platform.ready().then(() => {
  this.admobFree.interstitial.config(this.interstitialConfig);
  this.admobFree.interstitial.prepare()
    .then(() => { }).catch(e => alert(e));
});

this.admobFree.on('admob.interstitial.events.CLOSE').subscribe(() => {
  this.admobFree.interstitial.prepare()
    .then(() => {}).catch(e => alert(e));
});