4 ms·

Me, too thinks that this is just (a subset of) MIMO in disguise. The advantage of the antenna array you mention is, that it can precode/decode with any channel

Me, too thinks that this is just (a subset of) MIMO in disguise. The advantage of the antenna array you mention is, that it can precode/decode with any channel matrix, while these antennas implement one fixed matrix.

bjornsing 15y ago
I get the same feeling. MIMO basically exploits that there can be multiple propagation "paths" over the same frequency and that those can each carry information up to the Shannon capacity. Paths can be separated by geometry (reflections etc) or polarity. This Orbital Angular Momentum is probably just another way to create more "paths" that can be exploited with MIMO. In complex radio environments (like urban cellular networks) you get so many reflections that the limitation to MIMO is usually not too few separate paths but rather too many, with too much correlation between them. I'm not sure this would help very much there. But in simple radio environments, like point-to-point links you often are limited by the number of separate paths (you can have two based on Spin Angular Momentum aka polarity and there are lots of products out there that do that). In short; I think these guys are barking up the wrong tree in trying to adapt this for cellular, they should stay in point-to-point. They will be very welcome in that space if they can go from 2x2 to 4x4 MIMO across a point-to-point link.
- henryaym 15y ago
  This doesn't have to do with different propagation paths. The Orbital Angular Momentum (OAM) is an inherent wave property, like Wavelength or Frequency. Just like you can listen to radio on different frequencies (eg 660AM, 1010AM), you can think about this technology as "listening" on different OAM channels. The twist in the wavefront can be thought of as a newly available subset of channels. This has been a a field of research that has been primarily been developed in Optics, in fact, my PhD research included creating ultrafast (femtosecond), supercontinuum (white light) vortices that are capable of transmitting information over 2^L channels where L is the amount of twist the light has.
  - gallamine 15y ago
    Is this "twist" related to the circular polarization of light?
    - henryaym 15y ago
      Quick Answer: No. Longer Answer: Sorta. Circular polarization of light (or waves in general) corresponds to a spin angular momentum that arises as the polarization of the light is "spinning". This "twist" is the wavefronts phase rotating or being staggered as it travels forward. Think of the wavefront of the twisted wave as looking like a piece of spiral pasta. It is this spiraling that corresponds to orbital angular momentum.
      - jpdoctor 15y ago
        > orbital angular momentum Under what circumstance can you change the OAM in order to exert a torque? (or maybe vice-versa: What is an example of torque exerting a change in OAM on the wave?) TIA.
        henryaym 15y ago
        If I understand your question.. You can prepare waves in such a way that forces them into this spiraled waveform state. This can be done in a cavity (in the case of lasers) as there are solutions of the wave equation that give rise to OAM; or this can be done by using diffractive optics, like a hologram, that somewhat force the wave into this state. It is this case that you can think of a torque being exerted onto the waves. Waves too, can exert torque on small particles (micron sized polystyrene spheres for instance). Light with spin or orbital angular momentum can be used to make these small object rotate!
        jpdoctor 15y ago
        > Light with spin or orbital angular momentum can be used to make these small object rotate! Thank you, that was exactly the effect I was looking for. Any links handy? Fascinating stuff, this from an old microwave-digital guy.

bjornsing 15y ago

I get the same feeling. MIMO basically exploits that there can be multiple propagation "paths" over the same frequency and that those can each carry information up to the Shannon capacity. Paths can be separated by geometry (reflections etc) or polarity. This Orbital Angular Momentum is probably just another way to create more "paths" that can be exploited with MIMO. In complex radio environments (like urban cellular networks) you get so many reflections that the limitation to MIMO is usually not too few separate paths but rather too many, with too much correlation between them. I'm not sure this would help very much there. But in simple radio environments, like point-to-point links you often are limited by the number of separate paths (you can have two based on Spin Angular Momentum aka polarity and there are lots of products out there that do that). In short; I think these guys are barking up the wrong tree in trying to adapt this for cellular, they should stay in point-to-point. They will be very welcome in that space if they can go from 2x2 to 4x4 MIMO across a point-to-point link.

henryaym 15y ago
This doesn't have to do with different propagation paths. The Orbital Angular Momentum (OAM) is an inherent wave property, like Wavelength or Frequency. Just like you can listen to radio on different frequencies (eg 660AM, 1010AM), you can think about this technology as "listening" on different OAM channels. The twist in the wavefront can be thought of as a newly available subset of channels. This has been a a field of research that has been primarily been developed in Optics, in fact, my PhD research included creating ultrafast (femtosecond), supercontinuum (white light) vortices that are capable of transmitting information over 2^L channels where L is the amount of twist the light has.
- gallamine 15y ago
  Is this "twist" related to the circular polarization of light?
  - henryaym 15y ago
    Quick Answer: No. Longer Answer: Sorta. Circular polarization of light (or waves in general) corresponds to a spin angular momentum that arises as the polarization of the light is "spinning". This "twist" is the wavefronts phase rotating or being staggered as it travels forward. Think of the wavefront of the twisted wave as looking like a piece of spiral pasta. It is this spiraling that corresponds to orbital angular momentum.
    - jpdoctor 15y ago
      > orbital angular momentum Under what circumstance can you change the OAM in order to exert a torque? (or maybe vice-versa: What is an example of torque exerting a change in OAM on the wave?) TIA.
      - henryaym 15y ago
        If I understand your question.. You can prepare waves in such a way that forces them into this spiraled waveform state. This can be done in a cavity (in the case of lasers) as there are solutions of the wave equation that give rise to OAM; or this can be done by using diffractive optics, like a hologram, that somewhat force the wave into this state. It is this case that you can think of a torque being exerted onto the waves. Waves too, can exert torque on small particles (micron sized polystyrene spheres for instance). Light with spin or orbital angular momentum can be used to make these small object rotate!
        jpdoctor 15y ago
        > Light with spin or orbital angular momentum can be used to make these small object rotate! Thank you, that was exactly the effect I was looking for. Any links handy? Fascinating stuff, this from an old microwave-digital guy.