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CDMA Session 8 Nilesh Jha IS95 CDMA DIGITAL CELLULAR --- CDMA Same frequency allocations as TDMA (but different bandwidths per signal, so grouped differently) Started ... – PowerPoint PPT presentation

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Title: CDMA

  • Session 8
  • Nilesh Jha

1.25 MHz
  • up to 64 channels
  • (is the number of
  • orthogonal codes)
  • code 1.2288 Mchips/sec.
  • Voice coder 9.6 kb/s
  • (really 8.55 kbps plus overhead)
  • Cell power controlled at base stations to
    minimize interference (and near far problem)

  • Same frequency allocations as TDMA (but different
    bandwidths per signal, so grouped differently)
  • Started in 1996 -- 1997, standard called IS-95
  • New technology had doubters, has proven
  • More spectrally efficient than TDMA because of
    adaptive voice coding rates (which CDMA can adapt
    to), power and interference control leading to
    reuse factor of 1, more reliable because of soft
  • Uses 1.25 MHz bandwidth (BW) (a carrier with this
    BW) where multiple users share the bandwidth and
    are differentiated via each having a different
  • Max. 26 calls/MHz/cell or 780 calls/cell (in 30
    MHz) -- soft limit, could be less, sometimes
    stated at about half
  • Adopted in wideband form (about 5 MHz carriers)
    for 3G, with both a US version and a European
    version which seems to be the one most of the
    world is going to

Spread Spectrum Signal
  • Transmitted signal bandwidth gt gt information
  • Some function other than the information
    transmitted is used to determine resultant
    transmitted bandwidth
  • Called the Spreading Function
  • Determines Spreading Gain or Processing Gain
  • PG Bandwidth/Data Rate BW/R
  • Determines How Many Users Can Share Same
    Frequency Band Without Affecting Each Other After

General Model of SS
From Stallings
Advantages of CDMA Cellular
  • Frequency diversity frequency-dependent
    transmission impairments have less effect on
    signal (signal is spread over 1.25 MHz, frequency
    selective effects average out)
  • Multipath resistance chipping codes used for
    CDMA exhibit low cross correlation and low
    autocorrelation -- allows for multiple
    correlation receiver (called Rake receiver) to
    separate out multipath pieces of the signal
  • In TDMA multipath fading is handled through
    equalization, requires complex processing and not
    being as effective because it is narrowband
  • Rake does better --- uses inherent frequency
  • Privacy privacy is inherent since spread
    spectrum is obtained by use of noise-like signals
    -- need the codes to receive and decipher them
    -- a PN code is used like the A key is in TDMA,
    with a unique PN code assigned to a mobile
  • Graceful degradation system gradually degrades
    as more users access the system --- soft limit,
    no real hard limit

Drawbacks of CDMA Cellular
  • Some interference remains arriving
    transmissions from multiple users not aligned
    perfectly on chip boundaries unless users are
    perfectly synchronized
  • multipath signals not synchronized, are random
  • Near-far problem signals closer to the receiver
    are stronger than signals farther away
  • Requires fast and efficient closed loop power
    control to keep interference to weaker signals to
    a minimum
  • Soft handoff uses signals in two cells and thus
    increases interference and uses more than the
    minimum numbers of channels
  • Requires more complex transmitter and receiver
    for spread spectrum signal generation and
    reception --- more expensive -- still, nowadays,
    chipsets available

Mobile Wireless CDMA Considerations
  • RAKE receiver when multiple versions of a
    signal arrive more than one chip interval apart,
    RAKE receiver attempts to recover signals from
    multiple paths and combine them
  • This method achieves better performance than
    simply recovering dominant signal and treating
    remaining signals as noise
  • Soft Handoff mobile station temporarily
    connected to more than one base station
    simultaneously -- this is possible because
    frequency reuse is 1, and the RAKE receiver can
    combine signals from 2 different basestations, or
    pick the best in real time, or weight the
    strongest one more
  • Otherwise handoff and mobility management are
    done the same way as in the US TDMA system, using
    IS-41 for any intersystem messaging

Principle of RAKE Receiver
-Notice that the channel is modeled as multiple
paths with different time delays and
amplitude -Notice that at Rake it is necessary
to estimate those channel numbers -Each Rake
receiver path called a finger, includes a
-Forward channel maximum is 64, 64 Walsh
codes but reverse channels can be more, uses PN
codes -All limited by S/I
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From Garg
Types of Channels Supported by Forward Link
  • Pilot (channel 0) - allows the mobile unit to
    acquire timing information, provides phase
    reference, provides means for signal strength
  • Uses Walsh code 0 (null - pure sines and
    cosines), done 4-6 dB higher than others, used to
    acquire freq./phase reference, needed for
    coherent demodulation
  • Used by mobile for power measurements for handoff
  • Uses PN short code to identify BS, with time
    offset, 512 unique offsets
  • Synchronization (channel 32) - system time,
    system parameters
  • Also PN code offset for that BS, SID, network ID,
    long PN state
  • Paging (channels 1 to 7) - contain messages for
    one or more mobile stations
  • Traffic (channels 8 to 31 and 33 to 63) the
    forward channel supports 55 traffic channels

Forward Channels - more
  • SYNC -- Message can be long, in multiple frames,
    each 32 bits
  • Multiple superframes, each 3 framesmessage could
    be 1146 data bits, CRC
  • Message repeats -- has header, data, CRC ---
    system time from GPS
  • Paging -- Wsub0 to Wsub7, paging MSs
  • Messages can be 1184 bits, in timeslots of 80
    msec, organized so MS only looks at fraction, eg,
    1 in 16 (or up to 64), sleeps rest of time
  • Messages have header, data, CRC -- has called MS,
    calling , messages waiting, BS ID and other
    parameters, alerts, unlock, registration accepted
    or rejected, tune to new frequency, etc
  • Traffic -- data rates of 9.6 or 14.4 kbps (rate
    sets 1 and 2)
  • Voice at 8.55 kbps, error detection to 9.6 kbps,
    dropped to 1.2 kbps during quiet periods with
  • 20 msec frames with 1/2 FEC (to 19.2 kbps),
    interleaved, scrambled, spread, modulated
  • Each frame has 192 bits
  • Multiple codes inserted --- for BS ID, scrambling
    and spreading
  • Can be blanked or dimmed and signaling inserted

Forward Traffic Channel Processing Steps
  • Speech is encoded at a rate of 8000 bps
  • Additional bits added for error detection
  • Data transmitted in 20-ms blocks with forward
    error correction provided by a convolutional
  • Data interleaved in blocks to reduce effects of
  • Data bits are scrambled, serving as a privacy mask

Forward Traffic Channel Processing Steps (cont.)
  • Power control information inserted into traffic
  • DS-SS function spreads the 19.2 kbps to a rate of
    1.2288 Mcps (cpschips/sec) using one row of 64 x
    64 Walsh matrix
  • Digital bit stream modulated onto the carrier
    using QPSK modulation scheme

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From Garg
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Forward Channel -- Comments
  • Notice that BS transmits all channels
    synchronously -- the spreading codes, Ws, are
    orthogonal, and stay orthogonal as they all
    travel the same path to each user
  • Also simultaneously, they all are summed, and RF
    modulated and amplified simultaneously with a
    single RF transmitter
  • Ws used for orthogonal spreading, short PNs for
    BS ID, long PN for scrambling/privacy (each MS
    has its own)
  • Power control bit inserted 800 times/sec,
    puncturing the voice data

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Reverse Channel -- Comments
  • Important differences --- the MSs do not
    transmit synchronously, and moreover, the paths
    back to the BS are different so even if
    orthogonal codes they would NOT stay orthogonal
  • Ws used for modulation on reverse, taking 6 bits
    and turning them into a W row, one modulation
    symbol made up of 64 chips
  • Better demodulation -- better BER for Eb/Nsub0
  • Long PN code, unique to each MS, is used for
    spreading-- it determines the channel (on FWD it
    was the Ws)
  • Short PN code is used for phase sync
  • OQPSK is used, Q chip is half a chip offset, no
    pass tru 0

Logical Channels and Messages -- Some Features
  • Traffic channels can carry voice/data, or
  • Speech vocoder QCELP, at 8.55, 4, 2, .8 kbps
  • Variable rate -- When no or little voice it
    reduces the output rate --- voice activity
  • Signaling with blanking/dimming
  • Also power control
  • Messages
  • Paging and Access are like FOCC and RECC in AMPS,
  • Most messages have CRC and ARQ or selective ARQ
  • eg, Paging channels ACKs messages on Access

Some Network Operations Features
  • RRM
  • Power Control -- needed for near-far problem,
    open and closed loop
  • open loop is MS measures pilot power from BS and
    uses a message from BS that tries to keep MS
    power at some level wrt BS power
  • closed loop BS measures power form MS and sends
    messages to adjust up or down by 1 dB, at 800 Hz
  • Soft Handoff
  • MS tells BS when to start handoff, but MSC
    controls it
  • MS receives from 2 BS (up to 6), 2 physical
    channels, assigning at least one correlator to
    each --- at BS each of 2 BSs looks for that MS
    PN code
  • MS thus does diversity reception with Rake, MSC
    can combine or select
  • MS measures pilots in neighbors list and reports
    to BS
  • Exchange of info on traffic channels, as
  • Can handoff to AMPS -- hard

From Garg
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Capacity Comparison -- Ideal
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From IEC --- some would say the true numbers are
TDMA/GSM 3-4 to 1, CDMA 6-10 to 1
ITUs Standards for Third-Generation Systems (3G)
  • Voice quality comparable to the public switched
    telephone network
  • 144 kbps data rate available to users in
    high-speed motor vehicles over large areas
  • 384 kbps available to pedestrians standing or
    moving slowly over small areas
  • Support for 2.048 Mbps for office use
  • Symmetrical / asymmetrical data transmission
  • Support for both packet switched and circuit
    switched data services

ITUs Standards for Third-Generation Systems (3G)
  • An adaptive interface to the Internet to reflect
    efficiently the asymmetry between inbound and
    outbound traffic
  • More efficient use of the available spectrum in
  • Support for a wide variety of mobile equipment
  • Flexibility to allow the introduction of new
    services and technologies

Wideband CDMA Considerations
  • Bandwidth about 5 MHz
  • Chip rate depends on desired data rate, need
    for error control, and bandwidth limitations 3-4
  • Multirate advantage is that the system can
    flexibly support multiple simultaneous
    applications from a given user and can
    efficiently use available capacity by only
    providing the capacity required for each service

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