"If carrier is single, we are able send only one vector of symbols on the channel by using the carrier.
In the case of OFDM systems, we are going to pass multiple carriers through a single channel and each carrier carries vector of array. "
Is the above statement correct. because I have understand the OFDM as above?
Please clarify my doubt.
Thanking you
Dear Sir,
Yes, that is correct.
The goal of OFDM is to enlarge the symbol duration T. In order not to lower the data rate, multiple carriers are then used. E.g. suppose symbol duration T, then set
w = 2PI/T, then the carriers used are
exp(jkwt), k = 0 ... N
You can see these carriers are orthogonal, meaning that if you multiply any 2 of these carriers, and integrate the result over T, then the result is always zero.
To transfer information, these carriers are modulated. Typically, QPSK, QAM16, QAM64, QAM256 modulation is used.
Note that not all carriers need to be modulated. Sometimes, pilots are sent on some to help in synchronization.
Note the main advantage of OFDM is immunity for multipath distortion. (dispersion.) Due to dispersion/multipath , there is a variable delay in the receive signal, there being a max time difference of delta-t between the first received signal, and the last. Now, in order avoid intersymbol interference, the transmit symbol duration is at least delta-t larger than T. In this case, it is possible in the receiver to recover the received symbols without intersymbol interference. There is still a need to compensate the frequency-dependent fading, but this is rather easy.
You easily see, to have efficient transmission in case of rather long delta-t, you need long T, and then in order to have sufficient data throughput, you need many carriers.
(rather large N).
Best Regards,
Henri.
I think Henri Cloetens gave a good and thorough explanation. The only thing I might add is what makes OFDM different from a single carrier scheme, in terms of sending data.
In short, if you use the entire RF channel to send one stream of symbols, you can send those symbols a lot faster (more symbols per unit time) than you could if you subdivided the channel into many subcarriers. So OFDM does not gain you spectral efficiency, compared with single carrier schemes. And both can use n-QAM as a means for transferring digital values.
The advantage of OFDM is that because you are sending the symbols in each subcarrier quite slowly, compared with a single carrier scheme, you can afford to create guard intervals of useful duration between each symbol in each stream. Useful duration means a guard interval long enough to accommodate any pre or post echo you might experience in the propagation path. So this is a great way to accommodate intersymbol interference, without having to rely on fancy feedback equalizers with many many taps, to twist back those symbols to their original shape, in the single carrier method.
You can of course also create guard intervals in single carriers schemes. But would they be of usefully long duration? Not likely. To be useful, guard intervals have to be several 10s of usec duration at least, and even more than 100 usec in some cases. Pretty hard to do that if your symbol duration is on the order of nsec.
Every data, [Example datas are like 1+i,1-i,-1+i,-1-i]you have to take IFFT in the transmitter side. Then you get an orthogonal sub carriers.
by
R.Suresh Babu
OFDM is a block transmission system due to the block-wise DFT from the point view of discrete signal processing.
@Thomas Ketseoglou Dear Professor, If OFDM is not using actual subcarriers in hardware, how does the filter-bank produce the OFDM signal? I am sorry that I am not familiar with these.
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