Labels overlaps and more spaces between letters in cesium

I have a geojson loaded in cesium and also i had added labels for the layer with below code

positions = entity.polygon.hierarchy['_value'].positions;

        center = Cesium.BoundingSphere.fromPoints(positions).center;
        Cesium.Ellipsoid.WGS84.scaleToGeodeticSurface(center, center);
        entity.position = new Cesium.ConstantPositionProperty(center);
        label = {
                            font : 'bold 10px Helvetica',
                            fillColor : Cesium.Color.BLACK,
                            outlineColor : Cesium.Color.WHITE,
                            outlineWidth : 5,
                            style : Cesium.LabelStyle.FILL_AND_OUTLINE,
                            verticalOrigin : Cesium.VerticalOrigin.BOTTOM,
                            pixelOffset : new Cesium.Cartesian2(0, -9)

        entity.label = label;

I could see labels overlapping when zoomed out,more spaces between the letters and also some labels hidden inside the 3D building. Snapshot attached will make it clear. How this labels can be fixed?