Performance Analysis of Coherent and Noncoherent Modulation under I/Q Imbalance

In-phase/quadrature-phase Imbalance (IQI) is considered a major performance-limiting impairment in direct-conversion transceivers. Its effects become even more pronounced at higher carrier frequencies such as the millimeter-wave frequency bands being considered for 5G systems. In this paper, we quantify the effects of IQI on the performance of different modulation schemes under multipath fading channels. This is realized by developing a general framework for the symbol error rate (SER) analysis of coherent phase shift keying, noncoherent differential phase shift keying and noncoherent frequency shift keying under IQI effects. In this context, the moment generating function of the signal-to-interference-plus-noise-ratio is first derived for both single-carrier and multi-carrier systems suffering from transmitter (TX) IQI only, receiver (RX) IQI only and joint TX/RX IQI. Capitalizing on this, we derive analytic expressions for the SER of the different modulation schemes. These expressions are corroborated by comparisons with corresponding results from computer simulations and they provide insights into the dependence of IQI on the system parameters. We demonstrate that the effects of IQI differ considerably depending on the considered system as some cases of single-carrier transmission appear robust to IQI, whereas multi-carrier systems experiencing IQI at the RX require compensation in order to achieve a reliable communication link

Towards low-cost gigabit wireless systems at 60 GHz

The world-wide availability of the huge amount of license-free spectral space in the 60 GHz band provides wide room for gigabit-per-second (Gb/s) wireless applications. A commercial (read: low-cost) 60-GHz transceiver will, however, provide limited system performance due to the stringent link budget and the substantial RF imperfections. The work presented in this thesis is intended to support the design of low-cost 60-GHz transceivers for Gb/s transmission over short distances (a few meters). Typical applications are the transfer of high-definition streaming video and high-speed download. The presented work comprises research into the characteristics of typical 60-GHz channels, the evaluation of the transmission quality as well as the development of suitable baseband algorithms. This can be summarized as follows. In the first part, the characteristics of the wave propagation at 60 GHz are charted out by means of channel measurements and ray-tracing simulations for both narrow-beam and omni-directional configurations. Both line-of-sight (LOS) and non-line-of-sight (NLOS) are considered. This study reveals that antennas that produce a narrow beam can be used to boost the received power by tens of dBs when compared with omnidirectional configurations. Meanwhile, the time-domain dispersion of the channel is reduced to the order of nanoseconds, which facilitates Gb/s data transmission over 60-GHz channels considerably. Besides the execution of measurements and simulations, the influence of antenna radiation patterns is analyzed theoretically. It is indicated to what extent the signal-to-noise ratio, Rician-K factor and channel dispersion are improved by application of narrow-beam antennas and to what extent these parameters will be influenced by beam pointing errors. From both experimental and analytical work it can be concluded that the problem of the stringent link-budget can be solved effectively by application of beam-steering techniques. The second part treats wideband transmission methods and relevant baseband algorithms. The considered schemes include orthogonal frequency division multiplexing (OFDM), multi-carrier code division multiple access (MC-CDMA) and single carrier with frequency-domain equalization (SC-FDE), which are promising candidates for Gb/s wireless transmission. In particular, the optimal linear equalization in the frei quency domain and associated implementation issues such as synchronization and channel estimation are examined. Bit error rate (BER) expressions are derived to evaluate the transmission performance. Besides the linear equalization techniques, a low-complexity inter-symbol interference cancellation technique is proposed to achieve much better performance of code-spreading systems such as MC-CDMA and SC-FDE. Both theoretical analysis and simulations demonstrate that the proposed scheme offers great advantages as regards both complexity and performance. This makes it particularly suitable for 60-GHz applications in multipath environments. The third part treats the influence of quantization and RF imperfections on the considered transmission methods in the context of 60-GHz radios. First, expressions for the BER are derived and the influence of nonlinear distortions caused by the digital-to-analog converters, analog-to-digital converters and power amplifiers on the BER performance is examined. Next, the BER performance under the influence of phase noise and IQ imbalance is evaluated for the case that digital compensation techniques are applied in the receiver as well as for the case that such techniques are not applied. Finally, a baseline design of a low-cost Gb/s 60-GHz transceiver is presented. It is shown that, by application of beam-steering in combination with SC-FDE without advanced channel coding, a data rate in the order of 2 Gb/s can be achieved over a distance of 10 meters in a typical NLOS indoor scenario

Communication Subsystems for Emerging Wireless Technologies

The paper describes a multi-disciplinary design of modern communication systems. The design starts with the analysis of a system in order to define requirements on its individual components. The design exploits proper models of communication channels to adapt the systems to expected transmission conditions. Input filtering of signals both in the frequency domain and in the spatial domain is ensured by a properly designed antenna. Further signal processing (amplification and further filtering) is done by electronics circuits. Finally, signal processing techniques are applied to yield information about current properties of frequency spectrum and to distribute the transmission over free subcarrier channels

On the Capacity of the Wiener Phase-Noise Channel: Bounds and Capacity Achieving Distributions

In this paper, the capacity of the additive white Gaussian noise (AWGN) channel, affected by time-varying Wiener phase noise is investigated. Tight upper and lower bounds on the capacity of this channel are developed. The upper bound is obtained by using the duality approach, and considering a specific distribution over the output of the channel. In order to lower-bound the capacity, first a family of capacity-achieving input distributions is found by solving a functional optimization of the channel mutual information. Then, lower bounds on the capacity are obtained by drawing samples from the proposed distributions through Monte-Carlo simulations. The proposed capacity-achieving input distributions are circularly symmetric, non-Gaussian, and the input amplitudes are correlated over time. The evaluated capacity bounds are tight for a wide range of signal-to-noise-ratio (SNR) values, and thus they can be used to quantify the capacity. Specifically, the bounds follow the well-known AWGN capacity curve at low SNR, while at high SNR, they coincide with the high-SNR capacity result available in the literature for the phase-noise channel.Comment: IEEE Transactions on Communications, 201

Stacked Modulation Formats Enabling Highest-Sensitivity Optical Free-Space Communications

Die vorliegende Arbeit befasst sich mit hochempfindlichen optischen Kommunikationssystemen, wie sie z.B. bei Intersatellitenlinks verwendet werden. Theoretische Überlegungen zur Steigerung der Empfängerempfindlichkeit werden mit Simulations- und Messergebnissen ergänzt und verifiziert. Auf Grund der steigenden Nachfrage nach optischen Links zwischen Satelliten stellt sich die Frage, was sind geeignete Eckparameter, um ein solches System zu beschreiben. Die gigantischen Datenmengen, die von diversen Messgeräten, wie z.B. hochauflösende Kameras auf einem Satelliten generiert werden, bringen die Kapazitäten klassischer HF-Datenlinks an ihre Grenzen. Hier können optische Kommunikationssysteme auf Grund ihrer hohen Trägerfrequenz im Infrarotbereich sehr hohe Datenraten im Terabit/s Bereich ermöglichen. Systeme mit Radiowellen im GHz Bereich als Trägerfrequenz sind hier deutlich limitierter. [7] Linkdistanz, verfügbare Leistung, Pointinggenauigkeit und verfügbare Antennengröße sind einige Parameter, die einen wichtigen Einfluss auf die Leistungsfähigkeit des Systems haben. Je größer die Distanz und desto kleiner die verfügbare Antennengröße sowohl am Sender als auch am Empfänger sind, desto weniger Signalleistung wird den Detektor erreichen. Nimmt man dann noch ungenaues Pointing hinzu, d.h. Sender und Empfänger sind nicht exakt aufeinander ausgerichtet, treten zusätzliche Verluste auf. [7] Ziel dieser Arbeit ist es, ein vereinfachtes System zu implementieren und zu testen, das mit möglichst wenigen Photonen pro Bit bei einer gegebenen Bitfehlerwahrscheinlichkeit bei einer möglichst hohen Datenrate arbeiten kann. Hierfür werden alle Freiheitsgrade einer optischen Welle zur Modulation verwendet, um mit sog. „Stapeln“ von Modulationsformaten eine Empfindlichkeitssteigerung zu erreichen. Die Amplitude des Signals wird durch Pulspositionsmodulation (PPM) moduliert, wobei das zeitlich variable Vorhandensein eines Pulses innerhalb des Symbols die Information enthält. Dieses Modulationsformat weist bis dato die höchste Empfindlichkeit in Literatur und Experimenten auf [4]. Je mehr Möglichkeiten es gibt, einen Puls in einem Symbol zu platzieren, desto höher ist die zu erwartende Empfindlichkeit des Systems. Mit anderen Worten: Steigert man die zeitliche Dauer eines PPM-Symbols, so wächst ebenfalls die Empfängerempfindlichkeit. Da bei diesem Ansatz die Datenrate sinkt, wird in dieser Arbeit eine andere Methode vorgestellt, die Empfindlichkeit eines Übertragungssystems zu steigern, ohne die Symbollänge unnötig in die Länge zu ziehen. Diese Arbeit befasst sich mit dem Stapeln (sog. „Stacking“) von Modulationsformaten, in dem neben der Amplitudenmodulation weitere Freiheitsgrade, wie die Frequenz, Phase und Polarisation geschickt genutzt werden. Bei der Frequenzumtastung (FSK) wird die optische Frequenz je nach Symbol um ein gewisses Maß verschoben. Bei der polarisations-geschalteten Quadratur-Phasenumtastung (PS-QPSK) werden sowohl die Phase, als auch die Polarisation der optischen Welle moduliert [12]. Als Endergebnis erhält man PPM-FSK-PS-QPSK als Modulationsformat mit hoher Empfindlichkeit. Gegenüber dem reinen PPM wird eine theoretische Empfindlichkeitssteigerung von mehr als 1 dB erreicht. Sowohl Simulations- als auch Messergebnisse bestätigen den Empfindlichkeitsgewinn

Demonstration and evaluation of an optimized RFS comb for terabit flexible optical networks

We experimentally demonstrate and evaluate an optimization strategy of a recirculating frequency shifting (RFS) optical comb for terabit flexible optical networks. We achieve an increased optical signal-to-noise ratio (OSNR) with good stability (no system outage) by reducing erbium-doped-fiber amplifier gain in the shifting loop and deploying an in-loop noise suppression filter. We demonstrate that this source can support 20×200  Gb/s dual polarization Nyquist-16QAM transmission. With optimization, the RFS comb has greater and more uniform OSNR per channel. Flexible optical networks with software-defined networking are particularly suited to this enhanced RFS due to 1) programmable frequency spacing, 2) dense, stable spacing enabling very high spectral efficiency, 3) uniform performance across channels, and 4) sufficient OSNR for high-order modulation. The RFS can be used in short links when using low overhead forward error correction (FEC). Distances as great as 1150 km are achieved when using a 20% FEC overhead. Long-distance tests at 4 Tb/s result in a post-FEC net rate of 3.3 Tb/s and 6.3 bit/s/Hz of spectral efficiency