I. Introduction
TD-SCDMA is a time-division duplex (TDD) system that relies heavily on precise synchronization. Currently, the TD system uses the Global Positioning System (GPS) for timing synchronization. However, GPS-based timing systems have several limitations, such as inflexible deployment, limited accessibility, and potential security risks. To address these challenges, China Mobile has explored alternative solutions to replace GPS for time synchronization. The current mainstream approaches include using the 1588v2 protocol in wired networks and the independently developed Beidou satellite timing system in China. Despite their advantages, large-scale implementation of these alternatives has been hindered by issues like the complexity of 1588v2 network testing and significant interference problems with Beidou signals. In response, China Mobile, in collaboration with Datang Mobile, has developed an integrated satellite timing optimization system. This innovative solution aims to enhance traditional satellite timing systems while offering a practical complement to existing GPS alternatives. The development of this system has also contributed significantly to the broader use of satellite timing technologies.
II. Traditional Satellite Timing System
Among the three major 3G standards in mobile communications, both CDMA2000 and TD-SCDMA rely on base station synchronization. Accurate timing is essential for seamless handover and roaming between base stations, as any deviation can cause serious system interference. At present, the TD base station synchronization primarily depends on the GPS system. Due to concerns over GPS security, China has opted to implement the Beidou satellite timing system instead. The structure of the traditional satellite timing system is illustrated below:
[Image: Schematic diagram of GPS/Beidou timing in the TD-SCDMA system]
The satellite antenna receives GPS or Beidou signals, which are then transmitted via RF cables to the GPS/Beidou receiver located in the Node B. The receiver extracts the clock signal and outputs the PPS (Pulse Per Second) and TOD (Time of Day) to the phase-locked loop in the Node B. This locked time signal is then distributed to all relevant boards within the Node B that require synchronization.
However, traditional satellite timing systems face several challenges:
(1) Engineering Construction Issues
The installation of traditional GPS/Beidou antennas requires open spaces with minimal obstructions, such as trees, buildings, or towers. Antennas must be placed at least 2 meters away from large metal objects larger than 20 cm. Additionally, RF cables used for signal transmission are thick and rigid, making them difficult to bend and install, which limits aesthetic and flexible deployment.
(2) RF Feeder Extension Limitations
RF feeders suffer from signal attenuation, limiting the distance between the antenna and the receiver. For example, in the Beidou system, the signal power at the antenna port is typically -127 dBm, with a gain of 43 dB and a receiver sensitivity of -100 dBm. This allows only 16 dB of total loss. Using LMR-40 cables (which have a loss of 22 dB per 100 meters), the maximum allowable distance is about 72 meters. Beyond that, additional repeaters are needed, complicating construction and limiting the feasibility of full GPS/Beidou antenna installations. This poses significant challenges in dense urban areas and underground environments like subway tunnels.
(3) Interference Problems
Satellite antennas often face complex installation environments, leading to electromagnetic interference. Beidou, operating at 2.5 GHz, is particularly vulnerable to interference from nearby wireless systems. To mitigate this, engineers often implement anti-interference measures such as narrow-band filtering and strategic antenna placement, which add complexity to the installation process and increase the difficulty of deploying Beidou as a GPS alternative.
III. Integrated Satellite Timing Optimization System Solution
1. Overview of the Plan
To address the limitations of traditional satellite timing systems, China Mobile and Datang Mobile have introduced an integrated satellite timing optimization system. This solution integrates the GPS/Beidou antenna and receiver into a fiber-optic remote design. The receiver outputs PPS and TOD signals, which are transmitted via fiber to the Node B. The Node B's clock recovery module processes these signals and distributes them to the necessary components. This design eliminates the need to consider factors like the type, model, or manufacturer of the receiver, as long as the Node B and remote receiver follow the same interface and timing protocols.
[Image: GPS/Beidou receiver remote timing diagram]
The timing optimization focuses on reducing interference in co-located sites and improving the satellite timing antenna design. According to current interference analysis, most interference comes from nearby ground equipment, with different signal arrival angles compared to satellite signals. The integrated system combines frequency-domain and spatial filtering techniques. Frequency-domain filtering uses high-selective filters inside the low-noise amplifier to suppress out-of-band interference, while spatial filtering employs a modified antenna design. The system utilizes helical antennas, known for their directional characteristics, high gain, and wide bandwidth, especially in frequency coverage. This makes it easier to support future shared antennas for Beidou and GPS. Parameters such as beam width, gain, and impedance are easily controlled, and the design is simple to produce and debug.
[Image: Comparison of ordinary GPS antennas and spiral GPS antennas]
Figure 3 shows a comparison between a standard GPS antenna and a spiral GPS antenna. The helical antenna demonstrates strong suppression of in-band interference, making it ideal for improving signal quality in challenging environments.
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