I cannot find in any of the transmission examples for digital communication where the carrier frequency comes into play. I have tried to track it down, but cannot figure out where the actual "carrier modulation" is taking place. Any help?
on 2009-03-17 18:13
on 2009-03-17 18:23
On Tue, 2009-03-17 at 11:03 -0500, William H. wrote: > I cannot find in any of the transmission examples for digital > communication where the carrier frequency comes into play. I have > tried to track it down, but cannot figure out where the actual > "carrier modulation" is taking place. Any help? That's because there aren't any. The USRP is usually treated as a "complex baseband, direct conversion" system. The modulation is created in its baseband representation, with no carrier component, and sent to the USRP and converted to analog I and Q signals by the DAC. The analog daughterboards then upconvert these using a quadrature mixer to a passband frequency. Receiving works exactly the same way, in the opposite order. This a slightly simplified description, and there are some additional considerations on the receive side due to frequency and timing offset between the transmitter and receiver, but that's the gist of it. Johnathan
on 2009-03-17 22:05
William H. wrote: > I cannot find in any of the transmission examples for digital > communication where the carrier frequency comes into play. I have tried > to track it down, but cannot figure out where the actual "carrier > modulation" is taking place. Any help? GnuRadio is one of a class of radios known as "zero-IF" or direct conversion". To understand what this means, consider a classical receiver where a local oscillator (LO) is mixed with the carrier to form an intermediate frequency (IF) that is the difference between the LO and the carrier. Now pretend we slowly change the frequency of the LO to approach that of the carrier. As we do so, our IF gets lower and lower. When they are equal, our IF is centered at 0 Hz, with sidebands extending above and below zero. This is called the baseband signal. In order to represent both the positive and negative parts, it is necessary to to represent both the amplitude and the phase, hence the need to use a quadrature signal with I and Q components. All this is basic digital signal processing 101. I'm sure any DSP text can do a better job of explaining it than this. The USRP source block delivers digital samples that are already downconverted to a 0 Hz IF. The USRP sink block expects digital samples that are baseband, centered on 0 Hz. The USRP hardware contains the necessary down and up converters to receive or generate the correct carrier (sometimes with the help of a daughtercard). As a result of all this, modulation and demodulation do not necessarily look like you might expect. For example, to do a simple amplitude demodulator, you would compute sqrt(I^2 + Q^2) for each sample. Similarly, a phase demod would be done as atan(Q/I). Let's take a (slightly) more complex example. In QPSK (Quadrature Phase-Shift Keying), there are two bits encoded into every "symbol" that is transmitted. The "symbols" correspond to any one from a set of four phases (each 90 degrees apart). For simplicity, lets choose: 00 = 45 degrees 01 = 135 degrees 11 = 225 gegrees 10 = 315 degrees So, in order to modulate a digital bitstream using QPSK you break the bitstream up into groups of two bits and set I and Q accordingly: 00 => I=+1 , Q=+1 01 => I=-1 , Q=+1 11 => I=-1 , Q=-1 10 => I=+1 , Q=-1 This gives you a nice unit vector with the desired phase. Note that nowhere did the carrier frequency enter into the modulation (or the preceeding demodulations). This is a highly simplified explanation. Find a good DSP text to learn more. @(^.^)@ Ed