With the increasing number of TV programs supplied to customers and thus the rapid expansion of the TV reception distribution systems, there comes a demand for a stable and robust distribution system occupying a compact trunk space but being able to provide high quality transmission for broadband signals over a long distance to meet the needs for both today and future.
Here we introduce a reconstruction of satellite TV receive only (TVRO) front-end systems in AsiaSat Tai Po earth station. The original and new satellite TVRO front-end systems in Tai Po earth station are described, analyzed and compared. The new system is evaluated with respect to its performance and outstanding features.
Traditional satellite TVRO front-end systems
Original satellite TVRO front-end systems in Tai Po earth station are relatively traditional: a coaxial cable connects the LNB of a receiving antenna and a satellite receiver (IRD) in the RF room. The downlink signal is received, the DC power is supplied to the LNB and the satellite receiver through this coaxial cable. A linear amplifier is inserted to compensate for the attenuation of the coaxial cable especially when the receiving antenna is located far away, i.e. 60 meters or more from the RF room. Downlink signals also pass through a power divider to distribute the signal for multi-channel TV programs. A configuration diagram to describe the traditional satellite TVRO front-end system in Tai Po earth station is shown in Fig.1. Fig. 2 shows pictures of such a coaxial cable distribution system.
Fig. 1 The configuration diagram of traditional satellite TVRO front-end systems in Tai Po earth station
Fig. 2: Left: The picture of coaxial cables and arrestors in Tai Po earth station. Right: The picture of RG 11 coaxial cables inside the Tai Po trunk space
Coaxial cable distribution system is low cost and suitable for transmission in short distance less than 60 meters. However, there are some disadvantages of this type of satellite TVRO front-end systems:
- Coaxial cable is susceptible to EMI and RF interference at inter-connection points;
- Group delay and phase distortion are increased due to the latent capacitance & inductance of the coaxial cable;
- Large antenna for good reception can only be installed in a place where it is far away from the RF room and transmission in a long distance by a coaxial cable causes high level attenuation;
- In order to reduce the loss of coaxial cables in high frequency, a big coaxial cable such as RG11 cable is required which makes the system heavy and bulky. Table 1 lists typical transmission losses of coaxial cables in varied sizes vs. different frequencies;
- The linear amplifier can cause signal degradation, such as a performance deterioration of carrier-to-noise ratio, group delay, intermodulation distortion and so on;
- The power divider leads to a serious signal splitting loss.
Table 1 Typical transmission losses of coaxial cables in varied sizes vs. different frequencies
Optic fiber distribution systems for TVROs in Tai Po earth station
The above traditional satellite TVRO front-end system used to be economical, however with the increasing number of satellite TV programs and the demand for higher quality of TV reception, it is challenging for the traditional coaxial cable front-end system to meet the requirement.
With the development of advanced technologies and emergence of new products, a good solution is becoming feasible: building satellite TVRO front-end systems by employing optical transmission technology. Tai Po earth station has adopted and implemented this new technology for the past five years, and has benefited from the reliability and high quality transmission performance it provided.
Following a LNB, the coaxial cable is replaced by an optic fiber cable operating in single mode that connects a pair of L-band (950MHz-2150MHz) optic fiber transmitter and receiver to transmit the receiving signal in long distance from 100 meters to 2 kilometers without degradation of the signal. A configuration diagram of the optic fiber distribution system for TVROs in Tai Po earth station is shown in Fig. 3.
Fig. 3 The configuration diagram of the optic fiber distribution system for TVROs in Tai Po earth station.
Fig. 4 shows pictures of the Tai Po optic fiber distribution system in which the junction box of the outdoor fiber transmitter is mounted just besides the TVRO antenna on the roof of the earth station building. One chassis provides a 1-way to 4-way optical transmitter module to support 1-4 polarisation of the application for downlink reception of the earth station. No external linear amplifiers are required.
In some cases where large-scale TV reception is demanded, an outdoor unit cabinet is built on the roof of the building as the forefront of the workplace in the vicinity of the satellite antenna sites, normally within 60 meters, in order to avoid introducing linear amplifiers in this segment. Optic fiber transmitters and related devices are installed in the cabinet. This arrangement can reduce the RF interference and in addition, no longer occupies the trunk areas. The downlink signal is received and transmitted via an optic fiber cable to optic fiber receivers in the RF room.
Armored optic fiber cables are well protected and can be used anywhere without the potential for damage. 24-strand optic fiber cables, an equivalence to 24 RG11 coaxial cables in total, are used in the optic fiber distribution systems for TVROs in Tai Po earth station. The diameter of the 24-strand optic fiber is 10mm, the same as that of one single coaxial cable. As can be seen from Fig. 5 the optic fiber (2 inch cable tray) is occupying much less space than those coaxial cables (2ft.×4ft. for 15 pc RG11 coaxial cables) in the traditional distribution system as shown in Fig. 2.
Fig. 5 The picture of the 24-strand optic fiber cable
There are two general types of optic fibers used for communications: single mode and multimode fibers. Single mode fiber is used to transmit 1270 - 1650 nm light over long distances, most commonly at 1310 and 1550 nm wavelength for high data rates. Multimode fiber distribution system, cheaper than single mode fiber distribution system, is commonly used to transmit light at 850 and 1300 nm wavelength for shorter distances and moderate data rates. Approximate transmission losses of both single mode and multimode optic fibers transmitting in various wavelength are listed in Table 2.
Table 2 Typical transmission losses of optic fibers transmitting in various wavelength
Optic fibers in Tai Po earth station are single mode fibers. Typical loss of single mode transmission is 0.35 dB/km at 1310 nm which can almost be neglected. It indicates that after the transmission over a long distance, the signal level at the output of optic fiber receiver will remain the same as the input of optic fiber transmitter. Most importantly, minimum performance degradation of carrier-to-noise ratio, IMD intermodulation distortion, phase error and thus bit error rate (BER) is warranted. This is superior when compared to the transmission scheme by using a coaxial cable followed by a linear amplifier. After application of optic fibers, there would be no restriction on the distance between satellite receiving antennas and the RF room. Furthermore, it can prevent damage on devices in the RF room due to lightning shocks. These are the reasons why optic fiber distribution systems are constructed in Tai Po earth station to replace the traditional coaxial cable distribution systems for TVROs.
Table 3 summarizes the comparison of the characteristics of optic fiber and coaxial cable distribution systems. Fig. 6 demonstrates a significant improvement of transmission characteristics by using optic fiber distribution systems for TVROs in Tai Po earth station. The optic fiber distribution systems allow AsiaSat to offer high-quality video turn around services to our customers.
Table 3 Comparison between optic fiber and coaxial cable distribution systems
Fig. 6 Performance improvement of replacing coaxial cable with optic fiber distribution systems in Tai Po earth station
Application of optic fiber distribution systems for TVROs in Tai Po earth station of AsiaSat to replace the traditional satellite TVRO front-end systems using coaxial cables has been introduced.
It has been demonstrated that the advantages of the optic fiber over coaxial cable distribution systems for TVROs include low loss in high frequency, immunity to RF interference, light weight and compact in size, easy installation, less maintenance cost, being suitable for long-distance transmission. The optic fiber distribution system can provide stable signal level, high quality transmission, and greatly improve the reliability of the operation of the earth station. The optic fiber distribution systems in Tai Po earth station allow AsiaSat to offer high-quality and high-reliability video reception and turn around services to our customers.
About the Authors
Tommy Fu is AsiaSat’s Senior Communications Systems Engineer. He received Bachelor of Computer Science from Victoria University of Technology. Mr. Fu has more than 15 years of experience in SatCom industry. He is responsible for TT&C tracking antenna, Teleport communication antenna, RF systems and other Earth Station projects implementation.
Rui Zhang is AsiaSat’s Senior Communications Systems Engineer. She received the Bachelor and Master degrees from Nanjing University of Science and Technology, Nanjing, China, and the Ph.D. degree from the University of Waterloo, Waterloo, ON, Canada, all in Electrical Engineering. Dr. Zhang has been with AsiaSat for 8 years specializing in spectrum planning and management. She is responsible for frequency coordination, spectrum regulatory issues and protection of spectrum for satellite communications.