Microwave Vector Signal Research Articles

Non-orthogonal multiple access in a radio over fiber link based on optical-domain power allocation.

Non-orthogonal multiple access (NOMA) in a radio over fiber (RoF) link based on optical-domain power allocation is proposed and demonstrated. The NOMA is implemented at the RoF transmitter where two spectrum-overlapped microwave vector signals with an identical power level are modulated on an optical carrier to generate two orthogonally polarized optical signals. By passing the optical signals through a polarization controller (PC) and a polarizer, the power levels of the two optical signals are controlled to achieve optical power allocation (OPA). The optical signals are then transmitted over a fiber to the receiver. Since the power levels of the two microwave vector signals applied to the modulator are identical and the power allocation is implemented in the optical domain, the nonlinearity due to the higher-power input microwave vector signal is reduced, leading to an increase in the dynamic range. At the receiver, the two optical signals are detected at a photodetector (PD). To demultiplex the two microwave vector signals, a digital signal processing (DSP) algorithm is developed. The proposed approach is evaluated experimentally. The results show that the transmission performance in terms of error vector magnitude (EVM) is improved thanks to the increased dynamic range.

Simple polarization-insensitive coherent RoF link with increased capacity.

A simple polarization-insensitive coherent radio-over-fiber (RoF) link with increased spectrum efficiency and transmission capacity is proposed and demonstrated. Instead of using two polarization splitters (PBSs), two 90° hybrids, and four pairs of balanced photodetectors (PDs) in a conventional polarization-diversity coherent receiver (PDCR), a simplified PDCR with only one PBS, one optical coupler (OC), and two PDs is employed in the coherent RoF link. At the simplified receiver, a novel, to the best of our knowledge, digital signal processing (DSP) algorithm is proposed to achieve polarization-insensitive detection and demultiplexing of two spectrally overlapping microwave vector signals as well as the elimination of the joint phase noise originating from the transmitter and the local oscillator (LO) laser sources. An experiment is performed. The transmission and detection of two independent 16QAM microwave vector signals at identical microwave carrier frequencies of 3 GHz with a symbol rate of 0.5 GSym/s over a 25-km single-mode fiber (SMF) is demonstrated. Thanks to the spectrum superposition of the two microwave vector signals, the spectral efficiency as well as the data transmission capacity is increased.

An approach for linear optical phase demodulation using a single photodetector.

In this paper, we propose and experimentally verify a phase-modulated radio-over-fiber (RoF) link capable of transmitting the radio frequency (RF) signal linearly. By executing the Kramers-Kronig (KK) algorithm at the receiver, the proposed link can accomplish linear optical phase demodulation with a single photodetector rather than a coherent receiver. In the 16-quadrature amplitude modulation (16-QAM) and 64-QAM microwave vector signal transmission experiments, measured error vector magnitudes (EVMs) are 4.14% and 4.38%, respectively, after 25-km fiber transmission, and the measured spurious-free dynamic range (SFDR) is 114.5 dB·Hz2/3, which shows a good performance in linearity.

A coherent MMW-ROF link employing mixed modulations and photonic down-conversion

We propose and experimentally demonstrate a coherent millimeter wave radio-over-fiber (MMW-ROF) link employing mixed modulations with a simple digital signal processing (DSP) unit. At the transmitter, a light wave is intensity and phase-modulated by two microwave vector signals in a dual-parallel Mach–Zehnder modulator (DP-MZM). At the receiver, by using a 3-line optical frequency comb (OFC) signal as the optical local oscillator (OLO), the microwave vector signals can be photonically down-converted to intermediate frequency (IF) signals. Then, instead of a complex digital phase-locked loop (DPLL), simple algorithms are used to remove the laser phase fluctuation between two free-running lasers and do the demodulations. In the 25 km fiber transmission experiments, we successfully down-convert two 50 Msymbol/s 16-quadrature amplitude modulation (16-QAM) signals at 25 GHz and 24 GHz to intermediate frequencies of 2 GHz and 1 GHz. The transmission performance of the proposed link in terms of error vector magnitudes (EVMs) are also investigated. The results show that when the receiver optical power is -20 dBm, the measured EVMs are still less than 12.5% which satisfies the 3rd Generation Partnership Project (3GPP) standard.

Coherent-detection radio-over-fiber link with high spectral efficiency based on digital phase noise cancellation.

A radio-over-fiber (RoF) link with high spectral efficiency using digital phase cancellation is proposed and experimentally verified. The link can be utilized to transmit four microwave vector signals. The four microwave vector signals are modulated on optical signals by using a dual-polarization dual-parallel Mach-Zehnder modulator at the transmitter. The optical signals subsequently pass through a spool of single-mode fiber (SMF) and are sent into a coherent receiver. A digital signal processing (DSP) algorithm is proposed to eliminate the phase noise and frequency difference caused by two free-running lasers at transmitter and receiver. Two 16-quadrature amplitude modulation (16-QAM) vector signals at 6GHz and another two 16-QAM vector signals at 7GHz transmit over a 10-km SMF and are successfully recovered after a DSP algorithm. In order to evaluate the transmission performance of the RoF link, error vector magnitudes and bit error rates are also estimated.

A Microwave Photonic Link With Quadrupled Capacity Based on Coherent Detection and Digital Phase Noise Cancellation

A microwave photonic link (MPL) with quadrupled capacity based on coherent detection and digital phase noise cancellation is proposed and experimentally demonstrated. At the transmitter, a continuous-wave (CW) light wave is intensity modulated by four independent microwave vector signals with two having an identical center microwave frequency at a dual-parallel Mach-Zehnder modulator (DPMZM) consisting of two dual-drive MZMs (DEMZMs) with the sub-DEMEMs biased at the quadrature transmission point. Four intensity-modulated optical signals are generated and transmitted over a single-mode fiber (SMF) to a coherent receiver. To perform coherent detection, a second CW laser source as a local oscillator (LO) is also applied to the coherent receiver. To recover the microwave vector signals, a novel digital phase noise cancellation algorithm is developed and applied to eliminate the joint phase noise from the transmitter laser source and the LO laser source as well as the unstable offset frequency between the two laser sources. A theoretical analysis is performed to show the recovery of the microwave vector signals which is verified by an experiment. For four independent 16 quadrature amplitude modulation (16-QAM) microwave vector signals with a symbol rate of 0.5 GSymbol/s, error-free transmission over a 9-km SMF is achieved when the received optical power at the coherent receiver is higher than −18 dBm with forward error correction (FEC).

Microwave photonic link to transmit four microwave vector signals on a single optical carrier based on coherent detection and digital signal processing.

A microwave photonic link to transmit four independent microwave vector signals modulated on a single optical carrier based on coherent detection and digital signal processing (DSP) is proposed and experimentally demonstrated. At the transmitter, a continuous-wave (CW) light wave is modulated by four independent microwave vector signals with an identical center microwave frequency at a dual-polarization dual-drive Mach-Zehnder modulator (DP-DDMZM), to generate four intensity-modulated optical signals, with two signals sharing one of the two orthogonal polarization states. After transmission over a single-mode fiber (SMF), the optical signals are applied to a polarization- and phase- diversity coherent receiver to which a light wave from a second laser source as a local oscillator (LO) is also applied. To eliminate the joint phase noise and the unstable offset frequency from the transmitter and the LO laser sources and to perform polarization demultiplexing, a digital noise cancellation algorithm and a polarization demultiplexing algorithm are developed. The proposed approach is evaluated experimentally. For four independent 16 quadrature amplitude modulation (16-QAM) microwave vector signals at 4 GHz with a symbol rate of 0.5 GSymbol/s, error-free transmission over a 9-km SMF is achieved when the received optical power at the coherent receiver is higher than -21 dBm with forward error correction (FEC).

A High Spectral Efficiency Radio Over Fiber Link Based on Coherent Detection and Digital Phase Noise Cancellation

An approach to transmitting two independent microwave vector signals on a single optical carrier with one polarization state based on coherent detection and digital phase noise cancellation is proposed and experimentally demonstrated. At the transmitter, two independent microwave vector signals are modulated on an optical carrier via a dual-drive Mach-Zehnder modulator (DD-MZM). The modulated optical signals are transmitted over a single-mode fiber (SMF) and sent to a coherent receiver. At the receiver, the optical signals are detected where a local oscillator (LO) optical wave generated by a second free-running laser source is also applied. To recover the two microwave vector signals, a novel digital signal processing (DSP) algorithm is developed and applied to eliminate the joint phase noise terms from the transmitter and the LO laser sources as well as the unstable offset frequency between the two laser sources. An experiment is performed. The transmission of two independent 16 quadrature amplitude modulation (16-QAM) microwave vector signals at 4 GHz with a symbol rate of 1 GSymb/s over a 9-km SMF is demonstrated. The transmission performance in terms of error vector magnitudes (EVMs) and bit error rates (BERs) is also evaluated.

All-Optical Double Spectral-Efficient RoF Link With Compensation of Dispersion-Induced Power Fading

An all-optical radio-over-fiber (RoF) link with double spectral-efficient transmission and compensation of chromatic dispersion-induced power fading is proposed and experimentally demonstrated. At the central office, a commercial dual-polarization quadrature phase-shift keying modulator is utilized to produce a polarization-multiplexed signal composed of two modulated microwave vector signals and an unmodulated optical carrier. At the remote node, by adjusting the polarization controller to introduce a proper phase shift to the unmodulated optical carrier, the two vector signals can be demodulated respectively and dispersion-induced power fading can be compensated simultaneously. The link performance is evaluated by error vector magnitude and bit error rate.

A simplified coherent-detected radio-over-fiber link with intensity and phase modulation and simple digital phase noise cancellation

Reducing the complexity and the cost is the key to introducing high-performance coherent technologies into practical radio-over-fiber (RoF) links. In this paper, we propose a coherent-detected RoF link with low-complexity optical configuration and a light digital signal processing (DSP) unit. Based on a dual-parallel Mach–Zehnder modulator (DP-MZM) at the transmitter, the amplitude and phase of a light wave are modulated to achieve spectral-efficient use of the optical source. After transmission over a single optical fiber, the modulated signals are coherent-detected at the receiver by being interfered with an independent local oscillator laser and then are demodulated digitally. Compared with other two-optical-source coherent-detected RoF links, our proposed scheme can accomplish dual-channel RF signal transmission albeit with halved optical configuration. Additionally, the DSP algorithm to get rid of the laser phase fluctuation between two lasers is simple and efficient. An experimental demonstration is done. Two 16-QAM microwave vector signals at the same symbol rate of 50 Msymbol/s are transmitted over 25-km single mode fiber (SMF) and the error vector magnitudes (EVMs) of 3.15% and 3.53% are obtained respectively. To explore its potential, two 64-QAM microwave vector signals are transported with similar conditions and the measured EVMs are 4.05% and 5.88%, respectively.

M-QAM transmission over hybrid microwave photonic links at the K-band.

Two experimental configurations of a hybrid K-band (25 GHz) microwave photonic link (MPL) are investigated for seamless broadband wireless access networks. Experimental configurations consist of optical fiber, free-space optics (FSO) and radio frequency (RF) wireless channels. We analyze in detail the effects of channel impairments, namely fiber chromatic dispersion, atmospheric turbulence and multipath-induced fading on the transmission performance. In the first configuration, transmission of the 64-quadrature amplitude modulation (QAM) signal with 5, 20 and 50 MHz bandwidths over 5 km standard single-mode fiber (SSMF), 2 m turbulent FSO and 3 m RF wireless channels is investigated. We show that, for QAM with a high bandwidth, the link performance is being affected more by atmospheric turbulence. In the second configuration, the 20 MHz 4/16/64-QAM signals over a 50 km SSMF and 40 m FSO/RF wireless links are successfully transmitted with the measured error vector magnitude (EVM) values of 12, 9 and 7.9%, respectively. It is shown that, for all transmitted microwave vector signals, the bit error rate is lower than the hard-decision forward-error-correction limit of 3.8×10-3. Moreover, an extended FSO link span of 500 m for 25 GHz hybrid MPL with 16-QAM at 10 Gb/s under the weak and strong turbulence regimes is evaluated via simulation analysis to mimic a practical outdoor system.

A Novel Spectral-Efficient Coherent Radio-Over-Fiber Link With Linear Digital-Phase Demodulation

In this paper, a novel spectral-efficient coherent radio-over-fiber (RoF) link with linear digital phase demodulation is proposed and experimentally demonstrated. At the transmitter, to make an efficient use of the optical power and spectra, an intensity-modulated optical signal serving as the optical reference signal and a phase-modulated optical signal are polarization-multiplexed on a single optical carrier. At the receiver, the two optical signals are coherently detected with an optical local oscillator (OLO) and demodulated free of laser phase fluctuation through digital signal processing. Owing to simple and linear digital phase demodulation, an RF input signal is linearly demodulated from the optical phase without approximations and preconditions, which preserves the linearity of the phase-modulated RoF optical link from the transmitter end to the receiver end. The proposed scheme is experimentally verified by 25-km single-mode-fiber (SMF) transmission of two 16-QAM microwave vector signals at 2 GHz and 2.4 GHz, both with a symbol rate of 50 Msymbol/s. The transmission performance in term of error vector magnitude (EVM) is evaluated. Additionally, 25-km SMF transmission of the phase-modulated input signal with a spurious-free dynamic range (SFDR) of 112.8 dB·Hz2/3 is obtained.

Carrier-frequency-doubled photonic microwave vector signal generation based on PDM-MZM

A simple method based on an integrated polarization-division multiplexing Mach–Zehnder modulator (PDM-MZM) to produce a carrier-frequency-doubled vector signal is presented and experimentally demonstrated. In this structure, the upper sub-MZM is driven by I/Q signals and biased at the quadrature point to modulate a baseband vector signal on the optical carrier. The lower sub-MZM is driven by a LO signal and is biased at the maximum transmission point (MATP) to generate the optical carrier and second-order sidebands. By filtering the negative second-order sideband via an optical bandpass filter (OBPF) and suppressing the optical carrier via polarization controller (PC), a carrier-frequency-doubled vector signal is generated at the photodetector. Due to the adoption of LO frequency-doubled technique, the system’s requirement of a high-frequency LO is relaxed. In a proof-of-concept experiment, 300 MSym/s QPSK signal with a carrier frequency of 16 GHz is obtained by 8 GHz LO signal. Besides, 25-km fiber transmission of the generated QPSK signal is demonstrated. When the optical power obtained after fiber transmission is higher than -4 dBm, the BER after demodulation is less than 8×10−10. Moreover, generation of vector signal with other formats is verified.

A High Spectral Efficiency Microwave Photonic Link With Optimization of Chromatic-Dispersion-Induced Power Fading

A novel microwave photonic link to simultaneously realize high spectral efficiency and optimization of chromatic-dispersion-induced power fading based on dual-drive polarization division multiplexing Mach–Zehnder modulator (PDM-MZM) is proposed and demonstrated. Two microwave vector signals are IQ modulated on an optical carrier in one polarization state via the upper sub-MZM (IQ modulator) of the PDM-MZM. The optical carrier with orthogonal polarization state is unmodulated and transmitted together with the modulated optical signal. At the receiver, a polarization controller and polarizer are cascaded to change the phase of the optical carrier. By properly changing the phase of the optical carrier, the two microwave vector signals are respectively recovered and the chromatic-dispersion-induced power fading is optimized simultaneously. Since two microwave vector signals are transmitted over a single optical carrier, the spectral efficiency of the link is doubled. A proof-of-concept experiment is performed. The transmission of a 50-Msym/s 16-QAM signal and a 100-Msym/s QPSK signal both at 6 GHz over a 25-km SMF is realized, and its transmission performance in terms of error vector magnitude (EVM) and bit error rate (BER) is evaluated. The optimization of the chromatic-dispersion-induced power fading is also verified by the experiment.

A Spectral-Efficient Self-Homodyne-Detected Microwave Photonic Link With an Extended Fiber-Reach

In this letter, a simple and spectral-efficient self-homodyne-detected microwave photonic link is proposed and experimentally demonstrated. At the transmitter, a phase-modulated optical signal and an intensity-modulated optical local oscillator signal are polarization-multiplexed and delivered in a single fiber over long distance. At the receiver, the two signals are coherently detected by a self-homodyne interferometer and then demodulated through simple digital signal processing. The proposed scheme is experimentally validated by 25-km fiber transmission of the 50-Msymbol/s 16-QAM microwave vector signals at 2 and 2.5 GHz. The transmission performance compared with the conventional homodyne detection scheme in terms of error vector magnitude and bit error rate is evaluated. The proposed scheme can be a potential solution to wireless access networks for both high spectral efficiency and low-cost implementation.

Data Rate Quadrupled Coherent Microwave Photonic Link

A data rate quadrupled microwave photonic link (MPL) based on coherent detection and digital signal processing (DSP) is proposed and experimentally demonstrated. The data rate is quadrupled by employing both intensity and phase modulation and polarization multiplexing to transmit four microwave vector signals with an identical microwave center frequency over a single optical carrier. The recovery of the four microwave signals is implemented at a coherent receiver followed by DSP with a new algorithm, to separate the four microwave vector signals and to eliminate the phase noise from the transmitter and local oscillator laser sources and to cancel the unstable frequency offset between the two laser sources. Error-free transmission of four 2.5-Gpbs 16-QAM microwave vector signals with a total data rate of 10 Gbps if forward error correction (FEC) is employed. The total net data rate is 9.225 Gbps if the zero padding and the FEC (overhead 6.7%) are considered. The use of the proposed MPL for the implementation of $4\times 4$ MIMO is also discussed.

Wavelength Reuse in an RoF Link Based on CS-DSB, Coherent Detection and DSP

Wavelength reuse in a bidirectional radio over fiber link based on carrier-suppressed double-sideband (CS-DSB) modulation, coherent detection, and digital signal processing (DSP) is proposed and experimentally demonstrated. At the center office, an optical single-sideband with carrier (OSSB+C) signal is generated and distributed to the base station. The wavelength reuse at the base station is implemented by directly modulating the downstream OSSB+C signal with an upstream signal based on CS-DSB modulation. The modulated upstream signal is sent to the center office where a coherent receiver and a DSP unit are used to recover the upstream signal by eliminating the crosstalk from the downstream signal, and at the same time, to cancel the phase noise from the laser source. An experiment is performed. The transmission of a 16-QAM downstream microwave vector signal and a 16-QAM upstream microwave vector signal over a 17-km single-mode fiber (SMF) is experimentally demonstrated. The power penalty caused by the wavelength reuse is less than 0.8 dB.

Two Microwave Vector Signal Transmission on a Single Optical Carrier Based on PM-IM Conversion Using an On-chip Optical Hilbert Transformer

A new microwave photonic link to simultaneously transmit and detect two microwave vector signals modulated on a single optical carrier based on orthogonal modulation and optical Hilbert transformation is proposed and demonstrated, for the first time to the best of our knowledge. In the proposed system, two microwave vector signals are phase and intensity modulated on a single optical carrier at a dual-parallel Mach–Zehnder modulator and transmitted over a single-mode fiber (SMF). At the receiver, the optical signal is applied directly to a photodetector (PD) to recover the microwave vector signal that is intensity modulated and the second microwave vector signal is recovered by using an on-chip optical Hilbert transformer (OHT) to perform phase-modulation to intensity-modulation conversion followed by photodetection at a second PD. An experiment is performed. The transmission of a 16QAM and a QPSK signal with a total data rate of 2.4 Gb/s over a 10-km SMF is implemented. The performance of the transmission in terms of error vector magnitude, bit error rate, and energy efficiency is evaluated.

A High Spectral Efficiency Coherent RoF System Based on OSSB Modulation With Low-Cost Free-Running Laser Sources for UDWDM-PONs

A coherent radio over fiber (RoF) system based on optical single-sideband modulation with no optical carrier (OSSB) for ultradense wavelength division multiplexing passive optical networks (UDWDM-PONs) is proposed and experimentally demonstrated. For one channel of the UDWDM-PONs, at the transmitter, the OSSB RoF signal is generated using a dual parallel Mach–Zehnder modulator (DP-MZM) with a real-valued precoded microwave vector signal which is applied to the DP-MZM via the two electrodes with one having a 90° phase shift. Then, the OSSB RoF signal is sent to a coherent receiver over a single-mode fiber (SMF). The coherent receiver with a free-running laser source as a local oscillator (LO) is used to perform the coherent detection. To recover the standard microwave vector signals from the real-valued precoded microwave vector signal that is embedded in a strong phase noise introduced by both the transmitter laser source and the LO laser source, a digital signal processing algorithm is developed to perform phase noise cancellation. An experiment is performed. The transmission of a 1.25-Gbps quadrature phase shift keying, a 1.875-Gbps eight-phase shift keying (8-PSK), and a 2.5-Gbps 16 quadrature amplitude modulation (16-QAM) microwave vector signals with a channel spacing as narrow as 3 GHz over a 25-km SMF is experimentally demonstrated. For the transmission of the 2.5-Gbps 16-QAM microwave vector signal, the received optical sensitivity at forward error correction level over a 25-km SMF link is −24.8 dBm, while for the 1.875-Gbps 8-PSK microwave vector signal, it can reach −29.8 dBm which leaves an enough margin to accommodate the splitting losses in the optical distribution networks.

Wavelength Reuse in a Symmetrical Radio Over WDM-PON Based on Polarization Multiplexing and Coherent Detection

A symmetrical radio over a colorless wavelength-division multiplexing passive optical network with wavelength reuse based on polarization multiplexing and coherent detection incorporating digital phase noise cancellation is proposed and experimentally demonstrated. For the downlink, two optical signals are intensity modulated by two different 16-QAM downstream microwave vector signals that are polarization multiplexed at the optical line terminal (OLT). Then, the two orthogonally polarized optical signals are transmitted to the optical network unit (ONU). At the ONU, one of the two orthogonally polarized optical signals is selected to be reused for the uplink transmission. Then, the reused optical signal is split into two channels by a 3-dB coupler. One portion of the reused optical signal is phase modulated by a 16-QAM upstream microwave vector signal, while the other portion is intensity modulated by another 16-QAM upstream microwave vector signal. Again, the two portions of the optical signals are polarization multiplexed and transmitted back to the OLT. At the OLT, a coherent receiver with a digital signal processing (DSP) unit is used to detect the upstream optical signals. Two DSP algorithms are developed to recover the two 16-QAM upstream microwave vector signals. The transmission of two 2.5-Gb/s 16-QAM downstream microwave vector signals and two 2.5-Gb/s 16-QAM upstream microwave vector signals over a 10.5-km single-mode fiber is experimentally demonstrated. The error vector magnitudes for both the down and upstream transmissions are measured to be 7.3% and 8.65%, which are good enough to achieve error-free transmission with forward error correction.