TDMA and GSM

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TDMA and GSM

• Session 7
• Nilesh Jha

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Differences Between First and Second Generation
Systems

• Digital traffic channels first-generation
  systems are analog second-generation systems are
  digital
• Channel access second-generation systems use
  TDMA or CDMA, first uses FDMA
• First in 800-900 MHz band, second also there plus
  1800-2000 MHz band
• Encryption all second generation systems
  provide encryption to prevent eavesdropping
• Error detection and correction
  second-generation digital traffic allows for
  detection and correction, giving clear voice
  reception

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Cellular vs PCS Coverage
Free Space loss is proportional to 20log10f (f
in MHz) . Difference between PCS (1900 MHz) and
cellular (880 MHz) is around 7 db.
Cellular base station
PCS base stations
On average ratio of PCS stations to cellular 31
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Cellular vs PCS

• PCS goal is for a user not a place or vehicle
• PCS
• digital
• portable power
• 100 to 300 milliwatts
• TDMA access
• (IS-54/136 and GSM)
• CDMA access (IS-95)
• Often cells closer together

• Cellular designed for cars
• Cellular
• analog
• portable power
• 1/2 to 3 watts
• FDMA access
• Large cell sizes

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PCS License Auction Results

• Auctions raised about 20 billion
• Blocks A and B (30 MHz)- companies wanting a
  nationwide footprint (MTAs)
• Block C (30 MHz)- small companies- dominated by
  Nextwave which went bankrupt (MTAs) -- now
  re-organizing?
• Blocks D, E, F- (10 MHz) - mainly bought to fill
  coverage gaps (BTAs)

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DIGITAL CELLULAR DAMPS --- also called US TDMA

• IS-54 later renamed IS-136
• TDMA, 8 kb/s voice, x2 overhead
• Three 16 kb/s TDMA channels in 30 kHz --- Reuse
  factor 7 with sectoring
• 48 kb/s in 30 kHz 1.6 bits/sec/Hz
• 3 times more spectrum efficient than analog
  (AMPS)
• Approx. 7 calls/MHz/cell
• Approx. 210 max calls/cell
• Used by ATT, Cingular and others in US
• See TDMA Tutorial at
• http//www.iec.org/online/tutorials/tdma/
• See PCS Tutorial at
• http//www.iec.org/tutorials/pcs/index.html or
  at at http//www.iec.org/online/tutorials/pcs/

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IS-54 (IS136) TDMA
Slot N
6 time slots (interleaving of 2 voice samples) 3
users/ frame 324 bits/ time slot 6.667 ms/slot
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US TDMA Architecture

• US TDMA started as IS-54, dual mode terminals,
  after GSM
• Adopted MAHO, encryption, associated control
  channels (instead of FVC/RVC), but uses the AMPS
  forward and reverse control channels to set up
  calls and for MM
• Later established IS-136, with digital control
  channels (DCCH) separate from the AMPS control
  channels, and added sleep modes, allowing all
  digital phones, and various supplementary
  services like voice mail, caller ID, and short
  message service
• IS-136 also specifies an air interface, and a
  basestation, MSC and interworking function, and
  going to public, private or residential networks
  (PSTN, PBX, or cordless)
• Identifiers AMPS plus others A-key to each
  subscriber (for encryption and authentication),
  location areas (for easier location tracking and
  registration), IMSI (international mobile
  subscriber ID), others

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Radio Transmission

• 30 KHz, 6 slots per frame, each user 2 slots, 40
  msec frame
• Some time offset between reverse and forward to
  not transmit and receive at same time, still do
  full duplex
• 324 bits per slot, 6 slots/frame, in 40 msec48.6
  kbps
• Full rate channel is 2 slots/frame 16.2 kbps
  also half rate, 2X, 3X
• No fixed assignment of frequencies to control
  channels
• Uses DQPSK with possible 45 degree, 4590, 45180
  and -45 degree shifts from each phase angle, so 4
  possible next symbols, so 2 bits each, called
  pi/4 shifted DQPSK --- a 1.62 bps/Hz modulation
  spectral efficiency
• Fig. 5.4 --- also, differential, no absolute
  phase reference or detector needed
• But not very energy efficient -- BER for given
  Eb/Nsub0 not great, reuse still 7
• Mobile transmits .25 mw up to 4 w, in 4 dB steps,
  but only 1/3 the time
• Spectral efficiency in terms of voice calls
• About 3 better than AMPS ( a bit higher, if it
  uses 21 control channels for one provider in 25
  MHz, instead of 213), with 7 factor reuse

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Logical Channels

• Digital traffic channels
• Data (incl. Voice), associated control channels,
  sync and other information
• Typically all in one slot -- see fig. 5.6
• eg, 28 bit sync, 260 data bits, 12 SACCH, some
  guard time
• SYNC does frame sync, and is training sequence
  for equalizer
• SACH is control, at 600 b/s per user, like
  FVC/RVC in AMPS
• Mobile only transmits on its slot, power off rest
  of time
• FACCH does a blank and burst on the traffic
  channel
• Faster rate control for handoffs (about x6), with
  rate 1/4 code
• DCCH
• Forward are both broadcast as well as addressed
  to one, reverse are random access -- all have
  SYNC, some preamble, control data
• Organized hierarchically in half frames (blocks),
  superframes (32 frames) and hyperframes (64
  frames) --- control data is muxed in into
  superframes
• Different types of control data are called
  logical channels
• eg, SPACH is short message service, paging and
  access response channel
• Terminals listen to a specific paging subchannels
  in the SPACH, sleep otherwise

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Messages and Authentication

• On AMPS logical channels, on SACCH and FACCH, on
  DCCHs
• Table 5.5 for SACCH and FACCH -- includes call
  management RRM, authentication, handoff (Table
  5.6), etc
• eg, Handoff includes new frequency for handoff,
  power to radiate, half rate or full rate, time
  slot number, color code of new BS, other
• On DCCH system info on broadcast channels, call
  management messages, message waiting and paging
  on SPACH, authentication, etc
• Authentication and privacy in IS-136 due to
  A-key, in phone and in authentication center (AC)
• Used by both mobile and AC to generate a shared
  secret key, SSD, from A-key and a random number
  generator (random number is transmitted) -- but
  can not be reversed to A-key --- used for
  authentication and privacy

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MAHO -- MACA -- Some RRM

• Terminal measures signal quality on the active
  traffic channel
• During time slots it is not active it monitors
  other BSs
• Transmits channel quality information to its BS
  on the SACCH
• Mobile is told which other channels to monitor by
  BS -- 6 or 12
• Signal quality is from power level and BER
• BER is better than just power levels
  interference could give good power levels, but
  bad BER -- better than AMPS
• BS also measures signal quality on active traffic
  channel
• Since in TDMA the BS knows signal quality at
  nearby BSs it knows who to handoff to
• In TDMA most of the processing done at BS, in
  AMPS at MSC
• MACA is similar, for channel allocation, helping
  the BS assign channels the mobiles measure idle
  channels and tell the BS
• SACCH and FACCH have also power adjustment and
  time alignment messages

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GSM (Europe/US))Global System for Mobile

• Agreed TDMA standard devised for European
  environment
• 200 kHz channels with 270.833 kbits/s.
• eight TDMA users
• 13kb/s vocoder, 20kb/s w/overhead
• Reuse factor 3-4
• About 5 calls/MHz/cell with sectoring, or 150
  calls/cell (30 MHz)
• See GSM Tutorial
• Available at http//www.iec.org/tutorials/gsm/inde
  x.html

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Mobile Wireless TDMA Design Considerations ---
for GSM

• Number of logical channels (number of time slots
  in TDMA frame) 8
• Maximum cell radius (R) 35 km
• Frequency region around 900 MHz
• Maximum vehicle speed (Vm)250 km/hr
• Maximum coding delay approx. 20 ms
• Really, this is also max. speech sample delay so
  that one can not distinguish breaks
• Maximum delay spread (?m) 10 ?s
• Bandwidth Not to exceed 200 kHz (25 kHz per
  channel)

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Steps in Design of TDMA Timeslot
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Logic for GSM Rate and Modulation

• Max. delay of 20 msec gtgtgt How much data in 20
  msec?
• If 12 kbps speech codec, thats 260 bits
• Add rate 1/2 convolutional code, thats 480 bits
• Put in 8 speech slots, thats 8480 bits, all in
  20 msec
• Thats 192 kbps
• Notice that data rate is high enough that 20 msec
  worth of speech is included, multiplexed in with
  7 other users, for each users sample
• Really with 13 kbps and other overhead it turns
  into 270.8 kbps
• It uses GMSK modulation -- Gaussian weighted
  Minimum Shift Keying -- like FSK, but changes
  frequency while maintaining continuous phase, and
  shifts the minimum possible --- used because more
  spectrally efficient than PSK or FSK, and fits
  data rate into 200 KHz BW, but power efficient
  (see later)

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GSM Speech Signal Processing
-RPE-LPE (Linear Predictive Coding)
-In 20 msec, 260 bits, turned into (with rate 1/2
codingother) 189278456 bits, in 20 msec is
22.8 kbps (traffic channel) -Interleaved over
multiple slot timeperiods, within 20
msec protects against bursts -Encrypted 114 bits
at a time -Into time slots or bursts -GMSK
modulation
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Radio Transmission -- GSM

• 200 KHz carriers, so fewer transmitters and
  receivers at a BS
• GMSK does 1.35 bps/Hz, worse than US TDMA, but
  has better BER for a given Eb/Nsub0 (so better
  frequency reuse), and has constant envelope
  modulation which allows more efficient amplifiers
  and is better on battery drain than US TDMA
• Can do FH -- network directed
• Slot is .577 msec, then a frame is 8 slots at
  4.615 msec
• Slot has 257 bits of data, 26 bits training
  sequence (8 different ones, also used as SAT/DCC
  function), guard time and tail bits, flags
• Then organized as multiframes (26 or 51 frames),
  superframes(26 or 51 multiframes) and hyperframes
  (2048 superframes -- about 31/2 hours, used for
  encryption periods)
• Traffic multiframe (26 frames ) is 120 msec
• A full rate traffic channel (TCH/F) carries one
  time slot in 24 of 26 traffic frames, in every
  multiframe -- each TCH/F has its SACCH in one
  frame of every multiframe

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Spectrum Efficiency

• GMSK is more power efficient than US TDMA,
  providing good voice quality at S/I of about 7
  dB
• Thus allows frequency reuse of 3-4-5
• With 4 it is 5 calls/cell/MHz
• 8 calls/200 KHz or 40 in 1 MHz, one way
• 20 two ways, and with 4 reuse its 5
  calls/MHz/cell
• Actually one carrier left as guard, slightly
  smaller (4.96)

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TDMA Format Time Slot Fields -- GSM

• Trail bits 3 --- allow synchronization of
  transmissions from mobile units
• Encrypted bits encrypted data, same number of
  bits -- 114 in two groups of 57
• Stealing bit - indicates whether block contains
  data or is "stolen for control signaling
• Training sequence used to adapt parameters of
  receiver to the current path propagation
  characteristics -- in the middle
• Ground rule is that it 6max. delay spread for
  equalizer training -- thats 60usec, at 270 kbps
  or so its about 16 bits -- actually 26
• Guard bits used to avoid overlapping with other
  bursts

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Logical Channels

• Traffic channels, half and full rate
• Signaling channels
• Broadcast
• eg, frequency correction (pure sine wave, used to
  match the BS, SYNC, some control
• Common Control Channels
• Paging, Random access, Access
• Dedicated Control Channels
• Slow, fast, stand-alone

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GSM Network Architecture
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Mobile Station

• Mobile station communicates across Um interface
  (air interface) with base station transceiver in
  same cell as mobile unit
• Mobile equipment (ME) physical terminal, such
  as a telephone or PCS
• ME includes radio transceiver, digital signal
  processors and subscriber identity module (SIM)
• GSM subscriber units are generic until SIM is
  inserted
• SIMs roam, not necessarily the subscriber devices

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Base Station Subsystem (BSS)

• BSS consists of base station controller and one
  or more base transceiver stations (BTS)
• Each BTS defines a single cell
• Includes radio antenna, radio transceiver and a
  link to a base station controller (BSC)
• BSC reserves radio frequencies, manages handoff
  of mobile unit from one cell to another within
  BSS, and controls paging

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Network Subsystem (NS)

• NS provides link between cellular network and
  public switched telecommunications networks
• Controls handoffs between cells in different BSSs
• Authenticates users and validates accounts
• Enables worldwide roaming of mobile users
• Central element of NS is the mobile switching
  center (MSC)

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Mobile Switching Center (MSC) Databases

• Home location register (HLR) database stores
  information about each subscriber that belongs to
  it
• Visitor location register (VLR) database
  maintains information about subscribers currently
  physically in the region
• Authentication center database (AuC) used for
  authentication activities, holds encryption keys
• Equipment identity register database (EIR)
  keeps track of the type of equipment that exists
  at the mobile station

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GSM Signaling Protocol Architecture
(m - modified/mobile from ISDN) (Uses CRC, ARQ)
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Functions Provided by Protocols

• Protocols above the link layer of the GSM
  signaling protocol architecture provide specific
  functions
• Radio resource management
• Does radio channel management, including for
  handoffs
• Mobility management
• Roaming, location databases, authentication
• Connection management
• sets up calls between users
• Mobile application part (MAP) -- Core Network
  functions, like IS-41 in US systems
• BTS management
• SCCP and MTP are from SS7, for control signaling
• Signal connection control part, message transfer
  part

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