i work on modelling a OFDM system in a indoor optical wireless channel (LOS channel), since my channel is LOS there is no multi -path propagation , so the delay spread of my channel is much smaller than the length or duration of one OFDM symbol, Cyclic Prefix length is the duration of multiple of OFDM symbols , now i cant determine the suitable number of symbols needed to be inserted to the CP ?
CP is critically needed as it turns the linear convolution to circular convolution causing a much easier equalization technique in the frequency domain , how to determine the length of cyclic prefix in this case , thanks so much in advance .
Dear Moaaz,
First, let me clarify some points:
- Cyclic Prefix length is the duration of multiple of OFDM symbols --> no, CP is a fraction of OFDM symbol length, e.g. 1/16, 1/8, 1/4
- the suitable number of symbols needed to be inserted to the CP --> do you mean the number of samples to be inserted?
Second, note that LOS does not mean that you have a unique path from TX to RX. It means that at least one path does not involve reflection. However, it is likely that delayed paths occur (it depends on the directivity of your antenna).
Therefore I suggest toadd a CP composed of few samples. The best way to adjust this number is to measure the channel impulse response at the RX antenna.
Best,
Vincent
Dear Vincent Savaux
by OFDM symbols i mean the time domain samples coming as the IFFT output , the answer to the second question of yours is yes ^_^
the problem is that the max delay spread of my channel model is about 9 ns , at the same time the time domain sample duration ( OFDM symbols )is about 18 ns , thus for the receiver there is no ISI , i need help to build more accurate model or find a solution to increase the delay spread , i suppose .
Hi,
OK I better understand the problem you are facing. Note that increasing the number of subcarriers does not increase the OFDM symbol length, but it increases the number of samples the OFDM symbol compsed of. For instance, suppose a system with 128 subcarriers and a sampling rate of 18 ns. Then, the OFDM symbol length is 128 * 18 ns.
Then, if you want to divide the sampling rate by 2, just use an IFFT of size 256, by 4 use IFFT 512, and so on.
Then, you can use your channel model, and process subsampling at the receiver side in order to come back to 128 samples.
Best,
Vincent
CP is an overhead which reduces throughput, however it is essential to combat ISI, to design the length, it is necessary to optimize it, by keeping the duration of CP slightly over delay spread
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