Method and system for using orthogonal space projections to mitigate interference

METHOD ANO SYSTEM FOR USING ORTHOGONAL SPACE PROJECTIONS TO MITIGATE INTERFERENCE

CROSS-REFERENCE TO RELATED APPUCATIONS

[0001] This application is a nonprovisional application claiming the benefit of the filing date of a provisional application having application serial number 61/638,934, filed on April 26,2012 and entitled A Method and Apparatus for the Mitigation of Signal Interference, and of the filing date of a provisional application having application serial number 61/779,531; filed on March 13,2013 and entitled Orthogonal Space Projection For Mitigation interference.

TECHNICAL FIELD OF THE INVENTION

[0002] The invention relates to mitigating of interfering electromagnetic signals. More particularly, the invention relates to methods and systems for dynamically identifying and mitigating interfering electromagnetic signals in real-time by using one or more orthogonal projections of a received signa! to isolate and mitigate the interference signals.

BACKGROUND OF THE INVENTION

[0003] Electromagnetic interference occurring in a receiver modifies or disrupts a target electromagnetic signal in the receiver, resulting in degradation of the target signal. The interference may be natural or man-made. Natural electromagnetic interference sources include thermal noise sources, pulses emitted by lightning, astronomical sources, etc. Man-made electromagnetic interference sources may be unintentional sources, such as interference generated by industrial processes and household appliances, or intentional sources, such as jammers designed to reduce the effectiveness of a system, such asa cell phonesystem ora radar jammer system.

[0004] Such interference can seriously degrade the performance of a system that is configured to receive particular signals. For example, for a radar qr sonar system, interference can result in a failure to detect an imminent threat. For navigation and position receivers, interference can result in loss of accuracy or outright jamming. For communication systems, interference can increase the bit error rate.

235207/2 (0005] Work on mitigating electromagnetic interference dates back to the earliest days of radio.

Until approximately 1950, the majority of the work consisted of means and techniques for minimizing natural and mutual interference. Post 1950, efforts began in earnest on mitigating the effect of purposeful, man-made interference.

[0006] Known techniques and systems for mitigating interference typically use the ergodic, or statistical, properties of the interference with respect to the desired signal to identify the interference and the desired signal and to create a weighting function that optimizes a cost function. Separating the signal from the interference in this manner requires averaging the signal over a time period in order to estimate signal statistics. The averaging time period is iong relative to the reciprocal bandwidth of the desired signal and the interference signal. Requiring the summation over a relatively long time period assumes that the statistics of both the interference and the desired signal are stationary for the time period, which is not always a valid assumption and can lead to undesirable mitigation resuits.

U.S. Patent Application Publication No. 20120016921 Al describes a method and apparatus for compressive domain filtering and interference cancellation. All signals of interest are captured and processed together with interference in a small set of compressive samples. The interference is removed or attenuated from the compressive measurements. The step of obtaining compressive measurements applies a random or pseudorandom projection operator or a valid compressive sensing matrix. A random or pseudorandom projection could introduce undesired noise or interference that under some conditions may combine with a signal of interest.

U.S. Patent Application Publication No. 20020090025 Al is premised upon an algorithm involving integration of oblique correlators and RAKE filtering to nullify interference from spread spectrum signals. The oblique correlator is based on the non-orthogonal projections that are optimum for nulling structured signals such as spread spectrum signals. RAKE filtering is used to rapidly Steer a beam of the multi-antenna system and to mitigate effects of multi-path. Conventional RAKE filters do not operate to suppress interference and rather maximize a signal to interference plus noise ratio.

U.S. Patent No. 7,978,759 describes systems and techniques for processing information received from a spatially diverse transmission transmitted over multiple subcarriers. The techniques include recursively computing a slgnaPto-noise ratio, recursively updating a diagonal kernel matrix and generating an equalization matrix from the updated kernel matrix, the equalization matrix used in equalizing the received signal across the multiple subcarriers. The time required to recursively compute a signaPto-noise ratio, update a diagonal kerne! matrix and equalize multiple received signals across the subcarriers can lead to undesirable delays in identifying a signal of interest in an environment with interference.

235207/2 (0007] Accordingly, a need exists for a method and system for mitigating interference that overcome the shortcomings of known systems and methods.

SUMMARY OF THE INVENTION

[0008] The invention is directed to systems and methods for mitigating electromagnetic interference. In accordance with an illustrative embodiment, the system comprises M antenna efementsand M electrical processing circuits electrically coupled to M respective antenna elements of the array, where M is a positive integer that is great than or equal to 1. Each electrical processing circuit receives an electrical signal received by the respective antenna element and performs at least a first projection operation on the respective received electrical signal to project the received electrical signal into a respective orthogonal projection space that is orthogonal to, or nearly orthogonal to, a respective reference signal. A respective target signal and a respective interference signal are present in the respective received electrical signal, whereas the interference signal, but not the target signal, is present in the respective orthogonal projection space. At least one of the M electrical processing circuits performs a separation method that processes at least the orthogonal projection spaces to separate the interference signals from the target signals and to mitigate the interference signals.

[0009] In accordance with an illustrative embodiment the method comprises:

with an array of M spatially-separated antenna elements, receiving electrical signals;

with IVI electrical processing circuits electrically coupled to M respective antenna elements of the array of antenna elements: receiving an electrical signal from the respective antenna element in response to the respective antenna element receiving a respective electrical signal, performing at least a first projection operation on the respective received electrical signal to project the received electrical signal into a respective orthogonal projection space that is orthogonal to, or nearly orthogonal to, a respective reference signal, wherein a respective target signal and a respective interference signal are present in the respective received electrical signal, and wherein the interference signal, but not the target signal, is present in the respective !mage space, and with at least one of the M electrical processing circuits, performing a separation method that processes at least the orthogonal projection spaces to separate the interference signals from the target signals and to mitigate the interference signals.

235207/2

[0010] The invention also is directed to a npn-transitory computer-readable medium having code thereon for execution by a processor. The code includes first, second and third code segments. The first code segment receives a plurality of electrical signals that have been received by respective antenna elements of an array of M spatially-separated antenna elements, where M is a positive integer that is great than or equal to 1, The second code segment projects each received electrical signal into an orthogonal projection space that is orthogonal to, or nearly orthogonal to, a respective reference signal. A respective target signal and a respective interference signal are present in the respective received electrical signal, whereas the interference signal, but not the target signal, is present in the respective Orthogonal projection space. The third code Segment performs a separation method that processes at least the orthogonal projection spaces to separate the interference signals from the target signals and to mitigate the interference signals.

[0011] These and other features and advantages of the invention will become apparent from the following description, drawings and claims,

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Fig. 1 is a block diagram of one illustrative embodiment of an OSP System designed to mitigate interference in a phased array radar configuration.

[0013] Fig. 2. is a processing flow diagram that demonstrates OSP method performed by the system shown in Fig. 1.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

[0014] The invention is directed to various embodiments of systems and methods for mitigating natural and man-made interference through the use of one or more orthogonal, or nearlyorthogonal, subspace projections of the received signal, which is assumed to be contaminated with interference, into one or more image, or separation., spaces based on properties of the signal of interest, Once separated into image spaced), the system and method use information contained in the image space(s) to separate the signal of Interest, or target signal, from the interference and to mitigate the interference. The projection operation(s) separates the received signal, which Includes the target signal and interference, from the interference by projecting the received signal into an orthogonal subspace that is orthogonal, or neariy orthogonal, to the target signal. By definition, tne portion of the interference that remains after this orthogonal projection has been performed does not contain a significant amount of signal energy. A second projection operation that is matched to the target signal may also be performed on the received signal, or on the received signal after it has had the interference mitigated, but this is not always necessary or useful.

SR TS REF. N0. 16025.0003112

Rts! Some exmnplcs of signal spaces that are Useful for the projection operation are code spaces, frequency spaces. and time spaces. Some examples of signal spaces that are useful for image separation are angle, range and frequency. There are several advantages to the orthogonal space projection (OSP) approach described herein. One advantage is that the use of orthogonal projections produces a fepresentation of the interference that i s truly isolated from the target signal in that the projection containing the interference is orthogonal to, or nearly orthogonal to. the target, signal, This enables subsequent operations to be performed that optimally or nearly optimally mmove the interference. Another advantage is that the projection operation can be orthogonal to a large class of target signals, which makes it well suited for mtovfeg interference from :multiple target signals. Another advantage is that the orthogonal projection requires only one projection processing intervah which may be, for exampie, the compression interval of a spread spectrum signal This obviates the need to perform time averaging processes that depend on the ergodic nature of the interference with respect to desired signal to identify the interference and the desired signal and then create a weighting function that optimizes a cost function. [Q016J Various iltamrmive enibodiments are de®nh®d herein, incl uding, for example, embodiments that use orthogonal or nearly orthogonal projections both In modulation-angle subspaces as well as rmtge-Duppler subspaces, and embodiment thai use Only a single orthogonal or nearly orthogonal projeetirm operation on a set of reeeiwd signals to reduce the imerferenee for a large class of signals. As used herein, the following terms have the following meanings. The term “'matched projection’’ refers to a prajeciion that reaches its maximam value when opcratmg 0:11 the target signal, or ׳signal of interest. The terms ''orthogonal projection״ and fofos-matched projection” refer to a projectiun tliai is orthogonal to: or nearly orthogonal to a matched projection. The form “Image space׳’ refers to a parameter space representotion of the signal alter the projection operation(js) has been performed.

ίόόΠ] The O&P approach o f t he invention also addresses the ease where interference from one or more sources is present. Lefrmg Π., and 110 denote initial parameter spaces anti Jetting fh and Q$ denote image spaces, the signa! function 5 and interference function ./ that map the parameter space Ifo x 1¾ into the image space can be defined as:

£ ·' Ifo, d'L -> Ω, X ifo cr C*““'

J: 1fr x Ifo. -> Ω״

Signal

Imerferenee (0 (2}

4־23510

SRTS REF. NO. J 6025.000302

- 5 · J Signal plus taerferen.ee (3)

A sal of referen ce signal vectors is defined as «(¾ R₂ ll, .,. Λ. ].where L < N and for each i

R, m_׳J -> £1 c O'׳, wherep_־ e Π ,, > R; can be formed as shifted versions of a reference signal that is direetly related to the signal where p; is related to the relative degree of mismatch associated with each R! and the reference vector R. The vector p ··· [yx ρ, p. ... p; ] determines the admissible parameters that define the prefimage spaceH'^, c C . Also a set of vectors. If, where each vector Is orthogonal or nearly orthogonal to the vector £ in C^;v can be formed. Ihus. A' - fo:S;¥ and R~ are linearly mdependent. ft should be noted that this is not the only way to create 2?־ . Another example would be any set of K. waveforms that are orthogonal or nearly orthogonal to R. and linearly independent of one another. An operation can then be defined that takes the received signal and projects it. into a space that is parallel to the reference vector. 1ft addition, a set of operations can be defined that project the received signal info a set of spaces, each parallel, to a vector fo^! . A set of projection operations is defined as:

57] - R, SR: Π, x --> £ft e C* ί* Matched Signal Fmjection (4)

JR * A · A: Πκ IL ~> c 1^th Mllfefohed Projections (5)

Em־ example, if the projection space is a space of orthogonal modulations, then £ would be a: reference modulation and R j could be a smgte orthogonal modulation to R and the remaining Ry would be the AM circular shifts of Rf. Then, SR can be defined as:

(6) where, ♦, is a matrix multiply operation. The orthogonal pmjections are handled in a similar manner giving

JR ~ R'^{i !}* · (5 4׳ /) c C ׳^: x A¹ Orthogonal projection. (7)

Note, matrix multiplying by all the shifts of R or A'/ ;are equivalent to the commiutiom ®, of

SRTS REF. NO. 16025.0003U2

R or 7?/ with (5+.7).

Using the projected signal and the received signal to create the following outer products gives,

QCJP) C* x , and (8) (9) functions as a covariance matrix for the interfeiwe only using the mul tiple samples of R and R ^: that constitute a single processing interval. Thus, Q does not require a statistical process over multiple processing intervals. S imilarly, P Is the covariance of the original received signal, including the target signal and interference over a single processing interval. These variables are used to create an image space timet ton, Y, that depends an the β Λ and the separate on parameters (possibly through a transformation V). For each ft o Tl^ let ft - [ft ft, ft; ... ft_A. ] denote a set of admissible parameters that defined the pre-image space 0״ m . Then the image space associated with the received signal.־

Y * Τ(β P, i׳(ft),5P(p)) <4 C^; x C* for each ft s Πβ οΠ;, (10) where for each /.

F(ft) ־. IX -> % c C׳^w Steering Vector (I1) is a vector that conforms to a prfor/ signal strueture in the image space, sometimes referred to as a steering vector. A detection fisnetmn, 79(¾ cun be used to determine the parameters of the signal of interest. For example, if the detector is the absolute maximum fonction then applying the detection function over all projection and separation parameter values leads to the set of parameter values that optimize the signal reception, or

1,0 ft [- 0(1¾ma^(^., where and ,oefL. (12)

It should be noted that the projection of the received signal that Is orthogonal to, or nearly orthogonal to, the target signal, JP, can be formed in a potentiaily large number of ways. That is, there are a large number of orthogonal, or nearly orthogonal, proiecifon operations can be used to form IP. It is equally׳ valid, and useful in some cases, to combine many of these projections or even the inverted outer products from these projections to achieve further

2S5Z

SW REF. NO. 16025.()0031.)2 interference cancellation.

The signal can be reconstructed by

55> - ׳(p J) ־ 'f7 * Ω;. x O_$ c C**. (I3)

Fig. 1 is a block diagram of an illustrative embodiment of a system 100 for mi tigating electromagnetic interference in a recei ver where the orthogonal projection of the received signal is achieved by performing a mis-matched filter operation on the received signal to project it into an orthogonal image space. The system 100 includes an array 102of spatially-separated antenna elements, an analog electrical circuit 104 and a digital processor 116. This array may have only a single anteona element The analog electrical circuit '104 includes a low noise amplifier 106, a modulated refereitoe generator 108. a mixer 110, a bandpass filter 112, and ADC circuitry 114. Each signal channel has one of the antenna elements of the array 102 and one of the electrical eireniis 104 associated with it It will be understood by those skilled there.» multiple ways to accomplish this receiver. For ease of illustration, only one of the electrical circuit 104 Is shown in Fig. 1.

[001 s! rhe target signal is described by,

S4׳Sj ··· ¾,]⁷ (14)

Similarly, the interference signal is represented by,

J-p. /, ... ,/, J (15)

The actual signal received at the antenna element 102a is comprised of the sum of the target signal and the interference signal, which may be expressed mathematically as .¾- r .f- The system 100 operates as fol tows. An electrical signal is renewed at antenna element 102a. The antenna element 102a outputs an electrical signal (i.e., Sk 4· /0 to amplifier 106. Amplifier 106 amplifies the electrical signal and delivers tire amplified electrical signal to the mixer ΓΙ0. The mixer 110 mixes the amplified electrical signal with a reference signal that is generated by the modulated reference generator 108. The signal exiting the mixer HO passes through bandpass filter 112 and enters the ADC circuitry 114, which eonveils the analog signal into a digital signal. The digital signal is transferred to the processor 116 for interference mitigation processing, as will be described with reference to Fig. 2. The electrical circuit 104 and the processor 116 together form an electrical processing circuit for performing interference τ352־^τ /X

SRTS REF. NO. 16O25',(XX)3V2 mitigation. A memory device !17 that is is communication with the processor 116 stores computer code for execution by the processor I16 and typically also stores data.

[0020] The memory device 117 may be any computer-readable medium capable of storing program code and data thereon, such as, for example, a RAM device, a ROM device, a PROM device, an EPROM device., a flash memory device, a CD, a DVD, a hard disk drive, a tape· drive, and a memory card or stick. The processor 116 may be any type of processi ng device capable of processing computer code and data, such as, for example, a microprocessor, a micwemttroller, a PGA. a PLA, an ASIC, an S0C, an SIP, a DSP, and a combination or two or more of such devices. As will now be described with reference to Fig. 2, the processor 1I6 performs an interference mitigation process that includes a mis-matehed filtering operation that is used to project the recei ved signal into an. orthogonal image space and then uses mfermatfon contained in the image space to remdve interference from the received signal to obtain the target signal.

[0021] Fig. 2 is a flow diagram of the portion of the interference mitigation process 200 that is performed by the processor 116 shown in Fig. 1. The process begins with inputting the received signal 4· J_k into the processor 116 as the signal is outputted from the electrical circuit 104 shown in Fig. 1. This step is represented by block 202. At the step represented by block 203. a copy of the received signal is made. At the step represented by block 204, a matched filter sub-process is performed on the received signal to obtain the result | Jp (S v J) j. At the step represented by block 205, a mis-matehed filter sub-process is performed on the copy of the received signal to obtain the result I ^; (<$ + J) L The symbol in Fig, 2 represents a dot product mathematical operation.

[9022] At the sub-process represented by block 210, creates an image space, Y 211. One of a variety of methods may be used for this purpose. An exampie of the OST method of the invention will be provided with reference to a Space Time Adaptive process known,as Capon's Method, which uses the following equations:

h’-G'., and (16) (17)

In the traditional Capon’s Method, Q is a covariance matrix that is computed over many

~2'/f ס 2־ כ 2.3

SR CS REE NO. 16025.0003(12 processing intervals in: order to build up toe necessary statistic«- This processing assumes that the signal and the intefferenee stay statistically stationtoy over these intervals. in accordance with an illustrative embodiment of the invention, Q is computed over a single processing itoerval, thereby removing the time delay and the stationary requirement. Because ¥ is linear in V, an efficient way to calculate Y is to compute the DP T of SP-W.

in the case where the projection space is the fest time modulation and the image space is the angle space, then S and J can be expressed as:

' 0% :::->_, a»·.״·...-׳״;. <- n* :^!

Atier detection, this algorithm resolves into fp P1 - max(DFJ52)־ g ))>

(18) (19) (20)

Thus, the process performed by the system 100 depicted in Fig. 1 can include the modified Capon's Method algorithm represented by equations 16 and 17 that processes information over a single processing ·interval· to mitigate interference in the target signal.

[9023] Alternatively, assuming that the projection space is the fast time modulation and that the image space is the product of the fast and slow time Doppler Space, then S’, J, and ¥ can be expressed: as:

s 11 נx Π-> Q_z>,_iw X Ω,- c T z , (21) d · (1.-:..-,,.:.,..^x 11,׳ ;:?·.;<,>·׳~* Ω,, xΩ, .-.׳.™-,.,·,z c^: ' x v (22)

The !mage space is formed as follows, (23)

Again the Discrete Fourier Transform (DPT) can. he used to for the image· space which in this case is called: the Range־D^^^^ map, ·IT) (24) '23 5203/2.

SETS REF. NO. 16015.000302

[0024] As an example of yet another alternative. it is also possible to utilize the QSP technique in implementing other conventional adaptive array algorithms that require a covariance matrix P for the signal plus interference and the covariance matrix Q for the orthogonal eompfement to the signal piss interference,

[0026] Using the above formalism;, several known adaptive array processing algorithms. such as. for example, the Generalised Sidefohe Canceller (GSC) algorithm and the Mfolmuin Variance Distorifonless Response (MVDR) caneellei· algorithm can be reformulated to perform the OSP method of the invention. In addition, persons of skill in the art will understand how to extend these results to eigenst^ techniques utilizing eigenvectors and eigenvalues associated with the matrices P and Q The following demonstrates the manner in which these algorithms can be modified to achieve the OSP approach of the invention.

for:____fL ........ (יל)

F^/f>P<F' י (26)

[p ^mx(pl)

Notice dial Y is not linear in V, so the DFT would not work to compute the image space.

Look direction(2k)

Define B as the A/ x Λ/Τ dimensional space orthogonal to Wq ^,WQi))

If. ־־fo ·P P(/wf

W -Bf/ - Ρ(Ρ; · P j?(BPBf^! fo :.:(/ -4/ l/P31)״)

Υ^Ρ(ρ)·ϊ^(32) ii

Z35207 /2, SRTS REF. NO. I6025.0003U2 (33) |082§] The embodimenis described above make use of an observation that the received signal has both temporal and spatial properties that make it possible to filter the signal into separate, respective tillered signals that are orthogonal to one another. The spatial property may be, for example, angle or range. The temporal property may be, for example, code division multiple access (CDMA), time division rmdiipte access (TDMA) or frequency division multiple access (FDMA). The received signal is sampled in both dimensions and then filtered into the matched״filtered signal and into the nus-matched-filtered signal, which, is orthogonal to, or nearly orthogonal to, the matched-filtered signal. The interference signal is present In both the matched-filtered signal and the mis-matched-filtered signal whereas the target signal is present in only the matched-filtered signal. In the .matehed״filtered signal, the interference signal is different from the matched-filtered signal in the temporal property, bin is the same as the ־ matched-filtered signal in the spatial property. The aforementioned image space, fis obtained by processing fee match-filtered signal and the mis-matched-filtered signal in accordance with a method such as those presented above to separate the target signal $ from the interference signal J.

[0027] It should be noted that the invention has been desertbed wife reference to a few illustrative, or exemplary, embodiments in order to demonstrate the principles and concepts of fee invention. It will be understood by those skilled in the art that the invention is not limited to these, embodiments, but may be modi fied in a number of ways while still achieving the goals of the invention. For example, fee circuit elements, logic or processes described above with reference to Figs. I and 2 may be different .from those that are explicitly disclosed. For example, while fee system 100 shown in fig. I includes an array of antenna elements 102, the OSP precess could be performed using a single antenna element. Also, while, the OSP process depicted in Fig. 2 has been described as being performed almost entirely within the processor 116, some of the tasks could instead be performed in analog eircumy, such ׳as the inarched and mis-matched filtering operations mprescnted by blocks 204 and 205. Persons skilled in the art will understand, in view of fee description being provided herein, these and other modifications maybe made while still achieving the goals of fee invention and without deviating from the scope of the invention.