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Code Division Multiple Access (CDMA)

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The Coding and Modulation process in CDMA ... This is possible since CDMA is a wide band ... Studies have shown that 50% of the time there is no speech signal ... – PowerPoint PPT presentation

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Title: Code Division Multiple Access (CDMA)


1
Code Division Multiple Access(CDMA)
  • Prepared by
  • Anil Ramroop ID-0024144
  • Perapong Uttarapong ID-0026852

2
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).

3
cdmaOne overview and Terminology
Add check bits
A/D
mux
Information
Information Bits
FEC
Code Symbols
Chips
Chips
Spreading Code Generator
Spreader
PSK
4
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

5
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.

6
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
7
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 )

8
Direct Sequence CDMA
  • Multiply data with a Pseudo-random noise sequence
    (PN)

9
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.

15
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.

16
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.

17
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

18
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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34
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.

38
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.

39
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.

43
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.
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