1 / 8

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

lparkinson
Télécharger la présentation

Spread Spectrum Communications

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related