Lecture 11: Cellular Networks

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

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) • Hexagonal shape cells • 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 theaverage 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