Check out this 15 minute video from Grandstream on the advanced configuration of roaming among Wi-Fi access points. Learn when an access point needs to drop a connection so another one can take the connection and learn what settings need to be configured to optimise roaming.
Note that not all hand-held wireless devices like smart phones, tablets and laptops are created equal. Many have hardware limitations in how quickly they can switch from one access point to another and with the distance from which they can connect to a Wi-Fi network.