1. EEE 441 : Wireless And Mobile Communications
Lecture 03
EEE 441 : Wireless And Mobile Communications

2. Quiz1
Today

3. The Cellular Concept
Major breakthrough in solving the problem of spectral congestion due to finite radio spectrum
Replaces single high power transmitter (large cell) with many low power transmitters (small cells)‏

4. Frequency Reuse Base station in each cell allocated group of channels
A group of cells make a cluster
Total number of channels replicated in next cluster
Adjacent cells always assigned different channel groups from neighboring cells

5. Frequency Reuse (cont’d)‏
Cells with same letter use the same set of frequencies
Cluster outlined in bold, replicated over coverage area
Example:
cluster size N = 7
frequency reuse factor = 1/7, as each cell contains 1/7 of total available channels. B G C A F D E B B G C G C A A F D F D E E

6. Choice of Shape of a Cell
Factors:
Equal area
No overlap between cells
Distance from center to farthest point in cell
Possibilities: d d d C1
C 2 C3

7. For a given d: Area(C3) > Area(C1), Area(C2)‏
Hence, C3 provides maximum coverage area for a given value of d.
Signal attenuation governed by d
By using hexagonal geometry, the fewest number of cells are required to cover a given geographic region
Actual cellular footprint determined by the contour of a given transmitting antenna

8. Channel Capacity Let a cellular system have total of S duplex channels
The S channels are divided into the N cells in a cluster
Number of channels available to each cell is:
k = S/N

9. Channel Capacity (cont’d)‏
Let the cluster be replicated M times within the system
Total number of channels, or the system capacity, is:
C = MS
= MkN
i.e. The initial capacity, S, has been increased M-fold by forming M number of clusters

10. Design of cluster size N
Each cell is assigned fraction 1/N of the total number of channels available to the system (S)‏
1/N is the frequency reuse factor
In order to connect without gaps between adjacent cells, N must follow:
N = i2 + ij + j2
where i and j are positive integers
Example: i = 2, j = 1 implies N = 7

11. To Find the Nearest Co-channel Neighbor of Particular Cell
Algorithm:
Move i cells along any chain of hexagons.
Then turn 60 degrees counterclockwise and move j cells.

12. How to Locate Co-channel Cells in a Cellular System
In this example N = 19
( i.e. i = 3, j = 2 ) A A A A A A A

13. Problem
If a particular FDD cellular telephone system has a total assigned bandwidth of 33 MHz,
and if the phone system uses two 25 KHz simplex channels to provide full duplex voice and control channels,
compute the number of channels per cell if N = 7

14. Solution Total available system bandwidth = 33 MHz
Channel bandwidth = 25 KHz x 2 = 50 KHz
Number of channels, S = 33 MHz / 50 KHz = 660
Cluster size, N = 7
Channels per cell, k = S/N = 660/7 = 95

15. Channel Assignment Strategies
Fixed Channel Assignments
Each cell allocated a pre-determined set of voice channels
If all channels in a cell are occupied, then a new call is blocked
Variation includes a borrowing strategy: - a cell is allowed to borrow channels from a neighboring cell if all its own channels are occupied
- borrowing is supervised by the MSC

16. Channel Assignment Strategies (cont’d)
Dynamic Channel Assignments
Voice channels not permanently allocated to each cell
Cell base station requests a channel from MSC every time a new call request is made
MSC then allocates a channel to the requesting call, based on a decision algorithm
MSC needs real-time, system-wide info on channel occupancy, traffic distribution, roaming possibilities, signal strength …

17. Handoff
Required when
a subscriber travels between cells during a conversation
interference occurs on current channel, hence call must be assigned to new channel
MSC must handoff call/control from old channel in base station in old cell to new channel in base station in new cell
MSC must
identify the new cell
determine available channels in the new cell
perform handoff seamlessly
Unnecessary handoffs need to be avoided

18. Handoff Scenario (a) Improper Handoff Situation PA
Level at point A
Handoff threshold PA
Received signal level
Minimum acceptable signal to maintain the call PB
Level at point B (call is terminated)‏
Time A B
PA– PB = ∆
BS1
BS2

19. Handoff Timing PA – PB = ∆ ∆ should not be too large or too small
∆ too large: too many handoffs
∆ too small: chance of call being lost

20. Handoff Scenario (cont’d)‏
(b) Proper Handoff Situation
Level at point B
Received signal level
Level at which handoff is made
Time A B
BS1
BS2

21. Hard and Soft Handoffs Hard handoff Soft handoff
MSC breaks mobile’s connection to base station in old cell
makes makes new connection using new channel in new cell
seamless transfer that goes unnoticed by user
Soft handoff
Mobile always retains same channel across the system
MSC compares signals from neighbouring base stations to determine the strongest
MSC selects between instantaneous signals received from a number of base stations
This method is used in CDMA Spread Spectrum systems

22. Other Types of Handoff Mobile-Assisted Handoff (MAHO):
Every mobile measures the received power from surrounding base stations, and continuously reports value to serving base station
Faster hand-off rate than with the MSC-controlled type
Particularly suited for micro-cell environments
Inter-system Handoff:
One cellular system to a different cellular system

23. The Umbrella Cell Approach
Small microcells for low-speed mobiles
Large “umbrella” cell with high-powered tower for high-speed mobiles
Rappaport, Fig. 3.4

24. Notices
Readings
Rappaport, Ch 3.1 – 3.4