cdma tutorial
32 Pages
English

cdma tutorial

-

Downloading requires you to have access to the YouScribe library
Learn all about the services we offer

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 ...

Subjects

Informations

Published by
Reads 50
Language English
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
1
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 DataCEFredocMod EnInterleaver ulator Spreader
Output DataFEC Despreader rDeinterleav Demodulat Decoderer o
Channel
3
4
April 29, 2006
2
CDM. MSAo uTruytaolrial
   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
5
6
April 29, 20063
DCM. MSAo uTruytaoliral
 Implemented using linear shift registers and mod-2 adders Example: + + g0 input output D D g1 + rate:r=1/2 (doubles the sequence length) constraint length:K=3 generator polynomials:g0=(7)oct,g1=(5)oct
7
!" 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:g1=(15)oct,g0=(13)oct 8
April 29, 02064
CDM. MSAo uTruytaoliral
#For achieving higher code rates (reducing redundancy) Example: Puncturing of rate 1/3 turbo code Puncturing Matrix (rate 1/2) Info. Bits c01 1 RSCc11 0 Π RSCc20 1 Punctured output sequence: c0,1,c1,1,c0,2,c2,2,c0,3,c1,3,c0,4,c2,4,
  #$ % 100Synchronous DS CDMA Simulated BER w/o FEC EbN0=20 dBSimulated BER with FEC -1 10 10-2
-103 -4 10 r=1/3; constr. length 9 = -105 0 5 10 15 20 25 30 35 40 K (number of users)
9
10
Apir l92, 20065
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 taEnFcEoCderInterleaverModulator Spreader
Output DataFECeDetniaelrrevDemodulator Despreader Decoder
Channel
11
12
April 29, 2006
6
CD.M MSAo uTruytaoliral
% '  
Temporary decrease in received energy due to fading burst errors
Time Can be alleviated byinterleavingcoded symbols at the transmitter anddeinterleavingthem at the receiver
13
() 'Coded symbols (bits) are writtenTo Channel to the interleaver row-wise and read out to the channel column-rC NFrnsumoldecoEnom wiseX1 ... XN Deinterleaver performs reverseXN+1 ... X2N operation at receiver Adjacent symbols through the channel are separated byN positions in the coded sequence Cost: Additional memory Introduces delay
XN(M-1)+1 … XMN
14
Apir l92, 02067
DCM. MSAo uTruytaoliral
   
Input DataEnFcEoCderInterleaverulodoratMSrpaeedr
Output DataFEC Decoder DeinterleaverrtolDaedmuoDespreader
Channel
15
 Coherent Schemes Binary Phase Shift Keying (BPSK)ctral pe sngsieacrIn Quaternary Phase Shift Keying (QPSK)efficiency. 8-PSKieqd re toHgieh rISRNr u 16-QAMachieve a given BER. Non-coherent Schemes Orthogonal signals (e.g., Walsh functions) Differential PSK
16
April 29, 20068
DC.M MSAo uTruytaolrial
   
Input Data EnFcEoCderInterleaverModulatoredrSprea
Output Data FEC D De dulatoreradeDpser Decoder einterleaver mo
*
d(t)a(t) d(t) Channel A B C a(t)a(t)
Channel
17
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
April 29, 02069
CDM. MSAo uTruytaoliral
  $ +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 0N N 0 , ,H H1=[ ]H2=0 1 2N=HHNHHNExample 0 0 0 0 H H H4=2222=010111000110H H Walsh sequences are rows of Hadamard matrix
19
20
Apir l29, 200601
CDM. MSAo uTruytaoliral
Common Components Common Features/Functionality Power Control Diversity Soft Handoff System Particulars cdmaOne (IS-95) cdma2000
21
# 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
22
April 29, 200611