BER Test Set in OFDM systems

[alistu]

Good day!

In OFDM systems, the BER Test Set component takes 5 OFDM parameters into consideration for calculating bit rate and bit-error-rate:

– number of fft points;
– number of subcarriers in each OFDM symbol;
– number of cyclic prefix points;
– number of training symbols;
– number of pilot symbols.

I would like to know how these parameters are taken into consideration for calculating error. After all, all of them have to be taken care of by OFDM components themselves, so the input to BER Test Set should be a bit sequence with bits supposedly the same as those produced for transmission.

Also, BER Test Set calculates bit rate as the bit rate defined in layout parameters multiplied by “no. subcarriers in each OFDM symbol/no. fft points”. Why doesn’t this component consider cyclic prefix (which has a temporal effect on signal), training and pilot symbols when calculating bit rate?

Thank you in advance.

[Ahmad Atieh]

Hello,
All the information that you are asking about can be found in the BER Test Set datasheet that you may access it using right click on the visualizer, then choose “Component Properties…” and press the “? Help” button.
Regards,
Ahmad

[alistu]

Hi Ahmad,

Thank you for your response. Regarding BER for OFDM, the help section only emphasizes in a few lines the importance of assigning the same values to the component as the ones used in OFDM transceivers with no other explanations on how the parameters are considered in calculations.

[Kam]

Hi Alistu. I also want answer to your question. did you find out?

[Ahmad Atieh]

Dear Alistu,
The setting of these parameters is usually done by the designer. They affect the system performance and accuracy. The default values in the component is what is typically published. However, users can change them as needed in their system.
The BER Test set needs to know the number of subcarrier to determine number of bits per subcarrier and to synchronized the received bits. This includes knowing if there are prefix, training and pilot symbols.
There is no FFT points in BER Test Set.
I recommend you refer to text book on OFDM for more information.

regards,
Ahmad

[alistu]

Dear Ahmad,

Thank you very much for your response. I believe the number of FFT points are represented as “maximum number of subcarriers” in the BER Test Set.

I do have basic familiarity with OFDM theory, but the BER Test Set calculation method is still not clear to me. To be more specific, if I want to implement an OFDM-CDMA structure, where every bit of the binary input is first multiplied by a code word, it is easy to implement the multiplication block through Matlab Component after the BER Test Set – please refer to an OFDM sample. That way, since the sequence length in the layout parameters is fixed, the overall bit rate and the input word length are reduced. However, I cannot figure out how to define the BER Test Set bit sequence the same as the Matlab Component output, so it correctly calculates the error.

Another approach is to replace the BER Test Set with a suitable program via Matlab Component, but it would then require a solid knowledge of how the BER Test Set works in OptiSystem. And that is why I started the thread.

Thank you again for helping me out.

Best regards,
Alistu

[Kam]

Thank you for replying Sir. Can you please refer a text book on ofdm for beginners like me?

[shaymaa Tahhan]

need the answer to this question please.

[Ahmad Atieh]

you may use Advanced Optical and Wireless Communications Systems for example.

[Kam]

Thank you.

[shaymaa Tahhan]

whenever I got OFDM EVM less than 5% , the BER readings in the tester is 0
is that realistic or what should I do?
I have tried many times but the same!

[Ahmad Atieh]

yes, that is very realistic

[Ahmad Atieh]

Dear Alistu,
The process that is used is as follow.
You set the maximum possible subcarriers and the number of used carriers at the port. As a result, the number of bit in the sequence used in BERT will be a fraction of the Sequence length (Bit rate *80/128) set in the Layout Parameters “Sequence length”.
This number will be used in QAM Sequence Generator as it groups each four bits into a symbol.
Please use the example (OFDM Coherent Detection Single Port Single Polarization 16QAM.osd) in OptiSystem 17 that describes properly the setting of each component. The number of FFT points in the OFDM Modulator has to be larger than the number of subcarriers. This is only used in an old obsolete component. A replacement component is created in newer versions of OptiSystem that doesn’t set the FFT points.
Please send me an email at ahmad.atieh@optiwave.com for further discussions.
Regards,
Ahmad

[Author]

Posts