MSK modulation and DSSS

MSK modulation

Minimum Shift Keying (MSK) is a continuous phase binary FSK with modulation index h = 0.5, i.e. tone spacing Δf = R_b/2, the minimum spacing that keeps the two tones orthogonal. The phase changes linearly by ±π/2 per bit with no sudden jumps, so the envelope is constant. It can also be seen as OQPSK with half-sinusoidal pulse shaping. MSK has fast side-lobe roll-off and the same BER as BPSK.

Direct sequence spread spectrum (DSSS)

DSSS spreads the data over a wide band by multiplying each data bit with a fast PN code of chip period T_c ≪ T_b. Processing gain PG = T_b/T_c.

Transmitter

m(t) --->(x)--->(x)---> s(t) = m(t)p(t)cos ωct
          ^      ^
    [PN gen p(t)] [cos ωct]

• Data (±1) × PN code (±1) gives a chip stream at rate R_c.
• The chip stream BPSK-modulates the carrier; the bandwidth becomes ≈ 2R_c.

Receiver

r(t)->(x)--->(x)--->[∫ 0..Tb]->[Decision]-> m^(t)
       ^      ^
 [Local PN  ] [Carrier]
 [synchron. ] [recovery]

• Multiplying by the same synchronized PN code despreads the wanted signal (p² = 1).
• Interference is spread by the same step and mostly filtered out.
• Integrating over T_b and comparing with zero gives the data.

 Before despread      After despread
   ____________          /\   <- data
  |  wideband  |    ____/  \____ noise

DSSS gives anti-jam capability, multipath resistance (RAKE) and CDMA multiple access; it is used in IS-95, WCDMA and IEEE 802.11b.