Wireless Communication Basics
Which is the most appropriate main reason that the 4G LTE uplink (transmission from terminal to base station) adopted SC-FDMA (DFT-spread OFDM) rather than the OFDMA used in the downlink?
- AIt removes the need for processing at the base station receiver to compensate for multipath delay
- BThe ratio of peak to average power (PAPR) of the transmitted signal is small, so the terminal's power amplifier can run efficiently
- CIt can use a wider bandwidth than OFDMA without increasing the number of subcarriers
Show answer and explanation
Answer: B. The ratio of peak to average power (PAPR) of the transmitted signal is small, so the terminal's power amplifier can run efficiently
An OFDM signal is the sum of many subcarriers, so occasional peaks far above the average appear and PAPR is large. With a large PAPR, the power amplifier (PA) must be operated at reduced output to avoid distortion, lowering power efficiency and shortening battery life. SC-FDMA applies a discrete Fourier transform (DFT) to the data before mapping it onto subcarriers, producing a signal close to single-carrier with a small PAPR. The 5G NR uplink can use CP-OFDM or DFT-spread OFDM depending on the situation.
- A. SC-FDMA also uses a CP, and the receiver performs equalization in the frequency domain. Multipath compensation is still needed.
- B. Correct. The small PAPR lets the terminal's PA run more efficiently.
- C. The usable bandwidth is set by the number of subcarriers assigned; SC-FDMA does not widen it.