Direct sequence spread spectrum (DSSS)

Direct Sequence Spread Spectrum (DSSS) spreads a narrowband data signal over a wide bandwidth by multiplying each data bit with a fast pseudo-noise (PN) code whose chip period T_c is much smaller than the bit period T_b. The receiver multiplies again by the same synchronized code to recover the data.

s(t) = A m(t) p(t) cos(2π f_c t)
Processing gain PG = T_b / T_c = R_c / R_b

Transmitter

m(t) ±1 --->(x)--- m(t)p(t) --->(x)---> s(t)
  (Rb)       ^                  ^
             |                  |
       [PN code p(t)]     [cos ωct]
       [ rate Rc    ]

• Data m(t) (±1) is multiplied by PN code p(t) (±1). Each bit becomes N = T_b/T_c chips.
• The chip stream BPSK-modulates the carrier. Bandwidth grows from about 2R_b to 2R_c.

Receiver

r(t)->(x)------>(x)---->[∫ over Tb]->[Decision]->m^(t)
       ^         ^
  [Local PN ] [cos ωct]
  [synced   ]

• The received signal is multiplied by a locally generated PN code, aligned by an acquisition and tracking circuit.
• Since p(t)² = 1, the wanted signal is despread to its original data bandwidth.
• Narrowband interference or jamming is spread by the same multiplication and mostly filtered out.
• Coherent BPSK demodulation (multiply, integrate over T_b, threshold) gives the data.

Waveforms (one data bit = 7 chips)

m(t):   ‾‾‾‾‾‾‾‾‾‾‾‾‾‾|______________
p(t):   ‾|_|‾‾|_|‾|_‾ |‾|_|‾‾|_|‾|_
m·p :   ‾|_|‾‾|_|‾|_‾ |_|‾|__|‾|_|‾

Features

• Interference suppression by a factor equal to PG.
• Multipath rejection: delayed copies more than one chip late look like noise; a RAKE receiver can combine them.
• Allows CDMA: users share the band using different codes. Used in IS-95, WCDMA and IEEE 802.11b.