Single Carrier – Frequency Division Multiple Access

Description

Single Carrier – Frequency Division Multiple Access (SC-FDMA) is a multiple access technique fundamental to the Long-Term Evolution (LTE) and 5G NR uplink. It is a hybrid scheme that combines the low Peak-to-Average Power Ratio (PAPR) characteristics of single-carrier transmission with the multipath resistance and flexible frequency allocation of Orthogonal Frequency Division Multiple Access (OFDMA). The core principle involves a Discrete Fourier Transform (DFT) precoding step applied to the complex modulation symbols (e.g., QPSK, 16QAM) before they are mapped onto the orthogonal subcarriers of an OFDMA modulator. This DFT-spreading effectively converts the multi-carrier signal into a single-carrier-like waveform, significantly reducing its cubic metric and PAPR.

The architecture of an SC-FDMA transmitter involves several key stages. User data bits are first encoded, interleaved, and mapped to complex modulation symbols. These symbols are grouped into blocks, each of which undergoes an M-point DFT. The resulting frequency-domain samples are then mapped to a specific set of contiguous or distributed subcarriers allocated to that user within the system bandwidth. This mapped data is processed by a large Inverse Fast Fourier Transform (IFFT) to generate the time-domain OFDM signal, to which a cyclic prefix is added. The receiver performs the inverse operations: removing the cyclic prefix, applying an FFT to convert to the frequency domain, extracting and equalizing the user's subcarriers, applying an IDFT to despread the signal, and finally demodulating and decoding the original symbols.

SC-FDMA's role in the network is pivotal for the User Equipment (UE) transmitter design. The reduced PAPR allows the UE's power amplifier to operate closer to its saturation point with higher efficiency, translating directly into lower power consumption and extended battery life. It also improves uplink coverage because the UE can transmit at a higher average power without distortion from the power amplifier. While OFDMA is used for the downlink due to its superior spectral efficiency and resilience to multipath fading, the uplink's power constraints make SC-FDMA the optimal choice. In 5G NR, a similar concept known as DFT-s-OFDM (Discrete Fourier Transform spread OFDM) is used for the uplink, particularly for coverage-limited scenarios, maintaining the same core benefits for power-constrained devices.

Purpose & Motivation

SC-FDMA was developed to address a critical limitation of OFDMA when applied to the uplink of cellular systems: high Peak-to-Average Power Ratio (PAPR). OFDMA, while excellent for the downlink from base stations, creates signals with high amplitude variations. Transmitting such signals requires a power amplifier with a large linear range (or 'back-off') to avoid distortion, which is highly inefficient. For battery-powered User Equipment (UE), this inefficiency would drastically reduce battery life and limit the maximum transmit power, thereby shrinking uplink coverage area.

The historical context is the transition from 3G (UMTS/HSPA) to 4G (LTE). 3G used Wideband Code Division Multiple Access (WCDMA), a single-carrier spread spectrum technique with good power amplifier efficiency but limitations in spectral efficiency and flexibility. For LTE, OFDMA was chosen for the downlink for its high performance. A new uplink technology was needed that could match OFDMA's scheduling flexibility and multipath resistance while being suitable for mobile devices. SC-FDMA was the engineered solution, inheriting the orthogonal subcarrier structure and scheduling benefits of OFDMA but with a crucial precoding step to create a single-carrier property.

Thus, the primary motivation for SC-FDMA was to enable the high data rates and advanced features of LTE without compromising device cost, battery life, or uplink reach. It solved the fundamental problem of bringing OFDMA-like performance to the power-constrained uplink, making high-speed mobile broadband practically feasible for consumer handsets. It directly addresses the economic and practical constraints of handset design that pure OFDMA could not.