ECE 442 COMMUNICATION SYSTEM DESIGNLECTURE 4. SPREAD SPECTRUM Husheng Li Dept. of EECS The University of Tennessee
THE MOTHER OF CDMA Hedy Lamarr (1913---2000): a famous actress in Holly Wood. She invented CDMA in 1941
DIRECT-SEQUENCE CDMA Information signal • The signal of DS-CDMA: Wideband PN waveform
PN WAVEFORM C(T) • We consider rectangle pulses (called chips) • The correlation function is given by
OUTPUT SPECTRUM • The spectrum of the output signal is given by • is called the bandwidth expansion factor (also called processing gain), which ranges from 10 to 10,000. • The wideband noise-like signal would be hard to distinguish from background noise at an authorized receiver.
PERFORMANCE OF RECEIVER • When there is noise, the bit error rate is given by • When there is a single-tone jammer, the big error rate is given by • is called the jamming margin. Processing gain
MULTIPLE ACCESS • Suppose a user shares the spectrum with M-1 other users. The interference is given by • The bit error rate is then given by (noiseless case) • The interference is like background noise.
HOMEWORK • Problem 1: Problem 15.1-1 • Problem 2: Problem 15.1-2 • Problem 3: Compute the bit error rate of DS-CDMA using QPSK in presence of noise (we have obtained the expression for BPSK in Eq. (6), page 726)