Time-division multiplexing (TDM) is a method of sending several data streams over one shared channel by giving each stream its own recurring time slot. In free-space optical (FSO) links, where a laser beam carries data through the atmosphere or space, TDM is one way to raise capacity without adding more beams, wavelengths, or apertures.
Principle
The streams appear to travel simultaneously as sub-channels, but they actually take turns on the channel. Time is divided into recurrent fixed-length slots, one per sub-channel. A TDM frame holds one slot for each sub-channel, plus a synchronization channel and sometimes an error-correction channel.[1] The receiver uses the synchronization information to separate the slots back into the original streams.
Electrical and optical TDM
TDM can be applied in two places in an optical link:
- Electrical TDM combines the streams in the electronic domain before a single modulator puts the result on the laser carrier. It is simple and low-cost, but limited by electronics speed.
- Optical TDM (OTDM) interleaves short optical pulses from several lower-rate sources directly in the optical domain. This lets it go beyond the available electronic bandwidth, roughly 100 GHz today, and symbol rates up to 1.28 Tbaud per polarization have been demonstrated.[2]
History
Optical TDM is nearly as old as laser communication. In 1968, Bell Labs researchers Kinsel and Denton described a free-space system in which a He-Ne laser output was modulated and multiplexed to reach 448 Mbit/s.[2]
FSO applications and demonstrations
High-speed OTDM links. One experiment multiplexed eight 2.5 Gb/s base-rate signals from a repetition-tunable, actively mode-locked fiber ring laser into a 20 Gb/s OTDM packet. After 50 m of FSO transmission, the packet was demultiplexed with a polarization-insensitive four-wave-mixing scheme in a highly nonlinear fiber loop. The bit error rate reached the 10⁻⁹ level with steady operation over 24 hours.[3]
Low-cost audio and video links. A TDM-based FSO system using an acousto-optical modulator and a programmable microcontroller transmitted video and audio signals synchronously, and the authors reported it as flexible, practical and low cost.[4]
Hybrid multiplexing. TDM is often combined with other schemes. One patented approach time-division multiplexes digital streams, modulates them onto analog signals, frequency-division multiplexes those into one broadband signal, and phase-modulates it onto a laser carrier for free-space transmission.[5] Another proposal applies orbital angular momentum (OAM) multiplexing to the data streams that share a time slot, using free-space optics such as collimators, spiral phase plates, or q-plates.[1]
RF-over-FSO. Another patent describes separating an input radio signal into in-phase and quadrature components, quadrature-modulating them, and combining them in a time-division manner before driving an optical source.[6]
Position among other multiplexing techniques
FSO capacity is usually scaled by combining several domains, including time, wavelength, polarization and space.[7] Some authors argue that TDM, wavelength-division and polarization-division multiplexing have nearly reached their scalability limits, leaving the space domain (for example OAM or mode-division multiplexing) as the main route to further capacity.[8] In practice, TDM remains useful as a complement to those techniques.
See also
- Wavelength-division multiplexing (WDM)
- Orbital angular momentum multiplexing
- Free-space optical communication
- Optical time-division multiplexing
References
- “Time division multiplexed orbital angular momentum based communication.” US Patent 9,860,013. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/9860013
- “Optical Time Division Multiplexing – an overview.” ScienceDirect Topics. https://www.sciencedirect.com/topics/physics-and-astronomy/optical-time-division-multiplexing
- Hu, H., Qian, F., Xie, X., et al. “Demonstration of flexible optical time-division multiplexing system for high-speed free-space optical communications.” Journal of Optics (Springer). https://link.springer.com/article/10.1007/s12596-016-0318-5
- Wang, D., Zhou, W., Li, Z., et al. “Research on free-space optical communication based on time-division multiplexing.” Proc. SPIE 10020, 100201F (2016). https://doi.org/10.1117/12.2247801
- “System and method for free space optical communications using time division multiplexing of digital communication signals.” US Patent 6,246,498. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/6246498
- “Radio frequency signal transmission using free space optical communications.” US Patent 12,457,037. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/12457037
- Demonstration of flexible optical time-division multiplexing system for high-speed free-space optical communications (ResearchGate listing, which includes the survey of multiplexing domains). https://www.researchgate.net/publication/292678605
- “Optical time-division multiplexing: Topics.” Science.gov. https://www.science.gov/topicpages/o/optical+time-division+multiplexing