SC FDM (Single carrier frequency division multiplex)

Single Carrier Frequency Division Multiplexing (SC-FDM) is a modulation technique used in wireless communication systems to transmit multiple signals simultaneously over a single carrier frequency. It is a variation of Orthogonal Frequency Division Multiplexing (OFDM), which is widely used in various communication standards such as 4G LTE and Wi-Fi.

In SC-FDM, the information to be transmitted is divided into multiple parallel substreams, each occupying a different frequency subcarrier. These subcarriers are typically evenly spaced in the frequency domain. The substreams are modulated using complex-valued symbols, and the resulting modulated subcarriers are summed together to form the final transmitted waveform.

The SC-FDM modulation process can be divided into several steps:

1. Substream Encoding: The information to be transmitted is divided into multiple substreams, each representing a separate data stream. These substreams can carry different types of information, such as voice, data, or video.
2. Symbol Mapping: Each substream is then mapped to a specific set of complex-valued symbols. The choice of symbols depends on the modulation scheme used, such as Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK). The symbol mapping assigns a unique complex symbol to each substream, representing the information to be transmitted.
3. Inverse Fast Fourier Transform (IFFT): After symbol mapping, the complex symbols are transformed from the frequency domain to the time domain using an Inverse Fast Fourier Transform (IFFT). The IFFT converts the modulated subcarriers into a time-domain waveform.
4. Cyclic Prefix Insertion: To mitigate the effects of multipath interference, a cyclic prefix is inserted at the beginning of each time-domain symbol. The cyclic prefix is a copy of the trailing portion of the symbol, which helps in eliminating the inter-symbol interference caused by multipath propagation.
5. Digital-to-Analog Conversion (DAC): The time-domain waveform, including the cyclic prefix, is converted from the digital domain to the analog domain using a Digital-to-Analog Converter (DAC). This analog signal is then suitable for transmission over the air using a wireless antenna.

At the receiver end, the reverse process takes place to demodulate and decode the received signal:

1. Analog-to-Digital Conversion (ADC): The received analog signal is converted back to the digital domain using an Analog-to-Digital Converter (ADC). This allows further processing of the signal in the digital domain.
2. Cyclic Prefix Removal: The cyclic prefix is removed from each received symbol to obtain the original time-domain symbol.
3. Fast Fourier Transform (FFT): The received time-domain symbol is then transformed into the frequency domain using a Fast Fourier Transform (FFT). This process separates the different subcarriers from each other.
4. Symbol Demapping: Each subcarrier is demodulated by mapping the received frequency domain symbols back to the original substreams. This involves symbol demapping using the inverse of the symbol mapping scheme used at the transmitter.
5. Substream Decoding: The demodulated symbols are then processed to recover the original information carried by each substream. This typically involves error correction coding and decoding techniques to ensure accurate data recovery.

SC-FDM offers several advantages over traditional OFDM. It has lower peak-to-average power ratio (PAPR), which reduces the overall power requirements and enhances power efficiency. It also exhibits better resistance to frequency-selective fading and provides robustness against narrowband interference. However, SC-FDM may have slightly higher implementation complexity compared to OFDM due to the requirement of a more sophisticated equalization process at the receiver.

Overall, SC-FDM is a modulation scheme that enables efficient transmission of multiple signals over a single carrier frequency, making it suitable for various wireless communication systems, including 4G LTE, 5G, and beyond.