SC FDMA (Single carrier frequency division multiple access )

Single Carrier Frequency Division Multiple Access (SC-FDMA) is a modulation scheme and access method used in wireless communication systems, particularly in the uplink transmission of cellular networks. It is designed to address the limitations of traditional Frequency Division Multiple Access (FDMA) and Orthogonal Frequency Division Multiple Access (OFDMA) in terms of power efficiency and peak-to-average power ratio (PAPR).

Here is a detailed explanation of SC-FDMA:

1. Basic Principles: SC-FDMA is based on the principle of dividing the available frequency band into multiple subcarriers, just like in OFDMA. However, unlike OFDMA, where all subcarriers are active simultaneously, SC-FDMA allows only one subcarrier to be active at a time. This feature is what gives SC-FDMA its "single carrier" name.
2. Frequency Domain Equalization: SC-FDMA employs frequency domain equalization to combat frequency-selective fading caused by multipath propagation in wireless channels. Before transmission, the information-bearing data is modulated onto the selected subcarrier using a modulation scheme such as Quadrature Amplitude Modulation (QAM). Then, a discrete Fourier transform (DFT) is applied to convert the modulated symbols from the time domain to the frequency domain.
3. Low PAPR: One of the main advantages of SC-FDMA is its ability to reduce the Peak-to-Average Power Ratio (PAPR) compared to OFDMA. In OFDMA, the signal from all active subcarriers is combined, which can result in high peak power levels. SC-FDMA avoids this issue by transmitting only one subcarrier at a time, leading to a lower PAPR. This reduction in PAPR helps in improving power efficiency and increasing the coverage area of the base station.
4. Subcarrier Allocation: In SC-FDMA, subcarriers are allocated to different users in a resource block. The resource block is a time-frequency resource unit that contains a group of subcarriers over a specific time duration. Multiple users can be assigned to the same resource block, but they are allocated different subcarriers within that block. The allocation of subcarriers is typically done based on a scheduling algorithm that considers factors like channel conditions, user priorities, and fairness.
5. SC-FDMA vs. OFDMA: SC-FDMA offers several advantages over OFDMA in the uplink transmission. Firstly, SC-FDMA has a lower PAPR, as mentioned earlier, which helps in reducing power consumption and improving battery life for user devices. Secondly, SC-FDMA provides better frequency localization, making it more suitable for narrowband channels. Thirdly, SC-FDMA is advantageous in scenarios where a single user has a high transmit power, as it avoids interference to other users more effectively compared to OFDMA.
6. Applications: SC-FDMA is widely used in various cellular networks, particularly in the uplink transmission of Long Term Evolution (LTE) and its successor, 5G New Radio (NR). It enables efficient transmission of user data from mobile devices to the base station, ensuring reliable and high-quality communication.

In summary, Single Carrier Frequency Division Multiple Access (SC-FDMA) is a modulation scheme and access method that provides benefits such as low PAPR, power efficiency, and improved frequency localization. It is commonly used in the uplink transmission of cellular networks, especially in LTE and 5G NR systems.