This is a narrowband IF system, designed to have only a single carrier present in the IF processing. More filtering is applied in the digital domain, and the GMSK signal is digitally demodulated to recover the transmitted bit stream. The ADSL receiver has different requirements. In this system, equalization is done in both the time and frequency domains before the signal is demodulated. An important trend in this development is to do more and more of the signal processing in the digital domain.
Typical receiver architectures.
An example of this sophistication in modems is the use of echo cancellation . The spectrum of Figure 1d shows the strong interferer that dominates the dynamic range of the received signal. In the case of a modem, this interference is not a random signal, but the duplex signal that the modem is transmitting back upstream. Since this signal is known, signal processing could be used to synthesize the expected echo on the receive line, and subtract it from the received signal, thereby cancelling its interference. Unfortunately, the echo has a strong dependence on the line impedance, which varies from user to user- and even varies with the weather.