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SC-FDMA (Single Carrier – Frequency Division Multiple Access)

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SC-FDMA (Single Carrier - Frequency Division Multiple Access) is a digital modulation and access scheme used in wireless communication systems, particularly in the uplink transmission of cellular networks such as LTE (Long-Term Evolution) and 5G.

SC-FDMA is specifically designed to overcome the limitations of traditional OFDMA (Orthogonal Frequency Division Multiple Access) in terms of peak-to-average power ratio (PAPR). In OFDMA, multiple users share the available frequency spectrum by dividing it into subcarriers, which are then allocated to different users. However, OFDMA suffers from high PAPR, meaning that the signal's power fluctuates significantly, leading to potential distortion and reduced efficiency in power amplifiers.

SC-FDMA addresses this issue by combining the benefits of single-carrier modulation and multi-carrier access. It uses a single carrier waveform, similar to traditional single-carrier modulation schemes such as QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation), but applies frequency-domain equalization and multi-carrier access similar to OFDMA.

Here is a detailed explanation of the key components and operation of SC-FDMA:

  1. Subcarrier Allocation: Like OFDMA, the available frequency band is divided into subcarriers. The number of subcarriers and their spacing depend on the system's requirements, but they are typically evenly spaced across the bandwidth. Each subcarrier can carry a separate stream of data or be shared among multiple users.
  2. Frequency Domain Equalization (FDE): SC-FDMA uses FDE to combat the frequency-selective fading caused by multipath propagation. FDE performs equalization in the frequency domain, compensating for the channel variations across different subcarriers. This equalization ensures that the received signal quality is uniform across all subcarriers.
  3. Discrete Fourier Transform (DFT): SC-FDMA utilizes DFT to convert the time-domain signal into the frequency domain. This transformation allows for parallel processing of the subcarriers, enabling efficient implementation.
  4. SC-FDMA Symbols: The data to be transmitted is divided into SC-FDMA symbols, each containing a group of subcarriers. The symbols are typically smaller in duration compared to OFDMA symbols, allowing for reduced delay and lower PAPR.
  5. Precoding: SC-FDMA employs precoding techniques to reduce inter-symbol interference (ISI) caused by multipath propagation. Precoding schemes like DFT precoding or SVD (Singular Value Decomposition) precoding are applied to transform the data symbols before modulation, effectively mitigating the ISI.
  6. Modulation: The transformed data symbols are then modulated onto the assigned subcarriers using traditional modulation schemes such as QPSK, 16-QAM, or 64-QAM. The modulation scheme chosen depends on the system's requirements and the signal-to-noise ratio (SNR) of the channel.
  7. Inverse Discrete Fourier Transform (IDFT): After modulation, each subcarrier signal is converted back to the time domain using IDFT. This process combines the individual subcarrier signals into a single time-domain signal for transmission.
  8. Cyclic Prefix: A cyclic prefix is appended to each SC-FDMA symbol to combat inter-symbol interference caused by multipath propagation. The cyclic prefix is a copy of the end part of the symbol that is added to the beginning, creating a guard interval between consecutive symbols and mitigating the effects of ISI.
  9. Power Amplification: SC-FDMA offers advantages in terms of power efficiency compared to OFDMA. By using single-carrier modulation, the signal's peak power is reduced, resulting in a lower PAPR. This characteristic makes SC-FDMA well-suited for mobile devices with limited battery power and improves the overall efficiency of power amplifiers.
  10. Multiple Access: SC-FDMA supports multiple access by allowing multiple users to share the same frequency band using different subcarriers. Each user is allocated a subset of subcarriers for transmission, and orthogonal techniques, such as frequency domain or time domain scheduling, are employed to prevent interference between users.

Overall, SC-FDMA combines the benefits of single-carrier modulation and multi-carrier access, offering improved power efficiency, reduced PAPR, and effective mitigation of inter-symbol interference. These characteristics make SC-FDMA a suitable modulation and access scheme for uplink transmission in cellular networks.