1. Basics of Cellular Communications Grade of Service, Interference, & Capacity

2. The Cellular Concept Early Mobile Communications The Cellular Concept
Single, high powered transmitter with an antenna mounted on a tall tower
The Cellular Concept
Replace a single high power transmitter (large cell) with many low power transmitters (small cells) each providing coverage to only a small portion of the service area

3. Frequency Reuse
Cluster
Cluster
Each base station is allocated a group radio channels to be used within a small geographic area
Base station in adjacent cells are assigned channel groups which contain completely different channels than neighboring cells E D F A G B C
Cluster
Total Number of Channels in the System:
C=MkN=MS
M: Number of clusters within the system
K: Number of channels per cell
N: Cluster Size
S: Number of available physical channels
A Cluster: A Group of N cells that which collectively use the complete set of available frequencies

4. Locating Co-channel Cells
Number of Cells per Cluster N = i2+ij+j2, i, j are non-negative integers
To find nearest co-channel neighbor of a given cell
Move i cells along any chain of hexagons
Turn 600 counter clockwise and move j cells
i=3, j=2, N=19

5. Grade of Service and Capacity (i.e., Blocking Probability)

6. Trunking & Grade of Service
Trunked Radio System:
Each user is allocated a channel on per call basis, and upon termination of the call, the previously occupied channel is immediately returned to the pool of available channels.
Grade of Service:
A measure of the ability of a user to access the trunked system
GOS measures in cellular networks
Probability that a call is blocked
Probability a call experiences a delay greater than a certain queuing time

7. Traffic Intensity Traffic intensity generated by each user: Au Erlangs
Au = H
H : average duration of the call
λ : average number of call requests per unit time
For a system containing U users and unspecified number of channels, Total offered traffic intensity: A Erlangs
A = UAu
In C channel trunked system, if the traffic is equally distributed among the channels, Traffic intensity per channel : Ac Erlangs
Ac = UAu/C

8. Types of Trunked Systems
Calls Blocked Cleared Trunking System
No queuing provided for call requests and calls are blocked if no available channels
Calls Blocked Delayed Trunking System
Queuing is provided to hold call requests. Calls are blocked if no available channels for a certain delay
Assumptions:
Calls arrive as determined by Poisson distribution
Infinite number of users
Memoryless arrivals of requests : all users can request channel at any time
Probability of user occupying a channel is exponentially distributed
Finite number of channels available in the trunking pool

9. Erlang B Formula M/M/C/C Queuing System
Queue Size
C Servers and Exponential Service Times
Inter-arrival time distribution
Service time distribution
Number of Servers
Exponential Interarrival Time
(Poisson Arrival Process)

10. Erlang B Curves

11. Erlang C Formula

12. Erlang C Curves
Pr[delay>0]

13. Interference and Capacity in Cellular Systems

14. Interference & System Capacity
The wireless environment constitutes a shared medium
Interference is the major limiting factor in performance of wireless systems in general
Types of Interference:
Co-channel interference
Adjacent channel interference

15. Co-channel Interference
Frequency reuse implies that several cells use the same set of channels E D F A G B C E D F A G B C E D F A G B C E D F A G B C E D F A G B C
Frequency reuse = 7
Co-channel interfering cells for cell allocated with channel group A

16. Co-channel Interference, SIR & System Capacity
Co-channel Interfering Cells P2
H21
BS 2
H12 P1
H22
H11 R D
MT 2
BS 1 R
MT 1
BS: Base Station
MT: Mobile Terminal
Px: Transmitter power by base station x
Hxy: Small-scale & Large-scale channel between base station x and mobile terminal y
SIRy: Signal-to-Interference Ratio at mobile terminal y
Improving SIR1 by increasing P1 would result in a decrease in SIR2
Improving BOTH SIR1 & SIR2 is possible by increasing the distance separation between BS1 and BS2

17. Distance Separation between Base Stations
j(2R’)
i(2R’)
Q: Co-channel reuse Ratio

18. SIR Computations
Assume interference from first tier (ring) of co-channel interferers D D D X R D D D
Di: interfering distance from ith co-channel interference
NB No. of co-channel interfering sites

19. SIR Computations X Worst Case SIR
Assume interference from first tier (ring) of co-channel interferers
D+R
D+R D R X
Worst Case SIR
D-R D
D-R
Di: interfering distance from ith co-channel interference
NB No. of co-channel interfering sites

20. SIR & System Capacity
Improving SIR means increasing cluster size, which corresponds to a decrease in system capacity
Decreasing the cell size does not affect the SIR as Q=D/R remains constant. A decrease in cell size corresponds to an increase in system capacity

21.  Capacity reduction = 7/9
Example
In First Generation cellular systems, sufficient voice quality is achieved when SIR = 18 dB
N=7Q=4.6. Worst Case SIR = (17 dB)
To design cellular system with worst performance better than 18 dB, N=9
 Capacity reduction = 7/9

22. Adjacent Channel Interference
Adjacent channel interference results from imperfect receiver which allows nearby frequencies to leak into the passband
Adjacent channel interference can be minimized through careful filtering and channel assignments

23. Improving Coverage and Capacity in Cellular Systems: Cell Splitting
Subdividing a congested cell into smaller cells, each with its own base station and a corresponding reduction in antenna height and transmitter power
Cell splitting Increasing system capacity by increasing the number of clusters in a given area
Decreasing Transmitter Power
The SIR is independent of transmitted power as long as it is the same for all base stations
Why not make Transmitter Power as low as possible?
The SNR must be a above a minimum threshold controlled by Pr