Second, the traditional satellite timing system
Among the 3G standards in the field of mobile communications, both CDMA2000 and TD-SCDMA are base station synchronization systems, and accurate time control is required for switching and roaming of the base stations, otherwise serious system interference will occur. At present, the TD base station synchronization is mainly implemented by using the GPS system. Based on the consideration of GPS security issues, the Beidou satellite timing system independently developed by China will be adopted on the satellite timing system. The composition of the traditional satellite time service system is shown in the figure below:
Fig.1 Schematic diagram of GPS/Beidu timing in existing TD-SCDMA system
The satellite antenna receives GPS/BeiDou satellite signals and transmits the RF signals to the GPS/BeiDou receiver in Node B through the RF feeder. The GPS/BeiDou receiver recovers the clock and outputs PPS and TOD to the phase locked loop in Node B. The ring sends the locked time signal to each main board in Node B that needs time synchronization.
However, there are some problems that need to be considered in the traditional satellite time service system:
(1) Construction problems
Traditional GPS/BeiDou antenna installation environment is more stringent, and it needs to be installed in a relatively open position, and it is necessary to ensure that there are no large obstructions (such as trees, iron towers, buildings, etc.) around it, and the shielding of the antenna should not exceed 30 degrees. To avoid the influence of reflected waves, the GPS antenna should be as far as possible away from metal objects with a surrounding size greater than 20cm;
In addition, the traditional GPS/Beidou synchronization system adopts radio frequency cables for transmission. RF cables have relatively thick wire diameters and poor toughness, which is not conducive to construction construction and flexibility.
(2) Limitation of RF feeder extension
Traditional satellite timing systems use RF feeders to transmit clock signals. The attenuation of signals by RF feeders is large, resulting in very limited RF feeders. Take the Beidou time service system as an example, Beidou antenna port signal power is generally -127dbm, antenna gain is 43db, BeiDou receiver sensitivity is -100dbm, and only 16db (= -127+43+100) attenuation is allowed on the RF line. If LMR is used, -40 signal line (22dB/z), RF line can only pull away 72 meters (=16÷22×100) distance. If it is more than 72 meters, it is necessary to install the line-distributor, which causes troubles for the construction and it is difficult to implement the GPS/BeiDou co-pilot feeder. At present, there are a large number of scenes that are far away from the high-level coverage system in the dense urban area and the long-distance antenna and base station in the airport subway tunnel coverage, which greatly limits the installation of traditional satellite timing systems.
(3) Disturbance problems
Satellite antenna installation environment is generally more complex, common site construction is more common, electromagnetic interference is more serious, which puts a high degree of isolation requirements for construction and installation. Among them, the BeiDou timing system interference is particularly prominent. BeiDou work frequency is 2.5GHz, and it is very close to some of the wireless system frequency bands in use, so its receiving performance is more vulnerable to interference. In practical engineering applications, in order to receive BeiDou signals normally, BeiDou receivers will consider narrow-band filtering, shielding, and selecting appropriate antenna installation positions and other anti-jamming measures. This also brings difficulties to construction and at the same time makes BeiDou a GPS alternative. Increased difficulty.
Third, integrated satellite timing optimization system solutions
1. Introduction to the overall plan
In response to the problems existing in traditional satellite timing systems, in order to adapt to a wider deployment scenario, China Mobile, in conjunction with Datang Mobile, has creatively proposed an integrated satellite timing optimization system solution. The integration mainly refers to the GPS/Beidou fiber remote design. The surface part integrates the GPS/Beidou antenna and the receiver design. The PPS and TOD output by the receiver are transmitted to the base station Node B through the optical fiber remote method. The Node B clock recovery module recovers PPS and TOD and transmits to Node B modules that need to be synchronized. The base station design no longer needs to consider a series of problems such as the type of receiver (GPS/Beidou), model, manufacturer, and size. It is only necessary that the Node B and the remote receiver have the same interface standard and time transmission mechanism.
Figure 2 Schematic diagram of remote timing of GPS/Beidou receiver
Timing optimization mainly refers to the problem of interference in co-location construction and the modification plan for satellite timing antenna. From the current interference test analysis, we can see that the interference is basically caused by the ground equipment near the base station. The angle at which it arrives at the receiving antenna is different from the angle at which the satellite signal arrives at the receiving antenna. The frequency and airspace anti-interference measures can be comprehensively adopted. The integrated satellite timing optimization system uses a combination of frequency domain filtering and spatial filtering. The frequency domain filtering adopts a high selectivity filter to suppress out-of-band interference in the antenna low-noise amplifier, while the airspace filtering uses the modified satellite antenna solution. The integrated satellite timing optimization system uses a helical antenna as the basic antenna technology. The characteristics of the helical antenna are good directionality, high gain, and wide bandwidth, especially the frequency bandwidth. This provides the conditions for the future Beidou and GPS to share a group of antennas, and the various parameters are easy to control (such as: beamwidth, gain, impedance, axis ratio ), The device has good consistency and is convenient for production debugging. Figure 3 shows the comparison of the lobe diagram of a conventional GPS antenna with a helical GPS antenna. The spiral antenna has a strong inhibitory effect on in-band interference.
Fig. 3 Comparison of common GPS antenna and 4 helical antenna lobes
2. Product reliability design
As the integrated satellite timing optimization system has long-distance clock units RCU placed outdoors, and the device structure is more complicated than ordinary satellite antennas, its reliability design directly affects the stability of the work, and it is unique in the design of the program. The remote clock unit RCU reliability scheme includes design for heat and high temperature resistance, solar radiation resistance design, waterproof seal design, and lightning protection design.
High-temperature resistance design mainly adopts double-shell structure. The entire RCU outer shell consists of two shells. The two shells inside and outside form a “chimney effect†to speed up the air flow. The shell is also resistant to UV radiation. High and low temperature reinforced nylon material production; waterproof seal design, the use of high strength shell, light weight, corrosion resistance and high temperature and low temperature deformation under the alternation of aluminum alloy production; connection is to use high and low temperature, corrosion resistant rubber seal ring. The lightning protection design mainly considers installing TVS diodes (transient voltage suppressing diodes) at the entrance of the internal power supply of the remote clock unit RCU to absorb the differential mode induced voltage energy that enters the machine. In addition, it is used within the feed length of 1 km. Grounding lightning protection measures.
IV. Analysis of program advantages
1. Have a significant impact on project implementation
The traditional GPS time service system has a very limited distance. In the engineering design, it is often necessary to consider the distance between the antenna and the base station, which places great restrictions on the location of the engine room and the deployment of the GPS antenna. The integrated satellite timing optimization system adopts optical hybrid cable to extend the distance, and at least one kilometre can be pulled. At least 1 kilometer or more of the nearest power can be used to extend the optical fiber to 10 kilometers. This fundamentally solves the problem of traditional GPS remote control. The problem of limited site selection has provided great convenience for construction. Moreover, if the RRU and the RCU are set up together, the remote fiber of the RRU and the remote fiber of the GPS can be laid at the same time, which saves construction cost and time. Especially suitable for high-rise buildings, subway tunnels and large stadiums and other computer room and satellite antenna far away from the coverage scene.
2. Increased anti-jamming capability, increasing the operability of co-location construction
The integrated satellite timing optimization system comprehensively adopts frequency domain filtering and airspace filtering schemes, which can greatly improve timing and anti-interference characteristics of the timing system. Experiments show that, at the frequency of 1575.42 MHz, the integrated receiver can improve the anti-interference ability by more than 28 dB compared with the traditional GPS antenna. Using a specially designed anti-jamming spiral antenna, the integrated receiver can improve the suppression of terrestrial jamming signals from low elevation angle directions by approximately 20 dB compared with conventional GPS antennas.
For China's self-developed Beidou satellite time service system, the anti-interference ability has also been greatly improved. The following is the noise spectrum diagram under the current network test. It can be seen that the Beidou band 2.5 GHz accessory out-of-band interference and in-band interference have been eliminated. This gave the Beidou synchronization system a great boost as a GPS alternative.
Figure 4 Output noise spectrum in BeiDou mode using integrated satellite timing optimization system
The two figures are the output noise spectrum within the 100MHz bandwidth and 20MHz bandwidth of the Beidou frequency point, respectively. The figure on the right shows the spectrum amplification near the 2.5 GHz frequency point in Beidou center frequency on the left. It can be seen that the elimination of out-of-band interference is particularly prominent.
3. Can be used as one of the important GPS alternatives
GPS replacement has always been one of the key directions for TD system improvement. Among them, Beidou synchronization timing system and 1588v2 wired network synchronization scheme are two mainstream alternatives. Two GPS alternatives can solve the construction problems and safety problems of the GPS time service system. However, due to severe interference from the Beidou synchronous time service system and the 1588v2 live network test has not yet been conducted, the two GPS alternatives have not yet been implemented on a large scale. The integrated satellite timing optimization system is a practical solution that can effectively solve the GPS timing synchronization problem, and can be selected as one of the GPS alternative solutions.
The integrated satellite timing optimization system can help other GPS alternatives. First, the integrated satellite timing optimization system can be selectively added to the BeiDou module, or the GPS/BeiDou dual mode system is selected, and the system adopts an efficient anti-jamming solution, which has great advancing power for BeiDou as a GPS alternative solution. In addition, for the 1588v2 wired synchronization scheme, the integrated satellite timing optimization system can also be a beneficial supplement. The 1588v2 protocol transmits the clock signal through the transmission network, which can greatly reduce the satellite antenna requirements of the base station system. However, for the base station support in the 1588v2 system, it needs to receive the satellite signal as a clock source, or a more important base station system also needs to receive the satellite signal as a backup. The integrated satellite timing optimization system can also play an important role in improving the advantages of satellite timing system and anti-jamming, which can be a powerful complement to wired clock synchronization.
Other advantages
Base station clock interface standardization, base station hardware revision no longer need to consider the receiver problem;
Simple networking can be achieved so as to achieve the effect of satellite clock sharing;
Because the receiver is very close to the antenna, the demand for the antenna gain is reduced, the antenna is easy to design, and the cost is reduced.
Conclusion
To further verify the stability and reliability of the products, Datang Mobile took the lead in designing and developing the GPS/Beidou dual-mode timing optimization trial commercial products based on the development and verification of the prototypes, and it was used in Jiangsu Province and other provinces in the TD Phase II network. A small-scale network test and verification was conducted. Tests and field tests showed that the integrated antenna has good reliability and stability, and it reduces the loss of feeder joints, anti-interference ability, beautifying the appearance of the sky, and improving the installation and maintenance. Convenience and other aspects have obvious advantages over traditional GPS antennas and fully meet the requirements of large-scale commercial operators.
The integrated satellite timing optimization system is Datang Mobile's innovative optimization solution for the existing TD-SCDMA satellite timing system, which greatly improves the construction of the satellite timing system and the anti-jamming. At the same time, in other industry systems that require time synchronization, the same problem still exists. Therefore, the integrated satellite timing optimization system will be widely used by more industries as it matures in TD-SCDMA timing synchronization.