Dispersion-induced RF Power Fading Research Articles

Generation of Switchable Chirp Waveforms in the Photonic Domain with Immunity to Dispersion-Induced Power Fading

A novel photonic approach to generating switchable multi-format chirp waveforms using a dual-drive dual-parallel Mach–Zehnder modulator (DD-DPMZM) is proposed and experimentally demonstrated. By properly controlling the bias voltage on the DD-DPMZM, different chirp RF waveforms, including the dual-, down-, and up-chirp waveforms, can be obtained when an RF signal and a chirp RF signal are injected into the modulator. A main feature of this approach is that it can eliminate chromatic dispersion-induced RF power fading, which is highly desired in distributed multi-functional radars based on radio over fiber. There is no polarization control and optical filtering in the given scheme, which also improves the stability and feasibility of the approach. An experiment successfully demonstrated the generation and switching of the multi-format chirp waveforms and the capability of immunity to dispersion-induced power fading.

Asymmetric dual-SSB modulation for photonic co-frequency mm-wave signals generation and DSP-free receiver.

An asymmetric dual-single-sideband (SSB) modulation scheme for photonic co-frequency millimeter (mm)-wave signals generation and digital signal processing (DSP)-free receiver is experimentally demonstrated for the first time, to the best of our knowledge. To effectively avoid the sideband crosstalk in the traditional symmetric dual-SSB modulation scheme, not only two vector-modulated signals but also two unmodulated sidebands are modulated on the two asymmetric sides of an optical carrier in this scheme. An optical delay line interferometer could easily separate these two asymmetric dual-SSB signals simultaneously in the receiver, and thus the photonic frequency up-conversion is realized. Besides, this scheme is free of dispersion-induced RF power fading thanks to the SSB modulation. By this means, no digital compensation algorithms such as carrier phase recovery, fiber dispersion compensation, and channel equalization are required, contributing to the DSP-free receiver. In our experiment, two 32 GHz 3.2 Gb/s 16-ary quadrature amplitude modulation mm-wave signals are produced using two RF signals with the carrier frequencies of 12 GHz and 20 GHz. The error vector magnitude (EVM) performances of these two mm-wave signals after 25.5 km standard single-mode fiber transmission are better than 3rd Generation Partnership Project requirements without using any digital compensation algorithms.

Performance Analysis of Kramers–Kronig Receiver in the Presence of IQ Imbalance

We investigate the impact of IQ imbalance of the transmitter on the performance of the direct-detection system utilizing the Kramers–Kronig receiver. We show through theoretical analysis that the amplitude imbalance, timing misalignment, and IQ phase mismatch create double-sideband distortions, which, in turn, serve to violate the minimum phase condition and also lead to the dispersion-induced RF power fading after transmission. The theoretical analysis is verified by using numerical simulations. The results show that the amplitude imbalance and timing misalignment should be less than 2 dB and 2 ps, respectively, when the receiver sensitivity penalty is smaller than 1 dB for the transmission of a 112-Gb/s 16-QAM OFDM signal.

Direct-Detection OFDM Transmission Using Four-State Chirp Control With a Dual-Electrode MZM for Dispersion Compensation

We discuss a four-state chirp-control scheme using a dual-electrode Mach–Zehnder modulator to compensate dispersion-induced RF power fading when employing direct-detection-based orthogonal-frequency-division-multiplexed (OFDM) signals. We numerically confirm that in contrast to the previously proposed parallel transmitter employing an intensity/phase modulator, four-state chirp control can flatten frequency response and make the total performance uniform even though its structure is significantly simplified. To demonstrate this, we successfully transmitted a 20-GHz OFDM signal over an 80-km single-mode fiber using four-state chirp control, achieving a line bit rate of 78.41 Gb/s even in the presence of chromatic dispersion.

Broadband IF-Over-Fiber Transmission With Parallel IM/PM Transmitter Overcoming Dispersion-Induced RF Power Fading for High-Capacity Mobile Fronthaul Links

We demonstrate a broadband and long-distance intermediate frequency-over-fiber (IFoF) transmission scheme employing a transmitter composed of parallel intensity/phase (IM/PM) modulators with appropriate bandwidth allocations to IM and PM. Due to the proposed scheme, we can eliminate all the null frequencies caused by dispersion-induced RF power fading, which, in turn, enables us to significantly increase the available bandwidth. In addition, our system does not require any synchronization between IM and PM, which reduces complexity compared to conventional parallel transmitter architecture. We successfully transmitted 20 × 360 MHz filtered orthogonal-frequency-division-multiplexed signals corresponding to a common public radio interface equivalent data rate of 524.28 Gbps over a 30- and 40-km single-mode fiber satisfying the 8% threshold for the error-vector magnitude values for all the subcarriers. These results show that our proposed IFoF transmission scheme is scalable to long-distance mobile fronthaul links for 5G and beyond.

1.032-Tb/s CPRI-Equivalent Rate IF-Over-Fiber Transmission Using a Parallel IM/PM Transmitter for High-Capacity Mobile Fronthaul Links

We report high-capacity intermediate-frequency-over-fiber (IFoF) transmission using parallel intensity/phase modulators (IM/PM). Thanks to the parallel IM/PM transmitter, we can avoid all the null frequencies caused by dispersion-induced RF power fading. Compared to other techniques to avoid RF power fading, the parallel IM/PM transmitter offers a simple solution because it does not rely on any sophisticated digital signal processing. We transmitted 14 $\times$ 1.2-GHz orthogonal-frequency-division-multiplexed signals over a 20-km single-mode fiber using the parallel IM/PM transmitter and achieved a Common Public Radio Interface equivalent data rate of 1.032 Tb/s. Thus, an IFoF system with the parallel IM/PM transmitter shows great potential to be applied to high-capacity mobile fronthaul links in the upcoming fifth-generation mobile system and beyond.

Wideband Optical Multipath Interference Cancellation Based on a Dispersive Element

A novel scheme is proposed for analog multipath interference cancellation based on two polarization modulators (PolMs) and a dispersive element. Multiple optical compensation branches are formed by an array of tunable lasers. By adjusting the wavelength and the optical power of each tunable laser, the delay and the magnitude of the known interference signal can be finely tuned, so that the multipath interference can be accurately reconstructed for counter-phase cancellation. The dispersion-induced RF power fading in a dispersive optical link can also be compensated because of the PolM-based intensity modulation scheme, so the best cancellation depth can be obtained at any desired frequency. Experimental result shows that the proposed system can achieve more than 44-dB cancellation depth for wideband signals.

A Q-band radio-over-fiber system for distribution of uncompressed high-definition video signals

A Q-band radio-over-fiber (RoF) system for the distribution of uncompressed high-definition (HD) video signal is proposed and demonstrated. To eliminate the fiber dispersion induced RF power fading, which is a serious problem for the Q-band RoF system, a polarization modulator (PolM) based transmitter is applied, by which the peak of the frequency response of the fiber link can be shifted to any desired frequency. Besides, an injection-locked Q-band optoelectronic oscillator (OEO) is constructed in the receiver to perform the carrier extraction and frequency downconversion. A 40-GHz RoF system is established, which delivers a 720P uncompressed HD video signal to a targeted displayer through a 20-km fiber link and a ∼1-m wireless link.

Optical Single-Sideband Modulation With Tunable Optical Carrier to Sideband Ratio in Radio Over Fiber Systems

We show that an optical modulator that consists of an integrated dual parallel Mach-Zehnder modulator (dMZM) can be used for obtaining not only optical single-side band modulation but also tunability of optical carrier to sideband ratio (OCSR) simultaneously. Such a modulator will be vital for optimizing the performance of radio over fiber links by improving modulation efficiency and receiver sensitivity and by removing fiber chromatic dispersion induced RF power fading. It is shown that a wide range of OCSR tunability can be obtained by altering bias voltage of dMZM, and optimum OCSR, to maximize the output RF power, depends on RF modulation index and extinction ratio of the integrated dMZM. Good agreement between theory, simulation, and experiment is obtained. For typical extinction ratio and low modulation index, it is found that an OCSR of 0 dB is optimum to maximize RF carrier power. However, for multiband orthogonal frequency-division multiplexing (MB-OFDM) ultra-wideband (UWB) radio, the best error vector magnitude (EVM) of -21.8 dB is obtained experimentally at an OCSR of ~5.4 dB due to avoidance of clipping induced nonlinear distortion.

Precompensated Optical Double-Sideband Subcarrier Modulation Immune to Fiber Chromatic-Dispersion-Induced Radio Frequency Power Fading

We propose and demonstrate a new optical double-sideband modulation technique that is immune to fiber chromatic dispersion and thus free of dispersion-induced RF power fading. The proposed modulation technique also provides a 3 dB improvement of RF power compared with optical single-sideband modulation. The proposed modulation technique is analyzed in theory and simulation. It is shown that for a given fiber length an optimum electrical phase shift exists to completely cancel the dispersion-induced RF power fading. We verify this proposed modulation technique experimentally for a single-tone RF signal and for a multiband orthogonal frequency division multiplexing ultrawideband signal.

Soliton sampling of subcarrier multiplexed signals to suppress dispersion-induced RF power fading

By using a novel soliton sampling technique, we demonstrate the suppression of RF power fading over 94 km of single-mode fiber. We transmit a digital data signal that has been multiplexed on a subcarrier wave that ranges from 0.25-14 GHz. A 35-ps soliton pulse train is used to sample the data-modulated subcarrier. These inherently stable soliton pulses are then used to carry the information without suffering from dispersion. By using this technique, RF power fading is reduced from 22 dB to <2 dB over a 10-GHz frequency range.