Ph.D Thesis

Ph.D StudentTolmachev Alexander
SubjectDigitally Sub-Banded Coherent Optical OFDM Transmission
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Moshe Nazarathy


Coherent Optical communication has emerged as a go-to technology for next generation ultra-wide band photonic transmission, capitalizing on the multiple virtues of coherent transmission. However, a necessary enabler for this technology is the ability to process the vast data rates in currently available HW.

A central motif of this proposed thesis is to propose and investigate novel high performance yet computationally efficient methods of digital signal processing for coherent optical transmission.  A promising novel receiver Digital Signal Processing (DSP) approach has been developed, evaluated and experimentally demonstrated over laboratory and commercial optical links. We have developed novel methods for improved, more efficient digital partitioning of the transmission band into multiple sub-bands, leveraging modern DSP concepts in order to digitally process all the spectral sub-bands in parallel in an computationally efficient way.

We show that the complexity of digital signal processing in optical receivers scales super linearly with bandwidth (e.g., the complexity of processing twice a given bandwidth is much higher than the complexity of two given bandwidth systems). Therefore, frequency domain sub-band partitioning simultaneously slows down the required processing rate per sub-band, while also reducing the overall required computational complexity.  We invented a new under-decimated filter bank structure that takes advantages the orthogonality between of the adjacent sub-bands to achieve low complexity and effectively “brick-wall” ideal filter performance.

All functions of the sub-band receiver were reviewed and adapted for sub-band based processing. For some of these functions, namely carrier and phase recovery, novel efficient algorithms were developed and demonstrated.  In fact, these improved algorithms are not limited to sub-banded systems, but can also be applied in any high speed modem.

The resulting system was implemented in real-time over programmable hardware (HW) platform. We demonstrated a complete real-time receiver (Rx) over the fastest (180 Gb/s) filter-bank HW in just 3 FPGAs, at record 1.06 samples/symbol (7.3 b/Hz). The filter bank complexity and sampling rate savings imply that an ASIC implementation of our method for a long-range optical DFTS OFDM Rx would save up to 50% of the power of the DSP section of receiver ASIC.