex Borrowing Schemes
Basic strategy:
Divide the channels into two groups: one group assigned to each cell permanently and the second group kept reserved as donors.
The ratio between groups can be determines a priori or based on the estimated traffic in the system.
Alternative strategy:
Borrowing with channel ordering – assign priorities to all channels of each cell. Lowest priority channel for borrowing.

13. Fixed Channel Allocation Schemes(cont.)
Alternative strategy (cont.):
Borrowing with directional locking – borrow the available nearby cochannel cells with minimizing interference.
The basic sectoring technique can be used to allocate channels temporarily.
There are relative advantages and disadvantages of different complex schemes in term of: Total channel utilization,Total carried traffic, Allocation complexity.
In channel borrowing schemes, there are two step of verifying potential interference and potential prohibition:
Checking the reuse distance with other nearby clusters using those borrowed channels
Looking of sector directions of all cells not satisfying the reuse distance

14. Dynamic Channel Allocation Schemes
Channels are allocated dynamically as new calls arrive.
All free channels are kept in a central pool.
When call completed, channel will be returned to central pool.
Free channel can be allocated to any cell, as long as interference constraints in that cell can be satisfied.
The control can be centralized or distributed.
DCAS can be classified into two : centralized schemes and distributed schemes.

15. Dynamic Channel Allocation Schemes (cont.)
Centralized Dynamic Channel Allocation Schemes
Channel is selected for a new call from a central pool of free channel and a specific characterizing function is used to select one among candidate free channel.
The schemes:
Select the first available free channel that can satisfy the reuse distance – the simplest.
Select a free channel that can minimize the future blocking probability
Channel reuse optimization, maximizes the use of every channel

16. Dynamic Channel Allocation Schemes (cont.)
Distributed Dynamic Channel Allocation Schemes
The allocation is primarily based on one of these three parameters: cochannel distance, signal strength measurement and signal-to-noise interference ratio.
In cell-based distributed schemes:
Use a table to indicates if other cochannel cells in the neighborhood are not use and select one of them.
Cochannel interference scheme
Adjacent channel interference scheme
Signal strength measurement-based distributed schemes:
Channels are allocated to a new call if the anticipated CIR(Carrier-to-Interference Ratio) above a threshold.

17. Other Channel Allocation Schemes
Hybrid Channel Allocation Scheme
Combination of fixed and dynamic channel allocation scheme where channels are divided into fixed and dynamic sets.
Flexible Channel Allocation Scheme
Similar to HCAS but the channels are divided into fixed and flexible (emergency) sets.
Flexible scheme require central control with up-to-date traffic pattern.
With two different strategies:
Scheduled – a priori estimate the variation traffic needed to schedule emergency channel at predetermined traffic change
Predictive – traffic intensity and blocking probability is monitored in each cell all the time for assigning flexible channels.

18. Allocation in Specialized System Structure
Channel allocation also depends on the inherent characteristics of the system structure.
Example: schemes for freeway.
Channel Allocation in One Dimensional Systems
Can be used for a highway, where handoff and forced call termination do occur frequently due to small size of cell and speed of MSs located ini fast moving cars (figure 8.3).
Reuse Partitioning Based Channel Allocation
Cell is divided into multiple concentric, equal-size zones.
Basic idea: inner zone require lesser power to attain a desired CIR or SIR. The net effect – lower value of reuse distance for inner zones, as compared to outer zones (figure 8.4).

19. Allocation in Specialized System Structure (cont.)
Overlapped Cells-Based Allocation
Cells are split into seven microcells with separate BS and microwave tower for each microcells (figure 8.5).
Logical structure can be changed dynamically, starting with only the main cell for low traffic
As traffic increase, one or more microcell will turned on.
Another possibility to have an overlap of cell area between two adjacent cells (figure 8.6)