% "FIR Filter Design via Spectral Factorization and Convex Optimization" example
% by S.-P. Wu, S. Boyd, and L. Vandenberghe
% (figures are generated)
%
% Designs a uniform linear antenna array using spectral factorization method where:
% - it minimizes sidelobe level outside the beamwidth of the pattern
% - it has a constraint on the maximum ripple around unit gain in the beamwidth
%
%   minimize   max |y(theta)|                   for theta in the stop-beamwidth
%       s.t.   1/delta <= |y(theta)| <= delta   for theta in the pass-beamwidth
%
% We first replace the look-angle variable theta with the "frequency"
% variable omega, defined by omega = -2*pi*d/lambda*cos(theta).
% This transforms the antenna pattern y(theta) into a standard discrete
% Fourier transform of array weights w. Then we apply another change of
% variables: we replace w with its auto-correlation coefficients r.
%
% Now the problem can be solved via spectral factorization approach:
%
%   minimize   max R(omega)                        for omega in the stopband
%       s.t.   (1/delta)^2 <= R(omega) <= delta^2  for omega in the passband
%              R(omega) >= 0                       for all omega
%
% where R(omega) is the squared magnitude of the y(theta) array response
% (and the Fourier transform of the autocorrelation coefficients r).
% Variables are coefficients r. delta is the allowed passband ripple.
% This is a convex problem (can be formulated as an LP after sampling).
%
% Written for CVX by Almir Mutapcic 02/02/06

%********************************************************************
% problem specs: a uniform line array with inter-element spacing d
%                antenna element locations are at d*[0:n-1]
%                (the array pattern will be symmetric around origin)
%********************************************************************
n = 20;               % number of antenna elements
lambda = 1;           % wavelength
d = 0.45*lambda;      % inter-element spacing

% passband direction from 30 to 60 degrees (30 degrees bandwidth)
% transition band is 15 degrees on both sides of the passband
theta_pass = 40;
theta_stop = 50;

% passband max allowed ripple
ripple = 0.1; % in dB (+/- around the unit gain)

%********************************************************************
% construct optimization data
%********************************************************************
% number of frequency samples
m = 30*n;

% convert passband and stopband angles into omega frequencies
omega_zero = -2*pi*d/lambda;
omega_pass = -2*pi*d/lambda*cos(theta_pass*pi/180);
omega_stop = -2*pi*d/lambda*cos(theta_stop*pi/180);
omega_pi   = +2*pi*d/lambda;

% build matrix A that relates R(omega) and r, ie, R = A*r
omega = linspace(-pi,pi,m)';
A = exp( -j*omega(:)*[1-n:n-1] );

% passband constraint matrix
Ap = A(omega >= omega_zero & omega <= omega_pass,:);

% stopband constraint matrix
As = A(omega >= omega_stop & omega <= omega_pi,:);

%********************************************************************
% formulate and solve the magnitude design problem
%********************************************************************
cvx_begin
  variable r(2*n-1,1) complex
  % minimize stopband attenuation
  minimize( max( real( As*r ) ) )
  subject to
    % passband ripple constraints
    (10^(-ripple/20))^2 <= real( Ap*r ) <= (10^(+ripple/20))^2;
    % nonnegative-real constraint for all frequencies
    % a bit redundant: the passband frequencies are already constrained
    real( A*r ) >= 0;
    % auto-correlation symmetry constraints
    imag(r(n)) == 0;
    r(n-1:-1:1) == conj(r(n+1:end));
cvx_end

% check if problem was successfully solved
if ~strfind(cvx_status,'Solved')
    return
end

% find antenna weights by computing the spectral factorization
w = spectral_fact(r);

% divided by 2 since this is in PSD domain
min_sidelobe_level = 10*log10( cvx_optval );
fprintf(1,'The minimum sidelobe level is %3.2f dB.\n\n',...
          min_sidelobe_level);

%********************************************************************
% plots
%********************************************************************
% build matrix G that relates y(theta) and w, ie, y = G*w
theta = [-180:180]';
G = kron( cos(pi*theta/180), [0:n-1] );
G = exp(2*pi*i*d/lambda*G);
y = G*w;

% plot array pattern
figure(1), clf
ymin = -40; ymax = 5;
plot([-180:180], 20*log10(abs(y)), ...
     [theta_stop theta_stop],[ymin ymax],'r--',...
     [-theta_pass -theta_pass],[ymin ymax],'r--',...
     [-theta_stop -theta_stop],[ymin ymax],'r--',...
     [theta_pass theta_pass],[ymin ymax],'r--');
xlabel('look angle'), ylabel('mag y(theta) in dB');
axis([-180 180 ymin ymax]);

% polar plot
figure(2), 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_level + zerodB)*cos(pi*theta/180), ...
      (min_sidelobe_level + zerodB)*sin(pi*theta/180),'k:')  % min level
text(-zerodB,0,'0 dB')
text(-(min_sidelobe_level + zerodB),0,sprintf('%0.1f dB',min_sidelobe_level));
plot([0 60*cos(theta_pass*pi/180)], [0 60*sin(theta_pass*pi/180)], 'k:')
plot([0 60*cos(-theta_pass*pi/180)],[0 60*sin(-theta_pass*pi/180)],'k:')
plot([0 60*cos(theta_stop*pi/180)], [0 60*sin(theta_stop*pi/180)], 'k:')
plot([0 60*cos(-theta_stop*pi/180)],[0 60*sin(-theta_stop*pi/180)],'k:')
hold off

 
Calling Mosek 9.1.9: 1171 variables, 40 equality constraints
   For improved efficiency, Mosek is solving the dual problem.
------------------------------------------------------------

MOSEK Version 9.1.9 (Build date: 2019-11-21 11:32:15)
Copyright (c) MOSEK ApS, Denmark. WWW: mosek.com
Platform: MACOSX/64-X86

MOSEK warning 710: #19 (nearly) zero elements are specified in sparse col '' (571) of matrix 'A'.
MOSEK warning 710: #19 (nearly) zero elements are specified in sparse col '' (1170) of matrix 'A'.
Problem
  Name                   :                 
  Objective sense        : min             
  Type                   : LO (linear optimization problem)
  Constraints            : 40              
  Cones                  : 0               
  Scalar variables       : 1171            
  Matrix variables       : 0               
  Integer variables      : 0               

Optimizer started.
Presolve started.
Linear dependency checker started.
Linear dependency checker terminated.
Eliminator started.
Freed constraints in eliminator : 0
Eliminator terminated.
Eliminator - tries                  : 1                 time                   : 0.00            
Lin. dep.  - tries                  : 1                 time                   : 0.00            
Lin. dep.  - number                 : 0               
Presolve terminated. Time: 0.01    
Problem
  Name                   :                 
  Objective sense        : min             
  Type                   : LO (linear optimization problem)
  Constraints            : 40              
  Cones                  : 0               
  Scalar variables       : 1171            
  Matrix variables       : 0               
  Integer variables      : 0               

Optimizer  - threads                : 8               
Optimizer  - solved problem         : the primal      
Optimizer  - Constraints            : 40
Optimizer  - Cones                  : 0
Optimizer  - Scalar variables       : 1106              conic                  : 0               
Optimizer  - Semi-definite variables: 0                 scalarized             : 0               
Factor     - setup time             : 0.00              dense det. time        : 0.00            
Factor     - ML order time          : 0.00              GP order time          : 0.00            
Factor     - nonzeros before factor : 820               after factor           : 820             
Factor     - dense dim.             : 0                 flops                  : 1.68e+06        
ITE PFEAS    DFEAS    GFEAS    PRSTATUS   POBJ              DOBJ              MU       TIME  
0   8.9e+02  4.0e+00  7.9e+00  0.00e+00   2.947569365e+00   0.000000000e+00   4.0e+00  0.01  
1   1.2e+02  5.5e-01  1.1e+00  2.71e+00   -1.529008670e-01  -2.384358179e-01  5.5e-01  0.02  
2   4.9e+01  2.2e-01  4.4e-01  3.97e+00   -4.425618567e-02  -5.651547334e-02  2.2e-01  0.02  
3   2.1e+01  9.5e-02  2.0e-01  2.18e+00   -1.331325515e-02  -1.707565750e-02  9.6e-02  0.02  
4   1.1e+01  4.9e-02  1.0e-01  1.41e+00   -6.910651456e-03  -8.672733903e-03  4.9e-02  0.02  
5   4.0e+00  1.8e-02  3.8e-02  1.21e+00   -4.094147002e-03  -4.716241084e-03  1.8e-02  0.03  
6   2.1e+00  9.3e-03  1.9e-02  1.00e+00   -3.687505923e-03  -4.013387222e-03  9.4e-03  0.03  
7   8.6e-01  3.9e-03  8.0e-03  9.67e-01   -3.495981730e-03  -3.632753953e-03  3.9e-03  0.03  
8   4.4e-02  2.0e-04  4.1e-04  1.01e+00   -3.432986425e-03  -3.439912616e-03  2.0e-04  0.03  
9   1.3e-02  6.0e-05  1.2e-04  9.94e-01   -3.429045476e-03  -3.431146180e-03  6.1e-05  0.03  
10  1.3e-03  5.8e-06  1.2e-05  1.00e+00   -3.427957151e-03  -3.428160194e-03  5.9e-06  0.03  
11  2.7e-05  1.2e-07  2.5e-07  1.00e+00   -3.428141962e-03  -3.428146250e-03  1.2e-07  0.04  
12  2.7e-09  1.2e-11  2.6e-11  1.00e+00   -3.428146796e-03  -3.428146797e-03  1.3e-11  0.04  
Basis identification started.
Primal basis identification phase started.
Primal basis identification phase terminated. Time: 0.00
Dual basis identification phase started.
Dual basis identification phase terminated. Time: 0.00
Basis identification terminated. Time: 0.00
Optimizer terminated. Time: 0.05    


Interior-point solution summary
  Problem status  : PRIMAL_AND_DUAL_FEASIBLE
  Solution status : OPTIMAL
  Primal.  obj: -3.4281467964e-03   nrm: 1e+00    Viol.  con: 5e-11    var: 0e+00  
  Dual.    obj: -3.4281467968e-03   nrm: 1e+00    Viol.  con: 0e+00    var: 3e-15  

Basic solution summary
  Problem status  : PRIMAL_AND_DUAL_FEASIBLE
  Solution status : OPTIMAL
  Primal.  obj: -3.4281467969e-03   nrm: 1e+00    Viol.  con: 8e-16    var: 0e+00  
  Dual.    obj: -3.4281467968e-03   nrm: 1e+00    Viol.  con: 0e+00    var: 1e-13  
Optimizer summary
  Optimizer                 -                        time: 0.05    
    Interior-point          - iterations : 12        time: 0.04    
      Basis identification  -                        time: 0.00    
        Primal              - iterations : 0         time: 0.00    
        Dual                - iterations : 0         time: 0.00    
        Clean primal        - iterations : 0         time: 0.00    
        Clean dual          - iterations : 0         time: 0.00    
    Simplex                 -                        time: 0.00    
      Primal simplex        - iterations : 0         time: 0.00    
      Dual simplex          - iterations : 0         time: 0.00    
    Mixed integer           - relaxations: 0         time: 0.00    

------------------------------------------------------------
Status: Solved
Optimal value (cvx_optval): +0.00342815
 
The minimum sidelobe level is -24.65 dB.