882 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 8, APRIL 15, 2008 Multimode Fiber as Random Code Generator— Application to Massively Parallel MIMO Transmission Maxim Greenberg, Student Member, IEEE, Moshe Nazarathy, Senior Member, IEEE, and Meir Orenstein Abstract—We propose a novel multiple-input multiple-output (MIMO) scheme over multimode fiber, acting as a distributed random code generator fed by spatial codes, using silicon pho- tonics in the transmitter and maximum-likelihood (ML) electronic detection in the receiver, providing an alternative to coarse wave- length division multiplexing (CWDM) for implementation of ultrahigh speed parallel transmission over short-range optical interconnects. The optical MIMO system utilizes mutually co- herent transmission and conventional direct detection with one-bit quantization, facilitating cost-effective application to 100 Gb/s links over 50 m. Index Terms—Integrated optics, maximum-likelihood decoding, multimode fiber, 100-Gbs Ethernet, optical fiber communication, optical MIMO, phase shift keying (PSK). I. INTRODUCTION T HERE has been renewed interest in ultra-high-speed transmission over short-range multimode fiber optical interconnects. State-of-the-art high-speed (10 Gb/s) multimode fiber (MMF) transmission over longer ranges (300 m–1 km) is essentially ISI-limited. A vast amount of research has been devoted to mitigating ISI by means of equalization techniques [1] or improved modulation formats such as optical OFDM [2]. On the other hand, over very short distances 50 m for which the modal dispersion does not onset yet, conventional single-input single-output (SISO) transmission is unable to sup- port high-speed (tens of Gb/s) communications, due to the un- availability of cost-effective transmitters supporting such ultra- high rates. Current approaches towards 100-Gb/s Ethernet high- speed implementations envision combinations of wavelength, time-division multiplexing (TDM), polarization, phase multi- plexing/diversity. A promising approach is the parallelization of multiple channels over a single physical fiber, by MIMO techniques. Several MIMO optical interconnect schemes have recently been proposed [1]–[9], taking advantage of the multiplicity of propagation modes of MMF and the modal diversity to achieve improved tradeoffs between the transmission throughput and power efficiency. Manuscript received June 6, 2007; revised November 18, 2007. The authors are with the Electrical Engineering Department, Technion— Israel Institute of Technology, Haifa 32000, Israel (e-mail: eemaxim@tx. technion.ac.il; nazarat@ee.technion.ac.il; naza@ieee.org; meiro@ee.technion. ac.il). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2007.915272 This paper explores an alternative optical multiple-input- multiple-output (MIMO) transmission architecture for ultrahigh speed short-range interconnects over MMF, based on the insight that the MMF channel matrix (CM) random nature provides a unique opportunity to generate and manipulate random codes, reliably conveying vast amounts of information in parallel over a single MMF interconnect. We propose and detail a novel technique for massive parallel transmission over MMF, briefly previewed in [1] and [2], in effect converting the fiber into an extended random code generator providing a close-to-ideal novel optoelectronic realization of the mathematical construct of random coding introduced by Shannon [1]. Despite the fact that random codes ideally attain channel capacity, to the best of our knowledge, they have never been considered for the realization of any optical or wireless communication system apparently due to the unavailability of a means to share the random codebook realizations between the transmitter (TX) and receiver (RX), and also due the excessive complexity required for random decoding. Our random coding approach capitalizes on the statistical multipath characteristics of the MMF propagation modes [8], exploring a new insight that the fiber naturally acts as the gen- erator of random codes, as well as the conveyor of these codes to the receive side. The fiber natural random propagation char- acteristics enable the random codewords generation and the dis- tribution of the entire codebook to the RX-side by means of a training sequence procedure to be detailed, performed in each coherence interval during which the CM is stable. To further improve the performance of the random code gen- eration, we introduce a second level of coding: MIMO-oriented spatial (deterministic) coding at the input. The optoelectronic feasibility of our system is made possible at this juncture by advances in optoelectronic components, in particular silicon photonics [1], using potentially simpler op- tics, eliminating the multiple coarse wavelength division multi- plexing (CWDM) sources in favor of a single CW laser coupled to a silicon-based array of high-speed PSK waveguide modu- lators. Such simplified MIMO TX structure, as proposed in [4] launches into the input facet of the MMF a multitude of mutu- ally coherent synchronized optical signals derived from a single laser source. Such coherent transmission can evidently be co- herently detected by mixing with a stabilized optical local os- cillator [6], [7] but this would be too complicated for short-range interconnects. A related coherent-transmission direct-detection system [16] makes use of a spatial light modulator in a multiple-input 0733-8724/$25.00 © 2008 IEEE