cdma tutorial

Edmund Michael

Description

CDMA Tutorial April 29, 2006Michael SouryalApril 29, 2006 Common Components Encoding, modulation, spreading Common Features/Functionality Power control, diversity, soft handoff System Particulars cdmaOne (IS-95) cdma2000Sources:1. V. Vanghi, A. Damnjanovic, and B. Vojcic, The cdma2000 System for Mobile Communications (Prentice Hall PTR, 2004).2. V. K. Garg, IS-95 CDMA and cdma2000 (Prentice Hall PTR, 2000).2M. Souryal 1CDMA Tutorial April 29, 2006 A way for multiple users to share the channelTime Division Frequency Division Code Division Multiple Access Multiple Access Multiple Access3 InputData FEC Interleaver Modulator SpreaderEncoderChannelOutputDataFECDeinterleaver Demodulator DespreaderDecoder4M. Souryal 2CDMA Tutorial April 29, 2006 Standard component of most digital communications systems Especially important in fading and interference channels FEC encoder adds coded redundancy to the information data stream Decoder uses the redundancy to correct errors caused by channel impairments (FEC a.k.a. “channel coding”) Benefits Ability to operate at a lower bit error rate (BER), for a given signal-to-noise-and-interference ratio (SNIR) Ability to operate at a lower SNIR for a given BER (“coding gain”) Ability to tolerate more users in a multiuser system5 Costs ...

Informations

• Publié par: Omko
• Nombre de lectures: 50
• Langue: English

Extrait

CDMA Tutorial

M. Souryal

 



Michael Souryal April 29, 2006

 Common Components  Encoding, modulation, spreading  Common Features/Functionality  Power control, diversity, soft handoff  System Particulars  cdmaOne (IS-95)  cdma2000 Sources: 1. V. Vanghi, A. Damnjanovic, and B. Vojcic,The cdma2000 System for Mobile Communications(Prentice Hall PTR, 2004). 2. V. K. Garg,IS-95 CDMA and cdma2000(Prentice Hall PTR, 2000). 2

April 29, 2006

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CDMA Tutorial

M. Souryal

 

 A way for multiple users to share the channel

Time Division Frequency Division Code Division Multiple Access Multiple Access Multiple Access

   

Input Data CEF redoc Mod En Interleaver ulator Spreader

Output Data FEC  Despreader rDeinterleav Demodulat Decoder er o

Channel

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CD M.  M S A o   u T r u y t a o l rial

     Standard component of most digital communications systems  Especially important in fading and interference channels  FEC encoder adds coded redundancy to the information data stream  Decoder uses the redundancy to correct errors caused by channel impairments (FEC a.k.a. “channel coding”)  Benefits  Ability to operate at a lower bit error rate (BER), for a given signal-to-noise-and-interference ratio (SNIR)  Ability to operate at a lower SNIR for a given BER (“coding gain”)  Ability to tolerate more users in a multiuser system

    Costs  Added complexity of encoder/decoder  Increased bandwidth and/or reduced data rate (exception: trellis-coded modulation)  Types of codes  Block (e.g., Golay, Reed-Solomon)  Convolutional Turbo (i.e., concatenated with iterative decoding) used in current cellular CDMA standards

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   Implemented using linear shift registers and mod-2 adders  Example: + + g 0 input output D D g 1 + rate:r=1/2 (doubles the sequence length) constraint length:K=3 generator polynomials: g 0 =(7) oct ,g 1 =(5) oct

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!"   Example:r=1/3 parallel concatenated convolutional code X (systematic bit) + + Y (parity bit) X + D D D + Inter- leaver punctured Y' + + (parity bit) + D D D + Feed-forward and feedback generator polynomials:g 1 =(15) oct ,g 0 =(13) oct 8

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#  For achieving higher code rates (reducing redundancy)  Example: Puncturing of rate 1/3 turbo code Puncturing Matrix (rate 1/2) Info. Bits c 0 1 1 RSC c 1 1 0 Π RSC c 2 0 1 Punctured output sequence: c 0,1 ,c 1,1 ,c 0,2 ,c 2,2 ,c 0,3 ,c 1,3 ,c 0,4 ,c 2,4 ,

  #$ %  10 0 Synchronous DS CDMA Simulated BER w/o FEC E b N 0 =20 dB Simulated BER with FEC -1 10 10 -2

- 10 3 -4 10 r=1/3; constr. length 9 = - 10 5 0 5 10 15 20 25 30 35 40 K (number of users)

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CDMA Tutorial

M. Souyral

 &!   Turbo Codes  Higher coding gain (resilience to noise/interference)  Longer latency (better suited for data, not voice)  Greater complexity (decoder)

   

Input Da ta En F c E o C der InterleaverModulator Spreader

Output Data FEC eDetniaelrrevDemodulator Despreader Decoder

Channel

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% '  

Temporary decrease in received energy due to fading burst errors

Time  Can be alleviated byinterleavingcoded symbols at the transmitter anddeinterleavingthem at the receiver

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() '  Coded symbols (bits) are written To Channel to the interleaver row-wise and read out to the channel column- r C N Fr nsumol decoEn om wise X 1  ... X N  Deinterleaver performs reverse X N+1  ... X 2N operation at receiver  Adjacent symbols through the channel are separated byN positions in the coded sequence  Cost:  Additional memory  Introduces delay

X N(M-1)+1  … X MN

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   

Input Data En F c E o C der InterleaverulodoratMSrpaeedr

Output Data FEC Decoder DeinterleaverrtolDaedmuoDespreader

Channel

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  Coherent Schemes  Binary Phase Shift Keying (BPSK) ctral pe   s ngsieacrIn  Quaternary Phase Shift Keying (QPSK) efficiency.  8-PSK ieqd re to Hgieh rISRN r   u  16-QAM achieve a given BER.  Non-coherent Schemes  Orthogonal signals (e.g., Walsh functions)  Differential PSK

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   

Input Data En F c E o C der InterleaverModulatoredrSprea

Output Data FEC  D De dulatoreradeDpser Decoder einterleaver mo

*

d(t)a(t) d(t) Channel A B C a(t)a(t)

Channel

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 Each signal (user) is spread with a different spreading sequence (distributes signal power over wider bandwidth).  Different spreading sequences have low cross-correlation.  Despreading recovers desired signal while leaving other signals at lower power. 18

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  $ +  Maximal Length Shift Register (MLSR) sequences  Used for generating pseudo-noise (PN) sequences with random-like properties  Long periods  Gold sequences, …  Good cross-correlation properties  Orthogonal sequences  Zero cross-correlation (when synchronous)  Example: Walsh-Hadamard

,%-.+  Generation 0 0 N N 0 , ,H H 1 = [ ] H 2 =    0 1      2N = H H N H H N     Example 0 0 0 0 H H H 4 =    2 2 2 2    = 0101 1100 0110       H H  Walsh sequences are rows of Hadamard matrix

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  Common Components  Common Features/Functionality  Power Control  Diversity  Soft Handoff  System Particulars  cdmaOne (IS-95)  cdma2000

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#   CDMA performance is sensitive to relative received powers of the signals.  If one signal is too strong, it generates too much interference to the others (near-far effect).  Goals of Power Control  Maintain equal performance for all users  Minimize transmitted power to achieve desired QoS  Types of Power Control  Open Loop  Closed Loop

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