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Advanced Training Version - 6th Edition

Concepts of CDMA. Advanced Training Version - 6th Edition. Cellular Access Methods. Power. Time. Time. Power. FDMA. Frequency. Power. Time. Frequency. CDMA. TDMA. Frequency. 45 MHz. 45 MHz. CDMA is Also Full Duplex. US Cellular Channel 384. Amplitude. Forward Link.

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Advanced Training Version - 6th Edition

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  1. Concepts of CDMA Advanced Training Version - 6th Edition

  2. Cellular Access Methods Power Time Time Power FDMA Frequency Power Time Frequency CDMA TDMA Frequency

  3. 45 MHz 45 MHz CDMA is Also Full Duplex US Cellular Channel 384 Amplitude Forward Link Reverse Link AMPS Frequency 836.52 MHz 881.52 MHz Amplitude Reverse Link Forward Link CDMA Frequency 836.52 MHz 881.52 MHz

  4. Cellular Frequency Reuse Patterns 1 1 3 2 1 1 6 4 2 1 1 1 6 5 1 1 7 1 FDMA Reuse CDMA Reuse

  5. 42 calls (Using 1.5 MHz BW) CDMA = CDMA Capacity Gains (1) (1) (Chan BW) (Fr) Capacity = × × × Processing Gain Processing Gain (S/N) (Vaf) (Data Rate) (1,230,000) (1) (1) CDMA = (0.67) × × × (9,600) (5.01) (.40) AMPS = 1.5 MHz ÷ 30 kHz = 50 Channels Capacity = 50 Channels ÷ 7 (1/7Frequency Reuse) AMPS = 7 calls (Using 1.5 MHz BW)

  6. The CDMA Concept 10 kHz BW 1.23 MHz BW 10 kHz BW 1.23 MHz BW 0 f f 0 c c CDMA Transmitter CDMA Receiver Baseband Data Encoding & Interleaving Walsh Code Spreading Walsh Code Correlator Baseband Data Decode & De- Interleaving 9.6 kbps 19.2 kbps 19.2 kbps 1228.8 kbps 1228.8 kbps 9.6 kbps -113 dBm/1.23 MHz Spurious Signals 1.23 MHz BW 1.23 MHz BW f f f f c c c c External Interference Other Cell Interference Background Noise Other User Noise Interference Sources

  7. What is Correlation ? • Is a Measure of How Well a Given Signal Matches a Desired Code • The Desired Code is Compared to the Given Signal at Various Test Times Received Signal Correlation = 1 Correlation = 0 Time Correlation = 0 Correlation = 0

  8. Analog Analog CDMA CDMA CDMA Paradigm Shift • Multiple Users on One Frequency • Analog/TDMA Try to Prevent Multiple Users Interference • Channel is Defined by Code • Analog Systems Defined Channels by Frequency • Capacity Limit is Soft • Allows Degrading Voice Quality to Temporarily Increase Capacity • Reduce Surrounding Cell Capacity to Increase a Cell's Capacity

  9. CDMA Diversity • Spatial Diversity • Making Use of Differences in Position • Frequency Diversity • Making Use of Differences in Frequency • Time Diversity • Making Use of Differences in Time

  10. CDMA Spatial Diversity • Diversity Reception: • Multiple Antennas at Base Station • Each Antenna Is Affected by Multipath Differently Due to Their Different Location • Allows Selection of the Signal Least Affected by Multipath Fading • If Diversity Antennas Are Good, Why Not Use Base Stations as a Diversity Network? • Soft Handoff

  11. Spatial Diversity During Soft Handoff MTSO Land Link Vocoder / Selector Base Station 1 Base Station 2

  12. CDMA Frequency Diversity • Combats Fading, Caused by Multipath • Fading Acts like Notch Filter to a Wide Spectrum Signal • May Notch only Part of Signal Amplitude 1.23 MHz BW Frequency

  13. CDMA Time Diversity • Rake Receiver to Find and Demodulate Multipath Signals • Data is Interleaved • Spreads Adjacent Data in Time to Improve Error Correction Efficiency • Convolutional Encoding • Adds Error Correction and Detection • Viterbi Decoding • Most Likely Path Decoder for Convolutionaly Encoded Data

  14. Why Interleaving Works Original Data Frame Errors/Time TX 14 13 1 4 5 7 8 9 12 2 3 6 10 11 15 16 Errors/Time RX 14 13 1 4 5 7 8 9 12 2 3 6 10 11 15 16 Interleaved Data Frame Interleaved Data Frame Errors/Time TX 1 2 7 9 13 10 14 3 15 4 8 16 5 6 11 12 Errors/Time RX 14 13 1 4 5 2 3 7 8 9 11 12 16 6 10 15

  15. Amplitude Time Frequency The Rake Receiver

  16. Antenna Delay Taps T T T T T 0 1 2 3 4 Tap Weights W W W W W 0 1 3 4 2 + Output Rake Receiver Design

  17. Synchronization • All Direct Sequence, Spread Spectrum Systems Should be Accurately Synchronized for Efficient Searching • Finding the Desired Code Becomes Difficult Without Synchronization

  18. Reverse Link Power Control • Maximum System Capacity is Achieved if: • All Mobiles are Powered Controlled to the Minimum Power for Acceptable Signal Quality • As a result, all Mobiles are Received at About Equal Power at the Base Station Independent of Their Location • There are Two Types of Reverse Control: • Open Loop Power Control • Closed Loop Power Control • Open & Closed Loop Power Control are Always Both Active !

  19. Open Loop Power Control • Assumes Loss is Similar on Forward and Reverse Paths • Receive Power+Transmit Power = -73 • All powers in dBm • Example: • For a Received Power of -85 dBm • Transmit Power = (-73) - (-85) • Transmit Power = +12 dBm • Provides an Estimate of Reverse TX Power for Given Propagation Conditions

  20. Closed Loop Power Control • Directed by Base Station • Updated Every 1.25 msec • Commands Mobile to Change TX Power in +/-1 dB Step Size • Fine Tunes Open Loop Power Estimate • Power Control Bits are "Punctured" over the Encoded Voice Data • Puncture Period is two 19.2 kbps Symbol Periods = 103.6 usec

  21. CDMA Variable Rate Speech Coder • DSP Analyzes 20 Millisecond Blocks of Speech for Activity • Selects Encoding Rate Based On Activity: • High Activity: Full Data Rate Encoding (9600 bps) • Some Activity: Half Data Rate Encoding (4800 bps) • Low Activity: Quarter Date Rate Encoding (2400 bps) • No Activity: 1/8 Data Rate Encoding (1200 bps) • How Does This Improve Capacity? • Mobile Transmits in Bursts of 1.25 ms • System Capacity Increases by 1/Vaf

  22. Mobile Power Bursting • Each Frame is Divided Into 16 Power Control Groups • Each Power Control Group Contains 1536 Chips (represents 12 encoded voice bits) • Average Power Is Lowered 3dB for Each Lower Data Rate CDMA Frame = 20 ms Full Rate Half Rate Quarter Rate Eighth Rate

  23. Base Station Variable Rate Vocoder • Base Stations Do Not Pulse TX Channels • How Does the Base Station Handle Variable Rate Vocoding ? • Repeats Data Bits When Transmitting at Reduced Rates • Repeating Data Adds 3 dB Coding Gain • Lowers the TX Power 3 dB for Each Lower Rate

  24. Forward Link Traffic Channel Physical Layer Power Control Puncturing Vocoded Speech data Convolutional Encoder 1.2288 Mbps Interleaver I Short Code 800 bps Walsh Cover Long Code Scrambling 9.6 kbps 19.2 kbps 1/2 rate 1.2288 Mbps P.C. MUX I FIR 3/4 rate 19.2 kbps 19.2 kbps 19.2 kbps 19.2 kbps Short Code Scrambler 14.4 kbps Q FIR 1.2288 Mbps 20 msec blocks 19.2 kbps 800 bps Long Code Walsh Code Generator Q Short Code 1.2288 Mbps

  25. CDMA Vocoders • Vocoders Convert Voice to/from Analog Using Data Compression • There are Three CDMA Vocoders: • IS-96A Variable Rate (8 kbps maximum) • CDG Variable Rate (13 kbps maximum) • EVRC Variable Rate (improved 8 kbps) • Each Has Different Voice Quality: • IS-96A - moderate quality • EVRC - near toll quality • CDG - toll quality

  26. 14400 bps Frame Mixed Mode bit 7200 bps Frame Mixed Mode bit 3600 bps Frame Mixed Mode bit 1800 bps Frame Mixed Mode bit CDMA Frame Formats 288 bits in a 20 ms Frame 192 bits in a 20 ms Frame 9600 bps Frame 266 8 171 12 8 12 CRC CRC Mixed Mode bit Information Bits Information Bits 1-bit Reserved or Frame Erasure Encoder Tail Bits Encoder Tail Bits 144 bits in a 20 ms Frame 96 bits in a 20 ms Frame 4800 bps Frame 124 79 8 8 8 10 CRC CRC Mixed Mode bit Information Bits Information Bits 1-bit Reserved or Frame Erasure Encoder Tail Bits Encoder Tail Bits 48 bits in a 20 ms Frame 72 bits in a 20 ms Frame 2400 bps Frame 54 39 8 8 8 CRC Mixed Mode bit Encoder Tail Bits Information Bits Information Bits 1-bit Reserved or Frame Erasure Encoder Tail Bits 36 bits in a 20 ms Frame 24 bits in a 20 ms Frame 1200 bps Frame 20 15 8 8 6 CRC Mixed Mode bit Information Bits Information Bits Encoder Tail Bits 1-bit Reserved or Frame Erasure Encoder Tail Bits

  27. D D D D D D D D Forward Error Protection • Uses Half-Rate Convolutional Encoder • Outputs Two Bits of Encoded Data for Every Input Bit Data Out 9600 bps + Data In 9600 bps + Data Out 9600 bps

  28. 14.4 TCH Forward Link Modifications • Replaces 8 kbps Vocoder with a 13 kbps Vocoder (both Variable Rate) • Effects: • Provides Toll Quality Speech • Uses a 3/4 Rate Encoder • Reduces Processing Gain 1.76 dB • Results in Reduced Capacity or Smaller Cell Sizes Vocoded Speech data Convolutional Encoder 3/4 rate 14.4 kbps 19.2 kbps 20 msec blocks

  29. 12 11 1 10 2 3 9 4 8 5 7 6 CDMA System Time • How Does CDMA Achieve Synchronization for Efficient Searching ? • Use GPS Satellite System • Base Stations Use GPS Time via Satellite Receivers as a Common Time Reference • GPS Clock Drives the Long Code Generator

  30. Long Code Generation Long Code Output Modulo-2 Addition User Assigned Long Code Mask 42 bits 42 41 3 2 5 4 1 Long Code Generator

  31. Long Code Scrambling • User's Long Code Mask is Applied to the Long Code • Masked Long Code is Decimated Down to 19.2 kbps • Decimated Long Code is XOR'ed with Voice Data Bits • Scrambles the Data to Provide Voice Security XOR Encoded Voice Data 19.2 kbps 19.2 kbps 19.2 kbps Long Code Decimator Long Code Generator 1.2288 Mbps

  32. Closed Loop Power Control Puncturing • Long Code is Decimated Down to 800 bps • Decimated Long Code Controls the Puncture Location • Power Control Bits Replace Voice Data • Voice Data is Recovered by the Mobile's Viterbi Decoder Closed Loop Power Control Bits 800 bps Long Code Scrambled Voice Data 19.2 kbps 19.2 kbps P.C. Mux 800 bps Long Code Decimator Long Code Decimated Data 19.2 kbps

  33. Walsh Codes W = 0 1 0 0 0 1 W = W W W W 2 n n W = 0 0 0 0 0 1 0 1 0 0 1 1 0 1 1 0 2n n n W = 4

  34. Cross Correlation N agreements - N disagreements = N total_number_of_digits 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0 1 0 0 1 1 0 1 1 0 W = Y Y N N 4 Checking for Orthogonality 2 match - 2 don't =0

  35. Walsh Code Spreading XOR Encoded Voice Data 19.2 kbps 1.2288 Mbps What is the Spreading Rate Increase ? 1.2288 Mbps Walsh Code Generator

  36. I Channel Short Sequence Code Generator Why Spread Again with the Short Sequence ? • Provides a Cover to Hide the 64 Walsh Codes • Each Base Station is Assigned A Time Offset in its Short Sequences • Time Offsets Allow Mobiles to Distinguish Between Adjacent Cells • Also Allows Reuse of All Walsh Codes in Each Cell 1.2288 Mbps Walsh Coded Data at 1.2288 Mbps To I/Q Modulator Q Channel Short Sequence Code Generator 1.2288 Mbps

  37. Pseudo-Random Sequence 1 0 -1 chip 0 20 10 15 25 30 5 Auto-Correlation Versus Time Offset 0 0 20 10 15 25 30 5 Auto-Correlation • Is a Comparison of a Signal Against Itself • Good Pseudo-Random Patterns Have: • Strong Correlation at Zero Time Offset • Weak Correlation at Other Time Offsets chip offset

  38. Short Code Correlation • Short Codes Are Designed to Have: • Strong Auto-Correlation at Zero Time Offset • Weak Auto-Correlation at Other Offsets • Good Auto-Correlation In Very Poor Signal-to -Noise Ratio Environments • Allows Fast Acquisition in Real World Environment Auto-Correlation Versus Time Offset with 17 dB Noise Added 0 0 20 10 15 25 30 5 chip offset

  39. Forward Link Channel Format Walsh Code 0 I Data Convert to I/Q & Short Code Spreading All 0's FIR LP Filter & D/A Conversion 1228.8 kbps Pilot Channel Q Data Walsh Code 32 I I Data Convert to I/Q & Short Code Spreading FIR LP Filter & D/A Conversion 4.8 kbps 1228.8 kbps Sync Channel Q Data Walsh Codes 1 to 7 I Data Convert to I/Q & Short Code Spreading 1228.8 kbps 19.2 kbps FIR LP Filter & D/A Conversion Paging Channels 1 up to 7 Channels Q Data Walsh Codes 8-31, 33-63 Q I Data Convert to I/Q & Short Code Spreading FIR LP Filter & D/A Conversion 19.2 kbps 1228.8 kbps Traffic Channels 1 up to 55 Channels Q Data 1228.8 kbps

  40. +1 +1 -1 -1 0 0 +1 +1 1 1 -1 -1 +1 +1 -1 -1 0 0 1 1 0 0 0 0 1 0 0 1 0 1 1 0 Walsh Coding Example User A User B 0 0 0 1 - User A For a 0 Input, Use Code 00 W = For a 0 Input, Use Code 01 2 - User B 0 1 For a 1 Input, Use Code 11 For a 1 Input, use code 10 1 0 +1 +1 Channel A Voice Data Channel B Voice Data 0 0 + 0 1 0 0 1 0 0 1 Channel A Walsh Encoded Voice Data Channel B Walsh Encoded Voice Data Sum of A & B Walsh Encoded Data Streams +2 +1 -1 -2

  41. f f z = (t) dt (t) j i ij +2 +1 -1 -2 Walsh Decoding Example Original User B Voice Data Original User A Voice Data Correlation Coefficient +1 +1 T  1 0 0 1 0  0 1 0 1 0 0 User A + B Walsh Data User A + B Walsh Data T +2 0 +1 -1 -2 Multiply Summed Data with Desired Walsh Code Multiply Summed Data with Desired Walsh Code +2 +2 +2 +2 -  1 +1 +1  +1 1 +1 +1 +1 = = x = x = -1 -1 -1 -1 -1 -1 1 1 0 1 -2 -2 -2 -2

  42. +1 +1 -1 -1 0 0 1 1 0 0 0 0 1 0 0 1 0 1 1 0 +2 +1 -1 -2 What if Walsh Codes are Not Time Aligned ? Original Time Delayed + Channel A Walsh Encoded Voice Data Channel B Walsh Encoded Voice Data Sum of A & B Walsh Encoded Data Streams Multiply Summed Data with Desired Walsh Code Original Data Was 0 (-1), We Have Interference Now! +2 +2 -  +1 +1 +1 = = x 0.75 -1 -1 -1 1 1 -2 -2

  43. Pilot Channel Physical Layer • Uses Walsh Code 0: • All 64 bits are 0 • All Data into Walsh Modulator is 0 • Output of Walsh Modulator is Therefore all 0's • Pilot Channel is just the Short Codes 1.2288 Mbps Walsh Modulator I Short Code 1.2288 Mbps All 0 input I FIR Short Code Scrambler Q FIR 1.2288 Mbps Walsh Code Generator Q Short Code 1.2288 Mbps Walsh Code 0

  44. 2x Sync Channel Physical Layer Sync Channel Message Data 1.2288 Mbps Walsh 32 Cover Convolutional Encoder Symbol Repetition I Short Code Interleaver 1.2288 Mbps 1/2 rate I FIR 4.8 kbps 1.2 kbps 2.4 kbps Short Code Scrambler 4.8 kbps Q FIR 1.2288 Mbps Walsh Code Generator Q Short Code 1.2288 Mbps

  45. Long Code Scrambling 2x 19.2 kbps Paging Channel Long Code Paging Channel Physical Layer Paging Channel Message Data 1.2288 Mbps Convolutional Encoder Symbol Repetition Walsh 1 to 7 Cover I Short Code Interleaver 1.2288 Mbps 1/2 rate I FIR 19.2 kbps 19.2 kbps 4.8 kbps Short Code Scrambler 9.6 kbps 19.2 kbps Q FIR 1.2288 Mbps Walsh Code Generator Q Short Code 1.2288 Mbps

  46. 1/3 rate Reverse Link Traffic Channel Physical Layer 64-ary Modulator 1.2288 Mbps Long Code Modulator Convolutional Encoder 1 of 64 Walsh Codes I Short Code Vocoded Speech Data 1.2288 Mbps Walsh Code 63 Interleaver I Walsh Code 62 28.8 kbps FIR 9.6 kbps 307.2 kbps Walsh Code 61 Short Code Scrambler Q FIR t/2 1/2 rate Walsh Code 2 1/2 Chip Delay 14.4 kbps 28.8 kbps 28.8 kbps Walsh Code 1 Q Short Code 20 msec blocks Walsh Code 0 1.2288 Mbps 1.2288 Mbps Long Code

  47. D D D D D D D D Reverse Error Protection • Uses Third-Rate Convolutional Encoder • Outputs Three Bits for Every Input Bit Data Out 9600 bps + Data In 9600 bps + Data Out 9600 bps + Data Out 9600 bps

  48. 14.4 TCH Reverse Link Modifications • Replaces 8 kbps Vocoder with a 13 kbps Vocoder (both Variable Rate) • Effects: • Provides Toll Quality Speech • Uses a 1/2 Rate Encoder • Reduces Processing Gain 1.76 dB • Results in Reduced Capacity or Smaller Cell Sizes Vocoded Speech data Convolutional Encoder 1/2 rate 14.4 kbps 28.8 kbps 20 msec blocks

  49. Walsh Code 63 Walsh Code 62 Walsh Code 61 307.2 kbps Walsh Code 2 28.8 kbps Walsh Code 1 Walsh Code 0 64-ary Modulation • Every 6 Encoded Voice Data Bits Points to One of the 64 Walsh Codes • Spreads Data From 28.8 kbps to 307.2 kbps: • (28.8 kbps * 64 bits)/ 6 bits = 307.2 kbps) • Is Not the Channelization for the Reverse Link

  50. Why Aren't Walsh Codes Used for Reverse Channelization ? • All Walsh Codes Arrive Together in Time to All Mobiles From the Base Station • However, Transmissions from Mobiles DO NOT Arrive at the Same Time at the Base Station

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