ECE 5221 Personal Communication Systems Introduction to GSM

1. ECE 5221 Personal Communication SystemsIntroduction to GSM

2. Page 2 Course Outline Part 1: Introduction Historical overview Elements of network architecture Elements of air interface Part 2: Signal processing and network features Voice processing GSM Network features Part 3: Network design Coverage planning Capacity planning Migration towards 3G and beyond

3. Page 3 History Driving Factors: Incompatibility of the European analog cellular systems Reaching of capacity limits Costs of the equipment 1982, Conference of European Post and Telecommunications formed Group Speciale Mobile (GSM) 1987, 15 operators from 13 countries signed Memorandum of Understanding (MoU) 1991, Finland’s operator Radiolinia launched first GSM network in July 1991 1992, Massive deployment of GSM started By 2000 GSM became the most popular 2G technology worldwide GSM standard still evolving and enriched with new features and services

4. Page 4 Deployment worldwide 930 networks in 222 countries and regions More than 3 billion subscribers worldwide More than 80% worldwide market share

5. Page 5 GSM in the USA 1994, US FCC auctioned large blocks of spectrum in 1900MHz GSM started deployment in PCS band 1995, American Personal Communications launched first GSM network In 2002, 850 band opened for GSM Currently there are ~ 95M GSM subscribers Largest GSM operators ATT T-Mobile

6. Page 6 GSM Standards Divided into 12 series Standardization efforts coordinated by ETSI www.etsi.org Specifications available online – free of charge Standardization and public availability of specification - one of fundamental factors of GSM success

7. Page 7 GSM Network Layout GSM system layout is standardized Standardization involves: Elements of the network Communication Interfaces Standard layout allows for the use of equipment from different suppliers

8. Page 8 GSM Components and Interfaces Network has many functional components Components are integrated through a network protocol – MAP Standardized interfaces Um (air interface) A – GERAN interface A-Bis (somewhat standardized)

9. Page 9 Mobile Station (MS) Two functional parts HW and SW specific for GSM radio interface Subscriber Identity Module (SIM) SIM – detaches user identity from the mobile Stores user information Without SIM – only emergency calls

10. Page 10 Base Transceiver Station (BTS) BTS is a set of transceivers (TX/RX). GSM BTS can host up to 16 TX/RX. In GSM one TX/RX is shared by 8 users. The main role of TX/RX is to provide conversion between traffic data on the network side and RF communication on the MS side. Depending on the application, it can be configured as macrocell, microcell, omni, sectored, etc.

11. Page 11 BSC plays a role of a small digital exchange. It can be connected to many BTSs and it offloads a great deal of processing from MSC One BSC connects to several tens to couple of hundred BTS Some of BSC responsibilities: Handoff management MAHO management Power control Clock distribution Operation and maintenance TRAU is responsible for transcoding the user data from 16Kb/sec to standard ISDN rates of 64Kb/sec. It can physically reside on either BSC side or MSC side. If it resides on the MSC side, it provides substantial changes in the backhaul – 4 users over a single T-1/E-1 TDMA channel. TRAU, BSC and BTSs form Base Station Subsystem (BSS Base Station Controller (BSC) and TRAU

12. Page 12 Responsible for connecting the mobile to the landline side GSM MSC is commonly designed as a regular ISDN switch with some added functionality for mobility support GSM Network can have more than one MSC One of the MSC has an added functionality for communication with public network – Gateway MSC (GMSC) All calls from the “outside networks” are routed through GMSC Mobile Switching Center (MSC)

13. Page 13 Registry HLR/VLR HLR – Home Location Registry Database for permanent or semi-permanent data associated with the user Logically, there is only one HLR per network Typical information stored in HLR: International Mobile Service Identification Number (IMSI), service subscription information, supplementary services, current location of the subscriber, etc. HLR is usually implemented as an integral part of MSC VLR – Visitor Location registry Temporary database that keeps the information about the users within the service area of the MSC Usually there is one VLR per MSC The main task of the VLR is to reduce the number of queries to HLR. When the mobile, registers on the system its information is copied from HLR to VLR VLR is usually integrated with the switch

14. Page 14 AUC/EIR AUC – Authentication center Integral part of HLR GSM specifies elaborate encryption Three levels A5/1 USA + Europe A5/2 COCOM country list No encryption – rest of the world EIR – Equipment Identity Registry Responsible for tracking equipment and eligibility for service Maintains three lists White list – approved mobile types Black list – barred mobile types Gray list – tracked mobile types

15. Page 15 GSM Air Interface - Um Interface between the MS and the GSM network Subject to rigorous standardization process We examine: Channelization Multiple access scheme Interface organization: On the physical level On the logical level

16. Page 16 Frequency allocation For PCS-1900 band ARFCNul = (Fc-1850)/0.2+511; ARFCNdl = (Fc-1930)/0.2+511 For GSM-850 ARFCNul = (Fc-824)/0.2+127; ARFCNdl = (Fc-969)/0.2+127

17. Page 17 TDMA Access Scheme Multiple users operate on the same frequency, but not at the same time. Advantages of TDMA: Relatively low complexity MAHO Different user rates can be accommodated Easier integration with the landline Disadvantages: High sync overhead Guard times Heavily affected by the multipath propagation

18. Page 18 GSM as a TDMA system GSM is a combination of FDMA and TDMA TDMA supports: Up to 8 full rate users Up to 16 half rate users GSM uses Frequency Division Duplexing

19. Page 19 GSM bursts Data sent over one time slot = burst Five types: normal, frequency correction, synchronization, dummy, access Format of a burst defied by its function DL: normal, frequency correction, synchronization, dummy UL: normal, access

20. Page 20 Normal Burst Used to carry information on both control and traffic channels Mixture of data and overhead GSM defines 8 training sequences assigned in color code mode Both on the forward and reverse link

21. Page 21 Frequency Correction Burst Sometimes referred to as the F-burst Provides mobile with precise reference to the frequency of the broadcast control channel Inserting the F-bursts on the control channel produces spectral peak 67.7 KHz above the central frequency of the carrier Only on the forward link

22. Page 22 Synchronization Burst Facilitates the synchronization of the MS to the network at the base band Commonly referred to as S-burst Only on the forward link The same sync sequence is used in all GSM networks

23. Page 23 Dummy Burst Supports MAHO Used to ensure constant power level of the broadcast control channel Only on the forward link

24. Page 24 Access Burst Used when the MS is accessing the system Shorter in length – burst collision avoidance Extended synchronization sequence Used only on the reverse link

25. Page 25 GSM TDMA Hierarchical Organization

26. Page 26 GSM Time Division Duplex Communication on the forward and reverse link does not happen simultaneously Delay of three slots between TX and RX Time division duplexing avoids RF duplexer at the RF stage Reduces the cost of mobile Saves battery

27. Page 27 GSM Logical Channels

28. Page 28 Traffic channel carries speech and user data in both directions Full rate ~ 33.85 Kb/sec Half rate ~ 16.93 Kb/sec Full rate uses 1 slot in every frame Half rate uses 1 slot in every other frame Data rates differ due to differences in Error Control Coding Traffic Channels (TCH)

29. Page 29 Control Channels GSM Defines 3 types of Control Channels: Broadcast Channels (BCH) Broadcast information that helps mobile system acquisition, frame synchronization, etc. They advertise properties and services of the GSM network. Forward link only Common Control Channels (CCCH) Facilitate establishment of the link between MS and system Both forward and reverse link Dedicated Control Channels (DCCH) Provide for exchange the control information when the call is in progress Both forward and reverse – in band signaling

30. Page 30 Broadcast Channels (BCH) Three types of BCH: Synchronization channel (SCH) Provides a known sequence that helps mobile synchronization at the baseband Communicates with S-burst Broadcasts Base Station Identity Code (BSIC) Frequency Correction channel (FCH) Helps mobile tune its RF oscillator Communicates with F-burst Broadcast Control Channel (BCCH) Provides mobile with various information about network, its services, access parameters, neighbor list, etc.

31. Page 31 Broadcast Channels (BCH) cont’d. In general, the information sent over BCCH can be grouped into four categories: Information about the network Information describing control channel structure Information defining the options available at the particular cell Access parameters

32. Page 32 Common Control Channel (CCCH) Three types of CCCH: Random Access Channel (RACH) Used by mobile to initialize communication Mobiles use slotted ALOHA Reverse link only Paging Channel (PCH) Used by the system to inform the mobile about an incoming call Forward link only GSM Supports DRX Access Grant Channel (AGC) Used to send the response to the mobiles request for DCCH Forward link only

33. Page 33 Dedicated Control Channels (DCCH) Three types of DCCH: Stand Alone Dedicated Control Channel (SDCCH) Used to exchange overhead information when the call is not in progress Slow Associated Control Channel (SACCH) Used to exchange time delay tolerant overhead information when the call is in progress Fast Associated Control Channel (FACCH) Used to exchange time critical information when the call is in progress

34. Page 34 Logical Channels - Summary

35. Page 35 Timing Advance Mobiles randomly distributed in space Timing advance prevents burst collision on the reverse link Maximum advancement is 63 bits

36. Page 36 Signal Processing – From Voice to Radio Waves As a digital TDMA technology GSM implements extensive signal processing

37. Page 37 Sampling and Quantization Sampling Sampling theorem specifies conditions for discretization of band limited analog signals Voice needs to be sampled at the sampling rate greater then 8000Hz Quantization Discrete values assigned to continuous samples Quantization noise In GSM, voice is sampled at 8 K samples/sec and quantized with 8192 levels (13 bit words)

38. Page 38 Speech Source Encoding Speech coder reduces the data rate needed for voice signal representation GSM specifies operation of : Full rate vocoder 13Kb/sec Half rate vocoder 5.6Kb/sec Enhanced Full Rate (EFR) 12.2Kb/sec AMR (Adaptive multi rate) AMR-FR (4.75-12.2Kb/sec) AMR-HR (4.75-7.95Kb/sec) AMR rate - function of C/I

39. Page 39 Performance comparison of some commercial vocoders

40. Page 40 Error control coding (ECC) increases the robustness of the signal ECC increases the overhead and reduces the efficiency of the communication In GSM, the ECC increases the overhead per user by 57% Channel Encoding

41. Page 41 Interleaving In mobile communications, the errors are “bursty” Optimal performance from ECC is obtained for uniform error distribution Interleaving increases the performance of ECC in mobile environment

42. Page 42 Modulation: GMSK (Gaussian MSK) GMSK has excellent spectral characteristics Low sidelobes Robust to non- linearities Price paid is in the increased Inter Symbol Interference (ISI)

43. Page 43 Equalization Necessary due to the multipath propagation Needs to have : Fast convergence Low complexity Two modes of operation Training Equalization GSM equalizer capable of equalizing for two equal multi paths separated by 16 microseconds Introduces overhead of about 18%

44. Page 44 GSM Network Features Mobile Assist Handoff (MAHO) Discontinuous Transmission (DTX) Dynamic Power Control (DPC) Frequency Hopping (FH) Intercell Handoff

45. Page 45 Mobile Assisted Handoff (MAHO) GSM Implements MAHO In the process of evaluating handoff candidates, GSM systems evaluate measurements performed by both the MS and BTS

46. Page 46 MAHO - Signal Strength Measurements Performed on uplink and downlink Reported as a quantized value RXLEV: RXLEV = RSL[dBm] + 110 Minimum RXLEV: -110, MAX RXLEV = -47 On the downlink, measurement performed for both serving cell and up to 32 neighbors Up to 6 strongest neighbors are reported back to BTS through SACHH

47. Page 47 MAHO - Signal Strength Measurements Measurements of the neighbors are performed on the BCCH channels – not affected by the DTX Measurements on the serving channel – affected by the DTX. Perform over a subset of SACCH that guarantees transmission even in the case of active DTX Before processing, the RXLEV measurements are filtered to prevent unnecessary handoffs

48. Page 48 MAHO – Signal Quality Measurements Performed on uplink and downlink Only on the serving channel Reported as a quantized value RXQUAL For a good quality call RXQUAL < 3 Measurements are averaged before the handoff processing If DTX is active, the measurements are performed over the subset of SACCH that guarantees transmission

49. Page 49 Performed on uplink (BTS) Only on the serving channel Used by the BTS to estimate distance to the MS Expressed in number of bits of TX advancement Can be between 0 and 63 TA MAHO – Time Alignment Measurement

50. Page 50 Discontinuous Transmission (DTX) Typical voice activity is around 60% DTX discontinues transmission during silent periods Benefits of DTX Uplink: System interference reduction Lower battery consumption Downlink System interference reduction Reduction of the intermodulation products Lower power consumptions Downsides of DTX usage: MAHO measurements are less accurate Voice quality is degraded due to slowness of VAD

51. Page 51 Dynamic Power Control (DPC) There are three reasons for DPC: Reduction of battery consumption Elimination of “near-far” problem Reduction of system interference

52. Page 52 Dynamic Power Control (DPC) DPC for MS Depending on its power class, MS can adjust its power between the max and min value in 2dB steps MS can perform 13 adjustments every SACCH period, i.e., 480ms Large adjustments > 24 dB will not be completed before the arrival of new command Commonly implemented as BSC feature. Many vendors are moving it at the BTS level DPC for BTS Vendor specific Based on MAHO reports

53. Page 53 Hierarchical Cell Structure (HCS) Incorporates various cell sizes into layers of RF coverage Three common layers: Umbrella cells (HL = 0) Macrocells (HL = 1) Microcell (HL = 2) HCS provides a way to assign preference levels between the cells Very effective way for capacity and interference management

54. Page 54 Handling of Fast Moving Mobiles If the mobile is moving at a high speed, it will spend a short time in the coverage area of the microcell To prevent excessive handoffs, a temporal GSM introduces temporal penalty – prevents immediate handoff initialization If the duration of mobile stay within the coverage area is shorter than the temporal penalty, it will never initialize handoff

55. Page 55 Frequency Hopping (FH) FH - multiple carriers used over the course of radio transmission There are two kinds of FH: Slow Hopping – change of carrier frequency happens at the rate slower than the symbol rate Fast Hoping – carrier frequency changes faster than the symbol rate GSM implements slow FH Scheme Carrier frequency is changed once per time slot There are two reasons for frequency hopping Frequency Diversity Interference avoidance

56. Page 56 Frequency Diversity of FH Mobile environment is characterized with small scale fading The depth of signal fade is a function frequency If two signals are sufficiently separated in frequency domain they fade independently Frequency diversity gain diminishes for fast moving mobiles

57. Page 57 Interference Avoidance of FH FH averages interference Allows for tighter reuse of frequencies Increases the capacity of the system

58. Page 58 Baseband FH in GSM Each radio operates on a fixed frequency The bursts are routed to individual radios in accordance to their hopping sequence

59. Page 59 Synthesized FH in GSM Each radio is hopping in an independent way Radios retune – “real time”

60. Page 60 FH Algorithms Cyclic Hopping Frequencies are used in the consecutive order If the radio is performing cyclic FH the order of frequencies in the sequence goes from the lowest ARFCN to the highest ARFCN

61. Page 61 Intracell Handoff Measurement indicates: Poor RXQUAL Good RXLEV There is high probability that the call will improve with the handoff to different carrier within the same cell To avoid unnecessary handoffs, system introduces maximum number of intercell handoffs

62. Page 62 GSM RF Planning / Design Link Budget and Nominal Cell Radius Calculation Receiver Sensitivity Required C/I ratio Mobile Transmit Power Examples of Link Budget Calculation of a Nominal Cell Radius Frequency Planning and Reuse Strategies Frequency Planning Using Regular Schemes Automatic Frequency Planning Capacity of GSM Networks

63. Page 63 GSM Migration Towards 3G

64. Page 64 GSM 2+ Data Services GSM’s traffic channel can support the data transfer of a bit rate up to 9.6Kb/sec This data rate can be used for: Short messages Fax services E-mail, etc. Circuit switched data services Not suitable for Internet Too slow Too costly (user would pay for the “circuit” even if there is no traffic exchanged

65. Page 65 High Speed Circuit Switched Data (HSCSD) HSCSD is using existing GSM organization to provide data services of a somewhat higher data rates It can combine several existing traffic channels into a single connection, i.e., it allows for mobiles multislot operation HSCSD can be implemented through software upgrades on existing networks and no hardware upgrades are needed Seems to be less accepted by the service providers

66. Page 66 GPRS is another new transmission capability for GSM that will be especially developed to accommodate for high-bandwidth data traffic GPRS will handle rates from 14.4Kbps using just one TDMA slot, and up to 115Kbps and higher using all eight time slots It introduces packet switching - can accommodate the data traffic characteristics General Packed Radio Data (GPRS)

67. Page 67 GPRS Network architecture New type of node: GPRS Service Node (GSN)

68. Page 68 GPRS Call routing Routing is performed “parallel” to the GSM network

69. Page 69 Packet switched Upgrades the modulation scheme From GMSK to 8-PSK Maximum speed ~59 Kb/sec per time slot, ~473.6 Kb/sec for all 8 time slots Variable data rate – depending on the channel conditions Defines several different classes of service and mobile terminals Enhanced Data GSM Environment (EDGE)

70. Page 70 Practically achievable data rates Theoretical rates are constrained by mobile power and processing capabilities Most mobiles support less than the maximum allowed by standard

71. Page 71 UMTS – 3G cellular service Provides data rates up to 2Mb/sec Possibly standardized as W-CDMA Universal Mobile Telephone Service (UMTS)