Direct sequence and frequency hopped SS

Both are spread spectrum methods that use a PN code to spread a narrowband signal over a wide band.

Direct sequence (DSSS)

Each data bit is multiplied by a fast PN chip sequence (T_c ≪ T_b), and the product BPSK-modulates the carrier.

data->(x)->(x)-> s(t)      r(t)->(x)->(x)->[∫]-> data
       ^    ^                     ^    ^
     [PN] [cos]                 [PN] [cos]

The receiver multiplies by the same synchronized code to despread the signal. Processing gain PG = T_b/T_c. Used in IS-95 and 802.11b.

Frequency hopping (FHSS)

The carrier frequency of an FSK signal hops among many channels in a PN-controlled pattern.

data->[FSK]->(x)-> FH    FH->(x)->[FSK demod]-> data
              ^               ^
       [Synth <- PN]   [Synth <- PN]

The receiver hops in step to "de-hop" the signal. Slow or fast hopping. Used in Bluetooth.