Note: Descriptions are shown in the official language in which they were submitted.
11~9~)86
Field of the Invention
This inventi.on relates to communications systcms and, more
particularly, to coT~unications systems in which simple, relatively inexpensive
arrangements are utilized to automatically eliminate or reduce external
interference.
Background of the Invcntion
As is well known and understood, one of the major concerns of : .
designers of antenna system communication links is the elimination or reduction
of external interference sources, such as jamming, self-interference, atmos-
pheric noise, man-made noise, and acoustic noise. As is also well known, most
of the arrangements which attempt to resolve these problcms of external inter-
ference do so in a relatively complex manner, ortentimes utili~ing very large
directional antennas and/or wi.th antennas h~vi.n~ llundretls" or cvell more,
elements. This problem of external interference is especially prevalent in
the area of mobile communications systems where omnitlirecti.onal antennas are
employed, because of the large numbers of users operating in the same frequency
band and because of multipath. Use of very large directional antennas in such ~ ;
mobile arena will be seen to be almost a physical impossi~ility, and an economic
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Summary of the Invent10n
As w~ll become clear here~nafter, the ~nterference cancel1ng system of
the present lnvent~on affords an operat10n wh1ch slmulate~ that of a d1rect10n-
l al antenn~ ln ellmlnatlng, or reducing, external 1nterference sources, but w1th
; I slmple ~nd lnexpens1ve c1rcu1try, and w~th a small number of ~ntenna ele~ents
(typlcally, anywheres from 2 to 8). In a f1rst embod~ment of the tnvent~on,
orthogonal mult1plex1ng lt employed 1n con~unct~on w1th ~ notch ~ntenna to
cancel Interference arrlvtng from all d~rect10ns exceDt over the narrow be~m
~ldth port10n forred wlth the notched antenna. As w111 be seen, 1n such ver-
slon an almost omnldlrect1cnal antenna 1s ut11~zed w1th a narrow notch, or
: nulled-out beamwldth,along ~1th a full omnidirect10nal antenna to enable 1m-
plementat10n of the system w~th s1mple, small antennas. In A second embod1ment
of the 1nvent10n, the ampl1tude 1n element separat10n of an array antenna are
predeterm1ned to obta1n a steep slope 1n lts beam pattern, ~lth the output be-
lng progresslvel~ phise shlfted and comblned to provlde a dupl1cate beam pat-
tern for subsequent subtract10n so as to el1m1nate ~nterference from all d1-
rectlons except over a srall anQular sector. In such Yers10n, the requlrement
~or orthogonally mult1plexln~ two channels through the rece1ver 1s obv1ated,
so as to prov1de A further cost s~v1n~s.
Br1ef Descr1pt10n of the Drawln~
~hese and other features of the present 1nvent10n w111 be more read~ly
understood from a conslder~t10n of the followlnq descript10n taken 1n connec-
tlon ~lth the accompany1ng draw1ng ln wh1ch
¦ FIGURE l 1s a funct10nal block d1agram of a multtplexed lnterference
I cancellng recelver system constr~cted ln accordance w1th the lnvent10n;
; I FIGURE 2 ls a s~mpllfled 111ustrat10n of an antenn~ pattern helpful 1n
~ an understand~ng of the block dlagram of FI~URE l:
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l I FIGURES 3 and 4 are s1mpllfied 111ustrat~ons of an alternative antenna
! pattern and resultant beam processln~ appllcable to a mod1ficatlon of the ln-
terference cancel;n~ system of FIGlIRE l;
ll FIGURE 5 i5 a functional block dlaar~m of an 1nterference cancel1na
antenna system constructed 1n accordance wlth the ~nventlon: and
FIGURE 6 ls a slmpllfled 111ustratlon of the resoltant antenna ~eam
process1ng applicable to the lnterference cancelinn antenna system of FIGlIRE S
., .
Deta11ed Descr~ptlon of the Drawin~
~; lO The lnterference canceltnq system sho~m ln FTr~lJRE l ellmlnates, or
reduces 1nterference 1n a transm~sslon l~nk hy ut~ lnn ortho~onal multiplex-
in~ 1n conjunctlon w1th a notch antenna (i.e., an antenna which has a sln~le
¦ deep nu11 over a small angular beam w1dth), The inset alonnside Flr~llQE l
represents the beam pattern of an almost omn~d~rectlollal antenna ln havlng a
narrow notch or nulled-out beamw1dth, 1n wh1ch "~" represents the des1red s19-
nal recelved and "I ", "I ", "I ",........ "~ " represents external 1nterference~ l 2 3 n
; slgnals. The output of the notch antenna lO (channel ~) ls nrthoqonally multl-
plexed w1th the output of an omnldirectional antenna 12 (channel B), wlth the
multlplexin~ belnq of tlme, frequency or space methnd sn lonn as the slgnals
~ 20 From the antennas 10, 12 are rendered non-lnterferln~ wlth each other. The
ii;~, output from the multlplexer 14 is then ampl1fle~ in a recel~er lfi, and then
~, separated in a de-multlplexer 18, A subtractor 2n is emploved, to one lnput
of whlch is provlded the output of de-multlplexer l~ wh~ch cnrresponds to the
; channel B s19nal, as compr1s1n~ the des1red 1nformation, plus all of the 1nter-
fering si~nals from the external sources. nn the other hand, hy rotat1n~ the
; notch antenna 10 (or electronically scanninq lt) unt11 lts notch 1s po1nt1n~
1n the dlrect10n of the desired siqnal (as shown ln the ~nset), then the output
;~ from the de-multlplexer 18 whlch corresponds to the channel A sl~nal would con-
ta1n all of the 1nterferln~ s1gnals, but not the deslred slnnal. Thls second
output prov1des a col)erent and correlated rF~11ca of all the ~nterference
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l I assoc1ated wlth the sl~nal enter;n~ the omni~lrectiona1 antenna l~, and ls
prov1ded as the other ~nPut to the subtractor 2n. Rv ~ncludinq ~mpl1tude and
phase adjust~ng c1rcu1ts 22, 24 1n this second branch, to account for any 1n-
herent d1fferences ~n ga~n and del~y 1n the two ortho~onal channels, the de-
1 mult1plexed output from the notch antenna lO can he directlv suhtracted from
¦¦ the de-mult/plexed output from the omn~d~rect10nAl antenna 12 to yleld a
¦ totally 1nterference free si~nal.
A mafor advanta~e of th1s FIGlIRE I emhod1ment w111 be seen to be that
all 1nterference enterin~ the antenna l~ wh1ch is outslde of the notched beam
1s vlrtually el1m1nated w~thout requ~r~nn any complex adapt1ve processing, or
requ1r1ng a lar~e complex narrow beam ~ntenna. Th1s makes the FlGllRE 1 system
des1rable for mob~le commun1cat10ns usa~e, and for sm~11, ll~htwel~ht, tactlcal
communlcat10ns equ1pment. -
As wlll ~e readilv apparent to those sk111ed ln the art, the notch
antenna lO of FIGURE l represents the ma1n ele~ent 1n the 1nterference can-
; cel1ng system there descrlbed, and 1ntroduces d1fferent re(~ulrements for oper-
at10n than are normally encountered 1n typ1cal antenna des19n. For example,
lnstead of be1n~ concerned w1th a des~gn whtch forms a d~rectlve beam hav~n~
low s1delobes, or des1gn1ng an adapt1ve system hav~nq several movable nulls,
the antenna eng~neer lts here concerned w1th prov1d1n~ a f1xed pattern wh1ch
conta~ns un~fonm recept10n 1n all dtrect10ns, except for that 1n which the
normal beam slot po1nts. Add1ttonally, to be effect1ve, the antenna des1gn
needs a slope 1n the p~ttern developed, at the potnt of the null, and to be
as steep a 510pe as ts pract1cal. Such A requ/red ~attern 1s /llustrat1vely
~25 shown ln FIGURE 2.
General deslgn procedures for prov1d1n~ an array antenna hav1n~ the
type of pattern shown In FIGIIRE 2 are descr1bed 1n my tJn1ted States Patents
No. 3,130,410 and No. 3,605,106. As noted there1n, such patterns are made up
of products and/or sums of 5~n r funct10ns, and can be ~ch1eved by controll1nc
~30 ~ both the lltudes and spdc1nns of array antennd elements. As a result, the
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slope of a null in an antenna beam pattern can be made steep, either by pro-
viding one or more ~Sin mx]y ~erms or by appropriate amplitude and phase con-
Sin x
trols when summing several [Sin mx~ functions using sub-arrays. Further ex-
Sin xplanation can be had by referring to these patents, as well as to the article
"Coded-Linear-Array Antenna", published in Volume 39, No. 2, of Electrical
Communications Magazine.
An alternative version of a notch antenna that could be used with the
FIGURE 1 canceling system is one having a steep slope, but with a somewhat
wider nulled beamwidth, as shown in Figure 3. With this version, the antenna
pattern can be electronically scanned to provide a second received beam which
is angularly displaced by a small amount. The two beams 30, 32 (FIGURE 4)
could then be positioned so that the desired signal "S" lies near the edge of
one receiving beam, e.g. 30, while being nulled out of the second receiving
; beam, 32. Although this alternative scheme continues to null out the inter-
ference sources Il, I2, ....... I , it does introduce a second sector (sector B)
which is vulnerable to interferences. However, the beam width of sector A,
wherein the desired signal S is being received, (and therefore, sector B) can
-` be made very small. Such an alternative approach might be useful where it is
found more practical to design an array antenna with a steeper slope and wider
notch beamwidth than as illustrated in Figure 2.
The functional block diagram of Figure 5 is intended for use with a
second embodiment of the invention in which cancelation of the interference is
; provided at radio frequencies directly at the antenna elements themselves, and
`~ without the need for orthogonally multiplexing two channels. A significant
savings in cost will be seen to result, and again follows in part, from the
; ability to predetermine the amplitude and element separations of an array
antenna so as to obtain a steep slope and a narrow beam slot. The antenna
elements are here represented by the reference numerals 100, 200, 300, ..... N,
with each having its signal output being applied to an adder 50 by means of
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l ~ amplltude and ph~se control clrcu~ts K , K , K , ..... K . A second adder 60
1s employed, to the lnputs of ~hlch are recelved the outputs of the a~pl1tude
and phase control c1rcu1ts, but proqressively phase shlfted by clrcu1ts ~,
2 0, 3 p,.,.... n 0. The result of thls comblnat~on ~s to Drov~de a dupl1cate
beam pattern from the adder 60 whlch is scanned by the s~ll dnqular seqments,
p, as comp~red to the output of the adder 50. ~he two adder outputs are then
: coupled to an antenna pattern canceler 70, whereln the output of the scanned
pattern ls 11near1y subtracted from the or1~1nal notch beam p~ttern to provlde
a substant~ally complete cancelat~on of the Interference, w~th the resultant
~lO s1gnal then be1ng pro~1ded atrece1ver 72.
In hctuallty, the subtract~on ls not ~ complete one, as can be seen
from the resultant beam process1ngs of FlGlJRE 6. ~he rece~ved notch antenna
beam pattern ls shown as 80, whlle the scanned notch ~ntenna pattern ls shown
as B2. As ln the lllustrat~on of FIGtJRE 4, Interference ~s not com~letely can-
celed 1n the two, small angular sectors A and B. As ~111 be apprec~ated, If
: the notch 1n the antenna pattern 1s sufflclently narrow, then, If omnld1rect10n .
al antenna elements are utll1~ed, lt would only beco~e necessary to use the
center element as the second lnput to the pattern canceler ~n. To quaranty
. that the 1nterference from both adders 50, 6n would be ex~ctly ~n phase, ln-
dependent of the d1rect10n of arr1v~1 of the 1nterfer~nq slqnals, e1ther the
progress~ve phase sh1ft should be svmmetr1cal ahout such center element, or the
a compos1te 1n1tlal beam pattern should be phase shlfted by (n ~. As ~lth the
conf1guratlon of FlGURE l~ the construct~on accordlng to th1s embodlment of th
1nventlon could be ut~ ed to large advantAge ln any commun1catlons 11nk ~here
.25 1t 1s des1rable, or necessary, to ellm1nate or reduce external 1nterference,
. such as from a ~ammer, ~tmospher1c nolse, man-made nolse, etc. Anv quant1ty
of such 1nterferlng sources that enter the antenna from outs1de the overlapped
. narro~ beam sectors would thus be s1multaneously el1mlnated, ~lthout the re-
qu1rement of any complex adapt~ve process1na belnq needed.
~ ~h11e there have been descr~bed ~hat are cons1dered to be 111ustrat1ve
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l ~ embod~ments of the present 1nvent10n, it w111 he read11y apparent to those
sk111ed ln the art that mod1f1catlons may be made wlthout depart1ng from the
scope of the taach~nqs here1n of prov1d1nn 1nterference charaCter1st1cs com-
l mensurate w1th a d~rect10nal antenna hav1nq a very narrow beam, but of sub-
¦ stant1ally reduced sl~e -- analys~s show1nq that s1~n~f~cant 1mProvements can
¦¦ be atta1ned w1th Just a patr of antenna elements, and that substant1al ~m-
provement results w1th no more than the employ~ent of R such elements. Bes~des
be1ng easler to bu~ld, and 1mmensely cheaper, the 1nterference cancel1nq sys- ¦tems of the present 1nvent10n enable operat10n to cont~nue even at the lo~er
l frequenc1es where otherwlse very larqe d1rect10nal antennas arr requ1red. ~lth¦
the 1nventlon, ~t becomes but ~ s1mple matter to incorporate the systems 1n ~ ¦mob11e envlronment, as the physical constr~1nts no lonqer pre~ent employment
of the ant nna system on a mov~ng motor vehicle.
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