Digital predistortion of RF amplifiers using baseband injection for mobile broadband communications

Radio frequency (RF) power amplifiers (PAs) represent the most challenging design parts of wireless transmitters. In order to be more energy efficient, PAs should operate in nonlinear region where they produce distortion that significantly degrades the quality of signal at transmitter’s output. With the aim of reducing this distortion and improve signal quality, digital predistortion (DPD) techniques are widely used. This work focuses on improving the performances of DPDs in modern, next-generation wireless transmitters. A new adaptive DPD based on an iterative injection approach is developed and experimentally verified using a 4G signal. The signal performances at transmitter output are notably improved, while the proposed DPD does not require large digital signal processing memory resources and computational complexity. Moreover, the injection-based DPD theory is extended to be applicable in concurrent dual-band wireless transmitters. A cross-modulation problem specific to concurrent dual-band transmitters is investigated in detail and novel DPD based on simultaneous injection of intermodulation and cross-modulation distortion products is proposed. In order to mitigate distortion compensation limit phenomena and memory effects in highly nonlinear RF PAs, this DPD is further extended and complete generalised DPD system for concurrent dual-band transmitters is developed. It is clearly proved in experiments that the proposed predistorter remarkably improves the in-band and out-of-band performances of both signals. Furthermore, it does not depend on frequency separation between frequency bands and has significantly lower complexity in comparison with previously reported concurrent dual-band DPDs

Long-Range Imaging Radar for Autonomous Navigation

This thesis describes the theoretical and practical implementation of a long-range high-resolution millimetre wave imaging radar system to aid with the navigation and guidance of both airborne and ground-based autonomous vehicles. To achieve true autonomy, a vehicle must be able to sense its environment, comprehensively, over a broad range of scales. Objects in the immediate vicinity of the vehicle must be classified at high resolution to ensure that the vehicle can traverse the terrain. At slightly longer ranges, individual features such as trees and low branches must be resolved to allow for short-range path planning. At long range, general terrain characteristics must be known so that the vehicle can plan around difficult or impassable obstructions. Finally, at the largest scale, the vehicle must be aware of the direction to its objective. In the past, short-range sensors based on radar and laser technology have been capable of producing high-resolution maps in the immediate vicinity of the vehicle extending out to a few hundred metres at most. For path planning, and navigation applications where a vehicle must traverse many kilometres of unstructured terrain, a sensor capable of imaging out to at least 3km is required to permit mid and long-range motion planning. This thesis addresses this need by describing the development a high-resolution interrupted frequency modulated continuous wave (FMICW) radar operating at 94GHz. The contributions of this thesis include a comprehensive analysis of both FMCW and FMICW processes leading to an effective implementation of a radar prototype which is capable of producing high-resolution reflectivity images of the ground at low grazing angles. A number of techniques are described that use these images and some a priori knowledge of the area, for both feature and image based navigation. It is shown that sub-pixel registration accuracies can be achieved to achieve navigation accuracies from a single image that are superior to those available from GPS. For a ground vehicle to traverse unknown terrain effectively, it must select an appropriate path from as long a range as possible. This thesis describes a technique to use the reflectivity maps generated by the radar to plan a path up to 3km long over rough terrain. It makes the assumption that any change in the reflectivity characteristics of the terrain being traversed should be avoided if possible, and so, uses a modified form of the gradient-descent algorithm to plan a path to achieve this. The millimetre wave radar described here will improve the performance of autonomous vehicles by extending the range of their high-resolution sensing capability by an order of magnitude to 3km. This will in turn enable significantly enhanced capability and wider future application for these systems

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

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

Intermodulation distortion performance enhancement of microwave power amplifiers

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis reports the author's investigation of the effects of the injection of specific signals on the intermodulation distortion performance of microwave power amplifiers. Theory, simulation and practical results are presented, analysed and compared. The thesis gives the reader background knowledge of power amplifiers and their nonlinearities and go on to analyse the phenomena of intermodulation distortion product generation in power amplifiers. The analysis is based on a three-tone test since this highlights a second kind of third order intermodulation distortion (IMD3), which are in general higher in amplitude than the first kind of IMD3 found in a two-tone test. A mathematical analysis and a simulation of a MESFET amplifier are performed. It enables the comparison of the performance of IMD cancellation by injection of signals whose frequencies are chosen to be first, the second harmonic of the fundamental signals, second, the sum of the fundamental signal frequencies and finally the difference frequencies of the fundamental signals. A practical implementation of the difference frequency technique is then presented and practical results are compared to the other two techniques of second harmonic injection and the injection of the sum of fundamental frequencies. It is further shown that in practise these two techniques may be considered as a single technique

Optical free space feedforward non-linearity correction system

Recent years have seen unprecedented growth in the popularity and deployment of mobile phones. As this continues, so the strain on existing mobile cellular radio network has also increased, leading to the need to investigate new technologies to relieve this pressure. The problem is being further exacerbated by the introduction of the 3rd generation of mobile communications, otherwise known as UMTS (Universal Mobile Telecommunications System), with the aim of offering multimedia services on pocket sized portable receivers. A major cost of the mobile radio network, in terms of both financial and social/environmental aspects, is the need apparent need for more base transceiver stations (BTS), due to the increased number of services, and the density of them. Therefore, judicious use of fewer, but more "intelligent" base stations, thereby reducing the overall system costs, and extra flexibility in the design of mobile cells would be desirable. This can be achieved by having the BTS antennas remotely positioned from the BTS by transmitting the radio signals down an optical fibre or, as in this project, over free space. The main application for this is in densely urban heavy use areas, where there is extensive reuse of both cell and cell cluster. This, along with building shadowing, would require a BTS on every corner, and where extra cell design flexibility would be desirable. Also, in remote rural areas, where various natural features, such as rivers or mountains can cause similar cell design problems, there is a need for this flexibility. The problem with this requirement is that the electrical to optical conversion process, involving a laser diode driver unit, is inherently non-linear, and, unless this is resolved, the desired signal will become unusable due to distortion. To overcome these nonlinearities, a novel correction may be used, based on an optical feedforward correction technique. The prototype system employs off-the-shelf components, and has one Fabry Perot laser diode (FP-LD) providing two signals (via a beam splitter), for a main path and one for the error path loop. The error path signal is detected by a receiver circuit, then mixed with a reference signal to produce a 'pure' error signal, which then modulates the second FP-LD. In contrast with previous fibre feedforward systems, where the two LD outputs are then combined in the optical fibre pre-reception, this system has to combine the signals post-reception. After the main signal and error signal are received and recombined, the non-linearities of the main path are predominantly cancelled by those present in the error path signal, leaving only the desired signal, free of non-linearities