As 5G becomes commercially available, several countries have begun exploring the transformative potential of 6G. This next-generation technology is anticipated to support a wide range of future applications, including extended reality (XR), digital twins, autonomous driving, and smart homes. These advanced services will place unprecedented demands on data rates, energy consumption, mobility, and positioning accuracy. For example, 6G is expected to achieve peak data rates up to 50 times higher and sensing accuracy improvements up to 20 times greater than current 5G standards. However, meeting these ambitious targets with traditional microwave frequencies and established modulation techniques like OFDM/SC-FDE may prove quite challenging. In this context, initial efforts have been made to develop new modulation techniques from a physical-layer perspective, with the goal of meeting the diverse performance requirements needed for 6G networks.

Thus, as we edge closer to the rollout of sixth generation (6G) wireless networks, the quest for faster, more reliable, and more efficient communication technologies is intensifying. One of the critical areas driving this innovation is digital modulation techniques, which are essential for optimizing how data is transmitted over the airwaves, and recent advances are setting the stage for revolutionary improvements in 6G networks. For instance, to achieve terabits per second (Tbps) data rates, it may be necessary to utilize terahertz (THz) frequencies and beyond due to the limited spectrum available at microwave frequencies. Despite offering ultra-wide bandwidth and carrier frequencies exceeding 100 GHz, THz frequencies present significant challenges, including severe path loss, frequency-selective fading, and pronounced Doppler shifts, particularly at high mobility. These issues render existing modulation formats inadequate, prompting the development of new modulation schemes designed to improve resilience in such challenging channel conditions. Examples include orthogonal time frequency space (OTFS), orthogonal delay-Doppler division multiplexing (ODDM), orthogonal chirp division multiplexing (OCDM), and affine frequency division multiplexing (AFDM).

The Role of Digital Modulation in 6G

Digital modulation involves altering a carrier signal's properties—such as amplitude, frequency, or phase to encode data. In the context of 6G, which promises unprecedented data rates, ultra-low latency, and massive connectivity, advanced modulation techniques are crucial for meeting these demands.

Key Advances in Digital Modulation for 6G

       1.    Higher-Order Modulation Schemes

The development of higher-order modulation schemes is a significant trend in the 6G wireless networks. While 4G LTE predominantly uses 64-QAM (Quadrature Amplitude Modulation), 5G has pushed the envelope with 256-QAM. There has been increased focus of researchers on exploring even higher-order schemes such as 1024-QAM and beyond for 6G. 1024-QAM allows for a 25% increase in data rates for Wi-Fi 6 (802.11ax) access points and devices. By modulating both the phase and amplitude of the radio waves, this technology enhances spectral efficiency, enabling more data to be transmitted with each signal. These advanced schemes enable higher data rates by increasing the number of bits per symbol transmitted, but they also come with challenges in terms of signal-to-noise ratio and error performance.

       2.    Orthogonal Time Frequency Space (OTFS) Modulation

OTFS modulation is an innovative approach that enhances robustness against Doppler shifts and multipath fading—common issues in high-speed and mobile environments. OTFS, when combined with equalization, transforms the fading and time-varying wireless channel encountered by modulated signals like OFDM into a time-independent channel with a nearly constant complex channel gain for all symbols. This means that transmitter adaptation is unnecessary. By fully exploiting channel diversity, OTFS simplifies system operation and enhances performance, especially in scenarios involving high Doppler shifts, short packets, and large antenna arrays. Unlike traditional modulation schemes that rely on time-frequency resources, OTFS transforms the signal into an orthogonal basis that is more resilient to time and frequency variations. This is particularly beneficial for the high-speed mobility and diverse environments anticipated in 6G networks.

       3.    Non-Orthogonal Multiple Access (NOMA)

Non-Orthogonal Multiple Access (NOMA) is a key technology for improving spectrum efficiency and user connectivity in 6G. NOMA is proposed as an alternative to traditional orthogonal multiple access (OMA) because it enhances user capacity, bandwidth efficiency, and minimizes downtime. The core concept of NOMA is to enable multiple users to transmit signals concurrently within the same bandwidth block. Combining NOMA with MEC (Multi-access Edge Computing) has emerged as an effective approach for efficient transmission and processing across numerous devices. By using NOMA, multiple clients can simultaneously offload their computational tasks to an edge server, thereby boosting computational power and overall efficiency. NOMA stands out as one of the most promising electromagnetic transmission techniques for the next generation of mobile communication. It provides several significant enhancements over traditional orthogonal frequency division multiple access (OFDMA).

Recent advancements include the integration of NOMA with advanced modulation techniques like power-domain NOMA and code-domain NOMA, which promise to significantly boost the capacity and connectivity of 6G networks. NOMA maintains user fairness and diverse quality of service through flexible power allocation between strong and weak users. By allocating more power to users with weaker signals, NOMA enhances cell-edge throughput, thereby improving the overall user experience.

      4.     Machine Learning-Enhanced Modulation

Machine Learning (ML) plays a very important role in realizing and optimizing 6G network applications and it is gradually making its mark on digital modulation through adaptive modulation schemes that can dynamically adjust based on channel conditions and network load. With the use of ML algorithms, 6G systems can optimize modulation parameters in real-time, and hence improve both efficiency and performance. Techniques such as reinforcement learning, and deep neural networks are being explored to predict optimal modulation strategies and enhance error correction.

Combining machine learning estimators, such as CNN with Autoregressive Network (ARN) for predicting Channel State Information (CSI) and RNN for channel forecasting, can enhance performance in massive MIMO systems with channel aging characteristics. Results indicate that this proposed model improves prediction accuracy and user throughput gains in both low and high mobility scenarios.

       5.    Polarization-Domain Modulation

Polarization-domain modulation leverages the polarization properties of electromagnetic waves to encode information. This technique provides increased data rates and spectral efficiency by utilizing both the amplitude and polarization states of the carrier signal. With advancements in polarization-sensitive materials and detection techniques, this method is becoming more feasible for 6G systems.

The polarization-resolved, wire-gate structure enables the device to respond distinctively to terahertz waves with different polarizations, allowing it to modulate terahertz waves according to their polarization.

The preparation method is straightforward and cost-effective, making it ideal for 6G communication devices. It allows for large-scale production with high precision.

The integration of SPPs, polarization resolution, and ultra-fast response, coupled with precise fabrication and high-volume production capabilities, makes it highly suitable for applications such as 6G communication encoders.

       6.    Terahertz (THz) Communication Modulation

As 6G explores the use of terahertz frequencies (100 GHz to 10 THz), new modulation techniques are being developed to handle these high frequencies. THz communication requires novel modulation schemes that can cope with the unique challenges of ultra-high-frequency signals, including increased free-space loss and atmospheric absorption. Researchers are investigating THz-specific modulation techniques to achieve high data rates and extended range for 6G.

THz communication primarily relies on THz modulation and demodulation technologies, THz detection and reception methods, and THz generation techniques. Efficient use of modulators can significantly reduce the complexity, cost, and size of THz systems, making modulation technology a key area of research in THz communication. Signal modulation involves using a modulation signal to control one or more parameters (such as amplitude or phase) of the carrier signal. Recently, a variety of THz modulators utilizing different materials and structures have been developed to achieve high modulation depth, rapid modulation speed, and broad modulation bandwidth.