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NULL STEERING IN ANTENNA ARRAYS

NULL STEERING IN ANTENNA ARRAYS. BY VADIRAJ HARIBAL. INTRODUCTION. Null steering in antenna arrays has been a major area of research since a long time due to the ability to produce nulls in a given direction , to avoid unwanted directional interference.

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NULL STEERING IN ANTENNA ARRAYS

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  1. NULL STEERING IN ANTENNA ARRAYS BY VADIRAJ HARIBAL

  2. INTRODUCTION • Null steering in antenna arrays has been a major area of research since a long time due to the ability to produce nulls in a given direction, to avoid unwanted directional interference. • The increasing pollution of the electromagnetic environment has provided impetus to several null steering methods, that find applications in modern radar and communication systems. • Null steering mainly depends upon the key parameters of an array, namely; amplitude, phase and element spacing. • By changing each of these indivisual parameters or a combination of these we can obtain nulls in desired directions. • There are various null steering methods but in general these methods involve the search for appropriate complex weights of amplitude and phase.

  3. MATHEMATICAL BACKGROUND- SCHELKUNOFF POLYNOMIAL METHOD: Visible region(VR) and Invisible Region (IR) for β= 0 Visible region (VR) and Invisible region(IR) fro β=π/4

  4. Roots of the Array Factor If we can somehow succeed in producing the roots of z to fall in the visible region then each of the roots that falls in the visible region contributes to the null for that particular angle θ. This is because for that particular value of z which equals to the root/roots lying in visible region, the length between z and zn becomes zero, thus enforcing a null. This is a method to form nulls by changing d and keeping β constant. If the roots do not lie in the visible region, we may rotate the visible region such that the roots get positioned in the visible region, thus creating nulls at that angle θ. This method keeps a constant d but varies β.

  5. DIFFERENT NULL STEERING METHODS: A] Null steering by real weight control- A method of decoupling weights(NSWC): We can achieve nulls in a given direction without changing the phase by selecting zeroes that occur in conjugate pairs. a] N=even=2k In this case the array factor becomes • AF=(z+1)(z-z1)(z-z1*).......(z-zk-1)(z-zk-1*) b] N=odd=2k+1 Here, • AF=(z-z1)(z-z1*).....(z-zk)(z-zk*)

  6. B] Null steering by controlling the element positions(NSEP): This method places nulls by the perturbation of element positions such that there is even symmetry around the centre of array. • A small change Δn in the element spacing introduces a null. Thus, AF(u)= Where, kcosθ=u and kcosθs=us and AF(u)=AFo(u)+AFc(u) Where AFc(u)= is the cancellation term • We need M nulls to cancel M interferers.

  7. COMPARISON Nulls at 10 degrees 40 degrees and 70 degrees Null at 40 degrees

  8. Table 1 Table 1 represents the values of null depths obtained by these two methods On comparing the mathematical formulation it can be inferred that NSWC has lesser computational complexity. It requires minimal computations to steer nulls in a new direction in case of one interferer changing its direction, whereas the BSEP method has to recalculate the positions of the elements for steering a null in a new direction.

  9. END-ON ARRAYS WITH EQUISPACED NULL POINTS • For an end-on array, Ψ=βl(cosθ-1) • the field strength relative to the principal maximum at the point lying midway between the point of maximum and first null point is, ρ= π/(4(n-1)√2(n-1)) due to which the second maxima is 30[log(n-1)+5] dB below the principal maxima. Thus doubling the number of elements diminishes the first maxima by around 9dB.

  10. APPLICATIONS: • Null steering are highly required in GPS systems where it is important to reject interferences for better accuracy. • For example, Dual polarized adaptive antenna arrays use 2 different null steering techniques namely spatial and polarization null steering. • The adaptive polarization null steering method exploits the fact of mismatching the polarization antenna response to that of the incoming interfering signal. • The spatial null steering technique makes use of several adaptive beamforming methods to achieve nulls.

  11. REFERENCES: 1] “Antenna theory and design ”,Constantine balanis- Third edition 2] “A Mathematical Theory Of Arrays”, S.A.Schelkunoff- Bell System Technical Journal,22: 1. January1943 pp 80-107 3] “Method Of Null Steering Without Using Phase Shifters”, T.B.Vu- IEEE Proceedings, Vol. 131,Pt.H,No.4, August 1984 4] “Null Steering, A Comparative Analysis”, Rahim A. Qamar, Noor M. khan- Multitopic Conference, 2009, INMIC 2009, IEEE 13th International 5] “Null Steering LMS Dual Polarized Adaptive Antenna Arrays For GPS ”, W. C .Cheuk, M Trinkle, D.A.Gray-Journal Of Global Positioning Systems(2005) Vol 4, No.1-2: 258-267

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