Seamless data-rate change using punctured convolutional codes for time-varying signal-to-noise ratio

In a time-varying signal-to-noise ratio (SNR) environment, the symbol rate is often changed to maximize the data return. However, the symbol-rate change has some undesirable effects such as changing the transmission bandwidth and perhaps causing the receiver symbol loop to lose lock temporarily, thus losing some data. We propose an alternate way of varying the data rate without changing the symbol rate and therefore the transmission bandwidth. The data rate change is achieved in a seamless fashion by puncturing the convolutionally encoded symbol stream to adapt to the changing SNR environment. We have also derived an exact expression to enumerate the number of distinct puncturing patterns. To demonstrate this seamless rate-change capability, we searched for good puncturing patterns for the Galileo mission (14,1/4) convolutional code and changed the data rates by using the punctured codes to match the Galileo SNR profile of November 9, 1997. We show that this scheme reduces the symbol-rate changes from 8 to 2 and provides a comparable data return and a higher symbol SNR in a day.