OFDM Bandwidth

[Gokul Boro .]

Hi all,
The sample 4QAM OFDM system is attached below. The RF OFDM signal after RF upconversion (7.5GHz) has a bandwidth of 5Ghz. The bandwidth of the OFDM signal is 5GHz. Now how wan I theoretically calculate this 5GHz? Should I first calculate subcarrier spacing by applying the formula—–
delta_f= bitrate/(log2(M)*Nsc)…………. where, M=4 (for 4QAM) etc. Nsc—> No. of Subcarriers.

and then—>
OFDM bandwidth = delta_f * Number_of_Used_Subcarriers

If I consider this steps, I get delta_f=10*10^9/(log2(4)*512 = 0.009765 GHz

so, ofdm bandwith = 9.765 * 10^9 * 512 = 5 GHz.

Is this the calculaton procedure of OFDM BW or there is some other procedure?

Also if I want a 10 GHz BW then how do I do it?

Thanks.

Attachments:

1. Direct-Detection-OFDM-4-QAM4.osd

[Gokul Boro .]

May be I was wrong. I think

delta_f = Global_bit_rate*/((log2(M)*Max_Possible_Subcarrier)

[Duy Le Nguyen]

Hi Gokul!
I don’t work on the new OFDM modulation. But see it if it can help you.

what is happening?

Regards.

[Rajguru M. Mohan]

Hi Gokul,
In OFDM Modulation Finally, I and Q
generated analog waveforms are converted to real-valued waveforms by mixing with
a RF carrier.
The subcarrier frequencies are integer multiples of 1/Tsymbol, where Tsymbol is the
duration of an OFDM symbol, and in this case the frequency is 1.25 GHz. Since the
initial position defined by the OFDM modulator is 17 (position “Number of IFFT points
/ 2 = 16“ stands for a subcarrier frequency of 0), the initial subcarrier will be allocated
at 1.25 GHz, and the subsequent subcarriers will be at 2.5 GHz, 3.75 GHz, and 5
GHz, respectively. The allocation of subcarriers, can also be
visualized from the “Graphs“ property of OFDM Modulator in project browser, the spectrum of the In-phase signal at the OFDM output as well as the upconverted OFDM signal spectrum.

Thanks

[Rajguru M. Mohan]

Hi,
The bandwidth of OFDM is ideally infinite.
The channel bandwidth and FFT size alone don’t provide enough information to describe the entire structure of the OFDM signal. Recall the following relationship:

Δf=fs/N
Δf=fs/N
where Δf is the subcarrier spacing, fs is the sample rate used at the modulator input, and N is the FFT size.

For 802.11a, however, the FFT size is typically 64; with a sample rate of 20 MHz, this yields a subcarrier spacing of 312.5 kHz. Of these 64 potential subcarriers, 12 of them are unused (sometimes referred to as “virtual carriers”). Of the remaining 52 subcarriers, 4 contain pilot tones, so only 48 subcarriers carry information at any given time.

Thanks

[Author]

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