Constellation analysis of DVB-S2 8PSK Robert Sydee FSCTE, Technical Director Whyte Technologies
the SNR falls below this threshold, its error rate increases sharply. This sudden increase in errors can lead to a degradation or in the worst case a loss service. In many ways the constellation diagram is the most underused tool on the modern spectrum analyser but when trouble shooting problematic installations it is an important diagnostic technique. So how do we interpret a constellation diagram? The constellation diagram for DVB-S2 8PSK is an eight-point chart representing the possible states of a symbol. Each point corresponds to a unique combination of three bits transmitted on the I and Q channels. Ideally, the received symbols should land precisely on these points. Any deviations from the ideal I and Q positions on the constellation diagram can help us identify different types of problems. So, let’s look at this in detail: Scattering: A wide spread of points around the ideal positions suggests either noise or other interference affecting the signal measured, and the degree of scattering indicates the severity of the issue. An unexpectedly high Pre-Bit Error Ratio (Pre-BER) usually accompanies this scenario and closer inspection of the decoded video may reveal artifacts (pixelisation) along with audio chirping. Additionally, if you have significant Post-Bit Errors occurring, then this indicates a serious problem as the decoder in your analyser can no longer cope with the
Back in January 2024 the BBC and ITV services completed their transition to HD on Astra-2 and the remaining DVB-S QPSK services will be transitioning to DVB-S2 8PSK transponders by March. Of course, this does not mean Astra-2 becomes a completely HD platform. While it’s not entirely accurate to say 8PSK is inherently less stable than QPSK, their differing characteristics can be identified on a decent spectrum analyser. 8PSK modulation carries less power per bit when compared to QPSK and this makes the signal more susceptible to interference and attenuation. Multipath echoes/reflections from urban environments can also significantly impact 8PSK signal quality due to its denser constellation pattern leading to instability. In 8PSK modulation, information is encoded onto two carrier signals, the In-phase (I) and Quadrature-phase (Q) channels. These channels are orthogonal, meaning they are completely independent and do not interfere with each other. For optimal signal reception the I and Q channels need to be perfectly balanced in amplitude and phase. This ensures that the encoded symbols on each channel remain distinct and can be accurately decoded by the receiver. This is visually represented via the constellation diagram function on most modern spectrum analysers.
8PSK performs well above a certain minimum Signal-to-Noise Ratio (SNR). However, when
44
CAI Feedback Spring 2024
Made with FlippingBook - Online magazine maker