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DSSS bandwidth spreading

Short answer

DSSS multiplies each data bit by a higher-rate pseudo-noise code, converting bits to chips. The chip rate Rc = n × Rb, so the signal bandwidth expands by factor n. At the receiver, correlating with the same code despreads the desired signal and spreads narrowband interference, providing processing gain equal to n (e.g.,11 for Barker-11).

Explain how DSSS (Direct Sequence Spread Spectrum) achieves bandwidth spreading.

In DSSS every data bit is replaced by n smaller pulses called chips, taken from a pseudo-noise (PN) spreading code. The chip rate is n times the bit rate, so the bandwidth grows by the same factor n.

Steps

  1. Data signal has bit rate Rb and bandwidth about Rb.
  2. A PN code generator produces chips at rate Rc = n × Rb.
  3. Data and chips are combined by XOR (or multiplication of ±1 values), producing a signal at chip rate Rc.
  4. The result is modulated (usually BPSK), so the required bandwidth is proportional to Rc, i.e. n times wider.
 Data bit    : 1       0
 PN chips    : 1011 0111 000 (n = 11 Barker)
 Sent chips  : chips   inverted chips
 Rate  : Rc = n * Rb   =>  Bss = n * B

Example with Barker n = 11: a 1 Mbps stream becomes 11 Mchip/s, and the bandwidth rises about 11 times. Processing gain Gp = Bss/B = n = 11, i.e. 10 log10(11) ≈ 10.4 dB. At the receiver the same chips are used to despread the signal, which compresses the wanted signal back to narrow band while narrow-band interference is spread out and reduced.

✓ Answer checked by Claude Sonnet 5.5

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