Wireless Communication Basics
In OFDM, which is the main purpose of the cyclic prefix (CP), formed by copying part of the end of each symbol to its beginning?
- ATo lower the ratio of peak to average power (PAPR) of the transmitted signal and raise the efficiency of the power amplifier
- BTo repeat data at the start of the symbol so that errors can be detected instead of using error-correcting codes
- CTo absorb interference from the previous symbol caused by multipath waves that arrive late, and to preserve the orthogonality between subcarriers
Show answer and explanation
Answer: C. To absorb interference from the previous symbol caused by multipath waves that arrive late, and to preserve the orthogonality between subcarriers
Waves that arrive late via multipath overlap the end of the previous symbol onto the start of the next (intersymbol interference). If the CP is longer than the spread of delays (delay spread), this overlap stays within the CP, and the interference is removed when the receiver discards the CP before processing. In addition, because the CP is a copy of the end of the symbol, late-arriving waves also look periodic within the interval processed by the FFT, so orthogonality between subcarriers is preserved. In exchange, the CP adds time during which no data is sent, lowering transmission efficiency.
- A. The CP is not for lowering PAPR; adding it leaves PAPR almost unchanged.
- B. The CP is an interval the receiver discards and is not used for error detection.
- C. Correct. A CP longer than the delay spread prevents intersymbol interference.