OAM Multiplexing in 6G: Advantages and Disadvantages

This article explores the pros and cons of using OAM (Orbital Angular Momentum) in wireless communication, specifically within the context of 6G technology. It highlights the benefits and drawbacks of Wireless OAM Multiplexing.

What is Wireless OAM Multiplexing?

Electromagnetic (EM) waves are characterized by a helical phase front in the direction of propagation. This characteristic is exploited in OAM to create multiple independent channels. OAM is often referred to as a “Vortex EM wave” because it possesses a phase rotation factor of exp (-jlφ).

The beam velocity and phase singularity characteristics of EM waves make it theoretically possible to achieve an unlimited number of OAM modes, or eigenstates. Since OAM modes are orthogonal to each other, multiple channels can be used to increase both capacity and spectral efficiency.

Initially, OAM was used primarily for optical communication. However, its potential is now being explored in microwave, millimeter-wave (mmWave), and THz frequency ranges.

OAM multiplexing has various applications, including:

• Deep space and near-Earth optical communications
• LAN-to-LAN connections in campus or city environments
• Wireless communication to carry EM waves in 6G technology

Benefits or Advantages of OAM Multiplexing in Wireless Communication (6G)

Here are the advantages of using OAM Multiplexing in wireless communication, as envisioned for 6G technology:

• Increased Transmission Rate: OAM can significantly increase the transmission rate in point-to-point links, such as wireless backhaul and fronthaul connections.
• Improved Spectrum Efficiency: OAM enables EM wave multiplexing transmission, which increases the spectrum efficiency of future 6G wireless networks.
• Mode Division Multiple Access: OAM offers a new access method called Mode Division Multiple Access (MDMA). This allows supporting more users without requiring additional time or frequency resources.
• Enhanced Reliability: OAM offers improved reliability for anti-jamming techniques used in wireless communication.
• Suitable for Long-Distance Communication: OAM-based MIMO systems are well-suited for long-distance communication in open areas.

Drawbacks or Disadvantages of Wireless OAM Multiplexing

Here are the challenges and disadvantages associated with Wireless OAM Multiplexing:

• Complex Multiplexing/Demultiplexing: The multiplexing and demultiplexing processes for radio frequency waves are complex to implement.
• Difficult Beam Combining/Splitting: Combining and splitting OAM radio waves are challenging due to their much longer wavelength compared to optical waves. This makes coaxial transmission difficult to ensure.
• Specialized Devices and Losses: To overcome the above limitation, specialized devices are required, which can introduce insertion loss and reduce the system’s overall efficiency.
• Alignment Requirements: The transmitter and receiver must be carefully aligned with each other to ensure proper separation of different OAM modes.
• Sensitivity to Fading: Wavefront phases of OAM beams can be affected by fading phenomena such as rain, fog, and atmospheric turbulence at the receiver.
• Divergence and Distance Limitations: As the order of the OAM mode increases, the OAM beam becomes increasingly divergent. This severely reduces the effective transmission distance and decreases the spectrum efficiency of OAM-based wireless communication.
• Demodulation Challenges: Identifying different OAM modes and demodulating the information modulated onto these “eddy EM waves” are significant challenges.