An Improved Digital Timing Recovery Circuit for Burst-Mode Communications

Abstract

Burst-mode communication (or burst-mode transmission) has been increasingly adopted in modern wireless communication systems. It requires a digital timing recovery circuit with short acquisition time. The bang-bang phase detector is widely used in digital timing recovery circuits owing to its simple structure. However, these timing recovery circuits do not provide a quick acquisition. In this paper, an enhanced bang-bang phase detector is proposed, increasing the resolution of the timing error by over-sampling the data L times and therefore reducing the phase error variance by 1/L 2 times. This timing recovery scheme can be implemented using a Direct Digital Synchronizer (DDS). Both simulation and experimental results illustrate that the new scheme has a shorter acquisition time and a longer holding time, satisfying the need of burst-mode communications. Keywords: Burst-mode communication, bang-bang phase detector, phase lock loop (PLL), acquisition time1．Introduction Timing recovery circuit is an essential part of digital communication receivers. Bang-bang phase detector is a common structure adopted in various timing recovery circuits. Conventional bang-bang phase detector samples the NRZ input at a clock which is two times of the symbol rate and calculates the phase error signal by logic operations [1]. It has been widely used in optical receivers due to its simple structure and high processing speed. However, since the phase error information of a bang-bang phase detector is one-bit, this kind of timing recovery circuits usually has a long acquisition time and suffers large phase jitter [2]. M. Ramezaizi proposed an improved bang-bang phase detector for the optical receivers with four samples in one clock period by an analog phase shifter, and hence it generates two-bit phase error information by each operation [3]. The advantage obtained from the analog phase shifter is limited because the number of quantization steps is 4 (merely two-bit phase error information). Burst-mode transmission is a popular modulation technique used in wireless communication systems, such as in satellite communication, TDMA systems and military communication systems. In burst-mode communications, the timing recovery circuit requires a quick acquisition and long holding time. It is apparent that neither the original bang-bang phase detector nor the M-R improved scheme meets this requirement. Here, we propose an enhanced bang-bang phase detector to address these issues. This method increases the phase error information precision by over sampling the input data L times and provides a linear characteristic curve of the phase detection. The implementation of the resulting timing recovery circuit is consist of a direct digital synchronizer (DDS) and a digital loop filter. The DDS can fully utilize the phase error