# Minimize beamwidth of an array with arbitrary 2-D geometry

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    % "Convex optimization examples" lecture notes (EE364) by S. Boyd
    % "Antenna array pattern synthesis via convex optimization"
    % by H. Lebret and S. Boyd
    % (figures are generated)
    %
    % Designs an antenna array such that:
    % - it has unit sensitivity at some target direction
    % - it obeys a constraint on a minimum sidelobe level outside the beam
    % - it minimizes the beamwidth of the pattern
    %
    % This is a quasiconvex problem and can be solved using a bisection.
    %
    %   minimize   max |y(theta)|     for theta outside the beam
    %       s.t.   y(theta_tar) = 1
    %
    % where y is the antenna array gain pattern (complex function) and
    % variables are w (antenna array weights or shading coefficients).
    % Gain pattern is a linear function of w: y(theta) = w'*a(theta)
    % for some a(theta) describing antenna array configuration and specs.
    %
    % Written for CVX by Almir Mutapcic 02/02/06

    % select array geometry
    ARRAY_GEOMETRY = '2D_RANDOM';
    % ARRAY_GEOMETRY = '1D_UNIFORM_LINE';
    % ARRAY_GEOMETRY = '2D_UNIFORM_LATTICE';

    %********************************************************************
    % problem specs
    %********************************************************************
    lambda = 1;           % wavelength
    theta_tar = 60;       % target direction
    min_sidelobe = -20;   % maximum sidelobe level in dB

    max_half_beam = 50;   % starting half beamwidth (must be feasible)

    %********************************************************************
    % random array of n antenna elements
    %********************************************************************
    if strcmp( ARRAY_GEOMETRY, '2D_RANDOM' )
      % set random seed to repeat experiments
      rand('state',0);

      % (uniformly distributed on [0,L]-by-[0,L] square)
      n = 36;
      L = 5;
      loc = L*rand(n,2);

    %********************************************************************
    % uniform 1D array with n elements with inter-element spacing d
    %********************************************************************
    elseif strcmp( ARRAY_GEOMETRY, '1D_UNIFORM_LINE' )
      % (unifrom array on a line)
      n = 30;
      d = 0.45*lambda;
      loc = [d*[0:n-1]' zeros(n,1)];

    %********************************************************************
    % uniform 2D array with m-by-m element with d spacing
    %********************************************************************
    elseif strcmp( ARRAY_GEOMETRY, '2D_UNIFORM_LATTICE' )
      m = 6; n = m^2;
      d = 0.45*lambda;

      loc = zeros(n,2);
      for x = 0:m-1
        for y = 0:m-1
          loc(m*y+x+1,:) = [x y];
        end
      end
      loc = loc*d;

    else
      error('Undefined array geometry')
    end

    %********************************************************************
    % construct optimization data
    %********************************************************************
    % build matrix A that relates w and y(theta), ie, y = A*w
    theta = [1:360]';
    A = kron(cos(pi*theta/180), loc(:,1)') + kron(sin(pi*theta/180), loc(:,2)');
    A = exp(2*pi*i/lambda*A);

    % target constraint matrix
    [diff_closest, ind_closest] = min( abs(theta - theta_tar) );
    Atar = A(ind_closest,:);

    %*********************************************************************
    % use bisection algorithm to solve the problem
    %*********************************************************************

    halfbeam_bot = 1;
    halfbeam_top = max_half_beam;

    disp('We are only considering integer values of the half beam-width')
    disp('(since we are sampling the angle with 1 degree resolution).')
    disp(' ')

    while( halfbeam_top - halfbeam_bot > 1)
      % try to find a feasible design for given specs
      halfbeam_cur = ceil( (halfbeam_top + halfbeam_bot)/2 );

      % create optimization matrices for the stopband
      ind = find(theta <= (theta_tar-halfbeam_cur) | ...
                 theta >= (theta_tar+halfbeam_cur) );
      As = A(ind,:);

      % formulate and solve the feasibility antenna array problem
      cvx_begin quiet
        variable w(n) complex
        % feasibility problem
        Atar*w == 1;
        abs(As*w) <= 10^(min_sidelobe/20);
      cvx_end

      % bisection
      if strfind(cvx_status,'Solved') % feasible
        fprintf(1,'Problem is feasible for half beam-width = %d degress\n',...
                   halfbeam_cur);
        halfbeam_top = halfbeam_cur;
      else % not feasible
        fprintf(1,'Problem is not feasible for half beam-width = %d degress\n',...
                   halfbeam_cur);
        halfbeam_bot = halfbeam_cur;
      end
    end

    % optimal beamwidth
    halfbeam = halfbeam_top;
    fprintf(1,'\nOptimum half beam-width for given specs is %d.\n',halfbeam);

    % compute the minimum noise design for the optimal beamwidth
    ind = find(theta <= (theta_tar-halfbeam) | ...
               theta >= (theta_tar+halfbeam) );
    As = A(ind,:);
    cvx_begin quiet
      variable w(n) complex
      minimize( norm( w ) )
      subject to
        Atar*w == 1;
        abs(As*w) <= 10^(min_sidelobe/20);
    cvx_end

    %********************************************************************
    % plots
    %********************************************************************
    figure(1), clf
    plot(loc(:,1),loc(:,2),'o')
    title('Antenna locations')

    % plot array pattern
    y = A*w;

    figure(2), clf
    ymin = -40; ymax = 0;
    plot([1:360], 20*log10(abs(y)), ...
         [theta_tar theta_tar],[ymin ymax],'g--',...
         [theta_tar+halfbeam theta_tar+halfbeam],[ymin ymax],'r--',...
         [theta_tar-halfbeam theta_tar-halfbeam],[ymin ymax],'r--');
    xlabel('look angle'), ylabel('mag y(theta) in dB');
    axis([0 360 ymin ymax]);

    % polar plot
    figure(3), clf
    zerodB = 50;
    dBY = 20*log10(abs(y)) + zerodB;
    plot(dBY.*cos(pi*theta/180), dBY.*sin(pi*theta/180), '-');
    axis([-zerodB zerodB -zerodB zerodB]), axis('off'), axis('square')
    hold on
    plot(zerodB*cos(pi*theta/180),zerodB*sin(pi*theta/180),'k:') % 0 dB
    plot( (min_sidelobe + zerodB)*cos(pi*theta/180), ...
          (min_sidelobe + zerodB)*sin(pi*theta/180),'k:')  % min level
    text(-zerodB,0,'0 dB')
    text(-(min_sidelobe + zerodB),0,sprintf('%0.1f dB',min_sidelobe));
    theta_1 = theta_tar+halfbeam;
    theta_2 = theta_tar-halfbeam;
    plot([0 55*cos(theta_tar*pi/180)], [0 55*sin(theta_tar*pi/180)], 'k:')
    plot([0 55*cos(theta_1*pi/180)], [0 55*sin(theta_1*pi/180)], 'k:')
    plot([0 55*cos(theta_2*pi/180)], [0 55*sin(theta_2*pi/180)], 'k:')
    hold off

    We are only considering integer values of the half beam-width
    (since we are sampling the angle with 1 degree resolution).

    Problem is feasible for half beam-width = 26 degress
    Problem is feasible for half beam-width = 14 degress
    Problem is not feasible for half beam-width = 8 degress
    Problem is feasible for half beam-width = 11 degress
    Problem is feasible for half beam-width = 10 degress
    Problem is not feasible for half beam-width = 9 degress

    Optimum half beam-width for given specs is 10.