Wireless Communication Basics
In OFDM, the symbol length excluding the CP is the reciprocal of the subcarrier spacing. When the subcarrier spacing is widened from 15 kHz to 60 kHz, which is closest to the symbol length excluding the CP?
- A16.7 µs
- B66.7 µs
- C267 µs
Show answer and explanation
Answer: A. 16.7 µs
The symbol length excluding the CP is T = 1/Δf, so it is about 66.7 µs at 15 kHz and 1/(60×10³) ≈ 16.7 µs at 60 kHz. In 5G NR the subcarrier spacing can be chosen from 15 kHz×2^µ (15, 30, 60, 120 kHz and so on), and this choice is called numerology. Wider spacing shortens the symbols, which helps low-latency communication and tolerance to phase noise and Doppler shift in millimeter-wave bands, but the delay that a CP of the same proportion can absorb becomes shorter.
- A. Correct. 1/(60 kHz) ≈ 16.7 µs.
- B. This is the symbol length at the original 15 kHz.
- C. This multiplies by 4 on the assumption that symbol length is proportional to subcarrier spacing. Symbol length is inversely proportional to the spacing.