Average Error Vector Magnitude Research Articles

Quantum-Dash Semiconductor Optical Amplifier for Millimeter-Wave over Fibre Wireless Fronthaul Systems

This paper demonstrates a five-layer InAs/InP quantum-dash semiconductor optical amplifier (QDash-SOA), which will be integrated into microwave-photonic on-chip devices for millimeter-wave (mmWave) over fibre wireless networking systems. A thorough investigation of the QDash-SOA is conducted regarding its communication performance at different temperatures, bias currents, and input powers. The investigation shows a fibre-to-fibre (FtF) small-signal gain of 18.79 dB and a noise figure of 6.3 dB. In a common application with a 300 mA bias current and 25 °C temperature, the peak FtF gain is located at 1507.8 nm, which is 17.68 dB, with 3 dB gain bandwidth of 56.6 nm. Furthermore, the QDash-SOA is verified in a mmWave radio-over-fibre link with QAM (32 Gb/s 64-QAM 4-GBaud) and OFDM (250 MHz 64-QAM) signals. The average error vector magnitude of the QAM and OFDM signals after a 2 m wireless link could be as low as 8.29% and 6.78%, respectively. These findings highlight the QDash-SOA’s potential as a key amplifying component in future integrated microwave-photonic on-chip devices.

A Wideband Energy-Efficient Multi-Mode CMOS Digital Transmitter

This article presents a wideband, energy-efficient digital transmitter (DTX) suitable for multi-mode/multi-band wireless communication applications. It features various operation modes comprising Cartesian (Modes-1/-2) and multi-phase (Modes-3/-4) configurations utilizing LO clocks with different duty cycle in the interleaving/non-interleaving configurations. The multi-phase operation compromises polar and Cartesian features by mapping the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I/Q$ </tex-math></inline-formula> signals into two non-orthogonal basis vectors with a 45° relative phase difference and a 3-bit phase selector scheme. The different operation modes are extensively analyzed and compared. Fabricated in a 40-nm CMOS process with an off-chip matching network, the proposed DTX occupies a core area of 0.72 mm2 and delivers 23.18-dBm RF peak power at 2.1 GHz from a 0.95-V supply voltage with drain/system efficiencies of 66.26%/52.59%, respectively. Utilizing a simple memory-less digital pre-distortion (DPD) for a 160-MHz four-channel 64-quadrature amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM) signal, the DTX delivers an average <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P _{\text {Out}}$ </tex-math></inline-formula> of 13.5/11.4/7.7/9.4 dBm, achieving an adjacent channel (power) ratio (ACL(P)R) of better than −42/−40/−40/−38 dBc and an average error vector magnitude (EVM) of −36/−34/−34/−32 dB, operating in Modes-1/-2/-3/-4, respectively. While transmitting a 200-MHz single-channel 256 (1024)-QAM OFDM signal at 2.4 GHz in Modes-1/-4, the average delivered output power is 14.11/9.29 (12.23/7.32) dBm with average drain and system efficiencies of 33.17%/26.3% (23.82%/22.83%) and 24.81%/22.85% (19.34%/18.81%), while the ACLR and EVM are better than −42/−41 (−43/−43) dBc and −34.6/−33.1 (−33.5/−33.9) dB, respectively.

A Millimeter-Wave CMOS Series-Doherty Power Amplifier With Post-Silicon Inter-Stage Passive Validation

This article presents a wideband series-Doherty power amplifier (SDPA) for millimeter-wave (mm-wave) fifth-generation (5G) applications. It features a compact two-step impedance inverting-based series-Doherty power combiner that provides broadband close-to-perfect power back-off (PBO) efficiency enhancement. The amplitude-to-amplitude (AM–AM)/amplitude-to-phase (AM–PM) performance of the load-modulated Doherty power amplifier for broadband operation is analyzed. We also devise a post-silicon inter-stage passive validation (PSIV) approach to evaluate the mm-wave chip prototype utilizing the embedded voltage root mean square detectors. The proposed SDPA is realized in a 40-nm bulk CMOS, and it delivers 20.4 dBm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P_{\text {SAT}}$ </tex-math></inline-formula> with 39.1%/34% PAE at 0-/6-dB PBO. Over a 23.5–30 GHz band, its PAE is >24% at 6-dB PBO. At 27 GHz, applying a “2 GHz 16-quadratic-amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM)” signal, the proposed SDPA generates 10.2 dBm average power with 18.9% average PAE. The average error vector magnitude is better than −24.5 dB without digital predistortion for a “400-MHz 64-QAM OFDM” signal while generating an average output power of 8.8 dBm with 15% PAE. The AM–AM/AM–PM of the realized SDPA is investigated by employing a “50-MHz 64-QAM OFDM” signal, validating our analysis and showing that the linearity limitation of DPAs is systematic and predictable. Utilizing the proposed PSIV approach, the frequency response of the input/inter-stage passive circuits is measured, indicating an excellent agreement with 3-D electromagnetic (EM) simulation results.

A Millimeter-Wave Mutual-Coupling-Resilient Double-Quadrature Transmitter for 5G Applications

This article presents a wideband energy-efficient transmitter (TX) for 5G mm-wave phased-array systems. It features an advanced double-quadrature direct upconverter (DQ-DUC) to improve its in-band linearity and spectral purity. The proposed TX architecture incorporates an efficiency-enhanced balanced power amplifier (EEBPA) that mitigates VSWR fluctuations in phased-array systems while enhancing efficiency at power back-off (PBO). The EEBPA comprises two identical series-Doherty power amplifiers (PAs) combined through a quadrature hybrid coupler forming a balanced PA. The proposed DQ-DUC consists of a pair of I/Q modulators and the proposed EEBPA’s quadrature combiner to further suppress the I/Q image. To verify the proposed techniques, a 40-nm CMOS prototype is implemented. It delivers 20 dBm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P~_{\text {1 dB}}$ </tex-math></inline-formula> with 40%/31% drain efficiency at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P~_{\text {1 dB}}$ </tex-math></inline-formula> /6-dB PBO. The measured TX output reflection coefficient is better than −18 dB over a 22.5–30-GHz band. Its intrinsic LO feedthrough and image-rejection ratio for a 100-MHz tone spacing over a 24–30-GHz band are better than −45 dBc/50 dB, respectively, without calibration. The average error vector magnitude (EVM) is better than −27.1 dB without digital pre-distortion for an eight-carrier “100-MHz 64-QAM OFDM” signal with an 800-MHz aggregated bandwidth while generating an average output power of 8.4 dBm with 10.8% drain efficiency. Its maximum forward-power/EVM deviations are better than 0.3/1.65 dB, respectively, for a “100-MHz 64-QAM” signal under a voltage standing wave ratio of 3.

Design of Minimum Nonlinear Distortion Reconfigurable Antennas for Next-Generation Communication Systems.

Nonlinear effects in the radio front-end can degrade communication quality and system performance. In this paper we present a new design technique for reconfigurable antennas that minimizes the nonlinear distortion and maximizes power efficiency through the minimization of the coupling between the internal switching ports and the external feeding ports. As a nonlinear design and validation instance, we present the nonlinear characterization up to 50 GHz of a PIN diode commonly used as a switch for reconfigurable devices in the microwave band. Nonlinear models are extracted through X-parameter measurements supported by accurate calibration and de-embedding procedures. Nonlinear switch models are validated by S-parameter measurements in the low power signal regime and by harmonic measurements in the large-signal regime and are further used to predict the measured nonlinearities of a reconfigurable antenna. These models have the desired particularity of being integrated straightforwardly in the internal multi-port method formulation, which is used and extended to account for the power induced on the switching elements. A new figure of merit for the design of reconfigurable antennas is introduced—the power margin, that is, the power difference between the fed port and the switching elements, which combined with the nonlinear load models directly translates into nonlinearities and power-efficiency-related metrics. Therefore, beyond traditional antenna aspects such as port match, gain, and beam orientation, switch power criteria are included in the design methodology. Guidelines for the design of reconfigurable antennas and parasitic layers of minimum nonlinearity are provided as well as the inherent trade-offs. A particular antenna design suitable for 5G communications in the 3.5 GHz band is presented according to these guidelines, in which the specific switching states for a set of target performance metrics are obtained via a balancing of the available figures of merit with multi-objective separation criteria, which enables good control of the various design trade-offs. Average Error Vector Magnitude (EVM) and power efficiency improvement of 12 and 6 dB, respectively, are obtained with the application of this design approach. In summary, this paper introduces a new framework for the nonlinear modeling and design of reconfigurable antennas and provides a set of general-purpose tools applicable in cases beyond those used as examples and validation in this work. Additionally, the use of these models and guidelines is presented, demonstrating one of the most appealing advantages of the reconfigurable parasitic layer approach, their low nonlinearity.

Channel Modeling and Over-the-Air Signal Quality at 3.5 GHz for 5G New Radio

With the next generation of mobile communication being trialled across the world, 5G New Radio (NR) promises to provide a flexible radio interface that fits a diverse range of use cases. As trials and pilots progress, propagation studies are required that evaluate electromagnetic (EM) propagation effects for 5G NR signals. In this article, 5G NR signals are used to evaluate the over-the-air (OTA) error vector magnitude (EVM) within a line-of-sight (LoS) rural/urban environment. For the same receiver locations, time dispersion and propagation loss were measured via the root mean square (RMS) delay spread and path loss. The transmitter−receiver distance investigated ranged from $\mathrm {60~m}$ to $\mathrm {450~m}$ . From the measurement campaign, the path loss exponent (PLE) using a directional antenna at the receiver was 1.98, and 1.82 with an omnidirectional antenna. The root mean square error (RMSE) of the floating intercept/close-in (FI/CI) models for the path loss was 4.96/4.74 and 3.84/3.23 for directional and omnidirectional receivers. The delay spread using a cross polar configuration showed significant increase and a dependency of the delay spread on the path loss was observed. For the measurement route, the mean delay spread was $\mathrm {24~ns}$ and $\mathrm {35~ns}$ for directional and omnidirectional measurements. With regards to the EVM, 16 and 64-QAM transmissions were robust along the entire route for directional and omnidirectional antennas, whereas 256-QAM worked in locations where there was minimal obstruction to the propagation path. The average EVM (%) for 16, 64 and 256-QAM measured along the route was 4.5/5.3/3.9 and 3.4/4.3/3.6 for omnidirectional and directional antennas. The OTA results also show that using a directional antenna (as the receiver) significantly improves the obtainable signal quality of 5G NR signals and also reduces outages for higher modulation schemes. With the measurement results presented, system designers can design efficient receivers, estimate coverage and adequately provision services using 5G NR on the sub-6 GHz frequency band.

An investigation of 16-QAM signal transmission over turbulent RoFSO link modeled by gamma–gamma distribution

Abstract This paper outlines the performance of a 10 Gbit/s rectangular 16-quadrature amplitude modulation–based radio over free space optical communication system. Here, 60 GHz radio frequency–modulated signal is propagated through a 1550-nm free space optical link. The gamma–gamma distribution is used for the channel modeling of weak to strong atmospheric turbulence. The reported constellation plots and eye patterns are attributed to impairment factors in adverse conditions of atmosphere. The evaluation is carried out that the variation in average error vector magnitude in the range of 1.45–1.63% and equivalent symbol error rate of 0.019–0.023 are obtained for a clear atmosphere compared to the turbulent link of 0.2–1 km, respectively.

Experimental Demonstration of an SFO-Robustness Scheme With Fast OFDM for IMDD Passive Optical Network Systems

A sampling frequency offset (SFO)-robustness scheme with zero padding (ZP)-fast orthogonal frequency-division multiplexing (FOFDM) is proposed and experimentally demonstrated in intensity-modulated and direct-detection (IMDD) passive optical network (PON) systems. It can be used to mitigate the impact of SFO-induced inter-carrier interference (ICI). The performance comparison between the proposed FOFDM scheme and conventional OFDM scheme in the presence of SFO is presented. Experimental results indicate that, as SFO-induced subcarrier phase offset (SPO) is compensated within the estimated error of ±0.1 ppm, at the same spectral efficiency (SE) and bandwidth granularity, the error vector magnitude (EVM) of ZP-FOFDM is superior to the OFDM at different received optical power (ROP). In addition, when the ROP is −1 dBm, the average error vector magnitude (EVM) of all the data subcarriers for ZP-FOFDM is 3.3-dB superior to that of OFDM. It means that the ZP-FOFDM scheme can effectively improve the SFO-robustness compared with OFDM.

Performance evaluation and implementation complexity analysis framework for ZF based linear massive MIMO detection

This paper discusses a framework for algorithm-architecture synergy for (1) performance evaluation and (2) FPGA implementation complexity analysis of linear massive MIMO detection techniques. Three low complexity implementation techniques of the zero-forcing (ZF) based linear detection are evaluated, namely, Neumann series expansion (NSE), Gauss–Seidel (GS) and a proposed recursive Gram matrix inversion update (RGMIU) techniques. The performance analysis framework is based on software-defined radio platform. By extrapolating the real data measured average error vector magnitude versus a number of served single-antenna user terminals, GS and RGMIU are showing no performance degradation with respect to ZF with direct matrix inversion. It is shown that under high load regime NSE and GS require more processing iterations at the expense of increased processing latency. We, therefore, consider a unified approach for field-programmable gate array based implementation complexity analysis and discuss the required baseband processing resources for real-time transmission. Due to the wide differences of NSE, GS and RGMIU in terms of performance, processing complexity and latency, practical deployment and real-time implementation insights are derived.

Distributed Antenna System Using Sigma-Delta Intermediate-Frequency-Over-Fiber for Frequency Bands Above 24 GHz

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.

Distributed Multi-User MIMO Transmission Using Real-Time Sigma-Delta-Over-Fiber for Next Generation Fronthaul Interface

To achieve the massive device connectivity and high data rate demanded by 5G, wireless transmission with wider signal bandwidths and higher-order multiple-input multiple-output (MIMO) is inevitable. This work demonstrates a possible function split option for the next generation fronthaul interface (NGFI). The proof-of-concept downlink architecture consists of real-time sigma-delta modulated signal over fiber (SDoF) links in combination with distributed multi-user (MU) MIMO transmission. The setup is fully implemented using off-the-shelf and in-house developed components. A single SDoF link achieves an error vector magnitude (EVM) of 3.14% for a 163.84 MHz-bandwidth 256-QAM OFDM signal (958.64 Mbps) with a carrier frequency around 3.5 GHz transmitted over 100 m OM4 multi-mode fiber at 850 nm using a commercial QSFP module. The centralized architecture of the proposed setup introduces no frequency asynchronism among remote radio units. For most cases, the 2 × 2 MU-MIMO transmission has little performance degradation compared to SISO, 0.8 dB EVM degradation for 40.96 MHz-bandwidth signals and 1.4 dB for 163.84 MHz-bandwidth on average, implying that the wireless spectral efficiency almost doubles by exploiting spatial multiplexing. A 1.4 Gbps data rate (720 Mbps per user, 163.84 MHz-bandwidth, 64-QAM) is reached with an average EVM of 6.66%. The performance shows that this approach is feasible for the high-capacity hot-spot scenario.

A Compact Ka/Q Dual-Band GaAs MMIC Doherty Power Amplifier With Simplified Offset Lines for 5G Applications

In this paper, a Ka/Q dual-band Doherty power amplifier (DPA) with simplified offset lines is implemented in a 0.1-μm gallium arsenide (GaAs) process. It is found that the dual-band transmission lines (TLs) employed in the sub-6-GHz dual-band DPA are not suitable to be used as the offset lines in millimeter-wave (mm-wave) dual-band DPAs due to their large sizes and insert losses. An in-depth analysis reveals that the phase requirement of the offset lines can be relaxed, and the DPA exhibits a reasonable performance in a certain phase-shift range. A novel design method is proposed to realize offset lines using simple TLs, which can satisfy the phase-shift ranges in dual bands by choosing a proper electrical length. To enhance the gain of the DPA, a reversed uneven power splitter is adopted to deliver more power to the main power amplifier (PA). The fabricated DPA achieves an output power of 25.4/25.2 dBm, a peak power-added efficiency (PAE) of 33%/25%, and a 6-dB back-off PAE of 22%/17% at 29/46 GHz, respectively, with a compact size of 2.2 1.4 mm2. Applying a 20-MHZ 64-quadraticamplitude modulation (QAM) signal with a 7.7-dB peak-toaverage power ratio (PAPR), the measured average output power, PAE, and error vector magnitude (EVM) at 29/46 GHz are 18.4/19 dBm, 17%/15%, and 1.5%/1.1% after linearization, respectively. To the best of our knowledge, the proposed DPA is the first demonstration of mm-wave dual-band DPAs that do not require any additional switching or reconfiguration.

Performance Evaluation of OFDM Data Transmission Using a 2D Beamsteering Transmitarray

This paper presents the analysis and characterisation of modulated data transmission, over-the-air, using an electronically Reconfigurable Transmitarray (eRT) composed of Frequency Selective Surfaces (FSS). The eRT, which has already been presented by the authors in previous work, is employed as a transmitter antenna on astate-of-the-art Orthogonal Frequency-Division Multiplexing (OFDM)-based Software Defined Radio (SDR), to enable electronically controlled two-dimensional beamsteering. A comparative study is presented by transmitting OFDM with and without the eRT attached at the transmitter’s horn antenna, in order to evaluate the impact that such man-made manufactured structures may have on real data transmission. The study is conducted at 5.2 GHz, when both transmitter and receiver antennas are aligned to each other but also when they are physically misaligned and the eRT is compensating for such angular mismatch. Error Vector Magnitude (EVM) and Bit-Error-Rate (BER) are used as figures of merit to evaluate the system’s performance. Results experimentally obtained inside an anechoic chamber show an enhancement on the system’s performance of 20 dB of gain on the average EVM, confirming the electromagnetic properties of the eRT.

Digital Code-Division Multiplexing Channel Aggregation for Mobile Fronthaul Architecture With Low Complexity

A low-complexity mobile fronthaul architecture via digital code-division multiplexing (CDM) is proposed to enable channel aggregation of 4G-LTE signals. In comparison with traditional frequency division multiplexing based aggregation scheme, the fast Fourier transformation/inverse fast Fourier transformation operations are replaced by simple sign selection and addition, leading to the significant reduction of computational complexity. Moreover, synchronous transmission of both the I/Q waveforms of wireless signals and the control words (CWs) used for the purpose of control and management using the CDM approach is also presented to be compliant with the common public radio interface (CPRI). In a proof-of-concept experiment, we demonstrate the transmission of 48 × 20 MHz LTE signals with CPRI equivalent data rate of 59 Gb/s, achieving an average error vector magnitude (EVM) of ∼3.6% and ∼4.3% after 5 and 20 km transmission over standard single-mode fiber (SSMF), respectively. Furthermore, we successfully demonstrate the transmission of 32 × 20 MHz LTE signals together with CPRI-compliant CWs, corresponding to CPRI-equivalent data rate of 39.32 Gb/s, only using single optical wavelength channel with analog bandwidth of ∼1.96 GHz. After transmission over 5 km SSMF, CWs can be error-free recovered while the LTE signals are recovered with an EVM of ∼3.6%.

Experimental Demonstration of Amplified Feedback DFB Laser With Modulation Bandwidth Enhancement Based on the Reconstruction Equivalent Chirp Technique

A novel amplified feedback DFB laser with extended modulation bandwidth is experimentally demonstrated in this paper. All the three sections of laser cavity, including DFB section, phase section, and amplified feedback section, have the same active layer, which can avoid the butt-joint regrowth process. The gratings in both DFB section and amplified feedback section are fabricated by the reconstruction equivalent chirp technique, which can significantly decrease the difficulties in realizing precise grating structure. An enhanced −3 dB bandwidth of 19 GHz is achieved by means of a normal low-frequency wafer. Modulation linearity and relative intensity noise characteristics are experimentally investigated. 20 MSymbol/s 32-QAM signal with 19 GHz carrier is transmitted via 25-km radio-over-fiber link using the as-fabricated directly modulated DFB laser, and the average error vector magnitude of the whole link is 3.91%.

Modulation properties enhancement in a monolithic integrated two-section DFB laser utilizing side-mode injection locking method.

A monolithic optical injection-locked (MOIL) DFB laser with large stable injection locking range is experimentally demonstrated using the side-mode injection locking technique. The low-frequency roll-off in the MOIL DFB laser is suppressed significantly. The relaxation oscillation frequency is measured to be 26.84 GHz and the intrinsic 3-dB response bandwidth is more than 30 GHz, which is about 20 GHz higher than that of the free running DFB laser. The nonlinear distortions, including the 1-dB compression point, second harmonic distortion (2HD) and third-order intermodulation distortion (IMD3), are also suppressed significantly. A simple radio-over-fiber system transmitting 40 Msymbol/s 32-QAM signal with 6 GHz carrier is achieved using the MOIL DFB laser. After 50 km transmission, the average error vector magnitude (EVM) of the whole link is 2.94% in injection locked state, while the EVM in free running DFB laser is 5.25% as a comparison. To our knowledge, this is the first time that the MOIL DFB laser is realized utilizing the side-mode injection locking method.

Compact and portable 100-GHz microresonator-based optical frequency comb generation system for dense wavelength division multiplexing fiber optic communication

A compact and portable microresonator-based optical frequency comb generation system was developed to provide an array of multiwavelength laser sources for dense wavelength division multiplexing (DWDM) fiber optic communication. The reported comb generation system was capable of producing comb lines after 7 km of travel without environmental control, indicating the effectiveness of the packaging. The comb spacing is about 98.3 GHz (i.e., ∼0.79 nm around 1553 nm), thus the comb line frequencies coincide nicely with 100-GHz DWDM channel frequencies, assuming the suggested channel bandwidth is 0.3 nm. The quality of selected comb lines was evaluated individually based on error vector magnitude (EVM) measurements along with RF spectrum measurements. An average EVM value observed is as low as 3.8%, close to typical EVM values of less than 3% used for fiber optic communication.

ENHANCING POWER EFFICIENCY OF DOHERTY POWER AMPLIFIERS USING WINDOWING BASED CREST FACTOR REDUCTION TECHNIQUE

This paper investigates the performance of a Windowing Based Crest Factor Reduction (CFRWB) technique, to enhance the power efficiency of Radio Frequency (RF) power amplifiers. In particular, CFRWB is implemented on a Doherty Power Amplifier (DPA) in conjunction with Generalized Memory Polynomial (GMPDPD), and Volterra series based Digital Predistortion (VDPD) techniques. Key features like spectral regrowth, Peak to Average Power Ratio (PAPR) reduction, efficiency improvement and Error Vector Magnitude (EVM) have been used to measure the efficacy of the proposed method. Both simulation and experimental results show that the proposed combination of CFRWB technique with GMPDPD and VDPD is able to reduce the PAPR of the complex input signals by nearly 60%, with minimal degrading of the EVM and spectral regrowth. Moreover, such signal with reduced PAPR can be used to overdrive the DPA, allowing for a relevant average efficiency enhancement (i.e., up to 25%), while fulfilling the requirements of modern communication standards such as Wideband Code Division Multiple Access (WCDMA) and long-term evolution (LTE).

A Novel Two-Dimensional Crest Factor Reduction for Performance Improvement of RF Power Amplifiers

In this letter, a novel two-dimensional crest factor reduction (2D-CFR) technique is proposed. It works together with dual-band digital predistortion (DPD) to enhance the performance of concurrent dual-band or broadband power amplifiers (PAs). During peak detection process, this new method introduces the instantaneous difference between the two bands to determine the amount of reduction. Theoretical analysis is performed to compare the proposed approach and the conventional approaches. To evaluate the performance of the proposed method, a broadband PA is excited by a dual-band signal composed of a 5 MHz wideband code division multiple access (WCDMA) signal and a 5 MHz long-term evolution (LTE) signal with edge to edge separation of 85 MHz. The experimental results demonstrate that the average error vector magnitude (EVM) is reduced from 3.35% to 1.85% without sacrificing the performance of linearity and efficiency.

Study on Two-Section DFB Lasers and Laser Arrays Based on the Reconstruction Equivalent Chirp Technique and Their Application in Radio-Over-Fiber Systems

Two-section distributed feedback (TS-DFB) semiconductor lasers based on the reconstruction equivalent chirp technique is proposed. A four-channel TS-DFB laser array (TS-DFB-LA) is theoretically studied and experimentally demonstrated. Compared with the conventional one-section DFB lasers, the proposed TS-DFB lasers have much higher output efficiency and higher side-mode suppression ratio (SMSR) when both the facets of the laser cavity are coated with antireflection coatings. About 39% increase in output efficiency, more than 10 mA decrease in threshold current, and about 5 dB increase in SMSR have been achieved experimentally. Furthermore, experimental results show that the wavelength spacing accuracy of the four-channel TS-DFB-LA can be improved by tuning the injection current of one of the two sections. Using the two-section structure, the dynamic characteristics including small-signal frequency response, relative intensity noise, and spurious-free dynamic range are also improved significantly. A radio-over-fiber link utilizing the TS-DFB laser as directly modulated light source was experimentally achieved. About 50 MSymbol/s 64-QAM signal with 10 GHz carrier was transmitted from the TS-DFB laser. After 40 km transmission in single-mode fiber, the average error vector magnitude of the whole link is 2.97%.