U.S. patent application number 14/646893 was filed with the patent office on 2015-11-12 for system and method for cyclostationarity-based signal identification.
This patent application is currently assigned to CommScope Technologies LLC. The applicant listed for this patent is COMMSCOPE TECHNOLOGIES LLC. Invention is credited to Martin C. Alles, Andrew Beck, Thomas B. Gravely, Navin Srinivasan.
Application Number | 20150326334 14/646893 |
Document ID | / |
Family ID | 49917751 |
Filed Date | 2015-11-12 |
United States Patent
Application |
20150326334 |
Kind Code |
A1 |
Alles; Martin C. ; et
al. |
November 12, 2015 |
System and Method for Cyclostationarity-Based Signal
Identification
Abstract
The present disclosure describes systems and methods for
identifying a signal that is a product of two or more other
signals. In an embodiment, the presence of a particular signal is
determined and identified by applying a cyclostationarity detection
technique, such as comparing a cyclic autocorrelation function of a
product signal with the cyclic autocorrelation function of at least
one of the signals which formed the product signal.
Inventors: |
Alles; Martin C.; (Vienna,
VA) ; Srinivasan; Navin; (Fairfax, VA) ;
Gravely; Thomas B.; (Herndon, VA) ; Beck; Andrew;
(Ashburn, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
COMMSCOPE TECHNOLOGIES LLC |
Hickory |
NC |
US |
|
|
Assignee: |
CommScope Technologies LLC
Hickory
NC
|
Family ID: |
49917751 |
Appl. No.: |
14/646893 |
Filed: |
December 16, 2013 |
PCT Filed: |
December 16, 2013 |
PCT NO: |
PCT/US2013/075394 |
371 Date: |
May 22, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61737500 |
Dec 14, 2012 |
|
|
|
Current U.S.
Class: |
375/343 ;
375/340 |
Current CPC
Class: |
H04L 27/2678 20130101;
H04L 27/2666 20130101; H04L 27/2627 20130101; H04B 1/1027 20130101;
H04L 7/007 20130101; H04L 27/0012 20130101; H04J 11/0023
20130101 |
International
Class: |
H04J 11/00 20060101
H04J011/00; H04L 27/00 20060101 H04L027/00; H04L 27/26 20060101
H04L027/26 |
Claims
1. A method for identifying a signal, the method comprising the
steps of: (a) determining the presence of a first signal in a
predetermined frequency band wherein the first signal is a product
of a second and a third signal; (b) applying a predetermined
cyclostationarity detection technique to the first signal; and (c)
identifying the first signal from the application of the
predetermined cyclostationarity detection technique to the first
signal.
2. The method of claim 1 wherein the predetermined
cyclostationarity detection technique includes determining a cyclic
autocorrelation function.
3. The method of claim 1 wherein step (b) includes determining if
the first signal comprises a predetermined characteristic of either
the second or third signal.
4. The method of claim 3 wherein the predetermined characteristic
is selected from the group consisting of: cyclic prefix-induced
cyclostationarity, frame rate, and chip rate.
5. The method of claim 1 wherein at least one of the second and
third signals is selected from the group consisting of: a
communication signal, a communication signal in a wireless
communication system, an Orthogonal Frequency Division Multiplexed
("OFDM") signal, and a Long Term Evolution ("LTE") signal.
6. The method of claim 1 wherein at least one of the second and
third signals is a communication signal in a wireless communication
system and wherein the first signal is not a communication signal
in the wireless communication system.
7. The method of claim 1 wherein at least one of the second and
third signals is selected from the group consisting of: a tone, a
modulated carrier, and noise.
8. A system for identifying a signal, comprising: a first circuit
for determining the presence of a first signal in a predetermined
frequency band wherein the first signal is a product of a second
and a third signal; a detection circuit for applying a
predetermined cyclostationarity detection technique to the first
signal; and an identification circuit for identifying the first
signal from the application of the predetermined cyclostationarity
detection technique to the first signal.
9. The system of claim 8 wherein the detection circuit includes
circuitry for determining a cyclic autocorrelation function.
10. The system of claim 8 wherein the detection circuit includes
circuitry for determining if the first signal comprises a
predetermined characteristic of either the second or third
signal.
11. The system of claim 10 wherein the predetermined characteristic
is selected from the group consisting of: cyclic prefix-induced
cyclostationarity, frame rate, and chip rate.
12. The system of claim 8 wherein at least one of the second and
third signals is selected from the group consisting of: a
communication signal, a communication signal in a wireless
communication system, an Orthogonal Frequency Division Multiplexed
("OFDM") signal, and a Long Term Evolution ("LTE") signal.
13. The system of claim 8 wherein at least one of the second and
third signals is a communication signal in a wireless communication
system and wherein the first signal is not a communication signal
in the wireless communication system.
14. The system of claim 8 wherein at least one of the second and
third signals is selected from the group consisting of: a tone, a
modulated carrier, and noise.
15. A non-transitory machine-readable medium having stored thereon
a plurality of executable instructions to be executed by a
processor to implement a method of identifying a signal, the method
comprising the steps of: (a) determining the presence of a first
signal in a predetermined frequency band wherein the first signal
is a product of a second and a third signal; (b) applying a
predetermined cyclostationarity detection technique to the first
signal; and (c) identifying the first signal from the application
of the predetermined cyclostationarity detection technique to the
first signal.
16. The machine-readable medium of claim 15, wherein the
predetermined cyclostationarity detection technique includes
determining a cyclic autocorrelation function.
17. The machine-readable medium of claim 15 wherein step (b)
includes determining if the first signal comprises a predetermined
characteristic of either the second or third signal.
18. The machine-readable medium of claim 17 wherein the
predetermined characteristic is selected from the group consisting
of: cyclic prefix-induced cyclostationarity, frame rate, and chip
rate.
19. The machine-readable medium of claim 15 wherein at least one of
the second and third signals is selected from the group consisting
of: a communication signal, a communication signal in a wireless
communication system, an Orthogonal Frequency Division Multiplexed
("OFDM") signal, and a Long Term Evolution ("LTE") signal.
20. The machine-readable medium of claim 15 wherein at least one of
the second and third signals is a communication signal in a
wireless communication system and wherein the first signal is not a
communication signal in the wireless communication system.
21. The machine-readable medium of claim 15 wherein at least one of
the second and third signals is selected from the group consisting
of: a tone, a modulated carrier, and noise.
Description
RELATED AND CO-PENDING APPLICATIONS
[0001] This application is a U.S. national stage application of the
PCT Application entitled "System and Method for
Cyclostationarity-Based Signal Identification", Serial Number
PCT/US2013/075394 filed 16 Dec. 2013, which claims priority to U.S.
provisional application entitled "Cyclostationarity Based
Identification of Intermodulation Distortion", Ser. No. 61/737,500
filed 14 Dec. 2012. This application is related to each of the
following U.S. national stage applications, each of which is filed
concurrently herewith, of the following PCT applications: "System
and Method for Determining Intermodulation Distortion of a Radio
Frequency Product Signal", Serial Number PCT/US2013/075409, and
"System and Method for Determining Intermodulation Distortion in a
Radio Frequency Channel", Serial Number PCT/US2013/075420. The
entirety of each of the above applications is hereby incorporated
herein by reference.
BACKGROUND
[0002] Intermodulation distortion is a form of signal distortion
caused by an unwanted amplitude modulation of signals that occurs
due to passage through a non-linear channel. Such intermodulation
can cause additional signals that are present at various
combinations of sums and differences of the frequencies that
constituted the original source signals. Such intermodulation can
cause unwanted signal components to appear in other frequency bands
and cause interference to other useful signals. In addition, this
intermodulation can cause signal distortion throughout the original
band and thus reduce the quality of information-carrying signal
transmissions.
[0003] The presence of intermodulation distortion, particularly on
the downlink path, can be a serious problem for communication
network operators. This distortion, which may corrupt existing
communication signals and/or occupy frequency bands that are
allocated for other purposes, can cause the communication network
to fail to achieve its design throughput capacity. One known cause
of intermodulation distortion is due to cabling and/or connector
malfunction or is caused by other passive components. Such
intermodulation distortion is referred to herein as Passive
Intermodulation Distortion ("PIM"). Unfortunately, intermodulation
distortion is not easy to identify without active investigation,
such as disconnecting components and subjecting them to scrutiny
and/or injecting test signals via sophisticated diagnostics
equipment. Such investigation and testing is costly and
time-consuming and requires at least a portion of the communication
network to be out of operation for a period of time. Furthermore,
cables, connectors, and other passive circuit elements may
deteriorate over time due to a variety of reasons including weather
and the local operating environment. Thus, for example, cables and
connections that check out fine at the time of installation or
testing may deteriorate without notice until they cause a decrement
in network operation.
[0004] Intermodulation distortion is often present in many
operational communication networks and the network operator may not
be aware until the problem becomes large enough that is causes
major interference with data and/or voice carrying communication
channels.
[0005] Accordingly, there is a need for identifying signals which
may be the result of intermodulation distortion, determining
intermodulation distortion from two or more radio frequency ("RF")
signals, and determining intermodulation distortion in a
communication system which operates using known RF channels and a
known communication signal type.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a flow chart for identifying a signal according to
an embodiment of the present subject matter.
[0007] FIG. 2 is a functional block diagram for identifying a
signal according to an embodiment of the present subject
matter.
[0008] FIG. 3 is a flow chart for determining intermodulation
distortion of a radio frequency product signal according to an
embodiment of the present subject matter.
[0009] FIG. 4 is a functional block diagram for determining
intermodulation distortion of a radio frequency product signal
according to an embodiment of the present, subject matter.
[0010] FIG. 5 is a flow chart for determining radio frequency
signals causing intermodulation distortion according to an
embodiment of the present subject matter.
[0011] FIG. 6 is a functional block diagram for determining radio
frequency signals causing intermodulation distortion according to
an embodiment of the present subject matter.
DETAILED DESCRIPTION
[0012] The following description of the present subject matter is
provided as an enabling teaching of the present subject matter and
its best, currently-known embodiment. Those skilled in the art will
recognize that many changes can be made to the embodiments
described herein while still obtaining the beneficial results of
the present subject matter. It will also be apparent that for some
embodiments, some of the desired benefits of the present subject
matter can be obtained by selecting some of the features of the
present subject matter without utilizing other features.
Accordingly, those skilled in the art will recognize that many
modifications and adaptations of the present subject matter are
possible and may even be desirable in certain circumstances and are
part of the present subject matter. Thus, the following description
is provided as illustrative of the principles of the present
subject matter and not in limitation thereof and may include
modification thereto and permutations thereof. While the following
exemplary discussion of embodiments of the present subject matter
may be directed towards or reference specific systems and methods,
it is to be understood that the discussion is not intended to limit
the scope of the present subject matter in any way and that the
principles presented are equally applicable to other similar
systems and methods for identifying a signal that is a product of
two or more other signals.
[0013] Those skilled in the art will further appreciate that many
modifications to the exemplary embodiments described herein are
possible without departing from the spirit and scope of the present
subject matter. Thus, the description is not intended and should
not be construed to be limited to the examples given but should be
granted the full breadth of protection afforded by the appended
claims and equivalents thereto.
[0014] With reference to the figures where like elements have been
given like numerical designations to facilitate an understanding of
the present subject matter, various embodiments of a system and
method for identifying signals which may be the result of
intermodulation distortion, determining intermodulation distortion
from two or more radio frequency ("RF") signals, and determining
intermodulation distortion in a communication system which operates
using known RF channels and a known communication signal type, are
described. In an embodiment, the presence of a particular signal is
determined and identified by applying a cyclostationarity detection
technique. One such technique includes comparing a cyclic
autocorrelation function of a product signal with the cyclic
autocorrelation function of at least one of the signals which
formed the product signal. In another embodiment, intermodulation
distortion is determined by searching a frequency band for an RF
product signal and identifying the RF product signal as an
intermodulation distortion signal using a cyclostationarity
detection technique. In a further embodiment, the presence of
intermodulation distortion in a communication system is determined
by comparing a cyclic autocorrelation function ("CAF") of a complex
envelop of signal content in a frequency bin, comparing the
determined CAF with the CAF for a known signal type, and comparing
a frequency of the signal content with the frequency of an RF
channel in the communication system. In yet a further embodiment,
the presence of intermodulation distortion or other forms of
interference in a communication system is determined through
cyclostationary Spectral Correlation Density ("SCD"). The SCD is
used to further evaluate the nature of the interfering signal, such
as spectral energy, modulation type, etc. The SCD may be determined
by a Frequency Smoothing and/or Time Smoothing algorithm.
[0015] The disclosed systems and methods may be used to identify
intermodulation distortion, including passive intermodulation
distortion in a system such as, but not limited to, a communication
system including a wired or a wireless communication system. In an
embodiment, the techniques include classifying a signal or signals
according to their cyclostationary properties. Intermodulation
distortion is identified by searching for cyclostationary
properties of the product waveforms generated due to the underlying
distorting process. One or more regions of signal spectrum may be
sequentially examined for product waveforms with such properties.
By applying certain hypotheses tests, the likelihood of
intermodulation distortion is determined. The systems and methods
may be applied to any digital signal type.
[0016] Passive intermodulation distortion is a form of signal
distortion caused by an unwanted amplitude modulation of signals
that occurs due to their passage through a non-linear channel and
caused by passive components. Such intermodulation can cause
additional, generally undesirable signals to be generated at
various combinations of sums and differences of the frequencies
that constituted the original source signal. It can thus cause
unwanted signal components to appear in other frequency bands and
cause interference to other signals. In addition, this
intermodulation can cause signal distortion throughout the original
band and thus reduce the quality of the transmissions. While the
disclosure discusses examples using PIM, it will be readily
understood by those of skill in the art that the described
techniques and procedures are applicable to other types of
intermodulation distortion and are not limited to PIM.
[0017] Consider a channel within which the desired source signal
has frequency components at frequencies
f.sub.1,f.sub.2,f.sub.3, . . . f.sub.N.
[0018] Then, on passage through a non-linearity, the output signal
has frequency components of the form
a.sub.1f.sub.1+a.sub.2f.sub.2+a.sub.3f.sub.3+ . . .
+a.sub.Nf.sub.N
where the coefficients
a.sub.1,a.sub.2, . . . ,a.sub.N
[0019] are some arbitrary constants. One such case of particular
interest is where so called third order intermodulation products
are formed. This is the case where the absolute sum of the
coefficients of two or more of the output signal frequency
components equals three. For example:
|a.sub.1|+|a.sub.2|+|a.sub.3|=3,
[0020] and where spurious signals are now generated at frequencies
such as
(f.sub.1+f.sub.2-f.sub.3),(2f.sub.1-f.sub.2),(2f.sub.2-f.sub.3).
[0021] More generally the absolute sum of the terms a.sub.i is
referred to as the order of the intermodulation. Cases where this
order is equal to 3, 5, or 7 are the most deleterious, and the
third order terms are often the dominant contributors.
[0022] For a more detailed understanding of the process giving rise
to these spurious frequency terms, let us consider that we have a
channel in which we ideally have the signals
x.sub.1(t)sin(2.eta.f.sub.1t), x.sub.2(t)sin(2.pi.f.sub.2t),
x.sub.3(t)sin(2.pi.f.sub.3t), where x.sub.1(t), x.sub.2(t) and
x.sub.3(t) are narrow band signals relative to the total channel
bandwidth and f.sub.1, f.sub.2, and f.sub.3 are frequencies such
that the signals occur with no overlap within the channel in the
ideal case (the signals are spectrally disjoint). As an exemplary,
non-limiting, illustration of the concepts involved, assume that
there is a non-linearity at some point in the signal path which
cubes the entire channel content. It is noted that this is an
extreme situation, but useful to illustrate the concepts involved.
More generally, the cube typically occurs as one term of a sum of
terms in the channel output. Taking the cube of an expression is a
non-linear operation and the output of the channel is then
expressible as
(x.sub.1(t)sin.sup.3(2.pi.f.sub.1t)+x.sub.2(t)sin.sup.3(2.pi.f.sub.2t)+x-
.sub.3(t)sin.sup.3(2.pi.f.sub.3t).sup.3. (1.1)
[0023] The expansion of the terms in this equation give rise to the
following terms:
x.sub.1.sup.3(t)sin.sup.3(2.pi.f.sub.1t)+x.sub.2.sup.3(t)sin.sup.3(2.pi.-
1f.sub.2t)+x.sub.3.sup.3(t)sin.sup.3(2.pi.f.sub.3t)+2x.sub.1.sup.2(t)sin.s-
up.2(2.pi.f.sub.1t)x.sub.2(t)sin(2.pi.f.sub.2t)+2x.sub.1.sup.2(t)sin.sup.2-
(2.pi.f.sub.1t)x.sub.3(t)sin(2.pi.f.sub.3t)+2x.sub.2.sup.2(t)sin.sup.2(2.p-
i.f.sub.2t)x.sub.1(t)sin(2.pi.f.sub.1t)+2x.sub.2.sup.2(t)sin.sup.2(2.pi.f.-
sub.2t)x.sub.3(t)sin(2.pi.f.sub.3t)+2x.sub.3.sup.2(t)sin.sup.2(2.pi.f.sub.-
3t)x.sub.1(t)sin(2.pi.f.sub.1t)+2x.sub.3.sup.2(t)sin.sup.2(2.pi.f.sub.3t)x-
.sub.2(t)sin(2.pi.f.sub.2t)+6x.sub.1(t)sin(2.pi.f.sub.1t)x.sub.2(t)sin(2.p-
i.f.sub.2t)x.sub.3(t)sin(2.pi.f.sub.3t) (1.2)
[0024] Now consider the fourth term of this expansion. This is
2x.sub.1.sup.2(t)sin.sup.2(2.pi.f.sub.1t)x.sub.2(t)sin(2.pi.f.sub.2t),
[0025] which can be expanded using trigonometric identities as
x.sub.1.sup.2(t)x.sub.2(t)sin(2.pi.f.sub.2t)(1-cos(4.pi.f.sub.1t)).
[0026] This can be further simplified to give
x.sub.1.sup.2(t)x.sub.2(t)sin(2.pi.f.sub.2t)-1/2(x.sub.1.sup.2(t)x.sub.2-
(t)(sin(2.pi.f.sub.2t+4.pi.f.sub.1t))-1/2(t)x.sub.2(t)sin(2.pi.f.sub.2t-4.-
pi.f.sub.1t) (1.3)
[0027] Examining the three terms in equation 1.3, we note that the
first term is an interfering signal at the frequency f.sub.2, the
third term is a signal at frequency f.sub.2-2f.sub.1 and the second
term is a signal at frequency f.sub.2+2f.sub.1. The second term is
the most likely to lie outside the frequency band containing the
signals, and the second and third are examples where the absolute
sum of the multipliers on the frequency terms is three, as
mentioned earlier. We also observe that the signal content for
these terms is the product of the square of one of the original
signals and another original signal. Thus in the case of the second
term one signal has been squared and multiplied by another and the
whole product shifted out to a new frequency of f.sub.2+2f.sub.1.
We can also observe that this is one single term in the expansion
given by equation 1.2. The other terms will produce similar
frequency shifted products of the original signals. The effect of
intermodulation distortion can thus be interpreted in terms of the
multiplication of distinct signals having first been raised to
small integer powers (such as 1, 2, 3, etc.) and various frequency
shifts applied to the resulting product signals.
[0028] More typically, the entire channel content is not cubed; a
squared, cubed or other non-linear term adds to the channel
content. For example if the signal into the channel is y(t), the
output could be of the form:
z(t)=a.sub.1y(t)+a.sub.2y.sup.2(t)+a.sub.3y.sup.3(t)+ . . . ,
[0029] where a.sub.1, a.sub.2, a.sub.3 etc. are some constants. In
a channel with no intermodulation distortion a.sub.i is zero for
all values i>1 and if there is intermodulation distortion
a.sub.i is non zero for some values of i. Generally the cases of
concern have a.sub.i.noteq.0 for i=2,3.
[0030] As stated above, passive intermodulation distortion on the
downlink path due, e.g., to cabling and/or connector malfunction
can be a serious problem for communication system operators. This
distortion, by corrupting existing signals and by occupying bands
that are allocated for other purposes, can cause the network to
fail in achieving its designed throughput capacity.
[0031] Embodiments of the present disclosure propose a novel
solution to the problem of identifying intermodulation distortion,
including PIM, in a system by using techniques that classify
signals according to their cyclostationary properties. The focus of
the following description is on the LTE (Long Term Evolution)
wireless communication standard, however, those of skill in the art
will readily understand that the general principles can be equally
applied to any other protocol or signal type with distinguishing
cyclostationary features. In addition the described techniques and
procedures may also be applied to the products of one signal type
with another provided such product signals also exhibit
cyclostationary features.
[0032] Cyclostationarity of LTE Signals
[0033] Cyclostationarity techniques are used in the field of
cognitive radio to hypothesize the existence and parameters of
various wireless protocols. Since PIM can cause a signal to produce
artifacts of itself at frequencies other than intended, it is
sensible to examine whether such artifacts can be found using
cyclostationarity techniques. As we have observed in the previous
section, if the signals present in some communication channel are
corrupted due to the presence of a channel non-linearity, the
resultant aggregate signal can have various original signal
products shifted by various frequencies.
[0034] As discussed above, intermodulation distortion results in
the product of integer powers of distinct signals distributed both
in the original signal bandwidth and out of this bandwidth.
Embodiments of the present disclosure consider the intermodulation
products caused due to non-linearities (cabling defects, faulty
connections, etc.) throughout a signal reception or transmission
system. Other embodiments choose certain frequency bands to search
for "unexpected signals" using cyclostationarity techniques. To
clarify the meaning of an unexpected signal as used herein,
consider a downlink signal from a communication network operator.
The communication network operator typically has some assigned
bandwidth for the aggregate of all the communication signals in the
communication network, an aggregate that could be of the order of
several tens of MHz. In this bandwidth, the communication network
operator likely has multiple cellular protocols in operation. Some
of this bandwidth is assigned to LTE, other bandwidth is assigned
to other protocols, perhaps even some residual of protocols such as
GSM (Global System for Mobile Communications) or 1.times.RTT
(Single Carrier Radio Transmission Technology). There may also be
certain unused pieces of spectrum that the operator has license to
but does not use (guard bands, unallocated spectrum, etc.) There
also may be certain bands between or adjacent to the assigned
bandwidth that are reserved for special purposes (national
security, etc.) in which communication is restricted and which
bands are often not occupied by signals. Thus, an unexpected signal
is a signal component that the communication network operator had
no desire to generate but which is nonetheless produced by, for
example, defects in the communication network infrastructure.
[0035] Suppose several LTE signals are present in the source
signals of this communication network operator's downlink. When
intermodulation distortion occurs, the signals mutually amplitude
modulate each other and cause spurious signals to emerge elsewhere
as described above. As a simple, non-limiting, example, consider a
first LTE signal centered at frequency f.sub.1 and a second LTE
signal centered at frequency f.sub.2. The intermodulation
distortion (assuming third order terms exist) can now produce a
product signal at a frequency 2f.sub.2-f.sub.1. It can also produce
a product signal at 2f.sub.2+f.sub.1. Extending this to all
possible LTE signals at various frequencies we see that a whole
range of possible unwanted product signals can emerge at a whole
range of various frequencies.
[0036] As is known in the art, LTE signals are generated as
Orthogonal Frequency Division Multiplexed ("OFDM") signals. To
examine the features of the product waveforms more carefully,
consider two synchronized (i.e., symbol start times coincident)
OFDM signals s.sub.1(t) and s.sub.2(t) at two distinct frequencies
f.sub.1 and f.sub.2. Utilizing the complex envelope of each
waveform, we can then write these signals as:
s.sub.1(t)={{tilde over
(s)}.sub.1(t)e.sup.(j2.pi.f.sup.1.sup.t)}
s.sub.2(t)={{tilde over
(s)}.sub.2(t)e.sup.(j2.pi.f.sup.2.sup.t)}
[0037] Consider a typical term such as
p(t)=s.sub.1(t).sup.2s.sub.2(t) that may arise in the passage of
the sum of these OFDM signals through a non-linearity. For example,
p(t) could occur as a third order intermodulation distortion. Both
s.sub.1(t) and s.sub.2(t) exhibit non-conjugate cyclostationarity.
To see this, we can write {tilde over (s)}.sub.1(t) in the OFDM
format as
n = - .infin. .infin. 1 N c d ni ( j 2 .pi. .DELTA. f ( t - nT s -
) ) r ( t - nT s - ) ( 2.3 ) ##EQU00001##
[0038] where d.sub.ni are the complex symbols (QAM or PSK) in each
symbol time T.sub.s, n is the symbol counter, .epsilon. is the
unknown symbol timing, N.sub.c is the number of subcarriers,
.DELTA.f is the subcarrier frequency spacing, and r(t) is a
rectangular pulse of width T.sub.s. The symbol time
T.sub.s=T.sub.u+T.sub.g where T.sub.u is the useful symbol time and
T.sub.g is the guard time. The non-conjugate Cyclic Autocorrelation
Function ("CAF") for the complex signal {tilde over (s)}.sub.1(t)
is defined by
R s 1 s 1 .alpha. ( .tau. ) = lim T .fwdarw. .infin. 1 T .intg. - T
2 T 2 E { s ~ 1 ( t ) s ~ 1 * ( t + .tau. ) } ( j 2 .pi. .alpha. t
) t , ( 2.4 ) ##EQU00002##
[0039] where E{.} is the expectation operator. For a
non-cyclostationary signal,
R.sub.s.sub.1.sub.s.sub.1.sup..alpha.(t,.tau.)=0 for all
.alpha..noteq.0. Any nonzero value of .alpha. for which the CAF is
non-zero is called a cycle frequency of the signal s.sub.1(t). The
time varying non-conjugate autocorrelation of the OFDM signal
R.sub.s.sub.1.sub.s.sub.1(t,.tau.)=E{{tilde over
(s)}.sub.1(t){tilde over (s)}.sub.1*(t+.tau.)}
can be written as
R s 1 s 1 ( t , .tau. ) = E { n = - .infin. .infin. i = 1 N c d ni
( j 2 .pi. .DELTA. f ( t - nT s - ) ) r ( t - nT s - ) m = -
.infin. .infin. k = 1 N c d mk * ( - j 2 .pi. k .DELTA. f ( t +
.tau. - mT s - ) ) r ( t + .tau. - mT s - ) } ( 2.5 )
##EQU00003##
[0040] It may be noted that the only random quantities over which
the expectation operates are the constellation points d.sub.ni and
d.sub.mk*. The only surviving terms in equation (2.5) are those
terms where the complex modulation symbols are exact conjugates of
each other. These terms occur only when .tau.<T.sub.s and in
such cases the shifted pulse waveforms intersect in a new pulse
waveform of shortened duration resulting in a waveform that is
periodic in t. Thus equation (2.5) simplifies to give
m = - .infin. .infin. A sin ( .pi. N c .DELTA. f .tau. ) sin ( .pi.
.DELTA. f .tau. ) r ( t - nT s - ) r ( t + .tau. - nT s - ) ( 2.6 )
##EQU00004##
[0041] where A is a scalar real multiplier dependent on the
specific constellation d.sub.i and mapping of binary data to QAM or
PSK constellation points. This equation is clearly periodic in t
with period T.sub.s, and hence when the CAF is generated will show
spectral lines for certain values of .alpha.. It is instructive to
consider what happens when .tau.=T.sub.u. In this case the
fractional term in equation (2.6) results in unity. Thus when the
data is moved to overlap at exactly the cyclic prefix, the
amplitude of the autocorrelation is at a relative maximum. Thus the
existence of the cyclic prefix makes the cyclostationary features
of OFDM rise out of the noise floor and make it detectable.
[0042] For purposes of this discussion, it is not sufficient that
s.sub.1(t) and s.sub.2(t) exhibit cyclostationarity. We want to
take a particular third order term such as
s.sub.1.sup.2(t)s.sub.2(t) and show that it too exhibits
cyclostationarity. More generally, we want a product such as
p(t)=s.sub.1.sup.2(t+.beta.)s.sub.2(t) to also exhibit
cyclostationarity, at least when .beta. is a small fraction of
T.sub.s. To show this, we must first attempt to write the product
waveform in terms of its Complex Envelope ("CE"). Now s.sub.1(t)
can be written as
s.sub.1(t)={{tilde over
(s)}.sub.1(t)e.sup.(j2.pi.f.sup.1.sup.t)}=1/2[s.sub.1(t)e.sup.(j2.pi.f.su-
p.1.sup.t)+{tilde over (s)}.sub.1*(t)e.sup.(-j2.pi.f.sup.1.sup.t)],
(2.7)
and similarly
s.sub.2(t)=1/2[{tilde over
(s)}.sub.2(t)e.sup.(j2.pi.f.sup.2.sup.t)+{tilde over
(s)}.sub.2*(t)e.sup.(-j2.pi.f.sup.2.sup.t)] (2.8)
[0043] Now we can square the expression for s.sub.1(t) to
obtain
s.sub.1.sup.2(t)=1/4[{tilde over
(s)}.sub.1.sup.2(t)e.sup.(j4.pi.f.sup.1.sup.t)+{tilde over
(s)}.sub.1*.sup.2(t)e.sup.(-j4.pi.f.sup.1.sup.t)+2{tilde over
(s)}.sub.1(t){tilde over (s)}.sub.1*(t)] (2.9)
[0044] When we multiply out the terms in the expansion of
s.sub.1.sup.2(t)s.sub.2(t) we find that this product can be
generated from the complex envelopes given by {tilde over
(s)}.sub.1.sup.2(t){tilde over (s)}.sub.2*(t), {tilde over
(s)}.sub.1.sup.2(t), and |{tilde over (s)}.sub.1(t)|.sup.2{tilde
over (s)}.sub.2(t), inclusive of the conjugates of each term, and
where in the first case the applicable carrier frequency is
2f.sub.1-f.sub.2, in the second case the carrier frequency is
2f.sub.1+f.sub.2 and in the third case it is f.sub.2.
[0045] Thus, the question now becomes whether any of the above
three components of the product waveform is capable of exhibiting
cyclostationarity. It turns out that all three exhibit
cyclostationarity. Let us show this first in the simplified case
where .beta.=0 (synchronized signals).
[0046] We now present a novel and useful result which greatly
simplifies the task of exhibiting cyclostationarity for these
waveforms: the product of the CE's of any number of synchronized
OFDM signals is the CE of a different OFDM signal. To show this,
consider the product of two OFDM CEs,
n = - .infin. .infin. i = 1 N c d ni ( j 2 .pi. .DELTA. f ( t - nT
s - ) ) r ( t - nT s - ) m = - .infin. .infin. k = 1 N c d mk ( j 2
.pi. k .DELTA. f ( t - mT s - ) ) r ( t - mT s - ) , ( 2.10 )
##EQU00005##
[0047] which, noting that the only terms remaining when the two
sums are multiplied are the terms with the same index on the pulse
waveforms, results in
n = - .infin. .infin. { i = 1 N c d ni ( j 2 .pi. .DELTA. f ( t -
nT s - ) ) k = 1 N c d nk ( j 2 .pi. k .DELTA. f ( t - nT s - ) ) r
( t - nT s - ) } = n = - .infin. .infin. i = 1 N c k = 1 N c d nk d
ni ( j 2 .pi. .DELTA. f ( t - nT s - ) ) ( j 2 .pi. k .DELTA. f ( t
- nT s - ) ) r ( t - nT s - ) = n = - .infin. .infin. i = 1 N c k =
1 N c d nk d ni ( j 2 .pi. ( i + k ) .DELTA. f ( t - nT s - ) ) r (
t - nT s - ) = n = - .infin. .infin. m = 1 2 N c D n m ( j 2 .pi. m
.DELTA. f ( t - nT s - ) ) r ( t - nT s - ) , ( 2.11 )
##EQU00006##
[0048] in which m=(i+k) and {D.sub.(.)} is a new constellation.
Equation (2.11) can now be recognized as identical to the CE of a
different OFDM signal (e.g., equation (2.3)) whose constellation is
formed by the product of the individual constellations of the two
signals and where the span of the sub-carriers is the sum of the
previous spans (with the same sub-carrier spacing). The extension
to the product of more than two OFDM CEs follows by induction; for
three terms, take the first two, apply the result and then apply it
to the product of the first two and the third term.
[0049] This result makes it immediately obvious that all the terms
referred to earlier in the product waveform will exhibit
cyclostationarity with exactly the same cycle frequencies as the
base signals. Thus for OFDM signals, all the PIM products (of any
order) of synchronous signals will exhibit cyclostationarity. We
also note that self-products, namely terms which involve integer
powers of one particular signal, behave similarly.
[0050] Now consider the question of whether a product such as
p(t)=s.sub.1(t+.beta.)s.sub.2(t) also exhibits cyclostationarity
when .beta..noteq.0. This is harder to demonstrate mathematically
in an exact manner, so we will approach this differently.
[0051] Consider signals s.sub.1(t), s.sub.2(t) where the signal in
any symbol time is not a sum of subcarriers but rather a single
subcarrier. It is clear that the actual OFDM signals are then
obtained by aggregating N.sub.c such signals in each symbol time,
but it is easier to make the case by first focusing on the single
carrier signals. As previously, we assume that the cyclic prefix
fraction or guard time is the same for both signals. Now, placing
our attention on a single symbol time, if the signals were
synchronized, then the product signal content is the same in the
first [0,T.sub.g] segment of p(t) and the final
[T.sub.s-T.sub.g,T.sub.s], segment. That is, there is a repetition
of the signal content that occurs with a delay T.sub.u. This is
another way to argue for the cyclostationary properties in the
synchronized case.
[0052] Now let us assume that .beta. is a small fraction of
T.sub.g. Then, the effect of .beta. is to slightly stagger a symbol
time of s.sub.1(t) with respect to a symbol time of s.sub.2(t).
What we then see is that there is still a repetition of terms in
the product in the segments [0, T.sub.g-.beta.] and
[T.sub.s-T.sub.g+.beta.,T.sub.s]. Thus, the autocorrelation of the
product signal at delay will produce energy equal to that with zero
delay. This means that there is still a repeated component in the
product waveform, and hence implies the likely existence of a
cyclostationary feature that may be detectable. As .beta.
increases, this feature will diminish in size and when
.beta..gtoreq.T.sub.g the feature should disappear. The most
important observation here is that a cyclostationary feature may be
observable if the de-synchronization (expressed using .beta.) is
small relative to the guard time.
[0053] Identifying Passive Intermodulation Distortion on LTE
Signals
[0054] All of the possible frequencies or frequency bands where
intermodulation products could possibly exist are computable given
knowledge of the channel or frequency map used by the communication
network operator. Let this set of possible frequencies (or
frequency bands) at which undesirable LTE product signals occur be
denoted by F. Then one can propose examining each candidate
frequency or frequency band in F for the following hypotheses:
[0055] H1: Does it exhibit the presence of an LTE product signal?
More strictly does a cyclostationarity analysis of this candidate
frequency or frequency band exhibit a positive test for LTE?
[0056] H2: In an ideal situation, should this frequency or
frequency band exhibit LTE cyclostationary features?
[0057] If H1 is answered in the affirmative and H2 in the negative,
it is then possible for us to argue that the communication network
operator clearly has a defect in his network. One possible and
likely explanation for this defect is that he or she has
unrecognized intermodulation distortion actively degrading the
system.
[0058] One novel aspect in the exemplary embodiments is that we
search for products of LTE signals (i.e., one LTE signal raised to
some integer power times one or more other LTE signals raised to
some integer powers) in the Radio Frequency domain, optionally
convert these product signals to baseband and then apply
cyclostationarity detection techniques to answer the hypotheses
tests H1 and 112. Note that the signals are not always assumed to
be perfectly synchronized; that is, one signal could have some
offset in time with respect to where the other signal or signals
starts.
[0059] In an embodiment, if we were searching for an
intermodulation product at frequency 2f.sub.2+f.sub.1, where
f.sub.2 and f.sub.1 are the RF frequencies of the LTE signals, we
might downconvert the signals by suitable quadrature downconversion
using a frequency of 2f.sub.2+f.sub.1 and then examine this
baseband signal for LTE features. In another embodiment, the
intermodulation product at frequency 2f.sub.2+f.sub.1 need not be
downconverted. We note that there is a plethora of possible linear
frequency combinations that can be searched based mainly on a
positive response to the query in H2. Our finding is that the LTE
product signals exhibit a unique Cyclic Autocorrelation Function
("CAF") provided the cyclic prefix on the signals is not zero, and
provided that the signals are synchronized to within approximately
75% of the cyclic prefix length. For example, if the cyclic prefix
is of size 25% of the symbol time, we then need to be synchronized
to within about 18% of the symbol time. Since we can identify LTE
product signals in this manner, we then have a scheme that produces
a positive result in the presence of intermodulation distortion.
With respect to LTE, other embodiments can exploit the
cyclostationary features of the LTE reference signal, where when we
consider signal products we can additionally search for more
features if one of the LTE signals was a reference signal.
[0060] In some variations of the LTE communication signals it may
be necessary to provide further filtering of the signals in the
time or frequency domain prior to applying cyclostationarity
analysis. One such example is the case where LTE does not operate
in the extended mode. In such cases the relative size of the Cyclic
Prefix ("CP" or guard time) may change in some deterministic
manner. In all such cases, by suitable excision of signal segments
in the time domain the excised signals will preserve
cyclostationary features. Such excision operations in time or
filtering operations in frequency are contemplated herein and can
precede any of the techniques presented in this disclosure.
[0061] In the time domain, different time intervals within LTE are
expressed as multiples of a basic time unit T.sub.s=1/30720000
seconds. The radio frame has a length of 10 ms;
(T.sub.f=307200T.sub.s). Each frame is divided into ten equally
sized sub-frames of 1 ms in length. Scheduling is done on a
sub-frame basis for both the downlink and uplink. Each sub-frame
may consists of two equally sized slots of 0.5 ms in length. Each
slot in turn consists of a number of OFDM symbols which can be
either seven (normal cyclic prefix) or six (extended cyclic
prefix). The useful symbol time is
Tu=2048.times.T.sub.s.apprxeq.66.7 .mu.s. For the normal mode, the
first symbol has a cyclic prefix of length
TCP=160.times.T.sub.s.apprxeq.5.2 .mu.s. The remaining six symbols
have a cyclic prefix of length TCP=144.times.T.sub.s.apprxeq.4.7
.mu.s. The reason for a different CP length of the first symbol is
to make the overall slot length in terms of time units divisible by
15360. For the extended mode, the cyclic prefix is
TCP.sub.c=512.times.T.sub.s.apprxeq.116.7 .mu.s. By design, the CP
is longer than the typical delay spread of a few microseconds
typically encountered in practice. The normal cyclic prefix is used
in urban cells and high data rate applications while the extended
cyclic prefix is used in special cases like multi-cell broadcast
and in very large cells (e.g. rural areas, low data rate
applications). When the normal cyclic prefix is used, data
extracted for cyclostationary analysis can be excised by removing
the first symbol of every slot. In the extended mode, no such
modification of the data is needed. In general, therefore, it is
quite feasible, and contemplated herein, to accommodate small
variations in signaling formats that may occlude cyclostationary
features by intelligently modifying the extracted data prior to
analysis.
[0062] With attention drawn to FIG. 1, a flow chart 100 for
identifying a signal according to an embodiment of the present
subject matter is presented. At block 101, the presence of a first
signal in a frequency band is determined where the first signal is
the product of a second and third signal. At block 102, a
cyclostationarity detection technique is applied to the first
signal. At block 103, the first signal is identified from the
results of the application of the cyclostationarity detection
technique to the first signal. In an embodiment, the
cyclostationarity detection technique includes determining a cyclic
autocorrelation function of the first signal. In another
embodiment, the results of applying the cyclostationarity detection
technique to the first signal include determining if the first
signal includes a predetermined characteristic of either the second
or third signal. The predetermined characteristic may be one or
more of a cyclic prefix-induced cyclostationarity, a frame rate,
and a chip rate.
[0063] In yet another embodiment, one or both of the second and
third signals is a communication signal (e.g., a signal that is
intended by the operator of a communication system to carry useful
information), a communication signal in a wireless communication
system, an Orthogonal Frequency Division Multiplexed ("OFDM")
signal, or a Long Term Evolution ("LTE") signal. In still a further
embodiment, one or both of the second and third signals is a
communication signal in a wireless communication system but the
first signal is not a communication signal in the wireless
communication system. In yet still a further embodiment, one or
both of the second and third signals is a tone, a modulated
carrier, or noise.
[0064] Now turning to FIG. 2, a functional block diagram 400 for
identifying a signal according to an embodiment of the present
subject matter is depicted. In an embodiment, mobile device 210
communicates, via radio frequency ("RF") uplink and downlink
channels, as is known in the art, with wireless transmitter 220 in
a wireless communication network. The uplink and/or downlink
channel may be composed of one or more frequency bands. It will be
understood by those of skill in the art that the present exemplary
embodiment is non-limiting and that other embodiments of the
present disclosure, including use in a wired system, are
contemplated herein. The wireless communication network also
includes a processor 230 which is operatively connected to
transmitter 220 and a memory device 240. The processor 230 includes
a signal presence circuit 231, a detection circuit 232, and an
identification circuit 233.
[0065] In an embodiment, the signal presence circuit 231 determines
the presence of a first signal in a frequency band, such as, but
not limited to, a frequency band in a downlink channel, where the
first signal is the product of a second and third signal. The
detection circuit 232 applies a cyclostationarity detection
technique to the first signal. The identification circuit 233
identifies the first signal from the results of the application of
the cyclostationarity detection technique to the first signal in
the detection circuit 232. In an embodiment, the detection circuit
232 includes circuitry which determines a cyclic autocorrelation
function of the first signal. In another embodiment, the
identification circuit 232 includes circuitry which determines if
the first signal includes a predetermined characteristic of either
the second or third signal. The predetermined characteristic may be
one or more of a cyclic prefix-induced cyclostationarity, a frame
rate, and a chip rate. The predetermined characteristic may be
stored in the memory device 240.
[0066] In yet another embodiment, one or both of the second and
third signals is a communication signal (e.g., a signal that is
intended by the operator of a communication system to carry useful
information) in either an uplink or downlink channel, a
communication signal in a wireless communication system, an
Orthogonal Frequency Division Multiplexed ("OFDM") signal, or a
Long Term Evolution ("LTE") signal. In still a further embodiment,
one or both of the second and third signals is a communication
signal, in either an uplink or downlink channel, in a wireless
communication system but the first signal is not a communication
signal in the wireless communication system. In yet still a further
embodiment, one or both of the second and third signals is a tone,
a modulated carrier, or noise.
[0067] In another embodiment, the processor 230 is programmed using
a non-transitory machine-readable medium which stores executable
instructions to be executed by the processor 230 to implement a
method of identifying a signal. In an embodiment, the method
includes the steps of determining the presence of a first signal in
a frequency band where the first signal is the product of a second
and third signal, applying a cyclostationarity detection technique
to the first signal, and identifying the first signal from the
results of the application of the cyclostationarity detection
technique to the first signal. In an embodiment, the
cyclostationarity detection technique includes determining a cyclic
autocorrelation function of the first signal. In another
embodiment, the results of applying the cyclostationarity detection
technique to the first signal include determining if the first
signal includes a predetermined characteristic of either the second
or third signal. The predetermined characteristic may be one or
more of a cyclic prefix-induced cyclostationarity, a frame rate,
and a chip rate.
[0068] In yet another embodiment, one or both of the second and
third signals is a communication signal (e.g., a signal that is
intended by the operator of a communication system to carry useful
information), a communication signal in a wireless communication
system, an Orthogonal Frequency Division Multiplexed ("OFDM")
signal, or a Long Term Evolution ("LTE") signal. In still a further
embodiment, one or both of the second and third signals is a
communication signal in a wireless communication system but the
first signal is not a communication signal in the wireless
communication system. In yet still a further embodiment, one or
both of the second and third signals is a tone, a modulated
carrier, or noise.
[0069] FIG. 3 illustrates a flow chart 300 for determining
intermodulation distortion of a radio frequency product signal
according to an embodiment of the present subject matter. At block
301, a frequency or a frequency band, in either an uplink or
downlink channel, is searched for an RF product signal. The RF
product signal is a product of a first RF signal raised to the
power of a first integer and a second RF signal raised to the power
of a second integer. Additionally, the first RF signal is at a
first frequency or frequency band, the second RF signal is at a
second frequency or frequency band, and the RF product signal is at
a third frequency or frequency band. At block 302, a
cyclostationarity detection technique is applied to the RF product
signal. At block 303, the RF product signal is identified as an
intermodulation distortion signal from the results of the
application of the cyclostationarity detection technique to the RF
product signal. In an embodiment, the first and second RF signals
are transmitted by a communication system. In a further embodiment,
at block 304 a determination is made as to whether the frequency or
frequency band of the RF product signal is a frequency or frequency
band transmitted by the communication system, i.e., whether the
detected cyclostationary characteristic is expected in the
frequency band.
[0070] In an embodiment, the first RF signal is transmitted by a
first communication system and the second RF signal is transmitted
by a second communication system. In a further embodiment, a
determination is made as to whether the third frequency or
frequency band is a frequency or frequency band transmitted by
either the first or second communication system, i.e., whether the
detected cyclostationary characteristic is expected in the
frequency band. The first and second communication systems may
operate using different communication protocols.
[0071] As discussed above, the BY product signal is a product of a
first RF signal raised to the power of a first integer and a second
RF signal raised to the power of a second integer. In an
embodiment, the first and second integers are the same. In another
embodiment, the sum of the first and second integers equals
three.
[0072] In yet a further embodiment, the RF product signal includes
a cyclic prefix, as is known in the art. In still a further
embodiment, the first and second RF signals each include a cyclic
prefix having a predetermined length and the first and second RF
signals are synchronized to within at least 75% of the
predetermined length of the cyclic prefix.
[0073] In an embodiment; the cyclostationarity detection technique
includes determining a cyclic autocorrelation function of the first
signal. In another embodiment, the results of applying the
cyclostationarity detection technique to the RF product signal
include determining if the RF product signal includes a
predetermined characteristic of either the first or second RF
signal. The predetermined characteristic may be one or more of a
cyclic prefix-induced cyclostationarity, a frame rate, and a chip
rate.
[0074] In yet another embodiment, one or both of the first and
second RF signals is a communication signal (e.g., a signal that is
intended by the operator of a communication system to carry useful
information), a communication signal in a wireless communication
system, an Orthogonal Frequency Division Multiplexed ("OFDM")
signal, or a Long Term Evolution ("LTE") signal. In still a further
embodiment, one or both of the first and second RF signals is a
communication signal in a wireless communication system but the RF
product signal is not a communication signal in the wireless
communication system. In yet still a further embodiment, one or
both of the first and second RF signals is a tone, a modulated
carrier, or noise.
[0075] Considering FIG. 4, a functional block diagram 400 for
determining intermodulation distortion of a radio frequency product
signal according to an embodiment of the present subject matter is
depicted. In an embodiment, mobile device 410 communicates, via
radio frequency ("RF") uplink and downlink channels, as is known in
the art, with wireless transmitter 420 in a wireless communication
network. The uplink and/or downlink channel may be composed of one
or more frequency bands. It will be understood by those of skill in
the art that the present exemplary embodiment is non-limiting and
that other embodiments of the present disclosure, including use in
a wired system, are contemplated herein. The wireless communication
network also includes a processor 430 which is operatively
connected to transmitter 420 and a memory device 440. The processor
430 includes an RF product signal search circuit 431, a detection
circuit 432, and an identification circuit 433. In further
embodiments, the processor 430 further includes one or both of
circuit 434 and circuit 435, as discussed in further detail
below.
[0076] In an embodiment, the RF product signal search circuit 431
searches a frequency or a frequency band, in either an uplink or
downlink channel, for an RF product signal. The RF product signal
is a product of a first RF signal raised to the power of a first
integer and a second RF signal raised to the power of a second
integer. Additionally, the first RF signal is at a first frequency
or frequency band, the second RF signal is at a second frequency or
frequency band, and the RF product signal is at a third frequency
or frequency band. The detection circuit 432 applies a
cyclostationarity detection technique to the RF product signal. The
identification circuit 433 identifies the RF product signal as an
intermodulation distortion signal from the results of the
application of the cyclostationarity detection technique to the RF
product signal. In an embodiment, the first and second RF signals
are transmitted by a communication system. In a further embodiment,
circuit 434 determines whether the frequency or frequency band of
the RF product signal is a frequency or frequency band transmitted
by the communication system, i.e., whether the detected
cyclostationary characteristic is expected in the frequency
band.
[0077] In another embodiment, the first RF signal is transmitted by
a first communication system and the second RF signal is
transmitted by a second communication system. In a further
embodiment, circuit 435 determines whether the third frequency or
frequency band is a frequency or frequency band transmitted by
either the first or second communication system, i.e., whether the
detected cyclostationary characteristic is expected in the
frequency band. The first and second communication systems may
operate using different communication protocols.
[0078] As discussed above, the RF product signal is a product of a
first RF signal raised to the power of a first integer and a second
RF signal raised to the power of a second integer. In an
embodiment, the first and second integers are the same. In another
embodiment, the sum of the first and second integers equals
three.
[0079] In yet a further embodiment, the RF product signal includes
a cyclic prefix, as is known in the art. In still a further
embodiment, the first and second RF signals each include a cyclic
prefix having a predetermined length and the first and second RF
signals are synchronized to within at least 75% of the
predetermined length of the cyclic prefix.
[0080] In an embodiment, the cyclostationarity detection technique
applied by the detection circuit 432 includes circuitry which
determines a cyclic autocorrelation function of the RF product
signal. In another embodiment, the identification circuit 433
includes circuitry which determines if the RF product signal
includes a predetermined characteristic of either the first or
second RF signal. The predetermined characteristic may be one or
more of a cyclic prefix-induced cyclostationarity, a frame rate,
and a chip rate.
[0081] In yet another embodiment, one or both of the first and
second RF signals is a communication signal (e.g., a signal that is
intended by the operator of a communication system to carry useful
information), a communication signal in a wireless communication
system, an Orthogonal Frequency Division Multiplexed ("OFDM")
signal, or a Long Term Evolution ("LTE") signal. In still a further
embodiment, one or both of the first and second RF signals is a
communication signal in a wireless communication system but the RF
product signal is not a communication signal in the wireless
communication system. In yet still a further embodiment, one or
both of the first and second RF signals is a tone, a modulated
carrier, or noise.
[0082] In another embodiment, the processor 430 is programmed using
a non-transitory machine-readable medium which stores executable
instructions to be executed by the processor 430 to implement a
method of determining intermodulation distortion. In an embodiment,
the method includes the step of searching a frequency or a
frequency band, in either an uplink or downlink channel, for an RF
product signal. The RF product signal is a product of a first RF
signal raised to the power of a first integer and a second RF
signal raised to the power of a second integer. Additionally, the
first RF signal is at a first frequency or frequency band, the
second RF signal is at a second frequency or frequency band, and
the RF product signal is at a third frequency or frequency band.
The method also includes the steps of applying a cyclostationarity
detection technique to the RF product signal, and identifying the
RF product signal as an intermodulation distortion signal from the
results of the application of the cyclostationarity detection
technique to the RF product signal. In an embodiment, the first and
second RF signals are transmitted by a communication system. In a
further embodiment, the method further includes the step of
determining whether the frequency or frequency band of the RF
product signal is a frequency or frequency band transmitted by the
communication system, i.e., whether the detected cyclostationary
characteristic is expected in the frequency band.
[0083] In an embodiment, the first RF signal is transmitted by a
first communication system and the second RF signal is transmitted
by a second communication system. In a still further embodiment,
the method includes the step of determining whether the third
frequency or frequency band is a frequency or frequency band
transmitted by either the first or second communication system,
i.e., whether the detected cyclostationary characteristic is
expected in the frequency band. The first and second communication
systems may operate using different communication protocols.
[0084] As discussed above, the RF product signal is a product of a
first RF signal raised to the power of a first integer and a second
RF signal raised to the power of a second integer. In an
embodiment, the first and second integers are the same. In another
embodiment, the sum of the first and second integers equals
three.
[0085] In yet a further embodiment, the RF product signal includes
a cyclic prefix, as is known in the art. In still a further
embodiment, the first and second RF signals each include a cyclic
prefix having a predetermined length and the first and second RF
signals are synchronized to within at least 75% of the
predetermined length of the cyclic prefix.
[0086] In an embodiment, the cyclostationarity detection technique
includes determining a cyclic autocorrelation function of the first
signal. In another embodiment, the results of applying the
cyclostationarity detection technique to the RF product signal
include determining if the RF product signal includes a
predetermined characteristic of either the first or second RF
signal. The predetermined characteristic may be one or more of a
cyclic prefix-induced cyclostationarity, a frame rate, and a chip
rate.
[0087] In yet another embodiment, one or both of the first and
second RF signals is a communication signal (e.g., a signal that is
intended by the operator of a communication system to carry useful
information), a communication signal in a wireless communication
system, an Orthogonal Frequency Division Multiplexed ("OFDM")
signal, or a Long Term Evolution ("LTE") signal. In still a further
embodiment, one or both of the first and second RF signals is a
communication signal in a wireless communication system but the RF
product signal is not a communication signal in the wireless
communication system. In yet still a further embodiment, one or
both of the first and second RF signals is a tone, a modulated
carrier, or noise.
[0088] Identifying Passive Intermodulation Distortion on UMTS
Signals
[0089] The method to be applied for UMTS signals is identical to
that for LTE signals. UMTS signals have specific cyclostationary
signatures just as LTE signals have signatures such as those we
have discussed. Cyclic frequencies at multiples of both the inverse
chip time and the inverse frame time may be recognized in the
various order product signals that occur as a result of PIM. Once
again, these product signals will occur with frequency translations
corresponding to the PIM order.
[0090] Candidate Algorithms for Identifying PIM
[0091] We now consider two possible algorithms for identifying PIM
in the aggregate communication channels of a communication system.
Typically, in an embodiment, these may be the entire downlink
("DL") of a wireless system in a given cell or sector. In both of
these algorithms we operate under the assumption of operating in
co-operation with the network (communication system) operator.
Clearly, various other algorithms based on the techniques and
procedures disclosed herein are possible.
[0092] In an embodiment, the network frequency plan for, e.g., a
communication network is obtained. An assumption may be made
regarding the presence of some form of PIM generating
intermodulation of a specific form. Typically, the likelihood of
PIM is initially set to zero. Then for each existing LTE channel
pair compute where product signals could exist. These define a set
of candidate frequency bins. In an embodiment, for the content in
each such bin construct the complex envelope. In another
embodiment, the candidate frequency bin content is first
downconverted to baseband. With the complex envelope available,
subject the complex envelope to cyclostationarity analysis. In an
embodiment, one would tune to a particular bandwidth and run the
CAF generating routines for a configurable time. Running such a
routine for several minutes may be needed to draw cyclostationary
features out of the noise. The hypotheses tests H1 and H2 as
indicated above can now be performed. If one winds up with a
positive result for H1 and a negative result for H2, then the
likelihood of PIM in the candidate frequency bin is increased. In
an embodiment, this procedure may be repeated over all candidate
frequency bins. Finally, if the likelihood of PIM is greater than
some configurable threshold, a decision that PIM exists in this
aggregate of channels is made.
[0093] In an embodiment, the network frequency plan for, e.g., a
communication network is obtained. Typically, the likelihood of PIM
is initially set to zero. Partition the entire bandwidth to be
tested into a set of frequency bins. In an embodiment, one may
start with the smallest assignable LTE channel bandwidth and work
up to the largest possible. In an embodiment, for the content in
each such bin construct the complex envelope. In another
embodiment, the candidate frequency bin content is first
downconverted to baseband. With the complex envelope available,
subject the complex envelope to cyclostationarity analysis. In an
embodiment, one would tune to a particular bandwidth and run the
CAF generating routines for a configurable time. Running such a
routine for several minutes may be needed to draw cyclostationary
features out of the noise. The hypotheses tests H1 and H2 as
indicated above can now be performed. If one winds up with a
positive result for H1 and a negative result for H2, then, in an
embodiment, a computation is initiated. This computation calculates
which possible LTE channel pair in the frequency plan could
possibly generate a PIM product in the bin under test. The
calculation is repeated for various possible intermodulation
orders. If such a pair of valid LTE channels and a valid
intermodulation order exists, the likelihood of PIM is increased.
In an embodiment, the above procedure is repeated over all
candidate frequency bins. Finally, if the likelihood of PIM is
greater than some configurable threshold, a decision that PIM
exists in this aggregate of channels is made.
[0094] Considering now FIG. 5, a flow chart for determining radio
frequency signals causing intermodulation distortion according to
an embodiment of the present subject matter is presented. At block
501, a frequency bin is selected. The frequency bin may be selected
based on a pair of signals from the predetermined set of RF
signals. At block 502, a complex envelope is generated for a first
signal in the frequency bin. At block 503, a cyclic autocorrelation
function ("CAF") for the first signal is determined. At block 504,
the determined cyclic autocorrelation function is compared to a
cyclic autocorrelation function for the predetermined signal type.
At block 505, the frequency of the first signal is compared with
the frequency of the predetermined set of RF channels.
[0095] In a further embodiment, at block 506, a plurality of the RF
channels that produced the first signal is determined. In a still
further embodiment, at block 507, an intermodulation order for each
of the plurality of RF channels that produced the first signal is
determined.
[0096] In another embodiment, the step of comparing the determined
cyclic autocorrelation function to a cyclic autocorrelation
function for the predetermined signal type includes determining if
the first signal comprises a predetermined characteristic of the
predetermined signal type. The predetermined characteristic may be
one or more of a cyclic prefix-induced cyclostationarity, a frame
rate, and a chip rate.
[0097] In yet another embodiment, one or both of the plural RF
channels is a communication signal (i.e., a signal that is intended
by the operator of a communication system to carry useful
information) in either an uplink or downlink channel, an Orthogonal
Frequency Division Multiplexed ("OFDM") signal, or a Long Term
Evolution ("LTE") signal. In yet still another embodiment, the
first signal is not a communication signal in the communication
system. In a further embodiment, one or both of the plurality of RF
channels is a tone, a modulated carrier, or noise.
[0098] FIG. 6 depicts a functional block diagram for determining
radio frequency signals causing intermodulation distortion
according to an embodiment of the present subject matter. In an
embodiment, mobile device 610 communicates, via radio frequency
("RF") uplink and downlink channels, as is known in the art, with
wireless transmitter 620 in a wireless communication network. The
uplink and/or downlink channel may be composed of one or more
frequency bands. It will be understood by those of skill in the art
that the present exemplary embodiment is non-limiting and that
other embodiments of the present disclosure, including use in a
wired system, are contemplated herein. The wireless communication
network also includes a processor 630 which is operatively
connected to transmitter 620 and a memory device 640. The processor
630 includes frequency bin selection circuit 631, a complex
envelope generation circuit 632, a cyclic autocorrelation function
("CAF") determining circuit 633, a CAF comparison circuit 634, and
a frequency comparison circuit 635.
[0099] In another embodiment, circuit 631 includes circuitry for
selecting the frequency bin based on a pair of signals from the
predetermined set of RF signals.
[0100] In a further embodiment, processor 630 also includes circuit
636 which determines a plurality of the RF channels that produced
the first signal. In a still further embodiment, processor 630 also
includes circuit 637 which determines an intermodulation order for
each of the plurality of RF channels that produced the first signal
is determined.
[0101] In another embodiment, circuit 634 includes circuitry for
determining if the first signal comprises a predetermined
characteristic of the predetermined signal type. The predetermined
characteristic may be one or more of a cyclic prefix-induced
cyclostationarity, a frame rate, and a chip rate.
[0102] In yet another embodiment, one or both of the plural RF
channels is a communication signal (i.e., a signal that is intended
by the operator of a communication system to carry useful
information) in either an uplink or downlink channel, an Orthogonal
Frequency Division Multiplexed ("OFDM") signal, or a Long Term
Evolution ("LTE") signal. In yet still another embodiment, the
first signal is not a communication signal in the communication
system. In a further embodiment, one or both of the plurality of RF
channels is a tone, a modulated carrier, or noise.
[0103] In another embodiment, the processor 630 is programmed using
a non-transitory machine-readable medium which stores executable
instructions to be executed by the processor 630 to implement a
method for determining radio frequency signals causing
intermodulation distortion. In an embodiment, the method includes
the steps of selecting a frequency bin, where the frequency bin may
be selected based on a pair of signals from the predetermined set
of RF signals, generating a complex envelope for a first signal in
the frequency bin, determining a cyclic autocorrelation function
for the first signal, comparing the determined cyclic
autocorrelation function to a cyclic autocorrelation function for
the predetermined signal type, and comparing the frequency of the
first signal with the frequency of the predetermined set of RF
channels.
[0104] In a further embodiment, the method includes the step of
determining a plurality of the RF channels that produced the first
signal. In a still further embodiment, the method includes the step
of determining an intermodulation order for each of the plurality
of RF channels that produced the first signal.
[0105] In another embodiment, the step of comparing the determined
cyclic autocorrelation function to a cyclic autocorrelation
function for the predetermined signal type includes determining if
the first signal comprises a predetermined characteristic of the
predetermined signal type. The predetermined characteristic may be
one or more of a cyclic prefix-induced cyclostationarity, a frame
rate, and a chip rate.
[0106] In yet another embodiment, one or both of the plural RF
channels is a communication signal (i.e., a signal that is intended
by the operator of a communication system to carry useful
information) in either an uplink or downlink channel, an Orthogonal
Frequency Division Multiplexed ("OFDM") signal, or a Long Term
Evolution ("LTE") signal. In yet still another embodiment, the
first signal is not a communication signal in the communication
system. In a further embodiment, one or both of the plurality of RF
channels is a tone, a modulated carrier, or noise.
[0107] Optional Pre-Filtering
[0108] In an embodiment, we may also note that in those cases where
a region of spectrum to be examined contains a particular signal
that is not of interest to us, it may be possible to use well known
signal extraction techniques to first extract that particular
signal and then subject the residual to cyclostationarity tests for
the product signals of interest. Such methods may be viewed as
nulling nuisance or interferer signals in the spectrum prior to
searching for PIM. Note that such nulling may itself use a
cyclostationary technique to remove an interferer if the cycle
frequencies for this interferer are distinct from those of the
product waveforms.
[0109] Operation with Multiple Signal Types
[0110] In another embodiment, consider a case where a PIM product
of one signal type may occur in spectrum allocated to or containing
a different signal type. An example of such a situation may be
where an OFDM PIM product may occur in spectrum containing a UMTS
signal. In such cases, one can apply cyclostationary techniques
that ignore the second signal type. The CAF can be examined
directly for cyclic frequencies corresponding to the OFDM signals.
This is possible since the cyclic frequencies of the signal types
are distinct. Generally, therefore, if two or more signal types
coexist, provided they have different cyclostationary
characteristics, one can compute the CAF for the aggregate signal
(the total signal in the spectrum) and focus attention on the
features one expects for the product signals of interest.
[0111] Distinguishing Product Signals from Non-Product Signals
[0112] In an embodiment, consider a situation where a tentative
identification of a particular product signal type has been made
using cyclostationary methods. Now it may happen that there is some
non-zero probability that the examined spectrum may have contained
a non-product signal of the same type from some far transmitter. In
such cases, further processing may be needed to affirm or negate
the decision on the signal type. One method of excluding a
non-product signal would be to attempt to demodulate the signal in
the spectrum using a standard demodulator for that signal type.
Provided the SNR is high enough that such a demodulation can be
performed, a product signal may not generate a valid demodulated
signal. Thus a clean demodulation of the spectrum content may
indicate that this cannot be a product signal. Note that in the
case of OFDM, as we have shown in previous sections, the product
signal could exhibit an expanded constellation. So in such cases,
the demodulation will show a signal behavior that would not have
been expected as a standard OFDM signal. Secondary techniques such
as this or other modulation tests may need to be applied to further
confirm or negate a decision on whether the source of a signal in
some spectrum is PIM or some non-product signal (a regular
transmission).
[0113] Exploiting PIM Induced Subcarrier Interaction in OFDM
Signals
[0114] In an embodiment, when one examines the effect of PIM on a
single LTE signal, an interesting feature of the LTE signal can be
observed. An LTE signal, as discussed previously, is generated
using OFDM. At a very rudimentary level OFDM is simply an
aggregation of tones (subcarriers) with each tone having a
particular amplitude and phase. Thus, one can resort to the very
elementary exposition of PIM via tone interaction to argue that the
effect of PIM on an OFDM signal is that new tones are generated
with different amplitudes and phases.
[0115] Consider two subcarriers of an OFDM signal within an OFDM
symbol duration. Let these two subcarriers be the tones with
frequencies f.sub.1 and f.sub.2. If the subcarrier spacing is
.DELTA.f, then with some integer value k,
f.sub.2=f.sub.1+k.DELTA.f. Now assume there is third order PIM
present in the channel carrying the signal. This can cause a
product signal with frequency f.sub.PIM=2f.sub.1-f.sub.2.
Substituting for f.sub.2 this gives f.sub.PIM=f.sub.1-k.DELTA.f.
This tells us that a PIM product occurs at a frequency k.DELTA.f
below f.sub.1. If .DELTA.f=15 kHz, then a PIM product occurs an
integer multiple of 15 kHz below f.sub.1.
[0116] Now considering all the subcarriers in the OFDM signal we
see that these "self-product" terms generate signals at frequency
shifts with an integer multiple of the subcarrier spacing from the
original set of subcarriers. Some of the self-products may occur at
frequencies that are completely distinct from the set of OFDM
subcarriers. If these self-products can be recognized by some means
then that would increase the likelihood that PIM is present in the
signal channel.
[0117] Primary Synchronization Channel
[0118] As a particular example of how to exploit the PIM induced
self-product of OFDM signals, in an embodiment, one can consider
the Primary Synchronization Sequence ("PSS") in the Primary
Synchronization Channel ("P-SCH") of LTE. In a Frequency Division
Duplex ("FDD") LTE system, the PSS is mapped to the last symbol of
slot number 0 and slot number 10 in a particular radio frame. In a
Time Division Duplex ("TDD") system, the PSS is mapped to the third
OFDM symbol in sub-frames 1 and 6. In either case, the manner in
which this is done is to apply a specific complex domain Zadoff-Chu
sequence to 62 middle subcarriers of a 72 subcarrier window. Thus
there are 10 reserved subcarriers that do not have any applied
data. It follows that tones induced by PIM will result in frequency
components in these reserved subcarriers.
[0119] Since the PSS repeats cyclically every half-frame with
exactly the same data (the Zadoff-Chu sequence) then one can
propose the recognition of the PIM induced energy. To implement
this we can exploit the highly efficient and elegant aspect of OFDM
demodulation offered by the Fast Fourier Transform ("FFT").
Consider the 72-point FFT that may be applied to data demodulate
the content of the PSS carrying symbol. Clearly if there were no
PIM, there should be no data (energy) in the reserved subcarriers
after an FFT demodulation. With PIM on the other hand, there will
be data in these subcarriers, hence the FFT will result in some
frequency domain content in the reserve subcarriers. Since the PSS
is periodic, the frequency domain content will also be periodic.
Thus, one could consider the autocorrelation of the P-SCH in the
frequency domain which will then exhibit a cyclostationary feature.
One could also interpret this as a delay-multiply-integrate loop
where the delay is a half-frame and where the multiply is the
multiplication of the frequency bins corresponding to the reserve
subcarriers, and where the integration is simply a continuing
addition of output of the multiplier. The processing gain offered
by the integration is limited only by the drift of the PIM behavior
over time. If the PIM is static, the method should virtually
guarantee detection.
[0120] It should be noted that even if there were no reserved
subcarriers (essentially a set of frequency bins where no signal
exists), any periodically recurring PIM product of a fixed set of
subcarriers could be recognized by the above means. The requirement
for this to be possible is that the PIM product should not share
spectrum with some other signal which is also periodic with the
same period. Thus, if the PIM generated signal were to fall where
some other random non-periodic signal did, the PIM product should
still be detectable (albeit with greater processing effort).
[0121] Certain embodiments of the present disclosure may be
implemented by a general purpose computer programmed in accordance
with the principals discussed herein. It may be emphasized that the
above-described embodiments are merely possible examples of
implementations, merely set forth for a clear understanding of the
principles of the disclosure. Many variations and modifications may
be made to the above-described embodiments of the disclosure
without departing substantially from the spirit and principles of
the disclosure. All such modifications and variations are intended
to be included herein within the scope of this disclosure and
protected by the following claims.
[0122] Embodiments of the subject matter and the functional
operations described in this specification can be implemented in
digital electronic circuitry, or in computer software, firmware, or
hardware, including the structures disclosed in this specification
and their structural equivalents, or in combinations of one or more
of them. Embodiments of the subject matter described in this
specification can be implemented as one or more computer program
products, i.e., one or more modules of computer program
instructions encoded on a tangible program carrier for execution
by, or to control the operation of, data processing apparatus. The
tangible program carrier can be a computer readable medium. The
computer readable medium can be a machine-readable storage device,
a machine-readable storage substrate, a memory device, or a
combination of one or more of them.
[0123] The term "processor" encompasses all apparatus, devices, and
machines for processing data, including by way of example a
programmable processor, a computer, or multiple processors or
computers. The processor can include, in addition to hardware, code
that creates an execution environment for the computer program in
question, e.g., code that constitutes processor firmware, a
protocol stack, a database management system, an operating system,
or a combination of one or more of them.
[0124] A computer program (also known as a program, software,
software application, script, or code) can be written in any form
of programming language, including compiled or interpreted
languages, or declarative or procedural languages, and it can be
deployed in any form, including as a standalone program or as a
module, component, subroutine, or other unit suitable for use in a
computing environment. A computer program does not necessarily
correspond to a file in a file system. A program can be stored in a
portion of a file that holds other programs or data (e.g., one or
more scripts stored in a markup language document), in a single
file dedicated to the program in question, or in multiple
coordinated files (e.g., files that store one or more modules, sub
programs, or portions of code). A computer program can be deployed
to be executed on one computer or on multiple computers that are
located at one site or distributed across multiple sites and
interconnected by a communication network.
[0125] The processes and logic flows described in this
specification can be performed by one or more programmable
processors executing one or more computer programs to perform
functions by operating on input data and generating output. The
processes and logic flows can also be performed by, and apparatus
can also be implemented as, special purpose logic circuitry, e.g.,
a field programmable gate array (FPGA) or an application specific
integrated circuit (ASIC).
[0126] Processors suitable for the execution of a computer program
include, by way of example, both general and special purpose
microprocessors, and any one or more processors of any kind of
digital computer. Generally, a processor will receive instructions
and data from a read only memory or a random access memory or both.
The essential elements of a computer are a processor for performing
instructions and one or more data memory devices for storing
instructions and data. Generally, a computer will also include, or
be operatively coupled to receive data from or transfer data to, or
both, one or more mass storage devices for storing data, e.g.,
magnetic, magneto optical disks, or optical disks. However, a
computer need not have such devices.
[0127] Computer readable media suitable for storing computer
program instructions and data include all forms data memory
including non-volatile memory, media and memory devices, including
by way of example semiconductor memory devices, e.g., EPROM,
EEPROM, and flash memory devices; magnetic disks, e.g., internal
hard disks or removable disks; magneto optical disks; and CD ROM
and DVD-ROM disks. The processor and the memory can be supplemented
by, or incorporated in, special purpose logic circuitry.
[0128] While this specification contains many specifics, these
should not be construed as limitations on the scope of the claimed
subject matter, but rather as descriptions of features that may be
specific to particular embodiments. Certain features that are
described in this specification in the context of separate
embodiments can also be implemented in combination in a single
embodiment. Conversely, various features that are described in the
context of a single embodiment can also be implemented in multiple
embodiments separately or in any suitable subcombination. Moreover,
although features may be described above as acting in certain
combinations and even initially claimed as such, one or more
features from a claimed combination can in some cases be excised
from the combination, and the claimed combination may be directed
to a subcombination or variation of a subcombination.
[0129] Similarly, while operations are depicted in the drawings in
a particular order, this should not be understood as requiring that
such operations be performed in the particular order shown or in
sequential order, or that all illustrated operations be performed,
to achieve desirable results. In certain circumstances,
multitasking and parallel processing may be advantageous. Moreover,
the separation of various system components in the embodiments
described above should not be understood as requiring such
separation in all embodiments, and it should be understood that the
described program components and systems can generally be
integrated together in a single software product or packaged into
multiple software products.
[0130] While some embodiments of the present subject matter have
been described, it is to be understood that the embodiments
described are illustrative only and that the scope of the invention
is to be defined solely by the appended claims when accorded a full
range of equivalence, many variations and modifications naturally
occurring to those of skill in the art from a perusal hereof.
* * * * *