Chapter 7 The 2nd Generation Cellular Systems

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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) -gt 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 subscribers 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 Exercises

1. 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.