Citation
Abstract
The balanced modulator, which comprises two amplitude-modulation modules, is widely used in phase-modulated communication systems. In practice, the balance between these amplitude-modulation modules is di–cult to maintain, and the amplitude and phase imbalances can cause distortion in the signal constellation and introduce undesired interfering tone signal components when such an imperfect modulator is used to modulate the data directly onto the RF carrier. The rendered imperfection inevitably degrades the receiver performance and, in a quadrature phase-shift-keyed (QPSK) system, causes cross talk between the in-phase and quadrature-phase channels. This article summarizes an analysis of the impact of these modulator imbalances on an offset QPSK (OQPSK) communication system in which an OQPSK signal is coherently demodulated by using a carrier reference extracted from a modiffed QPSK carrier tracking loop. Both carrier-suppression level and bit-error performance are analyzed in this article. The results show that the current Consultative Committee for Space Data Systems (CCSDS) recommendations of 2-deg maximum permissible phase imbalance and 0.2-dB amplitude imbalance are su–cient to provide a 25-dB or more carrier suppression and a system degradation of 1 dB or less at the bit-error probability of 10^-4 when the OQPSK system is operated in a reasonable loop SNR region.
Keywords
Details
- Volume
- 42-135
- Published
- November 15, 1998
- Pages
- 1–15
- File Size
- 336.6 KB