Adaptive IIR Filters for Single Interference Suppression in a BPSK DS CDMA System In Rayleigh Fading Channel

In this paper, effect of a single narrow-band interference (NBI) on bit error rate (BER) performance for a binary phase shift  keying  (BPSK)  synchronous  direct-sequence  code-division  multiple  access  (DS  CDMA)  communication system operating  in  a  frequency  nonselective  Rayleigh  fading  channel  is  analyzed.  Second-order  adaptive  infinite  impulse response  (IIR)  notch  filters  with  plain  gradient  algorithm  (GA)  for  suppression  of  NBI  in  the  DS  CDMA system  are proposed. A general closed-form BER expression for the DS CDMA system with NBI suppression second order adaptive IIR  notch  filters  is  derived  based  on  the  standard Gaussian  approximation  (SGA)  method.  BER  expressions  are  then derived  for  the  allpass  filter-based  adaptive  IIR  notch  filter  and  adaptive  IIR  notch  filter  with  constrained  poles  and zeros,  the  two  structures  that  are  commonly  found  in  literature.  It  is  observed  that  both  adaptive  IIR  notch  filter structures  exhibit  comparable  BER  performance.  Extensive  computer  simulation  results  are  presented  to verify  the accuracy and limitations of the analysis

Adaptive Interference Mitigation in GPS Receivers

Satellite navigation systems (GNSS) are among the most complex radio-navigation systems, providing positioning, navigation, and timing (PNT) information. A growing number of public sector and commercial applications rely on the GNSS PNT service to support business growth, technical development, and the day-to-day operation of technology and socioeconomic systems. As GNSS signals have inherent limitations, they are highly vulnerable to intentional and unintentional interference. GNSS signals have spectral power densities far below ambient thermal noise. Consequently, GNSS receivers must meet high standards of reliability and integrity to be used within a broad spectrum of applications. GNSS receivers must employ effective interference mitigation techniques to ensure robust, accurate, and reliable PNT service. This research aims to evaluate the effectiveness of the Adaptive Notch Filter (ANF), a precorrelation mitigation technique that can be used to excise Continuous Wave Interference (CWI), hop-frequency and chirp-type interferences from GPS L1 signals. To mitigate unwanted interference, state-of-the-art ANFs typically adjust a single parameter, the notch centre frequency, and zeros are constrained extremely close to unity. Because of this, the notch centre frequency converges slowly to the target frequency. During this slow converge period, interference leaks into the acquisition block, thus sabotaging the operation of the acquisition block. Furthermore, if the CWI continuously hops within the GPS L1 in-band region, the subsequent interference frequency is locked onto after a delay, which means constant interference occurs in the receiver throughout the delay period. This research contributes to the field of interference mitigation at GNSS's receiver end using adaptive signal processing, predominately for GPS. This research can be divided into three stages. I first designed, modelled and developed a Simulink-based GPS L1 signal simulator, providing a homogenous test signal for existing and proposed interference mitigation algorithms. Simulink-based GPS L1 signal simulator provided great flexibility to change various parameters to generate GPS L1 signal under different conditions, e.g. Doppler Shift, code phase delay and amount of propagation degradation. Furthermore, I modelled three acquisition schemes for GPS signals and tested GPS L1 signals acquisition via coherent and non-coherent integration methods. As a next step, I modelled different types of interference signals precisely and implemented and evaluated existing adaptive notch filters in MATLAB in terms of Carrier to Noise Density (\u1d436/\u1d4410), Signal to Noise Ratio (SNR), Peak Degradation Metric, and Mean Square Error (MSE) at the output of the acquisition module in order to create benchmarks. Finally, I designed, developed and implemented a novel algorithm that simultaneously adapts both coefficients in lattice-based ANF. Mathematically, I derived the full-gradient term for the notch's bandwidth parameter adaptation and developed a framework for simultaneously adapting both coefficients of a lattice-based adaptive notch filter. I evaluated the performance of existing and proposed interference mitigation techniques under different types of interference signals. Moreover, I critically analysed different internal signals within the ANF structure in order to develop a new threshold parameter that resets the notch bandwidth at the start of each subsequent interference frequency. As a result, I further reduce the complexity of the structural implementation of lattice-based ANF, allowing for efficient hardware realisation and lower computational costs. It is concluded from extensive simulation results that the proposed fully adaptive lattice-based provides better interference mitigation performance and superior convergence properties to target frequency compared to traditional ANF algorithms. It is demonstrated that by employing the proposed algorithm, a receiver is able to operate with a higher dynamic range of JNR than is possible with existing methods. This research also presents the design and MATLAB implementation of a parameterisable Complex Adaptive Notch Filer (CANF). Present analysis on higher order CANF for detecting and mitigating various types of interference for complex baseband GPS L1 signals. In the end, further research was conducted to suppress interference in the GPS L1 signal by exploiting autocorrelation properties and discarding some portion of the main lobe of the GPS L1 signal. It is shown that by removing 30% spectrum of the main lobe, either from left, right, or centre, the GPS L1 signal is still acquirable

A Novel Optimization Algorithm for Notch Bandwidth in Lattice Based Adaptive Filter for the Tracking of Interference in GPS

The weak signal levels experienced at the reception of the messages transmitted by navigation satellites, makes Global Positioning System (GPS) vulnerable to unintentional and intentional interference. This calls for appropriate modelling of GPS signal sources and jammers to assess the anti-jamming and interference mitigation capabilities of algorithms developed to be implemented for GPS receivers. Using a practical simulation model, this work presents an anti-jamming technique based on a novel algorithm. A fully adaptive lattice based notch filter is presented that provides better performance when compared to existing adaptive notch filter based techniques, chosen from the literature, in terms of convergence speed whilst delivering superior performance in the excision of the interference signal. To justify the superiority of the proposed technique, the noise and interference signal power is varied for in a wide dynamic range assessing jamming-to-noise density versus effective carrier-to-noise density performance at the output of the correlator

A Fully Adaptive Lattice-based Notch Filter for Mitigation of Interference in GPS

Intentional interference presents a major threat to the operation of the Global Navigation Satellite Systems. Adaptive notch filtering provides an excellent countermeasure and deterrence against narrowband interference. This paper presents a comparative performance analysis of two adaptive notch filtering algorithms for GPS specific applications which are based on Direct form Second Order and Lattice-Based notch filter structures. Performance of each algorithm is evaluated considering the ratio of jamming to noise density against the effective signal to noise ratio at the output of the correlator. A fully adaptive lattice notch filter is proposed, which is able to simultaneously adapt its coefficients to alter the notch frequency along with the bandwidth of the notch filter. The filter demonstrated a superior tracking performance and convergence rate in comparison to an existing algorithm taken from the literature. Moreover, this paper describes the complete GPS modelling platform implemented in Simulink too

Narrowband Interference Suppression in Wireless OFDM Systems

Signal distortions in communication systems occur between the transmitter and the receiver; these distortions normally cause bit errors at the receiver. In addition interference by other signals may add to the deterioration in performance of the communication link. In order to achieve reliable communication, the effects of the communication channel distortion and interfering signals must be reduced using different techniques. The aim of this paper is to introduce the fundamentals of Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA), to review and examine the effects of interference in a digital data communication link and to explore methods for mitigating or compensating for these effects

Filtering techniques for mitigating microwave oven interference on 802.11b wireless local area networks

Thesis (M.Eng. and S.B.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.Includes bibliographical references (p. 165-169).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.With the increasing popularity and assimilation of wireless devices into the everyday lives of people, the issue of their feasibility for coexisting with other radio frequency (RF) devices arises. Particularly strong interferers for the IEEE 802.11b standard are microwave ovens, since both operate at 2.4 GHz. The interference mitigation techniques all exploit the differences between the interference and the signal, since the former is sinusoidal in nature while the latter can be viewed as noise. The first mitigation filter operates in the frequency domain and filters the received signal's Fast Fourier Transform (FFT) sequence by detecting and removing peak sinusoidal components over the flat 3- dB bandwidth of the signal. The second is a Least Mean Square (LMS) Adaptive filter that produces an estimate of the interference through a recursive approximation method and subtracts it out from the received signal. The third and last is the Adaptive Notch Filter (ANF) which implements a lattice structure and has a time-varying notch frequency parameter that converges to and tracks the frequency of the interference in the received signal. The three filters are shown to produce improvements in the bit error rate (BER) and frame error rate (FER) performance of the receiver under various relative strengths of the signal with respect to the interference.by Lorenzo M. Lorilla.M.Eng.and S.B

Hardware Implementation Of Tunable Heterodyne Band-Pass Filters

Modem wireless and satellite communication systems make use of spreadspectrum modulation concepts such as Frequency-hopping spread-spectrum (FHSS) and Direct sequence spread-spectrum (DSSS). The spread-spectrum modulation method inherently possesses anti-jamming and anti-interception properties due to the fact that the narrowband information signal is spread over a wide range of frequencies, masking the information-bearing signal as noise. Despite these properties, these communication channels can be severely corrupted by high-powered narrowband interference signals generated by local FM or AM transmitters which may cause complications when detecting the information signal at the receiver. Therefore, the communication system is made more efficient with the use of signal processing techniques for narrowband interference attenuation. Control systems is another area where the presence of narrowband interference signal due to mechanical resonance can be responsible for causing distortion in information signal. Any Band-pass, High-pass or a Low-pass filter may be converted into a tunable filter through the use of new Tunable Heterodyne Band-pass Filter concept in which the frequency of the heterodyne signal is adjusted thereby creating the effect of translating the entire transfer function of the fixed filter in frequency. In this thesis, hardware implementation techniques and results of the new Digital Tunable Heterodyne Band-pass filter is proposed that allows a prototype IIR or FIR filter to be shifted through the entire range of digital frequencies with a single parameter, the heterodyning frequency. The unique property of this new tunable filter is the range of tunability it possesses. With this technique, the fixed filter is tuned continuously using the concept of frequency translation. The images created by the heterodyne process are cancelled without the use of image canceling filters, which significantly contribute towards a hardware efficient design. In this thesis, simulation results are observed to illustrate the effects ofhaving the fixed prototype filter as a band-pass, high-pass, low-pass or notch filter. This thesis concentrates on the hardware implementation of the tunable heterodyne filter structure with a band-pass filter as the fixed prototype filter. Thus, simulation and experimental results show that if the fixed filter is a narrowband Bandpass filter, a much hardware efficient implementation can be achieved by using the new Tunable Heterodyne Band-pass filter to extract the narrowband interference from broadband communication or control systems as compared to the standard techniques used. The proposed heterodyne filter is suitable both as a tunable filter or to be implemented with standard algorithms to design adaptive digital filters. The new structure proposed is composed of three main components which can be implemented using Field Programmable Gate Arrays (FPGA) or easily be retargeted for an Application Specific Integrated Circuits (ASIC) standard cell technology or custom designed for Very Large Scale Integration (VLSI) processes. A prototype system is implemented using a single chip Xilinx Virtex Series Field Programmable Gate Arrays (FPGA) and thesimulation results are compared with the hardware data