Alex Tolmachev

Sub-banded / single-sub-carrier drop-demux and flexible spectral shaping with a fine resolution photonic processor

Spectral processor based on arrayed waveguide grating and free-space manipulation is capable of arbitrary filtering at record metrics of 0.8GHz resolution over 200GHz span. Narrowband coherent drop-demultiplexing and controlled optical shaping is demonstrated in unison with digital sub-banding.

Filter-bank based efficient transmission of reduced-guard-interval OFDM.

We propose a new way to structure the digital signal processing for reduced guard-interval (RGI) OFDM optical receivers. The idea is to digitally parallelize the processing over multiple parallel virtual sub-channels, occupying disjoint spectral sub-bands. This concept is well known in the optical or analog sub-carrier domains, but it turns out that it can also be performed efficiently in the digital domain. Here we apply critically sampled uniform analysis and synthesis DFT filter bank signal processing techniques in order to realize a novel hardware efficient variant of RGI OFDM, referred to as Multi-Sub-Band OFDM (MSB-OFDM), reducing by 10% receiver computational complexity, relative to a single-polarization version of the CD pre-equalizer. In addition to being more computationally efficient than a conventional RGI OFDM system, the signal flow architecture of our scheme is amenable to being more readily realized over multiple FPGAs, for experimental demonstrations or flexible prototyping.

OFDM/WDM PON with laserless, colorless 1 Gb/s ONUs based on Si-PIC and slow IC

We introduce a next-generation long-reach access optical network (35 dB loss budget +2 dB margin) delivering up to 40G/40G per passive 1:256 optical distribution network, supporting symmetrical 1 Gb/s rates per home user or up to 40 Gb/s for business users (e.g., enterprises, antenna sites). The proposed system is based on a novel spectrally efficient orthogonal frequency division multiplexing/wavelength division multiplexing OFDM/WDM architecture symmetrically using 16-QAM OFDM polarization diversity in both the downstream and upstream in order to serve low-cost energy-efficient symmetric 1 Gb/s optical network units (ONUs), which are self-coherent, laserless, colorless, and tunable-filter-free. Each ONU comprises a standard semiconductor optical amplifier (SOA), a silicon-based photonic integrated circuit (PIC), and mixed-signal electronic integrated circuits (ICs) performing the signal processing at a relatively slow rate as compared with the overall passive optical network (PON) throughput: digital to analog converters (DACs) and analog to digital converters (ADCs) at 417 MS/s for the home user ONUs.

Subbanded DSP Architectures Based on Underdecimated Filter Banks for Coherent OFDM Receivers: Overview and recent advances

Subbanded digital signal processing (DSP) with underdecimated (Udeci) filter banks (FBs), is a recent DSP technique whereby the optical channel bandwidth is digitally sliced into multiple spectrally disjoint subbands (SBs) to be processed in parallel. In terms of DSP hardware (HW) architecture, digital subbanding amounts to an alternative mode of parallelizing the receiver signal processing task to multiple slower processors, whereby the parallelization performed in the frequency domain (FD) rather than in the time domain (TD). We show that FD parallelization is especially suited to the long-haul optical fiber channel and present novel receiver DSP structures based on Udeci FBs, providing substantial complexity savings for ultra-high-speed optically coherent transmission.

Efficient multiplier-free FPGA demonstration of polar-domain multi-symbol-delay-detector (MSDD) for high performance phase recovery of 16-QAM

We eliminate all multipliers from the MSDD carrier recovery (CR) sub-system without performance penalty. The extreme-low-complexity CR is demonstrated in real-time in FPGA HW.

Multiplier-Free Phase Recovery With Polar-domain Multisymbol-Delay-Detector

We present a highly efficient carrier phase recovery module for coherent optical detection of single-carrier advanced modulation formats with differential precoding. Remarkably, we eliminate all multipliers from the multisymbol-delay-detector (MSDD) carrier recovery system, without incurring performance penalty relative to the previously shown complex-domain MSDD. We also introduce a novel polyblock hardware parallelization method for the MSDD, eliminating the “distant feedback” parallelization penalty of decision-feedback-based carrier recovery methods, such as MSDD. Simulations indicate extremely high nonlinear phase noise tolerance for the new polar MSDD carrier recovery system for 16-QAM 2000 km transmission. The ultralow-complexity CR is simulated and electrically demonstrated in real-time at 25 Gbaud symbol rate over FPGA HW for single-carrier QPSK and 16-QAM constellations, respectively.

Low-Complexity Multi-Band Polyphase Filter Bank for Reduced-Guard-Interval Coherent Optical OFDM

Smart multi-band signal processing yields substantial reduction of FDE+FFT complexity for recent Reduced Guard Interval (RGI) techniques emerging in ultra-broadband long-haul OFDM, providing the simplest high-performance QPSK-OFDM system.

Real-time FPGA implementation of efficient filter-banks for digitally sub-banded coherent DFT-S OFDM receiver

We demonstrate a real-time FPGA realization of dual polarization filter banks - this high-speed 2×25 GBd low-complexity core establishes feasibility of energy-efficient (multipliers count ~halved) HW architecture for a digitally sub-banded 170 Gb/s DFT-S OFDM receiver.

Oversampled digital filter banks simplify and improve signal processing in RGI OFDM receivers

The recent filter bank based digital sub-banding improves performance and reduces complexity of every aspect of coherent RGI OFDM reception: CD/PMD Equalization, Timing and Carrier recovery, Channel Estimation, Adaptive Tracking, Nonlinear Compensation.

Sub-Nyquist sampling of OFDM signals for cognitive radios

We investigate sampling and detection of orthogonal frequency-division multiplexing (OFDM) signals with unknown carriers at sub-Nyquist rates. Efficient acquisition and processing of such broadcast signals is a challenge but constitutes a crucial part of enabling cognitive radios. In order to alleviate both the analog and digital burden when treating wideband signals, we adapt the modulated wideband converter (MWC), a recently proposed sub-Nyquist sampling system, to fit OFDM signals. In particular, after detecting the active bands using the MWC, we use several different equalization methods in order to improve the bit-error rate (BER). We then show how to process the real sub-Nyquist samples in each band in order to recover the complex OFDM signal. A standard digital OFDM receiver is then used to detect the input symbols. To evaluate the performance of our system, we derive an analytical bound on the BER as a function of the received signal to noise ratio. Simulations validate the proposed system.