102L8cdma - CDMA 101 Scott Baxter CDMA 101 Outline CDMA...

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CDMA 101 Scott Baxter
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CDMA 101 Outline CDMA Basics u Multiple Access Technology Survey u CDMA coding principles u Spread Spectrum principles u Forward and Reverse Channel Structure Nortel CDMA System Architecture u MTX BSC BSM BTS CDMA Details and Operation u Power Control u Handoff mechanics u Optimization concepts
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CDMA Basics
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Multiple Access Technologies FDMA (example: AMPS) Frequency Division Multiple Access u each user has a private frequency TDMA (examples: IS-54/136, GSM) Time Division Multiple Access u each user has a private time on a private frequency CDMA (IS-95, J-Std. 008) Code Division Multiple Access u users co-mingle in time and frequency but each user has a private code Frequency Time Power Power Power FDMA TDMA CDMA
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Other Technologies: Other Technologies: Avoiding Interference Avoiding Interference AMPS, TDMA and GSM depend on physical distance separation to keep interference at low levels Co-channel users are kept at a safe distance by careful frequency planning Nearby users and cells must use different frequencies to avoid interference 2 3 4 56 7 4 6 4 72 7 2 5 3 5 3 6 1 1 1 1 1 1 1 AMPS-TDMA-GSM Figure of Merit: C/I (carrier/interference ratio ) AMPS: +17 dB TDMA: +14 to 17 dB GSM: +7 to 9 dB.
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CDMA: Using A New Dimension CDMA: Using A New Dimension All CDMA users occupy the same frequency at the same time! Time and frequency are not used as discriminators CDMA interference comes mainly from nearby users CDMA operates by using CODING to discriminate between users Each user is a small voice in a roaring crowd -- but with a uniquely recoverable code CDMA Figure of Merit: C/I (carrier/interference ratio ) AMPS: +17 dB TDMA: +14 to +17 dB GSM: +7 to 9 dB. CDMA: -10 to -17 dB. CDMA: E b /N o ~+6 dB.
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CDMA Uses Code Channels CDMA Uses Code Channels A CDMA signal uses many chips to convey just one bit of information Each user has a unique chip pattern, in effect a code channel To recover a bit, integrate a large number of chips interpreted by the user’s known code pattern Other users’ code patterns appear random and integrate toward low values, hence don’t disturb the bit decoding decision Building a Building a CDMA Signal CDMA Signal Bits from User’s Vocoder Symbols Chips Forward Error Correction Coding and Spreading
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CDMA is a Spread-Spectrum System Traditional technologies try to squeeze signal into minimum required bandwidth CDMA uses larger bandwidth but uses resulting processing gain to increase capacity Spread Spectrum Payoff : Processing Gain Spread Spectrum TRADITIONAL COMMUNICATIONS SYSTEM Slow Information Sent TX Slow Information Recovered RX Narrowband Signal SPREAD-SPECTRUM SYSTEM Fast Spreading Sequence Slow Information Sent TX Slow Information Recovered RX Fast Spreading Sequence Wideband Signal
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Spreading: What we do, we can undo 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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102L8cdma - CDMA 101 Scott Baxter CDMA 101 Outline CDMA...

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