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Low Power All-Digital Radio-Over-Fiber Transmission for 28-GHz Band Using Parallel Electro-Absorption Modulators
Abstract
We present a low-power all-digital radio-over-fiber (RoF) transmitter for the 28 GHz band using sigma-delta modulation. Using a parallel electro-absorption modulator (EAM) structure, the radio signal upconversion is split between the electrical and the optical domains. This halves the maximum bandwidth requirement of the driver circuit with respect to conventional implementations. Furthermore, the effect of chromatic dispersion can be mitigated by tuning the optical phase and amplitude applied to the individual modulators, such that transmission notches are partially removed. The modulator structure is described using simplified models and verified in VPI TransmissionMaker. Experimental results using a 140 mW non-return-to-zero (NRZ) driver and parallel EAMs are provided and yield an error vector magnitude (EVM) of 7.6% (5.2%) when transporting a radio signal modulated at 28 GHz with 5.25 Gb/s (2.625 Gb/s) 64-QAM over 10 km standard single mode fiber (SSMF) at 1560 nm.
Electroabsorption modulated lasers (EMLs) is sensitive to the residual facet reflection (RFR), which can cause a low-frequency drop (LFD) in the modulation response. The LFD may result in output waveform distortion, especially for high-data rate and multi-level modulation. We verify by simulation and experiments that the effect of LFD on the EML performance and propose to use LFD as one of the key performance indices for qualifying EMLs in high-speed optical transceivers. LFD can be suppressed by incorporating partial-corrugated-grating (PCG) in the laser section. With PCG-DFB structure, the EMLs can maintain high single-mode yield (SMY) and an excellent quality factor even with a strong modulator reflection from EAM section. This provides the robustness in applying the PCG-DFB to achieve a flat intensity modulation response or to enhance the output waveform under large signal modulation. By designing the PCG-EML to have about 60% of grating region, it can reach >80% of SMY and improve the average Q-value from the reduced LFD in the modulation response. The PCG-EML with reduced LFD can enhance the eye-opening through reduced waveform undershoot and overshoot for transmitting 56-Gbaud/s or beyond PAM-4 signals.
We demonstrate a 28 GHz radio-over-fiber system with laser-less, low-cost active antenna units using silicon photonics and a GaAs driver and LNA. 7-Gb/s downlink and uplink throughput was achieved over 2km SSMF and 5m wireless.
We present a low-power all-digital radio-over-fiber transmitter for beyond 28-GHz using sigma-delta modulation, a 140mW NRZ driver and parallel electro-absorption modulators. 5.25Gb/s (2.625Gb/s) 64-QAM is transported over 10-km SSMF at 1560nm with 7.6% (5.2%) EVM.
All-digital radio-over-fiber (RoF) transmission has attracted a significant amount of interest in digital-centric systems or centralized networks because it greatly simplifies the front-end hardware by using digital processing. The sigma-delta modulator (SDM)-based all-digital RoF approach pushes the digital signal processing as far as possible into the transmit chain. We present a real-time 100-GS/s fourth-order single-bit SDM for all-digital RoF transmission in the high-frequency band without the aid of analog/optical up-conversion. This is the fastest sigma-delta modulator reported and this is also the first real-time demonstration of sigma-delta modulated RoF in the frequency band above 24 GHz. 4.68 Gb/s (2.34 Gb/s) 64-QAM is transported over 10-km standard single-mode fiber in the C-band with 6.46% (4.73%) error vector magnitude and 3.13 Gb/s 256-QAM can be even received in an optical back-to-back configuration. The carrier frequency can be digitally tuned at run-time, covering a wide frequency range from 22.75 GHz to 27.5 GHz. Besides, this high-speed sigma-delta modulator introduces less than 1 μs latency in the transmit chain. Its all-digital nature enables network virtualization, making the transmitter compatible with different existing standards. The prominent performance corroborates the strong competitiveness of this SDM-based RoF approach in high-frequency radio-over-fiber 5G communication.
A 4-lane proof-of-concept real-time sigma-delta-over-fiber system targeting 5G C-RAN downlink is demonstrated using off-the-shelf components. Per lane, an EVM of 2.67% is achieved for a 3.5 Gbps 256-QAM signal modulated at 3.5 GHz over 20-km single-mode fiber at 1310 nm.
A new high-speed delta-sigma modulator (DSM) topology is proposed by cascading a bit reduction process with a multi-stage noise shaping MASH-1-1 DSM. This process converts the two-bit output sequence of the MASH-1-1 DSM to a single-bit sequence, merely compromising the DSM noise-shaping performance. Furthermore, the high clock frequency requirements are significantly relaxed by using parallel processing. This DSM topology facilitates the design of e.g. wideband software defined radio (SDR) transmitters and delta-sigma radio-over-fiber transmitters. Experimental results of the FPGA implementation show that the proposed low-pass DSM can operate at 21 GS/s, providing 520 MHz baseband bandwidth with 42.76 dB signal-to-noise-and-distortion ratio (SNDR) or 1.1 GHz bandwidth with 32.04 dB SNDR (based on continuous wave measurements). An all-digital transmitter based on this topology can generate 218.75 MBd 256-QAM over 200 m OM4 multimode fiber in real-time, with 7-GS/s sampling rate and an error vector magnitude below 1.89%.
Transceivers based on electro-absorption modulators are considered as a promising candidate for the next generation 400 GbE short-reach optical networks. They are capable of combining high bandwidth and low-power operation with a very compact layout, removing the need for traveling wave electrodes and dedicated 50 $\Omega$ termination. In this paper, we demonstrate the first silicon-based EAM, in combination with an in-house developed SiGe BiCMOS transceiver chipset, capable of transmitting single-lane 100 Gb/s non-return-to-zero in real-time. Transmission up to 500 m of standard single mode fiber and 2 km of non-zero dispersion shifted fiber is demonstrated, assuming a forward-error coding scheme with a bit-error rate limit of $3.8\times10^{-3}$ is used. Due to the high line rate, transmission over longer fiber spans was limited by the chromatic distortion in the fiber. As a possible solution, electrical duobinary modulation is proposed as it is more resilient to this type of fiber distortion by reducing the required optical bandwidth. We show improved performance for longer fiber spans with a 100 Gb/s electrical duobinary link, resulting in real-time, sub-FEC operation over more than 2 km of standard single-mode fiber without any digital signal processing. Finally, the possibility of a 100 Gb/s EAM-to-EAM link is investigated.
With the continuously increasing demand of cost effective, broadband wireless access, radio-over-fiber (RoF) starts to gain more and more momentum. Various techniques already exist, using analog (ARoF) or digitized (DRoF) radio signals over fiber. Each with their own advantages and disadvantages. By transmitting a sigma delta modulated signal over fiber (SDoF), a similar immunity to impairments as DRoF can be obtained while maintaining the low complexity of ARoF. This letter describes a detailed experimental comparison between ARoF and SDoF that quantifies the improvement in linearity and error vector magnitude (EVM) of SDoF over ARoF. The experiments were carried out using a 16QAM constellation with a baudrate from 20 to 125 Mbaud modulated on a central carrier frequency of 1 GHz. The sigma delta modulator (SDM) runs at 8 or 13.5 Gbps. A high speed VCSEL operating at 850 nm is used to transmit the signal over 200 m multimode fiber. The receiver amplifies the electrical signals and subsequently filters to recover the original RF-signal. Compared to ARoF, improvements exceeding 40 dB were measured on the third order intermodulation products when SDoF was employed, the EVM improves between 2.4 to 7.1 dB.
The advent of bandwidth-hungry wireless applications such as large file transfer, high definition video streaming, wireless display, and cellular data offload highlight the impending need for larger bandwidth and super speed WiFi links exceeding 1 Gb/s. This article introduces two emerging standards likely to shake the wireless world, namely IEEE 802.11ac and IEEE 802.11ad, and identifies the challenges in the path of multi-gigabit WiFi. We study the suitability of these standards for the new usage models enlisted in this article.
We demonstrate two techniques to reduce the effects of fiber
chromatic dispersion in fiber-wireless systems incorporating external
modulators. We theoretically and experimentally show that the achievable
link distance can be increased by varying the chirp parameter of the
modulator to give large negative chirp using a dual-electrode
Mach-Zehnder modulator (MZM) biased at quadrature. In addition, we show
that dispersion can be almost totally overcome by implementing a simple
method using the dual-electrode MZM to generate an optical carrier with
single sideband (SSB) modulation. We demonstrate the transmission of a
51.8-Mb/s pseudorandom bit sequence (PRBS) at 12 GHz over 80 km of
standard single-mode fiber using the SSB generator and measure a
bit-error-rate (BER) power penalty due to fiber dispersion of less than
0.5 dB for a BER equal to 10<sup>-9</sup>
The fifth generation (5G) cellular network is expected to include the millimeter wave spectrum, to increase base station density, and to employ higher-order multiple-antenna technologies. The centralized radio access network architectures combined with radio-over-fiber (RoF) links can be the key enabler to improve fronthaul networks. The sigma-delta modulated signal over fiber (SDoF) architecture has been proposed as a solution leveraging the benefits of both digitized and analog RoF. This work proposes a novel distributed antenna system using sigma-delta modulated intermediate-frequency signal over fiber (SDIFoF) links. The system has an adequately good optical bit-rate efficiency and high flexibility to switch between different carrier frequencies. The SDIFoF link transmits a signal centered at a 2.5 GHz intermediate frequency over a 100 m multi-mode fiber and the signal is up-converted to the radio frequency (24-29 GHz) at the remote radio unit. An average error vector magnitude (EVM) of 6.40% (-23.88 dB) is achieved over different carrier frequencies when transmitting a 300 MHz-bandwidth 64-QAM OFDM signal. The system performance is demonstrated by a 2×1 multiple-input single-output system transmitting 160 MHz-bandwidth 64-QAM OFDM signals centered at 25 GHz. Owing to transmit diversity, an average gain of 1.12 dB in EVM is observed. This work also evaluates the performance degradation caused by asynchronous phase noise between remote radio units. The performance shows that the proposed approach is a competitive solution for the 5G downlink fronthaul network for frequency bands above 24 GHz.
Migrating toward higher frequencies and densification of the communication cells are two key enablers for increased wireless data rates. To make these trends economically viable, centralized architectures based on radio-over-fiber (RoF) are explored. This paper describes the design of a photoreceiver that can be applied at the remote radio head in a 28 GHz analog RoF link. The devised photoreceiver comprises a Ge-on-Si photodetector and co-designed GaAs low noise amplifier offering 24 dB gain, corresponding to 224 V/W external conversion gain, over a 3-dB bandwidth between 23.5 and 31.5 GHz. The associated noise figure is 2.1 dB and an output referred third order intercept point up to 26.5dBm can be obtained with a power consumption of 303mW. Two possible applications are demonstrated in this paper. First, the photoreceiver is tested in a 5G New Radio environment resulting in rms-EVM values below 2.46/3.47% for 100/400-MBaud 16-QAM transmission over the 24.25–29.5 GHz band. Secondly, very high data rates can also be supported, demonstrated by a 36 Gb/s link with an rms-EVM of 5.2%.
As next-generation data center optical interconnects aim for 0.8 Tb/s or 1.6 Tb/s, serial rates up to 106 GBd are expected. However doubling the bandwidth of current 53 GBd 4-level pulse-amplitude modulation (PAM-4) transmitters is very challenging, and perhaps unfeasible with compact, non-travelling wave, lumped modulators or lasers. In [1]- [2], we presented an integrated 4:1 optical serializer with electro-absorption modulators (EAMs) in each path. Transmitter (TX) functionality was shown up to 104 GBd non-return-to-zero (NRZ) On-Off Keying (OOK) or PAM-4. However the performance for PAM-4 was limited by the distortion introduced by the EAM non-linearity. In [3], we presented a real-time DSP-free 128 Gb/s PAM-4 link with a silicon photonic transmitter using binary driven EAMs in a Mach-Zehnder interferometer (MZI) configuration. By combining two of such half-rate (53 GBd) transmitters in an integrated 2:1-serializer, improved 106GBd PAM-4 performance is expected without needing to compensate the inherent modulator nonlinearity and without requiring faster modulators or drivers. In this paper, we present a Silicon integrated 53 GBd PAM-4 TX as a candidate for integration into a 106GBd PAM-4 2:1 serialized TX. The presented TX consists of two EAMs in an MZI configuration, wirebonded to a low-power 55nm 4-channel SiGe BiCMOS driver [4], operating at 1.5 pJ/b (excluding laser). With a reference receiver (RX), transmission at or below the KP4- FEC threshold is shown beyond 1km standard single-mode fiber (SSMF) and up to 2km non-zero dispersion-shifted fiber (NZDSF) at 1565 nm. Furthermore, the integrated TX was combined with an Si-integrated RX consisting of the same EAM component, wirebonded to a 55nm SiGe BiCMOS transimpedance amplifier (TIA) [5] [6]. Both TX and RX were wirebonded on an RF-PCB, with electrical connectivity through transmission lines and 6-inch 50 GHz multi-coax cable connectors. With this electrically connectorized all-EAM TX and RX, PAM-4 link operation is shown up to 40GBd at 3.9 pJ/b (excluding laser), without using DSP or equalization.
We present a 70 Gb/s capable optical transmitter consisting of a 50 μm long GeSi electro-absorption modulator (integrated in Silicon Photonics) and a fully differential driver designed in a 55 nm SiGe BiCMOS technology. By properly unbalancing the output stage, the driver can be DC-coupled to the modulator thus avoiding the use of on-chip or external bias-Ts. At a wavelength of 1560 nm, open eye diagrams for 70 Gb/s after transmission over 2 km standard single-mode fiber were demonstrated. The total power consumption is 61 mW, corresponding to 0.87 pJ/bit at 70 Gb/s. Bit-error rate measurements at 50 Gb/s and 56 Gb/s (performed both back-to-back and with up to 2km standard single-mode fiber) demonstrate large (0.4 UI at a BER of
$10^{-12}$
) horizontal eye margins. This optical transmitter is ideally suited for datacenter applications which require densely integrated transceivers with a low power consumption.
A new function split option for the next generation fronthaul interface (NGFI) is demonstrated based on all-digital RF transmitter using bandpass delta-sigma modulation. Different from other low layer split (LLS) options, such as option 6 (MAC-PHY), 7 (high-low PHY), and 8 (CPRI), the proposed option 9 split implements RF functions in the digital domain, and splits within the RF layer, with high-RF layer centralized in the distributed unit (DU) and low-RF layer distributed in remote radio units (RRUs). A proof-of-concept all-digital RF transmitter based on real-time delta-sigma modulation is implemented using a Xilinx Virtex-7 FPGA. A 5-GSa/s delta-sigma modulator is demonstrated to encode LTE/5G signals with bandwidth up to 252 MHz and modulation format up to 1024-QAM to a 5-Gb/s OOK signal, which is transmitted over 30-km single-mode fiber from DU to RRU. To relax the FPGA speed requirement, a 32-pipeline architecture is designed. Two-carrier aggregation of 5G and 14-carrier aggregation of LTE signals are demonstrated with error vector magnitude (EVM) performance satisfying the 3GPP specifications. Compared with option 8 (CPRI), although the proposed option 9 split occurs at a lower level, it offers improved spectral efficiency and reduced NGFI data rate than CPRI. Moreover, other LLS options, such as 6, 7, 8, all require a complete RF layer implemented in the analog domain at remote cell sites; whereas option 9 realizes high-RF layer in the digital domain at DU, and eliminates the need of analog RF devices, e.g., DAC, local oscillator and mixer at RRU, which not only makes low-cost, energy-efficient, and small-footprint cell sites possible for the wide deployment of small cells, but also paves the road toward software defined radio (SDR) and virtualization of DU and RRU for improved compatibility and reconfigurability among multiple radio access technologies (multi-RATs). Given its centralized architecture and deterministic latency, option 9 is suitable for radio coordination applications, e.g., coordinated multipoint or joint Tx/Rx, and has potential in low-frequency narrowband scenarios with cost, power, and/or size sensitive cell sites, such as massive machine type communication (mMTC) and narrowband internet of things (NB-IoT).
The fifth generation (5G) wireless technology is designed to provide significantly faster Internet access, with lower latency, and ubiquitous mobile coverage compared to its predecessors. However, as there will be hundreds and thousands of wireless cells deployed in the 5G network, transporting the enormous volume of data instigated from high capacity 5G wireless cells to the core network in a cost-effective and greener manner with a low latency still remains one of the major hurdles. Therefore, in this paper, we exploit different optical transport network architectures and technologies that can be used to build an efficient fronthaul network for 5G, in order to carry data between wireless cells and the central office. Particularly, we equitably compare multiple fronthaul architectures in terms of major requirements of 5G fronthaul network such as bandwidth requirements, delay budgets, deployment costs, complexity of radio remote head (RRH), and the ability to support advanced wireless functions. In order to analyze the deployment cost of entire 5G transport and wireless networks, we develop a joint optimization framework to optimally deploy fiber-based fronthaul network and 5G wireless network simultaneously. Our framework is also capable of setting limits on the required network coverage and capacity. We analyze the optimal deployment costs under different deployment scenarios and different fronthaul technologies. Our analyses provide insights into modeling future-proof optical transport networks to realize 5G network deployment.
In this study, we demonstrate the intermediate-frequency-over-fiber (IFoF) based mobile fronthaul technology implemented in a 28 GHz millimeter-wave (mmWave) based 5G prototype supporting Giga-bit mobile broadband services. In order to confirm the technical feasibility of the proposed IFoF based mobile fronthaul, the clock signal for frequency synchronization and mobile payload data are simultaneously transmitted in the form of IF carriers. Phase noise (PN) and error vector magnitude (EVM) measurements were performed on separate and simultaneous clock transmission paths, respectively. The results confirm an acceptable impact on performance, and the simultaneous clock transmission method reduces system costs and complexity, and improves the flexibility and efficiency of wavelength operation. We also successfully demonstrate the operation of real-time Giga-bit mobile services such as 4K video streaming over IFoF based mobile fronthaul for 5G prototypes with a 28 GHz mmWave. The achieved peak data-rate per user is up to 1.5 Gb/s, which complies with the requirements of the 5G vision of IMT-2020.
Characteristics of noise in Erbium Doped Fiber Amplifier (EDFA) are theoretically analyzed and experimentally verified. Four discreet energy models are used for erbium ion transition. Spatially varying photon number for traveling optical field was defined in a spatially divided segment with finite length, whose length is decided by property of the spontaneous emission. Quantitative evaluation of the Langevin noise sources becomes possible by this model. The Amplified Spontaneous Emission (ASE) was included in terms of discrete longitudinal mode which is defined for whole length of the EDFA. It was found that the Relative Intensity Noise (RIN) in the EDFA hardly changes with noise frequency in the region higher than several kHz. The phase noise in EDFA also hardly changes with the noise frequency. The frequency noise of the signal light in the EDFA increases proportional to squared value of the noise frequency. The amount of RIN and the phase noise decrease in higher input optical power. Good correspondence between theoretical and experimental results were obtained.
Two ΔΣ-modulated digital radio-over-fiber (DRoF) transmission systems that employ a multi-pulse Manchester encoder are proposed and experimentally evaluated. With a two-step modulation process comprised of ΔΣ modulation and multi-pulse Manchester encoding, a high frequency replica or image of a ΔΣ-digitized analog communication signal can be transmitted without significant power loss. This is achieved by exploiting the spectral characteristics of the modified Manchester code. For comparative analysis, a conventional ΔΣ-modulation-based DRoF system is also evaluated. Based on the evaluation results, the proposed DRoF systems more significantly improve the reliability and flexibility of the RoF system by providing higher power margins or by making the DRoF system implementation more cost-effective and easier to perform on account of the low-frequency requirement for electronics and optical transceivers.
In this paper, we relate the error vector magnitude (EVM) bit error rate (BER) and signal to noise ratio (SNR). We also present the fact that with such relationship it would be possible to predict or in cases substitute EVM in places of BER or even SNR. In doing so, we first define EVM with normalization so that the definition stands for multi-modulation systems, viz. binary phas shift keying (BPSK), quadrature phase shift keying (QPSK) etc. We also compare among the different performance metrics and show that EVM can be equivalently useful as signal to noise ratio and bit error rate. The relationships are based on stream based communication systems. A few Monte Carlo simulations are carried out to illustrate the performance of EVM based on these relationships.
An all-digital RF signal generator using DeltaSigma modulation and targeted at transmitters for mobile communication terminals has been implemented in 90 nm CMOS. Techniques such as redundant logic and non-exact quantization allow operation at up to 4 GHz sample rate, providing a 50 MHz bandwidth at a 1 GHz center frequency. The peak output power into a 100 Omega diff. load is 3.1 dBm with 53.6 dB SNDR. By adjusting the sample rate, carriers from 50 MHz to 1 GHz can be synthesized. RF signals up to 3 GHz can be synthesized when using the first image band. As an example, UMTS standard can be addressed by using a 2.6 GHz clock frequency. The measured ACPR is then 44 dB for a 5 MHz WCDMA channel at 1.95 GHz with output power of -16 dBm and 3.4% EVM. At 4 GHz clock frequency the total power consumption is 120 mW (49 mW for DeltaSigma modulator core) on a 1 V supply voltage, total die area is 3.2 mm<sup>2</sup> (0.15 mm<sup>2</sup> for the active area).
The spectrum of an intensity modulated (IM) and a combined intensity-frequency modulated (IM-FM) monochromatic light source has been generated. The amplitudes of the modulated carrier and the first three pairs of sidebands are plotted showing the influence of IM on an FM signal. The effects of first order chromatic dispersion on the baseband amplitude response and harmonic distortion are determined. The manner in which modulation type and depth, modulating frequency, wavelength, and fiber length alter harmonic distortion is presented. Numerical examples giving the amplitude response of a single-mode fiber system as well as the magnitude of the second- and third-harmonic distortion caused by chromatic dispersion are presented. Based on this material, the limits placed on analog transmission due to chromatic dispersion may be assessed.
Low-power (1.5 pJ/b) silicon integrated
- J Lambrecht
- J Verbist
J. Lambrecht, J. Verbist et al., "Low-power (1.5 pJ/b) silicon integrated
Gb/s PAM-4 optical transmitter
Gb/s PAM-4 optical transmitter," Journal of Lightwave Technology,
vol. 38, no. 2, pp. 432-438, 2020.