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IS-95 CDMA
YOGESH SINGH
RAHUL KAUL
IS-95 CDMA
Special Features of IS-95 CDMA
System
• Bandwidth Recycling
-Enhancing the system capacity due to the increase of reuse
efficiency.
-Achieving higher bandwidth efficiency (interference limited)
and simplifying the system planning.
-Achieving flexibility due to the bandwidth on demand.
• Power Control
-Reducing the interference and increasing the talk time of mobile
station by using the efficient power control scheme.
• Soft handoffs
-Contributing to the achievement of the diversity and reduce
the chance of loss of link midway through the conversation.
Cnt’d
• Diversity
- Taking advantage of multiple levels of diversity:
frequency diversity (spreading), spatial diversity (multiple
antennas), path diversity (rake receiver) and time diversity
(block inter leaver), all of which reduce the interference
and improve speech quality.
• Variable Rate Vocoder
- Offering high speed coding and reducing background
noise and system interference based on the detection of
the voice activity.
• Coding Technique
- Enhancing the privacy and security.
Radio Aspects
• IS-95 is an air interface standard only
• System use FDD/FDMA/CDMA
• FDD - Uplink and Downlink channels separated according to
Cellular band or PCS band regulatory requirements
• FDMA – breaks up licensed spectrum into 1.25 MHz channels
• CDMA – multiple users share a 1.25 MHz channel by using
orthogonal spreading codes (Walsh codes)
Radio Aspects
• IS-95 uses several techniques adapted from military
• Direct Sequence Spread Spectrum (DSSS)
– Narrowband signal is multiplied by very large bandwidth signal
(spreading signal)
– Spreading signal is pseudo noise code sequence with chip rate
much greater than data rate of message
– DSSS provides resistance to narrowband interference, inter-symbol
interference and low power operation
• Code Division Multiple Access
– All users, each with own codeword approximately orthogonal to all
other codeword's, can transmit simultaneously with same carrier
frequency
Cnt’d
– Receiver performs a time correlation operation to detect only
desired codeword
• Rake Receiver
– Multiple parallel receivers used to combat multi-path
interference and inter-symbol interference
Radio Aspects
Modulation Quadrature phase shift
keying or variations
Channel/Chip Rate 1.2288 Mcps
Data rate 9.6 kbps
Filtered bandwidth 1.23 MHz -> 1.25 MHz with
guard band
Coding Convolution coding
Constraint length = 9
Viterbi decoding
Interleaving With 20 ms span
Multipath Combining
• Multipath: Reflection, diffraction, and dispersion of the
signal energy caused by natural obstacles such as buildings
or hills, or multiple copies of signals sent intentionally (e.g.,
soft handoff)
• Rake receiver used to combine different path components:
each path is dispread separately by “fingers” of the Rake
receiver and then combined
• Possible due to “low autocorrelation” of spreading code
IS-95 Cdma
IS-95 Cdma
Rake Receiver
Multipath and the RAKE Receiver
Codes used in IS-95
• Walsh codes
– They are the “orthogonal codes” used to create “logical
channels” on the up/downlink (at the same time and within the
same frequency band)
• PN (pseudo-noise) codes
– They are used to distinguish between transmissions from
different cells and are generated using “linear feedback shift
registers”
– Basically a pseudo-random number generator
– They have excellent autocorrelation properties
– Two short PN codes and a long PN code are used in IS 95 that
have periods of (2^15) – 1 and (2^42) – 1
• Convolution codes for error correction
• Block codes with interleaving and error correction
Physical Channel
• A CDMA system has 1.25 MHz wideband carriers
– Carrier bandwidth in AMPS is 30 kHz
– Carrier bandwidth in GSM is 200 kHz
– Carrier bandwidth in IS-95 is 1.23 MHz – 1.25MHz with guard
band
• One CDMA carrier can contain 41 AMPS channels of
spectrum
• In Cellular Band IS-95 carrier frequencies are denoted
in terms of the AMPS channel numbers
Logical Channels
• CDMA systems define multiple channels per frequency
channel
• Pilot channel
– Provides a reference to all signals (beacon)
• Sync channel
– Used for obtaining timing information
• Paging channel
– Used to “page” the mobile terminal when there is an incoming
call
• Traffic channel
– Carries actual voice or data traffic : fundamental code channel
• Up to seven supplemental code channels
IS-95 Cdma
IS-95 Cdma
Basic Spreading Procedure on the
Forward Channel in IS-95
• Symbols are generated at different rates
• For the spread signal to be at 1.2288 Mcps, the
incoming stream
must be at: 1.2288 x (10^6)/64 = 19.2 kbps
• What happens if the incoming stream is at a lower
rate?
– Example: Incoming stream is at 4.8 kbps
– Number of chips per bit = 1.2288 x (10^6)/4.8 x 103 = 256
– End result is greater spreading
IS-95 Cdma
Pilot Channel
• It is continuously transmitted by a BS on the forward
link
– Like a “beacon” (Compare BCCH in GSM)
– Acts as the reference signal for all MSs
– Used in demodulation and coherent detection
– Used to measure RSS for handoff and open loop power control
• It carries NO information but it is a very important
signal
• It has 4-6 dB higher transmit power than any other
channels
• The transmit power of the pilot channel is constant
(No power control)
• The I and Q PN sequences
– Are generated using a pseudorandom number generator of
length m = 15
– The period is (2^15) – 1 = 32767
– In time, one period is 32767 x 0.8138 μs = 26.7 ms
– Number of repetitions/second = 1/26.7 x (10^-3) = 37.5
– Number of repetitions in 2 seconds = 75
IS-95 Cdma
The Synch Channel
• The synch channel is locked to the offset of the PN
sequence used in the pilot channel
– It contains system information pertinent to the associated base
station
• Operates at a fixed data rate of 1.2 kbps
– After rate ½ convolution encoding, it becomes 2.4 kbps
– The symbols are repeated to 4.8 kbps and then transmitted
IS-95 Cdma
The Paging Channel
• Transmits control information to the MS
– Page message to indicate incoming call
– System information and instructions
• Handoff thresholds
• Maximum number of unsuccessful access attempts
• List of surrounding cells PN Offsets
• Channel assignment messages
– Acknowledgments to access requests
• It operates at either 4.8 kbps or 9.6 kbps
– It is passed through a rate ½ convolutional encoder to
go up to 9.6 kbps or 19.2 kbps
– If the output is 9.6 kbps, it is repeated to go up to
19.2 kbps
• MS chooses which slot to monitor within its cycle
based on its mobile identification number (MIN)
• The 19.2 kbps stream is block interleaved
– Block size is 20 ms (384 bits) but the information
is essentially a stream
• The data is scrambled by multiplying it with a
19.2 kbps stream generated by decimating a
long code generator output
IS-95 Cdma
Traffic Channel
• Carries user traffic and control messages to specific MSs,
dedicated exclusively to one MS
• assigned dynamically, in response to MS accesses, to specific
MS
• always carries data in 20 ms frames
• carry variable rate traffic frames, either 1, 1/2, 1/4, or 1/8 of
9600 bps or fixed 14.4 Kbps
• rate variation is accomplished by 1, 2, 4, or 8-way repetition of
code symbols, but the energy per bit approximately constant
• rate is independently variable in each 20 ms frame
• An 800 bps reverse link power control subchannel is carried on
the traffic channel by puncturing 2 from every 24 symbols
transmitted
IS-95 Cdma
IS-95 Cdma
IS-95 Traffic Channel Example
Forward Link Channel Parameters

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IS-95 Cdma

  • 3. Special Features of IS-95 CDMA System • Bandwidth Recycling -Enhancing the system capacity due to the increase of reuse efficiency. -Achieving higher bandwidth efficiency (interference limited) and simplifying the system planning. -Achieving flexibility due to the bandwidth on demand. • Power Control -Reducing the interference and increasing the talk time of mobile station by using the efficient power control scheme. • Soft handoffs -Contributing to the achievement of the diversity and reduce the chance of loss of link midway through the conversation.
  • 4. Cnt’d • Diversity - Taking advantage of multiple levels of diversity: frequency diversity (spreading), spatial diversity (multiple antennas), path diversity (rake receiver) and time diversity (block inter leaver), all of which reduce the interference and improve speech quality. • Variable Rate Vocoder - Offering high speed coding and reducing background noise and system interference based on the detection of the voice activity. • Coding Technique - Enhancing the privacy and security.
  • 5. Radio Aspects • IS-95 is an air interface standard only • System use FDD/FDMA/CDMA • FDD - Uplink and Downlink channels separated according to Cellular band or PCS band regulatory requirements • FDMA – breaks up licensed spectrum into 1.25 MHz channels • CDMA – multiple users share a 1.25 MHz channel by using orthogonal spreading codes (Walsh codes)
  • 6. Radio Aspects • IS-95 uses several techniques adapted from military • Direct Sequence Spread Spectrum (DSSS) – Narrowband signal is multiplied by very large bandwidth signal (spreading signal) – Spreading signal is pseudo noise code sequence with chip rate much greater than data rate of message – DSSS provides resistance to narrowband interference, inter-symbol interference and low power operation • Code Division Multiple Access – All users, each with own codeword approximately orthogonal to all other codeword's, can transmit simultaneously with same carrier frequency
  • 7. Cnt’d – Receiver performs a time correlation operation to detect only desired codeword • Rake Receiver – Multiple parallel receivers used to combat multi-path interference and inter-symbol interference
  • 8. Radio Aspects Modulation Quadrature phase shift keying or variations Channel/Chip Rate 1.2288 Mcps Data rate 9.6 kbps Filtered bandwidth 1.23 MHz -> 1.25 MHz with guard band Coding Convolution coding Constraint length = 9 Viterbi decoding Interleaving With 20 ms span
  • 9. Multipath Combining • Multipath: Reflection, diffraction, and dispersion of the signal energy caused by natural obstacles such as buildings or hills, or multiple copies of signals sent intentionally (e.g., soft handoff) • Rake receiver used to combine different path components: each path is dispread separately by “fingers” of the Rake receiver and then combined • Possible due to “low autocorrelation” of spreading code
  • 13. Multipath and the RAKE Receiver
  • 14. Codes used in IS-95 • Walsh codes – They are the “orthogonal codes” used to create “logical channels” on the up/downlink (at the same time and within the same frequency band) • PN (pseudo-noise) codes – They are used to distinguish between transmissions from different cells and are generated using “linear feedback shift registers” – Basically a pseudo-random number generator – They have excellent autocorrelation properties – Two short PN codes and a long PN code are used in IS 95 that have periods of (2^15) – 1 and (2^42) – 1
  • 15. • Convolution codes for error correction • Block codes with interleaving and error correction
  • 16. Physical Channel • A CDMA system has 1.25 MHz wideband carriers – Carrier bandwidth in AMPS is 30 kHz – Carrier bandwidth in GSM is 200 kHz – Carrier bandwidth in IS-95 is 1.23 MHz – 1.25MHz with guard band • One CDMA carrier can contain 41 AMPS channels of spectrum • In Cellular Band IS-95 carrier frequencies are denoted in terms of the AMPS channel numbers
  • 17. Logical Channels • CDMA systems define multiple channels per frequency channel • Pilot channel – Provides a reference to all signals (beacon) • Sync channel – Used for obtaining timing information • Paging channel – Used to “page” the mobile terminal when there is an incoming call • Traffic channel – Carries actual voice or data traffic : fundamental code channel • Up to seven supplemental code channels
  • 20. Basic Spreading Procedure on the Forward Channel in IS-95 • Symbols are generated at different rates • For the spread signal to be at 1.2288 Mcps, the incoming stream must be at: 1.2288 x (10^6)/64 = 19.2 kbps • What happens if the incoming stream is at a lower rate? – Example: Incoming stream is at 4.8 kbps – Number of chips per bit = 1.2288 x (10^6)/4.8 x 103 = 256 – End result is greater spreading
  • 22. Pilot Channel • It is continuously transmitted by a BS on the forward link – Like a “beacon” (Compare BCCH in GSM) – Acts as the reference signal for all MSs – Used in demodulation and coherent detection – Used to measure RSS for handoff and open loop power control
  • 23. • It carries NO information but it is a very important signal • It has 4-6 dB higher transmit power than any other channels • The transmit power of the pilot channel is constant (No power control) • The I and Q PN sequences – Are generated using a pseudorandom number generator of length m = 15 – The period is (2^15) – 1 = 32767 – In time, one period is 32767 x 0.8138 μs = 26.7 ms – Number of repetitions/second = 1/26.7 x (10^-3) = 37.5 – Number of repetitions in 2 seconds = 75
  • 25. The Synch Channel • The synch channel is locked to the offset of the PN sequence used in the pilot channel – It contains system information pertinent to the associated base station • Operates at a fixed data rate of 1.2 kbps – After rate ½ convolution encoding, it becomes 2.4 kbps – The symbols are repeated to 4.8 kbps and then transmitted
  • 27. The Paging Channel • Transmits control information to the MS – Page message to indicate incoming call – System information and instructions • Handoff thresholds • Maximum number of unsuccessful access attempts • List of surrounding cells PN Offsets • Channel assignment messages – Acknowledgments to access requests • It operates at either 4.8 kbps or 9.6 kbps – It is passed through a rate ½ convolutional encoder to go up to 9.6 kbps or 19.2 kbps
  • 28. – If the output is 9.6 kbps, it is repeated to go up to 19.2 kbps • MS chooses which slot to monitor within its cycle based on its mobile identification number (MIN) • The 19.2 kbps stream is block interleaved – Block size is 20 ms (384 bits) but the information is essentially a stream • The data is scrambled by multiplying it with a 19.2 kbps stream generated by decimating a long code generator output
  • 30. Traffic Channel • Carries user traffic and control messages to specific MSs, dedicated exclusively to one MS • assigned dynamically, in response to MS accesses, to specific MS • always carries data in 20 ms frames • carry variable rate traffic frames, either 1, 1/2, 1/4, or 1/8 of 9600 bps or fixed 14.4 Kbps • rate variation is accomplished by 1, 2, 4, or 8-way repetition of code symbols, but the energy per bit approximately constant • rate is independently variable in each 20 ms frame • An 800 bps reverse link power control subchannel is carried on the traffic channel by puncturing 2 from every 24 symbols transmitted
  • 34. Forward Link Channel Parameters