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L-Band Satellite Transport


Coax cable and hardline (coax with an outer copper or aluminum tube) are traditionally specified for RF transmission applications. While functional, this approach has several shortcomings. Copper coax and hardline are bulky and heavy, they conduct electricity, and they have low bandwidth, which seriously limits the maximum usable distance. Fiber optic RF transmission eliminates all of these shortcomings. Fiber optic cable weighs less than hardline or coax cable, and since single-mode fiber optic cable has only about 0.25 to 0.5 dB of signal loss per kilometer of fiber, an antenna may be located many kilometers from the receiver or transmitter. In addition, fiber's dielectric properties prevent lighting from following the fiber optic cable into the building that the antenna serves. Fiber optic transport of satellite signals may be used in a number of applications, including transport from the remote satellite dish farm to a broadcaster's headend, uplink and downlink applications, and DBS services.



Satellite Dish Farm to Headend Network

Teleport operators use fiber optic transport links to transmit RF signals from a remote satellite dish farm to their headend. Because of the remote location of some dish farms, L-Band transport equipment must be able to withstand wide environmental conditions. Signal quality is usually assured through some means of gain control, either manual, automatic, or fixed. Many Teleports also incorporate some form of remote monitoring and control using Simple Network Management Protocol (SNMP) tools to remotely monitor such parameters as RF signal level, optical output power (in fiber optic networks), gain settings, device status, and other parameters. Quite often, a redundant optical and/or RF system and antenna diversity are utilized to protect the satellite distribution system from the effects of weather-fade and sun-fade outages. Figure 1 illustrates an optically redundant system and Figure 2 illustrates an RF redundant signal.



Satellite System with Optical Redundancy

Figure 2 — Satellite System with RF Redundancy

Satellite System with RF Redundancy

Uplink and Downlink Applications

Satellite systems are rarely unidirectional. IF uplinks use satellite modems at the headend to transmit IF signals to the remote dish farm, while downlinks transport RF signals from the satellite dish farm to the headend. IF signals typically fall in the 10-200 MHz range. Figure 3 illustrates a typical uplink/downlink application.

Figure 3 — Typical Uplink/Downlink Application

Typical Uplink/Downlink Application

DBS System

A typical direct broadcast satellite (DBS) installation uses several satellites. This is necessary because each satellite can only transmit a small number of channels. In order to provide 500 channels, for example, the DBS provider would need additional satellites. These satellites hold a geosynchronous orbit over the equator and cover the northern hemisphere, from the United States to the northern part of South America. Regardless of the number of satellites needed in a system, the signal path configuration is the same. Figure 4 diagrams this signal path.

Figure 4 - DBS Signal Distribution

DBS Signal Distribution

A satellite signal is received by pointing the dish antenna toward a group of specific satellites. Digital broadcast satellite service providers usually include an on-screen interface for aiming the satellite dish. Once the antenna's course is aimed, the signal may be fine-tuned by moving the antenna until the signal meter is receiving at least 60% of the signal strength. This configuration applies to both copper coax set top box (STB) or a fiber optic satellite transport link.

Applications for an L-Band Satellite Transport System

Antenna remoting and headend relocation commonly appear at radio and television stations, although headend relocation to a CATV installation also applies. Most signals above 2 GHz are converted by the LNB converter at the dish antenna to an L-Band signal, which allows a microwave signal to be transported a considerable distance. Sending a 12 GHz signal over copper coax cable would result in extremely high loss; however, is the signal is blockdown converted to 2 GHz (L-Band frequency), the same signal can be transmitted over copper coax up to several hundred feet before the loss degrades the signal. Figure 5 illustrates this application. This configuration shows a redundant configuration where back up transmitters and receivers are connected to an A/B switch. In the event of failure on the primary path, the switch will activate the secondary path, keeping the network running at all times.

Figure 5— Antenna Remoting/Headend Relocation

Antenna Remoting/Headend Relocation

Fiber optic L-Band satellite transmission finds applications in multi-dwelling units (MDU's) such as apartment buildings, college dormitories, etc via a broadband amplifier and RF splitter. Figure 6 illustrates this application. The L-Band fiber optic transmitter accepts direct satellite LNB outputs in the range of 950 to 2200 MHz. The signal is converted from to an optical signal and transmitted to the L-Band fiber optic receiver. The receiver RF output is then connected to the RF input of a broadband distribution amplifier and fed to a 1 x 24 RF splitter. From the splitter, up to 24 signals may be distributed to 24 TV set top receivers. This distribution may go for a distance of 200 feet.

Figure 6 — MDU Application

DU Application

GPS positioning and timing offers another important application for L-Band satellite transmission links. Global positioning systems (GPS), used for navigation, relies on signals from satellites in a geosynchronous orbit around the earth. This synchronization is required for personal communications systems (PCS) such as cellular telephones. PCS base station signals using fiber optic transmitters and receivers experience low loss and high quality. In addition, smaller, less expensive power amplifiers can be mounted to the tower reducing transmission losses and system cost. Fiber's dielectric properties also reduce damage from lightning strikes at PCS base stations. Fiber optic L-Band links can also be used as an outside repeater in shopping malls, tunnels, and subways where PCS signals are non-existent or extremely weak, allowing L-Band satellite transport in these scenarios as well.