Pilot Tones Research Articles

PON transceiver technologies for ≥50 Gbits/s per λ: Alamouti coding and heterodyne detection [Invited

There has been an ongoing quest for transceiver technologies to be employed in next-generation passive optical networks (PONs) beyond 25G due to the growing number of subscribers and connected devices per subscriber and the ever-increasing bandwidth demand per device/application. Given the cost and loss/power budget requirements, the candidates seem to be digital signal processing (DSP)-aided intensity-modulation/direct detection (IM-DD) and low-complexity coherent transceivers. Here, we experimentally demonstrate a 100G coherent-lite PON using a novel transceiver DSP chain that utilizes a frequency-diverse dual-polarization RF pilot tone pair. The proposed scheme enables the implementation of Alamouti coding for single-carrier signaling, which is used in conjunction with heterodyne detection, achieving a significant complexity reduction in a coherent optical network unit (ONU) receiver. It consists of a single balanced photodiode followed by an analog-to-digital converter, offering comparable optical complexity to its DD counterpart, at the expense of DSP complexity in the ONU. The performance of a transceiver architecture facilitated by the proposed DSP was assessed in both back-to-back operation and up to 80 km standard single-mode fiber transmission using an ∼ 1 MHz linewidth distributed feedback laser as an ONU laser. At the hard-decision forward error correction (7% overhead) threshold, assumed to be 4 × 10 − 3 , a receiver sensitivity of − 29.6 dBm and a loss budget of 36.6 dB at a launch power of 7 dBm are achieved.

200 G Outdoor Free-Space-Optics Link Using a Single-Photodiode Receiver

We experimentally evaluate the performance of Kramers–Kronig (KK) receivers for free-space optical transmission. We transmit a dual-carrier signal, each carrier modulated as a 25 Gbaud 16/32 quadrature-amplitude modulation (QAM) signal. This dual-carrier signal, with overall symbol rate of 50 Gbaud, is detected in a single receiver with about 35 GHz bandwidth. Key parameters such as the carrier-to-signal power ratio, the frequency allocation of the pilot tone, and the frequency separation between both modulated carriers are optimized. In addition, we also study the effect of the receiver bandwidth limitations on the KK technique. We perform long-term measurements, beyond one hour, for both 16 and 32 QAM signals in a 55 m outdoor free-space link during daylight with sunny weather conditions. Our measurements demonstrate that the link operates below the BER threshold assuming 20% forward-error correction (FEC) for the 32QAM signal and 7% FEC overhead for the 16QAM signal. The resulting net data rates are 208 and 184 Gbit/s for the 32 and 16 QAM signals, respectively.

Phase Noise Cancellation in Coherent Communication Systems Using a Radio Frequency Pilot Tone

Long-haul optical fiber communication employing digital signal processing (DSP)-based dispersion compensation can be distorted by the phenomenon of equalization-enhanced phase noise (EEPN), due to the reciprocities between the dispersion compensation unit and the local oscillator (LO) laser phase noise (LPN). The impact of EEPN scales increases with the increase of the fiber dispersion, laser linewidths, symbol rates, signal bandwidths, and the order of modulation formats. In this work, the phase noise cancellation (PNC) employing a radio frequency (RF) pilot tone in coherent optical transmission systems has been investigated. A 28-Gsym/s QPSK optical transmission system with a significant EEPN has been implemented, where the carrier phase recovery (CPR) was realized using the one-tap normalized least-mean-square (NLMS) estimation and the differential phase detection (DPD), respectively. It is shown that the RF pilot tone can entirely eliminate the LPN and efficiently suppress the EEPN when it is applied prior to the CPR.

Sensing OFDM Signal: A Deep Learning Approach

Spectrum sensing plays a critical role in dynamic spectrum sharing, a promising technology to address the radio spectrum shortage. In particular, sensing of orthogonal frequency division multiplexing (OFDM) signals, a widely accepted multi-carrier transmission paradigm, has received paramount interest. Despite various efforts, noise uncertainty, timing delay and carrier frequency offset (CFO) still remain as challenging problems, significantly degrading the sensing performance. In this work, we develop two novel OFDM sensing frameworks utilizing the properties of deep learning networks. Specifically, we first propose a stacked autoencoder based spectrum sensing method (SAE-SS), in which a stacked autoencoder network is designed to extract the hidden features of OFDM signals for classifying the user’s activities. Compared to the conventional OFDM sensing methods, SAE-SS is significantly superior in the robustness to noise uncertainty, timing delay, and CFO. Moreover, SAE-SS requires neither any prior information of signals (e.g., signal structure, pilot tones, cyclic prefix) nor explicit feature extraction algorithms which however are essential for the conventional OFDM sensing methods. To further improve the sensing accuracy of SAE-SS, especially under low SNR conditions, we propose a stacked autoencoder based spectrum sensing method using time-frequency domain signals (SAE-TF). SAE-TF achieves higher sensing accuracy than SAE-SS using the features extracted from both time and frequency domains, at the cost of higher computational complexity. Through extensive simulation results, both SAE-SS and SAE-TF are shown to achieve notably higher sensing accuracy than that of state of the art approaches.

Enhanced Carrier to Noise Ratio by Brillouin Amplification for Optical Communications

The limits of narrowband Brillouin gain for improving the carrier to noise ratio (CNR) of spectral lines is evaluated for 64-QAM communications. Calculations of the change in CNR for practical gain, noise, and bandwidth parameters indicate the maximum obtainable CNR enhancement factor and output CNR as being a25 dB and a40∼43 dB/0.1 nm, respectively, limited by the narrowness of the Brillouin gain bandwidth and Brillouin amplifier noise, respectively. These limits determined from the relative power suppression of out of band noise contained within a 10 GHz channel bandwidth are relevant to spectral line amplification either before data modulation at the transmitter, or for a local oscillator (LO) at the receiver for coherent detection. A calibration measurement of the bit error rate (BER) of a 48 Gb/s 64-QAM signal versus CNR of a LO loaded with ASE noise is used to extrapolate the BER from the calculated CNR for Brillouin gain. The results predict the BER being minimally degraded for an input CNR of a>20 dB/0.1 nm and falling below the hard decision FEC limit for an input CNR as low as a0 dB/0.1 nm. The estimates are found consistent with 64-QAM signal experiments of pilot tone carrier recovery by Brillouin gain for both single channel and WDM cases. The insight aids the design and estimation of achievable performance improvement for applying Brillouin amplifiers to coherent communications.

RF pilot tone phase noise cancellation based on DD-MZM SSB modulation for optical heterodyne RoF link

The optical heterodyne radio over fiber (RoF) link using single-sideband (SSB) modulation has advantage in system scalability, tunability, as well as resistance to chromatic dispersion induced frequency selective fading. However, the phase noise which seriously degrades system performance, is a major obstacle to employ low-cost free-running lasers in the aforementioned link. In this paper, the RF pilot tone (RFP) phase noise cancellation (PNC) based on dual drive Mach–Zehnder modulator SSB modulation for optical heterodyne RoF link is investigated and demonstrated. Two kinds of RFP PNC schemes, carrier whole multiplication (CWM) and carrier signal multiplication (CSM), are both derived theoretically and verified experimentally. The impacts of carrier signal power ratio, carrier filter bandwidth and guard band on system performance are studied in detail. The results show that, the RFP-CSM scheme can improve the spectral efficiency by 62.5% and receiver sensitivity of 1.2 dB for 1.25 Gbps quadrature phase shift keying signal at a BER of 3.8 ×10−3, as compared with the RFP-CWM scheme and power detection scheme, mainly attributed to the absence of the second-order nonlinear intermodulation distortion. Furthermore, the RFP-CSM scheme demonstrates good phase noise robustness with different high-order modulation formats under two kinds of linewidth local oscillator lasers.

Develop of CFO in High Mobility Environments for OFDM-DMWT Based Mobile Wireless Applications

An “Orthogonal Frequency division Multiplexing (OFDM)” systems are sensitive to “service Frequency Offset (CFO)” mistakes. OFDM- “Discrete Multi-wavelet remodel (DMWT)” based totally cellular wireless with a sender, a provider Frequency Offset errors turns into similarly tough for cellular applications in view that power postpone outline of the channel may trade quick because of the sporadic start and dying of the channel paths. We discover analytical expressions for channel estimation mistakes inside the existence of CFO mistakes and excessive mobility. We display that the sensitivity of the channel estimator be capable of still be exploited to increase CFO in high mobility environments. A comparative have a look at amongst extraordinary techniques of service frequency offset estimation in OFDM systems is offered. We keep in mind strategies in frequency area are considered, namely; training image approach and Pilot tone technique. Mean square blunders (MSE) is the evaluation criteria used within the look at. Simulation consequences display that the CFO estimation strategies the schooling image technique outperforms the pilot tone approach with the aid of about 2dB. The contrast results are considered at MSE of 10-4 and normalized CFO of 0.2. Keywords— CFO, ICI, BER, SNR, OFDM, DMWT, MSE, Frequency domain. DOI : 10.7176/NCS/10-04 Publication date :July 31 st 2019

New Iterative Method for Estimation of Carrier Frequency in Multicarrier Systems

The key problem for OFDM (Orthogonal Frequency Division Multiple) systems is who to Estimate carrier frequency offset (CFO) with reduced complexity and acceptable performance. The CFO must be compensated before DFT (Discrete Fourier Transform) in order to restore data correctly and enhance the system performance. This paper, present a low complexity estimator of CFO with Semi-Blind (SB) criterion based on pilot tones and on virtual subcarriers, and with the aid of subspace based method. However, MUSIC and ESPRIT based semi-blind algorithms require a highly computational complexity. To overcome this drawback, we use Taylor’s series for the first order as developed in [1]. The present methods developed in this paper are very suitable for Multi- Carrier (MC) systems when the CFO are present. Simulation results demonstrated that the semi-blind (SB) approach outperforms the blind-based approach.

A Synchronous Baseband Receiver for High-Data-Rate Millimeter-Wave Communication Systems

A novel synchronous baseband receiver is presented in this letter. With a pilot tone insertion at the transmitter, the proposed baseband receiver can perform carrier recovery (CR) regardless of modulation scheme and/or baud rate. The synchronous baseband receiver has an analog–digital hybrid structure, where a low-cost digital signal processor controls an analog local oscillator (LO) in frequency and phase to achieve CR. This structure requires only one low-cost analog-to-digital converter (ADC) with a sampling rate of 100 MS/s. A proof-of-concept demonstration at E-band achieves 9-Gb/s 64-quadratic-amplitude modulation (QAM) and 16-Gb/s QPSK transmissions.

Experimental Investigation of Link Impairments in Pilot Tone Aided Superchannel Transmission

We investigate the performance of single-pilot-tone locked frequency comb-based superchannel transmission for distances up to 1200km. In our scheme, electro-optic transmitter and receiver combs are locked by leaving one of the transmitter carriers unmodulated and regenerating the receiver comb via optical injection locking. This approach significantly reduces carrier offsets and therefore leads to reduced digital signal processing complexity. We experimentally assess how transmission impairments such as noise added by optical amplifiers and fiber nonlinearities affect the quality of the comb regeneration. Our results show that while the operating conditions are more stringent at longer distances, the single pilot is robust to impairments. At optimal launch power, similar performance with respect to an intradyne receiver is observed, showing that the optical pilot tones can be co-transmitted with data channels even at distances spanning hundreds of kilometer. The total superchannel spectral efficiency (throughput), including the pilot tone and guardbands, is 9.6 bits/s/Hz (12 Tbit/s) after 480 km and 8.4 bits/s/Hz (10.5 Tbit/s) after 960km.

Reliability and Accessibility of Low-Latency V2I Channel Training Protocol Using Cover-Free Coding: Win–Win or Tradeoff ?

Future vehicle-to-infrastructure communications in a multi-vehicle environment highly depend on the acquisition of wireless orthogonal frequency division multiplexed (OFDM) channels through the conventional pilot-tone based channel training protocol. However, this protocol can be easily compromised by a pilot-aware attacker spoofing/jamming/nulling those pilot tones, which have been designated for multiple vehicles. To solve this issue, we propose merging the multi-vehicle signal characteristic into the well-known cover-free coding technique for securely and retrievably coding pilot tones of multiple vehicles. In particular, multiple vehicles encode and convey their own pilot tones into diversified subcarrier activation patterns (SAPs). Then, road side unit observes the superposition of those encoded SAPs and re-designs the decoding procedure using signal independence characteristic such that those SAPs, though camouflaged by malicious signals in wireless environment, could be separated, identified, and decoded into original pilots. The protocol reliability is formulated using pilot identification error probability (IEP), and the protocol $\xi$ -accessibility is defined by the ratio of the number of accessible vehicles to the consumed OFDM symbols under the constraint of IEP less than $\xi$ . We prove that a win–win relationship between reliability and accessibility always exists and also demonstrate how more number of antennas of the attacker could improve the reliability better. Finally, numerical results verify how the proposed method is enabled to support ultra-reliable ( $1-\xi \geq 99.999\%$ ) and ultra-low-latency (up to 1.5 ms) multi-vehicle access for channel training under an Long Term Evolution (LTE) 20 MHz bandwidth.

Hierarchical 2-D Feature Coding for Secure Pilot Authentication in Multi-User Multi-Antenna OFDM Systems: A Reliability Bound Contraction Perspective

Due to the publicly known and deterministic characteristic of pilot tones, pilot authentication (PA) in multi-user multi-antenna orthogonal frequency-division multiplexing systems is very susceptible to the jamming/nulling/spoofing behaviors. To solve this, in this paper, we develop a hierarchical 2-D feature (H2DF) coding theory that exploits the hidden pilot signal features, i.e., the energy feature and independence feature, to secure pilot information coding which is applied between legitimate parties through a well-designed five-layer hierarchical coding model to achieve secure multiuser PA (SMPA). The reliability of SMPA is characterized using the identification error probability (IEP) of pilot encoding and decoding with the exact closed-form upper and lower bounds. However, this phenomenon of non-tight bounds brings about the risk of long-term instability in SMPA. Therefore, a reliability bound contraction theory is developed to shrink the bound interval, and practically, this is done by an easy-to-implement technique, namely, codebook partition within the H2DF code. In this process, a tradeoff between the upper and lower bounds of IEP is identified and a problem of optimal upper and lower bound tradeoff is formulated, with the objective of optimizing the cardinality of sub-codebooks such that the upper and lower bounds coincide. Solving this, we finally derive an exact closed-form expression for IEP, which realizes a stable and highly reliable SMPA. Numerical results validate the stability and resilience of H2DF coding in SMPA.

A Digital Mode-Matching Control System Based on Feedback Calibration for a MEMS Gyroscope

A digital mode-matching control system based on feedback calibration, where two pilot tones are applied to actuate the sense mode by the robust feedback controller, is presented for a MEMS gyroscope in this paper. A dual-mass decoupled MEMS gyroscope with the integrated electrostatic frequency tuning mechanisms, the quadrature correction electrode, and the feedback electrode is adopted to implement mode-matching control. Compared with the previous mode-matching method of forward excitation calibration, the proposed mode-matching scheme based on feedback calibration has better adaptability to the variation in the frequency of calibration pilot tones and the quality factor of the sense mode. The influences of calibration pilot tone frequency and the amplitude ratio on tuning performance are studied in theory and simulation. The simulation results demonstrate that the tuning error due to the amplitude asymmetry of the sense mode increases with a frequency split between pilot tones and the drive mode and is significantly reduced by the amplitude correction technology of pilot tones. In addition, the influence of key parameters on the stability of the mode-matching system is deduced by using the average analysis method. The MATLAB simulation of the mode-matching control system illustrates that simulation results have a good consistency with theoretical analysis, which verifies the effectiveness of the closed-loop mode-matching control system. The entire mode-matching control system based on a FPGA device is implemented combined with a closed-loop self-excitation drive, closed-loop force feedback control, and quadrature error correction control. Experimental results demonstrate that the mode-matching prototype has a bias instability of 0.63°/h and ARW of 0.0056°/h1/2. Compared with the mode-mismatched MEMS gyroscope, the performances of bias instability and ARW are improved by 3.81 times and 4.20 times, respectively.

Independence-Checking Coding for OFDM Channel Training Authentication: Protocol Design, Security, Stability, and Tradeoff Analysis

In wireless OFDM communications systems, pilot tones, due to their publicly known and deterministic characteristic, suffer significant jamming/nulling/spoofing risks. Thus, the convectional channel training protocol using pilot tones could be attacked and paralyzed, which raises the issue of anti-attack channel training authentication (CTA), i.e., verifying the claims of identities of pilot tones and channel estimation samples. In this paper, we consider one-ring scattering scenarios with large-scale uniform linear arrays (ULA) and develop an independence-checking coding (ICC) theory to build a secure and stable CTA protocol, namely, ICC-based CTA (ICC-CTA) protocol. In this protocol, the pilot tones are not only merely randomized and inserted into subcarriers but also encoded as diversified subcarrier activation patterns (SAPs) simultaneously. Those encoded SAPs, though camouflaged by malicious signals, can be identified and decoded into original pilots for high-accuracy channel impulse response (CIR) estimation. The CTA security is first characterized by the error probability of identifying legitimate CIR estimation samples. The CTA instability is formulated as the function of probability of stably estimating CIR against all available diversified SAPs. A realistic tradeoff between the CTA security and instability under the discretely distributed AoA is identified and an optimally stable tradeoff problem is formulated, with the objective of optimizing the code rate to maximize security while maintaining maximum stability for ever. Solving this, we derive the closed-form expression of optimal code rate. Numerical results finally validate the resilience of proposed ICC-CTA protocol.

OFDM Modulated PNC in V2X Communications: An ICI-Aware Approach Against CFOs and Time-Frequency-Selective Channels

This paper studies the application of physical-layer network coding (PNC) in vehicle-to-everything communications to accommodate the time-critical nature of vehicular ad-hoc networks (VANETs). The idea can theoretically reduce the transmission latency by 50%, thus alleviating the short contact time issues caused by high-speed vehicle motion. Conventional studies of PNC primarily considered static networks. In highly mobile networks like VANETs, the carrier frequency offsets (CFOs) due to high-speed motion will lead to inter-carrier interference (ICI) in orthogonal frequency division multiplexing (OFDM) systems. Moreover, the vehicular environment with time–frequency-selective channels further undermines accurate channel estimation for multiple users. It is also worth noting that the CFO that exists in OFDM modulated PNC cannot be completely eliminated through CFO tracking and equalization as in conventional point-to-point transmissions. These critical issues can significantly increase the bit error rate at the receiver. To address these challenges, this paper proposes an ICI-aware approach that jointly achieves accurate channel estimation, signal detection, and channel decoding. We express the channel estimation and detection and decoding as two optimization problems and resolve them with the expectation–maximization algorithm and the belief propagation algorithm, respectively. The proposed approach can efficiently mitigate the negative effect of ICI by exploiting both pilot and data tones in channel estimation, detection, and decoding. Both simulation and experiment are conducted to evaluate the proposed approach, and the results reveal that the proposed algorithm outperforms the benchmark that simply treats ICI as Gaussian noise.

A Multi-User Detection Scheme Based on Polar Code Construction in Downlink Underwater Acoustic OFDM Communication System

In this paper, we propose a novel polar code construction scheme, suitable for the underwater acoustic (UWA) multi-user communication system using orthogonal frequency division multiplexing (OFDM). We use the polar codes because the code rate of a polar code can be arbitrarily designed. Moreover, the polar codes can also be used to create a nested code structure, divided into several subsets (codebook). We exploit this feature of the polar codes and place the information bits in the codebook to avoid superimposition of all the users. This helps to eliminate the influence of the power allocation coefficient and make full use of the power of the transducer at the transmitted node. Furthermore, the proposed scheme maximizes the utilization efficiency of the OFDM subcarrier resource for each user by avoiding null pilot tones that are usually required for channel equalization in the traditional schemes. The proposed scheme also does not depend on the channel state information (CSI) as prior information as each user can decode their own data independently by using their own codebook without successive interference cancellation (SIC). The simulation and tank experiment results demonstrate that the proposed scheme can significantly guarantee the reliability of all the users, and reduces the complexity of the receiver, thus making it most suitable for the downlink UWA multi-user communication systems.

A Time-Varying Filter for Doppler Compensation Applied to Underwater Acoustic OFDM.

This paper describes a Doppler compensation algorithm to improve the reliability of orthogonal frequency division multiplexing (OFDM). To compensate for the time-varying Doppler effect in a mobile deployment scenario, first the time-scaling factor over a wideband channel is estimated using pilot tones inserted in each OFDM symbol. Then, using a time-varying resampling technique, the Doppler effect is compensated during the reception of each OFDM symbol in the frame. To predict the performance of the system in relatively shallow waters, a software channel model is developed that is able to simulate a wide variety of dynamic shallow water deployment scenario. The performance of the algorithm was tested for two extreme frequency ranges during sea trials, the first at 2 kHz for a long-range application, and the second at 125 kHz for a short range telemetry link. For the 2-kHz system, a 16-bps mobile link in which the platform was moving at 1 m/s was demonstrated to have a bit error rate on the order of , while, for the 125-kHz telemetry application, a 2000-bps link was enabled with a bit error rate of 0.03 at a low SNR equal to 5.5 dB.

12 b/s/Hz Spectral Efficiency Over the C-band Based on Comb-Based Superchannels

We present transmission of three 50 × 24 Gbaud PM-256QAM superchannels with an overall spectral efficiency of 12 b/s/Hz over the entire C-band. Each comb-based superchannel combines a single shared optical pilot tone with a low-overhead pilot-based DSP. The pilot tone is used to lock the transmitter and the receiver comb seed lasers, suppressing carrier offsets. Combined with optimized insertion ratio of the digital pilot symbols, the total overhead is minimized, avoiding any excess loss in spectral efficiency. Our results demonstrate the feasibility to exploit the unique properties of frequency combs in the design of high spectral efficiency multi-Tb/s superchannels.

CSI estimation with pilot tone for scintillation effects mitigation on satellite optical communication

Abstract This study investigates a scintillation mitigation technique in the absence of perfect channel state information (CSI) for satellite optical communications (SOCs). Based on the spectrum property of the on–off keying (OOK) modulation scheme, a pilot tone was inserted into the transmitted signals to estimate turbulence information. Theoretical analysis subsequently demonstrated that the proposed scheme could extract CSI that was close to perfect CSI. The experiments showed that bit-error rate (BER) performance was similar to the results with perfect CSI for each scintillation index 0.1, 0.4 circumstance. It proves that the proposed technique was feasible to be adapted for practical SOC systems.

Code-Frequency Block Group Coding for Anti-Spoofing Pilot Authentication in Multi-Antenna OFDM Systems

A pilot spoofer can paralyze the channel estimation in multi-user orthogonal frequency-division multiplexing (OFDM) systems by using the same publicly known pilot tones as legitimate nodes. This causes the problem of pilot authentication (PA). To solve this, we propose, for a two-user multi-antenna OFDM system, a code-frequency block group (CFBG) coding-based PA mechanism. Here multi-user pilot information, after being randomized independently to avoid being spoofed, is converted into activation patterns of subcarrier-block groups on code-frequency domain. Those patterns, though overlapped and interfered mutually in the wireless transmission environment, are qualified to be separated and identified as the original pilots with high accuracy, by exploiting CFBG coding theory and channel characteristic. Particularly, we develop the CFBG code through two steps, i.e., 1) devising an ordered signal detection technique to recognize the number of signals coexisting on each subcarrier block, and encoding each subcarrier block with the detected number and 2) constructing a zero-false-drop code and block detection-based code via $k$ -dimensional Latin hypercubes and integrating those two codes into the CFBG code. This code can bring a desirable pilot separation error probability, inversely proportional to the number of occupied subcarriers and antennas with a power of $k$ . To apply the code to PA, a scheme of pilot conveying, separation, and identification is proposed. Based on this novel PA, a joint channel estimation and identification mechanism is proposed to achieve high-precision channel recovery and simultaneously enhance PA without occupying extra resources. Simulation results verify the effectiveness of our proposed mechanism.