OPTIMISATION OF CONVERGENCE OF SEQUENTIAL DECORRELATOR

Abstract: A sequential decorrelator arrangement for an adaptive antenna array comprising a plurality of antenna elements the outputs of which feed a cascaded beamforming network having a succession of stages, each stage having one less decorrelation cell than the preceding stage and the first stage having one less cell than the number of antenna elements. The network includes means for applying weighting to the signals applied as inputs to the cells of at least the first stage. The decorrelation cells in each stage comprise means for applying simple rotational transforms to the input data in accordance with a weighting factor common to all the cells in a stage, each stage further including means for deriving said weighting factor from the weighting factor deriving means of the previous stage and the output of one cell of the preceding stage. Each stage includes means for scaling the output of each cell in the stage by a scaling factor calculated from the weighting factor deriving means of the stage.

Description: This invention relates to sequential decorrelator arrangements such as are used in adaptive antenna arrays to perform beamforming operations. Adaptive beamforming provides a powerful means of enhancing the performance of a broad range of communication, navigation and radar systems in hostile electromagnetic environments. In essence, adaptive arrays are antenna systems which can automatically adjust their directional response to null interference or jamming and thus enhance the reception of wanted signals. In many applications, antenna platform dynamics, sophisticated jamming threats and agile waveform structures produce a requirement for adaptive systems having rapid convergence, high cancellation performance and operational flexibility. In recent years, there has been considerable interest in the application of direct solution or "open loop" techniques to adaptive antenna processing in order to accommodate these increasing demands. In the context of adaptive antenna processing these algorithms have the advantage of requiring only limited input data to accurately describe the external environment and provide an antenna pattern capable of suppressing a wide dynamic range of jamming signals. The objective of an optimal adaptive antenna system is to minimise the total noise residue (including jamming and receiver noise) at the array output whilst maintaining a fixed gain in the direction of the desired signal and hence lead to a maximisation of resultant signal to noise ratio. One way of implementing an adaptive beamforming algorithm is by the use of the so-called "sequential decorrelator". British patent No. 1,599,035 describes a sequential decorrelator using open loop decorrelation stages. Figs. 1 and 2 of the present specification illustrate a 5 element network as disclosed in this Patent and a simplified representation of the open loop decorrelation cell respectively. Only in the steady-state, in the limit of an infinite time average, will this network provide an effective weight transformation to the input data identical to the "optimal" least-squares solution as defined by McWhirter, J.G., "Recursive Least-Squares Minimization using a Systolic Array", Proc. SPIE, Vol. 431, Real-Time Signal Processing VI, 1983, discussed later. The convergence characteristics of the Sequential Decorrelator as described in patent 1,599,035 differ significantly from the optimal least-squares solution if the network is operated "on the fly" with data samples continuously applied to the processor. Optimal convergence can only be obtained by re-cycling input data through to network and by updating the decorrelation weights on a rank by rank basis. This mode of operation obviously detracts from real-time application. With the standard sequential decorrelator, each decorrelation cell adaptively combines the applied signals as shown by Fig. 2. The decorrelation weight is derived from the ratio of unnormalized, sampled data estimates of the cross and autocorrelation of the input signals. Hence, we have: where and and where * denotes a complex conjugate. Since the V²(k) factor is used by all decorrelation stages within a particular rank, then autocorrelation estimates in fact can be calculated by a separate processing stage as shown by Fig. 3. The arrangement of Fig. 3 utilises two types of processing cell referred to as "boundary cell" and "internal cell" respectively. Figs. 4a-4d show schematic diagrams of the different processing stages for the standard sequential decorrelator. Fig. 4b is a detailed expansion of the simple schematic stage shown in Fig. 4a and Fig. 4d is a detailed expansion of the simple schematic shown in Fig. 4c. For a fuller understanding of the standard Sequential Decorrelator and in particular the boundary and internal processing cell functions the reader is directed to IEE proceedings, Vol.131, Pt. F, No. 6, October 1984, pages 638-645, "Application of a systolic array to adaptive beamforming" Ward et al., especially Fig. 5 and 6 thereof. This paper discloses the basic form of triangular array of cascaded stages, each stage including a boundary cell and a group of internal cells, the group in each stage having one less internal cell than the group of the preceding stage and the first stage group having one less internal cell than the number of antenna elements. Note that in Fig. 4d of the present application the box labelled "half complex multiply" multiplies a complex number U(k) by a real number D. The basis of this invention is to show how the standard sequence decorrelator architecture (Figs. 3, 4(b) and 4(d) can be simply modified to provide an identical adaption characteristic as the optimal least-squares architecture which implements the QR decomposition algorithm. According to the present invention there is provided a sequential decorrelator arrangement for an adaptive antenna array, the array comprising a plurality of antenna elements the outputs of which feed a cascaded beamforming network having a succession of stages, each stage including a boundary cell and a group of internal cells, the internal cells in each stage comprising means for combining input data in accordance with a weighting factor (D) common to all the internal cells in a stage, the boundary cell associated with each stage comprising means for computing the weighting factor (D) to be applied to the internal cells of the stage from the output of one internal cell of the preceding stage and propagating an λ factor through successive boundary cells, the output of the last internal cell of the last stage being multiplied by the λ factor output of the last boundary cell, the group in each stage having one less internal cell than the group of the preceding stage and the first stage group having one less internal cell than the number of antenna elements, each internal cell of the first stage having as one input the output of a respective antenna element and as a second input an output of the associated boundary cell and each cell of each subsequent stage having as one input the output of a respective internal cell of the preceding stage and each internal cell having as a second input an output from the associated boundary cell of the same stage, characterised in that each stage includes means for scaling the output of each internal cell in the stage by a scaling factor (β) derived at each boundary cell from the ratio of the square root of the autocorrelation value of the input signal signal of said boundary cell at the last and current updates and and in that each boundary cell provides at its output the signal present on its input, the weighting factor (D) being a function of the autocorrelation value of said input signal and the λ factor itself being scaled by the reciprocal of the factor β in each boundary cell. For a clearer understanding of the invention, the term "weighting" as used herein denotes the application of a factor or numerical quantity which, when applied to a decorrelation cell, affects the processing of the applied signals in the cell. Likewise the term "scaling" as used herein denotes the application of a factor or numerical quantity which when applied to the output from a decorrelation cell, simply alters the value of the output. Although both a weighting factor and a scaling factor may be derived from the same boundary cell the two factors can be derived according to different mathematical processes and can therefore have different values. Embodiments of the invention will now be described with reference to the accompanying drawings, in which:- Fig. 1 illustrates a known standard sequential decorrelator arrangement, Fig. 2 illustrates a simplified representation of a known decorrelation cell, Fig. 3 illustrates a parallel architecture for a standard sequential decorrelator arrangement described in terms of boundary and internal processing cells. Figs. 4a-4d illustrate processing cells for a sequential decorrelator, Fig. 5 illustrates a basic adaptive antenna array, Fig. 6 illustrates a two channel stage using the QR decomposition algorithm, Fig. 7 illustrates obtaining the optimal Least Squares architecture which implements the QR decomposition algorithm, (prior art from J.G. McWhirter, supra). Figs. 8a-8b illustrate processing cell functions for the parallel architecture which implements the QR decomposition algorithm, Fig. 9 illustrates the structure of a sequential decorrelator arrangement according to the invention, Fig. 10 illustrates a boundary processing cell of the sequential decorrelator of Fig. 9. Referring to Fig. 5, the vector of residuals from the array is given by: e (k) = X (k) w (k) + y (k) where The "optimal" adaptive control law is defined as the weight solution which minimizes the norm of the residual vector, e k . Since the quantity is representative of the best estimate of the output power from the array after k data snapshots, the weight set which minimizes the norm of e k will in fact be the Maximum Likelihood estimate of the weight solution which minimizes the output power from the array. The optimal solution can be derived by the least-squares, QR decomposition processing algorithm. This technique performs a triangularization of the data matrix, X k using a sequence of pipelined Givens rotations and then involves a back substitution process to solve for the weight set w n . Kung, H.T. and Gentleman, W.M., "Matrix Triangularization by Systolic Arrays", Proc. SPIE, Vol. 298, Real-Time Signal Processing IV, 1981, have recently shown how a pair of processing arrays may be used to implement the triangularization stage and then provide back-substitution. McWhirter (supra) has described a modified version of Kung and Gentleman's QR processing array in which the least-squares residual is produced quite simply and directly at every stage without solving the corresponding triangular linear system. An analogy with this enhanced processing array is used to demonstrate how the Sequential Decorrelator as described originally by British patent No. 1,599,035 can be modified to provide an adaptive performance identical to the least-squares control law defined above. A decorrelation cell can be constructed with the QR decomposition algorithm and is shown by Fig. 6. It consists of two essential processing nodes; (i) the boundary stage, which computes the "rotation coefficients", and (ii) the internal processor, which performs the rotational transform. The terms V(k) and U(k) are effectively stored within the two processing stages and are resultant from the previous rotation following the application of the k th input data samples. Using the previous notation we define and When the samples, x(k+1) and y(k+1) are applied to the cell, a new transformation is computed whereby Now, the coefficients c and s denoting the rotation transform are: and This therefore gives for the resultant factors A and B and The important term of the transformed matrix described by equation (4) is α(K+1) since this will be an integral part of the required output from the decorrelation cell. Therefore, computing α(K+1) gives: and substituting for coefficients C and S gives Now U(k) = U(k+1) - x*(k+1) y(k+1) (10) so that This can be reduced to: Choosing λ(K+1) = c = V(k)/V(k+1) and forming the product α(k+1).λ(k+1) then gives It is therefore noted that the product α(K+1) . λ(K+1) is equivalent to a "beamformed" output: y'(k+1) = α(K+1) . λ(K+1) = y(k+1) + W(k+1).x(k+1) with the weight value W(k+1) given by: It should be noted that this result corresponds exactly to that for the 'conventional' decorrelation cell where the weight coefficient is computed from the quotient of recursively updated cross- and auto-covariance estimates. Previous work by McWhirter has shown how a number of these decorrelation stages (based on the QR decomposition algorithm) can be cascaded to form an arbitrary N element decorrelation network. A 4 input channel example capable of processing signals from 4 separate antenna elements is shown by Fig. 7 with corresponding cell descriptions given by Figs. 8a, 8b. Since the stored components in the networks shown by Figs. 3 and 7 are essentially identical, it is possible to deduce how the standard Sequential Decorrelator can be modified to provide an identical adaptation characteristic as the QR decomposition architecture. The modified sequential decorrelator is shown by Fig. 9, from which we note that; (i) the output from each internal (rectangular) stage is scaled to provide the α factor as produced by the optimal QR decomposition architecture (depicted in Fig. 7). The scaling factor, β is calculated in the boundary (circular) stage. (ii) the boundary stage is further modified to derive the producted λ factors transferred along the diagonal edge of the network. By factorising equation (12) we have that; Therefore, the scaling factor, β, is which can be readily evaluated with minimal modification to the boundary cell of the sequential decorrelator architecture. A schematic diagram detailing the internal operation of the boundary stage of the modified network is shown by Fig. 10. In this modified cell, further processing is required to handle the λ factor which is propagated along the boundary edge of the array. We thus have at the i th boundary cell of the modified sequential decorrelator architecture;

Claims: A sequential decorrelator arrangement for an adaptive antenna array, the array comprising a plurality of antenna elements the outputs of which feed a cascaded beamforming network having a succession of stages, each stage including a boundary cell and a group of internal cells, the internal cells in each stage comprising means for combining input data in accordance with a weighting factor (D) common to all the internal cells in a stage, the boundary cell associated with each stage comprising means for computing the weighting factor (D) to be applied to the internal cells of the stage from the output of one internal cell of the preceding stage and propagating an λ factor through successive boundary cells, the output of the last internal cell of the past stage being multiplied by the λ factor output of the last boundary cell, the group in each stage having one less internal cell than the group of the preceding stage and the first stage group having one less internal cell than the number of antenna elements, each internal cell of the first stage having as one input the output of a respective antenna element and as a second input an output of the associated boundary cell and each cell of each subsequent stage having as one input the output of a respective internal cell of the preceding stage and each internal cell having as a second input an output from the associated boundary cell of the same stage, characterised in that each stage includes means for scaling the output of each internal cell in the stage by a scaling factor (β) derived at each boundary cell from the ratio of the square root of the autocorrelation value of the input signal of said boundary cell at the last and current updates and in that each boundary cell provides at its output the signal present on its input, the weighting factor (D) being a function of the autocorrelation value of said input signal and the λ factor itself being scaled by the reciprocal of the factor β in each boundary cell,. A method of sequentially decorrelating by the least-squares QR decomposition algorithm signals received from an antenna array using cascaded stages of cells, wherein each stage includes a group of internal cells and a boundary cell, each internal cell of a stage being arranged to decorrelate an output of the boundary cell of that stage with the output of an internal cell of the preceding stage by applying rotational transforms thereto in accordance with a weighting factor (D) which is common to all the internal cells of the stage, each boundary cell being arranged to compute from the output of one internal cell of the preceding stage the weighting factor (D) to be applied to the internal cells of the stage and to propagate an λ factor through successive boundary cells, the output of the last internal cell of the last stage being multiplied by the λ factor output of the last boundary cell, characterised in that the method further includes the application of a scaling factor (β) for scaling the outputs of the internal cells of each stage, said scaling factor being derived at each boundary cell from the ratio of the square root of the autocorrelation value of the input signal of said boundary cell at the last and current updates, and in that each boundary cell is arranged to provide at its output the signal present on its input, and to compute the weighing factor (D) as a function of the autocorrelation value of said input signal, the λ factor itself being scaled by the reciprocal of the factor β in each boundary cell.

Inventors: ROBSON, ANTHONY JOHN, WARD, CHRISTOPHER ROBERT
Applicants: NORTHERN TELECOM LIMITED
Application year: 1985
Main classification: H01Q3
IPC codes: H01Q3