98 research outputs found

    Fifth-order Polynomial Predistortion for Mach-Zehnder Modulator Linearization

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    Modern wireless applications require access to ultra-wide instantaneous RF bandwidths to provide frequency agility and multi-band RF processing. Wireless communications, radar and electronic warfare are examples of applications that will benefit from wideband multi-function transceivers. The role of the front-end filtering is critical in order for the multi-function transceiver to achieve adequate RF performance. Integrated electric filters are unable to achieve the required frequency selectivity and tuning range mainly due to low Q of on-chip inductors. This renders a complete integrated solution impractical. Normally, high frequency and high selectivity filters are achieved with off-chip bulky SAW filters. The limitation of electrical filters has motivated the employment of RF photonic receivers. The main issue with photonics is the cost but in recent times the emergence of silicon photonics has enabled the potential of RF photonics receivers to be implemented at a low cost. The use of photonics gives access to devices that can achieve really high Q and high integration at high frequencies. At the heart of the photonic receiver is the Mach-Zehnder modulator (MZM). It modulates the received signal from the antenna to the optical carrier. The major issue with the MZM is: it is non-linear and wideband. The MZM is placed before the photonic filter and right after the antennae so interferers received with the desired RF signal generate intermodulation products at the output of the MZM. The intermodulation products can be very close to the desired RF signal so they cannot be filtered out by the photonic filter and may corrupt the desired RF signal. To curtail the effects of the MZM non-linearity, linearization schemes are implemented to reduce the amplitude of the intermodulation products generated when the MZM receives interferers. This thesis work focuses on two main issues, Firstly, analysis of the intermodulation products generated by the MZM when a two tone RF signal is applied. Secondly, a literature review is done to examine the existing linearization schemes. Based on the predistortion linearization scheme, a new fifth-order predistortion is proposed. The proposed fifth-order predistortion is fabricated in GP 65nm TSMC CMOS process. The proposed fifth-order linearization achieves high IM3 suppression~ 20dB at high modulation index ~49.7% with 49.2mW of power consumed

    Analog Pre-distortion Circuit for Simultaneous Suppression of Third and Fifth Order Intermodulation Distortion in Broadband Radio-over-Fiber Systems

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    ABSTRACT Analog Pre-distortion Circuit for Simultaneous Suppression of Third and Fifth Order Intermodulation Distortion in Broadband Radio-over-Fiber Systems Shuvasish Saha Rapid advance in wireless technologies coupled with the exponential increase in use of high bandwidth devices and applications have made it necessary to develop high capacity wireless transmission networks. Design of robust and cost-effective wireless signal transmission systems has become of paramount importance to keep up with the breakneck pace of wireless access demand, especially keeping in mind the future of massive multiple-input and multiple-output (MIMO) wireless. Radio-over-Fiber (RoF) transmission systems are at the forefront of the research topics being investigated right now as a possible solution to keep up with this exploding demand for wireless network access. Based on optical subcarrier modulation, RoF transmission systems combine the use of both optical fiber and radio transmission. Optical fibers are low cost. They are lightweight and suffer from low loss. They provide extremely high capacity and immunity from electromagnetic interference. Radio transmission over fiber makes it easy to assemble Remote Radio Units (RRUs) at antenna towers. However, RoF is an analog optical transmission system and it is susceptible to non-linear distortions caused by all the inline functional optical and electrical components. In case of front-haul RoF transmission systems of wireless access networks, two key specific functions: RF power amplification and optical subcarrier modulation, are the main contributors to the production of non-linear distortions. Non-linear distortions consist of harmonic distortions (HDs) and intermodulation distortions (IMDs). It is essential to suppress these distortions because they can introduce crosstalk if they fall in the passband of RF signals. In this thesis, the use of analog pre-distortion circuit (PDC) is investigated as a linearization technique for the purpose of suppressing non-linear distortions, especially the third order intermodulation distortion (IMD3) and the fifth order intermodulation distortion (IMD5) simultaneously. An analog pre-distortion circuit (PDC) is designed and fabricated based on the transmission characteristics of a modulator integrated distributed feed-back (DFB) laser (EML). The PDC is low cost, compact and has broad operational bandwidth. It is designed to operate in the bandwidth up to 6 GHz, using two beam-lead silicone Schottky diodes as predistorter. The linearization by this PDC is verified in EML modulated RoF transmission system. First, by using the EML modulated RoF, Spurious Free Dynamic Range (SFDR) improvement of over 11 dB is achieved related to IMD3 and over 3 dB related to IMD5 for the entire bandwidth. Similarly, the SFDR improved by more than 6 dB related to IMD3 and by more than 4 dB related to IMD5 for another EAM modulated RoF throughout the entire bandwidth. When Wi-Fi signals at 2.4 GHz and 5 GHz were transmitted through the EML modulated RoF, error vector magnitude (EVM) was improved by 1.3 dB at 2.4 GHz and by 1.55 dB at 5 GHz for back-to-back (BTB) transmission. For 10 km single mode fiber (SMF) transmission, EVM improved by 1.07 dB at 2.4 GHz and by 1.03 dB at 5 GHz. For the EAM modulated RoF, EVM was improved by 2.76 dB at 2.4 GHz and by 1.45 dB at 5 GHz for BTB transmission. For 10 km SMF transmission, EVM was improved by 2.08 dB at 2.4 GHz and by 1.44 dB at 5 GHz. Moreover, Wi-Fi signals were generated at a RF carrier of 2-5 GHz and EVM improvements were recorded. For the EML modulated RoF, EVM improvement was more than 1.4 dB for BTB transmission and more than 1 dB for 10 km SMF transmission. For the EAM modulated RoF, EVM improvement was more than 1.4 dB for both BTB and 10 km SMF transmission

    Broadband Linearization Technologies for Broadband Radio-over-Fiber Transmission Systems

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    Wireless access networks consist of three sections, i.e., back-haul, front-haul, and wireless transmission, where the front-haul transmission systems are to distribute radio frequency (RF) signals to antenna towers. For current low-capacity wireless access, RF signals over coaxial cables, digital fiber-optic transmission, microwave point-to-point transmission, and narrowband radio-over-fiber (RoF) transmission have been used for the front-haul transmission systems. However, with the increase of demand of high capacity wireless access and also use of massive multiple-input and multiple-output (MIMO) antennas, low-cost, simple and broadband front-haul transmission systems are required in current 4G and in particular the future 5G wireless. RoF transmission system, which is based on optical subcarrier modulation, combines the advantages of both optical fiber and radio transmission, where the optical fiber has low loss, low cost, extremely high capacity, lightweight, and immunity to electromagnetic interference, and the radio transmission simplifies remote radio units (RRUs) at antenna towers. Furthermore, radio transmission based front-haul is transparent to RF signal frequency and wireless protocol, i.e., upgradable, in addition to simplified RRUs. Unfortunately, RoF is an analog optical transmission, and it is well known that any analog transmission is susceptible to nonlinear distortion. To be more specific, nonlinear distortion is the major limit for RoF transmission. In fact, all inline functional optical and electrical components used in RoF transmission systems may induce the nonlinear distortion. Specifically in RoF based front-haul systems, two key functions, i.e., RF power amplification and optical subcarrier modulation, are the main factors in introducing nonlinear distortions. The nonlinear distortions from RF power amplifiers (PAs) have been studied for decades. Therefore, the nonlinear distortions from the optical subcarrier modulation are the main concern in this thesis. The nonlinear distortions include harmonic distortions (HDs) and intermodulation distortions (IMDs). For narrow band RF signals, the HDs can be suppressed by RF filtering, but it may be impossible for the IMDs to be filtered out. For broadband RF signals, both HDs and IMDs could fall in the passband of RF signals and introduce crosstalk, and therefore both of them are required to be suppressed, i.e., linearization required. In the past decades, linearization for RF PAs has been investigated extensively, mainly focusing on signal processing based linearization, i.e., digital linearization. Unfortunately, the digital linearization is typically limited to the RF signals with up to 20 MHz bandwidth. Based on the current technologies of signal processing hardware, linearization for 1 GHz RF signals can be done, but the complexity and cost are beyond the practical applications. In order to explore broadband RoF transmission systems that support broadband front-haul, simple, low cost, and broadband linearization is pivotal. In this thesis, two linearization technologies for RoF transmission systems are investigated comprehensively, i.e., analog predistortion circuit (PDC) and dual wavelength optical linearization. Two novel PDCs are designed and investigated to suppress 3rd order IMD (IMD3) of RoF transmission systems. The PDCs have the advantages of broad bandwidth, compact size, and low cost. The first PDC is designed to have a bandwidth from 7 to 18 GHz, using two zero-bias Gallium Arsenide (GaAs) Schottky diodes as predistorter. The linearization using this PDC is verified in externally modulated RoF transmission systems. When a Mach-Zehnder modulator (MZM) is used for the optical subcarrier modulation, the input power at 1 dB compression point (P1dB) of the RoF transmission system is improved by 0.4 and up to 2.2 dB from 7 to 18 GHz. The spurious-free dynamic range (SFDR) is improved by more than ~10 dB from 7 to 14 GHz and ~6 dB from 15 to 18 GHz. When an electro-absorption modulator (EAM) is used, the input P1dB is improved by 0.8 and up to 3.8 dB from 8 to 17 GHz. The SFDR is improved by more than ~9 dB from 7 to 14 GHz and ~4 dB from 15 to 18 GHz. The second PDC is designed to have an ultra broad bandwidth from 10 MHz to 30 GHz, using a dual Schottky diode as the predistorter. The linearization using this PDC is investigated in both directly and externally modulated RoF transmission systems. The SFDR at 8 GHz is improved by 11.9 dB for a directly modulated RoF transmission. The SFDR is improved by more than 10 dB from 1 to 5 GHz and more than 5 dB from 1 to 30 GHz for an externally EAM modulated RoF transmission. Similarly, the SFDR is improved by more than 12 dB from 2 to 5 GHz and more than 5 dB from 2 to 30 GHz for an externally MZM modulated RoF transmission. When WiFi signals are transmitted over the externally modulated RoF systems for back-to-back (BTB) and 20 km single mode fiber (SMF), the error vector magnitudes (EVMs) are improved by 0.4 and up to 5.1 dB by using the PDC. Dual wavelength linearization (DWL) technique is investigated compressively to suppress 2nd and 3rd order nonlinearities of externally modulated RoF transmission systems simultaneously, including HDs and IMDs. The linearization is verified in both EAM and MZM modulated RoF transmission systems. Theoretical analysis is given for the first time to understand DWL technique. The experimental results agree with the theoretical analyses. In the externally EAM modulated RoF transmission systems, when the 2nd order nonlinearity is maximally suppressed, 11.5 and 1.8 dB improvements of the SFDRs with respect to HD2 and HD3 respectively are achieved by using DWL simultaneously. 8.5 and 1.3 dB improvements of the SFDRs with respect to IMD2 and IMD3 respectively are also achieved. Correspondingly, 3 and 4 dB improvements of the input and output P1dBs respectively are obtained. When the 3rd order nonlinearity is maximally suppressed, the SFDRs with respect to HD3 and IMD3 are improved by 8.1 and 20.4 dB, respectively, and corresponding 7.7 and 11.7 dB improvements of the input and output P1dBs respectively are achieved. Furthermore, IMD5 is also suppressed, and the SFDR5 with respect to IMD5 is improved by 7.1 dB. Moreover, the RoF transmission of WiFi signals at 2.4 and 5 GHz are also linearized by using DWL technique. 3.5 dB at 2.4 GHz and 2.8 dB at 5 GHz improvements of the EVMs are obtained. For an externally MZM modulated RoF transmission system, DWL is also investigated theoretically and experimentally. In the system, it is found that the SFDRs with respect to HD2 and HD3 are both improved at the same time when the even order nonlinearities are suppressed, in which the power of the RF signal and 3rd order nonlinearity is increased by the same level. Thus, the SFDR3 is still improved even the 3rd order nonlinearity is increased. Compared to using a single 1553 nm laser, the SFDRs with respect to HD2 and HD3 are improved by 38.4 and 12.1 dB

    Adaptive optical feedforward linearization of optical transceiver for radio over fiber communication link

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    With the tremendous growth in numbers of mobile data subscribers and explosive demand for mobile data, the current wireless access network need to be augmented in order to keep up with the data speed promised by the future generation mobile network standards. Radio over fiber technology (RoF) is a cost effective solution because of its ability to support numerous numbers of simple structured base stations by consolidating the signal processing functions at the central station. RoF systems are analog systems where noise figure and spurious free dynamic range (SFDR) are important parameters in an RoF link. The nonlinearity of a laser transmitter is a major limiting factor to the performance of an RoF link, as it generates spurious spectral components, leading to intermodulation distortions (IMD), which limit the achievable SFDR of the analog RF wave transmissions. The device nonlinearity can be mitigated through various linearization schemes. The feedforward linearization technique offers a number of advantages compared to other techniques, as it offers good suppression of distortion products over a large bandwidth and supports high operating frequencies. On the other hand, feedforward linearization is a relatively sensitive scheme, where its performance is highly influenced by changing operating conditions such as laser aging, temperature effect, and input signal variations. Therefore, for practical implementations the feedforward system has to be real-time adaptive. This thesis aims to develop an adaptive optical feedforward linearization system for radio over fiber links. Mathematical analyses and computer simulations are performed to determine the most efficient algorithm for the adaptive controller for laser transmitter feedforward linearization system. Experimental setup and practical measurement are performed for an adaptive feedforward linearized laser transmitter and its performance is optimized. The adaptive optical feedforward linearization system has been modeled and simulated in MATLAB Simulink. The performances of two adaptive algorithms, which are related to the gradient signal method, such as least mean square (LMS) and recursive least square (RLS) have been compared. The LMS algorithm has been selected because of its robustness and simplicity. Finally, the adaptive optical feedforward linearization system has been set up with digital signal processor (DSP) as the control device, and practical measurement has been performed. The system has achieved a suppression of 14 dB in the third order IMD products over a bandwidth of 30 MHz, in a two-tone measurement at 1.7 GHz

    Digital Predistortion for Broadband Radio-over-Fiber Transmission Systems

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    Abstract Digital Predistortion for Broadband Radio-over-Fiber Transmission Systems Zichen Xuan Concordia University 2015 With the increase of the demand of high capacity wireless access, design of cost effective broadband wireless signal distribution system is required, particularly for future massive multi-input and multi-output (MIMO) wireless. Recently, Radio-over-Fiber (RoF) transmission systems have been revisited for broadband wireless signal distribution between central processing unit (CPU) and remote radio unit (RRU) (i.e., antenna towers). RoF, which is based on optical subcarrier modulation and thus an analog transmission system, fully utilize the advantages of broadband and low-loss fiber transmission, and also radio signal transmission. Unfortunately, RoF transmission systems are very susceptible to nonlinear distortions, which can be generated by all inline functional components of the RoF systems. However, two typical functions, i.e., optical subcarrier modulation and RF power amplification, are the two key sources of the nonlinear distortions. Various linearization techniques have been investigated for power RF amplifiers. It has been found that digital predistortion (DPD) linearization is one of the best approaches for RF bandwidth of up to 20 MHz. In this thesis, DPD linearization is explored for broadband RoF transmission systems. Instead of DPD implemented in baseband previously, a DPD linearization technique implemented in RF domain is investigated and demonstrated experimentally for broadband RoF transmission systems. Memory polynomial (MP) model is used for theoretical modeling of nonlinear RoF transmission systems, in which both nonlinear distortion and memory effect can be included. In order to implement the predistorter of the DPD using the MP model, least square (LS) method is used to extract the coefficients of the predistorter. Using the obtained coefficients, the trained predistorter is implemented and then verified in two experiments of directly modulated RoF transmission systems. In the first experiment, the DPD is verified in WiFi over fiber transmission systems, and more than 8 dB and 5.6 dB improvements of error vector magnitude (EVM) are achieved in back to back (BTB) and after 10 km single mode fiber (SMF) transmission. In the second experiment, both WiFi and ultra wide band (UWB) wireless signals are transmitted in the RoF system, which occupies over 2.4 GHz transmission bandwidth. It is shown that the implemented DPD leads to EVM improvements of 4.5 dB (BTB) and 3.1 dB (10 km SMF) for the WiFi signal, and 4.6 dB (BTB) and 4 dB (10 km SMF) for the UWB signal

    Envelope-Assisted RF Digital Predistortion for Broadband Radio-over-Fiber Systems with RF Power Amplifiers

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    Wireless communications play an extremely important role in our daily life. The ubiquitous cellphones and smartphones can provide not only voice service but also data service when they are connected to cellular network. With the enormous amount of mobile applications available to smartphones, people can listen to music, watch movies, check emails, update and share information on social media anytime and anywhere on their smartphones. All of these activities are possible because of the fast development of the wireless communication technology. Nowadays many people are enjoying the entertainments brought by 4G technology. In the meantime, companies and engineers in telecommunication industry have already been developing the 5G technology. To support the high data transmission rate of 4G or 5G technology, a complex and robust infrastructure is essential. Radio-over-Fiber (RoF) technology is a good solution to build such an infrastructure. RoF transmission can provide extremely high capacity with very low attenuation. Besides, RoF transmission system is much cheaper compared to the traditional way of transmission such as microwave coaxial cable or digital optical fiber transmission. However, like other analog systems, RoF system suffers from signal distortion induced by nonlinearities. RF power amplifier is needed to transmit the signal from a base station to the handsets because the attenuation of RF signal in the air is very high. Although many new types of RF power amplifiers have been designed to improve the power efficiency and the linearity, nonlinearity is still a big problem. To reduce the distortion of an RoF system, as well as that induced by an RF power amplifier, a new digital predistortion (DPD) technique, envelope-assisted RF digital predistortion, is presented in this thesis. This DPD model is evolved from the memory polynomial, and composed of an RF memory polynomial and a baseband memory polynomial. Therefore, this model takes both the RF signal and the baseband envelope as inputs to realize the digital predistortion. The RF polynomial is aimed to suppress the in-band nonlinearity, out-of-band nonlinearity and short-term memory effect, while the baseband polynomial is aimed to weaken the in-band nonlinearity and long-term memory effect. The indirect learning architecture is adopted and the least squares method is used to calculate the coefficients of the model. The validity of the proposed DPD has been proved by simulation and experiment. In the simulation system, an RoF model is built which can induce both short-term and long-term memory effects, as well as the in-band and out-of-band nonlinearities. The optimal RF and baseband memory depths of the proposed DPD adopted in the simulation system have been found in terms of error vector magnitude (EVM) improvement and complexity. After the establishment of the memory depths, three simulation cases have been run to evaluate the performance of the proposed DPD. The first case is a normal two-band test, where two 20 MHz LTE signals are located at 800 MHz and 900 MHz. Simulation results show that EVM is improved by 14.6 dB, adjacent channel power ratio (ACPR) is suppressed by 13.5 dB, and third order intermodulation distortion (IMD3) is weakened by 19.5 dB. In the second case, two LTE signals are closely located at 800 MHz and 840 MHz. Simulation results show that EVM is improved by 16.3 dB, ACPR is suppressed by 15.5 dB, and IMD3 is weakened by 27.5 dB. In the third case, three LTE signals are located at 800 MHz, 850 MHz and 900 MHz. EVM is improved by 15.7 dB, ACPR is suppressed by 15.2 dB and IMD3 is weakened by 14.1 dB. The performance of the proposed DPD has also been compared to that of a baseband 2D DPD. Comparison results show that 2D DPD is only better at improving the EVM and ACPR in the first case, and the proposed DPD is superior to the 2D DPD on all the other aspects. Especially, the proposed DPD is not limited by the number of signal bands, while the 2D DPD is limited to two-band scenarios. Experimentation has been conducted to further prove the effectiveness of the proposed DPD in a real RoF system. A similar test flow is adopted as in the simulation work. Firstly the optimal RF and baseband memory depths have been found. Then three test cases have been done. In the first case, EVM is improved by 6.9 dB, ACPR is suppressed by 14.8 dB, and IMD3 is weakened by 12.2 dB. In the second case, EVM is improved by 4.8 dB, ACPR is suppressed by 8.6 dB, and IMD3 is weakened by 9.5 dB. In the third case, EVM is improved by 8.9 dB, ACPR is suppressed by 16 dB, and IMD3 is weakened by 13.6 dB. Comparison with the 2D DPD shows the same result as in the simulation that the 2D DPD is only better at improving the EVM and ACPR in the first case. Influence of input power on EVM improvement has been studied. Experiment results show that the EVM improvement can be optimized at specific input power level. Sampling bandwidth has been reduced by down-converting the RF signal and sampling it at an intermediate frequency. Therefore, the implementation cost of the proposed DPD could be greatly reduced. In the two-band test, experiment results show that the performance of the proposed DPD with down conversion is even better than that of without down conversion. Influence of sampling bandwidth on EVM improvement has also been studied and the experiment results show that after down conversion, higher sampling bandwidth does not lead to significant EVM improvement

    Enabling Technologies for Distribution of Broadband Radio over Fiber

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    RÉSUMÉ La radio sur fibre (RoF) a Ă©tĂ© considĂ©rĂ©e comme une technologie prometteuse qui concurrencera de maniĂšre indisputable comme solution viable pour la distribution des systĂšmes de communication sans fil Ă  bande large actuels et futurs. La technologie RoF emploie la modulation d'onde sous-porteuse (SCM) pour moduler la lumiĂšre par un signal RF, qui Ă  son tour sera transmise par la fibre. Malheureusement, la transmission du signal RF sur la fibre est sujette Ă  un certain nombre de dĂ©fauts. Ces dĂ©fauts incluent le faible rendement de la conversion optique en Ă©lectrique, Ă  la dispersion chromatique de la fibre, et Ă  la non-linĂ©aritĂ© de l’émetteur optique. L'objectif de cette thĂšse est de dĂ©velopper des technologies de pointe pour la radio sur fibre Ă  large bande. Les conceptions proposĂ©es devraient adresser la dĂ©formation non linĂ©aire induite par l'Ă©metteur optique, combattre le problĂšme de l’affaiblissement de la puissance optique induit par la dispersion chromatique de la fibre, et amĂ©liorer l'efficacitĂ© de modulation optique au petit signal sans augmenter de maniĂšre significative le cout et la complexitĂ© du systĂšme RoF. Pour le signal RF Ă  large bande, nous considĂ©rons le signal Ă  bande ultra large utilisant le multiplexage par rĂ©partition orthogonale de la frĂ©quence (ULB MB-MROF), qui a Ă©tĂ© proposĂ© comme solution pour le rĂ©seau de secteur personnel sans fil d’IEEE 802.15.3a (WPAN). D'abord, la performance de la transmission de l'ULB MB-MROF par la fibre est Ă©tudiĂ©e en considĂ©rant l'impact de modulation et dĂ©modulation optique. L'analyse thĂ©orique de l'effet de la dispersion de la fibre, de la rĂ©ponse de l'Ă©metteur optique et du rĂ©cepteur optique sur la performance du systĂšme est effectuĂ©e en considĂ©rant la distorsion de la phase et de l'amplitude. Des expĂ©riences sont rĂ©alisĂ©es pour vĂ©rifier notre analyse thĂ©orique et une bonne concordance est obtenue. Il est constatĂ© que l'index de modulation RF de ~4% est optimum pour l'Ă©metteur optique avec le modulateur de Mach-Zehnder, et le rĂ©cepteur optique avec la rĂ©ponse de Tchebychev-II est le meilleur pour l'ULB MB-MROF sur fibre. Aussi, la performance de la transmission sans fil est limitĂ©e par la sensibilitĂ© du rĂ©cepteur ULB MB-MROF. Il est aussi trouvĂ© qu’une haute puissance optique reçue est exigĂ©e pour la transmission du signal de l'ULB MB-MROF sur fibre.----------ABSTRACT Radio over fiber (RoF) has been considered as a very promising technology that will indisputably compete as a viable solution for the distribution of current and future broadband wireless communication systems such as IEEE 802.15.3a WPAN using Multiband-Orthogonal Frequency Division Multiplexing Ultra-Wideband (MB-OFDM UWB) signal. The RoF technology makes use of subcarrier modulation (SCM) to modulate an RF signal on light, which in turn will be transmitted by optical fiber. Unfortunately, the transmission of RF signal over fiber is subject to a number of impairments. These impairments include: low optical to electrical conversion efficiency, fiber chromatic dispersion, and nonlinearity of the optical front end, etc.. The objective of this thesis is to develop enabling technologies for broadband RoF systems. The proposed design platforms and techniques should address nonlinear distortion induced by the optical transmitter; combat optical power fading issue induced by the chromatic dispersion; and improve modulation efficiency of the optical small-signal modulation without significantly adding excessive expense and complexity to the RoF system. First of all, the performance of MB-OFDM UWB wireless over fiber transmission system is investigated considering optical modulation and demodulation aspects. Theoretical analysis of the effects of fiber chromatic dispersion, relative intensity noise (RIN), optical transmitter and optical receiver response on system performance is carried out considering amplitude and phase distortion. Experiments are conducted, which have verified our theoretical analysis and a good agreement is obtained. It is found that low RF modulation index (4%) for optical transmitter with Mach-Zehnder modulator (MZM), and optical receiver with Chebyshev-II response is the best for MB-OFDM UWB over fiber. The wireless transmission performance is limited by the UWB receiver sensitivity. Moreover, a high received optical power is required for transmission of MB-OFDM UWB signal over fiber. It is also found that the parameters like laser output power, laser linewidth and fiber dispersion that control RIN, will critically affect the overall performance of a UWB over fiber system

    Injection-locked Semiconductor Lasers For Realization Of Novel Rf Photonics Components

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    This dissertation details the work has been done on a novel resonant cavity linear interferometric modulator and a direct phase detector with channel filtering capability using injection-locked semiconductor lasers for applications in RF photonics. First, examples of optical systems whose performance can be greatly enhanced by using a linear intensity modulator are presented and existing linearized modulator designs are reviewed. The novel linear interferometric optical intensity modulator based on an injection-locked laser as an arcsine phase modulator is introduced and followed by numerical simulations of the phase and amplitude response of an injection-locked semiconductor laser. The numerical model is then extended to study the effects of the injection ratio, nonlinear cavity response, depth of phase and amplitude modulation on the spur-free dynamic range of a semiconductor resonant cavity linear modulator. Experimental results of the performance of the linear modulator implemented with a multi-mode Fabry-Perot semiconductor laser as the resonant cavity are shown and compared with the theoretical model. The modulator performance using a vertical cavity surface emitting laser as the resonant cavity is investigated as well. Very low Vπ in the order of 1 mV, multi-gigahertz bandwidth (-10 dB bandwidth of 5 GHz) and a spur-free dynamic range of 120 dB.Hz2/3 were measured directly after the modulator. The performance of the modulator in an analog link is experimentally investigated and the results show no degradation of the modulator linearity after a 1 km of SMF. The focus of the work then shifts to applications of an injection-locked semiconductor laser as a direct phase detector and channel filter. This phase detection technique does not iv require a local oscillator. Experimental results showing the detection and channel filtering capability of an injection-locked semiconductor diode laser in a three channel system are shown. The detected electrical signal has a signal-to-noise ratio better than 60 dB/Hz. In chapter 4, the phase noise added by an injection-locked vertical cavity surface emitting laser is studied using a self-heterodyne technique. The results show the dependency of the added phase noise on the injection ratio and detuning frequency. The final chapter outlines the future works on the linear interferometric intensity modulator including integration of the modulator on a semiconductor chip and the design of the modulator for input pulsed light

    Feed-forward linearisation of a directly modulated semiconductor laser and broadband millimetre-wave wireless over fibre systems.

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    This thesis is concerned with reduction of non-linear distortion in a directly modulated uncooled semiconductor laser using feed-forward compensation and investigating the performance of broadband millimetre-wave wireless over fibre systems. One of the key elements that determine the performance in a fibre optic link is the linearity of the optical source. Direct modulation of an uncooled semiconductor laser diode is a simple and cost effective solution. However, the distortion and noise generated by the laser limit the achievable dynamic range and performance in a system. Feed-forward linearisation is demonstrated at 5 GHz, the highest operating frequency reported, with 26 dB third order intermodulation distortion suppression and simultaneous noise reduction leading to enhanced spurious free dynamic range of 107 dB (1Hz). The effectiveness of feed-forward in a multi-channel system is investigated. Laser non-linearity can cause spectral re-growth and interchannel distortion that can completely mask the adjacent channel. A significant 11 dB interchannel distortion suppression and 10.5 dB power advantage is obtained compared to the non-linearised case. These results suggest that feed-forward linearisation arrangement can make a practical multi-channel or multi-operator wireless over fibre system. In the second part of this thesis the first experimental transmission of wireless data over fibre with remote millimetre-wave local oscillator delivery using a bi-directional semiconductor optical amplifier in a full duplex system with 2.2 km coarse wavelength division multiplexing fibre ring architecture is demonstrated. The use of bi-directional SOAs in place of unidirectional erbium doped fibre amplifier or unidirectional SOAs, together with the use of CWDM and optical distribution of the local oscillator signal allow substantial reduction in overall complexity and cost. Successful transmission of data over 12.8 km fibre is achieved with clear and well defined constellations and eye diagrams as well as 10.5% and 7.8 % error vector magnitude values for the downlink and uplink directions, respectively. The thesis also presents an implementation and performance of a millimetre-wave gigabit wireless over fibre system. CWDM devices such as uncooled laser diodes and passive components are used for the first time in a gigabit system allowing cost savings compared to dense WDM. This makes such solutions more attractive for millimetre-wave access systems. Optically modulated gigabit wireless data signals to and from the base stations are distributed at 5 GHz and up-converted using a remotely delivered LO source. Eye diagrams and bit error rate are measured to assess the system performance
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