Unfortunately this is not possible. You could do is try disabling low MCS data rates but you need to have at least one legacy rate enabled. The best bet you can have is to enable rates 54Mbs and above.
Hope that helps.
802.11n cant work alone, it runs on either 802.11g or 802.11a. If you have the 802.11N supported APs and then if you have enabled N on the WLC global, then you wil get N speeds if you have configured the clients for either
Open Auth + no encryptions + all MCS rates enabled + WMM enabled
WPA2 (PSK/Enterprise) + AES encryption only + all MCS rates enabled + WMM enabled
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