Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems

Burts-by-burst (BbB) adaptive high-speed downlink packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER

A BP-MF-EP Based Iterative Receiver for Joint Phase Noise Estimation, Equalization and Decoding

In this work, with combined belief propagation (BP), mean field (MF) and expectation propagation (EP), an iterative receiver is designed for joint phase noise (PN) estimation, equalization and decoding in a coded communication system. The presence of the PN results in a nonlinear observation model. Conventionally, the nonlinear model is directly linearized by using the first-order Taylor approximation, e.g., in the state-of-the-art soft-input extended Kalman smoothing approach (soft-in EKS). In this work, MF is used to handle the factor due to the nonlinear model, and a second-order Taylor approximation is used to achieve Gaussian approximation to the MF messages, which is crucial to the low-complexity implementation of the receiver with BP and EP. It turns out that our approximation is more effective than the direct linearization in the soft-in EKS with similar complexity, leading to significant performance improvement as demonstrated by simulation results.Comment: 5 pages, 3 figures, Resubmitted to IEEE Signal Processing Letter

Iterative graphical algorithms for phase noise channels.

Doctoral Degree. University of KwaZulu-Natal, Durban.This thesis proposes algorithms based on graphical models to detect signals and charac- terise the performance of communication systems in the presence of Wiener phase noise. The algorithms exploit properties of phase noise and consequently use graphical models to develop low complexity approaches of signal detection. The contributions are presented in the form of papers. The first paper investigates the effect of message scheduling on the performance of graphical algorithms. A serial message scheduling is proposed for Orthogonal Frequency Division Multiplexing (OFDM) systems in the presence of carrier frequency offset and phase noise. The algorithm is shown to have better convergence compared to non-serial scheduling algorithms. The second paper introduces a concept referred to as circular random variables which is based on exploiting the properties of phase noise. An iterative algorithm is proposed to detect Low Density Parity Check (LDPC) codes in the presence of Wiener phase noise. The proposed algorithm is shown to have similar performance as existing algorithms with very low complexity. The third paper extends the concept of circular variables to detect coherent optical OFDM signals in the presence of residual carrier frequency offset and Wiener phase noise. The proposed iterative algorithm shows a significant improvement in complexity compared to existing algorithms. The fourth paper proposes two methods based on minimising the free energy function of graphical models. The first method combines the Belief Propagation (BP) and the Uniformly Re-weighted BP (URWBP) algorithms. The second method combines the Mean Field (MF) and the URWBP algorithms. The proposed methods are used to detect LDPC codes in Wiener phase noise channels. The proposed methods show good balance between complexity and performance compared to existing methods. The last paper proposes parameter based computation of the information bounds of the Wiener phase noise channel. The proposed methods compute the information lower and upper bounds using parameters of the Gaussian probability density function. The results show that these methods achieve similar performance as existing methods with low complexity

Multi-Relay Communications in the Presence of Phase Noise and Carrier Frequency Offsets

Impairments like time varying phase noise (PHN) and carrier frequency offset (CFO) result in loss of synchronization and poor performance of multi-relay communication systems. Joint estimation of these impairments is necessary in order to correctly decode the received signal at the destination. In this paper, we address spectrally-efficient multi-relay transmission scenarios where all the relays simultaneously communicate with the destination. We propose an iterative pilot-aided algorithm based on the expectation conditional maximization (ECM) for joint estimation of multipath channels, Wiener PHNs, and CFOs in decode-and-forward (DF) based multi-relay orthogonal frequency division multiplexing (OFDM) systems. Next, a new expression of the hybrid Cramér-Rao lower bound (HCRB) for the multi-parameter estimation problem is derived. Finally, an iterative receiver based on an extended Kalman filter (EKF) for joint data detection and PHN tracking is employed. Numerical results show that the proposed estimator outperforms existing algorithms and its mean square error performance is close to the derived HCRB at differnt signal-to-noise ratios (SNRs) for different PHN variances. In addition, the combined estimation algorithm and iterative receiver can significantly improve average bit-error rate (BER) performance compared to existing algorithms. In addition, the BER performance of the proposed system is close to the ideal case of perfect channel impulse responses (CIRs), PHNs and CFOs estimation

Enhanced PON Infrastructure Enabled by Silicon Photonics

Les systèmes de courte portée et de détection directe sont le dernier/premier kilomètre de la fourniture des services Internet d'aujourd'hui. Deux cas d'application sont abordés dans cette thèse, l'un concerne l'amélioration des performances des services Internet par la Fibre-To-TheHome ou les réseaux optiques passifs (PONs). L'autre est le radio access network (RAN) pour le fronthaul. Notre objectif pour RAN est de superposer les signaux 5G sur une infrastructure PON. Nous démontrons expérimentalement la génération d'un signal de répartition multiplexée de fréquences orthogonales (OFDM) à bande latérale unique en utilisant un modulateur IQ sur puce basé sur les photoniques au silicium à micro-anneau. Il s'agit d'une solution à coût bas permettant aux PONs d'augmenter les débits de données grâce à l'utilisation d'OFDM. Nous avons généré un signal OFDM à large bande avec un ratio de suppression de bande latérale de plus de 18 dB. Afin de confirmer la robustesse de la dispersion chromatique (CD), nous transmettons le signal généré OFDM SSB dans plus de 20 km de fibre de monomode standard. Aucun fading induit par la CD n'a été observé et le taux d'erreur sur les bits était bon. Nous proposons une solution de photoniques au silicium pour un réseau optique passif afin de mitiger l'interférence de battement signal-signal (SSBI) dans la transmission OFDM, et de récupérer une partie des porteuses de la liaison descendante pour une utilisation dans la liaison montante. Le sous-système recrée les interférences à une entrée du détecteur équilibré ; le signal de données corrompu par SSBI est à la deuxième entrée. L'annulation se produit via la soustraction dans la détection équilibrée. Comme notre solution de photoniques au silicium (SiP) ne peut pas filtrer les signaux idéalement, nous examinons un facteur d'échelle introduit dans la détection équilibrée qui peut balancer les effets de filtrage non idéaux. Nous montrons expérimentalement l'annulation de l'interférence donne de bonnes performances même avec une porteuse faible, soit pour un ratio porteuse/signal ultra bas de 0 dB. Bien que notre solution soit sensible aux effets de la température, notre démonstration expérimentale montre que le réglage de la fréquence résonante peut dériver jusqu'à 12 GHz de la valeur ciblée et présenter toujours de bonnes performances. Nous effectuons des simulations extensives du schéma d'annulation SSBI proposé, et suggérons une diverse conception polarisée pour le sous-système SiP. Nous examinons via la simulation la vulnérabilité à la variation de température et introduisons une nouvelle métrique de performance : Q-facteur minimum garanti. Nous nous servons de cette métrique pour évaluer la robustesse d'annulation SSBI contre la dérive de fréquence induite par les changements de température. Nous maximisons l'efficacité spectrale sous différentes conditions du système en balayant les paramètres de conception contrôlables. Finalement, les résultats de la simulation du système fournissent des indications sur la conception du résonateur micro-anneau, ainsi que sur le choix de la bande de garde et du format de modulation pour obtenir la plus grande efficacité spectrale. Finalement, nous nous concentrons sur la superposition des signaux 5G sur une infrastructure PON pour RAN. Nous expérimentalement validons un sous-système photonique au silicium conçu pour les réseaux optiques passifs avec réutilisation de porteuses et compatibilité radiosur-fibre (RoF) analogique 5G. Le sous-système permet la détection simultanée des signaux RoF et du signal PON transmis dans une seule tranche assignée de longueur d'onde. Tout en maintenant une qualité suffisante de détection des signaux RoF et PON, il n'y a que la puissance minimale de la porteuse qui est extraite pour chaque détection, ce qui conserve ainsi la puissance de la porteuse pour la modulation de liaison montante. Nous réalisons une suppression efficace du signal de liaison descendante en laissant une porteuse propre et forte pour la remodulation. Nous démontrons expérimentalement le signal RoF de liaison montante via un modulateur à micro-anneau. Nous avons détecté avec succès un signal à large bande de 8 GHz et cinq signaux RoF de 125 MHz simultanément. Et deux signaux RoF de 125 MHz sont remodulés sur la même porteuse. Le signal RoF de liaison montante généré est de 13 dB de plus que les signaux de liaison descendante, ce qui indique leur robustesse contre la diaphonie des signaux résiduels de la liaison descendante.Short reach, direct detection systems are the last/first mile of today's internet service provision. Two use cases are addressed in this thesis, one is for enhancing performance of Internet services on fiber-to-the-home or passive optical networks (PON). The other is radio access networks (RAN) for fronthaul. Our focus for RAN is to overlay 5G signals on a PON infrastructure. We experimentally demonstrate the generation of a single-sideband orthogonal frequency division multiplexed (OFDM) signal using an on-chip silicon photonics microring-based IQ modulator. This is a low cost solution enabling PONs to increase data rates through the use of OFDM. We generated a wideband OFDM signal with over 18 dB sideband suppression ratio. To confirm chromatic dispersion (CD) robustness, we transmit the generated SSB OFDM signal over 20 km of standard single mode fiber. No CD-induced fading was observed and bit error rate was good. We propose a silicon photonics solution for a passive optical network to mitigate signal-signal beat interference (SSBI) in OFDM transmission, and to recuperate a part of the downlink carrier for use in the uplink. The subsystem recreates the interference at one balanced detector input; the data signal corrupted with SSBI is at the second input. Cancellation occurs via subtraction in the balanced detection. As our silicon photonics (SiP) solution cannot filter the signals ideally, we examine a scaling factor to be introduced to the balanced detection that can trade-off the non-ideal filtering effects. We show experimentally that the interference is cancelled, allowing good performance even with a weak carrier, that is, for ultra low carrier to signal ratio of 0 dB. Although our solution is sensitive to temperature effects, our experimental demonstration shows the tuning of the resonant frequency can drift by as much as 12 GHz from the targeted value and still provide good performance. We perform extensive simulations of the proposed SSBI cancellation scheme, and suggest a polarization diverse design for the SiP subsystem. We examine via simulation the vulnerability to temperature variation and introduce a new performance metric: minimum guaranteed Qfactor. We use this metric to evaluate the SSBI cancellation robustness against the frequency drift induced by temperature changes. We maximize the spectral efficiency under different system conditions by sweeping the controllable design parameters. Finally the system simulation results provide guidance on the microring resonator design, as well as choice of guard band and modulation format to achieve the highest spectral efficiency. Finally, we turn to focus on overlay 5G signals on a PON infrastructure for RAN. We experimentally validate a silicon photonic subsystem designed for passive optical networks with carrier reuse and 5G analog radio-over-fiber (RoF) compatibility. The subsystem enables the simultaneous detection of RoF signals and a PON signal transmitted in a single assigned wavelength slot. While maintaining sufficient quality of RoF and PON signal detection, only the minimum carrier power is leached off for each detection, thus conserving carrier power for uplink modulation. We realize effective downlink signal suppression to leave a clean and strong carrier for remodulation. We demonstrate experimentally the RoF uplink signal via a micro ring modulator. We successfully detected an 8 GHz broadband signal and five 125 MHz RoF signals simultaneously. And two 125 MHz radio over fiber signals are remodulated onto the same carrier. The generated uplink RoF signal is 13 dB over the downlink signals, indicating their robustness against the crosstalk from residual downlink signals

Optical Time-Frequency Packing: Principles, Design, Implementation, and Experimental Demonstration

Time-frequency packing (TFP) transmission provides the highest achievable spectral efficiency with a constrained symbol alphabet and detector complexity. In this work, the application of the TFP technique to fiber-optic systems is investigated and experimentally demonstrated. The main theoretical aspects, design guidelines, and implementation issues are discussed, focusing on those aspects which are peculiar to TFP systems. In particular, adaptive compensation of propagation impairments, matched filtering, and maximum a posteriori probability detection are obtained by a combination of a butterfly equalizer and four 8-state parallel Bahl-Cocke-Jelinek-Raviv (BCJR) detectors. A novel algorithm that ensures adaptive equalization, channel estimation, and a proper distribution of tasks between the equalizer and BCJR detectors is proposed. A set of irregular low-density parity-check codes with different rates is designed to operate at low error rates and approach the spectral efficiency limit achievable by TFP at different signal-to-noise ratios. An experimental demonstration of the designed system is finally provided with five dual-polarization QPSK-modulated optical carriers, densely packed in a 100 GHz bandwidth, employing a recirculating loop to test the performance of the system at different transmission distances.Comment: This paper has been accepted for publication in the IEEE/OSA Journal of Lightwave Technolog

On the Impact of Phase Noise in Communication Systems –- Performance Analysis and Algorithms

The mobile industry is preparing to scale up the network capacity by a factor of 1000x in order to cope with the staggering growth in mobile traffic. As a consequence, there is a tremendous pressure on the network infrastructure, where more cost-effective, flexible, high speed connectivity solutions are being sought for. In this regard, massive multiple-input multiple-output (MIMO) systems, and millimeter-wave communication systems are new physical layer technologies, which promise to facilitate the 1000 fold increase in network capacity. However, these technologies are extremely prone to hardware impairments like phase noise caused by noisy oscillators. Furthermore, wireless backhaul networks are an effective solution to transport data by using high-order signal constellations, which are also susceptible to phase noise impairments. Analyzing the performance of wireless communication systems impaired by oscillator phase noise, and designing systems to operate efficiently in strong phase noise conditions are critical problems in communication theory. The criticality of these problems is accentuated with the growing interest in new physical layer technologies, and the deployment of wireless backhaul networks. This forms the main motivation for this thesis where we analyze the impact of phase noise on the system performance, and we also design algorithms in order to mitigate phase noise and its effects. First, we address the problem of maximum a posteriori (MAP) detection of data in the presence of strong phase noise in single-antenna systems. This is achieved by designing a low-complexity joint phase-estimator data-detector. We show that the proposed method outperforms existing detectors, especially when high order signal constellations are used. Then, in order to further improve system performance, we consider the problem of optimizing signal constellations for transmission over channels impaired by phase noise. Specifically, we design signal constellations such that the error rate performance of the system is minimized, and the information rate of the system is maximized. We observe that these optimized constellations significantly improve the system performance, when compared to conventional constellations, and those proposed in the literature. Next, we derive the MAP symbol detector for a MIMO system where each antenna at the transceiver has its own oscillator. We propose three suboptimal, low-complexity algorithms for approximately implementing the MAP symbol detector, which involve joint phase noise estimation and data detection. We observe that the proposed techniques significantly outperform the other algorithms in prior works. Finally, we study the impact of phase noise on the performance of a massive MIMO system, where we analyze both uplink and downlink performances. Based on rigorous analyses of the achievable rates, we provide interesting insights for the following question: how should oscillators be connected to the antennas at a base station, which employs a large number of antennas