GSM Fundamentals. Copyright 2000, Agilent Technologies All Rights Reserved

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Theodore Lambert
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GSM History 1981 Analogue cellular introduced Franco-German study of digital pan- European cellular system 1987 MoU signed by over 18 countries 1989 GSM

3 GSM History 1981 Analogue cellular introduced Franco-German study of digital pan- European cellular system 1987 MoU signed by over 18 countries 1989 GSM was moved into the ETSI organization GSM name changed to Global System for Mobile communications DCS1800 (edited GSM900) specification developed Page 3

4 GSM History 1981 Analogue cellular introduced Franco-German study of digital pan- European cellular system 1987 MoU signed by over 18 countries 1989 GSM was moved into the ETSI organization GSM name changed to Global System for Mobile communications DCS1800 (edited GSM900) specification developed Page 4

5 GSM Around the World Canada: PCS 1900 USA: PCS1900 Argentina, Brazil, Chile: DCS 1800 UK: GSM900 DCS1800 Thailand: DCS1800 CIS, Denmark, Finland, France, Germany, Greece, Hungary, Poland, Norway, Sweden, Switzerland: GSM900 DCS 1800 Hong Kong: PCS 1900 Singapore : DCS1800 Malaysia: DCS 1800 Page 5

6 # * GSM Network NMC BTS BTS BSC OMC OMC MS MS BTS BSS BTS BTS BSC BTS MSC MSC VLR VLR EIR AUC HLR BSS Interface to other networks Page 6

7 * # * # * # GSM Network Interfaces Um Interface LAP-Dm Abis Interface LAP-D SS#7 SS#7 BTS BSC MSC BTS BSC MSC PSTN BTS BSC MSC Rf Test Equipment Page 7

8 SIM Card Breakout for micro SIM IMSI: IMSI Locator for correct insertion Active chip of smart card Page 8

9 SIM Card Plugs into Every GSM Mobile Page 9 Two Sizes - Standard (credit card) Micro (postage stamp) Holds All Unique Subscriber Information IMSI (International Mobile Subscriber) Lists of Networks Allowed For User K i - Ciphering Key Stores Information on Last Location Stores User Information Speed Dial Lists, Memories, etc.

10 9 # * GSM Bands Uplink Downlink Page 10

11 GSM Channel Plans Phase 1 GSM900 Phase 2 GSM900 Phase 1 DCS1800 Phase 2 DCS1800 PCS1900 Uplink Frequency Range 890 to 915MHz 880 to 915MHz 1710 to 1785MHz 1710 to 1785MHz 1850 to 1910MHz Downlink Frequency Range 935 to 960MHz 925 to 960MHz 1805 to 1880MHz 1805 to 1880MHz 1930 to 1990MHz ARFCN Range and Tx/Rx Spacing (MHz) Page 11

12 GSM FDMA and TDMA Time Amplitude Timeslot # ARFCN # Frequency Page 12

13 GSM Bursts Time Amplitude Overshoot Lobes due to modulation Raised lobes due to ramp up Frequency Page 13

14 Downlink and Uplink Timeslots DOWNLINK ARFCN MHz UPLINK ARFCN Timeslots 1 Frame Page 14

15 Downlink and Uplink Uplink Lags Downlink by 3 Timeslot periods Uplink and Downlink use same Timeslot Number Uplink and Downlink use same Channel Number (ARFCN) Uplink and Downlink use different bands (45MHz apart for GSM900) Page 15

16 GSM Cell Plan Page 16

17 GSM Cell Types Omnidirectional Antenna Cell Sectored Antenna Cell Sector Page 17

18 # * GSM Broadcast Channel BCH 3 BCH 2 BCH 1 Page 18

19 # * GSM Traffic Channel BCH TCH Page 19

20 # * GSM Hand-Off BCH Mobile constantly monitors received TCH quality (RxQual Page 20 & TCH

21 Measuring Adj BCH Power DOWNLINK UPLINK ADJACENT CELL BC H Page 21

22 6 9 # * GSM Power Steps Too much Power Low Talk Time Too little Power Dropped Calls Mobile adjusted according to Received Signal Strength and Quality at Base Station. Page 22

23 Mobile Power Levels Phase 1 GSM900 Phase 2 GSM900 Phase 1 DCS1800 Phase 2 DCS1800 PCS1900 Mobile Max Power 20W (8W used) 43dBm/39dBm 8W / 39dBm 1W / 30dBm 4W / 36dBm 2W / 33dBm Mobile Min Power 20mW /13dBm 3mW / 5dBm 1mW/ 0dBm 1mW / 0dBm 1mW / 0dBm Mobile Power Control Steps ,30,31 Page 23

24 GSM Burst - TDMA Amplitude 148 Bits in the Useful Part of the Burst Time Page 24

25 GSM Time Delay Cell radius < 35 km Signal travel or time 117 usec BTS Page 25

26 GSM Time Delay Ideal Change noted in 1/4 bit periods, but only changed if change in excess Page 26 of 4/4 bit Delayed

27 GSM Midamble Equalisation Reference Color Code (Midamble) Actual Received Signal Page 27

28 Midamble or Training Bits 8 Midamble Patterns (Colour Codes) of 26 bits Equalizer Estimates Channel Impulse Response From Midamble Mathematically Construct Inverse Filter Uses Inverse to Decode Data Bits Page 28

29 Frequency Diversity db Wavelength Page 29

30 Hopping Sequences DOWNLINK C C2 C3 UPLINK C1 C2 C ADJACENT CELL BCH Page 30

31 Hopping Sequences Control Channels NEVER hop Can employ cyclic or pseudo-random hopping sequences Different mobiles in a cell have different MAIO s (Mobile Allocation Index Offset) Page 31

33 BCH - Frames Super-frame 26 Multiframes 6.12 s Multi-frame 51 Frames 120 ms Frame Timeslot (normal burst) 4.615ms us 8 Timeslots Bits Page 33

34 GSM BCH Broadcast Channel One BCH on all the time, in every cell BCH Information carried in Timeslot 0 Identifies Network Carries, Paging and other Control Information Page 34

35 Broadcast Channel Control Channel Organization Control Channel BCH CCCH DCCH Page 35

36 BCH - Subchannels Broadcast Channel Organization BCH Broadcast Channel FCCH Frequency Correction SCH Synchronization BCCH Broadcast Control Page 36

37 SCH - The S Burst Tail Bits Information Bits Midamble (training sequence) Information Bits Tail Bits Page 37

38 FCCH - The F Burst Bits set to ZERO 3 Tail Bits Information Bits Tail Bits Page 38

39 CCCH - subchannels Common Control Channel Organization CCCH Common Control Channel PCH RACH AGCH CBCH Paging Random Access Access Grant Cell Broadcast Page 39

40 9 # * # * # * SMS Cell Broadcast All mobiles in a cell receive the same message from the base station Page 40

41 CBCH Mobiles can receive SMS messages in IDLE and DEDICATED modes: Cell Broadcast Channels (CBCH s) are part of CCCH so IDLE state mobiles can receive SMS. FACCH sets up SMS send and acknowledge. SACCH carries data for mobiles in DEDICATED mode. Page 41

42 9 # * # * # * SMS Point to Point X X Only a single mobile in a cell receives the message from the service center Page 42

43 SMS Point to Point This is a message that comes to a particular user, from a service center, or to the service center from the user. Sent and Received as data on the TCH as a call. Send and Receive transparent to the user. Page 43

44 DCCH - subchannels Dedicated Control Channel Organization DCCH Dedicated Control Channel SDCCH Stand-Alone Dedicated SACCH Slow Associated FACCH Fast Associated Page 44

45 GSM SDCCH Stand Alone Dedicated Control Channel Used during Call Setup Stepping Stone between BCH and TCH Used for Authentication Page 45

46 BCH Organization Timeslot FCCH SCH BCCH CCCH DOWNLINK Page 46

47 BCH Organization Timeslot RACCH UPLINK Page 47

48 GSM RACH Random Access Channel Used by the mobile to get attention SHORT BURST Mobile sends normal burst after getting Timing Advance Page 48

49 Frame Format - RACH Tail Bits Midamble (training sequence) Information Bits Tail Bits Page 49

50 Page 50 DRX DRX * # * # * # * # ZZZzzzzzz ZZZzzzzzz Look for Pages ZZZzzzzzz

51 DRX Only looks for pages during its assigned Paging Group Paging Group is a function of the mobile IMSI Reduces power consumption as receiver can sleep between pages Page 51

53 TCH - Frames Super-frame 51 Multiframes 6.12 s Multi-frame 26 Frames 120 ms Frame Timeslot (normal burst) 4.615ms us 8 Timeslots Bits Page 53

54 Frames format - TCH Tail Bits Data Bits Stealing Flag Midamble Bits Stealing Flag Data Bits Tail Bits Page 54

55 TCH Organization Timeslot TCH SACCH IDLE UPLINK Page 55

56 GSM SACCH Slow Associated Control Channel DOWNLINK Mobile Tx Power Commands Mobile Timing Advance Page 56 Cell s Channel Configuration UPLINK RXQual report RXLev report

57 GSM SACCH Timeout Slow Associated Control Channel SACCH delivers RxQual and RxLev, which starts Radio_Link_Timeout counter. If no reports within this timeout period, then the call is dropped. Page 57

58 SACCH DOWNLINK UPLINK * Timing * Adjacent BCH * Power Ctrl. * Channel Power * Call configuration * Rx Level * Hopping frequency * Rx Qual. Page 58

59 GSM FACCH Fast Associated Control Channel Used by BS and MS to send large amounts of data FAST Hand-offs Set Stealing Flags Page 59

60 9 # * DTX Normal Mode SID Discontinuous Mode Page 60

61 DTX Instead of transmitting useless background noise, the mobile sends Silence Descriptor Frames (SIDs) Reduces the power consumption by the mobile Reduces the noise in the Spectrum for other users Page 61

63 GSM Voice Path Vocoder Codec Interleave Modulate Burst Page 63

64 GSM Vocoder Vocoder 20ms Speech makes 260 Bits Output 13 kbit/s RELP - Residually Excited Linear Predictive LTP - Long Term Predictive 3 Classes of bits output Page 64

65 Speech Coder 20 ms Block Speech Coder 260 Bits Page 65

66 Speech Coder LongTerm Prediction Linear Predictive Coding ADPCM - Adaptive PCM MOS - Mean Opinion Scoring Page 66

67 Speech Coder 260 Bits Bits Ordered 260 Bits 50 Bits type 1a 132 Bits type 1b 78 Bits type 2 Page 67

68 GSM Codec 13 kbit/s 260 bits CODEC 13 kbit/s 456 bits 3 classes of error correction Class 1a bits: Error Correction and CRC bits Class 1b bits: Error Correction only Class II bits: No Error Correction Page 68

69 Error Correction 50 Bits type 1a 132 Bits type 1b 78 Bits type bits type 1a CRC type 1b type 2 3 bit lock Code CRC inserted 263 bits Page 69

70 Error Correction 263 bits type 1a CRC type 1b type Bits Re-ordered 4 Tail Bits Inserted 267 bits Page 70

71 Error Correction 267 bits /2 Rate Convolutional Coding bits Page 71

72 GSM Interleave 13 kbit/s 456 bits 456 bits Interleave 13 kbit/s 456 bits 456 bits Page 72

73 Diagonal Interleaving TCH Bursts carry two 57 bit blocks 120ms of speech = 456 x 6 = 2736 bits 2736 / 114 = 24 bursts needed to transmit TCH Multiframe is 26 frames and 120ms Now have 2 spare bursts: Use one for the SACCH Page 73

74 CRC Capable of correcting single errors Not able to correct multiple bit errors Page 74

75 Coding: Channel Types TCH/F different classes & coding 260 in, 456 out Parity (3 bits) Convolution 1/2 TCH/F9.6 (data) 240 in, 456 out Convolution 1/2 punctured 1/15 bit Page 75

76 Coding: Channel Types TCH/F4,8 (data) + 32 null bits 120 in, 456 out Convolution 1/3 TCH/F2.4 (data) 72 in, 456 out Convolution 1/6 Page 76

77 Coding: Channel Types SCH (control) Parity 10 bits 25 in, 78 out Convolution 1/2 RACH (control) Parity 6 8 in, 36 out Convolution 1/2 FACCH (control) 224/184 Firecode Page in, 456 out Convolution

78 Interleaving: Channel Types TCH/F TCH/F9.6 portions TCH/F4.8 portions TCH/F2.4 SCH (control) RACH (control) FACCH (control) Page 78 8 half-bursts 22 unequal 22 unequal 8 half-bursts 1 S burst 1 Access burst 8 half-bursts

79 Ciphering CLEAR TEXT (data) CIPHERING SEQUENCE XOR CIPHERED TEXT Page 79

80 Ciphering Clear Text Cipher Sequence XOR Ciphered Text Page 80

81 Ciphering BS Controls if active or not After Interleaving, before Burst Building Very tightly controlled algorithms Requires K i Cyphering Key from HLR Similar to techniques used by DOD Page 81

82 GSM Modulate Input 13 kbits/s (Error Corrected, Interleaved) Modulate Output kbits/s 0.3GMSK (Time Divided) Page 82

83 GSM Burst Input kbits/s 0.3GMSK (Time Divided) Burst kbits/s Output 0.3GMSK (bursted) Page 83

85 Mobile Turn-On Mobile Searches for Broadcast Channels (BCH) Synchronizes Frequency and Timing Decodes BCH sub-channels (BCCH) Checks if Network Allowed by SIM Location Update Authentication Page 85

86 MS Location Update Channel Request Immediate Assignment BTS Location Update Request Location Updating Request Authentication Request Authentication Response Cipher Mode Command Cipher Mode Complete Location Updating Accept BSC MSC 1 OMC VLR HLR AUC PSTN Page 86

87 IMSI Attach Forced Location Update on Power On Only if LAC on SIM is different Forced Location Update on Power Off Page 87

88 Mobile Originated Call Mobile Sends RACH Channel Assignment Posted on BCH (AGCH) Mobile and Base Station communicate on SDCCH Authentication Mobile Assigned to Traffic Channel (TCH) Page 88 Speech Data sent and received

89 Mobile Originated Call (1) LR s MS BTS BSC MSC PSTN Channel Request Immediate Assign to DCCH OMC Asynchronous Mode Unnumbered Acknowledge Set-up Indication Authentication Request Authentication Response Cipher Mode Cipher Mode Complete Page 89

90 Mobile Originated Call (2) LR s MS BTS BSC MSC PSTN OMC Call Set Up Assign Command Assign Complete Alert (Ringing) Call Connect Speech Speech Page 90

91 Mobile Terminated Call Mobile Sees Page Mobile Sends RACH Channel Assignment Posted on BCH (AGCH) Mobile and Base Station communicate on SDCCH Authentication Mobile Assigned to Traffic Channel (TCH) Page 91

92 Mobile Terminated Call (1) LR s MS BTS BSC MSC PSTN Paging Request Channel Request (RACH) Immediate Assign to DCCH OMC Asynchronous Mode Unnumbered Acknowledge Set-up Indication Authentication Request Authentication Response Cipher Mode Cipher Mode Complete Page 92

93 Mobile Terminated Call (2) LR s MS BTS BSC MSC PSTN Call Set Up OMC Call Confirmed Alerting Assign Command Assign Complete Call Connect Connect Acknowledge Speech Page 93

94 # * * # Mobile Handoff BSC old switch point BSC new Hand-over required Start of path establishment End of path establish and hand over command to MS Allocation and activation of radio channel Hand-over command to MS MS has accessed cell B Release of Path Page 94

Chapter 2: Global System for Mobile Communication

Chapter 2: Global System for Mobile Communication (22 Marks) Introduction- GSM services and features, GSM architecture, GSM channel types, Example of GSM Call: GSM to PSTN call, PSTN to GSM call. GSM frame