How to keep running the task even if the app is already closed?

Currently, i have built a service to do httpRequest to my php server every 15 seconds. The code works well but the problem is when i close the app the httpRequest is stop working and does not send data anymore. what i want to do is when i close the app, the httpRequest still running. what should i change in my code?

here is the code i have been working on...

public class NotificationService extends Service {

public String response;

@Override
public IBinder onBind(Intent intent) {
    return null;
}

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    Toast.makeText(this, "on start command", Toast.LENGTH_SHORT).show();
    new MyAsyncTask().execute("muhammad sappe");
    return START_NOT_STICKY;
}

private class MyAsyncTask extends AsyncTask<String, String, String>{

  @Override
  protected String doInBackground(String... params) {
    // TODO Auto-generated method stub
    postData(params[0]);
    return null;
  }

  @Override
  protected void onPostExecute(String result) {
      new Handler().postDelayed(new Runnable() {
            public void run() {
                new MyAsyncTask().execute("muhammad sappe");
            }
        }, 15000);
    super.onPostExecute(result);
  }

}

public void postData(String MyName){
    HttpClient httpclient = new DefaultHttpClient();
    HttpPost httppost = new HttpPost("http://mydomain/getData.php");
    try {   
        // Add your data
        List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(1);
        nameValuePairs.add(new BasicNameValuePair("action", MyName));

        httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
        // Execute HTTP Post Request
        ResponseHandler<String> responseHandler = new BasicResponseHandler();
        response = httpclient.execute(httppost, responseHandler);
    }catch(ClientProtocolException e){
        // TODO Auto-generated catch block
    }catch(IOException e) {
        // TODO Auto-generated catch block
    }
}

public void onDestroy() {
    super.onDestroy();
}
}