RECEIVER
SOFTWARE
Receiver software is developed in LabVIEW. The receiver software
performs the following tasks:
- RTL-SDR parameters configuration and signal acquisition from RTL-SDR.
- Frequency offset correction and decimation.
- Demodulation, decoding, packet formation and data log.
- Received data analysis and bit error rate computation.
RTL-SDR drivers for LabVIEW are not available therefore USB to TCP
pipe is used to communicate with RTl-SDR as given in [11]. Once
communication with RTL-SDR is established its centre frequency,
amplifier gains and sample rate can be controlled from the receiver
software. Since occupied bandwidth of most of the digital radiosondes
is typically less than 20 KHz therefore IQ data is sampled at lowest
possible sample rate of 250 KSPS.
Demodulation is performed as shown in Figure 7. Plot-A in Figure 8
shows the spectrum of the received modulated signal. Following two
characteristics of the received signal can be observed from the
spectrum- first, there is a frequency offset of about 25 kHz in
received signal due to imperfection in crystal oscillator used in
tuner. Second, presence of DC noise due to IQ imbalance. These
problems are rectified by first applying a bandpass filter centered at
offset frequency of 25 KHz to remove DC noise and then shifting centre
frequency to zero hertz. These functions are performed usingDownconvert Passband VI available in modulation toolkit of
LabVIEW. This VI is also used to limit the signal bandwidth to 15 KHz
to remove out of band noise. Frequency spectrum of output of these
functions is shown in plot-B of Figure 8. Further to reduce the
computational load of the demodulator processes complex baseband
signal is resampled. Resampling rate is computed by multiplying symbol
rate with required sample per symbol. Required samples per symbol are
chosen as 16. An even number of samples per symbol is required because
timing recovery process in demodulator is optimized for four or more
samples per symbol.
Figure 7: Frequency estimation and
demodulation of 2FSK modulated signal.
Demodulation of the resampled complex baseband signal is performed by
FSK demodulator VI. After resampling, signal s[m] is computed
as given in equation-1 below: