Cellular Mobile Communications-
III Introduction to GSM Dr. Nasir D. Gohar
AN INTRODUCTION TO GSM Second Generation Cellular Mobile Radio Systems-Digital Cellular Telephony:
Main Advantages of Digital Cellular Networks
Higher Spectrum Efficiency Speech Encoding Better Access Technologies: TDMA and CDMA
Higher System Capacity Aggressive Frequency Reuse
Low Infra-Structure and Terminal Cost Application of VLSI/VVLSI Chips Many Traffic Channels per Carrier Channel [Transceivers are shared among several users) Low S/I Ratio (SIR) Allowing Smarter Equipment
Better Integration with Digital PSTN [ISDN] New Services such as Data Communications, Fax, and SMS Better Privacy due to Encryption
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AN INTRODUCTION TO GSM Digital Cellular Standards Table-L13.1: Overview of Second Generation Standards Region Europe
USA
Japan
Standard
Developing Organization
Remarks
GSM
ETSI
Harmonized European Standard
DCS-1800
ETSI
GSM Varient for 1800 MHz Band
D-AMPS, D-AMPS 1900 TIA [IS-54B, IS-136]
Digital AMPS works in 800 MHZ and 1900 MHz Bands
IS-95 CDMA
TIA/QualComm[IS-95]
Downward Compatible with AMPS, later modified to work with 1900 MHz Band
PCS-1900
ETSI
GSM Vairent to work with 1900 MHz Band
PDC
NTT
Works both in 800 MHz and 1500 MHz
Why So Many Standards? T.S. Rappaport Ch 11
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AN INTRODUCTION TO GSM Digital Cellular Standards
Table-L13.2: Overview of Important Technical Parameters of Digital Cellular Standards Parametrs Standard Standard
Frequency band(Mhz)
Air Interface Forward Ch.
Reverse Ch.
Duplex Separation
Access Tech. Channel BW
Mod. Rate
Speech Encoding
Tx Power of MS
H/O
GSM/DCS-1800/PCS1900
900/1800 /1900
935-960/1805- 890-915/17101880/19301785/18501970 1890
45/95/80
TDMA, 8 Time Slots, Later 16 Time Slots
200 KHz
271 Kbps
LPC,13 kbps
2W/1W/1W
MAHO
D-AMPS
800/1900
869-894/1930- 824-849/18501970 1890
45/80
TDMA, 3 Slots
30 KHz
48.6 Kpbps
VSELP, 7.95 kpps
1.2 W
MAHO
0.2 W
Soft Handover
0.8 W
MAHO
IS-95 CDMA
800/1900
869-894/1930- 824-849/18501970 1890
45/80
CDMA
1.25 MHz
PDC
800-1500
810-826/1477- 940-956/14291489 + 1501- 1441 + 14531513 1465
130/48
TDMA, 3 Slots, later 6 Slots
25 KHz
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1.2 to 14.4 Kbps QSELP, 1.2 to Transmitted 8 kbps at 1.339 Mchips/s 42 kbps
VSELP, 11.2 kbps
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AN INTRODUCTION TO GSM Overview of GSM Milestones 1982 1986 made.
1987
1989 1992 1994 1996
GSM Group, set up by CEPT, started Investigation to reserve a Spectrum in 900 MHz band for Pan-European Cellular Digital Telephony Two 25 MHz frequency Blocks [890-915 MHz, 935-960 MHz] reserved by EC, July 01, 1991 set as Introduction date. Choice of Digital and Narrow band [Approx. 200 KHz] TDMA 13 Operators from 12 European Countries signed MOU to Support GSM ETSI established which took over GSM from CEPT Introduction of first Commercial GSM Network 51 Commercial Networks Established More Than 20 Million Subscribers in 191 Networks, More than Double of all Subscribers of other Digital Systems [D-AMPS,
PDC, IS-95 CDMA] 2006 More than 2 billion subscribers (30% of world population and 82% of cellular market) T.S. Rappaport Ch 11
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AN INTRODUCTION TO GSM Implementation Phases of GSM Phase-1: Most Essential Services [Voice, Data, SMS] Implemented. Mostly all Current GSM Systems belong to this Phase. Phase-2: Half-Rate Speech Encoding, Doubling the System Capacity, Several Encryption Algorithms, Conference Call Facility[ up to 5 Participants] , Call Waiting Facility, Information on Call Charges, Calling or Called Part etc., Cell-Broadcast [Point to Multi-Point SMS]. Many of the Phase-2 Facilities have already been Implemented by some Manufacturers.
Phase-2 + GPRS[General Packet radio Service] for Packet Switched Data Transmission to Support LAN and Internet Traffic. [Considerably Delayed due to Lack of Interest on part of Network required for Infra-Structure and MS T.S. Rappaport Ch 11 Operators due to Large NDG Investment Notes 6
AN INTRODUCTION TO GSM 2G EVOLUTION TO 3G
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AN INTRODUCTION TO GSM
GSM EVOLUTION
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AN INTRODUCTION TO GSM GSM SYSTEM ARCHITECTURE MS
MANAGEMENT SUBSYSTEM
SIM Card EIN/ESN
Fault Management
Cost Management
Base Station Subsystem (BSS)
BTS BTS BTS
Several Dozens of BTS
BTS BTS BTS
Network &Switching Subsystem
MTX [up to 1 M users] Call set up , all Other
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Security Management
Performance Management
B S C
HLR = Home HLR Location Register
MTX
under one BSC Each BTS has 3-5 Carrier Channels
Functions
Config. Management
B S C
BTS = Base Transceiver Station BSC = base Station Controller
MS
BASE STATION SUBSYSTEM[BSS]
GMTX = Gateway MTX IWF = Inter Working Function
VLR = Visitor VLR Location Register
GMTX/ IWF
To Other Networks
NETWORK & SWITCHING SUBSYSTEM
•GMTX: An Interface to Other Networks •HLR: Keeps Record of System’s Own Users •VLR: Keeps Temporary Record of Visitors / Roamers [SID, ST,SVCES] •Au C/EIR: Manages the Sub. Authentication and Encryption Data NDG Notes
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AN INTRODUCTION TO GSM GSM SYSTEM ARCHITECTURE-2 MANAGEMENT SUBSYSTEM
Management Subsystem [Operation Subsystem (OSS)]
Fault Management
Cost Management
BTS BTS
Directly or Indirectly
Connected to all Other Subsystems Fault Management System Configuration Performance Management Cost Management Security Management
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Config. Management
BTS
Security Management
Performance Management
B S C
HLR = Home HLR Location Register
MTX BTS BTS BTS
B S C
BTS = Base Transceiver Station BSC = base Station Controller
MS
BASE STATION SUBSYSTEM[BSS]
NDG Notes
GMTX = Gateway MTX IWF = Inter Working Function
VLR = Visitor VLR Location Register
GMTX/ IWF
To Other Networks
NETWORK & SWITCHING SUBSYSTEM
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AN INTRODUCTION TO GSM GSM SYSTEM ARCHITECTURE-3 GSM Interfaces
Radio Interface:
Describes Data Interchange between MS and BSS Abis Interface: Describes Data Communications between BTS and BSC, allows Various Manufacturers Equipment A Interface: Describes Data Interchange between BSS and NSS
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AN INTRODUCTION TO GSM LOGICAL
CHANNELS
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AN INTRODUCTION TO GSM
Control Channels Control channels fall into three categories: Broadcast:: BCCH, FCCH, SCH One way, from base to mobile
Common Control: RACH, AGCH, PCH One way, some from base to mobile and some from mobile to the base
Dedicated: SDCCH, SACCG, FACCH Two-way, stand-alone or embedded in the traffic channels
All signaling channels share one carrier in a cell the dedicated control channels may be transmitted on traffic carriers T.S. Rappaport Ch 11
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AN INTRODUCTION TO GSM
Broadcast Channels Frequency Correction Channel (FCCH) Carries information for frequency correction Synchronization Channel (SCH) Carries information for frame synchronization and for identification of the BTS
Broadcast Control Channel (BCCH) Broadcasts general information on the BTS Broadcasts cell-specific information, e.g. Control channel organization, frequency hopping sequences, cell identification, etc. T.S. Rappaport Ch 11
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AN INTRODUCTION TO GSM
Common Control Channels Paging Channel (PCH) - downlink only
for paging purposes Random Access Channel (RACH) - uplink only
used by any MS to request allocation of a signaling channel (SDCCH) a slotted Aloha protocol is used, so collisions among MSs may happen
Access Grant Channel (AGCH) - downlink only
used to allocate a SDCCH or a TCH Notification Channel (NCH) - downlink only
notify MS of voice group and voice broadcast call (ASCI feature)
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AN INTRODUCTION TO GSM
Dedicated Control Channels Stand Alone Dedicated Control Channel (SDCCH) used for call setup (authentication, signaling,, traffic channel assignment), location updates and SMS
Slow Associated Control Channel (SACCH) always coupled with a SDCCH or TCH for communicating measurement data and control parameters
Fast Associated Control Channel (FACCH) to respond to increased signaling demand, e.g.
during handover bandwidth (bit slots) are stolen from the associated TCH (traffic data are preempted)
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AN INTRODUCTION TO GSM
Traffic Channels GSM support two types of traffic channels full rate (TCH/F): 22.8 kbps half rate (TCH/H): 11.4 kbps Mapping to physical channel full rate traffic channel - 1 timeslot half rate traffic channel - 1 timeslot in alternating frames
Full rate channel may carry 13 kbps speech or data at 2.4, 4.8 or 9.6 kbps Half rate channel may carry 6.5 kbps speech or data at 2.4 or 9.6 kbps T.S. Rappaport Ch 11
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AN INTRODUCTION TO GSM Channel Usage & MS Terminating Calls
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AN INTRODUCTION TO GSM GSM Call Setup Procedure Locating the Subscriber HLR keeps record of the MSC area which the Subscriber last registered VLR keeps record of the Location Area[LA] in which Subscriber last registered Location Area: A Group of Neighboring Cells having the same LAC LAC: Each Cell in the Area Broadcasts this Code Each Mobile itself periodically registers itself with MSC Paging and Random Access Procedure [RAP] GMSC interrogates HLR for MSC the Subscriber last registered Call is Switched to that MSC VLR tells about the LA the user last registered Ch MSC in all the Cells in the LA T.S. Rappaport 11 arranges a Paging NDG Notes 19
AN INTRODUCTION TO GSM GSM Call Setup Procedure-2 Paging and Random Access Procedure[RAP]-Cont’d The Mobiles listen to the Paging Channels for their Number If received a Paging Message, the Mobile starts a RAP It sends a Random Number [RN] + Brief Guide to describe the Purpose The System [MSC] responds to it by repeating the RN and providing the decided Channel The Mobile listens to this RN, if it is the same, it gets hold of the allocated Channel Reconfirmation is done by the System by sending again the Mobile Number [in case some other Mobile has also sent the same RN at the same time] If the Mobile hears its own number, it responds to the System and thus it gets connected to the incoming Call, If it was a wrong number [other than its own number] it must leave that Channel. In case, a Mobile listens no same RN from the System, it may repeat RAP at some time later randomly.
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AN INTRODUCTION TO GSM CALL PROCESSING & MOBILE TERMINATING CALLS
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AN INTRODUCTION TO GSM CALL PROCESSING & MOBILE ORIGINATING
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AN INTRODUCTION TO GSM GSM Handover and Routing Handover
The Mobile keeps updating MSC about the RSS levels from the
neighboring Cells. MSC, while looking at the quality of the existing link, decides whether a Handoff/ Handover is necessary or not. If yes, it asks the Candidate Cell to prepare the Channel [ GSM uses Fixed Channel Assignment] When Channel is allocated, it tells the Mobile to move to that Channel When Mobile has moved, it deactivates the old Channel
Routing
GMSC interrogates HLR to locate the Subscriber by telling the MSC where the Mobile last registered. PROBLEM: Call originated from PSTN in CANADA for a US Mobile roaming in CANADA will result in two International Calls “TROMBONE PROBLEM” ???? [Don’t Worry, Be Happy, Other People will Take Care of this Problem] T.S. Rappaport Ch 11
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AN INTRODUCTION TO GSM
HANDOVER TYPES Intra-BSC Old and new BTSs are controlled by the same BSC The MSC is not involved Intra-MSC Old and new BTSs are attached to different BSCs The BSCs are attached to the same MSC Inter-MSC Handover to a new MSC Serving MSC becomes anchor MSC IMT (Inter Machine Trunk) T.S. Rappaport Ch 11 NDG Notes is
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AN INTRODUCTION TO GSM GSM Numbering Scheme and Spectrum Efficiency GSM Numbering Scheme Each GSM Mobile has at least three Numbers: MSISDN [Mobile Station Integrated Services Digital Network] stored in SIM Card
SIM Card holds Subscriber ID[MSISDN, IMSI], some Extra Memory to store phone numbers and Encryption Algorithms
IMSI[International Mobile Subscriber Identity] not known to the User, HLR does translation between MSISDN to IMSI and vice versa. ESN or IMEI:Permanently stored/wired in the Mobile Station
Spectrum Efficiency
TDMA, 200 KHz Channel BW, 8-Time Slots per Carrier Channel Radio Spectrum = 2 x 25 MHz bands can support 125[124] Duplex
Carrier Channels and 1000 Traffic Channels [1000 Simultaneous Calls]. A typical GSM System Cluster Size = 12, so each Cell have Approx. 10 Carrier Channels and a Capacity of 80 Simultaneous Call [Traffic Channels] T.S. Rappaport Ch 11 NDG Notes 25 In case of half-rate Coder, Spectrum Efficiency will be Doubled.
AN INTRODUCTION TO GSM Technical Realization of GSM GSM Speech Communications
User Speech is Digitized using Adaptive PCM Speech Encoding using Linear Predictive Coding [LPC] User data flow = 13 kbps [ 6.5 kbps in case of half-rate coding] Adding an overhead of 9.8 kbps[for error detection, error correction,
and synchronization], we get an over all user data flow of 22.8 kbps. This user data flow is subdivided into short data blocks each of 456 bits, which is divided into 8 sub-blocks, each of 57 bits, 2 such subblocks are used to make a burst [to fit into a time slot of 0.58 ms]. See frame structure. These bursts from a user are interleaved over 8 time slots spread over 8 frames. TDMA/FDMA Mechanism Each Carrier Channels carries data burst of eight (8) users, each in its allocated time slot in the frame. Time slots are transmitted on several Channels [Carrier Channel or Frequency] Each Carrier Channel transmits the data of 8 users at 271 kbps.
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AN INTRODUCTION TO GSM Technical Realization of GSM [ Cont’d] GSM Speech Communications-2
GSM Carrier Frequencies are numbered 1-124 Channels[Duplex] Forward Channel = 935.2 MHz + (n-1) * 200 KHz Reverse Channel = 890.2 MHz + (n-1) * 200 KHz
Due to slightly more than 200 KHz bandwidth of the modulated
signal, Consecutive Frequencies are not used in the same System Also, 1 and 124 Channels are generally not used. The Spectrum of 124 Channels is generally not allotted to one Service Provider. Separation between two Duplex paired Channels is 45 MHz. Each user’s bursts are transmitted every 4.6ms apart. Frequency
97 96 95 94 93 92
1
2
3
4
5
6
7
8
1
2
3
5
6
7
8
1
2
3
4
5
6
7
8
Time
Frame [4.6ms]
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AN INTRODUCTION TO GSM
SPEECH PROCESSING
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AN INTRODUCTION TO GSM
SPEECH CODING
SPEECH ENCODER
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AN INTRODUCTION TO GSM
SPEECH DATA PROCESSING
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AN INTRODUCTION TO GSM
CHANNEL ENCODING
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AN INTRODUCTION TO GSM INTERLEAVING
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AN INTRODUCTION TO GSM VOICE TRANSMISSION PATH
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AN INTRODUCTION TO GSM Technical Realization of GSM [ Cont’d] GSM Framing Structure Hyper Frame 3 Hrs 28 Min
2048
1
1 1
51 26
One Super Frame Occurs when when the Speech and Control Channel restart at the same time.
Super Frame 6.12 Sec Multi-Frame 120 ms
Multi-Frame 235 ms 26
1
26 x Frames 4.61 ms
Speech Traffic
Control Traffic
Each Frame is sub-divided into 8 time-slots 0.58 ms
8
1
51
1
Preamble 3
57 bits User Speech Data
26 bits
57 bits User Speech Data
Time Slot 0.58 ms
3
GuardTime Block = 456 Bits 1
1
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81
2
. . 8 sub-blocks 57 bits each
8
81 81 81 8 1 81 8 x Frames 4.61 ms, Slot-2 of each Frame is Allocated to the User
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Blocked User Signal Inter-Leaved into 8 Time Slots over 8 Frames
8 1
8
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AN INTRODUCTION TO GSM TIME-SLOT STRUCTURE
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AN INTRODUCTION TO GSM FRAME HIERARCHY
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AN INTRODUCTION TO GSM BURST
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AN INTRODUCTION TO GSM TYPES OF BURST
Five different types of bursts Normal burst Traffic and control payload Frequency correction burst All zeroes sequence Synchronization burst A special fixed sequence Random access burst Extended guard period of 68.25 bitts (252 μs) Dummy burst T.S. Rappaport Ch 11
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AN INTRODUCTION TO GSM BURST STRUCTURES
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AN INTRODUCTION TO GSM Technical Realization of GSM [ Cont’d] GSM Modulation Scheme Gaussian Minimum Shift Keying [GMSK]
(For Detail see Text Book)
Ch-5 of the No Speech, No Transmission Saves Energy in MS, Reduces Ave. Interference Comfort Noise added for the Listener’s Pleasure.
GSM MS Power Classes Class 1: 20 W Not (yet) Available Class 2: 8 W Car Phone Class 3: 5 W Class 4: 2 W Normal Pocket Phone Class 5: 0.8 W Limited Coverage Phone [Urban Areas Only] Maximum Cell Size: Depends on Max. Permitted Delay, 35 km. Maximum Mobile Speed: 250 km/hr
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AN INTRODUCTION TO GSM Technical Realization of GSM [ Cont’d] GSM SMS Individual Messages: up to 160 Characters long can be sent and received by MS SMS uses Signaling Channel, thus, it can be received during current Communication Session Max. rate 600 bps
Cell Broadcast: up to 93 Character long message can be sent to all users in a given area [Cells]. Uses Communication Channel, so, can’t be received during current Communication Session Neither Addressed Nor Encrypted
Applications of SMS: .Network Operator Messages, Third
Party Messages, Public Interest Information Messages SMS Cost: Differs from Operator to Operator and depends on type of Application T.S. Rappaport Ch 11
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AN INTRODUCTION TO GSM Technical Realization of GSM [ Cont’d]
GSM Security Aspects GSM provides security about the identity of MS Guards against Eavesdropping Implementation User Authentication [A3 Algorithm]: 128 bit Secret Number, Ki, assigned to each User, stored in SIM Card as well as AuC, is used to create SRES using Ki and 128 bit RAND number from the network, if SRES of MS matches with SRES of the T.S. Rappaport Ch 11 the MS is NDG Notes network,
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AN INTRODUCTION TO GSM
Data Encryption [A8+A5 Algorithm]: Frame # + (RAND+Ki(Alg-A8))(Alg-A5) -> 144 bit Code Train ⊗ 1 44 bit user Data Train -> [Network] ⊗ Frame # + (RAND+Ki(AlgA8))(Alg-A5) ->Original Message
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AN INTRODUCTION TO GSM Technical Realization of GSM [Cont’d] GSM Inter-Connection with Other Networks: PSTN/ISDN for Voice GSM Speech data is Digital and Compressed Converted into audio of 3.1 KHz BW
PSTN/ISDN for Data For Data Interconnection, Modem Pools are used in GSM Network. Many Telephone Modems at GMSC/IWF supporting all the important telephone modem standards V.21[0.3 kbps, AS], V.22[1.2 kbps, AS and S], V.32 [4.8 kbps, S] etc.
PSDN (Basic PAD or Dedicated PAD) for Data 300 bps to 9600 bps on AS/S links
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NEW DEVELOPMENTS IN GSM Frequency Shortage and Extended GSM Band In some European countries, up to half GSM band is occupied by Interim Systems; NMT, ETACS etc. Additional frequencies adjacent to GSM band is recommended by ERO[1996], new terminals for this extended band are not available in bulk
New Encoding Techniques Half-Rate Speech Encoding
Instead of 13 kbps only 6.5 kbps will be required for the same speech quality Standardized in early 1995, but, only few manufacturers have implemented lack of interest on part of system suppliers fear of investment decrease in network expansion [uCell]
Enhanced Full Rate [EFR]:Developed by GSM and DCS-1800 Suppliers
Same 13 kbps speed, but, considerable improvement in speech
quality T.S. Rappaport Ch 11 NDG Notes More Immunity to transmission errors
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NEW DEVELOPMENTS IN GSM Any Cellular Phone may have many Speech Coders,
including FR/HR/EFR, it must be able to switch, and at least support FR for roaming purpose.
Data Communications over GSM
General Packet Radio Service[GPRS] offers packet-switched data
communications suitable for LAN and Internet Applications[PVC] Requires GPRS enabled Handsets and Changes at BSS[addition of Packet Control Unit, PCU] Option of Upgrading of BTS to support Enhanced Data rate for GSM Evolution, EDGE [8-PSK system]
Multi-band Terminals
Cellular phones to support more than frequency bands [GSM and DSC-1800 or GSM and PCS-1900 or GSM/DCS/PCS or many more combinations]
GSM Moving into 3G Mode
GSM extends itself into 3G as Universal Mobile Telecomm. Services [UMTS]
Radio Interface will use WCDMA technology [UMTS Terrestrial Radio Access, UTRA] in two different modes: FDD [two different frequencies for uplink and downlink] or TDD[same frequency for both uplink and T.S. Rappaport Ch 11 NDG Notes 46 downlink but time-shared]
The Market Share of GSM Suppliers [1/97] Networking & Switching Subsystems [NSS] Ericsson [48 %], Siemens [21 %], Nokia [14 %], and Alcatel [10%] = 93 % Others (Lucent, Motorola, Nortel, etc] = 7%
Basestation Sub-System [BSS] Nokia [22%], Motorola [13%], Alcatel [10%] and Ericsson [7%] = 52% Others [Italtel, Lucent, Matra, Philips etc] = 48%
GSM / DCS-1800 / PCS-1900 Mobile Terminals Ericsson [25 %], Nokia [24%], MOTOROLA [20 %], SIEMENS [9%] =78 % Others [Alcatel, Panasonic, Nortel, etc.] = 22 %
Due to Licensing Problems, Unfair and Restricted T.S. Rappaport Ch 11 NDG Notes Competition in GSM Markets
47
GSM and Health Problems GSM Terminals Cause Interference with Hearing Aid Devices and Pacemakers [Instruments used to regulate the heart functionality of Heart Patients], and Some Research show that GSM phones cause Brain Tumors * Inherent in GSM TDMA setup to generate strongly pulsating transmission signals [Continuously Tx is switched on and off that generates LF signals 217, 434, 651 Hz] * Extensive Research required to prove the validity of GSM Cellular Phones being the cause of Brain Tumors or Cancer.
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GSM Derivative Systems: DCS-1800 & PCS-1900
Digital Cellular System[DCS]-1800 Originally started [in 1990] as a separate system, but, later on became just a GSM variant Main modifications were made only in Air Interface Developed [by ETSI] particularly for densely populated urban areas 1.7 - 2.3 GHZ band [ 2 x 75 MHz spectrum, 1710- 1785 + 18051888 MHz] Duplex separation is 95 MHz, Channel BW is 200 KHz, 374 duplex channels Much smaller cells [cells within a building], lower power BTS and MS as compared to GSM handoff problems are much cleverly settled using hierarchical Cell structure Max Cell size 8 km with Class 1 MS [1W] Max Cell size with Class II[0.25W] even smaller [0.5 -4/5 km] International as well as National roaming is possible [Country to T.S. Rappaport Ch 11 Network to Network, NDG Notes 49 Country, and Intra-Network]
GSM Derivative Systems: DCS-1800 & PCS1900 Personal Communication Services [PCS]-1900
A GSM variant to adapt to US Market Frequency band 1900 MHz [1850-1890, 1930-1970 MHz] 2 x 40 MHz bands with Duplex Separation of 80 MHz Channel BW is 200 KHz, 200 Duplex Channels TDMA 8 time-slots EFR speech encoding is getting more Interest from US Service Providers
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GSM Facing Challenges Higher Costs incurred on Network Capacity Increase Business Market Saturation More Money on Advertisement and Subsidies/Customer Discounts
Interconnection Costs [to PSTN/ISDN] are very high Leased Line Costs to Interconnect own Infra-structural elements are very high High License Costs
With Implementation of De-regulation policies this
will CHANGE. T.S. Rappaport Ch 11
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REFERENCES
www.utdallas.edu/~nhutnn/cs6v81/LECTURE http://www.gsmworld.com Dr. Veselin Rakocevic http://www.staff.city.ac.uk/~veselin/Wireless Comms
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