OFDM

[Dhananjay Patel]

I am working on OFDM system whose design is attached here with. I have certain query to make:

1. What is the significance of these global parameters:

Sequence length
Samples per bit
Guard Bits
Symbol rate
Number of samples

2. What is the use of position array in OFDM block.

3. What is the relationship between the number of sub carrier, position array, number of FFT point

4. How to decide the roll of rate of the LP cosine filter?

5. Also in my input and output constellation, I am finding a huge difference, kindly let me know the reason behind it.

Attachments:

1. OFDM-basic.osd

[Ashu verma]

Hello Dhanjay

I try to explain the points and question you raised about the parameters of OFDM

First of all, sequence length is the no. of bits transmit and the sequence length is directly relates to the word length that you can set from PRBS. In the case of OFDM the sequence length is long.

Samples per bit are the instants to sample the the one bit and total sampling freq. is your bit rate x sample per bits.Sample per bit in the case of OFDM are chosen to be less due to long sequence length which results into high no. of samples.

Gaurd bits are used to reduce ISI in the OFDM system.

Symbol rate quantifies the number of symbols transmitted per second.

No. of sample=Seq. length x Sample per bit

[Ashu verma]

In the addition to the above points,for the FFT points go to the link i have given below

Optisystem OFDM Block

For Understanding of Lp cosine filter and roll of factor visit the given link
https://en.wikipedia.org/wiki/Raised-cosine_filter

For the last one,can you attach the system setup screen shot along with constellation diagrams as i am not able to open the file you have attached.
Hope you got your all answers
Thankyou

[alistu]

Hi Dhananjay,

I believe I gave you the answer for some of the questions you have asked above in the forum below:

Design error

Your constellation on receiver side looks strange! After all, you have only sent 4 symbols but you are receiving more!

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