1. Chapter 7 The 2nd Generation Cellular Systems
GSM: Pan-European Digital Cellular System
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Prof. Huei-Wen Ferng

2. Background and Goals GSM (Global System for Mobile Communications)
Beginning from 1982
European standard
Full roaming in Europe
A purely digital system
Goals (principal/ original) -> Phase 1, 2, 2+:
full international roaming
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Prof. Huei-Wen Ferng

3. Background and Goals
provision for national variations in charging and rates
efficient interoperation with ISDN systems
signal quality better than or equal to that of existing mobile systems
traffic capacity higher than or equal to that of present systems
lower cost than existing systems
accommodation of non-voice services and portable terminals
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Prof. Huei-Wen Ferng

4. Architecture Network elements Three databases
Mobile stations, base stations, and mobile switching center
Three databases
Home location registers (HLR)
Visitor location registers (VLR)
Equipment identity registers (EIR)
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6. Architecture
In contrast to the original cellular, micro cells are used in GSM
A BS separates into two parts: BTS (base transceiver station) and BSC (base station controller)
Typically, a BSC controls several BTS
To reduce the cost with the greatest possible service extent
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8. Architecture Subscriber identity module (SIM)
Two types: one like credit card and the one smaller
An important GSM innovation
A removable card that stores information, including ID number, abbreviated dialing code, and subscriber’s service plan
Easy to change telephones
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Prof. Huei-Wen Ferng

9. Architecture As in the other systems, GSM uses a variety of ID codes
GSM Identifiers
International Mobile Subscriber Identity (15 digits)
Temporary Mobile Subscriber Identity (32 bits)
Advantages: Privacy and save BW
International Mobile Equipment Identifier (15 digits)
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Prof. Huei-Wen Ferng

10. Architecture Authentication Key (max = 128 bits) Cipher key (64 bits):
Terminal and network use authentication key to compute the cipher key
Mobile station classmark including:
Version of the GSM standard
RF power capability (power levels available)
Encryption method
Other properties of terminal
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Prof. Huei-Wen Ferng

11. Architecture Training Sequence (26 bits)
help a terminal verify that it receives information from the correct BS rather than another BS using the same physical channel
BS Identity Code (6 bits)
Location Area Identity (40 bits) including:
A mobile country code, network code, and area code
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13. Radio Transmission GSM Spectrum Physical Channels
There are two 25 MHz bands separated by 45 MHz
Initial GSM systems operate in the upper 10 MHz
Physical Channels
GSM is a Hybrid FDMA/TDMA system
Each GSM band has carriers spaced at 200 kHz
The frame duration is 120/26 = 4.62 ms
Each frame contains 8 time slots
There are 25 MHz/200 k Hz = 125 carriers in per direction
GSM specifies only 124 carriers (one is used as guard band)
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17. Radio Transmission GSM time interval
A hyperframe = 2048 superframe = 3 h 28 m s
A superframe = 51 traffic multiframes = 26 control multiframes = 6.12 s
A traffic multiframe = 26 frames = 120 ms
A control multiframe = 51 frames = ms
A frame = 8 time slots = ms
A slot = bits = 577 µs
A bit = 3.69 µs
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19. Physical Channels Traffic Channels
A full-rate traffic channel (TCH/F) occupies one time slot in 24 of 26 frames in every multiframe
Traffic channel information travels in frames 0-11 and 13-24
Control information travels in frames 12 and 25
The SACCH occupies one frame in every traffic multiframe
A SACCH associated with a full-rate traffic channel alternatively occupies one slot in frame 12 and one slot in frame 25
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21. Physical Channels
A half-rate traffic channel (TCH/H) occupies a specific time slot in 12 of 26 frames in every multiframe
Each carrier can carry up to 16 half-rate traffic channels
Eight of these traffic channels have a SACCH in frame 12 and the other eight half-rate channel have a SACCH in frame 25
2018/12/2
Prof. Huei-Wen Ferng

22. GSM Bit Stream The contents of a GSM time slot is shown in Fig. 7.8
26 bits of training sequence serves as a purpose similar to that of the SYNC field in NA-TDMA
GSM specifies 8 different training sequences with low mutual cross-correlation
Network operators assign different training sequences to nearby cells that use the same carrier
The two DATA fields carry either user information or network control information
2018/12/2
Prof. Huei-Wen Ferng

23. Radio Transmission
The FLAG indicates whether the DATA field contains user information or control one
The TAIL bits all set to 0
There is also a guard time 0f 30.5 µs
The GSM transmission rate is kb/s
The modulation scheme in GSM is GMSK a form of frequency shift keying
The modulation efficiency of GSM is 1.35 b/s/Hz
GSM BS turn off its transmitter at the end of each time slot. It resume transmitting after a pause of 30.5µs to send to another terminal in the next time slot
The BS turn off its transmitter in unassigned time slots
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25. Slow Frequency Hopping
The signal moves from one frequency to another in every frame
The purpose of FH is to reduce the transmission impairments
Without FH, the entire signal is subject to distortion whenever the assigned carrier is impaired
Network operator assigns different hopping patterns to different cells
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27. Radiated Power
GSM specifies 5 classes of mobile stations transmitting power, ranging from 20 W (43 dBm) to 0.8 W (29 dBm)
Typically, vehicle-mounted terminal is 8 W and portable terminals is 2 W
2018/12/2
Prof. Huei-Wen Ferng

28. Spectrum Efficiency The reuse factor of N = 3 or 4
The number of physical channel is 124 carriers x 8 channels/carriers = 992 physical channels
The efficiency of GSM is E = 992 channels/4 cells/cluster/50 MHz = 4.96 conversation/cell/MHz (N= 4) or
The efficiency of GSM is E = 992 channels/3 cells/cluster/50 MHz = 6.61 conversation/cell/MHz (N= 3)
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Prof. Huei-Wen Ferng

29. Logical Channels Traffic channels (two-way)
Broadcast channels (base-to-mobile)
Common control channels (base-to-mobile or mobile-to-base)
Dedicated control channels (two-way)
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31. Broadcast channels and Common control channels
The broadcast channels always occupy time slot 0
The common control channels can also occupy time slots 0
Control Multiframe
There are 5 groups of frames, each containing ten frames beginning with a frequency-correction frame and a synchronization frame
In the reverse direction, time slot 0 is assigned to random access channels in all 51 frames
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32. Figure 7.11 shows the contents of time slot 0 in each of the 51 frames
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33. Logical Channels Frequency Correction Channel (FCCH)
The FCCH simply transmits 148 0s
The FCCH always occupies time slot 0 in a frame of 8 time slots
A terminal without a call in progress searches for a FCCH
Synchronization Channel (SCH)
A BS transmits a SCH in time slot 0 of every frame that follows a frame containing an FCCH
The SCH contains a TRAINING sequence
The DATA fields contain BS identity code (6 bits) and the present frame number
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35. Logical Channels Broadcast Control Channel (BCCH)
BS use the BCCH to transmit the information that terminals need to set up a call, including the control channel configuration and the access protocol
The message length is 184 bits and the encoded message is 456 bits occupying 4 time slots
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37. Logical Channels Paging Channel (PCH) and Access Grant Channel (AGCH)
The purpose of the AGCH is to direct a terminal to a stand-alone dedicated control channel (SDCCH)
Both channels use the same coding scheme as the BCCH
They occupy 36 frames of time slot 0 per multiframe
2018/12/2
Prof. Huei-Wen Ferng

38. Logical Channels Random Access Channel (RACH)
GSM terminals send messages on the RACH to originate phone calls, initiate transmissions of short messages, respond to paging messages, and register their locations
Terminals with information to transmit use the slotted ALOHA protocol to gain access to the time slot
The Ack directs the terminal to a stand-alone dedicated control channel (SDCCH) to be used for further communications
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Prof. Huei-Wen Ferng

39. Logical Channels The RACH slot includes a 41-bit TRAIN and 36-bit DATA
The 36-bit DATA field carries a simple 8-bit message
Three of the 8 bits indicate the purpose of the access attempt and the other 5 bits are produced by a random number generator
The 5-bit random code is likely (with probability 31/32) to distinguish the successful terminal from the other
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42. Logical Channels Stand-Alone Dedicated Control Channel (SDCCH)
SDCCH is a two-way channel assigned to a specific terminal
The physical channel used by an SDCCH is a set of four time slots in each 51-frame control multiframe
With 114 data bits per time slot, the data rate of the SDCCH is b/s (see eq. 7.7)
Each SDCCH has a slow associated control channel
The SACCH occupies an average of two time slots per control multiframe (969 b/s)
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Prof. Huei-Wen Ferng

43. Logical Channels Traffic Channels (TCH)
GSM defines two traffic channels, a full-rate channel occupies 24 time slots in every 26-frame and a half-rate channel
The bit rate of a full-rate traffic channel is 22,800 b/s
SACCH occupies time slots in frames 12 or 25 of each 26-frame traffic multiframe
The transmission rate of a traffic SACCH is 950 b/s
With 456 bits transmitted per message, a message spans four traffic multiframes, a time interval of 480 ms
2018/12/2
Prof. Huei-Wen Ferng

44. Logical Channels Fast Associated Control Channel (FACCH)
Use the traffic channel to transmit control information, which is an in-band signaling channel
Each FACCH message is multiplexed with user information and interleaved over 8 frames. Therefore, the transmission time of an FACCH message is approximately 40 ms
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46. Messages GSM Protocol Layers Message Structure
GSM provides a large number of open interfaces
Message Structure
All of the signaling message length is 184 bits with the exception of the FCCH, SCH, and RACH
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49. Network Operations Call to a GSM Terminal
Terminal uses the frequency correction channel (FCCH) to synchronize its local oscillator
It then gains timing information from the SCH
The terminal then obtains important information from broadcast control channel (BCCH)
After the initialization procedure, the terminal monitors a paging channel (PCH)
Eventually, it detects a paging request message and this message cause the terminal to transmit a channel request message on the random access channel (RACH)
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Prof. Huei-Wen Ferng

50. Network Operations
The network response this request by transmitting an immediate assignment message on an access grant channel (AGCH)
This message established a stand-alone dedicated control channel (SDCCH) to be used for exchange of mobility management messages and call management messages
When terminal moves to SDCCH, it transmits a paging response message to BS
The BS then initiates the GSM authentication procedure
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52. Network Operations Authentication and Encryption Procedure
The terminal received a 128-bit random number (RAND) from BS
Then it applies a GSM encryption algorithm A3 to compute a 32-bit signed response, SRES
The inputs of A3 are RAND and secret key Ki
The secret key Ki is stored in the subscriber information module (SIM)
The terminal applies another encryption algorithm A8 to compute a 64-bit ciphering key Kc from SRES and Ki
The network also uses A3 to compute SRES from RAND and Ki
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53. Network Operations
If the two values of SRES are identical, the network accept the the user as an authorized subscriber
To encrypt user information and network control information, the BS and network derive a 114-bit mask to be added (modulo 2) to the two DATA fields
The inputs of A5 are the 64-bit ciphering key Kc and the current 22-bit frame number
Because A5 uses the frame number to compute the ciphering mask, the mask change from frame to frame
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55. Network Operations To Setup a Call
BS transmits a setup message to the terminal
The terminal Ack this message with a call confirmed
The terminal then send a connect message to the network
In response, the network moves the call to a traffic channel by means of an assignment command message
Note that, GSM assigns a traffic channel after the mobile subscriber accepts the call
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56. Network Operations Location-Based Registration
Terminal registers its location when it moves to a new cell
Mobile-Assisted Handover
When mobile terminal finds a channel quality is better than present one the handover procedures will be executed
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59. Status of GSM GSM operates in 900 MHz, 1800 MHz, and 1900 MHz bands
GPRS (generalized packet radio service) with 100+ kbits/s data rate
Enhanced Data Rate for GSM Evolution (EDGE) with 300+ kbits/s data rate
Universal Mobile Telecommunication Services (UMTS): 3G telecommunication technology up to 2 Mbits/s data rate using WCDMA or TD/CDMA (IMT2000) transceiver
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60. Review Exercises
What is the advantage of transmitting the training sequence in the middle of a slot?
What is the benefit of using the frame number to calculate encryption marks?
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61. References
D. J. Goodman “Wireless Personal Communications Systems,” Chapter 7.
2018/12/2
Prof. Huei-Wen Ferng