I. Introduction
TD-SCDMA is a TDD-based system that relies heavily on precise synchronization. Currently, the TD system uses the Global Positioning System (GPS) for timing synchronization. However, GPS has several limitations, including inflexible deployment, limited accessibility, and security vulnerabilities. To address these issues, China Mobile has explored alternatives to GPS for time synchronization. The current mainstream solutions include the use of 1588v2 in wired networks and the independently developed Beidou satellite timing system. Despite their potential, large-scale implementation of these alternatives has been hindered by challenges such as network testing of 1588v2 and significant interference issues with Beidou. In response, China Mobile, in collaboration with Datang Mobile, has developed an integrated satellite timing optimization system. This innovative approach aims to complement existing GPS alternatives and improve the practicality of satellite-based timing solutions. Ongoing advancements in satellite timing technology have further supported this development.
II. Traditional Satellite Timing System
Among the three major 3G standards in mobile communications, both CDMA2000 and TD-SCDMA rely on base station synchronization. Precise time control is essential for seamless handover and roaming between base stations, as any delay can cause serious system interference. Currently, TD base stations primarily use the GPS system for synchronization. Due to concerns over GPS security, China has opted to implement its own Beidou satellite timing system. The traditional satellite timing system consists of a GPS/Beidou antenna, receiver, and phase-locked loop modules within the Node B. The satellite antenna receives signals from GPS and Beidou satellites, which are then transmitted via RF cables to the receiver. The receiver extracts the clock signal and outputs PPS (Pulse Per Second) and TOD (Time of Day) to the phase-locked loop, which then distributes the synchronized time to various boards within the Node B.
However, there are several issues with traditional satellite timing systems:
(1) Engineering Construction Challenges
The installation of GPS/Beidou antennas requires a clear line of sight, with no large obstructions nearby. The antenna must be placed at least 2 meters away from metal objects larger than 20 cm. Additionally, the thick and rigid RF cables used in traditional systems are difficult to manage, limiting flexibility and aesthetics during installation.
(2) RF Feeder Extension Limitations
RF feeders suffer from significant signal attenuation, which limits the distance between the antenna and the receiver. For example, in a Beidou system, the allowed signal loss is only 16 dB. Using LMR-40 cables, which have a loss of 22 dB per 100 meters, the maximum distance is limited to about 72 meters. Beyond that, additional repeaters are needed, complicating the setup. This limitation is especially problematic in dense urban areas or underground environments like subway tunnels.
(3) Interference Issues
Satellite antennas often face electromagnetic interference due to their complex installation environments. Beidou’s operating frequency of 2.5 GHz is close to many wireless communication bands, making it vulnerable to interference. To mitigate this, engineers implement anti-interference measures such as narrowband filtering and careful antenna placement, which add complexity to the installation process and reduce the feasibility of using Beidou as a GPS alternative.
III. Integrated Satellite Timing Optimization System Solution
1. Overview of the Plan
To overcome the limitations of traditional satellite timing systems, China Mobile and Datang Mobile have introduced an integrated satellite timing optimization solution. This system integrates GPS and Beidou antennas and receivers into a single unit, transmitting the PPS and TOD signals via fiber optics to the Node B. The Node B then recovers the clock signal and distributes it to all necessary components. This design eliminates the need to consider factors like receiver type, model, or manufacturer, as long as the interface and timing mechanism are standardized.
2. Product Reliability Design
The system incorporates advanced filtering techniques to reduce interference, especially in co-location scenarios. It uses a combination of frequency domain filtering—through high-selectivity filters in the low-noise amplifier—and spatial filtering, achieved through a modified helical antenna design. Helical antennas offer excellent directionality, high gain, and wide bandwidth, making them ideal for future integration of both Beidou and GPS signals. Their controllable parameters, such as beam width and impedance, make them easier to produce and maintain. These features enhance the system's reliability and performance in real-world conditions.
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