Wireless Communication Basics
Which describes OFDM (orthogonal frequency-division multiplexing)?
- AA scheme that splits data across many mutually orthogonal subcarriers sent in parallel; by setting the subcarrier spacing to the reciprocal of the symbol length, the subcarriers can be separated without interfering even though their spectra overlap
- BA scheme that modulates a single carrier at a high symbol rate, which is resistant to multipath delay because the symbols are short
- CA scheme in which each user's signal is multiplied by a different spreading code and sent at the same time on the same frequency
Show answer and explanation
Answer: A. A scheme that splits data across many mutually orthogonal subcarriers sent in parallel; by setting the subcarrier spacing to the reciprocal of the symbol length, the subcarriers can be separated without interfering even though their spectra overlap
OFDM splits the data across many subcarriers, modulates each at a low symbol rate and sends them in parallel. When the subcarrier spacing is the reciprocal of the symbol length, the subcarriers are mutually orthogonal even though their spectra overlap, so the receiver can separate them and the spectrum is used efficiently. Because each subcarrier's symbols are long, the scheme is resistant to multipath delay. Processing is done efficiently with an inverse fast Fourier transform (IFFT) at the transmitter and a fast Fourier transform (FFT) at the receiver, and OFDM is used in wireless LANs, 4G LTE, 5G NR, digital terrestrial broadcasting and more.
- A. Correct. Orthogonal subcarriers are overlapped side by side to use the spectrum efficiently.
- B. This describes single-carrier transmission, sending fast on one carrier. The shorter the symbols, the more easily late-arriving waves overlap the next symbol, so multipath has a strong effect.
- C. This describes CDMA (code-division multiple access).