Why show error unexpected namespace prefix xmlns in android?

I want to display Google ads in my app but it shows error `unexpected namespace prefix xmlns` at this tag `xmlns:ads="http://schemas.android.com/apk/lib/com.google.ads"`. If I remove this line then it shows `Error parsing XML:unbound prefix`. Please help me to identify that where things go wrong. Here is my code. ``` <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:id="@+id/LinearLayout1" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" android:background="@drawable/background_port" tools:context=".MainActivity"> <LinearLayout xmlns:ads="http://schemas.android.com/apk/lib/com.google.ads" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="wrap_content" android:gravity="center" > <com.google.ads.AdView android:id="@+id/adView" android:layout_width="wrap_content" android:layout_height="wrap_content" ads:adUnitId="a151b801b7c1d6b" ads:adSize="BANNER" ads:testDevices="TEST_EMULATOR, TEST_DEVICE_ID" ads:loadAdOnCreate="true"/> </LinearLayout> -------------------- ---- More views ---- -------------------- </LinearLayout> ```