HF Radio-Frequency Interference Mitigation Gang Chen,Zhengyu Zhao,Guoqiang Zhu,Yujie Huang,and Ting Li
Abstract—The den radio-frequency interference(RFI)dis-tributed on the spectrum in the high-frequency band greatly degrades the performance of radio systems and makes it urgent to find an effective and speedy method to eliminate the interferences. The interference mitigation method introduced in this letter is implemented before any further signal processing.It reconstructs the interference by the exactly estimated frequency,amplitude, and pha parameters in the signal spectrum and then subtracts the artificial interference from the input time-domain complex signal.The procedure steps of the method are illustrated in de-tail.The data stream of continuous oblique-incident ionospheric detection has been procesd effectively and rapidly in real time. Several delay-Doppler ionograms verely contaminated by RFI are lected and prented.Comparisons of the original and mit-igated ionograms and the signal-to-noi ratio(SNR)data show that the RFIs are eliminated perfectly and that the SNR is greatly improved.英语语法学习
Index Terms—High-frequency(HF)radar,interference sup-pression,ionogram,radar signal processing.
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后裔的意思I.I NTRODUCTION
W HEN Marconi succeeded with thefirst radio commu-nication between the U.K.and Canada in1901,there were no problems with radio-frequency interference(RFI).Due to the incread u of radio waves for communication and detection,there are more and more radio energy congestion in the environment around us.Like all other radio systems, the ground-bad ionospheric remote-nsing systems need to share frequency bands,which are denly populated due to the world wide propagation and u of radio frequencies.Particu-larly in the high-frequency(HF)band(3–30MHz),there is a strong influence of the Earth’s ionosphere,which reflects radio waves to enable over-the-horizon propagation.On the other hand,other signals in all directions reflected by the ionosphere or propagating along Earth’s surface form inten RFIs to limit the HF radar’s performance in range and time coverage.
For high data rate or pul compression gain,wideband radio systems are widely applied.The large bandwidth makes the receiver susceptible to a variety of contaminating radio emis-sions that are received together with the signal.A wideband signal is usually mixed with two kinds of radio contamina-tions,namely,wideband background noi and narrowband interferences.The background noi can be mitigated by many
Manuscript received October1,2009;revid November10,2009.Date of publication February8,2010;
踟蹰怎么读date of current version April29,2010.This work was supported by the National Natural Science Foundation of China (40804042).
The authors are with the School of Electronic Information,Wuhan Univer-sity,Wuhan430079,China(e-mail:g.chen@whu.edu).
Color versions of one or more of thefigures in this paper are available online at ieeexplore.ieee.
Digital Object Identifier10.1109/LGRS.2009.2039340methods such as signal integration/accumulation,pul coding, or incread transmission power.However,it is a significant challenge to remove narrowband interferences[1],[2],which are typically signals from other transmitters(in many cas, from broadcasting stations)with unpredictable occurrences at a priori unknown frequencies.
In ionospheric remote nsing,detection is rapidly repeated, and real-time data processing and ionogram display are re-quired.Therefore,data processing algorithms with high per-formance,but are time consuming,are inapplicable.In this letter,a speedy and effective RFI mitigation method is intro-duced,and its performance has been tested with the Wuhan Ionospheric Oblique-Incident Sounding 英语学习王
System(WIOISS)[3]. The recorded data without and with RFI mitigation are dis-played and compared.rubberband
II.P RINCIPLE OF RFI M ITIGATION
The frequency,occurrence time,and pattern of the inter-ference are all unpredictable,and it is a common ca that veral different interferences contaminate the signal at the same time.A wideband signal was recorded with12.2-MHz operating frequency,39-kHz bandwidth,and64repeated times, and the signal spectrum for one time is shown in Fig.1(a).The wideband signal is located between[−19.5kHz,+19.5kHz] frequency range,and there are two powerful interferences mix-ing in.One distributes between−15.12and−12.86kHz,and the other appears on9.6-kHz frequency.After pul compres-sion and frequency integration,the delay-Doppler ionogram is obtained with[−5.1Hz,+5.1Hz]Doppler range,as shown in Fig.1(b).The energy of the signal is integrated on time and frequency,respectively,to form peaks on certain range bins and spectral lines.The energy of the interferences is folded on the narrow Doppler spectrum to form the bandings arranged along the range bins or the spots distributed on the delay-Doppler ionogram.
Comparing Fig.1(a)and(b),the features of the RFI are more obvious in the signal spectrum;therefore,
the RFI mit-igation algorithm is ud to remove the interferences before pul compression and spectral transformation are done.The algorithm divides the sampled data into data ts with very short periods.The narrowband interference mixed in one short data t can be regarded as a pure continuous wave with constant amplitude and pha,regardless of the modulation pattern.We obtain the exact frequency,pha,and amplitude parameters of interference to reconstruct it and then subtract it from the time-domain complex signal.
First,the fast Fourier transform(FFT)calculates the signal spectrum of one data t.Given that the signal spectrum is compod by N spectral lines with interval f s,the interference
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Fig.1.Typical example of(a)contaminated signal spectrum and(b)delay-Doppler ionogram.coreana
frequency f I will not be a multiple integral of f s generally but can be calculated by[4]
f I=f A−f s·
B
A+B
·f A
|f A|(1)
where f A is the frequency of the highest spectral line of interference and A is the corresponding amplitude.There are two spectral lines neighboring the highest one on the right and left,and the amplitude B comes from the one clo to the center. Once the preci interference frequency isfigured out,a signal-line FFT determines the amplitude and pha of the interference
˜C I =
1
N+1
N
n=0
˜S
n
exp(j2πf I n)(2)
where˜C I is the complex spectral amplitude,˜S n is the time samples of the complex signal,and N is the total number of samples.The inver transform of(2)gives the preci time-domain prentation of the interference
˜I
n
=˜C I exp(−j2πf I n).(3) Finally,RFI mitigation is simply to subtract˜I n from the time-domain complex s
ignal.
The mitigation procedure steps are summarized in Fig.2. The algorithm is applied to the strongest spectral line,whether it comes from the interference or not.The interference
recon-Fig.2.Procedure steps for RFI mitigation.
struction and subtraction will be constantly repeated until the amplitude A n of the strongest spectral line begins to increa. In undisturbed conditions,the strongest special line cannot be decread,and so,the procedure steps will end in thefirst cycle and will output the unmitigated data.
The RFI mitigation method has been tested with Wuhan ionospheric detection systems.It is proved to be an effective and speedy method by processing the data stream of continuous ionospheric detection in real time.wagons
III.D ATA P ROCESSING AND C OMPARISON
The ionospheric obrvation station in Wuhan University is in the downtown area,and all kinds of interferences are jamming in the HF radio spectrum.WIOISS implemented the swept-frequency detection from6to25MHz with200-kHz step tofind the contaminated ionograms on March28,2009. The detection signal of each operating frequency was the 255-bit pha coding pul train with76.8-μs pulwidth and 20%duty cycle.The pul trains were transmitted from Hainan province,reflected by the F-layer ionosphere and then received by the HF receiver in Wuhan with39-kHz bandwidth.A global positioning system was applied for clock and frequency standard synchronization between the
transmitter and the receiver.After pul compression and FFT,the delay-Doppler ionograms of each operating frequency were obtained withsport
CHEN et al.:HF RADIO-FREQUENCY INTERFERENCE MITIGATION481
Fig.3.Comparison of the(right)original and(left)mitigated delay-Doppler ionograms.The operation frequencies of each data are9.8,11.6,12.6,13.6,14,and 15.4MHz,respectively.
482IEEE GEOSCIENCE AND REMOTE SENSING LETTERS,VOL.7,NO.3,JULY2010 TABLE I
SNR OF THE O RIGINAL AND M ITIGATEDbilly joel
S IGNALS AND SNR I MPROVEMENT
