Code Division Multiple Access (CDMA)

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Code Division Multiple Access(CDMA)

• Prepared by
• Anil Ramroop ID-0024144
• Perapong Uttarapong ID-0026852

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Code Division Multiple Access(CDMA)

• Multiple Access is a technique by which multiple
  users use the same physical resource.
• The most prevalent multiple access techniques are
  TDMA,FDMA and CDMA.
• CDMA is based on Spread Spectrum which evolved
  some 50 years back.
• Each traffic channel is multiplied by a unique
  high speed bit stream to spread the channel in
  the frequency domain.
• At the receiver end the spread signal is
  multiplied by the same high speed stream to
  retrieve the data.
• Out of the CDMA implementations cdmaOne is the
  one which is most widely deployed commercially.
• cdmaOne is based on the IS-95(1993) standard and
  is a trade mark of CDMA development group (CDG).

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cdmaOne overview and Terminology
Add check bits
A/D
mux
Information
Information Bits
FEC
Code Symbols
Chips
Chips
Spreading Code Generator
Spreader
PSK
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CDMA Cellular Reuse

• Same Frequency is used in every cell
• Interference becomes low power noise
• Spectral efficiency much higher than AMPS.
• 20 times theoretically.
• 58 times in practice.
• CDMA Design Parameters ( Same as AMPS )
• Forward Channel Frequency ? 869 894 Mhz
• Reverse Channel Frequency ? 824 849 Mhz
• Tx/Rx Frequency Spacing ? 45 Mhz

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IS-95 CDMA

• Existing 12.5 Mhz assigned cellular bands are
  used to derive 10 different CDMA bands ? 1.25 Mhz
  per band.
• Frequency Reuse factor in CDMA 1.
• Channel Rate 1.2288 Mcps (cycles per sec).
• Multipath Fading exploited in CDMA.
• Rake receivers are used to combine the output of
  several received signals.
• Fading does occur on the individual signals, but
  each signal is affected differently and so using
  several of them to make a decision improves the
  probability of obtaining a correct decision
  Multipath Diversity combining.
• At Mobile
• Three correlators used to receive three different
  signals. With a fourth one used as a roving
  finger which is used to detect new strong
  incoming signals. Process ensures that the Rake
  receiver always used the three strongest signals.
  
• At Base Station
• All four correlators are used to receive signals
  ? Antenna Diversity.

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The Rake receiver

• One of the main advantages of the CDMA system is
  its ability to resolve different multipath
  components.
• This is possible since CDMA is a wide band
  system.(??)
• In order to resolve multipath signals the
  subscriber unit/BTS should make use of multiple
  receivers
• operating at different phases. Each of these
  receivers are called fingers.
• The outputs of these fingers are added to form a
  strong output.

Correlator 1
Correlator 1
Output
Correlator 2
Combiner
Correlator 2
Combiner
Input
Input
Correlator 3
Correlator 3
Searcher
Searcher
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The Coding and Modulation process in CDMA

• 64 bit Walsh Codes are used to provide 64
  channels within each frequency band.
• Walsh codes used for spreading in the forward
  link.
• Walsh codes used to provide orthogonal modulation
  and not spreading to the full 1.2288 rate in the
  reverse link.
• Besides Walsh codes, 2 other codes are used in
  IS-95
• Long PN code generated from a 42 bit shift
  register having (242 1) 4.398 x 1012
  different codes. A mask is used to overlay the
  codes, the mask differs from channel to channel.
  The chip rate is 1.2288Mcps. These codes are used
  for
• Data scrambling/encryption in the forward path
• Data spreading and encryption in the reverse path
• Short PN code generated from a pair of 15 bit
  shift registers having 215 1 32767 codes.
  These codes are used for synchronization in the
  forward and reverse links and cell identification
  in the forward link
• Each cell uses one of 512 possible offsets.
• Adjacent cells must use different offsets.
• Chip Rate is 1.2288Mcps ( i.e., not used for
  spreading )

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Direct Sequence CDMA

• Multiply data with a Pseudo-random noise sequence
  (PN)

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Hadamard-Walsh Code
  The four orthogonal sequences in this Walsh
code set are taken from the rows of the matrix H4
that is,   W0 0 0 0 0 W1 0 1
0 1 W2 0 0 1 1 W3 0 1 1 0

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CDMA Channels

• Forward and Reverse Channels are separated by 45
  Mhz.
• Forward Channel comprises of the following
  channels
• Pilot channel (always uses Walsh code W0 )
• Paging channel(s) ( use Walsh code W1 W7 )
• Sync channel (always uses Walsh code W32 )
• Traffic channels ( use Walsh codes W8 W31 and
  W33 W63 )
• Reverse Channel comprises of the following
  channels
• Access channel
• Traffic channel
• Link Protocol can be summarized as follows
• Mobile acquires phase, timing and signal strength
  via the Pilot channel
• Mobile synchronizes to Base Station via the Sync.
  Channel
• Mobile gets system parameters via the paging
  channel.
• Mobile and BS communicates over the traffic
  channels during a connection.
• Mobile and BS communicate over the access and
  paging channels during system acquisition and
  paging.

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Forward/Reverse Channel Spreading and Scrambling
Process

• Forward channels are spread using one of 64
  orthogonal Walsh functions. Note Perfect
  separation between the channels in the absence of
  multipath interference.
• To reduce interference between mobiles that use
  the same Walsh function in neighboring cells, all
  signals in a particular cell are scrambled using
  the the short PN sequence for cell
  identification.
• For the paging and traffic channels, the long PN
  sequence is used to scramble the signal before
  spreading.
• Reverse channels are spread using the long PN
  sequence.
• All 64 orthogonal Walsh functions are used to
  provide orthogonal modulation.
• The stream is then scrambled using the short PN
  sequence for cell identification purposes.

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CDMA Vocoder Transmission Rates

• IS-95 supports different transmission rates. The
  vocoder (QCELP) outputs 9.6 Kbps when there is a
  full speech signal and 1.2 Kbps when a silent
  period is detected. (Note 1)
• Intermediate rates such as 4.8Kbps and 2.4 Kbps
  are progressively used to either increase or
  decrease rates based on the speech signal
  content.
• Rate decisions are made every 20msec interval (
  the interval over which samples are collected and
  processed).
• In CDMA A signal (rate set 1) is always sent
  for it takes too long for the receivers to ramp
  up again for reception.
• To accommodate all the different data rates using
  the same air interface, bits in the lower bit
  rate streams are repeated to bring the rate up to
  9.6Kbps.
• However the bits are output at a correspondingly
  lower power. For example the 1.2 Kbps bits are
  repeated 8 times to bring it up to 9.6 Kbps, but,
  the signal strength is reduced to 1/8 the power.

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CDMA Vocoder Transmission Rates(Cont.)

• In 1995, Qualcomm introduced a higher rate coder
  (QCELP13) called Rate Set 2 that produces a 14.4
  Kbps speech signal and 1.8 Kbps when a silent
  period is detected. The other intermediate rates
  are 7.2 Kbps and 3.6 Kbps.
• So as not to change the air interface and the
  transmitters and receivers (in particular the
  interleaver), the following were done
• Reverse link ? rate set 2(RC2) signal is encoded
  at 1/2 rate as opposed to 1/3 rate used in rate
  set 1(RC1).
• Forward Link ? puncturing of the code is used to
  reduce it from ½ to ¾ (i.e., 2 symbols from every
  6 encoded symbols are dropped).
• IS-95 also supports variable rate transmission on
  the reverse link as follows
• Instead of repeating the symbols and sending them
  at 9.6 or 14.4 Kbps, the repeated symbols are
  randomly deleted from the frame (after
  interleaving).
• Thus, mobiles transmitting at the same rate do
  not have all their bits arrive at the same time
  at the BS which reduces interference.
• When this mode is used, the symbols are sent at
  full power as oppose to reduce power when using
  repetition.

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Forward Logical Channels

• Pilot Channel
• Transmitted at all times ( sequence of 0s ).
• Uses Walsh Code W0.
• Provides phase and timing reference to the mobile
  terminal.
• Provides signal strength to the mobile for
  channel acquisition.
• Re-used in every cell and sector with different
  short PN code offset.
• Sync Channel -- can be received by a mobile after
  it locks on to a pilot channel. Features of the
  Sync Channel
• Operates at 1200 bps.
• Has a frame length of 26.666 msec.
• Uses Walsh code W32 and uses the same PN sequence
  offset as the Pilot channel.
• Provides timing information to the mobile for
  synchronization.
• Provides pilot PN offset.
• Provides system time ( needed for the short PN
  sequence generation ).
• Provides system and network Ids.
• Provides paging channel rates.
• Provides BS protocol revision level.
• CDMA channel number

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Forward Logical Channels (Cont.)

• Paging Channel is used to page mobiles and
  transmit system information.
• Bit rate of 9600 or 4800 bps.
• Frame Length 80msec messages can occupy several
  slots (1-4).
• Use Walsh codes W1 W7 ( System can use 17
  paging channels depending on traffic load ).
• Transmit the system parameter message
  registration information, BS class, BS
  longitude/latitude, power control thresholds,
  etc.
• Transmit the access parameter message of
  access channels, initial access power
  requirements, of access attempts,
  authentication info., etc.
• Carry the channel assignment for a traffic
  channel to mobile.

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Forward Logical Channels (Cont.)

• Forward Traffic Channels are used to carry user
  data and signaling data. Features are as follows
• Bit rates up to 9600bps (rate set 1) and up to
  14.4Kbps (rate set 2).
• Frame length of 20ms (192 bits for rate set 1 and
  288 bits for rate set 2)
• Use Walsh codes W8 W31 and W33 W63.
• Can be used in two modes Blank Burst or Dim
  Burst
• Blank Burst is similar to NA-TDMA, signaling
  data replaces speech data
• Dim Burst multiplexes signaling data or a
  secondary data stream with speech data (speech
  data sent at 4.8, 2.4 or 1.2 Kbps for RC1 and
  7.2, 3.6 or 1.8Kbps for RC2.

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Reverse Logical Channels

• Access Channel is a random access channel used
  by mobiles to send information (not user data) to
  the BS.
• One or more access channels are paired with a
  paging channel (max. is 32 in total)
• Mobiles respond to paging messages on their
  corresponding access channels.
• Bit rate is 4800bps.
• Long PN code mask consists of
• Access channel number, BS identifier,
  corresponding paging channel number, PN_offset
  (No PN offset is used for the quadrature spread).
• Mobiles compete for access as follows
• Mobile chooses an access channel at random from
  the set associated with the paging channel.
• If two mobiles choose the same access channel and
  PN time alignment ? their transmissions will
  interfere with each other Thus, the BS will not
  be able to distinguish between them.
• No channel sensing for collision avoidance.
• If a mobile does not get an ACK back before the
  timer expires it makes another attempt (at a
  higher power level) after a random wait. It
  repeats this process for a max. number of times,
  if it does not succeed, it waits a random time
  and then restarts the process all over again.

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Reverse Logical Channels

• Reverse Traffic Channel used to carry user data
  (primary and secondary) and signaling data. A BS
  will support up to 61 channels.
• Data transfers at 4 different levels within a
  rate set supported.
• Signaling information is multiplexed with the
  user data, where possible (i.e. if variable data
  rates are supported). If not possible, then the
  signaling information takes over the channel
  briefly to transmit a message (blank and burst)
• Instead of signaling information, a secondary
  traffic stream can be multiplexed (i.e., voice is
  primary and data is secondary).
• Long PN mask is used to uniquely identify a
  mobile. Can be of two types
• Public consists of the mobiles ESN.
• Private derived from the encryption and
  authentication process.
• Orthogonal modulation consists of sending one of
  64 possible Walsh functions for each group of 6
  coded bits.
• Walsh Function number C0 2C1 4C2 8C3
  16C4 32C5 where the Cs represent the coded
  bits. Output rate is 28.8 x 64 / 6 307.2Kbps.

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Power Control

• Power control is of paramount importance for a
  CDMA system. In order to have max. efficiency,
  the power received at the BS from all Mobiles
  must be nearly equal.
• Mobiles power too low, then many bit errors will
  occur.
• Mobiles power too high, then the interference
  level increases.
• Power Control at Mobile
• Closed Loop power control information is sent to
  the mobile from the BS. Puncturing is used, 2
  data symbols are replaced by one power control
  symbol (double the power). This bit either
  indicates a transition up or down in power in 1db
  increments. The power bit is sent 16 times per
  20ms (every 1.25ms) (Pclosed.)
• Open Loop The mobile senses the strength of the
  pilot signal and can adjust its power based upon
  that. If the signal is very strong, the
  assumption can be made that the mobile is very
  close to BS and the power should be dropped. The
  mobile uses Ptarget sent in the access param.
  Msg. (Popen).
• The transmitted power at the mobile (in units of
  dBm) is Ptran Popen Pclosed
• Power Control at BS
• The BS decreases its power level gradually and
  wait to hear from the mobile what the frame error
  rate (FER) is (power measurement report). If
  high, the BS then increases its power level.

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Handoffs

• CDMA supports three types of handoffs
• Hard handoff ( Similar to the NA-TDMA (IS-136) )
• Soft handoff
• Handoff between two different cells (between two
  different sites) operating on the same frequency.
• Softer handoff
• Handoff between two different sectors of the same
  cellular site.
• Mobile assists in the handoff process, therefore
  it is referred to as Mobile Assisted Hand Off
  (MAHO).
• Mobile report signal measurements to the BS. The
  roving finger of the Rake receiver is used to
  measure the pilot signals of neighboring BSs
  (neighbor list messages sent to mobiles
  periodically).
• During call setup, a mobile is given a list of
  handoff thresholds and a list of likely new
  cells. The mobile keeps track of those cell that
  fall above the threshold and sends this
  information to the MSC whenever requested.
• Mobile and MSC classify the neighboring BSs to
  keep track of the handoff process.
• Based upon data received from the mobile the MSC
  constantly re-classifies the BSs with regard to
  the mobile
• Active list contains BSs currently used for
  communication at least one BS.
• Candidate list contains list of BSs that could
  be used for communication based upon current
  signal strength measurements.

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Handoffs (Cont)

• Neighbor list contains a list of BSs that could
  soon be promoted to candidate list.
• Remaining list all other BSs that do not
  qualify.
• When the MSC moves a BS from the candidate list
  into the active list, it directs BS to serve the
  mobile.
• MSC informs both the new BS and the mobile and
  assigns a forward channel number (Walsh code) for
  communication.
• Soft handoffs consist of the mobile being served
  by two BSs. This means the following
• Mobile receives the signal from two BSs.
• Two BSs also receives the signal from the mobile.
• Soft handoffs also eliminate the ping pong effect
  (i.e., when traveling along the boundary of two
  cells) as the mobile is being served by two BSs
  and does not have to switch BSs until absolutely
  necessary.

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Handoffs (Cont)

• Mobile initiates the handoff
• The mobile analyze the measurements and inform
  the MSC when a handoff might be necessary. (If
  one BSs signal strength becomes much higher that
  the other).
• Handoff process is controlled by the MSC.
• When a handoff occurs all three correlators are
  switched over to the new cell and used as a Rake
  receiver again.
• The connection to the current BS is cutoff and
  the new BS becomes the current BS.
• Summary of handoff process is as follows
• Mobile communicates with original/current BS.
• Mobile communicates with current cell BS and new
  cell BS.
• Mobile communicates with the new cell BS (which
  becomes the current cell).

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Mobile Management

• Mobiles must register with a system if they want
  to receive or make calls.
• There are 5 different types of autonomous
  registration messages in IS-95. System msgs on
  the Sync channel indicates will ones are in
  effect.
• Power up
• Power down (de-registration)
• Timer exceeds a threshold
• Distance between new and old BS exceeds a certain
  limit.
• BSs sends out GPS info. in systems messages
  which includes distance threshold.
• New zone (cells under one MSC are clustered in
  zones).
• There are 4 other types of registration that are
  not mobile initiated, i.e., BS asks for it
  mobile changes some parameter and informs the BS
  implicitly in the page response.

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Mobile Management (Cont.)

• When a mobile registers it also will indicate
  which slots it will listen to when the paging
  channel is in slotted mode.
• It also provides other parameters such as
  protocol version and class type that it is using
  so that the MSC knows how to communicates with it
  and what services to provide.
• Roaming CDMA system consists of system Ids (SID)
  and network Ids (NID).
• System has many networks within it so a mobile
  has to keep track of the SID/NID pair of the area
  it is in (broadcast by the BSs).
• Each mobile has a list of home SIDs and NIDs. If
  it enters an area that has an NID that is not on
  the list, but the SID is ? classified as NID
  roaming.
• If the SID is not on the list ? it is SID
  roaming.
• Once the mobile knows its a roamer it will
  figure out what kind of services it will be able
  to access in this foreign (non home) environment.