This is good start:

 http://www2.ece.gatech.edu/research/labs/sarl/tutorials/OFDM/Tutorial_web.pdf

I'm not sure what your starting point is or resources are.  There are several OFDM simulators out there for matlab - eg simulink and quite a few published examples. 

Because your total bandwidth is 80 MHz and each band is 8 MHz wide, there are 10 bands, which appears to be a low number. Moreover -not that this is vital- it is no power of 2, which is typically used because of the efficient implementation of the DFT using the FFT algorithm.

Anyway, generating the OFDM signal is rather easy:

- map the data onto constellation points for each sub channel, resulting in a complex vector of length 10.

- take the IFFT of this vector to generate the time-domain representation

- copy the final elements of this resulting vector and insert these in front to make a cyclic prefix (CP). For transmission without inter symbol interference or inter carrier-interference, the CP should at least be as long as the order of the channel (when represented as a finite impulse response filter)

You have now generated one block in the time domain. Note that this block contains complex elements, because it will be (quadrature amplitude) modulated around a carrier. The complete signal consists of a serialisation of such blocks.

Good luck!

@Gert Cuypers: You have perfectly described the system diagram, however, I have an objection on one part: CP does not remove the inter-carreir interference (ICI); ICI results from the time varying nature of the wireless channels which is irrelevant to how the symbols are managed inside the OFDM block.

The wideband assumption is quite important for the decoder structure. You did not specify the carrier frequency so we can not judge how the decoder will be designed but generally speaking you will have demodulator followed by parser and an FFT block and decoder.

 Thank you so much for the prompt answers. 

@Gert Cuypers:  Apologies, if I have not explained my case very well. The real scenario is as follows:

We have a baseband from -40MHz to 40 MHz, contain 10 non- overlapping channels of equal bandwidth of 8 MHz. Each channel is possibly occupied by a transmission signal that uses OFDM modulation.Each 8 MHz OFDM symbol has 8192 frequency tones and a cyclic prefix length of 1024.  The occupancy ratio of the total 80 MHz band is 50%, i.e., 5 out of 10 channels are occupied by licensed transmission signals and the remaining 5 channels are unoccupied.The received signal is corrupted by additive white Gaussian noise (AWGN) with a variance of 0.2. The received signal to noise ratios (SNR) of the 5 active channels are 7dB, 4dB, 7dB, 7dB, and 4dB, respectively. I want to plot the power  spectral density of the above mentioned system. 

From your answer what I could gather is  '10' refers to the number of OFDM sub carriers, which is not actually the case. Rather, it refers to the number of sub-channels. 

Kindly share your views.

@Ahmad Elmoslimany: Its clearly mentioned in the paper that I'm trying to simulate that its a baseband channel from -40Mhz to +40 MHz. In that case, I'm not sure whether the carrier frequency is significant.

@Patrick Minix : Thank you for your assistance. 

Ahmad Elmoslimany: my remark was under the (as far as I knew, common) assumption that the channel does not vary significantly during the transmission of one block. If it does, this not only results in ICI, but also in an improper equalization of the desired signal, unless you include a clever training scheme, which I am not fmiliar with (though you may be of course). In any case, ICI also results if the CP is not long enough.

Yamuna K Moorthy: ok, this seems much more logical now :-)

If it weren't for the CP (and possible coding), the spectra would look like white noise. The CP has some influence on this. We have attempted to describe the spectrum (DMT case) in section 2 of the below article, which may be helpful:

http://www.asp.eurasipjournals.com/content/pdf/1687-6180-2006-070387.pdf

Would be interested in seeing the solution once it's worked out.

Sorry, I didn't know where you starting from ... can you provide a reference to the paper you trying to simulate?

For a full implementation (assuming you have access to Matlab), you can have a look at the Simulink model found at http://uk.mathworks.com/help/comm/examples/256-channel-adsl.html?refresh=true. This is a baseband system though (it is DMT, as in gert's case).