DFT-s-OFDM Signal Model

DFT-s-OFDM Signal Model

5G courses
  • In the current NR specifications [2], [3], Discrete Fourier transform-spread orthogonal frequency-division multiplexing (DFT-s-OFDM) [14] is used for the uplink transmission, especially in coverage limited scenarios.
  • This waveform is also referred to as single-carrier FDM waveform (SCFDM) in the literature.
  • In 3GPP 5G NR, QAM modulation symbols with modulation order (4, 16, 64, 256) can be transmitted using the DFT-s-OFDM
  • When compared to LTE, a new modulation scheme, namely, π 2 -BPSK was introduced in 5G NR
  • This is a special constellation-rotated BPSK modulation, such that even-numbered symbols are transmitted as in BPSK and the odd-numbered data symbols are phase rotated by π
  • The π 2 -BPSK modulation scheme when transmitted using DFT-s-OFDM has a low PAPR when compared to higher-order modulation schemes including QPSK as the zero-crossing transitions are avoided
  • The PAPR for various modulation schemes is shown in above, which clearly shows the low PAPR behavior of the π 2 -BPSK modulation scheme. Note that, although the constellation is similar to QPSK, we can only transmit 1-bit on one π 2 -BPSK modulation symbol.
  • For a cellular network, uplink transmissions define the coverage area.
  • This is because the transmission power in the uplink is limited to 23 dBm at the user equipment (UE) owing to hardware limitations (such a battery size) and regulatory constraints as opposed to 43 dBm at the base station in the downlink.
  • This limited transmission power in the uplink must, therefore, be used carefully to enhance cell coverage without increasing the CAPEX/OPEX costs of deploying more cell sites.
  • Therefore the uplink design of a cellular standard is crucial in enabling uplink transmissions at high powers without saturating the power amplifier, which otherwise results in unwanted non-linear distortions.

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