ECE Department Florida Institute of Technology ECE 5221 Personal Communication Systems Introduction to GSM

ECE Department Florida Institute of Technology Page 2 Course Outline  Part 1: Introduction oHistorical overview oElements of network architecture oElements of air interface  Part 2: Signal processing and network features oVoice processing oGSM Network features  Part 3: Network design oCoverage planning oCapacity planning oMigration towards 3G and beyond The GSM logo used on numerous handsets and by carries who wish to identify a GSM product

ECE Department Florida Institute of Technology 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 GSM = Global System for Mobile communications (GSM: originally from Groupe Spécial Mobile)

ECE Department Florida Institute of Technology Page 4 Deployment worldwide  930 networks in 222 countries and regions  More than 3 billion subscribers worldwide  More than 80% worldwide market share Worldwide map of GSM coverage (source

ECE Department Florida Institute of Technology 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 ATT coverage map T-Mobile coverage map

ECE Department Florida Institute of Technology Page 6 GSM Standards  Divided into 12 series  Standardization efforts coordinated by ETSI   Specifications available online – free of charge  Standardization and public availability of specification - one of fundamental factors of GSM success SeriesSpecifications area 01General 02Service aspects 03Network aspects 04MS-BS interface and protocol 05Physical layer and radio path 06Speech coding specification 07Terminal adapter for MS 08BS-MSC interface 09Network internetworking 10Service internetworking 11Equipment and type approval specification 12Operation and maintenance GSM Standard

ECE Department Florida Institute of Technology Page 7 GSM Network Layout  GSM system layout is standardized oStandardization involves:  Elements of the network  Communication Interfaces oStandard layout allows for the use of equipment from different suppliers

ECE Department Florida Institute of Technology 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)

ECE Department Florida Institute of Technology Page 9 Mobile Station (MS)  Two functional parts oHW and SW specific for GSM radio interface oSubscriber Identity Module (SIM)  SIM – detaches user identity from the mobile oStores user information oWithout SIM – only emergency calls Functional diagram of GSM mobile SIM card Most popular GSM phone Nokia 1100 – 200M+ sold

ECE Department Florida Institute of Technology 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. Typical BTS installation BTS antenna system Macrocell BTS radio cabinet hosts TX/RX Femto-cell

ECE Department Florida Institute of Technology 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: oHandoff management oMAHO management oPower control oClock distribution oOperation 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 Typical BSC TRAU = Transcoding and Rate Adaptation Unit

ECE Department Florida Institute of Technology 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)

ECE Department Florida Institute of Technology 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

ECE Department Florida Institute of Technology Page 14 AUC/EIR  AUC – Authentication center  Integral part of HLR  GSM specifies elaborate encryption  Three levels oA5/1 USA + Europe oA5/2 COCOM country list oNo encryption – rest of the world  EIR – Equipment Identity Registry  Responsible for tracking equipment and eligibility for service  Maintains three lists oWhite list – approved mobile types oBlack list – barred mobile types oGray list – tracked mobile types Over years – many other vendor specific features added to the system

ECE Department Florida Institute of Technology Page 15 GSM Air Interface - Um  Interface between the MS and the GSM network  Subject to rigorous standardization process  We examine: oChannelization oMultiple access scheme oInterface organization:  On the physical level  On the logical level

ECE Department Florida Institute of Technology Page 16 Frequency allocation  For PCS-1900 band oARFCNul = (Fc-1850)/ ; ARFCNdl = (Fc-1930)/  For GSM-850 oARFCNul = (Fc-824)/ ; ARFCNdl = (Fc-969)/ Mapping formulas GSM is FDD technology

ECE Department Florida Institute of Technology Page 17 TDMA Access Scheme  Multiple users operate on the same frequency, but not at the same time.  Advantages of TDMA: oRelatively low complexity oMAHO oDifferent user rates can be accommodated oEasier integration with the landline  Disadvantages: oHigh sync overhead oGuard times oHeavily affected by the multipath propagation TDMA = Time Division Multiple Access

ECE Department Florida Institute of Technology Page 18 GSM as a TDMA system  GSM is a combination of FDMA and TDMA  TDMA supports: oUp to 8 full rate users oUp to 16 half rate users  GSM uses Frequency Division Duplexing

ECE Department Florida Institute of Technology 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 Time/Frequency/Amplitude diagram for GSM normal burst

ECE Department Florida Institute of Technology 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 Total of 114 encoded user information bits Total of 34 overhead bits Normal burst

ECE Department Florida Institute of Technology 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 Spectral characteristics of the control channel. The peak in the spectrum allows for easier MS network acquisition Format of the F-burst Fixed sequence consists of all zeros Frequency correction burst

ECE Department Florida Institute of Technology 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 Synchronization burst

ECE Department Florida Institute of Technology Page 23 Dummy Burst  Supports MAHO  Used to ensure constant power level of the broadcast control channel  Only on the forward link Dummy burst

ECE Department Florida Institute of Technology 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  GSM mobiles use slotted ALOHA to access the system  In the case of collision – a hashing algorithm is provided Access burst

ECE Department Florida Institute of Technology Page 25 GSM TDMA Hierarchical Organization

ECE Department Florida Institute of Technology 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 oReduces the cost of mobile oSaves battery

ECE Department Florida Institute of Technology Page 27 GSM Logical Channels

ECE Department Florida Institute of Technology Page 28  Traffic channel carries speech and user data in both directions oFull rate ~ Kb/sec oHalf rate ~ Kb/sec oFull rate uses 1 slot in every frame oHalf rate uses 1 slot in every other frame  Data rates differ due to differences in Error Control Coding Traffic Channels (TCH) Full Rate TCH can carry: Voice (13 Kb/sec) Date at rates: -9.6 Kb/sec -4.8 Kb/sec -2.4 Kb/sec Half Rate TCH can carry: Voice (6.5 Kb/sec) Date at rates: Kb/sec -2.4 Kb/sec

ECE Department Florida Institute of Technology Page 29 Control Channels  GSM Defines 3 types of Control Channels: 1.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 2.Common Control Channels (CCCH)  Facilitate establishment of the link between MS and system  Both forward and reverse link 3.Dedicated Control Channels (DCCH)  Provide for exchange the control information when the call is in progress  Both forward and reverse – in band signaling

ECE Department Florida Institute of Technology Page 30 Broadcast Channels (BCH)  Three types of BCH: 1.Synchronization channel (SCH)  Provides a known sequence that helps mobile synchronization at the baseband  Communicates with S-burst  Broadcasts Base Station Identity Code (BSIC) 2.Frequency Correction channel (FCH)  Helps mobile tune its RF oscillator  Communicates with F-burst 3.Broadcast Control Channel (BCCH)  Provides mobile with various information about network, its services, access parameters, neighbor list, etc.

ECE Department Florida Institute of Technology Page 31 Broadcast Channels (BCH) cont’d.  In general, the information sent over BCCH can be grouped into four categories: 1)Information about the network 2)Information describing control channel structure 3)Information defining the options available at the particular cell 4)Access parameters Some BCCH messages

ECE Department Florida Institute of Technology Page 32 Common Control Channel (CCCH)  Three types of CCCH: 1.Random Access Channel (RACH)  Used by mobile to initialize communication  Mobiles use slotted ALOHA  Reverse link only 2.Paging Channel (PCH)  Used by the system to inform the mobile about an incoming call  Forward link only  GSM Supports DRX 3.Access Grant Channel (AGC)  Used to send the response to the mobiles request for DCCH  Forward link only

ECE Department Florida Institute of Technology Page 33 Dedicated Control Channels (DCCH)  Three types of DCCH: 1.Stand Alone Dedicated Control Channel (SDCCH)  Used to exchange overhead information when the call is not in progress 2.Slow Associated Control Channel (SACCH)  Used to exchange time delay tolerant overhead information when the call is in progress 3.Fast Associated Control Channel (FACCH)  Used to exchange time critical information when the call is in progress DCCH SDCCH SACCH FACCH

ECE Department Florida Institute of Technology Page 34 Logical Channels - Summary UL - Uplink DL - Downlink ChannelUL onlyDL onlyUL/DLPoint to point BroadcastDedicatedShared BCCHXXX FCCHXXX SCHXXX RACHXXX PCHXXX AGCHXXX SDDCHXXX SACCHXXX FACCHXXX TCHXXX

ECE Department Florida Institute of Technology Page 35 Timing Advance  Mobiles randomly distributed in space  Timing advance prevents burst collision on the reverse link  Maximum advancement is 63 bits

ECE Department Florida Institute of Technology Page 36 Signal Processing – From Voice to Radio Waves  As a digital TDMA technology GSM implements extensive signal processing

ECE Department Florida Institute of Technology Page 37 Sampling and Quantization  Sampling oSampling theorem specifies conditions for discretization of band limited analog signals oVoice needs to be sampled at the sampling rate greater then 8000Hz  Quantization oDiscrete values assigned to continuous samples oQuantization noise oIn GSM, voice is sampled at 8 K samples/sec and quantized with 8192 levels (13 bit words)

ECE Department Florida Institute of Technology Page 38 Speech Source Encoding  Speech coder reduces the data rate needed for voice signal representation  GSM specifies operation of : oFull rate vocoder  13Kb/sec oHalf rate vocoder  5.6Kb/sec oEnhanced Full Rate (EFR)  12.2Kb/sec oAMR (Adaptive multi rate)  AMR-FR ( Kb/sec)  AMR-HR ( Kb/sec)  AMR rate - function of C/I Vocoders enable efficient channel utilization

ECE Department Florida Institute of Technology Page 39 Performance comparison of some commercial vocoders Mean Opinion Scores (MOS) - Voice Quality source IIR. The First Annual CDMA Congress London, Oct , Clean Speech20dB SNR Babble 20dB SNR Car 15dB SNR Street Mu-PCM 8Kb/s EVRC (CDMA) 13Kb/s CELP (CDMA) IS-136 ACELP GSM EFR

ECE Department Florida Institute of Technology 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

ECE Department Florida Institute of Technology 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

ECE Department Florida Institute of Technology Page 42 Modulation: GMSK (Gaussian MSK)  GMSK has excellent spectral characteristics oLow sidelobes oRobust to non- linearities  Price paid is in the increased Inter Symbol Interference (ISI) Simplified GMSK block diagram Spectral characteristics of GMSK

ECE Department Florida Institute of Technology Page 43 Sequence used for equalizer training Equalization  Necessary due to the multipath propagation  Needs to have : oFast convergence oLow complexity  Two modes of operation 1.Training 2.Equalization  GSM equalizer capable of equalizing for two equal multi paths separated by 16 microseconds  Introduces overhead of about 18%

ECE Department Florida Institute of Technology Page 44 GSM Network Features  Mobile Assist Handoff (MAHO)  Discontinuous Transmission (DTX)  Dynamic Power Control (DPC)  Frequency Hopping (FH)  Intercell Handoff

ECE Department Florida Institute of Technology 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  There are three types of measurements: 1.Signal Strength Measurements 2.Signal Quality Measurements 3.Timing Advance Measurements

ECE Department Florida Institute of Technology Page 46 MAHO - Signal Strength Measurements  Performed on uplink and downlink  Reported as a quantized value RXLEV: RXLEV = RSL[dBm]  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 Example measurement report

ECE Department Florida Institute of Technology 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 Example RSL measurement

ECE Department Florida Institute of Technology 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 RXQUAL mapping table RXQUAL measurements Measurement report

ECE Department Florida Institute of Technology 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

ECE Department Florida Institute of Technology Page 50 Discontinuous Transmission (DTX)  Typical voice activity is around 60%  DTX discontinues transmission during silent periods  Benefits of DTX oUplink:  System interference reduction  Lower battery consumption oDownlink  System interference reduction  Reduction of the intermodulation products  Lower power consumptions  Downsides of DTX usage : oMAHO measurements are less accurate oVoice quality is degraded due to slowness of VAD Mobile stationEnvironmentTypical voice activity HandsetQuiet location55% HandsetModerate office noise with voice interference 60% HandsetStrong voice interference (ex. airport, railway station) 65-70% Hands free / handset Variable vehicle noise60%

ECE Department Florida Institute of Technology Page 51 Dynamic Power Control (DPC)  There are three reasons for DPC: 1.Reduction of battery consumption 2.Elimination of “near-far” problem 3.Reduction of system interference (1) Not available commercially

ECE Department Florida Institute of Technology Page 52 Dynamic Power Control (DPC)  DPC for MS oDepending on its power class, MS can adjust its power between the max and min value in 2dB steps oMS can perform 13 adjustments every SACCH period, i.e., 480ms oLarge adjustments > 24 dB will not be completed before the arrival of new command oCommonly implemented as BSC feature. Many vendors are moving it at the BTS level  DPC for BTS oVendor specific oBased on MAHO reports

ECE Department Florida Institute of Technology Page 53 Hierarchical Cell Structure (HCS)  Incorporates various cell sizes into layers of RF coverage  Three common layers: 1.Umbrella cells (HL = 0) 2.Macrocells (HL = 1) 3.Microcell (HL = 2)  HCS provides a way to assign preference levels between the cells  Very effective way for capacity and interference management

ECE Department Florida Institute of Technology 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

ECE Department Florida Institute of Technology Page 55 Frequency Hopping (FH)  FH - multiple carriers used over the course of radio transmission oThere are two kinds of FH: 1.Slow Hopping – change of carrier frequency happens at the rate slower than the symbol rate 2.Fast Hoping – carrier frequency changes faster than the symbol rate oGSM implements slow FH Scheme oCarrier frequency is changed once per time slot oThere are two reasons for frequency hopping 1.Frequency Diversity 2.Interference avoidance

ECE Department Florida Institute of Technology 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

ECE Department Florida Institute of Technology Page 57 Interference Avoidance of FH  FH averages interference  Allows for tighter reuse of frequencies  Increases the capacity of the system

ECE Department Florida Institute of Technology 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  Advantages of baseband hopping  No need to “real time” retune – simpler radios  More efficient combiners  Disadvantage of baseband hopping  Number of hopping frequencies limited by the number of radios

ECE Department Florida Institute of Technology Page 59 Synthesized FH in GSM  Each radio is hopping in an independent way  Radios retune – “real time”  Advantages of synthesized hopping: Set of the hopping frequencies can be assigned in an arbitrary way  Disadvantage of synthesized hopping: Need for expensive and lossy combiners

ECE Department Florida Institute of Technology Page 60 FH Algorithms Random Hopping Implemented in a pseudo – random way Uses one of 63 available pseudorandom sequences The actual frequency is obtained as a modulo operation with number of available frequencies in allocation list (FH group)  Cyclic Hopping oFrequencies are used in the consecutive order oIf the radio is performing cyclic FH the order of frequencies in the sequence goes from the lowest ARFCN to the highest ARFCN

ECE Department Florida Institute of Technology Page 61 Intracell Handoff High Interference  Measurement indicates: oPoor RXQUAL oGood 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

ECE Department Florida Institute of Technology 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

ECE Department Florida Institute of Technology Page 63 Migration: 1.High speed circuits switched data (HSCSD) 2.Packet switched data (GPRS,EDGE) 3.Integrated packet services – possibly under different access scheme (UMTS) GSM Migration Towards 3G

ECE Department Florida Institute of Technology Page 64 GSM 2+ Data Services  GSM’s traffic channel can support the data transfer of a bit rate up to 9.6Kb/sec oThis data rate can be used for:  Short messages  Fax services  , etc. oCircuit switched data services oNot suitable for Internet  Too slow  Too costly (user would pay for the “circuit” even if there is no traffic exchanged

ECE Department Florida Institute of Technology 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

ECE Department Florida Institute of Technology 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)

ECE Department Florida Institute of Technology Page 67 GPRS Network architecture  New type of node: GPRS Service Node (GSN)

ECE Department Florida Institute of Technology Page 68 GPRS Call routing  Routing is performed “parallel” to the GSM network

ECE Department Florida Institute of Technology Page 69  Packet switched  Upgrades the modulation scheme oFrom GMSK to 8-PSK oMaximum speed ~59 Kb/sec per time slot, ~473.6 Kb/sec for all 8 time slots oVariable data rate – depending on the channel conditions  Defines several different classes of service and mobile terminals Enhanced Data GSM Environment (EDGE) EDGE enabled data mobile

ECE Department Florida Institute of Technology 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 Practically achievable data rates

ECE Department Florida Institute of Technology Page 71  UMTS – 3G cellular service  Provides data rates up to 2Mb/sec  Possibly standardized as W-CDMA Universal Mobile Telephone Service (UMTS) Outline of UMTS (WCDMA) network