Techniques for applying interference cancelers to high-frequency band wireless link systems employing multilevel modulation

Abstract

SUMMARYWireless link systems have been investigated for use as base station networks in future mobile communication systems. They utilize a high-frequency band and multilevel modulation scheme to satisfy the system requirements for high capacity. An interference canceler (IC) is an effective method of enhancing the system frequency utilization efficiency. In such a system, however, performance of the IC is severely degraded by the frequency offset between the desired and interference signals since the frequency offset due to the inaccuracy of local oscillators in each base station becomes extremely large in a high-frequency band. Moreover, signal constellation ambiguity frequently occurs and this impairs the IC performance severely when multilevel modulation is employed. This paper proposes two methods to address the aforementioned problems. One method estimates and compensates for the frequency offset by adjusting the frequency of the desired signal to that of the interference signal. Another method controls the transmission power or the phase of the desired signal in order to avoid the signal constellation ambiguity. Simulation results show that the proposed methods effectively suppress the frequency offset and the signal constellation ambiguity and that the IC is effective when using the proposed methods even when employing a high-frequency signal and multilevel modulation.