Spread spectrum and DSSS transmitter/receiver

Spread spectrum modulation

Spread spectrum (SS) modulation is a technique in which the transmitted signal occupies a bandwidth much larger than the minimum needed for the data. The spreading is done by a pseudo-noise (PN) code that is independent of the data, and the receiver uses the same synchronized code to despread it. The ratio of spread bandwidth to data rate is the processing gain PG = B_ss/R_b = T_b/T_c. SS gives resistance to jamming, interference and multipath, low probability of interception, and allows multiple access (CDMA).

Direct sequence spread spectrum (DSSS)

In DSSS, each data bit is multiplied directly by a fast PN sequence with chip period T_c ≪ T_b. The product is then BPSK modulated on the carrier.

s(t) = √(2E_s/T_s) m(t) p(t) cos(2π f_c t + θ)

where m(t) is the data and p(t) the PN code.

Transmitter

data m(t) ---> (x) ------> (x) ------> s(t)
  Rb            ^           ^        (wideband)
                |           |
          [PN generator] [Oscillator]
           (chip Rc)      cos ωct

1. Data m(t) (±1, rate R_b) is multiplied by the PN code p(t) (±1, rate R_c = N R_b).
2. The product is a wideband chip stream at rate R_c.
3. This chip stream BPSK-modulates the carrier, giving an RF bandwidth of about 2R_c, and is transmitted.

Receiver

r(t)->[BPF]->(x)--->(x)--->[Integrate]->[Decision]-> m^(t)
              ^      ^      [0..Tb    ]
              |      |
       [PN generator] [Carrier]
       [+ sync     ]  [recovery]

1. The received signal r(t) = s(t) + interference + noise is filtered.
2. It is multiplied by a locally generated, synchronized copy of p(t). Because p(t)·p(t) = 1, the wanted signal collapses back to its narrow data bandwidth (despreading).
3. Any narrowband interference gets multiplied by p(t) and is spread over the wide band, so most of it is removed by the following filter.
4. Coherent demodulation (multiply by carrier, integrate over T_b, decide) recovers the data.

 Spectrum at receiver:
 before despreading     after despreading
   jammer                 data
     |                     /\
  ___|___               __/  \__   jammer spread
 /  data \___         ___________  (low level)

Example: IS-95 CDMA uses a chip rate of 1.2288 Mcps for 9.6 kbps data, giving PG = 128 (21 dB). IEEE 802.11b also uses DSSS.