Compressive Sensing for Spread Spectrum Receivers

With the advent of ubiquitous computing there are two design parameters of wireless communication devices that become very important power: efficiency and production cost. Compressive sensing enables the receiver in such devices to sample below the Shannon-Nyquist sampling rate, which may lead to a decrease in the two design parameters. This paper investigates the use of Compressive Sensing (CS) in a general Code Division Multiple Access (CDMA) receiver. We show that when using spread spectrum codes in the signal domain, the CS measurement matrix may be simplified. This measurement scheme, named Compressive Spread Spectrum (CSS), allows for a simple, effective receiver design. Furthermore, we numerically evaluate the proposed receiver in terms of bit error rate under different signal to noise ratio conditions and compare it with other receiver structures. These numerical experiments show that though the bit error rate performance is degraded by the subsampling in the CS-enabled receivers, this may be remedied by including quantization in the receiver model. We also study the computational complexity of the proposed receiver design under different sparsity and measurement ratios. Our work shows that it is possible to subsample a CDMA signal using CSS and that in one example the CSS receiver outperforms the classical receiver.Comment: 11 pages, 11 figures, 1 table, accepted for publication in IEEE Transactions on Wireless Communication

DEVELOPMENT OF AN UWB RADAR SYSTEM

An ultra-wideband radar system is built at the University of Tennessee with the goal to develop a ground penetrating radar (GPR). The radar is required to transmit and receive a very narrow pulse signal in the time domain. The bistatic radar transmits a pulse through an ultrawide spiral antenna and receives the pulse by a similar antenna. Direct sampling is used to improve the performance of the impulse radar allowing up to 1.5 GHz of bandwidth to be used for signal processing and target detection with high resolution. Using direct sampling offers a less complex system design than traditional lower sample rate, super-heterodyne systems using continuous wave or step frequency methods while offering faster results than conventional equivalent time sampling techniques that require multiple data sets and significant post-processing. These two points are particularly important for a system that may be used in the field in potentially dangerous environments. Direct sampling radar systems, while still frequency limited, are continually improving their upper frequencies boundaries due to more power efficient, higher sampling rate analog to digital converters (ADCs) which relates directly to better subsurface resolution for potential target detection

Modular Software-Defined Radio

<p>In view of the technical and commercial boundary conditions for software-defined radio (SDR), it is suggestive to reconsider the concept anew from an unconventional point of view. The organizational principles of signal processing (rather than the signal processing algorithms themselves) are the main focus of this work on modular software-defined radio. Modularity and flexibility are just two key characteristics of the SDR environment which extend smoothly into the modeling of hardware and software. In particular, the proposed model of signal processing software includes irregular, connected, directed, acyclic graphs with random node weights and random edges. Several approaches for mapping such software to a given hardware are discussed. Taking into account previous findings as well as new results from system simulations presented here, the paper finally concludes with the utility of pipelining as a general design guideline for modular software-defined radio.</p

What are the limits to increasing the dynamic range of RF ADCS?

The dynamic range of an analogue to digital converter (ADC) can be specified in several ways which are application dependent. This problem has been addressed in industry by the International Electro-technical Commission (IEC) standard for dynamic specifications for ADCs [1]. The most often used specification for RF applications is Spurious Free Dynamic Range (SFDR). This is specified in the frequency domain through analysis of a suitable test signal such as two tones of equal amplitude. SFDR then indicates the range in amplitude between the maximum test signal amplitude and the amplitude of any unwanted or spurious signal. These spurious signals may be related to the presence of the test signal or they may be from other sources

Linearization techniques to suppress optical nonlinearity

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis is shown the implementation of the linearization techniques such as feedforward and pre-distortion feedback linearization to suppress the optical components nonlinearities caused by the fibre and semiconductor optical amplifier (SOA). The simulation verified these two linearization techniques for single tone direct modulation, two tone indirect modulation and ultra wideband input to the optical fibre. These techniques uses the amplified spontaneously emission (ASE) noise reduction in two loops of SOA by a feed-forward and predistortion linearizer and is shown more than 6dB improvement. Also it investigates linearization for the SOA amplifier to cancel out the third order harmonics or inter-modulation distortion (IMD) or four waves mixing. In this project, more than 20 dB reductions is seen in the spectral re-growth caused by the SOA. Amplifier non-linearity becomes more severe with two strong input channels leading to inter-channel distortion which can completely mask a third adjacent channel. The simulations detailed above were performed utilizing optimum settings for the variable gain, phase and delay components in the error correction loop of the feed forward and Predistortion systems and hence represent the ideal situation of a perfect feed-forward and Predistortion system. Therefore it should be consider that complexity of circuit will increase due to amplitude, phase and delay mismatches in practical design. Also it has describe the compatibility of Software Defined Radio with Hybrid Fibre Radio with simulation model of wired optical networks to be used for future research investigation, based on the star and ring topologies for different modulation schemes, and providing the performance for these configurations

Quadrature sigma-delta modulators for reconfigurable A/D interface and dynamic spectrum access: analysis, design principles and digital post-processing

In the course of development of wireless communications and its modern applications, such as cloud technologies and increased consumption and sharing of multimedia, the radio spectrum has become increasingly congested. However, temporarily and spatially underused spectrum exists at the same time. For increasing the efficiency of spectrum usage, the concept of dynamic spectrum access (DSA) has been proposed. Ultimately, the DSA principle should be exploited also in cognitive radio (CR) receivers. Herein, this paradigm is approached from the receiver architecture point-of-view, considering software-defined radio (SDR) as a platform for the future DSA and CR devices. Particularly, an analog-to-digital converter (ADC) architecture exploiting quadrature ΣΔ modulator (QΣΔM) is studied in detail and proposed as a solution for the A/D interface, being identified as a performance bottleneck in SDRs. By exploiting a complex valued noise transfer function (NTF) enabled by the QΣΔM, the quantization precision of the ADC can be efficiently and flexibly focused on the frequency channels and the signals to be received and detected. At the same time, with a traditional non-noise-shaping ADC, the precision is distributed equally for the whole digitized frequency band containing also noninteresting signals. With a single QΣΔM, it is also possible to design a multiband NTF, allowing reception of multiple noncontiguous frequency channels without parallel receiver chains. Furthermore, with the help of digital control, the QΣΔM response can be reconfigured during operation. These capabilities fit in especially well with the above mentioned DSA and CR schemes, where the temporarily and spatially available channels might be scattered in frequency. From the implementation point-of-view, the effects of inherent implementation inaccuracies in the QΣΔM design need to be thoroughly understood. In this thesis, novel closed-form matrix-algebraic expressions are presented for analyzing the transfer functions of a general multistage QΣΔM with arbitrary number of arbitrary-order stages. Altogether, the signal response of an I/Q mismatched QΣΔM has four components. These are the NTF, an image noise transfer function, a signal transfer function (STF) and an image signal transfer function. The image transfer functions are provoked by the I/Q mismatches and define the frequency profile of the generated mirror-frequency interference (MFI), potentially deteriorating the quality of the received signal. This contribution of the thesis increases the understanding of different QΣΔM structures and allows the designers to study the effects of the implementation inaccuracies in closed form. In order to mitigate the MFI and improve the signal reception, a mirror-frequency rejecting STF design is proposed herein. This design is found to be effective against I/Q mismatches taking place in the feedback branches of the QΣΔM. This is shown with help of the closed-form analysis and confirmed with computer simulations on realistic reception scenarios. When a mismatch location independent MFI suppression is the desired option, it is a logical choice to do this processing in a digital domain, after the whole analog receiver front-end. However, this sets demands for the information to be digitized, i.e., the source of the MFI should be available also in the digital domain. For this purpose, a novel multiband transfer function design is proposed herein. In addition, a QΣΔM specific digital MFI compensation algorithm is developed. The compensation performance is illustrated in practical single- and multiband reception scenarios, considering desired signal bandwidths up to 20 MHz. In the multiband scenario, allowing reception and detection of noncontiguous frequency channels with a single receiver chain, the digital compensation processing is done sub-bandwise, securing reliable functionality also under strongly frequency-selective interference. In the applied single- and multistage QΣΔM architectures, the I/Q mismatches are considered in all the QΣΔM branches as well as in the preceding receiver front-end, modeling the challenging and realistic scenario where the whole receiver chain includes cascaded in-phase/quadrature (I/Q) mismatch sources. As a whole, developing digital MFI compensation is a significant step towards practical receiver implementations with QΣΔM ADCs. In consequence, this allows the exploitation of the multiband and reconfigurability properties. The proposed design can be implemented without additional analog components and is straightforwardly reconfigurable in dynamic signal conditions typical for DSA and CR systems, e.g., in case of frequency hand-off because of a primary user appearance. In addition, the digital post-compensation of the MFI eases the strict demands for the matching of the analog circuits in SDRs