Cisco AP 1252AG Cannot setup More than 54Mb/s speed in radio 0 (2.4G)

Question

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I have a problem when I configure CISCO AP 1252, it cannot support more than 54Mb/s speed but support 802.n which speed can up to 300Mb/s. See below :
here is dot11radio 0 speed option: (The MAX speed is 54Mb/s by manually, how can i setup more than 54Mb/s, because of this AP support 802.n. So the speed should be more than 54M/s, like 300M)
AP#sho inter dot11Radio 0
Dot11Radio0 is up, line protocol is up
   Hardware is 802.11N 2.4Ghz Radio, address is *************
   MTU 1500 bytes, BW 54000 Kbit, DLY 1000 usec,
AP(config)#inter dot11Radio 0
AP(config-if)#speed ?
   1.0         Allow 1.0 Mb/s rate
   11.0        Allow 11.0 Mb/s rate
   12.0        Allow 12.0 Mb/s rate
   18.0        Allow 18.0 Mb/s rate
   2.0         Allow 2.0 Mb/s rate
   24.0        Allow 24.0 Mb/s rate
   36.0        Allow 36.0 Mb/s rate
   48.0        Allow 48.0 Mb/s rate
   5.5         Allow 5.5 Mb/s rate
   54.0        Allow 54.0 Mb/s rate
***(why here without more than 54M/s rate, CISCO AP 1252 support IEEE802/n)
   6.0         Allow 6.0 Mb/s rate
   9.0         Allow 9.0 Mb/s rate
   basic-1.0   Require 1.0 Mb/s rate
   basic-11.0  Require 11.0 Mb/s rate
   basic-12.0  Require 12.0 Mb/s rate
   basic-18.0  Require 18.0 Mb/s rate
   basic-2.0   Require 2.0 Mb/s rate
   basic-24.0  Require 24.0 Mb/s rate
   basic-36.0  Require 36.0 Mb/s rate
   basic-48.0  Require 48.0 Mb/s rate
   basic-5.5   Require 5.5 Mb/s rate
   basic-54.0  Require 54.0 Mb/s rate
***(why here without over than 54Mb/s Rate?, it support IEEE802.n)
   basic-6.0   Require 6.0 Mb/s rate
   basic-9.0   Require 9.0 Mb/s rate
   default     Set default rates
   m0-7        Allow MCS rate indices 0-7
   m0.         Allow MCS rate index 0
   m1.         Allow MCS rate index 1
   m10.        Allow MCS rate index 10
   m11.        Allow MCS rate index 11
   m12.        Allow MCS rate index 12
   m13.        Allow MCS rate index 13
   m14.        Allow MCS rate index 14
   m15.        Allow MCS rate index 15
   m2.         Allow MCS rate index 2
   m3.         Allow MCS rate index 3
   m4.         Allow MCS rate index 4
   m5.         Allow MCS rate index 5
   m6.         Allow MCS rate index 6
   m7.         Allow MCS rate index 7
   m8-15       Allow MCS rate indices 8-15
   m8.         Allow MCS rate index 8
   m9.         Allow MCS rate index 9
   ofdm        How to place OFDM rates in rates elements
   only-ofdm   Set rates for best OFDM throughput (no ERP protection will be used)
   range       Set rates for best range
   throughput  Set rates for best throughput (includes non-OFDM rates and may cause ERP
Why this device cannot support  300Mb/s rate in radio 0 of speed ?
i want to setup more than 54 Mb/s when i use my laptop to connect this AP.

Answer 1

The 802.11n HT rates apply only to no encryption  or WPA2/AES encryption. They do not apply to WEP or WPA encryption. If  WEP or TKIP encryption is used, the 1250 series access points and any  802.11n Draft 2.0 clients will not transmit at the HT rates. Legacy  rates (802.11a/b/g) will be used for any clients using WEP or TKIP  encryption.
Ref.: Release Notes
40MHz Operation
If the  throughput you get from a 20-MHz channel isn't enough for your speedy  users, you can flip each 5-GHz 11n radio into a bonded-channel  configuration.
You can configure 40-MHz operation on the 2.4-GHz  radio of the 1250 Series AP, but this isn't recommended or supported.  The primary reason against it is that with a wide channel in a band that  has so little available spectrum (and so much interference), you make a  single AP capable of higher throughput at the expense of performance  for all neighboring access points-certainly not the way enterprise WLANs  ought to be designed. Also, the vast majority of client chipsets  (Intel's, for one) will not support 40-MHz operation in the 2.4-GHz  space.
Ref.: Cisco  802.11n Design and Deployment Guidelines
Making the Most of Things
Once you're  connected at 802.11n HT rates, you'll want to make sure you don't lose  the throughput gains you should be able to achieve. Not all may be  within your control, but if you understand the performance implications  of these variables, it will help you in baselining your WLAN  capabilities.
When your  laptops are unplugged, the 11n client will likely try aggressively to  save battery power. Depending on client radio and chipset, it's not  uncommon to see performance drop by as much as 50 percent. Unless you  know how to fully disable this, keep your laptops plugged in if you want  maximum performance,
Note: Even if you managed to disable this power save function, most laptop  motherboards/chipsets have automated power-saving features that kick in  when the laptop is unplugged. At best, these are difficult to disable.  Thus, it is strongly recommended that you not do performance testing  when clients are battery operated.
As detailed  previously, throughput performance may vary as legacy devices are  introduced into the 802.11n environment. This is to be expected, but  make sure that you find a channel void of all legacy transmissions if  you want to test the high-water mark of your new 11n WLAN. If that isn't  possible, adjust your expectations accordingly.
Ref.: Cisco 802.11n Design and Deployment Guidelines
Regards,
André