Cooperative Modulation: Pre-Equalization and Coordinated Constellation Formation

The growing demand for spectrum resources necessitates the urgent need for innovative physical-layer solutions that maximize spectral efficiency. Over the past decades, multiple access (MA) schemes have been extensively developed to improve spectral utilization and support more users. However, existing standards are based solely on orthogonal multiple access (OMA) schemes, which restrict multiple users from sharing the same channel, inherently limiting overall spectral efficiency. Although recent research has proposed various non-orthogonal multiple access (NOMA) alternatives, these approaches face significant practical challenges, including receiver complexity, fairness issues, and scalability concerns, which cast doubt on their short-term feasibility. To address these limitations, a new uplink MA framework, termed cooperative modulation (CoM), is introduced. The scheme enables simultaneous multiuser transmission over a single channel through user-side channel pre-equalization combined with a deterministic phase-rotation design. In CoM, users transmit precoded modulated symbols that cooperatively form a structured composite constellation at the receiver. This precoding requires no information exchange among users; each user independently applies channel pre-equalization and a fixed phase shift prior to transmission. Unlike existing rotation-based or NOMA schemes, the phase-rotation optimization in CoM is rendered channel-independent due to pre-equalization, allowing the design to scale to arbitrary numbers of users and modulation orders through offline lookup tables. It is also noted that practical limits on the number of users arise from computational complexity and the minimum Euclidean distance of the composite constellation rather than from theoretical constraints. Unlike conventional NOMA, CoM avoids iterative decoding and successive interference cancellation, making it more suitable for practical large-scale deployments. The phase-shift design is identified as a critical performance factor and is examined in detail. Through theoretical analysis and extensive simulations, including the impact of imperfect CSI across a wide range of SNR values and user densities, substantial gains in error performance, spectral efficiency, and computational complexity are demonstrated relative to OMA, NOMA, and recent index-modulated MA schemes. These results position CoM as a promising approach for future spectrally efficient uplink communication systems.