RECEIVER SOFTWARE

Receiver software is developed in LabVIEW. The receiver software performs the following tasks:
  1. RTL-SDR parameters configuration and signal acquisition from RTL-SDR.
  2. Frequency offset correction and decimation.
  3. Demodulation, decoding, packet formation and data log.
  4. 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: