Ch1 Fundamental of CDMA

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Ch1 Fundamental of CDMA

  1. 1. Course Outline• Introduction• Ch1: Fundamental Of CDMA• Ch2: UMTS Evolution & Air Interface.• Ch3: The UMTS Network.• Ch4: UMTS Transmission Chain.• Ch5: UMTS Protocols• HSPA Introduction
  2. 2. Transmission Medium
  3. 3. As AMPS and NMT
  4. 4. As GSM and IS-136
  5. 5. FDMA TDMA •low cost hardware technology •Permits a flexible bit rate •No need for network timingAdvantages •Easy for mobile or base stations to •No restriction regarding the type of base initiate and execute hands off band (voice or data) or type of modulation •The presence of guard bands •Requires network-wide timing •Requires Ideal RF filtering to synchronizationDisadvantages minimize adjacent channel interference •Requires signal processing for advancing
  6. 6. Why do we need a new technology? Band Saturation.Capacity (more than ½ The population of the globe use mobilephones). More BW efficiency. New service.
  7. 7. What about SDMA?
  8. 8. Switched Beam System Adaptive Array Desired Active User Beam Antenna Interfering Array User Antenna Array
  9. 9. Signal Spreading Fast Frequency Hopping
  10. 10. 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.
  11. 11. Spread Spectrum Anti-Jamming System S(f) S(f) Signal Signal f0 f f0 f Before spreading After spreadingS(f) S(f) Interfering noise Signal signal Interfering noise f0 f Before despreading f0 f After despreading
  12. 12. CDMA Spread Spectrum NO More necessaryAs Channel BW get smaller  More Capacity
  13. 13. Sharing the frequency by using Different codes
  14. 14. Signal Spreading Preview Fast Frequency Hopping This approach assumes that the two stations have: A common knowledge of the frequency hopping plan. Synchronized clocks
  15. 15. CDMA optionsDirect sequence and freq hopping CDMA
  16. 16. 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
  17. 17. The Strengths of CDMA Increased system capacity. Cell sites can be larger. Enhanced privacy due to digital coding of CDMA signals. Enhanced call quality. Lower transmit RF power levels, longer battery life, and increased talk time for hand-held units. Simplified System Planning. Bandwidth on Demand
  18. 18. 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).
  19. 19. Bit Stream Multiplication Concepts b(t) · c(t) = y(t) b(t) · c(t) · c(t) = b(t)
  20. 20. Bit Stream Multiplication Concepts Consider the difference between spreading and scrambling:
  21. 21. 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
  22. 22. Bit Stream Multiplication Concepts Receiver and Transmitter use identical code at same time offset Input Data +1 -1 +1 x x x PN code used in Transmitter +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 Transmitter = = = Transmitted +1 –1 +1 +1 –1 -1 +1 -1 -1 +1 -1 -1 +1 +1 -1 +1 +1 –1 +1 +1 –1 -1 +1 -1 Sequence x x x PN Code +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1Used in Receiver = = = +1 +1 +1 +1 +1 +1 +1 +1 -1 –1 –1 –1 –1 –1 –1 -1 +1 +1 +1 +1 +1 +1 +1 +1 Receiver Integrate Integrate Integrate Integrate Result +8 -8 +8 Divide by Code Length +1 -1 +1
  23. 23. Coding Concept… Receiver and Transmitter use two uncorrelated codes at same time offset Input Data +1 -1 +1 x x xOrthogonal code in Transmitter +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 Transmitter = = = Transmitted +1 –1 +1 +1 –1 -1 +1 -1 -1 +1 -1 -1 +1 +1 -1 +1 +1 –1 +1 +1 –1 -1 +1 -1 Sequence x x x Orthogonal +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 different Codeused in Receiver = = = +1 –1 +1 +1 –1 -1 +1 -1 -1 +1 -1 -1 +1 +1 -1 +1 +1 –1 +1 +1 –1 -1 +1 -1 Receiver Integrate Integrate Integrate Integrate Result 0 0 0 Divide by Code Length 0 0 0
  24. 24. 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: Fb =1/Tb (the bit rate of the input signal) Fc =1/Tc (the chip rate of the spreading code) G (processing gain) = Fc/Fb =Tb/Tc G = Spreading factor
  25. 25. Scrambling and Descrambling Scrambling is often used for privacy. When scrambling is used for encryption, then descrambling is used for decryption.
  26. 26. Repeated Spreading and ScramblingCDMA systems use scrambling and spreading in several ways for: Encryption for privacy Channel identification Base station identification
  27. 27. PN Codes A binary sequence with fixed length and has noise-like randomness Nearly equal number of zeros and ones 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
  28. 28. Code Generation Circuitry Long code = 242-1 Short code = 2 15-1
  29. 29. Code Generation CircuitryD D DFlip Flop Flip Flop Flip Flop
  30. 30. Quick Review
  31. 31. 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. Chip = .81 µsec
  32. 32. RF Transmission Delay and its Effect
  33. 33. Locking On to a Particular PN Sequence Cont.
  34. 34. Locking On to a Particular PN Sequence Cont.
  35. 35. Locking On to a Particular PN Sequence Cont.
  36. 36. Signal Correlation With Offsets Less Than 1 Time Chip
  37. 37. Signal Correlation With Offsets Less Than 1 Time Chip
  38. 38. Multi-User Downlink Overview
  39. 39. Multi-User Downlink Overview
  40. 40. Message detectionC3
  41. 41. Multi-User Uplink Overview
  42. 42. Interference limited systemTraditional Systems Performance is measured by signal-to-noise ratio (S/N). The desired users signal versus noise.CDMA Systems Performance is measured by signal-to- interference ratio (S/I). The desired users signal versus interference from other users signals.
  43. 43. Interference limited system
  44. 44. Multipath and Delay Spread
  45. 45. Non-RAKE CDMA Reception
  46. 46. RAKE Receiver Operation
  47. 47. Related Terms and Definitions Term Definition Narrow Band Signal Signal occupies a relatively small bandwidth i.e. (GSM signal has 200KHz bandwidth) Wide Band Signal Signal occupies relatively wide bandwidth i.e. (WCDMA signal has 5 MHz bandwidth) Pseudo Noise Signal Signal has a noise like behaviour - actual noise never repeats - Spreading Converting a signal with low bit rate into another signal with much higher bit rate Scrambling Converting a signal into another coded version of it keeping the same bit rate
  48. 48. Related Terms and Definitions Term Definition  Auto Correlation Measurement for how much a signal is related to another version of itself  Cross Correlation Measurement for how a signal is related to another different signal Orthogonal Codes Codes has Auto Correlation = 1 and Cross Correlation = 0 Pseudo Noise Codes Codes has Auto Correlation very close to 1 and Cross Correlation very close to 0

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