Channel Assignment Schemes for Cellular Mobile Systems

1
Channel Assignment Schemes for Cellular Mobile
Systems

• Fawzi Ali Sdudah
• Data Networks
• Winter 05

2
Outline

• Introductory Background
• Handoff
• Allocation Schemes
• Other schemes
• Cell Performance Modeling
• Conclusion

3
Introduction
4
Cell Physical Model
Ideal cell area (2-10 km radius) Circle
Alternative shape of a cell. Rectangle, Triangle,
BS
Cell
MS
Hexagonal cell area used in most models.
MS
Mobile Station
5
Co-channel Interference
First tier co-channel BS
Serving BS.
D6
D5
D1
R
D4
Mobile Station
D2
D3
C is channel received signal I is co-channel
interference
Co-channel interference ratio CIR
A channel can be reused if CIR in each
co-channel is gt CIRmin
6
Reuse Distance
Cluster
All system channels (S) are divided into sets (N)
F2
F7
Each cell within a cluster is assigned a set
Sum of Fx All System Channels
F3
F1
F6
F1
F4
F5
F2
F7
F3
F1
F6
F1
F4
F5
Reuse distance D
7
Frequency Reuse
Cells that can reuse the same channel with low
interference
Interfering cells

• For hexagonal cells, the reuse distance is
  given by

Cell x
D
R cell radius N the reuse pattern (Cluster
size) of cells per cluster.(e.g 7)
7 cell reuse cluster
Reuse distance D
8
Handoff
9
Handoff

• Hand-off is the process of switching from one
  frequency channel to another by the user in midst
  of a communication

BS-1
BS-2
10
Handoff Types
Intra-Cell
Inter-Cell
Soft Handoff
Hard Handoff
11
Channel Allocation Schemes
12
1- Fixed Channel Assignment(FCA)

• A set of nominal channels permanently allocated
  to each cell for its exclusive use.

13

• Simple Borrowing- A channel is borrowed from a
  neighboring cell provided it does not interfere
  with existing calls- The borrowed channel is
  locked in the non-co-channel cells of the
  borrower cell

Borrower
E
C
X
B
B
C
A
Bb1, b2,.. a3
A
D
G
E
D
F
a3 Aa1, a2,
F
B
E
A
E
B
C
X
C
A
G
F
G
D
D
E
F
(b)
(a)
Donor
14
2- Dynamic Channel Assignment(DCA)
Channels are allocated to a user on demand from a
central pool. No cell has a proprietary set of
channels.
15
Centralized DCA
Centralized DCA scheme involves a single
controller selecting a channel for each cell.
16
Distributed DCA

• Distributed DCA scheme involves a number of
  controllers scattered across the network (MSCs).

17
Distributed DCA
Moving Direction (MD)One-dimensional Systems
Call initiated
1 2 3 4 5
6 7 8
d
a
b
c
e
Reuse distance D
The set of mobiles moving in the same direction
are assigned the same channel.
18
3- Hybrid Channel Assignment (HCA)

• HCA ? FCA DCA

Shortage of channels is taken from the pool
Permanently owned Channels
Optimal ratio 31 (Ratio of fixed to dynamic)

• For traffic lt 50 ? performs better than fixed
  scheme.
• Beyond gt 50 ? Fixed scheme performs
  better.

19
Other Schemes
20
Overlapped Cells-based Allocation
Cell For high mobility
7
2
6
1
Microcell For low mobility
3
5
4
21
Use of Overlapped Cell Areas

• If MS in shared area does not find a free channel
  in cell A, it can take the free channel from cell
  B.

22
Cell Performance Modeling
23
A generic system model for a cell with Handoff
Calls
S . SC . . 2 1
S of All channels in system
?H
SR of Reserved channels for Handoff
SR
Handoffs
SC of shared channels in system for
new calls handoff calls
?
?O
System model with reserved channels for handoff
(No blocking till less than SC channels are busy)
Originated (New) calls
Channels
?O ?H
?H
?O ?H
?H


0
SC
S
?
(SC1)?
SC?
S?
State transition diagram
24

• The blocking probability BO for an originating
  call is given by
• (at least SC channels busy)( gt Sc busy
  channels)
• The blocking probability BH for a handoff call is
  
• (all S channels are busy)

25
Conclusion

• Fixed Channel Assignment (FCA)
• Low computational cost
• Used as a reference in Performance Comparison.
• Not flexible, poor utilization
• High blocking rate for non-uniform traffic
• Requires complex frequency planning
• Not desirable in micro-cell architectures
• Dynamic Channel Assignment (DCA)
• High utilization of channels
• High computational complexity
• Suitable for non-uniform traffic and micro-cell
  architectures
• Hybrid Channel Assignment (HCA)
• Combination of FCA and DCA
• Converges to FCA performance under heavy traffic

26
Thank You