Frequency selective fading channel

[khmaies]

Dear All,

I am working on OFDM and different type of noises. Narrowband
interference, frequency selective fading channel and AWGN.

I would like to understand the case of the frequency selective fading channel.
My question is:

Can we predict, at which is frequency I will have fading( like in the case of a Narrowband interference we can know which is the frequency will be affected). Is there a type of frequency selective fading channel where I can predict such noise.
In the case of frequency selective fading channel, how my received data will look like. Like in Narrowband, the output is saturated, what will be the case in the frequency selective channel.
Do anybody can help with references or persons can help with my question.


Thank you very much.
Kind regards
Khmaies

[Karn]

Hi Khmaies,

I am also studying the topics u have referred too. But I am sorry at the moment i am inable to reply tour queries. But here is a reference for u in this regard.

Chapter 4 in Wireless Communications: Principle and Practice. by T.S. Rappaport-

Hope this helps..

Karn

[hunter006]

Karn,

Thank you for your additional reference material suggestion. I am studying OFDM as my project for next year (OFDMA transmitter, receiver and several channel designs in Matlab).

Are there any other reference materials that either you, Khmaies or anyone else can suggest to look at? I'm using Haykin's Communication Systems, 4th ed. but it's not very useful...

[hunter006]

[kavitha]

Hey can u send some good material regarding ofdm . I want to do this in MATLAB[glow=red,2,300][/glow]

thanks

[hunter006]

That's actually what my initial steps of the project are, then I move on to OFDMA (Single user OFDM then moving on to OFDMA with multiple users in MATLAB 7 using M-Files).

Although it's not specifically OFDM material, Discrete Multitone Modulation (DMT) is often mentioned in many texts dealing with OFDM, and there's a great DMT example built in to Matlab 7. You can check out the design and execution by typing in the following in to Matlab (may take a while to bring up):

dmt_sim (www.mathworks.com/access/helpdesk_r13/help/toolbox/commblks/usersguide/moredem2.html)
dmt_sim_alt (www.mathworks.com/access/helpdesk/help/toolbox/commblks/ug/fp38199.html)

Please note that while Wikipedia states OFDM is sometimes called DMT [1], the two are not one and the same, however there are many parallels that can be drawn between the two, particularly in the transmission scheme.

Haykin (mentioned in my previous post) draws only three differences in the design used in an OFDM transmission/receiving scheme when comparing the two. Much like the relationship between the IFFT and the FFT mentioned in the OFDM tutorial on ComplexToReal.com, two differences are the use of an upconverter and a downconverter on the receiver and transmitter (one to translate the to transmitted frequency, and one to undo the translation) and the third difference is that "the signal constellation encoder does not include a loading algorithm for bit allocation" [2].

Thus Haykin's OFDM data-transmission system block diagram is as follows -
Transmitter:
Binary data input - Demultiplexer - Constellation encoder - IDFT - Parallel to Serial converter and guard-interval provider - DAC converter - Upconverter

Receiver:
Downconverter - ADC converter - Serial to Parallel Converter and guard-interval remover - DFT - Decoder - Multiplexer [2]

Note that this also doesn't coincide with what the DMT example of Matlab 7. It is simply a referential block diagram. I guess that if you were to ask several experts in the field, they would all have their own preferences, and this is one of those things. Maybe Charan Langton will have her own very interesting input to mention here.

Ref:
[1] en.wikipedia.org/wiki/OFDM
[2] S. Haykin, "Orthogonal Frequency Division Multiplexing," in Communications Systems, 4th edition, New Jersey: John Wiley & Sons, 2001 pp 444-448

[cindy]

The channel becomes frequency selective when the transmitted signal's BW is comparative to the coherent bandwidth of the channel. It is different from the narrowband interfence which occurs at a certain frequency.

[cindy]

Besides, when the channel is dispersive relatively to your imput signal, the output will be distorted.

Cindy