Usage of non-linear phase filters in RF receivers
All analog filters used in RF receiver frontends are in the class of Analog Infinite Impulse Response filters and, therefore, they can’t have stability and linear-phase properties at the same time. In your application, the signal has a constant envelope and all the data is stored in the phase of the signal. This signal is affected by so many parameters in the path that it is almost impossible to have a successful BPSK transmission in the first place! A part of these problems (including AM to PM distortion caused be nonlinear-phase filters and nonlinear amplifier/mixers) is solved using a Training Sequence. These are pre-known data streams that are initiated with the purpose of tuning the equalizers and vector analyzers. For example, imagine a pre-known stream of 10101010 is sent before the actual data transmission. The processor tunes the equalizer coefficients and the vector analyzer so that the constellation of the received signal looks like the intended pattern. After this calibration, actual data transmission can begin. To the best of my knowledge, this process is done 30 times a second in the GSM mobile technology.