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Digital modulation

Digital modulation comes in many forms. Some deliver high capacity data like QAM modulation used in digital TV and others deliver lower latency data but are more resilient to noise and interference like QPSK. A particular signal will have a link requirement in the form of a Signal to Noise Ratio (SNR) measured in decibels (dB). Generally the higher the data rate, the higher the required SNR. An SNR value can be increased by increasing effective radiated power through either greater RF power or a higher gain antenna.

SNR and BER

As well as SNR, the number of acceptable errors, known as the Bit Error Rate (BER) is another key variable for digital modulation. The BER can be considered as a reliability value, for example one error every 1000 bits (BER 0.001) is considered only 50% reliable but an error every million bits (1E-6) is 100% reliable. An ideal digital signal would have a low SNR and a low BER. If the SNR is high but the BER is also high then the link, whilst strong, might be unworkable due to errors.

CloudRF allows you to model different types of digital modulation with the Bit Error Rate and modulation options. In the following examples, coverage for a low power 868MHz signal at an airport is rendered with different modulation types. As the SNR increases the range decreases. The receiver threshold is determined by adding the SNR value to the noise floor which for a 1MHz wide signal is around -114dBm. The noise floor will vary depending on the signal's bandwidth. Expert users can set this with the API.

QPSK

QPSK modulation with 11dB SNR and 0.001 BER.

4PAM

4PAM modulation with 14dB SNR and 0.001 BER.

16PSK

16PSK modulation with 18dB SNR and 0.001 BER.

32PSK

32PSK modulation with 24dB SNR and 0.001 BER.

digital.txt · Last modified: 2016/10/09 21:59 by alex