iqbal: Improve convergence of the optimization

 - We need to only take the general direction of the gradient
   and not the actual value (hence the division by sum of abs)

 - We take new values even if the score is equal, it might get
   us out of a dead lock

 - When reducing the step size, we look at how much we overshoot
   to get a better new step size

 - If the gain was lower than 1%, we quit, but we still take the
   new value

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

1 files changed, 8 insertions, 7 deletions

diff --git a/src/iqbal.c b/src/iqbal.c
index f4c29c6..0bb6a54 100644
--- a/src/iqbal.c
+++ b/src/iqbal.c
@@ -319,26 +319,27 @@ osmo_iqbal_cxvec_optimize(const struct osmo_cxvec *sig, float *mag, float *phase
 	}
 
 	cv = _iqbal_objfn_val_gradient(state, cx, cgrad);
-	step = 0.1f * cv / (fabs(cgrad[0]) + fabs(cgrad[1]));
+	step = cv / (fabs(cgrad[0]) + fabs(cgrad[1]));
 
 	for (i=0; i<opts->max_iter; i++)
 	{
-		nx[0] = cx[0] - step * cgrad[0];
-		nx[1] = cx[1] - step * cgrad[1];
+		nx[0] = cx[0] - step * (cgrad[0] / (fabs(cgrad[0]) + fabs(cgrad[1])));
+		nx[1] = cx[1] - step * (cgrad[1] / (fabs(cgrad[0]) + fabs(cgrad[1])));
 
 		nv = _iqbal_objfn_value(state, nx);
 
-		if (nv < cv) {
+		if (nv <= cv) {
 			p = (cv - nv) / cv;
-			if (p < 0.01f)
-				break;
 
 			cx[0] = nx[0];
 			cx[1] = nx[1];
 			cv = nv;
 			_iqbal_objfn_gradient(state, cx, cv, cgrad);
+
+			if (p < 0.01f)
+				break;
 		} else {
-			step /= 2.0f;
+			step /= 2.0 * (nv / cv);
 		}
 	}