5G/NR - PHY Candidate

 

 

 

DFT-s-OFDM(DFT spread OFDM)

 You would know of SC-OFDM because this is the waveform that we use in current LTE. If you are familiar with SC-FDMA, you can easily understand DFT-s-OFDM(Discrete Fourier Transform spread Orthogonal Frequency Division Multiplexing) . High level view of DFT-s-OFDM waveform generation is illustrated as follows. You would notice that this is almost same as high level SC-OFDM process.

Below is the breakdown and brief description of each of the blocks

  • Serial to Parallel Conversion:
    • The input data stream (usually modulated symbols such as QPSK, 16-QAM, or 64-QAM) is split into multiple parallel streams.
    • This step organizes the data to be processed simultaneously.
  • DFT (K-point):
    • A K-point Discrete Fourier Transform is applied to the input symbols.
    • This step spreads the input symbols across the frequency domain.
    • This operation is what gives DFT-s-OFDM its "single-carrier-like" properties because it creates a frequency-domain representation of the signal while retaining low PAPR characteristics.
  • Subcarrier Allocation:
    • The DFT output is mapped onto a subset of the available subcarriers in the frequency domain.
    • The unused subcarriers remain empty (zero-padding) to fit the desired bandwidth and avoid interference.
    • This ensures efficient spectral usage and helps in uplink resource allocation.
  • IFFT (M-point):
    • An Inverse Fast Fourier Transform (IFFT) is applied to the mapped subcarriers, converting the signal back to the time domain.
    • This creates the OFDM waveform that is transmitted over the air.
    • Here, M is typically larger than K, as only a subset of the subcarriers are occupied.
  • Parallel to Serial Conversion:
    • The parallel time-domain samples are converted back into a single serial data stream for transmission.

Key Characteristics of DFT-s-OFDM:

DFT-s-OFDM combines the benefits of both single-carrier and multi-carrier techniques. This hybrid approach leverages the efficiency and flexibility of OFDM while addressing its limitations, such as high Peak-to-Average Power Ratio (PAPR), through the integration of a Discrete Fourier Transform (DFT) step. By preserving single-carrier-like characteristics and enabling efficient resource allocation, DFT-s-OFDM ensures optimal performance in power-constrained mobile devices. Additionally, its inherent robustness to multipath fading makes it well-suited for maintaining reliable communication in diverse and dynamic wireless environments.

  • Low PAPR:
    • Compared to standard OFDM, DFT-s-OFDM has a lower Peak-to-Average Power Ratio, which is crucial for efficient power amplifier operation in the uplink.
  • Single-Carrier-like Behavior:
    • The DFT step introduces frequency-domain spreading, making the transmitted signal behave similarly to single-carrier transmission, which is beneficial for uplink transmission where mobile devices have limited power.
  • Efficient Resource Utilization:
    • The subcarrier allocation step allows flexible and efficient assignment of frequency resources, making DFT-s-OFDM ideal for LTE’s uplink scheduling.
  • Robustness to Multipath:
    • Like standard OFDM, DFT-s-OFDM provides robustness against multipath fading, as the IFFT converts the signal to the time domain while maintaining orthogonality between subcarriers.

Reference

[1]  Frequency-Domain Equalization and Single-Carrier Transmission in OFDM Framework by Markku Renfors