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Introduction to CDMA

Introduction to CDMA

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Ch1 intro to cdma Presentation Transcript

  • 1. Introduction toCDMA
    By Eng. Waleed El Safoury
  • 2. Multiple Access
    Different multiple access methods use different strategies to:
     
    Divide the radio spectrum into channels (Time or Freq.)
    Allocate those channels among users (One channel or more)
    Identify different users on an RF carrier (Fixed or dynamic)
  • 3. Frequency Division Multiple Access (FDMA)
    As AMPS and NMT
  • 4. Time Division Multiple Access (TDMA)
    As GSM and IS-136
  • 5. Why Do we need new access techniques?
    • Band Saturation
    • 6. MoreBW efficiency
    • 7. Capacity (more than ½ The population of the globe use mobile phones)
  • What about SDMA?
    Pure SDMA need LOS
  • 8. CDMA Spread Spectrum
    Originally developed for military and navigation purposes
    NO More necessary
    As Channel BW get smaller  More Capacity
  • 9. CDMA Spread Spectrum
    • Researches started even from 1950’s
    • 10. Cellular spread-spectrum application was suggested by Cooper and Nettleton in 1978
    • 11. IS-95, the narrow band CDMA mobile network, has been standardized in 1993 and commercial networks were introduced in 1995
    • 12. 3G wideband CDMA systems, such as CDMA2000 in U.S. and European WCDMA developed from 1990s and still ongoing
  • CDMA Spread Spectrum
  • 13. CDMA Spread Spectrum
  • 14. CDMA Spread Spectrum
  • 15. Signal Spreading Preview
    In DS spreading, the incoming digital speech signal is multiplied by a digital pseudo-noise (PN) code through a process called bit stream multiplication.
    The result of DS spreading is an encoded, spread spectrum, CDMA signal.
  • 16. Signal Spreading Preview
    Fast frequency hopping
    This approach assumes that the two stations have:
    A common knowledge of the frequency hopping plan.
    Synchronized clocks
  • 17. Signal Spreading Preview
    Time Hopping
    Time hopping assumes that the two stations in communication have:
    A common knowledge of the time-hopping "plan."
    Synchronized clocks
  • 18. The Strengths of CDMA
    • Increased system capacity.
    • 19. Cell sites can be larger.
    • 20. Enhanced privacy due to digital coding of CDMA signals.
    • 21. Enhanced call quality
  • Bit Stream Multiplication Concepts
    Bit stream multiplication is the process where an input bit stream of information (usually a digital speech signal) called b(t) is multiplied by a pseudo-noise (PN) code called c(t) to produce a new composite output signal called y(t).
  • 22. Bit Stream Multiplication Concepts
    Consider the difference between spreading and scrambling:
  • 23. Bit Stream Multiplication Concepts
    01110101111010111101
    Data
    10110101001010101011
    Code
    10110101001010101011
    Code
    00110101001010101001
    Not Data
  • 24. Bit Stream Multiplication Concepts
    b(t) · c(t) = y(t)
    b(t) · c(t) · c(t) = b(t)
  • 25. Spreading and Despreading
    Remember that spreading occurs when a lower bit rate input signal b(t) is multiplied by a higher chip rate spreading code c(t) as shown.
    Bit
    Chip
  • 26. Spreading and Despreading Cont.
    despreading takes y(t) and multiplies again by the same spreading PN code c(t) to recover the digital information signal b(t).
  • 27. Scrambling and Descrambling
    Scrambling is often used for privacy. When scrambling is used for encryption, then descrambling is used for decryption.
  • 28. Spreading and Power Spectral Density
    Spreading does not change total power. Spreading changes how the power is distributed over frequency.
    The following formulas hold true:
     G (processing gain) = Fc/Fb
    SF(Spreading Factor)=Rc/Rb =Tb/Tc
  • 29. Orthogonallity & correlation
  • 30. PN Codes
    A binary sequence with fixed length and has noise-like randomness
    Low correlation between shifted versions of the sequence
    Low correlation with other user signals (interference) and noise
    Good autocorrelation properties with own signal in synchronization
  • 31. Code Generation Circuitry
  • 32. Code Generation Circuitry
    D
    Flip Flop
    D
    Flip Flop
    D
    Flip Flop
  • 33. Repeated Spreading and Scrambling
    CDMA systems use scrambling and spreading in several ways for:
    Encryption for privacy
    Channel identification
    Base station identification
  • 34. Repeated Spreading and Scrambling
    C5
    C1
    C2
    C1
    C3
    C4
  • 35. Quick Review
  • 36. RF Transmission Delay and its Effect
    Radio signals typically travel at a rate of about five microseconds to the mile. Assuming that a mobile is ten miles away from the base station, a 50 microsecond (µs) delay would be the result.
    192
    Chip = 260.4 nsec
  • 37. RF Transmission Delay and its Effect
  • 38. Locking On to a Particular PN Sequence Cont.
  • 39. Locking On to a Particular PN Sequence Cont.
  • 40. Locking On to a Particular PN Sequence Cont.
  • 41. Signal Correlation With Offsets Less Than 1 Time Chip
  • 42. Signal Correlation With Offsets Less Than 1 Time Chip
  • 43. Signal Correlation
  • 44. Multi-User Downlink Overview
  • 45. Multi-User Downlink Overview
  • 46. Multi-User Downlink Overview
    C3
  • 47. Multi-User Uplink Overview
  • 48. System Performance
    Traditional Systems Performance is measured by signal-to-noise ratio (S/N). The desired user's signal versus noise.
     CDMA Systems Performance is measured by signal-to-interference ratio (S/I). The desired user's signal versus interference from other users signals. 
  • 49. System Performance
  • 50. Frequency Reuse
  • 51. Multipaths and Delay Spread
  • 52. Non-RAKE CDMA Reception
  • 53. Non-RAKE CDMA Reception
    Differential Delay > 1 Tc
  • 54. Non-RAKE CDMA Reception
    Differential Delay < 1 Tc
  • 55. RAKE Receiver Operation
  • 56. RAKE Receiver
    Less multipath effect
    No ISI problem
    Better signal reception
    Less power needed