On OFDM Symbols and OFDM Frames

[Vinicius Mesquita]

Hi all,

I hope you are doing well.

I am trying to understand how many OFDM symbols I'll have per OFDM frame.

I am using exactly these .grc:

https://github.com/gnuradio/gnuradio/blob/master/gr-digital/examples/ofdm/tx_ofdm.grc

https://github.com/gnuradio/gnuradio/blob/master/gr-digital/examples/ofdm/rx_ofdm.grc

Some info: fft_len = 64, packet_len = 96, payload_mod = QPSK, header_mod = BPSK.

Since here https://wiki.gnuradio.org/index.php/Basic_OFDM_Tutorial

it says that "the OFDM frame is thus in the memory in matrix form" I made a custom block to check (and save) the whole OFDM frame after the OFDM Carrier Allocator in the transmitter and after the FFT block in the receiver.

I was surprised to see that after the OFDM Carrier Allocator, what is passed does not have a fixed shape, it keeps varying. For example: 

...
(12, 64)
(24, 64)
(36, 64)
(48, 64)
(12, 64)
(36, 64)
(24, 64)
(24, 64)
...

But in the receiver, the FFT in the Payload stream, we have a stream of matrices with a (almost) not changing shape 

...
(9, 64)
(9, 64)
(18, 64)
(9, 64)
(9, 64)
(9, 64)
(9, 64)
(9, 64)

...

I don't understand why the Carrier Allocator output keeps changing shape.
And I really would appreciate it if anyone could elucidate how many OFDM symbols I should expect in the receiver. How can I calculate that number?

I'd like to thank you in advance for your help.

Many thanks.
Best regards,
Vinicius.