Question E8B07:
Orthogonal frequency-division multiplexing (OFDM) is a technique used for which types of amateur communication?  

Choices:  
A: Digital modes  
B: Extremely low-power contacts  
C: EME  
D: OFDM signals are not allowed on amateur bands    

Correct Answer is  A: Digital modes  

OFDM is a multicarrier digital modulation/multiplexing method. In amateur radio it’s used for digital communications such as digital amateur TV (DVB‑T/COFDM) and high-speed multimedia (Wi‑Fi/AREDN links). Therefore, the correct answer is A: Digital modes.

Why A is correct
- OFDM divides the channel into many orthogonal subcarriers, each carrying digitally modulated data (e.g., QPSK/QAM). That’s inherently a digital mode technique.

Why the others are incorrect
- B: Extremely low-power contacts (QRP/QRPp) are about transmit power, not a specific modulation. OFDM is not uniquely suited to ultra-low power and its high peak-to-average power ratio can be challenging at very low power with non-linear amps.
- C: EME can use digital modes, but commonly uses very narrowband weak-signal modes like JT65, Q65, or FT8, not wideband OFDM. OFDM isn’t specifically an EME technique.
- D: OFDM is allowed on amateur bands where the bandwidth and emission type are permitted. It’s used legally for DATV and HSMM/AREDN.

Useful formulas and rules of thumb
- Orthogonality (subcarrier spacing vs. symbol time):
  $$\Delta f = \frac{1}{T_s}$$
- Approximate occupied bandwidth for $N$ subcarriers:
  $$B \approx N \,\Delta f$$
- OFDM has high crest factor (PAPR). Amplifier back-off (in dB) should be roughly the crest factor to maintain linearity, often:
  $$\text{Back-off} \approx 6\text{–}10 \ \text{dB}$$
This keeps intermodulation products down and the signal clean.