Bit-Error-Rate-Minimizing Channel Shortening Using Post-FEQ Diversity Combining and a Genetic Algorithm

In advanced wireline or wireless communication systems, i.e., DSL, IEEE 802.11a/g, HIPERLAN/2, etc., a cyclic prefix which is proportional to the channel impulse response is needed to append a multicarrier modulation (MCM) frame for operating the MCM accurately. This prefix is used to combat inter symbol interference (ISI). In some cases, the channel impulse response can be longer than the cyclic prefix (CP). One of the most useful techniques to mitigate this problem is reuse of a Channel Shortening Equalizer (CSE) as a linear preprocessor before the MCM receiver in order to shorten the effective channel length. Channel shortening filter design is a widely examined topic in the literature. Most channel shortening equalizer proposals depend on perfect channel state information (CSI). However, this information may not be available in all situations. In cases where channel state information is not needed, blind adaptive equalization techniques are appropriate. In wireline communication systems (such as DMT), the CSE design is based on maximizing the bit rate, but in wireless systems (OFDM), there is a fixed bit loading algorithm, and the performance metric is Bit Error Rate (BER) minimization. In this work, a CSE is developed for multicarrier and single-carrier cyclic prefixed (SCCP) systems which attempts to minimize the BER. To minimize the BER, a Genetic Algorithm (GA), which is an optimization method based on the principles of natural selection and genetics, is used. If the CSI is shorter than the CP, the equalization can be done by a frequency domain equalizer (FEQ), which is a bank of complex scalars. However, in the literature the adaptive FEQ design has not been well examined. The second phase of this thesis focuses on different types of algorithms for adapting the FEQ and modifying the FEQ architecture to obtain a lower BER. Simulation results show that this modified architecture yields a 20 dB improvement in BER

Design and implementation of low complexity adaptive optical OFDM systems for software-defined transmission in elastic optical networks

Due to the increasing global IP traffic and the exponential growing demand for broadband services, optical networks are experimenting significant changes. Advanced modulation formats are being implemented at the Digital Signal Processing (DSP) level as key enablers for high data rate transmission. Whereas in the network layer, flexi Dense Wavelength-Division Multiplexing (DWDM) grids are being investigated in order to efficiently use the optical spectrum according to the traffic demand. Enabling these capabilities makes high data rate transmission more feasible. Hence, introducing flexibility in the system is one of the main goals of this thesis. Furthermore, minimizing the cost and enhancing the Spectral Efficiency (SE) of the system are two crucial issues to consider in the transceiver design. This dissertation investigates the use of Optical Orthogonal Frequency Division Multiplexing (O-OFDM) based either on the Fast Fourier Transform (FFT) or the Fast Hartley Transform (FHT) and flexi-grid technology to allow high data rate transmission over the fiber. Different cost-effective solutions for Elastic Optical Networks (EON) are provided. On the one hand, Direct Detection (DD) systems are investigated and proposed to cope with present and future traffic demand. After an introduction to the principles of OFDM and its application in optical systems, the main problems of such modulation is introduced. In particular, Peak-to-Average Power Ratio (PAPR) is presented as a limitation in OFDM systems, as well as clipping and quantization noise. Hence, PAPR reduction techniques are proposed to mitigate these impairments. Additionally, Low Complexity (LC) PAPR reduction techniques based on the FHT have also been presented with a simplified DSP. On the other hand, loading schemes have also been introduced in the analyzed system to combat Chromatic Dispersion (CD) when transmitting over the optical link. Moreover, thanks to Bit Loading (BL) and Power Loading (PL), flexible and software-defined transceivers can be implemented maximizing the spectral efficiency by adapting the data rate to the current demand and the actual network conditions. Specifically, OFDM symbols are created by mapping the different subcarriers with different modulation formats according to the channel profile. Experimental validation of the proposed flexible transceivers is also provided in this dissertation. The benefits of including loading capabilities in the design, such as enabling high data rate and software-defined transmission, are highlighted.Degut al creixement del tràfic IP i de la demanda de serveis de banda ampla, les xarxes òptiques estan experimentant canvis significatius. Els formats avançats de modulació, implementats a nivell de processat del senyal digital, habiliten la transmissió a alta velocitat. Mentre que a la capa de xarxa, l'espectre òptic es dividit en ranures flexibles ocupant l'ample de banda necessari segons la demanda de tràfic. La transmissió a alta velocitat és fa més tangible un cop habilitades totes aquestes funcionalitats. D'aquesta manera un dels principals objectius d'aquesta tesis es introduir flexibilitat al sistema. A demés, minimitzar el cost i maximitzar l'eficiència espectral del sistema són també dos aspectes crucials a considerar en el disseny del transmissor i receptor. Aquesta tesis investiga l'ús de la tecnologia Optical Orthogonal Frequency Division Multiplexing (OFDM) basada en la transformada de Fourier (FFT) i la de Hartley (FHT) per tal de dissenyar un sistema flexible i capaç de transmetre a alta velocitat a través de la fibra òptica. Per tant, es proposen diferent solucions de baix cost vàlides per a utilitzar en xarxes òptiques elàstiques. En primer lloc, s'investiguen i es proposen sistemes basats en detecció directa per tal de suportar la present i futura demanda. Després d'una introducció dels principis d' OFDM i la seva aplicació als sistemes òptics, s'introdueixen alguns dels problemes d'aquesta modulació. En particular, es presenten el Peak-to-Average Power Ratio (PAPR) i els sorolls de clipping i de quantizació com a limitació dels sistemes OFDM. S'analitzen tècniques de reducció de PAPR per tal de reduir l'impacte d'aquests impediments. També es proposen tècniques de baixa complexitat per a reduir el PAPR basades en la FHT. Finalment, s'utilitzen algoritmes d'assignació de bits i de potència, Bit Loading (BL) i Power Loading (PL), per tal de combatre la dispersió cromàtica quan es transmet pel canal òptic. Amb la implementació dels algoritmes de BL i PL, es poden dissenyar transmissors i receptors flexibles adaptant la velocitat a la demanda del moment i a les actuals condicions de la xarxa. En particular, els símbols OFDM es creen mapejant cada portadora amb un format de modulació diferent segons el perfil del canal. El sistema és validat experimentalment mostrant les prestacions i els beneficis d'incloure flexibilitat per tal de facilitar la transmissió a alta velocitat i cobrir les necessitats de l'Internet del futurDebido al crecimiento del tráfico IP y de la demanda de servicios de banda ancha, las redes ópticas están experimentando cambios significativos. Los formatos avanzados de modulación, implementados a nivel de procesado de la señal digital, habilitan la transmisión a alta velocidad. Mientras que en la capa de red, el espectro óptico se divide en ranuras flexibles ocupando el ancho de banda necesario según la demanda de tráfico. La transmisión a alta velocidad es más tangible una vez habilitadas todas estas funcionalidades. De este modo uno de los principales objetivos de esta tesis es introducir flexibilidad en el sistema. Además, minimizar el coste y maximizar la eficiencia espectral del sistema son también dos aspectos cruciales a considerar en el diseño del transmisor y receptor. Esta tesis investiga el uso de la tecnologia Optical Orthogonal Frequency Division Multiplexing (OFDM) basada en la transformada de Fourier (FFT) y en la de Hartley (FHT) con tal de diseñar un sistema flexible y capaz de transmitir a alta velocidad a través de la fibra óptica. Por lo tanto, se proponen distintas soluciones de bajo coste válidas para utilizar en redes ópticas elásticas. En primer lugar, se investigan y se proponen sistemas basados en detección directa con tal de soportar la presente y futura demanda. Después de una introducción de los principios de OFDM y su aplicación en los sistemas ópticos, se introduce el principal problema de esta modulación. En particular se presentan el Peak-to-Average Power Ratio (PAPR) y los ruidos de clipping y cuantización como limitaciones de los sistemas OFDM. Se analizan técnicas de reducción de PAPR con tal de reducir el impacto de estos impedimentos. También se proponen técnicas de baja complejidad para reducir el PAPR basadas en la FHT. Finalmente, se utilizan algoritmos de asignación de bits y potencia, Bit Loading (BL) y Power Loading (PL), con tal de combatir la dispersión cromática cuando se transmite por el canal óptico. Con la implementación de los algoritmos de BL y PL, se pueden diseñar transmisores y receptores flexibles adaptando la velocidad a la demanda del momento y a las actuales condiciones de la red. En particular, los símbolos OFDM se crean mapeando cada portadora con un formato de modulaci_on distinto según el perfil del canal. El sistema se valida experimentalmente mostrando las prestaciones y los beneficios de incluir flexibilidad con tal de facilitar la transmisión a alta velocidad y cubrir las necesidades de Internet del futuro

Millimetre-Wave Fibre-Wireless Technologies for 5G Mobile Fronthaul

The unprecedented growth in mobile data traffic, driven primarily by bandwidth rich applications and high definition video is accelerating the development of fifth generation (5G) mobile network. As mobile access network evolves towards centralisation, mobile fronthaul (MFH) architecture becomes essential in providing high capacity, ubiquitous and yet affordable services to subscribers. In order to meet the demand for high data rates in the access, Millimetre-wave (mmWave) has been highlighted as an essential technology in the development of 5G-new radio (5G-NR). In the present MFH architecture which is typically based on common public radio interface (CPRI) protocol, baseband signals are digitised before fibre transmission, featuring high overhead data and stringent synchronisation requirements. A direct application of mmWave 5G-NR to CPRI digital MFH, where signal bandwidth is expected to be up to 1GHz will be challenging, due to the increased complexity of the digitising interface and huge overhead data that will be required for such bandwidth. Alternatively, radio over fibre (RoF) technique can be employed in the transportation of mmWave wireless signals via the MFH link, thereby avoiding the expensive digitisation interface and excessive overhead associated with its implementation. Additionally, mmWave carrier can be realised with the aid of photonic components employed in the RoF link, further reducing the system complexity. However, noise and nonlinearities inherent to analog transmission presents implementation challenges, limiting the system dynamic range. Therefore, it is important to investigate the effects of these impairments in RoF based MFH architecture. This thesis presents extensive research on the impact of noise and nonlinearities on 5G candidate waveforms, in mmWave 5G fibre wireless MFH. Besides orthogonal frequency division multiplexing (OFDM), another radio access technology (RAT) that has received significant attention is filter bank multicarrier (FBMC), particularly due to its high spectral containment and excellent performance in asynchronous transmission. Hence, FBMC waveform is adopted in this work to study the impact of noise and nonlinearities on the mmWave fibre-wireless MFH architecture. Since OFDM is widely deployed and it has been adopted for 5G-NR, the performance of OFDM and FBMC based 5G mmWave RAT in fibre wireless MFH architecture is compared for several implementations and transmission scenarios. To this extent, an end to end transmission testbed is designed and implemented using industry standard VPI Transmission Maker® to investigate five mmWave upconversion techniques. Simulation results show that the impact of noise is higher in FBMC when the signal to-noise (SNR) is low, however, FBMC exhibits better performance compared to OFDM as the SNR improved. More importantly, an evaluation of the contribution of each noise component to the overall system SNR is carried out. It is observed in the investigation that noise contribution from the optical carriers employed in the heterodyne upconversion of intermediate frequency (IF) signals to mmWave frequency dominate the system noise. An adaptive modulation technique is employed to optimise the system throughput based on the received SNR. The throughput of FBMC based system reduced significantly compared to OFDM, due to laser phase noise and chromatic dispersion (CD). Additionally, it is shown that by employing frequency domain averaging technique to enhance the channel estimation (CE), the throughput of FBMC is significantly increased and consequently, a comparable performance is obtained for both waveforms. Furthermore, several coexistence scenarios for multi service transmission are studied, considering OFDM and FBMC based RATs to evaluate the impact inter band interference (IBI), due to power amplifier (PA) nonlinearity on the system performance. The low out of band (OOB) emission in FBMC plays an important role in minimising IBI to adjacent services. Therefore, FBMC requires less guardband in coexistence with multiple services in 5G fibre-wireless MFH. Conversely, OFDM introduced significant OOB to adjacent services requiring large guardband in multi-service coexistence transmission scenario. Finally, a novel transmission scheme is proposed and investigated to simultaneously generate multiple mmWave signals using laser heterodyning mmWave upconversion technique. With appropriate IF and optical frequency plan, several mmWave signals can be realised. Simulation results demonstrate successful simultaneous realisation of 28GHz, 38GHz, and 60GHz mmWave signals

Joint transceiver design for MIMO channel shortening.

Channel shortening equalizers can be employed to shorten the effective impulse response of a long intersymbol interference (ISI) channel in order, for example, to decrease the computational complexity of a maximum-likelihood sequence estimator (MLSE) or to increase the throughput efficiency of an orthogonal frequency-division multiplexing (OFDM) transmission scheme. In this paper, the issue of joint transmitter–receiver filter design is addressed for shortening multiple-input multiple-output (MIMO) ISI channels. A frequency-domain approach is adopted for the transceiver design which is effectively equivalent to an infinite-length time-domain design. A practical space–frequency waterfilling algorithm is also provided. It is demonstrated that the channel shortening equalizer designed according to the time-domain approach suffers from an error-floor effect. However, the proposed techniques are shown to overcome this problem and outperform the time-domain channel shortening filter design. We also demonstrate that the proposed transceiver design can be considered as a MIMO broadband beamformer with constraints on the time-domain multipath length. Hence, a significant diversity gain could also be achieved by choosing strong eigenmodes of the MIMO channel. It is also found that the proposed frequency-domain methods have considerably low computational complexity as compared with their time-domain counterparts