Question E8D02:
What spread spectrum communications technique uses a high-speed binary bit stream to shift the phase of an RF carrier?  

Choices:  
A: Frequency hopping  
B: Direct sequence  
C: Binary phase-shift keying  
D: Phase compandored spread spectrum    

Correct Answer is  B: Direct sequence  

Direct sequence (DSSS) is correct.

Why it’s correct:
- In DSSS, the data bits are multiplied by a high-rate pseudo-noise (PN) chip stream. With a BPSK implementation, each chip causes the RF carrier to be either in-phase or inverted (0° or 180°), i.e., the PN bit stream rapidly shifts the carrier’s phase at the chip rate.
- This rapid phase flipping spreads the signal’s spectrum.

A simple signal model:
$$s(t) = A\,c(t)\,d(t)\,\cos(\omega_c t)$$
where:
- $d(t) \in \{\pm1\}$ = data,
- $c(t) \in \{\pm1\}$ = PN chip sequence at rate $R_c \gg R_b$,
- Multiplication by $c(t)$ toggles the RF phase: $+\cos(\omega_c t)$ or $-\cos(\omega_c t)=\cos(\omega_c t+\pi)$.

Why the other choices are incorrect:
- A: Frequency hopping — Changes the carrier frequency among channels according to a PN sequence; it does not rapidly shift the phase at the chip rate.
- C: Binary phase-shift keying — A modulation method, not a spread spectrum technique by itself. BPSK alone doesn’t spread; DSSS uses BPSK with a high-rate PN sequence to achieve spreading.
- D: Phase compandored spread spectrum — An uncommon technique involving phase companding, not the standard “high-speed bit stream shifts carrier phase” mechanism used in DSSS.

Useful rules/formulas:
- DSSS bandwidth is set by the chip rate: $B \approx R_c$.
- Processing gain: 
  $$G_p = \frac{R_c}{R_b}, \quad G_{p,\text{dB}} \approx 10\log_{10}\!\left(\frac{R_c}{R_b}\right)$$
- Key idea: High-rate PN chips cause 0°/180° phase shifts of the RF carrier, spreading the signal’s energy.