CDMA

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CDMA

• Session 8
• Nilesh Jha

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IS95 CDMA
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)

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DIGITAL CELLULAR --- CDMA

• 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
  successful
• 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
  handoff
• 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
  code
• 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

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Spread Spectrum Signal

• Transmitted signal bandwidth gt gt information
  bandwidth
• 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
  Despreading

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General Model of SS
Spreading
Despreading
From Stallings
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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
  diversity
• 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
  terminal
• Graceful degradation system gradually degrades
  as more users access the system --- soft limit,
  no real hard limit

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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

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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

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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
correlator
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-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
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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
  comparison
• 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

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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
  VOICE ACTIVITY DETECTION
• 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

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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
  encoder
• Data interleaved in blocks to reduce effects of
  errors
• Data bits are scrambled, serving as a privacy mask

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Forward Traffic Channel Processing Steps (cont.)

• Power control information inserted into traffic
  channel
• 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

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Logical Channels and Messages -- Some Features

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

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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
  rate
• 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
  signaling
• Can handoff to AMPS -- hard

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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
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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
  rates
• Support for both packet switched and circuit
  switched data services

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ITUs Standards for Third-Generation Systems (3G)
(cont.)

• An adaptive interface to the Internet to reflect
  efficiently the asymmetry between inbound and
  outbound traffic
• More efficient use of the available spectrum in
  general
• Support for a wide variety of mobile equipment
• Flexibility to allow the introduction of new
  services and technologies

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Wideband CDMA Considerations

• Bandwidth about 5 MHz
• Chip rate depends on desired data rate, need
  for error control, and bandwidth limitations 3-4
  Mcps
• 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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Alternative Interfaces