Question E8B10:
What is frequency division multiplexing (FDM)?  

Choices:  
A: The transmitted signal jumps from band to band at a predetermined rate  
B: Dividing the transmitted signal into separate frequency bands that each carry a different data stream  
C: The transmitted signal is divided into packets of information  
D: Two or more information streams are merged into a digital combiner, which then pulse position modulates the transmitter    

Correct Answer is  B: Dividing the transmitted signal into separate frequency bands that each carry a different data stream  

Correct answer: B — Dividing the transmitted signal into separate frequency bands that each carry a different data stream

1) Why B is correct
- Frequency Division Multiplexing (FDM) sends multiple information streams at the same time by assigning each stream to a different, non-overlapping frequency band (subcarrier).
- Each sub-band is filtered so the channels don’t overlap, allowing simultaneous transmission.

2) Why the other choices are incorrect
- A: “Jumps from band to band” describes frequency-hopping spread spectrum (FHSS), not FDM. FHSS uses one data stream that rapidly changes frequency; it doesn’t send multiple streams simultaneously in separate bands.
- C: “Divided into packets” is generic packetization and often associated with time-division multiplexing (TDM), where multiple streams share time slots on the same frequency, not different frequency bands.
- D: “Merged and pulse position modulated” describes a specific digital modulation/combining method, not multiplexing by frequency. It’s not FDM.

3) Useful formulas and rules of thumb
- Total occupied bandwidth for N FDM channels:
  $$B_{\text{total}} \approx \sum_{i=1}^{N} B_i + \text{guard bands} \approx N B_c + (N-1)G$$
  where:
  - $B_i$ or $B_c$ = bandwidth of each channel
  - $G$ = guard band between adjacent channels to limit interference
- In orthogonal FDM (OFDM), subcarrier spacing is set for orthogonality:
  $$\Delta f = \frac{1}{T}$$
  where $T$ is the symbol duration of each subcarrier. This minimizes inter-channel interference without large guard bands.

Key idea: FDM = multiple streams at once, each on its own frequency slice.