Joint Digital Signal Processing Research Articles

Multipoint Overlapping Optical Networks

Coherent transmission has drastically evolved in the last decade to provide high flexibility to improve spectral efficiency. This yields a point-to-point capacity close to Shannon's limit. In this work, we present the multipoint overlapping optical network architecture in a multi-transmitter to single-receiver context to boost the network capacity. It overlaps two optical signals, partly reusing network resources, here frequency slots of 12.5 GHz width. We show that a joint digital signal processing, based on multi-stage parallel interference cancellation efficiently compensates inter-channel interference up to 39% spectrum overlap with a 1 dB SNR penalty for PDM-QPSK signals. To investigate the impact of the residual interference penalty, we model the proposed solution as an additional power-independent Gaussian-noise term introduced in the transmission. Experiments and simulations have successfully validated the model and the extra penalty is shown to be dependent only on the chosen overlapping given factor. Finally, with such a model, we study the network gains brought by the proposed architecture. We investigate different networking scenarios assuming either 2-slot and 3-slot of overlapping, resulting in ∼20% and ∼40% of overlapping. We demonstrate a network capacity gain of up to 17%.

150 m/500 Mbps Underwater Wireless Optical Communication Enabled by Sensitive Detection and the Combination of Receiver-Side Partial Response Shaping and TCM Technology

In this paper, a joint digital signal processing (DSP) scheme including partial response shaping and trellis coded modulation (TCM) technology is proposed to mitigate the bandwidth limitation of the long-reach underwater wireless optical communication (UWOC) system. The similarity of spectral responses between the partial response filter and the communication system is utilized to suppress the enhancement of noise and distortion induced by the full-response feed-forward equalizer. A random interleaver and a precoder are applied to whiten the noise and distortion. In a laboratory experiment, the minimal required received optical power of -29.8 dBm (at a BER of 3.80 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> - 3</sup> ) is realized at a data rate of 500 Mbps. In a standard swimming pool, the proposed system successfully achieves a data rate of 500 Mbps with a transmission distance up to 150 m, at which the total attenuation is measured to be 53.47 dB. Such a data rate, enabled by the proposed joint DSP scheme, is 8.0%, 9.2%, and 14.2% higher than those achieved by solely using partial response shaping scheme, TCM scheme, and least square equalization scheme, respectively. To the best of our knowledge, this is the first time to employ the combination of partial response shaping and TCM technology in UWOC for high-speed long-reach transmission.

909.5 Tbit/s Dense SDM and WDM Transmission Based on a Single Source Optical Frequency Comb and Kramers-Kronig Detection

Space-division multiplexing (SDM) has become a promising technology for optical communications to seek a sustainable increment in data capacity to keep up with the ever-increasing bandwidth demand. SDM could provide massive parallelization in addition to existing WDM technology for a high capacity without compromising the signal-to-noise ratio requirement of the transmitter. However, the WDM-SDM combination requires arrays of laser sources with associated active wavelength controls. This is not favorable in terms of energy consumption and cost. Single source optical frequency comb (SS-OFC) is an appealing candidate to provide coherent WDM sources, replacing the laser arrays and surpassing the performance by providing the possibility of joint digital signal processing across multiple channels. In this paper, we have generated an SS-OFC based on highly nonlinear fibers (HNLF) for short-reach applications such as optical interconnects, taking advantage of the high pump-to-comb efficiency. The 50-GHz spaced SS-OFC across the whole C band provides 99 comb lines as WDM channel sources. Combining adaptive-rate modulation, a 7.9-km 37-core fiber, and a polarization diversity Kramers-Kronig receiver, we have successfully transmitted 3663 channels (37 SDM × 99 WDM) with reliably error-free performance, achieving 909.5 Tbit/s net rate with an aggregated spectral efficiency of 184.42 bit/s/Hz.

Laser Frequency Combs for Coherent Optical Communications

Laser frequency combs with repetition rates on the order of 10 GHz and higher can be used as multi-carrier sources in wavelength-division multiplexing (WDM). They allow replacing tens of tunable continuous-wave lasers by a single laser source. In addition, the comb's line spacing stability and broadband phase coherence enable signal processing beyond what is possible with an array of independent lasers. Modern WDM systems operate with advanced modulation formats and coherent receivers. This introduces stringent requirements in terms of signal-to-noise ratio, power per line, and optical linewidth which can be challenging to attain for frequency comb sources. Here, we set quantitative benchmarks for these characteristics and discuss tradeoffs in terms of transmission reach and achievable data rates. We also highlight recent achievements for comb-based superchannels, including >10 Tb/s transmission with extremely high spectral efficiency, and the possibility to significantly simplify the coherent receiver by realizing joint digital signal processing. We finally discuss advances with microresonator frequency combs and compare their performance in terms of flatness and conversion efficiency against state-of-the-art electro-optic frequency comb generators. This contribution provides guidelines for developing frequency comb sources in coherent fiber-optic communication systems.

Frequency Comb-Based WDM Transmission Systems Enabling Joint Signal Processing

We review the use of optical frequency combs in wavelength-division multiplexed (WDM) fiber optic communication systems. In particular, we focus on the unique possibilities that are opened up by the stability of the comb-line spacing and the phase coherence between the lines. We give an overview of different techniques for the generation of optical frequency combs and review their use in WDM systems. We discuss the benefits of the stable line spacing of frequency combs for creating densely-packed optical superchannels with high spectral efficiency. Additionally, we discuss practical considerations when implementing frequency-comb-based transmitters. Furthermore, we describe several techniques for comb-based superchannel receivers that enables the phase coherence between the lines to be used to simplify or increase the performance of the digital carrier recovery. The first set of receiver techniques is based on comb-regeneration from optical pilot tones, enabling low-overhead self-homodyne detection. The second set of techniques takes advantage of the phase coherence by sharing phase information between the channels through joint digital signal processing (DSP) schemes. This enables a lower DSP complexity or a higher phase-noise tolerance.

Linewidth-Tolerant Joint Digital Signal Processing for 16QAM Nyquist WDM Superchannel

The performance of Nyquist wavelength division multiplexing (WDM) superchannel, using 16-ary quadrature amplitude modulation (16QAM), is severely degraded due to the existence of inter-channel interference and full-response equalization-enhanced in-band noise. Here, we propose a joint digital signal processing (DSP) technique incorporating digital spectral shaping and maximum likelihood sequence detection for 16QAM Nyquist WDM superchannel with a symbol rate of 28 GBd. Compared with traditional joint DSP, we can obtain 1.8-dB required-OSNR improvement at a bit error rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for back-to-back measurement. In addition, 0.9-dB peak Q-factor improvement is also secured after 960-km standard single mode fiber transmission. Furthermore, using the proposed joint DSP technique, linewidth tolerance is relaxed from 80 to 230 kHz given 1-dB required-OSNR penalty. Meanwhile, reduction of block-size for carrier phase recovery is also secured, taking 100-kHz linewidth into account.

Joint digital signal processing of Nyquist-wavelength division multiplexing superchannel with group detection

To relax the limited sampling rate of an analog-to-digital converter (ADC) and to reduce the complexity of conventional fiber-optical superchannel coherent detection, we propose and demonstrate a joint digital signal processing (DSP) technique of Nyquist-wavelength division multiplexing superchannel with group detection. At the receiver side, three Nyquist-spaced channels with 12.5 Gbaud polarization multiplexing-quadrature phase shift keying signals are group detected with a sampling rate per channel of 1.33 times over the normal sampling rate. A modified carrier separation technique is then put forward to retrieve the high-frequency interference component of both the designated channel and its adjacent channels, which can subsequently be used to recover the designated channel with new constant modulus algorithm-based joint multiinput-multioutput equalizers. The results show that the proposed group detection and joint DSP algorithm can simultaneously improve the transmission performance and reduce the complexity of both the transmitter and receiver, regardless of bandwidth restrictions from the waveshaper, ADC module, and coherent receiver.

Digital Signal Processing Techniques Enabling Multi-Tb\/s Superchannel Transmission: An overview of recent advances in DSP-enabled superchannels

Digital signal processing (DSP) combined with coherent detection has played a central role in the recent capacity expansion of optical networks. Optical superchannels aim to increase per-channel interface rates as well as per-fiber capacities of wavelength-division multiplexed (WDM) systems in a cost-effective manner. Superchannels circumvent the electronic bottleneck via optical parallelism and provide high-per-channel data rates and better spectral utilization, especially in transparent optical mesh networks. This article reviews recent advances in the generation, detection, and transmission of optical superchannels with channel data rates on the order of terabits per second (Tb/s). Enabling DSP techniques such as Nyquist-WDM, orthogonal frequency-division multiplexing (OFDM), software-defined modulation and detection, advanced coding, and joint DSP among the superchannel constituents are presented. Future prospects of DSP techniques for high-capacity superchannel transmission are also discussed.

Joint digital signal processing for superchannel coherent optical communication systems

Ultra-high-speed optical communication systems which can support ≥ 1Tb/s per channel transmission will soon be required to meet the increasing capacity demand. However, 1Tb/s over a single carrier requires either or both a high-level modulation format (i.e. 1024QAM) and a high baud rate. Alternatively, grouping a number of tightly spaced "sub-carriers" to form a terabit superchannel increases channel capacity while minimizing the need for high-level modulation formats and high baud rate, which may allow existing formats, baud rate and components to be exploited. In ideal Nyquist-WDM superchannel systems, optical subcarriers with rectangular spectra are tightly packed at a channel spacing equal to the baud rate, thus achieving the Nyquist bandwidth limit. However, in practical Nyquist-WDM systems, precise electrical or optical control of channel spectra is required to avoid strong inter-channel interference (ICI). Here, we propose and demonstrate a new "super receiver" architecture for practical Nyquist-WDM systems, which jointly detects and demodulates multiple channels simultaneously and mitigates the penalties associated with the limitations of generating ideal Nyquist-WDM spectra. Our receiver-side solution relaxes the filter requirements imposed on the transmitter. Two joint DSP algorithms are developed for linear ICI cancellation and joint carrier-phase recovery. Improved system performance is observed with both experimental and simulation data. Performance analysis under different system configurations is conducted to demonstrate the feasibility and robustness of the proposed joint DSP algorithms.

Joint Digital Signal Processing Receivers for Spatial Superchannels

We discuss the advantages of spatial superchannels for future terabit networks based on space-division multiplexing (SDM), and demonstrate reception of spatial superchannels by a coherent receiver utilizing joint digital signal processing (DSP). In a spatial superchannel, the SDM modes at a given wavelength are routed together, allowing a simplified design of both transponders and optical routing equipment. For example, common-mode impairments can be exploited to streamline the receiver's DSP. Our laboratory measurements reveal that the phase fluctuations between the cores of a multicore fiber are strongly correlated, and therefore constitute such a common-mode impairment. We implement master-slave phase recovery of two simultaneous 112-Gbps subchannels in a seven-core fiber, demonstrating reduced processing complexity with no increase in the bit-error ratio. Furthermore, we investigate the feasibility of applying this technique to subchannels carried on separate single-mode fibers, a potential transition strategy to evolve today's fiber networks toward future networks using multicore fibers.