1. Radio & Telecommunications Systems 1 Basic cellular system Propagation Cellular Radio Principle

2. Radio & Telecommunications Systems 2 Basic Cellular System consists of three parts: (Fig. 1) mobile unit cell site mobile telephone switching office (MTSO)

3. Radio & Telecommunications Systems 3 Fig. 1 Cellular System

4. Radio & Telecommunications Systems 4

5. 5 Mobile units unit contains control unit transceiver antenna system Cell site provides interface between the MTSO and the mobile units, it has control unit radio cabinets antennas power plant

6. Radio & Telecommunications Systems 6 MTSO is the switching office, coordinating element for all cell sites, contains cellular processor cellular switch interfaces with telephone company zone offices controls call processing handles billing activities each mobile unit can only use one channel at a time for its communication link; channel is not fixed, can lie any one in the entire band assigned by the serving area

7. Radio & Telecommunications Systems 7 Propagation incident angle of the direct wave is  1 and the incident angle of the reflected wave is  2,  1 is also called the elevation angle (Fig. 2) C is inversely proportional to R 4 C  R -4 =  R -4 where C = received carrier power R = distance measured from the transmitter to the receiver  = is a constant

8. Radio & Telecommunications Systems 8 Fig.2 Mobile radio transmission

9. Radio & Telecommunications Systems 9 Fading antenna of the mobile unit is lower than its typical surroundings multi-path waves are generated at the mobile unit the sum of the multi-path waves causes a signal- fading phenomenon (Fig. 3)

10. Radio & Telecommunications Systems 10 Fig. 3 Typical fading signal

11. Radio & Telecommunications Systems 11 signal fluctuates about 40 dB (10 dB above and 30 dB below the average signal) if the mobile unit moves fast, the rate of fluctuation is fast Rayleigh fading is also called multi-path fading in the mobile radio environment (Fig. 4) multi-path waves bounce back and forth due to the buildings and houses summed together and become an irregular way fading structure

12. Radio & Telecommunications Systems 12 Fig. 4 Multi-path fading

13. Radio & Telecommunications Systems 13 Frequency Re-Use Reuse channels Reuse distance Cochannel interference Cell splitting

14. Radio & Telecommunications Systems 14 Concept of Frequency Reuse Channels radio channel consists of a pair of frequencies one for each direction of transmission for full-duplex operation a particular radio channel F 1 used in one geographic zone to call a cell C 1 coverage radius R can be used in another cell with the same coverage radius at a distance D away (Fig. 1)

15. Radio & Telecommunications Systems 15 Fig. 1 D/R ratio Where q is called the cochannel interference reduction factor

16. Radio & Telecommunications Systems 16 Frequency reuse system users in different geographic locations (different cells) simultaneously use the same frequency channel increase the spectrum efficiency if the system is not properly designed serious interference may occur interference due to the common use of the same channel called co-channel interference

17. Radio & Telecommunications Systems 17 Frequency reuse distance minimum distance which allows the same frequency to be reused depend on number of co-channel cells in the vicinity of the center cell type of geographic terrain contour antenna height transmitted power at each cell site

18. Radio & Telecommunications Systems 18 the frequency reuse distance D can be determined from D = (3K) 1/2 R where K is the frequency reuse pattern R is the radius of cell assume all the cell sites transmit the same power if K increases the frequency reuse distance D increases increases D => reduces cochannel interference

19. Radio & Telecommunications Systems 19 a large K is desired, however the total number of allocated channels is fixed when K is too large number of channels assigned to each of K cells becomes small smallest value of K is 3, obtained by setting i = 1, j = 1 then K = i 2 + ij +j 2 (see Fig. 2a-2d)

20. Radio & Telecommunications Systems 20 Fig. 2a 4-cell reuse pattern

21. Radio & Telecommunications Systems 21 Fig. 2b 7-cell reuse pattern

22. Radio & Telecommunications Systems 22 Fig. 2c 12-cell reuse pattern

23. Radio & Telecommunications Systems 23 Fig. 2d 19-cell reuse pattern

24. Radio & Telecommunications Systems 24 Cochannel Interference Reduction Factor to find the minimum frequency reuse distance in order to reduce cochannel interference co-channel interference is a function of a parameter q defined as: q = D / R where q is the cochannel interference reduction factor the ratio q increases, cochannel interference decreases

25. Radio & Telecommunications Systems 25 Operation of Cellular Systems Call Process Handoff Procedure

26. Radio & Telecommunications Systems 26 Call Processing Mobile unit initialization: power ON user activates the receiver of the mobile unit the receiver scans set-up channels selects the strongest and locks on for a certain time each site is assigned a different set-up channel Mobile originated call user places the called number into an originating register in the mobile unit request for service is sent on a selected set-up channel obtained from a self-location scheme

27. Radio & Telecommunications Systems 27 cell site receives it, and in directional cell sites, selects the best directive antenna for the voice channel to use cell site sends a request to the mobile telephone switching office (MTSO) MTSO selects an appropriate voice channel for the call the cell site acts on it through the best directive antenna to link the mobile unit the MTSO also connects the wire-line party through the telephone company zone office

28. Radio & Telecommunications Systems 28 Network originated call land-line party dials a mobile unit number telephone company forwards the call to the MTSO MTSO sends a paging message to certain cell sites based on the mobile unit number each cell site transmits the page on its own set-up channel mobile unit recognizes its own identification on a strong set-up channel, locks onto it, and responds to the cell site mobile unit also follows the instruction to tune to an assigned voice channel and initiate user alert

29. Radio & Telecommunications Systems 29 Call termination mobile user terminates the call a particular signal transmits to the cell site both sides free the voice channel mobile unit resumes monitoring pages through the strongest set-up channel

30. Radio & Telecommunications Systems 30 Handoff procedure during the call, two parties are on a voice channel mobile unit moves out of the coverage area of a particular cell site, the reception becomes weak (Fig. 1) present cell site requests a handoff the system switches the call to a new channel in a new cell site without either interrupting the call or alerting the user call continues as long as the user is talking

31. Radio & Telecommunications Systems 31 Fig 1 Occurrence of handoff CS: Cell Site

32. Radio & Telecommunications Systems 32 Mobile Assisted Handoff (MAHO) the mobile receiver is capable of monitoring the signal strength of the setup channels of the neighboring cells while serving a call Soft Handoff applied to CDMA systems all cells use the same radio carrier, change from one code to another code

33. Radio & Telecommunications Systems 33 Mobile and Base Stations Structure of a Mobile Station Structure of a Base Station

34. Radio & Telecommunications Systems 34 Structure of a Mobile Station Most portable phones are divided into two parts: (Fig. 1) RF part handles the receiving, transmitting, and modulation tasks digital part takes care of the data processing, control, and signaling functions

35. Radio & Telecommunications Systems 35 Structure of a Mobile Station

36. Radio & Telecommunications Systems 36 Fig 1 Block diagram of a mobile station

37. Radio & Telecommunications Systems 37 antenna combiner couples the receiving and the transmitting paths onto the single antenna connector or a fixed antenna receiver contains the front end, a receiving filter network, and a mixer to down-convert the input signal onto an IF that is eventually converted into the data domain by the ADC equalizer due to multipath propagation and other reflections, the signals arriving at the receiver are distorted the equalizer compensate distortions

38. Radio & Telecommunications Systems 38 demodulator extracts the bit stream from the IF demultiplexer sorts the received information from the different time slots and frames channel codec Channel coding is necessary to reduce probability of errors

39. Radio & Telecommunications Systems 39 speech codec compresses digitized speech coming from the ADC before being encoded minimizes bandwidth requirement control and signaling unit performs all the control functions of the mobile station

40. Radio & Telecommunications Systems 40 multiplexer assigns each individual burst to a time slot within a numbered frame modulator imparts information onto the IF carrier transmitter a mixer up-converts the modulated IF signals an amplifier increases the level of the signal output filters limit the bandwidth of the output to its assigned channel

41. Radio & Telecommunications Systems 41 synthesizer provides the internal timing references for the bit and frame clock as well as for the RF sources in the transmitter and the receiver voltage controlled oscillator (VCO) provides a stable operating frequency

42. Radio & Telecommunications Systems 42 Structure of a Base Station The general structure of a base station consists of base station control function (BCF) one to sixteen transceivers (TRX)

43. Radio & Telecommunications Systems 43 Fig 2 Block diagram of a base station

44. Radio & Telecommunications Systems 44 receiver contains the receiving filter which blocks frequencies other than the desired receiving band the signals are down-converted to an IF or directly to the baseband frequency, where the signals are sampled and quantized with an ADC equalizer compensates for the influences coming from the mobiles demodulator extracts the bit stream from the equalized signal and passes it to the demultiplexer

45. Radio & Telecommunications Systems 45 channel codec detect errors that have been introduced into the RF path and correct them speech codec compresses digitized speech to minimize bandwidth requirement signaling unit the logical interface for the control messages between the network and the mobile stations

46. Radio & Telecommunications Systems 46 control unit performs all the internal control tasks of the base station multiplexer maps the single bursts onto the single time slots bound for the individual mobile stations modulator modulates the digital signals onto the radio frequency carrier

47. Radio & Telecommunications Systems 47 transmitter contains the output filters to band-limit the signals controls the output level depending on the base station's power class synthesizer provides the necessary frequencies for the different entities in the BTS (Base Transceiver Station) usually synchronized with the clock from the BSC (Base Station Controller) alternatively, it is possible to have a local clock reference in each base station poor system performance (high hand-off failure rate) & low reliability due to sync problem

48. Radio & Telecommunications Systems 48 Multiple Access Methods Frequency Division Multiple Access Time Division Multiple Access Code Division Multiple Access

49. Radio & Telecommunications Systems 49 Frequency Division Multiple Access multiple access scheme for land mobile communication systems (Fig. 1 - 3) analog cellular systems use FDMA assigned system bandwidth is divided into bands with its bandwidth of W ch guard space to prevent spectrum overlapping each user sends a call request to the BS BS assigns one of the unused channels to the user channel is used exclusively by that user during a call when the call is terminated, the channel is reassigned to a different user

50. Radio & Telecommunications Systems 50 Fig.1 Basic Concept of FDMA system - spectrum

51. Radio & Telecommunications Systems 51 Fig. 2 Call initiation and holding model

52. Radio & Telecommunications Systems 52 Fig. 3 FDMA channel assignment

53. Radio & Telecommunications Systems 53 Time Division Multiple Access enables users to access the assigned bandwidth on a time basis (Fig. 4-6) each channel occupies the whole system bandwidth occupies only a fraction of the time, called slot, on a periodic basis one frame consists of N ch slots frame length is T second

54. Radio & Telecommunications Systems 54 Fig. 4 Concept of TDMA operation

55. Radio & Telecommunications Systems 55 Uplink each terminal transmits information using an assigned slot in each frame each terminal has to transmit its slot exactly in the assigned slot timing to prevent signal collisions Downlink all the slot signals are transmitted by the BS

56. Radio & Telecommunications Systems 56 Fig 6 Call initiation and holding model in TDMA

57. Radio & Telecommunications Systems 57 Fig 7 Slot assignment in TDMA

58. Radio & Telecommunications Systems 58 Code Division Multiple Access all the transmitted signals other than the desired signal are regarded as cochannel interference (CCI) signals (Fig. 7 - 9) at the CDMA receiver the desired signa1 can be picked up by taking correlation between the received signal and a code used at the transmitter (code#1)

59. Radio & Telecommunications Systems 59 Fig 7 Configuration of CDMA system

60. Radio & Telecommunications Systems 60 spreading code sequence for the receiver and that used at the transmitter are synchronized the resultant signal spectrum becomes the same as that for the source signal signal bandwidth of the interference signals still remains the same bandwidth even after taking correlation

61. Radio & Telecommunications Systems 61 all the terminals share the whole system bandwidth each terminal signal is discriminated by the code when each user sends a call request to the BS BS assigns one of the spreading codes to the user

62. Radio & Telecommunications Systems 62 Fig 8 Call initiation and holding model

63. Radio & Telecommunications Systems 63 Fig 9 Channel assignment in CDMA

64. Radio & Telecommunications Systems 64

65. Radio & Telecommunications Systems 65

66. Radio & Telecommunications Systems 66 Reference Seiichi Sampei, “Applications of Digital Wireless Technologies to Global Wireless Communications,” Prentice-Hall, 1997 Siegmund M. Redl, et. Al., “An Introduction to GSM,” Artech House, 1995 Lee, William C. Y., “Mobile cellular telecommunications : analog and digital systems,” McGraw-Hill, 1995 http://www.radiodesign.com/cellwrks.htm