CDMA2000-1x????

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Basic knowledge of CDMA
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Contents

• World Trends
• The Cellular Concept and Multiple Access
  Techniques
• Principles of CDMA
• CDMA Major Features
• Comparison with Other Technologies

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GSM and CDMA Coverage
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• Evolution of CDMA

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Technologies of Mobile Communications
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CDMA Past, Present, and Future

• The origins of spread spectrum are in military
  field and navigation systems
• In 1949, John Pierce wrote a technical memorandum
  where he described a multiplexing system in which
  a common medium carries coded signals that need
  not be synchronized. This system can be
  classified as a time hopping spread spectrum
  multiple access system
• In 1949, Claude Shannon and Robert Pierce
  introduced the basic ideas of CDMA by describing
  the interference averaging effect and the
  graceful degradation of CDMA
• In 1950, De Rosa-Rogoff proposed a direct
  sequence spread spectrum system and introduced
  the processing gain equation and noise
  multiplexing idea

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CDMA Past, Present, and Future

• In 1956, Price and Green filed for the
  anti-multipath "RAKE" patent . Signals arriving
  over different propagation paths can be resolved
  by a wideband spread spectrum signal and combined
  by the RAKE receiver.
• In 1961 ,The near-far problem (i.e., a high
  interference overwhelming a weaker spread
  spectrum signal) was first mentioned by Magnuski
• For cellular application spread spectrum was
  suggested by Cooper and Nettleton in 1978
• During the 1980s Qualcomm investigated DS-CDMA
  techniques, which finally led to the
  commercialization of cellular spread spectrum
  communications in the form of the narrowband CDMA
  IS-95 standard in July 1993
• Commercial operation of IS-95 systems started in
  1996.

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CDMA Past, Present, and Future

• Multiuser detection (MUD) has been subject to
  extensive research since 1986 when Verdu
  formulated an optimum multiuser detection for the
  additive white Gaussian noise (AWGN) channel,
  maximum likelihood sequence estimator (MLSE)
• During the 1990s ,wideband CDMA techniques with a
  bandwidth of 5 MHz or more have been studied
  intensively throughout the world, and several
  trial systems have been built and tested
• Based on the above description, the CDMA era is
  divided into three periods
• 1. the pioneer CDMA era
• 2. the narrowband CDMA era
• 3. the wideband CDMA era

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Table 1. CDMA era
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Evolution of Mobile communication system
N-BAND
W-BAND
Analog
Digital
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Evolution from 2G to 3G
WCDMA
GSM
GPRS
TD-SCDMA
EDGE
cdma2000-3x 1X-EVDV
IS-95A
cdma2000-1x
HRPD
IS-95B

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CDMA Concepts - Multiple Access Techniques -
Description of CDMA - Two Types of CDMA -
How does CDMA work? - DSSS Spreading
Time-Domain View - Spreading from a
Frequency-Domain View - CDMA Spread Spectrum
Payoff - CDMAs Nested Spreading Sequences
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Multiple Access Techniques

• FDMAFrequency division multiple access
• Featureeach user is allocated a unique
  frequency band or channel
• TACS AMPS
• TDMATime Division Multiple Access
• FeatureRadio spectrum is divided into time
  slots,and in each slot only one user is allowed
  to either transmit or receive.
• GSM DAMPS
• CDMACode division multiple access
• FeatureIn CDMA each user is assigned a
  unique code sequence it uses to encode its
  information-bearing signal
• IS-95/CDMA2000 /WCDMA/TD-SCDMA

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Description of CDMA

• The spreading signal is a pseudo-noise code
  sequence that has a chip rate which is greater
  than the data rate of the message.
• All CDMA users occupy the same frequency at the
  same time! Frequency and time are not used as
  discriminators.
• In cdma systems,the narrowband message signal is
  multiplied by a very large bandwidth signal
  called the spreading signal.
• CDMA operates by using CODING to
• discriminate between users.
• Each user has its own pseudorandom codeword which
  is approximately orthogonal to all other
  codewords.
• CDMA interference comes mainly from
• nearby users
• The receiver performs a time correlation
  operation to detect only the specific desired
  codeword.All other codewords appear as noise due
  to decorrelation.

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Two Types of CDMA
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How does CDMA work?

• Sender combines data with a fast spreading
• sequence, transmits spread data stream
• Receiver intercepts the stream, uses same
• spreading sequence to extract original data

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Direct Sequence CDMA
For example Data rate 240 kb/s Spreading factor
16 Chip rate 3.84 Mchip/s
Data signal bit Code signal chip
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DSSS Spreading Time-Domain View
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Spreading from a Frequency-Domain View
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CDMA Spread Spectrum Payoff
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CDMAs Nested Spreading Sequences
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Principle of spread-spectrum multiple access

• Code signal consists of a number of code bits
  called "chips" that can be either 1 or 1.
• Chip rate of the code signal must be much higher
  than the rate of the information signal.
• In this figure, 10 code chips per information
  symbol are transmitted (the code chip rate is 10
  times the data rate) so the processing gain is
  equal to 10.

Block diagram of a DS-SS transmitter
Generation of a BPSK-modulated SS signal
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Spreading the Spectrum
Original narrowband bit rate information
Fc Carrier frequency rb Bit rate rc Chip rate
Encoded wideband signal at symbol rate
Frequency
Fc
Fc rc
Fc rb
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Receiver of a DS-SS signal
Receiver of a DS-SS signal

• The receiver uses coherent demodulation to
  despread the SS signal, using a locally generated
  code sequence.
• To be able to perform the despreading operation,
  the receiver must not only know the code sequence
  used to spread the signal, but the codes of the
  received signal and the locally generated code
  must also be synchronized.
• This synchronization must be accomplished at the
  beginning of the reception and maintained until
  the whole signal has been received.
• The code synchronization/tracking block performs
  this operation
• After despreading a data modulated signal
  results, and after demodulation the original data
  can be recovered.

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Spreading Sequences

• Sequence properties
• High ACF peak
• Low ACF sidelobe ?inter-symbol interference
  (ISI)
• Low CCF ?multi-user interference (MUI)

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Block of DS-SS communication system
B
A
C
Information Demodulation
Spreading Modulation
Information Modulation
Despreading
b(t)
C(t)
fc
fc
Interference
PN
PN
A Point
B Point
C Point
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• Spreading spectrum Sequences(PN)
• - Property of PN
• - m-sequences
• -Walsh Code

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Property of PN

• Correlation - The Rule of Sequence design
• The ideal Sequence
• The side peak value of auto-correlation(ACF) is
  zero
• The value of cross-correlation(CCF) is zero
• Ideal Sequence can eliminate co-channel
• interference(MAI)
• Unfortunately,We cant find the ideal
  sequence
• So,our target is to find such sequence
• The smaller of the side peak value of
  auto-correlation and the value of
  cross-correlation ,the better of the sequence.

Auto-correlation Function
Cross-correlation Function
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Definition of ACF and CCF

Periodic sequence The Periodic ACF The
Periodic CCF
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m-sequences

• Good periodic ACF properties
• Bad periodic CCF properties
• CDMA operates by using different offset of the
  same m-sequence to discriminate between users for
  the bad periodic CCF properties
• In IS-95 and IS2000 standard ,two kinds of
  m-sequences are used
• short code and long code
• The m-sequences are generated using shift
  register
• The normative ACF of m-sequence

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Spreading / Despreading
Despreading of mobile 1
Terminal transmission
Base station reception
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Interference Rejection
Before despreading
After despreading
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An Example of periodic ACF of m-sequence

• length 15 m-sequence (- - - - - - - -
  )
• here , - means 1,means -1

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Introduction of the short code
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Introduce of the long code
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Walsh Code- orthogonal sequence
Walsh codes are generated by applying Hadamard
transform upon 0 repeatedly. Hadamard transform
is given by

• most important feature Orthogonal
• Walsh Sequence is Orthogonal when
  synchronized.
• ACF and CCF of Walsh Sequence are not
  ideal when not synchronized.
• Walsh function Set is self-contained.
• Walsh Sequence with Length n can
  constitute n Sequences orthogonal one
  another.

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Features of CDMA- Multiple Access Capability -
Protection Against Multipath Interference -
Privacy - Interference Rejection - Anti-Jamming
Capability, Especially Narrowband Jamming -
Low Probability of Interception(LPI)
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Multiple Access Capability

• If multiple users transmit a spread-spectrum
  signal at the same time, the receiver will still
  be able to distinguish between the users provided
  each user has a unique code that has a
  sufficiently low cross-correlation with the other
  codes.
• Correlating the received signal with a code
  signal from a certain user will then only
  despread the signal of this user, while the other
  spread-spectrum signals will remain
• spread over a large bandwidth.
• Within the information bandwidth
• the power of the desired user will be
• larger than the interfering power
• provided there are not too many
• interferers, and the desired signal
• can be extracted.
• At the receiver 1 only the signal of
• user 1 is "despread" and the data
• recovered.

Principle of spread-spectrum multiple access
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Protection Against Multipath Interference

• The signals of the different paths
• are all copies of the same transmitted
• signal but with different amplitudes,
• phases, delays, and arrival angles.
• Adding these signals at the receiver
• will be constructive at some of the
• frequencies and destructive at others. In
  the time domain, this results in a dispersed
  signal.
• If the code sequence has an ideal autocorrelation
  function, then the correlation function is zero
  outside the interval Tc,Tc, where Tc is the
  chip duration. This means that if the desired
  signal and a version that is delayed for more
  than 2Tc are received, coherent demodulation will
  treat the delayed version as an interfering
  signal, putting only a small part of the power in
  the information bandwidth.

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Privacy

• Privacy -- The transmitted signal can only be
  despread and the data recovered if the code is
  known to the receiver.

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Interference Rejection

• Cross-correlating the code signal with a
  narrowband signal will spread the power of the
  narrowband signal thereby reducing the
  interfering power in the information bandwidth.
• The spread-spectrum signal (s) receives a
  narrowband interference (i). At the receiver the
  SS signal is "despread" while the interference
  signal is spread, making it appear as background
  noise compared to the despread signal.

Interference rejection
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Anti-Jamming Capability

• This is more or less the same as interference
  rejection except the interference is now
  willfully inflicted on the system. It is this
  property, together with the next one-LPI, that
  makes spread-spectrum modulation attractive for
  military applications.

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Low Probability of Interception(LPI)
S(f)
Signal

• Because of its low power density, the
  spread-spectrum signal is difficult to detect and
  intercept by a hostile listener.

Signal
f
f0
f
f0
Signal Frequency Before SS
Signal Frequency after SS
S(f)
S(f)
Signal
Noise
Noise
Signal
f
f0
f
f0
Signal Frequency Before Decoding
Signal Frequency After Decoding
Signal
Pulse Noise
Other Noise
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Features of cdma2000-1X

• Reverse Pilot Support
• FunctionPhase reference,Coherent
  demodulation
• For each cdma2000 user, either Turbo or
  Convolutional codes can be used.
• Fast 800 Hz forward and reverse link power
  control.The reverse power control subchannel can
  be divided into two independent power control
  channel,the power control rate can be 400/400bps
  or 200/600bps for FCH/SCH.
• Double the capacity vs. IS-95-A/B
• Provide higher data rates more efficiently (up to
  307.2 kbps)

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Features of cdma2000-1X

• Support for Quasi Orthogonal Functions (QOF)
  increasing available forward channels
• Quick paging channel. This allows the mobile to
  wake up for a shorter period of time before
  entering sleep mode, thereby increasing the
  standby time of phones.
• OTD(Orthogonal Transmit Diversity) and STS(Space
  Time Spreading) transmit diversity

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Power Control

• Near-far ambiguity
• Transmit just enough power to achieve min. SIR
• Open loop path loss and shadow fading
• Inner closed loop fast adjustment to achieve
  target SIR, compensates some fast fading
  (Rayleigh) and lower required SIR

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Soft and Softer Handovers

• CDMA has the ability to talk to multiple cells
  simultaneously a soft handover
• When youre in a soft handover, its likely that
  theres at least one good link into the system
  nice!
• Question What two characteristics of CDMA make
  this possible?

Soft handover region
Freq1
Freq1
Softer handover is when a terminal talks to
two sectors of the same cell
Answer Universal Frequency Re-use and Rake
Receivers
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Soft/ Softer Handover

• Soft Handover
• The terminal is connected to (at least) two cells
  
• The signals are combined in the RNC by means of
  e.g. selection combining using CRC
• Softer Handover
• The terminal is connected to two sectors within
  one cell
• More efficient combining in the uplink is
  possible like maximum ratio combining (MRC) in
  the BTS.

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Soft Handover Principle
soft handover area
Measurement Quantity
CPICH 1
CPICH 2
time
Link to 1
Link to 1 2
Link to 2
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Multipath and Fading

• Multipath signals can cancel each other, causing
  fading
• Two receive antennas largely takes care of this
  problem because different places have different
  fading profiles
• Fine for the uplink, but youve still got a
  problem on the downlink

Transmitter
Receiver
R Ant0
R Ant1
X
X
Distance
Base Station
Combiner
To receiver
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RAKE Receiver for CDMA
Correlator 1
To De-Interleaver, Viterbi Decoder
Combiner
Correlator 2
Correlator 3
Search Correlator
Multipath Delay Components ( 150 ms gt Dt gt 1ms)
Rake receiver can isolate multipath spaced gt 1
chip length. Should one path fade for a short
while, another is probably still available
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CDMA Advantages Vast Coverage

• The coverage radius is 2 times of standard GSM.
• Coverage of 1000 km2 GSM needs 200 BSs, while
  CDMA requires only 50.
• Under the same coverage conditions, the number of
  BSs is greatly decreased.

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CDMA Advantages Full Use of Spectrum

• Spectrum of 10MHz (5MHz for transmitting, and
  5MHz for receiving)
• Parameter CDMA WLL FIXED GSM
• Bandwidth 1.25MHz 0.20MHz
• Number of CF 3 25
• Frequency reuse 1 4
• Effective CF 3/13 25/46.25
  
• Voice call/CF 25 to 40 7.25
• Voice call/cell 75 to 120
  7.256.2545
• Sector/cell 3 3
• Voice call/sector 75 to 120 45/315
  
• Erlangs/sector 64 to 107E 9.01E
• Optimal situation, no guard band for GSM and
  AMPS
• 0.75 fewer than 8 voice call/CF, which is used
  for overhead (e.g., control/pilot)
• Based on 2 of air block

5 times of GSM!
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CDMA Advantages High Quality Voice
Voice quality
64k PCM
8k CDMA
present GSM
13k CDMA
8k EVRC CDMA
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CDMA Advantages Green handset
Low transmission power Accurate power control,
handoff control, voice activation
Power absorbed in the body is 2dB (0.4 mW in CDMA
and 25mW in GSM)
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Thanks