The Critical Technic on The Effect of Digital Communications Quality

• Main Authors: CHANG, CHUNG-LIANG, 張仲樑
• Other Authors: Tu, Te-Ming
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/41705068746003106633

博士 === 國防大學中正理工學院 === 國防科學研究所 === 95 === ABSTRACT The essay researches the effect of the critical technologies in quality of digital communications. The study is mainly divided into the effect of aperture jitter on the sampling system, the synchronization errors on the multi-carrier code division multiple access (MC-CDMA) communication system, and the combining technic in the fast frequency-hopped binary frequency shift keying (FFH/BFSK) communication system. First, this article examines aperture jitter of the sampling system and its effect on communication systems in additive white Gaussian noise (AWGN) and fading channels. Previous studies have claimed that AWGN power is directly proportional to the jitter noise power. We demonstrate that the aperture jitter can influence the input signal of a sampling system, but is independent of AWGN. The noise power due to aperture jitter in a frequency-nonselective, slowly fading channel, is shown to be a function of the input signal, the aperture jitter, and the channel envelope. The frequency-selective, slowly fading channel involves another parameter, analyzable paths. The effect of aperture jitter on the bit error probability (BEP) of a binary phase-shift keying (BPSK) digital communication system is also considered. The received signal-to-noise ratio (SNR) is calculated first and its probability density function is derived. Then, the average BEP is evaluated as a function of SNR. Simulation results indicate that the aperture jitter noise severely degrades the average BEP by reducing the received SNR. Second, this work derives the average bit error rate (BER) of the uplink and downlink MC-CDMA systems using maximum ratio combining (MRC) and equal gain combining (EGC) with synchronization errors over fading channels. The derived equation can simultaneously incorporate the parameters of the fading channel and all of the synchronization errors, including frequency offset, carrier phase jitter, and timing jitter. Numerical results indicate that those two combining schemes on the uplink and downlink MC-CDMA systems are degraded by all of the normalized synchronization errors over . The comparison outcomes between MRC and EGC reveal that the MRC generally outperforms EGC in the uplink MC-CDMA system. However, EGC achieves better performance when the number of users is small, the normalized synchronization errors are low and the signal to noise ratio (SNR) is high. In the downlink system, EGC mainly outperforms MRC when the SNR and the number of users are gradually increased and the normalized synchronization errors are low. Therefore, the selection of MRC or EGC depends on the SNR, the synchronization errors and the number of users in uplink and downlink MC-CDMA systems. Finally, this study investigates how independent Rician fading channels affect both FFH/BFSK product-combining receiver (PCR) and partial-band jammer (PBJ). We demonstrate that the average bit error rate (BER) expressions of FFH/BFSK PCR can be derived with the aid of an approximation by quantizing the product-combiner’s output. Numerical results show that system performance of the FFH/BFSK PCR over fading channels and PBJ is lower than that in non-faded PBJ at a low signal-to-jammer density ratio .