A modem/codec for cellular telephony

A base-band modem/codec chip for the dual-mode North American IS-54 standard is shown. In the digital mode of operation, this chip acts as a modem. On the transmit side, it performs pi /4-shifted DQPSK modulation, square-root raised-cosine filtering, and waveform synthesis. The receive section helps to demodulate the signal by minimizing the intersymbol interference using square-root raised-cosine filters. In the analog mode of operation, the chip behaves like a codec. The transmit section supports part of the frequency modulation process while the receive section acts as an analog-to-digital interface. In addition to integrating the signal-processing sections of the transmit and receive paths, the chip also offers auxiliary controls for such functions as automatic gain control, automatic frequency control, and transmit power control. A wideband data detector is provided to decode the Manchester-encoded forward analog control channel messages in stand-by mode or forward analog voice channel messages.<>