Coherent Receiver Architecture Research Articles

Coherent EML+TIA Detector for DSP-Free 1 Gb/s/λ Reception Over Extended Budget

The continuing drive towards broadband access necessitates cost-effective optical access and mobile fronthaul solutions. While the fixed-mobile convergence motivates high-speeds for the antenna remoting through broadband digitized radio-over-fiber, the access segment still uses time division multiplexed access equipment. Although the steadily increasing line rates call for 50 Gb/s solutions, the unshared per-user rate remains in the 1-Gb/s range. It is a question whether such data rate can be facilitated without high-speed components, at the same time supporting higher passive splits for the optical distribution networks. We experimentally demonstrate a simplified receiver based on an electro-absorption modulated laser, co-integrated on a die-level with a transimpedance amplifier. We investigate its sensitivity for the phase-agnostic coherent heterodyne downstream detection of data signals in a passive optical network scenario. We will show that despite the omission of a high-performance local oscillator, balanced detection and digital signal processing, the proposed receiver accomplishes reception of 1 Gb/s phase-modulated signals over a link reach of 57 km and an optical budget of 48 dB, thus enabling high passive splits of 1:128. We study the implications raised by this single-ended coherent receiver architecture under multi-channel transmission and investigate the susceptibility of its integrated local oscillator to optical feedback.

Inter-polarization mixers for coherent detection of optical signals.

Electro-magnetic (EM) mixers are fundamental building blocks in communication systems. They are used in frequency/wavelength filters, interferometric modulators, amplitude-phase receivers, to name a few. Traditional EM mixers have two or more input ports and work only for co-polarized signal and local-oscillator (LO) incident on its inputs. Here we report on novel designs, in silicon, of inter-polarization EM mixers operating at 1550 nm wavelength. The 180-degree optical mixer comprising a single input port is demonstrated to coherently mix orthogonally polarized signal and LO. Using the proposed 180-degree mixer, we report on a novel design for a 90-degree optical mixer on silicon with small footprint, broadband response, low loss and good fabrication tolerance. It exploits birefringence of a waveguide to achieve broadband and fabrication-tolerant 90° phase difference between the signal/LO relative phase in the in-phase and quadrature components. A monolithic silicon photonics coherent receiver is demonstrated using the reported 90-degree mixer, and its operation at 22 Gbaud and 44 Gbaud is shown. These mixers pave the way for novel coherent receiver architectures in long-haul, metro, passive optical networks and data-center interconnect applications.

Design of Low-Power DSP-Free Coherent Receivers for Data Center Links

Coherent detection offers high spectral efficiency and receiver sensitivity, but digital signal processing (DSP)-based coherent receivers may be prohibitively power hungry for data centers even when optimized for short-reach applications, where fiber propagation impairments are less severe. We propose and evaluate low-power DSP-free homodyne coherent receiver architectures for dual-polarization quadrature phase shift keying (DP-QPSK) for inter- and intradata center links. We propose a novel optical polarization demultiplexing technique, for DP-QPSK and higher-order modulation formats, with three cascaded phase shifters driven by marker tone detection circuitry. We consider carrier recovery based on either optical or electrical phase-locked loops (PLLs). We propose a novel multiplier-free phase detector based on XOR gates, which exhibits less than 0.5 dB power penalty relative to a conventional Costas loop phase detector. We also study the relative performance of homodyne DP-differential QPSK, for which carrier phase recovery is unnecessary. Our proposed DSP-free architectures exhibit ∼1 dB power penalty at small chromatic dispersion compared to their DSP-based counterparts. We estimate conservatively that the high-speed analog electronics of an electrical PLL-based coherent receiver consume nearly 4 W for 200 Gbit/s DP-QPSK, assuming a 90-nm complementary metal-oxide semiconductor process.

A Factor Graph Approach to Iterative Channel Estimation, Detection, and Decoding for Two-Path Successive Relay Networks

We consider cooperative communications between a source node and a destination node with the help of two relays. In order to overcome the half-duplex constraint, the amplify-and-forward two-path relaying protocol is used. We adopt orthogonal frequency-division multiplexing modulation to combat frequency-selective channels and asynchronous reception at the destination node. In this paper, we develop a coherent receiver architecture suitable for unknown block fading channels. A factor graph representing the joint a posteriori probability of the coded symbols and the channels in the frequency domain is introduced. Then, we derive a Bayesian inference algorithm based on message passing over the factor graph. The resulting iterative receiver maintains low-complexity, based on the interaction between an off-the-shelf soft-input soft-output decoder and a newly introduced per-subcarrier processor for two-path relaying channel estimation and symbol detection. Simulation results show that the proposed solution maintains the full diversity order, even with a limited number of training blocks.

Polarization-beam-splitter-less integrated dual-polarization coherent receiver.

Conventional dual-polarization coherent receivers require polarization beam splitters for either the signal or the local oscillator path. This severely hinders monolithic integration, since integrated polarization splitting devices often exhibit stringent fabrication tolerances. Here we propose a dual-polarization monolithically integrated coherent receiver architecture that completely avoids the use of polarization splitting elements. Polarization management is instead achieved by adequately engineering the birefringence of the interconnecting waveguides. The resultant receiver is highly tolerant to fabrication deviations and thus offers a completely new route for monolithic integration of dual-polarization receivers without any type of active tuning.

Generation and transmission of 8 × 112-Gb/s WDM PDM-16QAM on a 25-GHz grid with simplified heterodyne detection

We propose and experimentally demonstrate a coherent receiver based on simplified heterodyne detection for 100 G polarization division multiplexing (PDM) signal. Compared to the conventional homodyne detection, only two balanced photo detectors (PDs) and two analog-to-digital converters (ADCs) are used in the simplified heterodyne detection. Compared to the conventional hybrid for homodyne detection, the polarization-diversity hybrid here is also simplified. The in-phase/quadrature (I/Q) separation and corresponding digital signal processing (DSP) following downconversion are realized in digital domain after ADCs. Using this scheme, we successfully demonstrated 8 × 112-Gb/s wavelength-division-multiplexing (WDM) polarization-division-multiplexing 16-ary quadrature amplitude modulation (PDM-16QAM) over 720-km single-mode fiber (SMF)-28 with heterodyne detection based on DSP and erbium-doped fiber amplifier (EDFA)-only amplification. Although the required analog bandwidth and sampling speed of the PDs and ADCs are significantly increased for heterodyne detection, the benefits from the simplified coherent receiver architecture and effective DSP in digital frequency domain are experimentally demonstrated.

Partially-coherent receivers architectures for QAM communications in the presence of non-constant phase estimation error

In this paper, we present a new partially coherent receiver architecture motivated by optimum detection of quadrature amplitude modulation (QAM) signals in the presence of time-varying Tikhonov-distributed residual phase estimation error due to phase-locked loop (PLL)-aided phase tracking scheme. Performance is established in terms of bit error rate (BER). In this paper, an approximate performance measure motivated by union bound is presented for the proposed receiver architecture for 8- and 16-QAM constellations. The performance measures are assessed via simulation and analytical means for additive white Gaussian noise (AWGN) as well as for Rayleigh and Rician fading channels. The performance measures are shown to follow those of the optimum receiver over a wide range of signal-to-noise ratio (SNR), while outperforming a standard coherent receiver operating in the presence of residual phase error by as much as 2 dB.