Quadrature Amplitude Modulation Transmission Research Articles

SNR Improved Digital-PCM Radio-Over-Fiber Scheme Supporting 65536 QAM for Mobile Fronthaul

We propose a digital pulse-code-modulation radio-over-fiber (DP-RoF) scheme and verify it experimentally in a 10-km standard single-mode fiber (SSMF) coherent system. The 20-Gbaud DP-RoF signal with a common public radio interface equivalent data rate (CPRI-EDR) of 112 Gb/s is successfully transmitted in the system with excellent performance. The recovered signal-to-noise ratio (SNR) of wireless signal in DP-RoF scheme is 50.14 dB, which is 26.62, 20.01 and 13.28 dB higher than that of analog radio-over-fiber (A-RoF), digital-analog radio-over-fiber (DA-RoF) and digital radio-over-fiber (D-RoF), respectively. The DP-RoF scheme meets the error vector magnitude (EVM) requirement of 65536 quadrature amplitude modulation (QAM), while A-RoF and DA-RoF schemes cannot reach the EVM threshold of 1024 QAM. With improved SNR, the proposed DP-RoF system is a promising scheme for ultra-high-order QAM transmission in future 6G fronthaul.

Experimental demonstration of 201.6-Gbit/s coherent probabilistic shaping QAM transmission with quantum noise stream cipher over a 1200-km standard single mode fiber.

A probabilistic shaping (PS) quadrature amplitude modulation (QAM) based on Y-00 quantum noise stream cipher (QNSC) has been proposed. We experimentally demonstrated this scheme with data rate of 201.6Gbit/s over a 1200-km standard single mode fiber (SSMF) under a 20% SD-FEC threshold. Accounting for the 20% FEC and 6.25% pilot overhead, the achieved net data rate is ∼160Gbit/s. In the proposed scheme, a mathematical cipher (Y-00 protocol) is utilized to convert the original low-order modulation PS-16 (22 × 22) QAM into ultra-dense high-order modulation PS-65536 (28 × 28) QAM. Then, the physical randomness of quantum (shot) noise at photodetection and amplified spontaneous emission (ASE) noise from optical amplifiers are employed to mask the encrypted ultra-dense high-order signal for further improving the security. We further analyze the security performance by two metrics known in the reported QNSC systems, namely the number of masked signals (NMS) of noise and the detection failure probability (DFP). Experimental results show it is difficult or even impossible to extract transmission signals from quantum or ASE noise for an eavesdropper (Eve). We believe that the proposed PS-QAM/QNSC secure transmission scheme has the potential to be compatible with existing high-speed long-distance optical fiber communication systems.

INVESTIGATION OF PSK/QAM MODULATED HYBRID FIBER OPTIC AND FREE SPACE OPTIC SYSTEM UNDER VARIOUS ATMOSPHERIC LOSSES

This research aims to investigate how effectively digital modulation performs to transfer data via hybrid fiber optic and free space optic (FO-FSO) channels for last-mile communication systems. Digital modulation techniques investigated are phase-shift keying (PSK) 8-PSK, 16-PSK, quadrature amplitude modulation (QAM) 4-QAM, 16-QAM, and 64-QAM. FSO is line-of-sight technology being a solution to a last-mile communication system for the challenge of fiber optic cables installation. Combining FSO with fiber optic technology avoids bending losses and the implementation challenges of fiber optic cables in metropolitan areas and between buildings. But atmospheric challenges such as haze, fog, and rain cause signal loss while transmitting data in free space. Also, atmospheric turbulence and antenna misalignments lead to signal error. This paper proposed the models of QAM and PSK digital modulated signal transmission over FO-FSO and compared the results to better fiber optic-FSO links to achieve a last-mile communication network. This fiber optic and FSO model is analyzed for the distance up to 100 km of fiber optic channel and 0.4-1.5 km of FSO channel. Constellation diagrams and error-vector magnitude (EVM) concerning received power at the receiver are compared to analyze FSO channel capacity for various weather attenuations.

Joint estimation of dynamic polarization and carrier phase with pilot-based adaptive equalizer in PDM-64 QAM transmission system

A pilot-based adaptive equalizer is investigated for high cardinality polarization-division-multiplexing quadrature amplitude modulation transmission systems. Pilot symbols are periodically inserted for joint estimation of the dynamic state of polarization (SOP) and carrier phase, in a least mean square (LMS) sense. Compared to decision-directed least mean square (DDLMS) equalization and radially-directed equalization, the proposed equalizer can achieve robust equalization and phase estimation, especially in low optical signal-to-noise ratio (OSNR) scenarios. In an experiment on 56 GBaud PDM-64 QAM transmission over 400 km standard single-mode fiber, we obtained at least 0.35 bit per symbol generalized mutual information (GMI) improvement compared with other training symbol-based equalization when tracking 600 krad/s dynamic SOP. With the joint estimation scheme, the equalization performance will not be compromised even if the SOP speed reaches 600 krad/s or the laser linewidth approaches 2 MHz. For the first time, it is demonstrated that the pilot-based equalizer can track dynamic SOP rotation and compensate for fiber linear impairments without any cycle slips under extreme conditions.

Asymmetric constellation transmission for a coherent free-space optical system with spatial diversity.

Spatial diversity is widely recognized as an effective technique for free-space optical (FSO) communication to combat channel fading. Most existing works on spatial diversity in FSO systems focus on intensity modulation and direct detection, while fewer mention coherent optical detection, which excels in optical receiver responsivity. Spatial diversity enables the FSO coherent system to demodulate the signal components of in-phase (I) and quadrature (Q) separately and thus simplify the coherent detector. More importantly, due to the differential channel conditions between I and Q components, we are offered a chance to realize an asymmetric quadrature amplitude modulation (QAM) constellation that can utilize the channel capacity of both I and Q components more sufficiently. Performance results demonstrate that the simplified structure and improved spectrum efficiency can be realized in the asymmetric QAM transmission system with an acceptable bit-error-rate loss.

Simulation and Performance Study of Quadrature Amplitude Modulation and Demodulation System

Both traditional frequency modulation and phase modulation digital modulation methods have low spectral utilization, poor multipath fading resistance, slow power spectrum attenuation, and severe out-of-band radiation. Quadrature Amplitude Modulation (QAM) is a digital modulation technique that combines phase and amplitude control. It not only achieves higher spectral efficiency, but also transmits higher rate data in a defined frequency band. In this paper, the basic principle, system structure and performance parameters of QAM modulation and demodulation are studied in depth to realize simulink simulation and performance analysis of QAM modulation and demodulation system. The basic theory and implementation method of analog signal digitization are analyzed in detail to realize simimlink of differential pulse code modulation. Based on the above theory, the analog source QAM transmission system is constructed, and simulink is used for modeling and simulation and performance verification.The simulation results show that the constructed QAM digital transmission system can achieve good transmission of analog signals.

QAM Transmission over RoF Using Fabry Perot Laser Diode and Continuous Wave Laser Diode

This paper introduces the Quadrature Amplitude Modulation (QAM) Radio over Fiber (RoF) framework dependent on Optical Frequency Comb (OFC) generation utilizing Fabry-Perot Laser Diode (FPLD) and Continuous Wave Laser Diode (CWLD). In this, optical Millimeter-Wave (MMW) can be created ether utilizing ease FPLD with an outside optical infusion or a continuous wave laser diode. At the base, optical MMW can be recognized by utilizing photodetector in the two transmissions. The experimental results are explained by using the constellation and eye diagram.

Silicon Photonic IQ Modulators for 400 Gb/s and Beyond

Silicon photonics has enormous potential for ultrahigh-capacity coherent optical transceivers. We demonstrate an in-phase and quadrature (IQ) modulator using silicon photonic traveling-wave modulators optimized for higher order quadrature amplitude modulation (QAM). Its optical and RF characteristics are studied thoroughly in simulation and experiment. We propose a system-orientated approach to optimization of the silicon photonic IQ modulator, which minimizes modulator-induced power penalty in a QAM transmission link. We examine the tradeoff between modulation efficiency and bandwidth for the optimal combination of modulator length and bias voltage to maximize the clear distance between adjacent constellation points. This optimum depends on baud rate and modulation format, as well as achievable driving voltage swing. Measured results confirm our prediction using the proposed methodology. Without precompensating bandwidth limitation of the modulator, net data rates up to 232 Gb/s (70 Gbaud 16-QAM) on single polarization are captured, indicating great potential for 400+ Gb/s dual-polarization transmission.

Enhanced Performance Utilizing Joint Processing Algorithm for CAP Signals

In this paper, we propose a joint processing algorithm for carrierless amplitude and phase modulation (CAP) signal to improve the system performance in both short-reach and metro network. Utilizing our proposed method, 112 Gb/s/λ CAP-64 quadrature amplitude modulation (QAM)-based transmission over 50 km of standard single mode fiber (SSMF) is experimentally achieved with a bandwidth-limited direct-detection receiver under the hard decision forward error correction (HD-FEC) threshold of 3.8 × 10–3. Furthermore, we experimentally demonstrate 112 Gb/s/λ CAP-16 QAM transmission over 480 km of SSMF with direct detection. With our proposed joint processing algorithm, both nonlinearity and In-phase/quadrature (IQ) imbalance can be mitigated. More than 3 dB receiver sensitivity gain is achieved at the HD-FEC threshold after 400-km transmission compared with the result using traditional nonlinear compensation.

Single-channel 200 Gbit/s, 10 Gsymbol/s-1024 QAM injection-locked coherent transmission over 160 km with a pilot-assisted adaptive equalizer.

We demonstrate a 10 Gsymbol/s, 1024 quadrature amplitude modulation (QAM) 160 km coherent transmission with an injection locking technique. Our newly developed, pilot-assisted adaptive equalizer has greatly improved the precision of waveform distortion compensation, and this has enabled us to increase the symbol rate to 10 Gsymbol/s in a 1024 QAM transmission. Thus, we could realize a 200 Gbit/s, 1024 QAM transmission over 160 km with a potential spectral efficiency of 12.6 bit/s/Hz.

Performance optimization of PM-16QAM transmission system enabled by real-time self-adaptive coding.

We experimentally demonstrate self-adaptive coded 5×100 Gb/s WDM polarization multiplexed 16 quadrature amplitude modulation transmission over a 100km fiber link, which is enabled by a real-time control plane. The real-time optical signal-to-noise ratio (OSNR) is measured using an optical performance monitoring device. The OSNR measurement is processed and fed back using control plane logic and messaging to the transmitter side for code adaptation, where the binary data are adaptively encoded with three types of low-density parity-check (LDPC) codes with code rates of 0.8, 0.75, and 0.7 of large girth. The total code-adaptation latency is measured to be 2273ms. Compared with transmission without adaptation, average net capacity improvements of 102%, 36%, and 7.5% are obtained, respectively, by adaptive LDPC coding.

Transmission of 51.2 Gb/s 16 QAM single carrier signal in a MIMO radio‐over‐fiber system at W‐band

AbstractWe experimentally demonstrate a 2 × 2 Multiple‐Input Multiple‐Output radio‐over‐fiber system at W‐band and successfully realize a single carrier 16 quadrature amplitude modulation transmission through 100 km single‐mode fiber and 40m W‐band wireless link with a net bitrate of 51.2 Gb/s. Two pairs of Cassegrain antennas are used in the wireless link, between which the isolation can be promised by adopting antenna polarization (H&V) multiplexing.

A Generic Reconfigurable Mixed Time and Frequency Domain QAM Transmitter with Forward Error Correction

In the past three decades, Field Programmable Gate Arrays (FPGAs) have emerged to be the backbone of digital signal processing, especially in high-speed communication systems. However, today, these devices are clocked below 1GHz and improvement in performance stays a big challenge on all abstraction layers, right from system architecture down to physical technology. Far and wide, myriad number of researches are done on methodologies and techniques which can deliver higher throughput with lower operating frequencies. Towards this projected objective, in this paper an efficient modulation technique like Quadrature Amplitude Modulation (QAM) along with mixed time and frequency domain approach and Forward Error Correction (FEC) technique have been utilized to employ a generic scalable FPGA based QAM transmitter with filter parallelization being executed in mixed domain. The system developed in this paper achieves an effective throughput of 12.8Gb/s for 256-QAM with 16 parallel inputs having an operating frequency of 201.25MHz, while a 18.7Gb/s effective throughput is realized with 32 parallel inputs at 146MHz. Thereby, it paves down a promising methodology for applications where having higher clock frequencies is a hard limit.

Layered Multiplexed-Coded Relaying in Wireless Multicast Using QAM Transmissions

We propose layered multiplexed-coded decode-and-forward (LMDF) relaying for a wireless multicast network. Splitting the message into two layers, the source encodes them separately and maps them to a quadrature amplitude modulation (QAM) constellation to achieve superposition, thus allowing partial message recovery at the relay and the destinations. The key component of LMDF is multiplexed coding at the relay that enables each destination to divert all channel resources toward decoding the layers that remain unrecoverable from the source’s transmission. We derive information theoretic achievable rates for LMDF and consider network outage as a performance measure. For a setup with 15 users uniformly spread around the relay, simulations indicate that LMDF outperforms conventional unlayered DF by 1.7 dB and superposition coding-based DF by 1.1 dB, while the respective gains in outage rate at individual users are up to 48.6% and 31.4%.

QAM Vector Signal Generation by Optical Carrier Suppression and Precoding Techniques

We numerically and experimentally investigate and compare photonic constant- and multi-amplitude quadrature-amplitude-modulation (QAM) vector signal generation at radio frequency (RF) bands enabled by a Mach-Zehnder modulator (MZM)-based optical carrier suppression (OCS) modulation. In order to attain an electrical vector RF signal displaying multi-amplitude QAM modulation, such as 8 QAM and 16 QAM, after square-law photodiode detection, the driving RF signal carrying multiamplitude QAM transmitter data, should be both amplitude- and phase-precoded before used to drive the MZM. However, for constant-amplitude QAM modulation, such as quadrature-phase-shift-keying (QPSK), only phase precoding is needed. We experimentally demonstrate 1-GBd vector signal generation at 12 GHz enabled by an MZM-based OSC modulation, adopting QPSK, 8 QAM, and 16 QAM modulation, respectively. We also experimentally compare the bit error rate performance of the three different vector signals, and the QPSK case is the best, while the 16 QAM case is the worst.

W-Band 8QAM Vector Signal Generation by MZM-Based Photonic Frequency Octupling

We propose a novel scheme for photonic multiamplitude quadrature-amplitude-modulation (QAM) vector signal generation at microwave/millimeter-wave bands enabled by Mach-Zehnder modulator (MZM)-based adaptive photonic frequency multiplication. In order to attain an electrical millimeter-wave vector signal displaying multiamplitude QAM modulation, such as 8QAM, after square-law photodiode detection, the driving radio-frequency (RF) signal, carrying multiamplitude QAM transmitter data, should be both amplitude- and phase-precoded before used to drive the MZM. We experimentally demonstrate 8QAM vector signal generation at W-band adopting photonic frequency octupling (x8) enabled by our proposed scheme. The MZM is driven by a 12-GHz precoded RF signal carrying 1-GBaud 8QAM transmitter data. The generated 1-GBaud 8QAM vector signal at W-band is air transmitted over 2-m distance. To the best of our knowledge, it is the first time to realize the generation and reception of multiamplitude QAM vector signal by one external modulator at W-band.

Digital Preemphasis in Optical Communication Systems: On the DAC Requirements for Terabit Transmission Applications

Next-generation coherent optical systems are geared to employ high-speed digital-to-analog converters (DAC), allowing for digital preprocessing of the signal and flexible optical transport networks. However, one of the major obstacles in such architectures is the limited resolution (less than 5.5 effective bits) and –3 dB bandwidth of commercial DACs, typically limited to half of the currently commercial baud rates, and even relatively reduced in case of higher baud rate transponders (400 Gb/s and 1 Tb/s). In this paper, we propose a simple digital preemphasis (DPE) algorithm to compensate for DAC-induced signal distortions, and exhaustively investigate the impact of DAC specifications on system performance, both with and without DPE. As an outcome, performance improvements are established across various DAC hardware requirements (effective number of bits and bandwidth) and channel baud rates, for m-state quadrature amplitude modulation (QAM) formats. In particular, we show that lower order modulation formats are least affected by DAC limitations, however, they benefit the most from DPE in extremely challenging hardware conditions. On the contrary, higher order formats are severely limited by DAC distortions, and moderately benefit from DPE across a wide range of DAC specifications. Moreover, effective number of bit requirements are established for m-state QAM, assuming low and high baud rate transmission regimes. Finally, we discuss the application scenarios for the proposed DPE in next-generation terabit transmission systems, and establish maximum transportable baud rates, which are shown to be used toward increasing channel baud rates to reduce terabit subcarrier count or toward increasing forward error correction (FEC) overheads to reduce the pre-FEC bit error rate threshold. Maximum baud rates after DPE are summarized here for polarization multiplexed BPSK, QPSK, 8QAM, and 16QAM, assuming two DACs: Current commercial DACs (5.5 effective bits, 16 GHz bandwidth) 57, 54, 51, and 48 Gbaud, respectively. Next-generation DACs (7 effective bits, 22 GHz bandwidth): 62, 61, 60, and 58 Gbaud, respectively.

Non-maximally decimated filter bank-based single-carrier receiver: a pathway to next-generation wideband communication

We present the design of a wideband digital modem based on non-maximally decimated filter bank (NMDFB) with perfect reconstruction (PR) property. The PR-NMDFB contains an analysis filter bank (AFB) and a synthesis filter bank (SFB) whose efficient polyphase forms are named as polyphase analysis channelizer (PAC) and polyphase synthesis channelizer (PSC). The waveform being processed is the legacy square root Nyquist-shaped quadrature amplitude modulation (QAM). In contrast to orthogonal frequency division multiplexing (OFDM) systems, the shaped QAM transmission has much superior performance properties in throughput, peak-to-average power ratio (PAPR), and synchronization. We will show the PR-NMDFB is capable of efficiently performing several key tasks of a digital receiver with dramatic workload reduction. This includes digital filtering, carrier recovery, and symbol timing recovery. Moreover, the nature of NMDFB allows the signal processing to operate a significantly reduced sample rate, which is a desired characteristic for replacing current FIR implementation in wideband systems.

All-Optical Simultaneous Multilevel Amplitude and Phase Regeneration

Simultaneous amplitude and phase noise reduction of multiple signal states using a nonlinear amplifying loop mirror with integrated directional phase-sensitive amplifier are presented for the star-eight quadrature amplitude modulation transmission format as an example. The performance of this combined regenerator scheme is compared with that of a cascade of separate phase and amplitude regenerators. It could be shown that an improvement in the error vector magnitude of 4 dB for the high-power states with simultaneous improvement of 5 dB for the low-power states is possible in both cases. Transmission improvement by regeneration is considered for two noise types: 1) amplified spontaneous emission and 2) nonlinear phase noise. Both schemes can either improve the bit error rate by an order of magnitude or enable an increase of the fiber launch power by 3 dB.

Modulation-Format-Independent Carrier Phase Estimation for Square M-QAM Systems

A modulation-format-independent or universal carrier phase estimation (U-CPE) technique is proposed for square 4-, 16-, and 64-ary quadrature amplitude modulation transmission systems for future elastic optical networks. The proposed U-CPE is based on a feedback architecture and eliminates the need to know the signal modulation format (MF) by using a cost function in the phase estimation update that is common to all square-shaped constellations. Compared with standard decision directed phase lock loop (S-DD-PLL) that requires the knowledge of the MF, the U-CPE method maintains close OSNR performance and linewidth tolerance. When MF is agnostic, the proposed U-CPE outperforms a modulation format oblivious (MFO)-DD-PLL in which QPSK decision rules are always used irrespective of actual MF of the signal.