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Spread Spectrum Communications. speech. speech. sample and quantize. linear predictive coding. error correction coding. message. 13 kbps. (analog). 64 kbps. 8 kbps. Sprint PCS. Speech compression and coding in transmitter Transmit message signal using spread system

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Sprint pcs

speech

speech

sample and quantize

linearpredictivecoding

errorcorrectioncoding

message

13 kbps

(analog)

64 kbps

8 kbps

Sprint PCS

  • Speech compression and coding in transmitter

  • Transmit message signal using spread system

    • For every message bit, generate L = 64 bits of a pseudo noise sequence with user’s code as initial value

    • Send and receive the L bits bit-by-bit using 2-PAM on a radio frequency carrier of 1.9 GHz

  • Speech decompression and decoding in receiver


Matched filtering for 2 pam
Matched Filtering for 2-PAM

  • Transmit equally probable bits, ai {-1, 1}

  • Send single pulse,ignore noise n(t),and assume thatchannel d(t)hasbeen equalized

xi(t)

zi(t)

yi(t)

channeld(t)

ai

g(t)

g*(T-t)

ri

T

Digital

Analog

Analog

Digital

g(t)

n(t)

t

AWGN, mn = 0

Sn(f) = N0/2

-T/2

T/2


Probability of error for 2 pam

Pri(ri)

ri

-

0

Probability of Error for 2-PAM

  • General case: one bit in isolation down channel

  • Since ai {-1, 1}, ri clusters around +Eband -Eb

    • Determine which bit was sent: threshold at 0

    • Bit errors due to noise (when tails of Gaussians overlap)

    • For chain of bits, assume each bit is independent


Probability of error for 2 pam1
Probability of Error for 2-PAM

  • Probability that tail of ri centered at +Ebis positive and tail of ri centered at -Eb is negative


Spread spectrum communications1

bi{-1, 1}

ai{-1, 1}

ri

rate = 1/T

cij, rate = 1/Tc

cij

Spread Spectrum Communications

  • Enhance modulator/demodulator to spread spectrum to make it look more like noise and convert it from narrowband to a wider band

    T/Tc = Lc = number of chips

    cij is pseudo-noise sequence generated by Galois Field (GF) binary polynomials

    cij are known in advance and must be synchronized

Pre-processing (digital)

Post-processing (digital)


Spread spectrum communications2

x4

x3

x1

x0

x2

D Q

D Q

D Q

D Q

D Q

out

CLK

CLK

CLK

CLK

CLK

XOR

Spread Spectrum Communications

  • g(t) scaled in time by Lc : system has same Pe

  • GF(N) generates sequences of N-1 bits

    Almost uncorrelated noise (pseudo-noise):

    Polynomials and polynomial variable takebinary values of 0 and 1

    Fast hardware implementations using D flip-flops

  • GF(32); 32 = 25; p(x) = x5 + x2 + 1. Note x0 = 1.


Cdma qualcomm standard

800 & 1900 MHz bands

Each user

Has unique spreading code

Receives from 2 closest base stations (handoff is robust)

Reverse link (from users to base station)

Walsh codes for M-ary mod

Power adjust in user trans-mission: base receiver sees all users at equal power

Forward link (base station to user)

Transmitter uses Walsh codes for each user

User signals orthogonal: requires each user to be synchronized to xmitter, but not to each other

Transmission power increases as number of users increase

CDMA QualComm Standard


Other applications of pn sequences
Other Applications of PN Sequences

  • Training for wireline transceivers (voiceband, ADSL, etc.)

  • From search of “pseudo noise sequence” in an on-line database

    Echo cancellers

    Pulse compression sonar

    Analysis of tape recorders

  • IEEE online database

    http://ieeexplore.ieee.org/Xplore/DynWel.jsp