Experimental evaluation of 3D printed spiral phase plates for enabling an orbital angular momentum multiplexed radio system

This paper evaluates the performance of three-dimensionally (3D) printed spiral phase plates (SPPs) for enabling an orbital angular momentum (OAM) multiplexed radio system. The design and realization of the SPPs by means of additive manufacturing exploiting a high-permittivity material is described. Modes 1 and 2 SPPs are then evaluated at 15 GHz in terms of 3D complex radiation pattern, mode purity and beam collimation by means of a 3D printed dielectric lens. The results with the lens yield a crosstalk of −8 dB for between modes 1 and −1, and −11.4 dB for between modes 2 and −2. We suggest a mode multiplexer architecture that is expected to further reduce the crosstalk for each mode. An additional loss of 4.2 dB is incurred with the SPPs inserted into the communication link, which is undesirable for obtaining reliable LTE-based communications. Thus, we suggest: using lower loss materials, seeking ways to reduce material interface reflections or alternative ways of OAM multiplexing to realize a viable OAM multiplexed radio system.

3-Given that OAMs are non-plane waves, is there a scope for use of reflective martials without comprising the orthogonality of OAMs in non-line of sight channels? 4-On the crosstalk levels, would the use of spatial filters (digital eg using the null-space) help in reducing crosstalk, this is related to comment no 2 5-Page 4, first line, it's grammatically more accurate to use "that" in "means it is important..." 6-In Page 3, lines 39-42, spaces are required between the number and the start of the lines 7-The authors are recommended to include a channel scattering matrix to demonstrate the individual coupling coefficients for a sample frequency with and without lens to improve readability 8-Equation (4) seems fundamental to the orthogonality of OAMs, although the authors discuss angular fourier transform, it's not entirely clear equation (5) dictates equation (4), would be useful to clarify this. As a reader, I expect the optimal solution for equation (4) is fourier-based. 9-It's clear that the lens improved the channel gain of the diagonal channel coefficients however the impact on the off-diagonal is unclear. The authors may want to rephrase, addressing (7) will resolve this point.
Overall, the paper is definitely worth publishing and the subject is timely, relevant and stimulating.

28-Oct-2019
Dear Dr Allen On behalf of the Editors, I am pleased to inform you that your Manuscript RSOS-191419 entitled "Experimental Evaluation of 3D Printed Spiral Phase Plates for enabling an Orbital Angular Momentum Multiplexed Radio System" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the referees' comments at the end of this email.
The reviewers and handling editors have recommended publication, but also suggest some minor revisions to your manuscript. Therefore, I invite you to respond to the comments and revise your manuscript.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. Comments to the Author(s) Some minor comments: The authors presented the PPS and lens for OAM generation, and implementation and measurement results are provided. 1. The authors are suggested to correct some grammatically issues in the paper. 'We suggest a mode multiplexer architecture that is expected to further reduce the crosstalk for each mode is employed.' Which appeared twice in the paper.
In summary, 'seek ways to reduce material' should be 'seeking ways …'; 'topological charge' should be 'topological change' 'This compares with 13 dB achieved', should it be -13 dB?
On page 10, 'with this lens and this a larger diameter lens or sub …', please rewrite.
2. The authors use 'S' to represent two different parameters in the paper, please change one of them.

Reviewer: 2
Comments to the Author(s) Use of OAM in the millimetre wave (mmwave) regime is certainly a promising approach to deliver increased spectral efficiency, especially given that both OAM and mmwave require lineof-sight to achieve meaningful gain in system capacity. The paper is very well-written and easy to 3-Given that OAMs are non-plane waves, is there a scope for use of reflective martials without comprising the orthogonality of OAMs in non-line of sight channels? 4-On the crosstalk levels, would the use of spatial filters (digital eg using the null-space) help in reducing crosstalk, this is related to comment no 2 5-Page 4, first line, it's grammatically more accurate to use "that" in "means it is important..." 6-In Page 3, lines 39-42, spaces are required between the number and the start of the lines 7-The authors are recommended to include a channel scattering matrix to demonstrate the individual coupling coefficients for a sample frequency with and without lens to improve readability 8-Equation (4) seems fundamental to the orthogonality of OAMs, although the authors discuss angular fourier transform, it's not entirely clear equation (5) dictates equation (4), would be useful to clarify this. As a reader, I expect the optimal solution for equation (4) is fourier-based. 9-It's clear that the lens improved the channel gain of the diagonal channel coefficients however the impact on the off-diagonal is unclear. The authors may want to rephrase, addressing (7) will resolve this point.
Overall, the paper is definitely worth publishing and the subject is timely, relevant and stimulating.

14-Nov-2019
Dear Dr Allen, It is a pleasure to accept your manuscript entitled "Experimental Evaluation of 3D Printed Spiral Phase Plates for enabling an Orbital Angular Momentum Multiplexed Radio System" in its current form for publication in Royal Society Open Science. The comments of the reviewer(s) who reviewed your manuscript are included at the foot of this letter.
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