CP-OFDM

CP-OFDM is plain OFDM with a cyclic prefix bolted onto each symbol, and it's the workhorse waveform of 5G NR — used on both downlink and uplink, which is the notable change from LTE. The "cyclic prefix" is a copy of the symbol's tail pasted onto its front. That sounds wasteful, but it does two jobs at once: it acts as a guard interval that absorbs multipath echoes spilling over from the previous symbol, and the cyclic structure turns the channel's linear convolution into a circular one, which is what lets the receiver equalise the channel with a simple per-subcarrier operation.

The length of the cyclic prefix is a deliberate trade-off: it has to exceed the channel's delay spread or inter-symbol interference leaks through, yet every sample spent on it is overhead. NR's numerology ties CP length to subcarrier spacing — wider spacing means shorter symbols and a shorter CP — which is part of how the same waveform stretches from low-band wide-area to mmWave. LTE used CP-OFDM downlink but switched to SC-FDMA uplink for power reasons; NR kept CP-OFDM uplink as the default and offers DFT-s-OFDM as the low-PAPR alternative when coverage is tight.