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Orbital Angular Momentum for Wireless Communications
Wenchi Cheng, Wei Zhang, Haiyue Jing, Shanghua Gao, Hailin Zhang
TL;DR
Wireless networks face growing capacity and user demands as traditional resources become efficiently utilized. This paper surveys OAM wireless communications and proposes OAM-mode multiuser access, finding higher spectrum efficiency than traditional FDMA.
Problem
Growing data traffic and user demand challenge wireless networks as traditional resources such as frequency, time, and space become efficiently utilized.
Method
The paper surveys fundamental OAM issues and proposes an OAM-mode-based orthogonal multiuser access framework for wireless communications.
Results
OAM-mode transmission has larger spectrum efficiency than traditional FDMA, with efficiency increasing as more than one OAM-mode is used.
Takeaways & Limitations
OAM provides a new mode domain for wireless networks and opportunities for future wireless communications.
Takeaways & Limitations
Designing an efficient antenna structure for OAM communications remains difficult.
Abstract
from arXiv · showhide
As the traditional resources (frequency, time, space, etc.) are efficiently utilized, it becomes more and more challenging to satisfy the ever-lasting capacity-growing and users-boosting demand in wireless networks. Recently, the electromagnetic (EM) wave was found to possess not only linear momentum, but also angular momentum. The orbital angular momentum (OAM) is a kind of wavefront with helical phase. The OAM-based vortex wave has different topological charges, which are orthogonal to each other, bridging a new way for multiple access in wireless communications. In this article, we introduce the fundamental theory of OAM and the OAM based wireless communications. The research challenges regarding OAM signal generation, OAM beam converging, and OAM signal reception are discussed. Further, we propose a new multiuser access with different OAM-modes in wireless networks, where multiple OAM-modes are used as a new orthogonal dimension for interference avoidance. Simulation results reveal the inherent property of OAM waves and show that OAM based radio transmission can significantly increase the spectrum efficiency in wireless networks.
I. INTRODUCTION
Wireless networks face growing capacity and user-demand pressures despite extensive use of frequency, time, and spatial resources. The paper surveys OAM communications and proposes OAM-mode-based orthogonal multiuser access to improve spectrum efficiency.
- Explosive data traffic makes higher capacity and support for more users increasingly difficult with traditional frequency-, time-, and space-based resources.
- Different OAM-modes are orthogonal and can be multiplexed or demultiplexed, providing an additional access dimension beyond time and frequency.
- Experiments have demonstrated the feasibility of OAM wireless communications, including shared-frequency modes and high-capacity mmWave transmission.
- Key unresolved challenges include generating and receiving multiple mixed OAM-modes, converging divergent hollow beams, and estimating phase errors from misalignment or fading.
- The paper surveys OAM fundamentals and challenges, then proposes an OAM-mode-based orthogonal multiuser access framework and evaluates its spectrum efficiency.
II. WHAT IS OAM?
OAM describes electromagnetic waves with spiral phase structure and an orbital rotational degree of freedom. Its mode-dependent wavefronts are orthogonal, while higher-order modes become more hollow and lose power gain, creating a transmission challenge.
- OAM is an electromagnetic-wave property in which the phase varies as a spiral around the propagation direction rather than as a plane.
- OAM waves use the phase factor exp(ilϕ), where l is the mode order or index and ϕ is the azimuthal angle.
- Pure OAM-modes have integer orders, and different modes are orthogonal; non-integer modes can be represented as sums of orthogonal OAM-modes.
- In a 16-element uniform circular array, the illustrated modes are 0, 1, 2, and 3, with mode 0 representing the plane-electromagnetic wave.
- As OAM-mode order increases, the phase spiral becomes more complex, the central hollow grows, and power gain decreases.
- Direct use of OAM-modes is unsuitable for long-distance transmission because higher-order beams lose power gain; beam convergence is therefore needed.
III. THE OAM BASED WIRELESS COMMUNICATIONS
OAM-based wireless communication uses orthogonal mode-domain resources to provide additional access and spectrum-efficiency opportunities. The paper reviews generation, transmission, convergence, and reception methods while identifying unresolved practical challenges.
- Advantages: Different OAM-modes are orthogonal, enabling parallel transmission and increased spectrum efficiency without consuming additional traditional frequency, time, code, or power-domain resources.Mode-domain resources can also be jointly used with frequency/time/code-domain resources.
- Advantages: Mode division multiple access assigns different OAM-modes to different users for orthogonal access without consuming more frequency and time resources.The paper contrasts this mode-domain approach with power-domain non-orthogonal multiple access.
- Advantages: OAM-mode hopping is proposed as a potential anti-jamming technique, either within a narrow band or jointly with frequency hopping in a wide band.The paper motivates this approach by limitations of conventional frequency hopping under increasing spectrum pressure.
- Research challenges: Three unresolved challenge categories concern radio vortex signal generation, transmission, and reception.Transmission challenges include alignment, fading, and beam convergence; reception requires phase detection to distinguish OAM-mode orders.
- Signal generation: SPP, UCA, and metasurface antennas generate radio vortex signals, but each has distinct trade-offs involving frequency range, simultaneous modes, divergence, attenuation, or phase control.SPP offers small divergence and low attenuation but cannot support relatively low frequencies or simultaneous multiple modes; UCA supports multiple modes but produces divergent, centrally hollow beams; metasurfaces are compact and inexpensive but have phase-control limitations.
- Beam convergence: Lens antennas effectively converge OAM beams, reducing the central hollow and increasing beam intensity, while higher-order unconverged modes exhibit larger central hollows.The illustrated comparison uses OAM modes 0, 1, 2, and 3 at 35 GHz.
IV. OAM-MODES BASED MULTIPLE ACCESS: A CASE STUDY
The case study uses OAM modes as an additional orthogonal access dimension, assigning modes across macrocells and small cells to reduce interference. Simulations in 3GPP Rel-12 small-cell scenarios show higher spectrum efficiency than traditional FDMA, while practical challenges constrain deployment.
- Access design: The network model contains one macrocell and several small cells, with PE beams used in the macrocell and OAM beams assigned to small-cell groups.The design allows macrocells and small cells to use PE and OAM waves, respectively, without cross-layer interference.
- Access design: OAM-mode allocation and scheduling assign different modes to neighboring small cells, while distant cells may reuse the same modes to minimize interference.The proposed access scheme uses mode, frequency, and time domains jointly when available OAM modes are insufficient.
- Performance evaluation: More than one OAM mode yields higher spectrum efficiency than traditional FDMA, and efficiency increases as the number of OAM modes grows.The comparison uses received SNR, user density 0.2 users/m2, and two frequency-orthogonal channels in the stated evaluation setting.
- Performance evaluation: At relatively large SNR, spectrum efficiency reaches a ceiling because interference from other users increases with operating conditions.The reported ceiling is attributed to residual multiuser interference rather than a loss of OAM-mode orthogonality.
- Performance evaluation: With increasing user density, four OAM modes provide much higher spectrum efficiency than one mode because more users can be interference-free.The study also reports that one OAM mode at 1.0 users/m2 is similar to one mode at 1.2 users/m2.
V. CONCLUSIONS
The article introduces OAM theory and wireless-communication applications, then proposes OAM-based multiuser access and studies it in two-tier networks. Results verify beam convergence and show enhanced spectrum efficiency, positioning OAM as a new mode domain for future wireless research.
- The article introduces the fundamental theory of OAM and its application to wireless communications, including associated benefits and challenges.
- It proposes multiuser access based on OAM beams and studies the approach in two-tier wireless networks.
- Performance results verify convergence for OAM beams and show enhanced spectrum efficiency for wireless networks.
- The conclusion identifies OAM as a new mode domain offering opportunities for future wireless-communications research.