다중 경로 페이딩 채널에서의 심벌 타이밍 복구 알고리즘의 성능 분석
Performance analysis of symbol timing recovery algorithms in the multipath fading channel
다중경로페이딩채널 심벌타이밍복구 이동통신 multipath fading channel;
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Today, digital receiver is using symbol timing recovery algorithms to exactly detect signals that arrive from the transmitter. Symbol timing recovery is considered as one of the basic functions required for constructing digital communication receivers. To improve receiver's performance, we have to use a proper symbol timing recovery scheme and timing error detector(TED). Symbol timing recovery configuration of the receiver can be divided into two groups. One is feedback timing recovery configuration and the other is feedforward timing recovery configuration. The feedforward scheme is based on block operations and is suitable for burst transmissions where fast acquisition of symbol timing is required. In the feedback scheme, symbol timing is measured by a TED and is fed back to the timing corrector recursively. The feedback scheme has generally better tracking performance after a timing acquisition process is completed. TED measures timing error signal, then this signal is compensated for when the next time signal is calculated. Many kind of TEDs are used nowadays. As for decision-directed TEDs of the feedback scheme, there are kinds of ML(Maximum Likelihood)-based TED, zero-crossing TED (ZCD), early-late TED(ELD) and Mueller & Mueller TED(MMD). And as for non-data-aided TEDs of the feedback scheme, there are kinds of non-data-aided ML-based TED, Gardner TED(GAD) and non-data-aided early-late TED(NEL). O&M timing estimator is representatively used in feedforward scheme. Multipath fading is one of the big problems that cause decrease of system's performance in mobile communication. The transmission path between the transmitter and the receiver can vary from simple line-of-sight to one that is severely obstructed by buildings, mountains, foliage, etc. The radio wave propagation process over the mobile communication channels can be explained in terms of large-scale path loss and small-scale multipath fading. In this thesis, effect and analysis aspects of symbol timing recovery in multipath fading were considered. Computer simulation of symbol timing recovery scheme was carried out using ◁수식 삽입▷(원문을 참조하세요)-shift DQPSK modulation method in the feedback scheme using Gardner timing error detector and the feedforward scheme using O&M timing estimator, and then we analyzed bit error rate(BER) of the schemes. From the simulation results, we found that the performance of the two schemes is almost identical. In the multipath fading, we found that with the increasing normalized doppler frequency, performance becomes worse. In case the normalized doppler frequency is 0.05, 0.01 and 0.005 for the two schemes, we found that the upper bound of bit error rate is 2.3×10^(-2), 10^(-3) and 3×10^(-4) respectively. But the performance does not vary according to rolloff factor α. Practically, even though large memory is required, in case of burst transmissions, the feedforward scheme is profitable because it can acquire fast acquisition. But even though we can't expect fast acquisition, the feedback scheme is suitable for continuous transmissions because it has generally better tracking performance after a timing acquisition process is completed.