Antennas used for satellite TV reception (direct-to-home aka DTH) in the Ku bands typically have a diameter smaller than 1 m, usually 60 to 80 cm. Due to the small diameter of the reflector, the beam width, measured between −3 dB points, is about 2 degrees, and provides attenuation of 10 dB at 2 degrees off the main lobe (4 degrees beam width). Geosynchronous communication satellites are typically separated by 2 degrees from each other. Therefore, for DTH receive-only operation, assuming the antennas are perfectly tuned and receiving the DTH service properly, these antennae provide sufficient rejection of the signals coming from the adjacent satellites. The modern digital signals used for TV transmission do not require signal-to-noise ratio larger than 10 dB.{mosimage}

In the DTH environment, if the antenna is not aligned properly, the gain toward the main satellite providing the service will be reduced, and the rejection of signals coming from adjacent satellite will reduce. In an extreme case of 1-degree misalignment, the antenna will have equal gain toward both satellites and, therefore, the reception of the signal from the main satellite may be blocked. {mosimage}

In the DTH receive-only environment, if this happens, the only implication is that the end user suffers either a low-quality reception, or even a complete blockage. In such cases, usually the end user will fine tune the antenna and get his service back.

Technological innovations enabling the use of existing receive-only DTH antenna for interactive TV and consumer-grade low-cost VSAT are in the foray. With the development of the RFModem technology, smart LNB, and CG-VSAT enables this. Due to the antenna radiation pattern, most of the power will be transmitted toward the main satellite, but still some power will be transmitted toward the adjacent satellites, potentially causing interference to other services. In order to avoid such interference, some measures need to be provisioned.

Assuming all satellites are with equal performance, the gain of an antenna outside its main lobe has to comply with the following formula: 29-25 log (Theta), where Theta is the distance from the antenna beam center in degrees. Table 1 compares the allowed gain versus typical 80 and 60 cm antennas gain, with and without misalignment. From the Table, it is clearly evident that a DTH antenna, whether 80 or 60 cm cannot be used for transmission without additional measures.

The RFModem implements a special code division multiple access (CDMA) technology, which utilizes the spread spectrum technique to overcome this limitation. CDMA was not used so far in satellite communications due to its low efficiency in utilizing the satellite resources. However, the RFModem implements a dedicated detection technology at the system hub to overcome this limitation and enables implementation of a high-efficiency usage of satellite bandwidth with CDMA. This makes CDMA a viable solution for Internet access over satellite in general and interactive TV in particular. In the implementation of RFModem as smart LNBs CDMA is provided with spreading factor ranging from 16 to 256. Table 2 shows the effect of using this waveform with spreading factor of 16 on the antenna gain toward an adjacent satellite.

To conclude, use of DTH antennas, for Internet access and interactive TV is made possible by usage of the unique CDMA technology implemented by RFModem.




BroadcastAsia 2017

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