טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAgmon Amos
SubjectMIMO Communication over Multimode Optical Interconnects
by Model Beamforming
DepartmentDepartment of Electrical Engineering
Supervisor Professor Moshe Nazarathy
Full Thesis textFull thesis text - English Version


Abstract

MultiMode Fiber (MMF) is the first guided optical medium to have been introduced for data transmission some 30 years ago. While Single Mode Fiber is the medium of choice for the long-haul transmission, there has recently been a renaissance in MMF techniques, aimed at increasing the performance of MMF links to multi-Gb/s rates.

The advent of Multiple Input Multiple Output (MIMO) techniques in wireless transmission has inspired porting MIMO to guided multimode optical media. However optical MIMO differs from its wireless counterpart in that it is preferable to use square-law optical direct detection rather than coherent detection, preventing straightforward porting RF MIMO techniques into the optical arena, but providing a new MIMO paradigm.

Most optical MIMO works to date, tacitly assume incoherent transmission, i.e. the signals coming from different transmitters are orthogonal (e.g. driven by independent mutually incoherent optical sources), satisfying optical power superposition. This work explicitly addresses two optical MIMO communication regimes both using direct detection receivers, but differing in the optical transmitter: incoherent transmission direct detection vs. coherent transmission direct detection. We derive an extensive quadratic MIMO channel model making use of quadratic forms instead of linear responses, and extending the description to multi-mode detectors capturing optical field multiple degrees of freedom in their apertures.

We treat the optical MIMO broadcast channel for the first time, namely a point-to-multipoint optical communication system signaling simultaneously to different users over the same frequency band (without wavelength division multiplexing). We port to optical MIMO the Zero Forcing BeamForming (ZFBF) techniques recently emerging in wireless MIMO broadcast.  However, unique to the optical predicament, our approach resolves the complication of field interference cross-terms resulting from quadratic detection of mutually coherent modulated optical sources.

The proposed ZFBF optical MIMO broadcast system provides multiple decoupled virtual sub-channels to independent receivers over multimode optical fibers or waveguides. A practical application of the proposed ZFBF technique to short-reach Passive Optical Networks (PON) makes use of a combined space-time division Multiplexing, improving upon the conventional time division multiplexing PON approach, mitigating the inherent power inefficiency of passive splitters and showing significant multiplexing gains for the same transmit power.

The ZFBF technique is finally applied to orthogonal polarizations over a single mode fiber. A novel practical application to SMF long-reach PONs is proposed, theoretically demonstrating a multiplexing gain of 2 with another key advantage of increased flexibility in user power allocation.