Capacity limits of spatially multiplexed free-space communication

Journal name:
Nature Photonics
Volume:
9,
Pages:
822–826
Year published:
DOI:
doi:10.1038/nphoton.2015.214
Received
Accepted
Published online
Corrected online

Increasing the information capacity per unit bandwidth has been a perennial goal of scientists and engineers1. Multiplexing of independent degrees of freedom, such as wavelength, polarization and more recently space, has been a preferred method to increase capacity2, 3 in both radiofrequency and optical communication. Orbital angular momentum, a physical property of electromagnetic waves discovered recently4, has been proposed as a new degree of freedom for multiplexing to achieve capacity beyond conventional multiplexing techniques5, 6, 7, 8, 9, and has generated widespread and significant interest in the scientific community10, 11, 12, 13, 14. However, the capacity of orbital angular momentum multiplexing has not been established or compared to other multiplexing techniques. Here, we show that orbital angular momentum multiplexing is not an optimal technique for realizing the capacity limits of a free-space communication channel15, 16, 17 and is outperformed by both conventional line-of-sight multi-input multi-output transmission and spatial-mode multiplexing.

At a glance

Figures

  1. Schematic of transmission systems.
    Figure 1: Schematic of transmission systems.

    a, Schematic of a canonical LOS system. b, Transmitter arrangement for conventional LOS MIMO, where each small circle represents one of the parallel beams. c, Receiver arrangement, where the receiver aperture is divided into hexagonal cells to maximize the fill factor.

  2. Comparison of singular values of the transmission matrix in descending order for different multiplexing/demultiplexing methods.
    Figure 2: Comparison of singular values of the transmission matrix in descending order for different multiplexing/demultiplexing methods.

    ac, Singular values for M= 3 (a), M = 9 (b) and M = 21 (c). SMM and conventional LOS MIMO have much larger singular values than OAM when the SBP is large, as in b and c.

  3. Spectral efficiencies and effective degrees of freedom (EDOF) for different multiplexing/demultiplexing methods.
    Figure 3: Spectral efficiencies and effective degrees of freedom (EDOF) for different multiplexing/demultiplexing methods.

    ac, Spectral efficiencies with M = 3 (a), M = 9 (b) and M = 21 (c). df, EDOF with M = 3 (d), M = 9 (e) and M = 21 (f). SMM and conventional LOS MIMO offer much larger spectral efficiencies and EDOF for large SBPs, as in c and f.

Change history

Corrected online 27 November 2015
In the version of this Letter originally published, in Fig. 3a,b, the x axis labels should have been ‘SNR (dB)’. This has now been corrected in the online versions of the Letter.

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Author information

Affiliations

  1. Key Laboratory of Optoelectronics Information Technology of Ministry of Education, College of Precision Instrument and Opto-Electronic Engineering, Tianjin University, Tianjin, China

    • Ningbo Zhao,
    • Xiaoying Li &
    • Guifang Li
  2. CREOL, The College of Optics & Photonics, University of Central Florida, Orlando, Florida 32816-2700, USA

    • Guifang Li
  3. Edward L. Ginzton Laboratory, Stanford University, 348 Via Pueblo Mall, Stanford, California 94305-4088, USA

    • Joseph M. Kahn

Contributions

N.Z., G.L. and J.M.K. conceived and designed the theoretical model. N.Z. and X.L. performed the simulations. N.Z., G.L. and J.M.K. analysed the data. N.Z., G.L. and J.M.K. co-wrote the paper.

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The authors declare no competing financial interests.

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