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TDMA and GSM

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... and a basestation, MSC and interworking ... TDMA US TDMA Architecture Slide 9 Slide 10 Radio Transmission Slide 12 Slide 13 Slide 14 Logical Channels ... – PowerPoint PPT presentation

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Title: TDMA and GSM


1
TDMA and GSM
  • Session 7
  • Nilesh Jha

2
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

3
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
4
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

5
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)

6
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/

7
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
8
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

9
From Goodman
10
From Goodman
11
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

12
From Goodman
13
From Goodman
14
From Goodman
15
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

16
From Goodman
17
From Goodman
18
From Goodman
19
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

20
From Goodman
21
From Goodman
22
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

23
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

24
From Goodman
25
From Goodman
26
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)

27
Steps in Design of TDMA Timeslot
28
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)

29
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
30
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

31
From Goodman
32
From Goodman
33
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)

34
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

35
From Goodman
36
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

37
GSM Network Architecture
38
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

39
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

40
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)

41
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

42
GSM Signaling Protocol Architecture
(m - modified/mobile from ISDN) (Uses CRC, ARQ)
43
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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