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Spread Spectrum Communications

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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Spread Spectrum Communications

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  1. Spread Spectrum Communications

  2. 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

  3. 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

  4. 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

  5. Probability of Error for 2-PAM • Probability that tail of ri centered at +Ebis positive and tail of ri centered at -Eb is negative

  6. 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)

  7. 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.

  8. 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

  9. 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

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