1. Lecture 11: Cellular Networks
Introduction
Principle of wireless networks
The principle of frequency reuse
Cellular system overview
Ben Slimane

2. Cellular Networks
The purpose of wireless networks is to provide wireless access to the fixed network (PSTN)

5. Cellular Networks
Multiple low-power transmitters (100 W or less) are used
The service area is divided into cells
Each cell is served by its own antenna
Each base station consists of a transmitter, a receiver, and control unit
Base station placed in the middle or at the border of the cell
Each base station is allocated a certain frequency band (frequency allocation)

6. Cellular Geometries

7. Cellular Geometries
The most common model used for wireless networks is uniform hexagonal shape areas
A base station with omni-directional antenna is placed in the middle of the cell

8. Cellular Geometries Cells are classified based on their sizes
Macrocells with radius of 1km or more (wide area)
Hexagonal shape cells
Microcells with radius of 100m or more (cities)
Manhattan (city) type cell structure
Picocells with radius in the meters (indoor)
Shape depends on the room

9. Design of Wireless Networks
The design is done in two steps
Area coverage planning
Channel (Frequency) allocation
Outage area
Coverage area

10. Frequency Reuse
An efficient way of managing the radio spectrum is by reusing the same frequency, within the service area, as often as possible
This frequency reuse is possible thanks to the propagation properties of radio waves

11. Frequency Reuse We form a cluster of cells
Divide the total number of channels (frequencies) between the cells of the cluster.
All the channels within the cluster are orthogonal
No interference between cells of the same cluster
We repeat the cluster over the service area
The distance between the clusters is called the reuse distance D
The design reduces to finding D!

12. Frequency Reuse
For hexagonal cells, the number of cells in the cluster is given by

13. Frequency Reuse Pattern
Frequency reuse pattern for N=3

14. Frequency Reuse Patterns
Frequency reuse pattern for N=7

15. Capacity of Wireless Networks
The capacity of a wireless network is measured as the average of simultaneous radio links supported by the systems
η=C/N, users/cell
The area capacity is defined as
η=C/(NxAcell), users/unit area
Acell is the cell area

16. Approaches of Increasing Capacity
Adding new channels
Frequency borrowing – frequencies are taken from adjacent cells by congested cells
Cell splitting – cells in areas of high usage can be split into smaller cells
Directional antennas – cells are divided into a number of wedge-shaped sectors, each with their own set of channels
Microcells – antennas move to buildings, hills, and lamp posts

17. Cellular System Overview

18. Cellular Systems Terms
Base Station (BS) – includes an antenna, a controller, and a number of transceivers
Mobile telecommunications switching office (MTSO) – connects calls between mobile units
Two types of channels available between mobile unit and BS
Control channels – used to exchange information having to do with setting up and maintaining calls
Traffic channels – carry voice or data connection between users

19. Steps in an MTSO Controlled Call between Mobile Users
Mobile unit initialization
Mobile-originated call
Paging
Call accepted
Ongoing call
Handoff

20. Examples of Mobil Cellular Calls

21. Examples of Mobile Cellular Calls

22. Examples of Mobile Cellular Calls

23. Additional Functions in an MTSO Controlled Call
Call blocking
Call termination
Call drop
Calls to/from fixed and remote mobile subscriber

24. Mobile Radio Propagation Effects
Signal strength
Must be strong enough between base station and mobile unit to maintain signal quality at the receiver
Must not be so strong as to create too much cochannel interference with channels in another cell using the same frequency band
Fading
Signal propagation effects may disrupt the signal and cause errors

25. Radio Resource Allocation problem
To each active terminal assign
- Base station
- Channel (“Frequency”) - Transmitter power
such that Link Quality & power constraints are satisfied for as many terminals as possible

26. Handover Performance Metrics
Cell blocking probability – probability of a new call being blocked
Call dropping probability – probability that a call is terminated due to a handover
Call completion probability – probability that an admitted call is not dropped before it terminates
Probability of unsuccessful handover – probability that a handover is executed while the reception conditions are inadequate

27. Handover Performance Metrics
Handoff blocking probability – probability that a handoff cannot be successfully completed
Handoff probability – probability that a handoff occurs before call termination
Rate of handoff – number of handoffs per unit time
Interruption duration – duration of time during a handoff in which a mobile is not connected to either base station
Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur

28. Handover Strategies Used to Determine Instant of Handover
Relative signal strength
Relative signal strength with threshold
Relative signal strength with hysteresis
Relative signal strength with hysteresis and threshold
Prediction techniques

29. Handover decision

30. Transmitter Power Control
Why transmitter power control?
Reduce terminal power consumption
Reduce interference within the cellular system and improve quality
Efficient handling of mobility
In SS systems using CDMA, it’s desirable to equalize the received power level from all mobile units at the BS
Reduce near-far problem

31. Types of Power Control Open-loop power control
Depends solely on mobile unit
No feedback from BS
Not as accurate as closed-loop, but can react quicker to fluctuations in signal strength
Closed-loop power control
Adjusts signal strength in reverse channel based on metric of performance
BS makes power adjustment decision and communicates to mobile on control channel

32. Traffic Engineering
In cellular systems, the number of active users (calls) is random.
Ideally, available channels would equal number of subscribers active at any time
Not possible in practice
For N channels per cell and L active subscribers per cell we have
L < N non-blocking system
L > N blocking system

33. Performance Questions
Blocking Probability
Probability that a call request is blocked?
System capacity for a given blocking probability?
What is the average delay?
System capacity for a certain average delay?

34. Traffic Intensity
In cellular systems, calls are Poisson distributed with  calls/s
The traffic load of the system is
 is the number of calls per seconds
h is the average call duration in seconds
A = average number of calls arriving during average holding period (in Erlangs)

35. Factors that Determine the Nature of the Traffic Model
Manner in which blocked calls are handled
Lost calls delayed (LCD) – blocked calls put in a queue awaiting a free channel
Blocked calls rejected and dropped
Lost calls cleared (LCC) – user waits before another attempt
Lost calls held (LCH) – user repeatedly attempts calling
Number of traffic sources
Whether number of users is assumed to be finite or infinite