What is gaurd Interval
It makes sure that Inter Symbol Interference at the receiver is minimized, if the channel exhibits delay spread.
Delay spread is caused by dispersive medium, Ex:





Why is 802.11ac requires every frame to be sent in AMPDU format ?

An  MPDU  contains  only  packet duration  but  not  length  information.   Because  the delimiter  in  an  A-MPDU  contains  MPDU  length  information,  802.11ac  requires  that  even  a  packet with a  single  MPDU  is  transmitted in an A-MPDU format to provide packet length information in bytes.

What are the differences between 11n & 11ac
Removed features:
1. green Field
2. RIFS
3. PCO
4. LSig protection
5. Unequal modulation
6. Dual CTS protection

Added
1. Enhanced CCA on Secondary channels by increasing the sensivity to -72dBm from -62 dBm.
2. Static & Dynamic Band width selection.
The  basic  Request-to-Send  (RTS)  and  Clear-toSend  (CTS)  mechanism  of  802.11  is  modified  to
improve  dynamic  channel  width  operation.
Consider  an  interference  scenario  illustrated  in
Figure  3,  whereby  STA2  is  transmitting  to  AP2  and
AP1  is  communicating  with  STA1.   AP2  is
occupying overlapping channels of the secondary 40
MHz channel of AP1.  STA1 and STA2 can interfere
with each other, but the interference is not heard by
the  two  APs.  To  address  this  situation,  bandwidth
signaling is added to the RTS and CTS frames.  As
illustrated  in  Figure  4,  AP1 sends an RTS with the
bandwidth of the intended transmission,  which is 80
MHz comprised of channels 36, 40 44, and 48 in this
example.  Before STA1 replies with a CTS frame, it
senses  the  medium  on  all  secondary  channels  for
PIFS. If the secondary 40MHz channel is not free,
STA1  sends  a  CTS  response  with  the  BW  of  the
clear  channels,  i.e.  40  MHz  comprised  of  channels
36 and 40 in this example. Then AP1 sends data to
STA1  only  on the clear channels  and STA1 replies
with Block Ack (BA) frames that are duplicated over
the clear channels.




However,  if  the  interference  in  this  example  is
frequent, another new mechanism may be employed.
In  such  a  case,  STA1  can  send  a  Operating  Mode
Notification  frame  to  AP1  to  tell  the  AP  that  the
client  is  changing  its  bandwidth  on  which  it
operates.   For  example,  STA1  can  change  its
operating bandwidth from 80 MHz to 40 MHz  with
the  constraint  that  the  client  still  needs  to  use  the
same primary channel as the AP. Subsequently the
AP  will  only  send  data  frames  at  this  reduced
bandwidth.


What is VHT-Single MPDU


Normal Ack or Implicit Block Ack Request.
In a frame that is a non-A-MPDU frame or VHT single MPDU:
The addressed recipient returns an ACK or QoS +CF-Ack frame after a short
interframe space (SIFS) period, according to the procedures defined in 9.3.2.8 and
9.19.3.5. For a non-DMG STA, this is the only permissible value for the Ack Policy
subfield for individually addressed QoS Null (no data) frames.
In a frame that is part of an A-MPDUOtherwise:
The addressed recipient returns a BlockAck MPDU, either individually or as part of
an A-MPDU starting a SIFS after the PPDU carrying the frame


PLCP Signal _ legacy (Length field):
The PLCP LENGTH field shall be an unsigned 12-bit integer that indicates the number of octets in the PSDU that the MAC is currently requesting the PHY to transmit.

IN VHT
802.11ac devices set the data rate to 6 Mbps and derive a spoofed length in bytes so that when any receiver calculates its length, it matches the time duration required for the 802.11ac frame.

The MPDU Delimiter for each sub frame contains the length of the AMPDU sub frame.