Abstract: Adaptive broadband modem is achieved by that, the receiver signals the estimated channel quality via an available return channel to gateway to select the best modulation and coding scheme to adapt the channel change timely. In this paper, some key algorithms were studied for the adaptive broadband modem system, including the algorithm for carrier recovery, SNRs estimation, frame synchronization, and adaptive threshold switching. The architecture and hardware implementation schemes of the wideband adaptive coding and modulation demodulator were presented. The traditional hardware realization structure was improved for carrier frequency recovery and high-order modulations de-mapping based on soft decision. Performance of the implemented modem was tested and analyzed. The test results show that the prototype can work normally at symbol rates from 2 MS/s to 45 MS/s. The difference between the BER performance of base band loop and the theoretical limit is less than 0.2 dB, the difference between the BER performance of the intermediate frequency loop and the theoretical limit is less than 1 dB. The system link adaptive algorithm can track and match the imitative rain-field 0.65 dB/s channel attenuation well.