Kuo Zhang

Elastic optical ring with flexible spectrum ROADMs

Data center networks (DCN) are facing significant increase in both scale and bandwidth requirement, and various intra-DCN optical circuit switching architectures have been proposed to handle the inter-rack traffic. As the size of data centers are becoming larger, ring based architectures have received considerable interests, because a ring can provide connectivity to a large number of racks that are distributed far apart in a data center. However, due to the specific data center traffic pattern, fixed spectrum allocation scheme in existing ring based DCNs will lead to low spectrum utilization and hence low system throughput. In this paper, we propose to introduce flexible spectrum allocation into ring based intra-DCN optical switching, so that spectrum utilization can be improved. We design an elastic ring intra-DCN optical circuit switching architecture with flexible spectrum reconfigurable optical add-drop multiplexers (FS-ROADMs). Adopting flexible spectrum allocation in elastic ring, a spectrum allocation problem which is essentially a very complex yet interesting optimization problem arises. For a given set of inter-rack traffic requests, we develop an integer linear program formulation as well as a heuristic algorithm to solve the flexible spectrum allocation problem. We study how the inter-rack traffic pattern may affect the system performances in both fixed and elastic ring cases. For elastic ring, we also analyze and evaluate the computational complexity in solving the spectrum allocation problem. Our results demonstrate that, to handle uneven inter-rack traffic pattern in DCNs, elastic ring can remarkably improve system throughput, though its computational complexity of spectrum allocation problem is slightly higher than that of the fixed ring.

Soft-stacked PON for soft C-RAN

Cloud radio access networks (C-RANs) feature central pooling baseband processing units (BBUs) and lowcost remote radio headends (RRHs) with fiber-connected fronthauls between them, and they are promising for 4G. Researchers are redesigning the C-RAN with different split approaches to relax the fronthaul capacity for 5G with massive antennas, which is termed soft C-RAN. In this paper, we propose a soft-stacked passive optical network (PON) to support the soft C-RAN as an integrated solution. Two approaches are investigated in this paper. The first one is realized by employing an arrayed waveguide grating router (AWGR) and directly modulated tunable lasers, and it is promising for digital fronthaul and midhaul transmission and switching. The second is realized by employing wavelength-selective switches (WSSs) to allocate one-toone and many-to-many connections, and it is promising for analog fronthaul transmission and switching. Their performances are analyzed and measured with directly modulated lasers (DMLs) that are either fixed or tunable. A delay interferometer is used for the DMLs chirp and fiber dispersion management. The result is significant for future soft C-RANs and stacked PON.

Fidelity enhancement in high-data-rate digital mobile fronthaul with sample bits interleaving and unequally-spaced PAM4

Driven by continuously growing mobile traffic, line rate of digital mobile fronthaul (MFH) network keeps surging. 4-level pulse amplitude modulation (PAM4) is a promising format to provide such high data capacity, due to its bandwidth and cost efficiency. In this paper, we propose an improved method to reduce mobile signal distortion caused by bit error of optical transmission. The concept comes from the characteristic that high order sample bit error induces far severer performance degradation to radio signal than low order one. In the solution, high order sample bits and low order sample bits are interleaved, so that they are respectively mapped to first bit (1stb) and second bit (2ndb) of PAM4 symbol. On the other hand, amplitude levels of PAM4 are set unequal to broaden the middle eye, levering accuracy of 1stb. Hence, the total fidelity of mobile signal is enhanced. The feasibility is confirmed both theoretically and experimentally. The investigation is based on two typical digital systems, i.e., 16-bit uniform quantizing and 8-bit nonuniform quantizing. Experimental results indicate that, EVM of LTE-A like radio signal decreases by up to 13dB in uniform quantizing system, and by up to 5dB in nonuniform quantizing system, compared with conventional equally-spaced PAM4.

Low-cost WDM fronthaul enabled by partitioned asymmetric AWGR with simultaneous flexible transceiver assignment and chirp management

The explosion of mobile traffic requires fronthaul networks to provide huge data capacity. One straightforward solution is to use wavelength division multiplexing (WDM) technologies with a sufficient number of transceivers, but the massive deployment of fronthaul networks inevitably adds transceiver cost. In this paper, we propose and demonstrate a low-cost scheme using a partitioned asymmetric arrayed waveguide grating router (AWGR) and distributed feedback Bragg (DFB) directly modulated lasers (DMLs) for WDM fronthaul. First, the centralization feature of a cloud radio access network (C-RAN) puts the transceivers together by nature, offering the potential to reduce the number of deployed transceivers based on optical switching fabric. We design what we believe is a novel asymmetric N × M AWGR-based switching fabric featuring a partitioned cyclic routing property that (i) enables flexible contention-free transceiver assignment; (ii) relaxes the requirement of tuning range, which allows the use of DFB lasers with moderate tuning range; and (iii) changes the diversity of the tuning range from N types to N∕M types. We also investigate the scalability of the scheme and show its rearrangeable contention-free assignment feature. Second, to adopt cost-effective DMLs, the intrinsic Gaussian-shape AWGRs edge filtering is employed to manage the frequency chirp of DFB DMLs, avoiding additional optical or electrical processing modules. Experiments are conducted to demonstrate the robustness of this scheme. In particular, we report that the allowable optical signals frequency deviation ranges from −7.5 GHz to 7 GHz, and the induced additional crosstalk to AWGR’s adjacent port due to a deliberate frequency offset is far below the system design limit. In addition, the scheme is compatible with common 50 GHz, 100 GHz, and 200 GHz channel spacing AWGRs

A novel wavelength sharing TWDM-PON architecture with tunable laser and multi-free-spectral-range AWGR

We propose and experimentally demonstrate a novel TWDM-PON architecture with shared tunable laser and multi-free-spectral-range AWGR. It supports wavelength sharing within the same and among different ODNs. 40Gbps capacity and 40dB power budget are achieved.

25Gbps two-dimensional trellis coded PAM4 TDM-PON transmission based on 10G optics

We demonstrate an alternative approach to 25Gbps 2D TCM PAM4 for TDM-PON with 10GHz optics, which employs M-algorithm at its decoder. Compared to PAM4, 2D TCM PAM4 brings the power budget up by ∼3dB with low complexity.

Intensity directed equalizer for the mitigation of DML chirp induced distortion in dispersion-unmanaged C-band PAM transmission

The directly modulated laser (DML) is one of the most cost-effective transmitter options in optical communication systems, but it introduces an additional impairment caused by the interaction between frequency chirp and chromatic dispersion for C-band transmission. In this paper, we propose a low-complexity intensity directed equalizer based on feedforward equalizer and decision feedback equalizer (FFE/DFE) to mitigate the chirp induced distortions, and remarkably improve the transmission performance of PAM signals generated by DML. The equalizer is based on the fact that the directly modulated PAM symbols with different intensity levels have different chirp frequencies, which will lead to different inter-symbol interference (ISI) contributions to their adjacent symbols due to the velocity difference caused by chromatic dispersion. To address this phenomenon, the proposed equalizer employs multiple sets of tap coefficients according to the intensity levels of PAM signals. With this equalizer and a commercial 16.8GHz DML, we demonstrate a 56Gb/s PAM4 transmission over a record 43km SSMF in the C-band without optical dispersion compensation under the 3.8 × 10−3 HD-FEC BER threshold.

Multi-FSR AWGR assisted uplink load aggregation in fronthaul with parallel signal detection

We proposed an uplink load aggregation scheme for fronthaul based on Multi-FSR AWGR and parallel uplink signal detection, in which receiver can serve one or multiple cell sites according to different traffic demands to save energy.

Multi-Dimensional Coded PAM4 Modulation for TDM-PON based on 10G Optical Devices

We demonstrate an alternative approach of 12.5 GBaud 2D coded PAM4 for TDM-PON with 10 GHz DML and 10 GHz PIN-TIA. Compared with PAM4, 2D coded PAM4 shows superiority in power budget with low complexity.

AWGR-based large port count data center architecture with extended frequency range

Arrayed Waveguide Grating Router (AWGR) is currently one of the most promising components in data center interconnections for its passive and non-blocking feature. However, the scalability is restricted by the limited channel count. In this paper, by using multiple free spectral ranges (FSRs) of AWGR and couplers, we report a novel data center architecture with extended frequency range. By simulation, we prove that, with limited FSRs, tens of thousands port count data center can be realized using a single 32 × 32 AWGR with less than 1 ms average delay and less than 0.1% blocking rate.