High speed modem suitable for operating with a switched network

Abstract

A modem capable of achieving essentially 9600 bit per second information transfer rate through the switched telephone network. The high bit rate is achieved notwithstanding the variations in the characteristics of the switched telephone network by phase and amplitude modulating a plurality of sub-carriers at a relatively low baud rate. Correlation detection is employed at the receiver to extract or demodulate the information contained in the various sub-carriers. To allow correlation detection with orthogonal signals, each of the sub-carriers is harmonically related to a fundamental which fundamental is, however, slightly higher than the baud rate. This allows the receiver to delay correlation for a 'gap' time, which reduces the effect of transients produced by modulation and also provides greater tolerance to inter-symbol distortion. Since the fundamental of all the sub-carriers is higher than the baud rate, the receiver 'sees' orthogonal signals during the integration or correlation time which is maintained as the period of the fundamental. In addition to the information sub-carriers, a pair of unmodulated sub-carriers are also transmitted to enable the receiver to maintain the correlation interval in proper synchronization with the transmitter, and to correct for phase distortion and frequency offset.