An advanced groupband data modem based on the orthogonally multiplexed QAM technique is proposed. Its performance is evaluated through experiments. In the proposed modem, data are transmitted by a number of orthogonally multiplexed parallel subchannels. Because of this orthogonally multiplexing parallelism, the proposed modem becomes more immune to Gaussian noises, impulsive noises, and channel distortions, compared to the conventional Class IV partial response VSB modem. Furthermore, the orthogonally multiplexed QAM modem retains high flexibility in multiplexing a variety of data terminals and in carrying out fallback operation to a wide range of data rates. Experimental results demonstrate the robustness of the proposed groupband data modem.

This paper describes CMOS-VLSI architecture for a rate conversion digital filter which interpolates an input discrete-time audio signal at the twice times higher sampling rate. In the proposed architecture, two discrete-time audio signals with 16-bit word length are multiplexed into a serial bit stream and are simultaneously interpolated through parallel table look up array multiplications. Based on the architecture study, the VLSI interpolator which contains 34,000 transistors has been fabricated with 3 µm CMOS technologies. Its power consumption is less than 100 mW and the maximum input sampling rate is 50 kHz. The interpolator quality is specified in terms of in-band amplitude ripple and out-band attenuation each of which is ± 0.13 dB or 80 dB, respectively.

A maximum likelihood receiver for an orthogonally multiplexed QAM (OQAM) system has been derived based on the mathematical structure of an OQAM signal. The resulting receiver provides the minimum variance estimates of carrier and timing phases under the disturbance caused by not only Gaussian noise but also random data. It has also been proven that the carrier and timing joint control system is asymptotically stable almost everywhere in a state space. The equilibrium points are where the parallel synchronization over all subchannels in the OQAM system is established. Simulations have been carried out in order to verify the maximum likelihood receiver performances, where 9600 bit/s data are transmitted over 750-2450 Hz voiceband with 8×8 staggered QAM signal constellations. It has been shown that sufficient eye opening is obtained under 10 Hz carrier frequency offset and 10-4timing frequency stability.

This paper introduces a modified likelihood function and then provides the suboptimal maximum likelihood (SML) timing estimator which is applicable to PCM regenerative repeaters because of its structural simplicity. Since the SML timing estimator tracks a difference waveform balancing point, as opposed to its conventional maximum likelihood (ML) counterpart, which tracks a derivative waveform zero-crossing point, its performance is rather immune against channel phase distortion. Analyses and experiments are also presented to validate the SML timing estimator for timing extraction from a band-limited PAM signal.

An orthogonally multiplexed QAM (O-QAM) system is a multichannel system with a baud rate spacing between adjacent carrier frequencies; this property is desirable to digitally implement the system using the discrete Fourier transformation (DFT). This paper provides a novel digital signal processing method based on anN/2-point DFT processing in the O-QAM system. A complexity comparison between a digital O-QAM system and a digital singlechannel QAM system shows that the digital O-QAM system using the new method is more economical than the digitally implemented conventional single-channel data transmission system.

A simple structure of a per-channel automatic equalizer for an orthogonally multiplexed QAM system is proposed and the performance is analyzed. It is shown that a dual automatic equalizer withT/2-long tapped delay lines, whereTis a period of individual baseband baud, can equalize not only the transmission channel distortions, but the timing deviations and demodulating carrier phase deviations. Despite its high efficiency, the orthogonally multiplexed QAM system has been regarded as overly costly. However, by using the dual automatic equalizer with fairly simple structure, the system is competitive with conventional single channel data transmission systems.