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The sampling rate of 30.72 MHz refers to a complex sampling rate of the baseband signal. For a bandwidth of 20 MHz, the single sided bandwidth would be 10 MHz and the minimum sampling rate according to Nyquist would be 20 MHz (and we would do something larger than this for practical reasons). So there is no issue here with the 30.72 MHz sampling rate ...

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Your questions are quite broad and hence difficult to answer. If the communication system can afford the bandwidth for a pilot signal, then a pilot can be used. Yes, channel estimation would be needed. Typically, SNRs would be calculated in the digital domain. However, these answers won't help you and you'll need to delve into the theory more deeply. If you ...

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A DC offset in the frequency domain will be an impulse at $f=0$. The interpretation of this is a single tone interference: "DC" is no different spectrally than any other frequency when viewed as the Fourier Transform of a complex signal with positive and negative frequencies as the baseband spectrum centered about $f=0$ is identical to that ...

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Each IFFT complex result bin represents an amplitude and a phase for some frequency. You can use the sum of the a sine and a cosine in the appropriate ratios to produce a sinusoid of any phase (see trig identity). The IQ inphase and quadrature modulators produce the sine and cosine in the appropriate ratios to produce the transmit phase as needed. This ...

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Because the output of the IFFT operation is a vector of complex numbers. An RF signal is a real thing, so you can't just multiply a sine wave by a complex number and get something sensible. However, you can map a complex number onto an RF carrier by multiplying the real part by $\cos \omega t$, and the imaginary part by $\sin \omega t$. This is I/Q ...

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