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SC-FDM single carrier frequency division multiplexing

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Single Carrier Frequency Division Multiplexing (SC-FDM) is a modulation technique used in wireless communication systems to transmit multiple data streams simultaneously over a shared channel. It is an alternative to Orthogonal Frequency Division Multiplexing (OFDM), which is commonly used in many wireless standards such as Wi-Fi and LTE.

SC-FDM operates by dividing the available frequency spectrum into multiple subcarriers, each carrying a separate data stream. These subcarriers are closely spaced and typically have overlapping frequency bands. Unlike OFDM, which uses orthogonal subcarriers, SC-FDM employs overlapping subcarriers to achieve spectral efficiency and reduce the overall system complexity.

The SC-FDM modulation process can be divided into two main steps: subcarrier mapping and transmission.

Subcarrier Mapping:

  • Input Data: The data to be transmitted is divided into multiple parallel streams, which can represent different users or different parts of a single data stream.
  • Subcarrier Allocation: The available frequency spectrum is divided into a set of subcarriers. The number of subcarriers depends on factors such as bandwidth availability and system requirements.
  • Modulation: Each data stream is modulated onto its assigned subcarriers using a suitable modulation scheme, such as Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK).

Transmission:

  • IFFT: The modulated subcarriers are then converted from the frequency domain to the time domain using an Inverse Fast Fourier Transform (IFFT). This process converts the parallel subcarrier signals into a single time-domain waveform.
  • Cyclic Prefix: A cyclic prefix is added to the beginning of each time-domain symbol. The cyclic prefix is a copy of the last portion of the symbol and helps mitigate inter-symbol interference caused by multipath propagation.
  • DAC and RF Upconversion: The time-domain signal is converted from the digital domain to the analog domain using a Digital-to-Analog Converter (DAC). The analog signal is then upconverted to the desired radio frequency (RF) for transmission.
  • RF Filtering and Amplification: The upconverted signal is passed through RF filters to limit the out-of-band interference. It is then amplified to an appropriate power level for transmission.
  • Antenna Transmission: The amplified signal is fed to the antenna for wireless transmission.

At the receiver side, the SC-FDM signal undergoes the reverse process:

  • Antenna Reception: The transmitted signal is received by the antenna.
  • RF Filtering and Low-Noise Amplification: The received signal is filtered and amplified to improve its quality and level.
  • ADC and Baseband Processing: The analog signal is converted back to the digital domain using an Analog-to-Digital Converter (ADC). It then undergoes baseband processing, including synchronization, channel estimation, equalization, and demodulation.
  • FFT: A Fast Fourier Transform (FFT) is applied to the demodulated symbols to convert them from the time domain to the frequency domain.
  • Subcarrier Demapping: The demodulated symbols are allocated to their respective subcarriers and streams based on the predefined subcarrier mapping.
  • Data Recovery: The demodulated symbols are processed further to recover the original data streams.

Overall, SC-FDM provides an efficient method to transmit multiple data streams simultaneously, while maintaining spectral efficiency and managing interference. It has been adopted in various communication systems, including 4G LTE, as an alternative to OFDM.