Single Carrier

Description

Single Carrier (SC) modulation is a radio transmission technique where information is encoded in the amplitude and/or phase of a single, continuous radio frequency (RF) carrier wave. This is in contrast to multi-carrier schemes like Orthogonal Frequency Division Multiplexing (OFDM), which splits data across many closely spaced subcarriers. In a pure SC system, the entire allocated channel bandwidth is used by one signal stream. Classic examples include Gaussian Minimum Shift Keying (GMSK) used in GSM and various forms of Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM) used in other systems.

The operation of an SC system involves several key stages. At the transmitter, a digital bit stream is mapped to symbols (e.g., QAM symbols). These symbols are then pulse-shaped using a filter (like a Root Raised Cosine filter) to limit the signal's bandwidth and minimize Inter-Symbol Interference (ISI) in a dispersive channel. The filtered, discrete-time symbol stream is then converted to a continuous-time analog signal, which is used to modulate (e.g., via quadrature modulation) the single RF carrier frequency. At the receiver, the process is reversed: the RF signal is demodulated, sampled, filtered, and equalized to recover the transmitted symbols and ultimately the original bit stream. A critical component in SC receivers for wideband channels is an equalizer (e.g., a linear or decision-feedback equalizer) to combat the frequency-selective fading caused by multipath propagation, which distorts the single carrier signal.

SC's role in modern networks, particularly 3GPP standards, is multifaceted. While LTE and 5G NR's downlink and uplink for wide area coverage primarily use OFDMA and SC-FDMA/CP-OFDM respectively, the SC concept remains vital. For instance, the 5G NR uplink offers a "CP-OFDM" option which is a multi-carrier scheme, but the term "Single Carrier" historically refers to the specific DFT-s-OFDM (Discrete Fourier Transform Spread OFDM) waveform, often called SC-FDMA, used in LTE uplink and NR uplink for its lower Peak-to-Average Power Ratio (PAPR). Furthermore, SC modulation is fundamental in narrowband IoT (NB-IoT) and LTE-M technologies for efficient, low-power transmission. It also serves as the basic building block; advanced multi-carrier systems can be understood as a collection of parallel, closely spaced SC subcarriers. The simplicity of SC makes it robust and power-efficient for certain applications, especially where transmitter power amplifier efficiency is critical, as in mobile device uplink.

Purpose & Motivation

Single Carrier transmission is one of the oldest and most fundamental radio communication techniques, predating cellular systems. Its initial purpose in early digital cellular (like GSM) was to provide a robust and relatively simple method for transmitting digital voice over radio channels. The primary problem it solved was enabling efficient spectrum usage for digital modulation within the technological constraints of the late 1980s and early 1990s, where digital signal processing power was limited.

Compared to earlier analog FM systems, SC digital modulation offered improved spectral efficiency, better noise immunity, and enabled advanced services like SMS. The motivation for its continued use and evolution within 3GPP standards stems from its inherent advantages in specific scenarios. A key strength is its low Peak-to-Average Power Ratio (PAPR). A signal with low PAPR allows the power amplifier in a mobile device to operate more efficiently and linearly, conserving battery life and reducing cost and heat dissipation. This is why SC-FDMA (a single-carrier-like waveform) was chosen for the LTE uplink. SC techniques also provide robustness in certain channel conditions and are conceptually simpler for narrowband applications. While multi-carrier OFDM excels in high-speed, wideband scenarios by simplifying equalization, SC modulation remains the optimal or necessary choice for power-limited devices (IoT), for backward compatibility with legacy systems, and as the conceptual foundation upon which more complex multi-carrier systems are built and understood.