We studied the influence of frequency and phase noise of semiconductor laser on the performance of photonics assisted terahertz wave system. The laser spectrum is Lorentzian intrinsic shape when only Gaussian white noise exists. The phase noise of the laser increases with the increase of the Lorentzian spectral linewidth of the laser. When considering 1 f noise, the laser frequency will be superimposed with 1 f noise. The 1 f noise in the low-frequency band will make the reconstructed laser spectrum of the whole frequency noise tend to be non-Lorentzian shape, which has a serious impact on system performance. When the system is affected by the resonance frequency, the side lobes appearing on both sides of the main peak of the laser spectrum also have a certain impact on the system performance. We simulated dual-polarization (DP) 16-ary quadrature amplitude modulation (16QAM) signal transmission up to 60Gbaud on a 50km standard single mode fiber (SSMF) at 300GHz.
To combine the merits of fiber communication and wireless communication, photonics-aided terahertz-wave (THz-wave) technology has become a popular technology in recent years. In this paper, a 92 Gbit/s photonics-aided 0.3-THz wireless transmission system based on digital sub-carrier multiplexing (DSCM) is proposed and simulated. In order to compensate the phase noise, both Viterbi-Viterbi and maximum likelihood (VV&ML) algorithm and decision directed digital phase lock loop (DD-PLL) method are considered and compared. According to the simulation results, DSCM based scheme can provide better performance than single carrier (SC) based scheme in the case of low received optical power (ROP) or input optical power (IOP) while there is no advantage in the case of high ROP or IOP. For SC based scheme, DD-PLL has 1 dB sensitivity gain compared with VV&ML when BER drops from 10-3 to 10-4, while 2 dB sensitivity gain was obtained for DSCM based scheme for ROP. For SCM based scheme, DD-PLL has 1 dB sensitivity gain compared with VV&ML when BER drops from 10-3 to 10-4. On the whole, DD-PLL outperforms VV&ML in terms of performance in high ROP case for both SC and SCM based scheme. Besides, DD-PLL has lower computation complexity than VV&ML.
KEYWORDS: Video, Extremely high frequency, Video compression, Ka band, Phased array optics, Sensors, Data transmission, Cameras, Antennas, Heterodyning
With the large-scale commercialization of 5G and the continuous evolution of the ultra-high-definition video industry, the next five years will be a period of strategic opportunities for the technological development and achievement transformation of the ultra-high-definition video industry. 5G mm-wave such as 28GHz in Ka-band will drive the rapid development of ultra-high-definition video industry applications. In this paper, we experimentally demonstrate a real-time full-duplex photonic-assisted 28GHz mm-wave communication system for video services. The experimental results show that our system supports real-time data error-free transmission of 1.25Gbaud and bit error rate level of 10-12 at 2.125 Gbaud under the case of 5km fiber and 1.6m wireless distances. Additionally, we also demonstrate the real-time full-duplex transmission of 1080p uncompressed video with the overall bandwidth of 1.485Gbps. It means this system can enable at least 8 channels of 8K video or 20 channels of 4K video to be live and on-demand at the same time after using video compression techniques. According to the above results, we believe that this system can promote the development of 5G mm-wave real-time ultra-high-definition video services for indoor or outdoor scenarios.
KEYWORDS: Digital signal processing, Data centers, Optical amplifiers, Transmitters, Signal detection, Modulation, Signal processing, Picosecond phenomena, Modulators, Forward error correction
Propelled by bandwidth-hungry cloud services, the ongoing growth of intra-datacenter traffic drives the development of high-speed short-reach transceivers, which calls for next generation optical interfaces of 800-GE or even 1.6-TbE. Conventional intensity-modulation direct-detection (IM/DD) systems still dominate the market for high speed short reach optical interconnects due to its simplicity and low cost compared with coherent solutions. Several advanced techniques to achieving net data rates around 200∼250 Gbps have been demonstrated. Effective digital signal processing (DSP) for signal recovery are always used in these systems, including digital pre-distortion, digital timing recovery, feed-forward and decision feedback equalization (FFE/DFE) and stronger forward error correction. Probabilistic shaping (PS) has been introduced for 200G+ per lane IM/DD systems. Semiconductor optical amplifier (SOA) and PS can be potentially used for 200G+ per lane IM/DD systems at O-band over 10 km SMF. There are two main transmission impairments: the nonlinear impairments from the nonlinear region of the electro-optical components, and linear impairments from the bandwidth constraint of the optoelectronic devices and chromatic dispersion. Single-lane 200G+ transmission is difficult to realize due to the nonlinear impairments and the strong bandwidth constraint of optoelectronic devices. In recent years, we have experimentally demonstrated several 200G+ per lane IM/DD short-reach transmission system, making it a promising scheme for data center short-reach applications.
In this paper, 28GHz millimeter wave (MMW) in n257 band which is one of the recommended frequency bands for beyond 5G, has been demonstrated by experiment for optical wireless access. The MMW antenna with the bandwidth of 26.5 ~ 29.5GHz and an envelope detector with a 3dB bandwidth of about 500MHz are used to enable 4-ary pulse amplitude modulation (PAM4) signal transmission over 5km fiber and 1.6m wireless distances in our experiment. In order to compensate the linear and nonlinear impairments of the optical wireless links, the long short-term memory (LSTM) neural network nonlinear equalizer is adopted in the receiver DSP. Additionally, the traditional linear equalizer (LE) and Volterra equalizer (VE) are also conducted for comparison. The results show that neither the performances of LE nor VE can reach the 7% hard-decision forward error correction (HD-FEC) threshold (3.8×10-3) in the case of 5 Gbaud PAM4 transmission over 5km fiber and 1.6m wireless distances. Instead, after adopting the LSTM equalizer, the bit error rate can be reduced to approximate 1×10-3, which reveals a noticeable performance improvement. Moreover, the performances of the three kinds of equalizers at different transmission rates are further studied. We find that the LSTM can help improve the system capacity from below 9Gbps to above 10Gbps at 7% HD-FEC threshold, which means more than 10 percent improvement has been achieved. According to the above results, we believe that the LSTM equalizer will facilitate the large-capacity communication for the upcoming 5G MMW in fiber wireless access networks.
Wireless communication and sensing show more and more similarities in system design and signal processing flows. A common transmitter sharing is preferred for communication and sensing to reduce equipment cost, hardware size, and power consumption. In this paper, we propose and demonstrate a joint photonic transmitter for wireless communication and sensing at mmW-band based on heterodyne detection. The key device of the photonic transmitter is a dual-drive Mach- Zehnder modulator (DMZM) and a multichannel tunable optical filter (MTOF). The two arms of the DMZM are driven by a specially coded sine wave and a specially coded single chirped linear frequency modulation (LFM) wave, respectively. Besides, both of the two amplitude-coded waves are simultaneously modulated onto the two lasers coupled into the DMZM. By filtering out appropriate sidebands using the MTOF, an amplitude-shift-keying (ASK) signal for wireless communication and a dual-chirp LFM for sensing at mmW-band can be simultaneously generated after heterodyne detection. When the product of the two baseband signals used to code the sine and LFM waves is fixed, the envelope of the mmW LFM wave for radar sensing keeps constant. The simulation results show that a 11.5-Gbit/s ASK signal and a dual-chirp LFM signal with constant envelope are successfully generated at the same time. By properly setting the frequencies of the lasers and the sine wave, the generated ASK signal can be centered at 28GHz for 5G mmW communication, and the generated LFM wave can be distributed around 79GHz for vehicle radar sensing. Thanks to the dual chirps of the mmW LFM wave, range-Doppler decoupling is achieved.
KEYWORDS: Laser sintering, Receivers, Modulation, Digital signal processing, Signal processing, Telecommunications, Data communications, Signal detection, Absorbance, Active optics
The intensity modulation and direct detection (IM/DD) systems have been widely investigated and demonstrated to fulfil the requirement of short reach data communication links with simple implementation. DMLs are a low cost solution for IM/DD systems due to their low power dissipation, small footprint and high output optical power. However, for DMLs, the driving current can influence the optical density at its active region, hence the intrinsic chirp affects the generated optical carrier and results in distortions of the signals, which reduces transmission rates and signals decision accuracy. We propose a machine learning-based decision technique to mitigate nonlinear distortions of the DMLs without using any nonlinear processing, and demonstrate a 60-Gb/s PAM-8 IM/DD system using a DML. About 0.6-dB receiver sensitivity improvement is achieved after 2km transmission.
Millimeter wave (mm-wave) and microwave frequency has become a hot research topic in recent years. Comparing to traditional wireless communication frequency, microwave possesses larger available bandwidth, which is up to tens of gigahertz, so that it can support advanced digital services with ultra-high bit rate. To support the transmission rate over 100Gbit/s in an optical wireless system, forward error correction (FEC) is adopted in real-time communication systems to correct bit errors. Polar code is a kind of FEC which can theoretically achieve channel capacity as the code length tends to infinity. In this paper, we experimentally demonstrate a photonics-aided microwave communication system at K-band. With polar coding, 20-Gbit/s signal is transmitted over 20m wireless link. Our experimental results show that BER performance of such optical wireless system can be improved largely after we employ polar coding.
KEYWORDS: Signal generators, Digital signal processing, Eye, Receivers, Modulation, Signal detection, Multiplexers, Oscilloscopes, Modulators, Binary data
With the popularization of data center and other bandwidth hungry inter-connect applications, the desired capacity of short reach optical network has exponentially increased. In order to realize high-speed transmission, a few modulation formats or schemes, such as PAM4 and DMT are proposed and experimentally demonstrated. However, these modulation formats need expensive DAC and ADC as well as DSP procession. OOK modulation has simple architecture and high receiver sensitivity. Duo-binary signal is a special OOK signal. Here we experimentally demonstrate a record bit rate of 160-Gb/s OOK electrical signal generation, and realize a duobinary optical signal at a bit rate of 160Gb/s transmission and detection.
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