OFDMModulator() returns a time-domain OFDM signal with size = None (in training)

[kassankar]

Describe the bug
The class OFDMModulator() returns a time-domain OFDM signal with size = None while Training the Keras model. (only while training the model)

To Reproduce
Please find a gist link : "https://gist.github.com/kassankar/70d62384d05b00fa8b9486ed862a4784" to reproduce the error.

The bug occurred while running block number 7 due to OFDMModulator() in block number 6.

Expected behavior
Based on the OFDMModulator description, the function should return an output size = num_ofdm_symbols*(fft_size+cyclic_prefix_length)

Screenshots

Additional context
The problem is directly connected to the function flatten_last_dims() used in the OFDMModulator() class. More precisely, in the last two lines new_shape = tf.concat([shape[:-num_dims], [-1]], 0), return tf.reshape(tensor, new_shape). I think the problem is occuring due to the use of [-1] in the tf.reshape() function. In the training the batch_size is equal to None, and based on the description of the tf.reshape() function with an axis shape = [-1], the dimension is computed so that the total size remains constant. However, due to batch_size = None, the tf.reshape() return another 'None' dimension for the last output.

Note: tf.reshape() with [-1] is also used in OFDMDemodulator(), in
new_shape = tf.concat([tf.shape(inputs)[:-1], [-1], [self.fft_size + self.cyclic_prefix_length]], 0)

[jhoydis]

Thank you for this bug report. Could you please provide a gist instead of a pdf, so that we can easily copy the code? Could you provide a code example that isolates the problem as much as possible, i.e., the shortest possible code snippet that allows us to reproduce the bug?

[kassankar]

Thank you for this bug report. Could you please provide a gist instead of a pdf, so that we can easily copy the code? Could you provide a code example that isolates the problem as much as possible, i.e., the shortest possible code snippet that allows us to reproduce the bug?

Done!
I hope it is better now.

[jhoydis]

Hi, I think that there is an issue with you gist. The model in cell 6 does not seem to return anything. I have created an alternative gist, similar to yours, which works nicely: https://gist.github.com/jhoydis/0f09064c89accdb55c8135afcb1982e9
Could you check which modifications would need to be done in this code to reproduce the error?

[kassankar]

Hi,
Thanks for the gist modification. It looks way better.
Please, find the new gist (similar to yours), with three lines added after the line number 54 :

• print(x_time.shape)
• x_f = self._ofdm_demod(x_time)
• return x_f.
  The print is only used to show the None that appears in the dimension.

(BTW, I did not add a return to the function since the print(x_time.shape) alongside x_f = self._ofdm_demod(x_time) are sufficient to present the bug. But I can understand my gist is not well-arranged :) )

gist v2: https://gist.github.com/kassankar/15842eaa59c4cbb969ad0aded2ff5613

[jhoydis]

I was able to reproduce the error in this minimal gist: https://gist.github.com/jhoydis/7720a7a2af236e5061d907e969084e85
The bug will be fixed in the next release.