Monday, November 24, 2014

Forward Channel Considerations

  • the forward channel operates at much higher spectral efficiency than the return channel
  • the forward channel can aggregate multiple channels 
    • with no technical limit to aggregated data rate
  • a single forward channel operates under down link regulatory limitations
    • no advantage to spot beam
    • Ka band can operate over 10 dB higher PSD than is typical over Ku band
  • a single forward channel transponder supported symbol rate is increasing dramatically
    • a companion benefit of spot beams and frequency reuse
  • a single forward channel powerful spot beam uses less PEB than an equivalent wide beam
    • Arguably an economic benefit and also offering greater flexibility
    • Has no effect on signal strength due to common downlink limitation
  • Forward Channel uplink carrier to interference is higher with spot beams
    • 3 dB reduction in received signal strength

The forward channel delivers information to the remote terminal.

The good news, there is no technical limit to the forward channel.

A remote terminal can receive the benefits from one or many transponders (using one or many modems) without any physical challenge other than the receiving antenna system instantaneous bandwidth must span all the servicing transponder downlink frequency bands and you need at least one modem per transponder.

Cost is always a practical factor.

Hosting one modem is much easier than hosting two.

Bringing down more than one transponder requires frequency agility in spanning the RF bandwidth to reach the service channels and a means to stack and accommodate the high composite IF bandwidth.

There may be different services, for example interactive IP and television broadcast.

Generally, a transponder set aside for television broadcast coupled with one of a set of transponders for interactive IP is the most scalable.

A single channel is fundamentally limited in downlink, relating to coordinated, overlapping services by satellites spaced as closely as two degrees apart along the geo-stationary orbit (GSO).

FCC filed link budgets from Ku-band and Ka-band spot beam service providers model high levels of uplink interference, resulting in about a three dB reduction in the signal strength received by the remote terminal.  One provider did not model this level of interference, and this may be due to their relative beam sizes.  In any case, the three dB reduction is a disappointing factor.

Peter Lemme

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