Orbital Angular Momentum (OAM) Multiplexing for 6G
- Venkateshu
- 4 days ago
- 4 min read
Introduction
As 6G research gathers momentum, the demand for higher data rates, massive connectivity, and extreme spectral efficiency is pushing the boundaries of wireless communication. One of the most promising and futuristic solutions is Orbital Angular Momentum (OAM) multiplexing — a revolutionary technique that uses the twisting nature of electromagnetic (EM) waves to create multiple data channels over the same frequency.
What is Orbital Angular Momentum (OAM)?
Basic Wave Properties
Electromagnetic waves can carry energy and momentum. Two fundamental forms of angular momentum in EM waves are:
Spin Angular Momentum (SAM)
Associated with polarization (right-hand(RCP) or left-hand circular polarization(LCP)).
Limited to 2 modes: left circular or right circular (±1 spin).
Example: A flat, round disc spinning in place. No pattern on it — just rotation.
Orbital Angular Momentum (OAM)
Associated with the spatial structure of the wavefront.
An EM wave carrying OAM has a helical or twisted phase front.
You can have many OAM modes (l = 0, ±1, ±2, …), each one adding a different amount of twist
Example: Spiral staircase — twisting outward in space as it moves forward.
What Does a Twisted Wave Mean?
A beam with OAM has a helical phase front — imagine a screw or a spiral staircase. The wavefront doesn't propagate as a flat sheet but spirals around the direction of propagation.
Mathematically, this can be represented as exp(ilϕ) Where:
l is the topological charge (OAM mode number),
Φ is the azimuthal angle,
exp(ilϕ) gives the beam its helical phase.
Each integer l corresponds to a unique OAM mode, and these modes are orthogonal — a critical property that allows OAM multiplexing.
Think of Twisted Beams Like Spiral Staircases
Imagine multiple people walking up different spiral staircases that are twisted but never collide — because each staircase spirals differently. Similarly, each OAM mode represents a unique helical twist, allowing multiple beams to coexist in the same spatial, spectral, and polarization domain.
How OAM Multiplexing Works
Multiplexing Explained
Multiplexing is a technique to send multiple signals simultaneously using the same channel — by differentiating them in time (TDM), frequency (FDM), code (CDM), or space (SDM).
OAM multiplexing transmission is a technique to increase the number (multiplexing number) of data signals to be transmitted at the same time by transmitting different signals using radio waves having different OAM modes [1, 2]. The number of phase rotations is called an OAM mode. When an electromagnetic wave has this OAM property, the trace of the same phase takes on a helical shape in the direction of propagation.
OAM introduces mode-division multiplexing (MDM):Instead of slicing time or frequency, we twist the beam differently for each data stream.
Technical Process
Generation:
An antenna or optical device (like a spiral phase plate or metasurface) generates beams with different OAM modes (e.g., l=0,±1,±2,…l = 0, m1, m2, l=0,±1,±2,…).
Each mode is assigned a unique data stream.
Propagation:
All modes propagate simultaneously in the same frequency band and polarization.
Reception:
At the receiver, mode demultiplexing is performed using spatial filtering or inverse-phase devices to extract each data stream separately.
Why is OAM Multiplexing Possible?
The key enabler is orthogonality. Just as sine and cosine functions don't interfere, OAM modes with different topological charges are mutually orthogonal in the spatial domain.
This means:
No crosstalk (ideally) between modes.
Multiple independent data streams can be transmitted without consuming more bandwidth.
Why OAM for 6G?
With 6G pushing into sub-THz and THz frequency ranges and requiring massive spatial multiplexing, OAM fits perfectly:
Works best in line-of-sight (LoS) or quasi-LoS environments (like UAVs, satellite-to-ground, or backhaul links).
Can be combined with Massive MIMO, beamforming, and AI-based adaptive optics for mobile scenarios.
Exploits the spatial structure of beams — an untapped degree of freedom in traditional wireless systems.
Benefits of OAM in 6G
Feature | Advantage |
High Capacity | Multiple OAM modes = parallel data streams over same frequency |
Spectral Efficiency | No need to allocate separate time or frequency resources |
Low Interference | Orthogonality minimizes inter-stream interference |
Scalability | Potential to use dozens of modes, limited mainly by hardware and environment |
Implementing OAM in 6G Systems
Practical Techniques:
Spiral Phase Plates: Introduce a helical phase shift.
Metasurfaces: Programmable 2D surfaces that dynamically shape beam profiles.
Phased Arrays: Modify element phasing to generate and steer OAM beams.
Hybrid Architectures:
OAM + MIMO: Use spatial multiplexing plus mode multiplexing.
OAM + Beamforming: Focus each twisted beam to minimize divergence.
Role of AI:
Dynamic mode tracking
Real-time correction of atmospheric distortions
Optimized demultiplexing and decoding
Real-World Examples
Case Study 1: Terahertz OAM Communication
Frequency: 0.3 THz
Setup: 6 OAM modes used to transmit data over 100 meters
Throughput: Over 100 Gbps
Application: 6G backhaul, indoor massive data offloading
Case Study 2: Optical Wireless OAM
Using twisted light beams (laser-based), researchers transmitted 2.5 Tbps using 26 OAM modes — all within a single optical link.
Challenges to Overcome
Challenge | Description |
Mode Crosstalk | Caused by atmospheric turbulence or reflection |
Beam Divergence | Higher-order OAM beams tend to spread more |
Line-of-Sight Dependency | Needs precise alignment or adaptive optics |
Hardware Maturity | Compact, tunable OAM antennas are still evolving |
Conclusion
OAM multiplexing transforms the spatial structure of EM waves into a new communication dimension. It offers a unique path to meet 6G’s ambitious performance goals—especially in high-frequency, high-capacity scenarios like UAV networks, backhaul, and satellite links.
As enabling technologies mature—OAM antennas, beam tracking, THz systems, and AI-driven optics—we're not just adding more capacity, we're redefining how we use the wave itself.
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