Sent and received signals

From Tools4SDR

These tools (the server and the DLL) are mainly design to send and receive signals from the USRP. On the computer, you simply deal with arrays of data (with matlab as with the DLL). This section explains the link between these arrays and the real signals.

The received signals

The receive script works as follows:

• Demodulates the signal around $f_0$ with band $1/T_e$
• Samples at $T_e$ the I and Q components of the signal
• Send them to the PC through the USB links
• The DLL computes the I and Q signals, the matlab interface computes the complex envelop.

This description is note exactly what appends in the USRP, but is equivalent to it.

The frequency $f_0$ is set with a setFreq command. $T_e$ is set with a setDecimRate command. The following relation exists:

• DecimRate = 128, $1/T_e$ = 500kHz
• DecimRate = 64, $1/T_e$ = 1MHz
• DecimRate = 32, $1/T_e$ = 2MHz
• DecimRate = 16, $1/T_e$ = 4MHz
• DecimRate = 8, $1/T_e$ = 8MHz

In general, the value DecimRate = 128 is used in the proposed tutorials.

The sent signals

To send a signal, you first describe it. Within matlab, it is described with a complex array. Within a windows software, it is described with two arrays, one for the I component, and one for the Q component.

These arrays are then passed through the USRP. The following operations are performed:

• The signal is interpolated with rate $T_e$.
• The signal is transmitted at frequency $f_0$

As for the receiver, the frequency $f_0$ is set with a setFreq command. About the interpolation, if $y[n]$ is an input array (complex, I or Q component), the following time continuous signal is generated:

$y(nT_e) = y[n]$ and $\forall 0<t<T_e$, $y(nT_e+t) = y[n]$

The following relation exists:

• InterRate = 256, $1/T_e$ = 500kHz
• InterRate = 128, $1/T_e$ = 1MHz
• InterRate = 64, $1/T_e$ = 2MHz
• InterRate = 32, $1/T_e$ = 4MHz
• InterRate = 16, $1/T_e$ = 8MHz