Guard interval and ISI-free OFDM transmission
This experiment illustrates the ISI-free transmission of OFDM systems under this condition:
The guard time exceeds the channel's maximum delay spread:
OFDM symbols are generated by an IFFT with size
. A guard interval (GI) is appended at the start of each symbol. This cyclic prefix comprises the last
samples of each symbol.
In this example the guard time is
. Further settings from IEEE 802.11ac are used.
|Number of subcarriers, FFT size
|Discrete length of the guard interval
|Extended OFDM symbol period
|Guard interval duration
The simulation uses a single echo channel and starts with
(echo delay). This results in an ISI-free detection.
ISI free detection for
Now, modify the echo delay. When restarting the simulation the channel estimation and equalizer initialization is performed.
Simulation - Setup (F12): modify “Echo delay” (
) and click on “OK”.
Approve the ISI free condition
for different echo delays.
Note that the transmission sampling time is
and thus the echo delay can be adjusted in this granularity, e.g.
The initial setup uses 52 data QPSK symbols (2 bit) per OFDM symbol (4us). This yields to a source symbol duration of
and a bit rate of 26 Mbit.
Now select a different OFDM configuration in Simulation - Setup (F12).
Is this transmission ISI-free?
Check the ISI-free condition