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thavamaran
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[MODDERATOR NOTE: The OP has a follow up question. I added it to the end of this thread.]
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Not a lot of detail but I know it involves sending a multiplex of many signals (i.e. a wide and signal) as lower bandwidth signals on a set of different carriers. The system has been used by BBC Digital Audio Broadcasting, as an example. It avoids the multi path problems of a single carrier with very wide band modulation. The wiki article gives a description. (Hope this link insert works with my iPhone)anorlunda said:ping @sophiecentaur , @berkeman . Do you know about Orthogonal frequency-division multiplexing?
To demodulate just one of the carriers, first divide it into two paths. Mix one path with a local carrier which has the exact frequency. This will give you a data stream. Then mix the second path with the same local carrier but 90 degrees shifted in phase. This gives a second data stream. The two data streams are decoded using logic gates to obtain one stream which is faster.thavamaran said:Hi guys, I have this question, I know in OFDM system, when you upconvert the signal with certain carrier frequency with certain sampling frequency and when you downconvert it, if the carrier frequency is same, but the sampling is not, then you will have a sampling frequency offset and that will cause unsyncronized problem, like you will get a phase rotation or shifted constellation.
But how actually this sampling difference causes the intermixing in subcarrier or even changes the phase of the signal? In theoretical perspective, I don't really get it. Please someone explain it to me, I went through books, and still I don't get the idea. thank you very much!
The sampling issue is not related to the data channel. Once you have a data stream, phase errors will just compromise the channel noise performance ( ‘eye pattern’)thavamaran said:But how actually this sampling difference causes the intermixing in subcarrier or even changes the phase of the signal?
thavamaran said:Hi guys, I am designing my OFDM system, actually have designed, I am using 128 subcarriers, 72 of it carriers my data, and 56 are zero padded, 27 subcarriers of zeros at the beginning which are at DC, then 36 subcarries of data, then 2 subcarries of zeros in the middle, then again with 36 subcarriers of data, at the end another 27 subcarries with zeros.
So 27 beginning, 2 in the middle and 27 at the end (Nyquist frequency). Correct me if I padded it wrongly.
Then now I am having problem at the end of my constellation, I have a phase shift because I am using a nonlinear laser model in between the channel, so to do the channel estimation now, I need to add Pilot symbol. I went through a few books and papers, but I still don't get the idea of Pilot symbol.
Can anyone rigorously explain to me why pilot tone, how to include them in subcarrier and a bit of explanation on that, please, thank you very much.
OFDM stands for Orthogonal Frequency Division Multiplexing. It is a popular modulation technique used in communication systems to transmit large amounts of data over a given bandwidth. OFDM divides the available bandwidth into multiple smaller subcarriers, allowing for more efficient use of the spectrum and increasing the data rate.
The sampling frequency in OFDM determines the spacing between the subcarriers. Changing the sampling frequency can affect the orthogonality of the subcarriers, which can lead to inter-carrier interference and degrade the performance of the system. Higher sampling frequencies can improve the spectral efficiency, but also increase the complexity of the system.
Increasing the sampling frequency in OFDM can increase the data rate and improve the spectral efficiency. It can also help to reduce the effects of channel impairments, such as frequency selective fading, and improve the overall system performance.
One potential drawback of increasing the sampling frequency in OFDM is the increased complexity and cost of the system. Higher sampling frequencies also require more precise timing synchronization and can be more susceptible to phase noise and other timing errors, which can degrade the system performance.
The sampling frequency is an important design parameter in OFDM systems and should be carefully chosen to balance the trade-off between spectral efficiency, complexity, and performance. The selection of the sampling frequency also affects the choice of subcarrier spacing, guard intervals, and other system parameters that can impact the overall performance of the system.