# Why would we simulate baseband rather than passband

My question is, Why do we usually use baseband in simulation rather than passband? however, if we want to test our simulated idea in real environment, it must be sometimes in passband !!

So, should simulation in baseband give the same results if we made it in passband? for term of BER, MSE and so on

Here is an example as asked in the first comments,

In baseband, Suppose you have ofdm system, with $$s$$ is the $$n^{th}$$ block with N symbol, when performing N-by-N FFT to get $$x = FFT^Hs$$, after adding a CP as guard interval with length P, we will have $$M = x + P$$ is the transmitted signal as baseband.

But in Passband, we will not transmit the $$M$$ as it's, we will have additional process when having $$M$$, this process is summarized conversion --> matches filter --> carrier modulation by multiplying (.*exp(-j*2*pifct), fc is the carrier frequency t is the time period, and them transmit real part of this results, then in the receiver side, we follow the inverse of these steps till getting the baseband signal $$M$$ then, both passband and baseband will follow the same steps.

Thank you

• Could you please provide an example that motivated this question? – A_A Oct 7 '18 at 11:12
• @A_A an example is added into the question – New_student Oct 7 '18 at 14:19

In OFDM or any other system, if you understood what is the difference between the baseband and passband, then you would decide by yourself if there is a difference or no. However, its already mentioned in above comments, you shouldn't have any major difference between simulating base-band and pass-band.

Now let me explain what are the base-band and pass-band, first be aware that the main difference between them is when using baseband, it means we transmit the signal as it is without doing modulation, but when using pass-band, we start doing modulation and shift the signal to be transmitted in frequency to be higher frequency and then transmit it. So you can notice, the pass-band signal is, in reality, a base-band. The alone thing to differentiate between them is that frequency shifting (Modulation ).

Here is additional explanation for them,

Almost all sources of information generate baseband signals. Baseband signals are those that have frequencies relatively close to zero such as the human voice (20 Hz – 5 kHz) and the video signal from a TV camera (0 Hz – 5.5 MHz). A plot of an audio signal and its frequency spectrum are shown below, where it is seen that the most of the power of the audio signal is concentrated in the frequency range from (0 – 4 kHz). The telephone system used for homes and offices, for example, may transmit the baseband audio signal as it is when the call is local (from your home to your neighbor’s home). However, when the telephone call is a long–distance call that is transmitted via microwave or satellite links, the baseband audio signal becomes unsuitable for transmission and the communication system becomes a passband system. Similarly, transmitting the video signal from your camera to your TV using a wire represents a baseband communication while transmitting that video signal via satellites passband transmission. Therefore, baseband transmission, which is easier than passband transmission, is usually used when communicating over wires, while over–the–air transmission requires passband transmission. Notice that even over wires, the transmission may be passband transmission in specific applications.

One very obvious advantage of baseband simulation of communication systems is the low sampling rate it requires.

Considering a narrowband communication system having a few MHz of bandwidth to a few GHz of carrier frequency, it would be much easier (in terms of computation) to simulate it in the baseband sampling rate.

The reliability and the quality of a baseband simulation depends mostly on how accurate and realistic your mathematical modeling of the actual physical channel and the modulation system using it.

• Thank you so much .. but now, how about if we simulated the same signal in baseband and then in passband, will that give same results or almost the same results? regardless the sampling rate which will be higher when upsampling the transmitted signal. – New_student Oct 8 '18 at 1:07
• The results should not depend on the sampling rate. But since every practical simulation has some limitations, therefore there could be (minor) differences.@Eng.Badr – Fat32 Oct 8 '18 at 7:40