1. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 1 Wireless Personal Communications Systems – CSE5807 Topic – Cellular Wireless Networks Lecture: 04 Stephen Giles and Satha K. Sathananthan Faculty of Information Technology Monash University Modified by Peter Granville August 2006 These slides contain figures from Stallings, and are based on a set developed by Tom Fronckowiak.

2. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 2 Cellular technology is the foundation of mobile wireless communications and supports users in locations that are not easily served by wired networks.

3. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 3 Wireless Networks  Limited bandwidth.  Noisy channel and Multipath propagation. >> Interference.  Limited coverage => Roaming  Security.  Power consumption.

4. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 4 Wireless Networks 1001010.1 Mbps Vehicle Walk Fixed Walk Fixed Indoor Outdoor Wideband Cellular GSM, CDMA WLAN WPAN LAN WAN 3G 2G WPAN – Wireless Personal Area Network

5. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 5 Cellular Systems First Generation Systems: Analog –Advanced Mobile Phone Service (AMPS): US, Australia, Southeast Asia. –Total Access Communication System (TACS): EU –Nippon Telephone and Telegraph (NTT): Japan –Being phased out Second Generation Systems: Digital –Global System of Mobile communications (GSM): Europe, Asia –Code Division Multiple Access (CDMA) systems (IS-95): US, Asia Third Generation Systems: Digital & Packet switching –High Speed Transmission –Wideband CDMA –CDMA2000

6. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 6 Wireless Channel Compared to wire/fiber, mobile radio channels have major problems with noise and interference. - Environmental effects. - Large amounts of noise. - Leakage from adjacent channels and distant transmitters on the same channel. - Multi-path fading (Rayleigh) and Doppler effect. Signal coverage: - Essential for deployment of wireless networks. - Influenced by the radio frequency of operation, transmitted power and the terrain.

7. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 7 Wireless Channel Tx Rx Diffraction Reflection Scattering

8. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 8 Multipath Propagation - Effects System Performance Reflection –A signal can be reflected by obstacles (obstacle > signal wave) so that multiple copies of the signal with varying delays can be received. –Depending on the differences in the path lengths of the direct and reflected waves, the composite signal can be either larger or smaller than the direct signal. Scattering –Occurs when Size of obstacle <= size of signal wave –An incoming signal is scattered into several weaker outgoing signals Diffraction –Occurs at the edge of an impenetrable body (large cf signal). –When a radio wave encounters such an edge, waves propagate in different directions with the edge as the source. –Thus signals can be received even when there is no unobstructed LOS from transmitter.

9. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 9 Cellular Concepts The concept of cellular radio emerged in the late 1940s: high power transmitter 80km radius cell 25 channels To get around the limitations on available frequencies, the following approach was implemented: A large number of low-power transmitters (<=100W) with shorter radius Because the range of the transmitter is small, an area can be divided into cells one base station per cell - transmitter, receiver, control unit A range of frequencies allocated to each cell.

10. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 10 Cellular Concepts Frequency allocation such that co-channel interference is limited. Adjacent cells assigned different frequencies. Cells sufficiently distant from each other to allow frequency reuse “Hand-over“ (handoff) techniques for mobile units moving from cell to cell.

11. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 11 Cell Shape Square Cell Pattern –If cell width is d, then a cell has four neighbours at a distance d and 4 neighbours at a distance SQRT(2d) from cell centre –As a mobile user moves towards cell’s boundaries, it is best if all adjacent antennas are equidistant. This simplifies the task of determining when to switch the user to an adjacent antenna and which antenna to use. Hexagonal Cell Pattern –Provides for equidistant antennas –Radius R, is radius of circle that circumscribes it –Distance between all adjacent cell centres is d = SQRT(3R) Refer Stallings fig 10.1

12. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 12 Cell Shape Variations from ideal hexagon are due to: –Topographical limitations –Local signal propagation conditions –Practical limitations on siting antennas

13. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 13 Cellular Concepts Areas divided into cells: –Each served by its own antenna => multiple low-power transmitters. –Served by base station consisting of transmitter, receiver, and control unit. –Band of frequencies allocated. –Adjacent cells assigned different frequencies to avoid interference or crosstalk. =>> Frequency reuse.

14. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 14 Cellular Concepts: Frequency Reuse coverage Base station (BS) coverage Backbone Network To transmit signals on a particular frequency band with “limited power” so that the same frequency band can be reused in other location. Fig 10.2 Stallings

15. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 15 Cellular Concepts: Frequency Reuse 4-cell frequency reuse (N=4) 7-cell frequency reuse (N=7)

16. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 16 Cellular Concepts: Frequency Reuse N = Frequency reuse factor R = Radius of a cell d adj = Distance between centers of adjacent cells D min = Minimum distance between centers of cells that use the same band of frequencies (cochannel) K = Total number of channels (frequency bands) allocated to the systems. R d D

17. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 17 Cellular Concepts: Frequency Reuse In a hexagonal cell pattern: If each cell is assigned equal number of channels, then the number of channels per cell:

18. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 18 Calculation D min = 1.6 x SQRT(3 x 7) = 1.6 x 4.58 = 7.3 km d adj = SQRT(3 x 1.6) = 2.19 km Given a cell radius of R = 1.6 km, N reuse = 7 calculate: 1) Minimum distance D min, between centers of cells that use the same band of frequencies (co-channels) 2) Distance d adj, between centers of adjacent cells

19. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 19 Calculation Given a system with N cells = 32, R =1.6 km, total number of traffic channels K = 336, N reuse = 7, calculate: N cpc number of traffic channels per cell T calls total number of concurrent calls that can be handled T calls = N cpc x N cells N cpc = K/N reuse N cpc = 336/7 = 48 traffic channels per cell T calls = 48 x 32 = 1536 concurrent calls (ie 1536 channels available)

20. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 20 Cellular Concepts: Interference and Capacity Interference affects reuse plan. Major interference: - Cochannel (or same frequency interference): => most important. - Adjacent Channel: => less important. The smaller value of N reuse the: Wider available bandwidth in each cell.  Higher interference. For AMPS, K=395 and N reuse = 7 is the smallest pattern that can provide sufficient isolation between two uses of the same frequency, about 57 frequency channels per cell

21. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 21 Cellular Concepts: Cochannel Interference 7-cell frequency reuse (weaker) 4-cell frequency reuse (stronger) Calculate D min with R=1.6km

22. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 22 Cellular Concepts: Adjacent Channel Interference wanted Interference … power Adjacent channel interference Frequency band of the mobile phone

23. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 23 Cellular Concepts: Increasing Capacity In time as more customers use the system, traffic may build up so that there are not enough frequencies assigned to a cell to handle its calls.

24. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 24 Cellular Concepts: 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. –Power level used must be reduced to keep the signal within the cell –1.5 km close to practical limit –As cells get smaller, handoffs become more frequent –Fig 10.3 Stallings Cell sectoring: –Cells are divided into a number of wedge-shaped sectors, each with their own set of channels.

25. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 25 Cellular Concepts: Cells Different sizes and types of cells are used in a cellular network. The choices of a cell depend on the bandwidth usage in a region. - Macrocells: - Used to serve low density traffic area. - Tens of kilometers, served by base stations. - Microcells: - Used to serve high density traffic area. - 100m to 1km, base station antenna moved to rooftops of small buildings and finally to lamp posts. - Sectored cells: - Used to reduce cochannel interference. - Umbrella cells: - Used to reduce the need for handover in microcells.

26. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 26 Cellular Concepts: Cells Large cells are used to serve low traffic areas. Microcells are used for high traffic demand regions. Umbrella cells are used in areas where users are moving fast from one cell to another (eg. freeways)

27. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 27 Cellular Concepts: Sectored Cells 3 or 6 sectors per cell: The output power of an antenna in a sectored cell: -3dB coverage antenna Rhombic Hexagonal Triangular

28. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 28 Cellular Concepts: Interference in Sectored Cells interference 7-cell frequency reuse Each sector is operated at a different frequency band. The number of main interference is reduced from 6 to 2

29. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 29 Spatial Diversity  Multipath in wireless transmissions results in “Rayleigh Fading” (or fast fading).  Multiple antennas are used to receive signals from a mobile phone to reduce Fading effects.  Base station tower Two receivers

30. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 30 Operation of Cellular System

31. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 31 Operation of Cellular System fig 10.6 Stallings Mobile equipment (ME): –Physical terminal, includes radio transceiver, digital signal processors and subscriber identity module (SIM). Base Station (BS): –Includes antenna, controller, and a number of receivers. –Use multiple low-power transmitters. –Areas divided into cells, and each served by its own antenna. –Band of frequencies allocated. 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 for setting up and maintaining calls. –Traffic channels: carry voice or data connection between users.

32. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 32 Operation of Cellular System Mobile unit initialization: –Scanning => Select the strongest setup channel. –Handshake => Identify and register location. Mobile-originated call: –Request for connection on the pre-selected setup channel. Paging: –MTSO sends message to certain BSs to identify the called number. Call accepted: –Mobile recognizes the call respond to BS -> MTSO. –MTSO assigns traffic channels. Ongoing call: –Monitoring stage. Handoff:

33. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 33 Handoff or Handover  Handoff occurs when a mobile phone moves from one cell to another.  Power levels are constantly measured by base stations and/or mobile phones to decide whether a handoff is needed.  cell boundary Base station f1f1 f2f2

34. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 34 Handoff or Handover Handoff types –Network initiated, based on received signals from the mobile unit. –Mobile assisted, via providing info to network concerning signals received at mobile unit Performance metrics for handoff decision. –Cell blocking probability: probability of a new call being blocked, due to heavy load on Base Station traffic capacity. What happens ? –Call dropping probability: probability that a call is terminated due to a handoff. –Call completion probability: probability that an admitted call is not dropped before it terminates.

35. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 35 Handoff or Handover Principal parameter used to make the Handoff Decision is measured signal strength from the mobile at BS. BS averages the signal strength over a moving window of time to remove the rapid fluctuations due to multi-path effects Handoff strategies

36. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 36 Handoff Strategies Refer fig 10.7 Stallings: Relative signal strength. –The mobile unit is handed off from BS A to BS B when signal strength at B first exceeds that at A - L 1 –Can result in ping-pong effect

37. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 37 Handoff Strategies Relative signal strength with threshold. –Handoff only occurs if : (1)the signal at current BS is sufficiently weak (less than a pre- defined threshold) and (2)the other signal is the stronger of the two –The intention is that so long as the signal at current BS is adequate, handoff is unnecessary –High Threshold Th 1 - handoff at L 1 as above –Th 2 - handoff at L 2 –Th 3 - handoff at at L 4, reduces quality of the link and may result in

38. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 38 Operation of Cellular System Call blocking: –No free traffic channels, busy tone returned to user. Call termination: –MTSO is informed, traffic channels released Call drop: –Resulting from weak signal, MSTO is informed Calls to/from fixed and remote mobile subscriber: –MTSO sets up the connection.

39. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 39 Power Control Dynamic power control in a cellular system. –Received power must be sufficiently above the background noise for effective communication –Desirable to minimize power in the transmitted signal from the mobile Reduce cochannel interference Alleviate health concerns Save battery power –In spread spectrum systems using CDMA, it’s desirable to equalize the received power level from all mobile units at the BS.

40. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 40 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: Received signal power level. Received signal to noise ratio. Received bit error rate. –BS makes power adjustment decision and communicates to mobile on control channel. –Also used to adjust power in forward channel. Mobile provides power adjustment information to BS. Refer table 10.2 Stallings

41. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 41 Traffic Engineering Ideally, available channels would equal number of subscribers active at one time In practice, not feasible to have capacity to handle all possible users. For N simultaneous user capacity and L subscribers L non-blocking system L > N => blocking system Blocking system: –Blocking probability (B): Probability that call request is blocked. –What capacity is needed to achieve a certain upper bound on probability of blocking, B?

42. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 42 Traffic Engineering Traffic intensity (A): –Load presented to a system: = mean rate of calls attempted per unit time. h = mean holding time per successful call. A = traffic intensity = * h Erlang 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.

43. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 43 Traffic Engineering - Example h = mean holding time per successful call = 3mins = mean rate of calls attempted per unit time = 20 calls/min Hence A = traffic intensity = 3 * 20 = 60 Erlang Erlang a measure of traffic intensity Number of channels needed ? –120 expect 50% utilization –50 clearly inadequate –60 would meet average demand, but would not cater for fluctuations around the mean rate A

44. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 44 Traffic Engineering - Example Refer fig 10.8 Stallings Cell capacity 10 channels = Mean Rate of calls per min = 97/60 = 1.62 h = Mean Call holding time = 294/97 = 3 mins Hence A = traffic intensity = 3 * 1.62 = 4.86 Erlang, so an average of 4.86 channels are engaged

45. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 45 Traffic Engineering Performance is measured by the call blocking probability. Blockage rate depends on the number of lines available, the number of initiated calls, and the length of the call. Erlang B formula: - Calls are lost if a line is not available.

46. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 46 Call Blocking - Example Installing a Base Station You are required to determine how many lines the Base Station is required to support Project a 50% growth in cell traffic over the next five years What probability of call blocking are you prepared to accept ? Currently mobile users are generating an average of 2 calls/min (the call arrival rate) and the average connection time per call is 3 mins (the call holding time)

47. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 47 Call Blocking - Example (Call Arrival Rate) = 2 * 1.5 = 3 calls/min (Call Holding Time) = 3 mins You need to calculate the traffic load that the Base Station will be required to handle A – the Traffic Intensity –A is a measure of incoming traffic = * = 9 Erlang The Probability of a Call Blocking is given by: where N is the number of outgoing lines

48. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 48 Call Blocking - Example Results of Calculations Let N = 3 outgoing lines n! = n x n-1 x n-2 x......x 1, 0! = 1, eg 3! = 3 x 2 x 1 = 6 Result = 1 + 9 + 40.5 + 121.5 = 172 P B = 121.5/172 = 0.71 A/N the utilisation per link = 9/3 = 3 as A/N exceeds 1 the probability of blocking rises rapidly

49. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 49 Traffic Engineering - Example Refer table 10.3 Stallings A larger capacity system is more efficient than a smaller capacity system for a given grade of service: –Consider two cells: each with a capacity of 10 channels they have a joint capacity of 20 channels can handle a combined offered traffic intensity of (2 * 3.43) 6.86 Erlangs for a grade of service of 0.002 –However, a single cell: capacity 20 channels can handle 10.07 Erlangs at a grade of 0.002

50. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 50 Traffic Engineering - Example Refer table 10.3 Stallings A larger capacity system is more susceptible to reduction of the grade of service as traffic increases: –Consider a cell of 10 channels: giving a grade of service of 0.002 for load of 3.43 Erlangs A 30% increase in traffic to 4.46 Erlangs reduces the grade to 0.01 –However, a cell of capacity 70 channels: giving a grade of service of 0.002 for a load of 51 Erlangs only a 10% increase in traffic to 56.1 reduces the grade to 0.01

51. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 51 Analog Cellular Networks (1 G)  An analog cellular network is operated at 900 MHz.  FDMA is used to allow multiple mobile phones to share a single base station in a cell.  Voice signals are transmitted with no coding scheme.  The major analogue cellular systems are based on the original AMPS design: - System bandwidth: 25MHz - 25 or 30 kHz channels - AMPS (USA, EIA-553) 800MHz - TACS (UK) 900MHz - NMT (Nordic countries) 450 and 900 MHz

52. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 52 AMPS Parameters Downlink: 869 to 894 MHz Uplink: 824 to 849 MHz Channel bandwidth: 30 kHz Spacing between forward and reverse channel: 45 MHz Number of full-duplex voice channels: 790 Number of full-duplex control channels: 42 Mobile unit maximum power: 3 watts Cell radius: 2 to 20 km Data transmission rate: 10 kbps Modulation schemes: FM and FSK

53. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 53 AMPS Operation Subscriber initiates call by keying in phone number and presses send key. MTSO verifies number and authorizes user. MTSO issues message to user’s cell phone indicating send and receive traffic channels. MTSO sends ringing signal to called party. Party answers: –MTSO establishes circuit and initiates billing information. Either party hangs up: – MTSO releases circuit, frees channels, completes billing

54. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 54 1 G and 2 G Comparison 1G2G System AnalogDigital Multiple Access FDMATDMA, CDMA Scheme:FDMA Voice quality:LowGood Bandwidth efficiency:LowHigh Power consumptionHighLow on mobile phones: Security:LowHigh Value added service: DifficultEasy System complexity:LowHigh

55. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 55 GSM Use of several carrier frequencies. Cell sizes vary : 100 m up to 35 km (user density, geography, transceiver power etc). Multiple Access = FDMA/TDMA FDMA  200kHz TDMA  8 slots in a frame ie. each channel = 200kHz/8 = 25kHz (Bandwidth) AB 012345670…012345670…time freq f3f2f1f3f2f1 cell A

56. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 56 GSM Parameters Downlink: 935 to 960 MHz Uplink: 890 to 915 MHz Channel bandwidth: 200 kHz Users per channel: 8 Spacing between forward and reverse channel: 45 MHz Number of duplex channels: 125 Mobile unit maximum power: 20 watts Speech coding bit rate: 13 kbps Modulation schemes: GMSK

57. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 57 GSM Speech Signal Processing

58. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 58 GSM Network Architecture

59. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 59 GSM: Mobile Station Mobile station communicates across Um interface (air interface) with base station transceiver in same cell as mobile unit. Mobile equipment (ME) – physical terminal, such as a telephone or PCS. –ME includes radio transceiver, digital signal processors and subscriber identity module (SIM). GSM subscriber units are generic until SIM is inserted.

60. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 60 GSM: Base Station Subsystem (BSS) BSS consists of base station controller and one or more base transceiver stations (BTS). Each BTS defines a single cell. –Includes radio antenna, radio transceiver and a link to a base station controller (BSC). BSC reserves radio frequencies, manages handoff of mobile unit from one cell to another within BSS, and controls paging.

61. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 61 GSM: Network Subsystem (NS) NS provides link between cellular network and public switched telecommunications networks: –Controls handoffs between cells in different BSSs. –Authenticates users and validates accounts. –Enables worldwide roaming of mobile users. Central element of NS is the mobile switching center (MSC).

62. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 62 GSM: Mobile Switching Center (MSC) Databases Home location register (HLR) database: –Stores information about each subscriber that belongs to it. Visitor location register (VLR) database: –Maintains information about subscribers currently physically in the region. Authentication center database (AuC): –Used for authentication activities, holds encryption keys. Equipment identity register database (EIR): –Keeps track of the type of equipment that exists at the mobile station

63. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 63 GSM Network Architecture fixed network BSC MSC GMSC VLR HLR NSS with OSS RSS VLR BTS MS

64. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 64 GSM Network Architecture NSS MS BTS BSC GMSC IWF OMC BTS BSC MSC A bis UmUm EIR HLR VLR A BSS PDN ISDN, PSTN RSS radio cell MS AUC OSS signaling O

65. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 65 GSM: Channels  Physical: TCH (Traffic Channel)  Logical channels, they are used for controls and signaling. Some examples:  Synchronization channel (SCH): - to supply mobile phones with training sequence to achieve synchronization.  Random access channel (RACH): - to allow a mobile phone to request for a channel.  Paging channel (PCH): - for a base station to page individual mobile phones and many others.

66. Wireless & Personal Communication Systems – CSE5807 Lecture: 04 66 Required Reading W. Stallings, “ Wireless Communications and Networks ” Prentice-Hall, 2000. >> Chapter 10 Optional Reference K. Pahlavan and K. Krishnamurthy “Principles of Wireless Networks”, Prentice-Hall, 2002.