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Chapter 7 The 2nd Generation Cellular Systems

GSM: Pan-European Digital Cellular System

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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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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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Architecture Network elements

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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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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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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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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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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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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Radio Transmission GSM Spectrum

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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Radio Transmission GSM time interval

A hyperframe = 2048 superframe = 3 h 28 m 53.76 s A superframe = 51 traffic multiframes = 26 control multiframes = 6.12 s A traffic multiframe = 26 frames = 120 ms A control multiframe = 51 frames = 235.4 ms A frame = 8 time slots = 4.615 ms A slot = 156.25 bits = 577 µs A bit = 3.69 µs

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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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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

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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

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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 270.833 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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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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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

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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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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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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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Figure 7.11 shows the contents of time slot 0 in each of the 51 frames

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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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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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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

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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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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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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 1937.25 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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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

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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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Messages

GSM Protocol Layers 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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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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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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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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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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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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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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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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Review Exercises1. What is the advantage of transmitting

the training sequence in the middle of a slot?

2. What is the benefit of using the frame number to calculate encryption marks?

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References D. J. Goodman “Wireless Personal

Communications Systems,” Chapter 7.