Link System Performance: Chapter 5

• Chapter 5 discusses Link System Performance.

Link Considerations

• The primary parameter of interest: carrier-to-noise ratio (c/n) or carrier-to-noise density (c/no).
• Three typical satellite link configurations should be considered: fixed antenna size link, fixed antenna gain link, and fixed antenna gain/size link.

Fixed Antenna Size Link

• Satellite link with a fixed antenna size at both ends.
• An example is a satellite network with identical antennas for each ground terminal, such as a VSAT network.
• The performance increases with frequency, implying better performance in a higher frequency band if other parameters are equal; effects of frequency dependent losses are neglected here.

Fixed Antenna Gain Link

• A satellite application is needed that maintains a specific antenna beamwidth on the ground.
• Critical applications: fixed area coverage systems like mobile satellite networks (MSS) and broadcast satellite services (BSS).
• Lowest possible frequency band should be used when beamwidth constraints are important on both link ends.
• Performance depends on p, r, and t(s), but improves as frequency decreases.

Fixed Antenna Gain and Size Link

• Antenna gain is fixed at one end for coverage area, and antenna size is maximized on the other end.
• Typical in a communication satellite downlink where the satellite transmit antenna gain is determined by the desired size of the coverage area.
• Ground terminal receive antenna is made as large as possible based on cost/location.
• Could apply to Fixed Satellite Service (FSS) user terminals or large feeder link hub terminals.
• Solid angle Ω, which subtends an area A(s) on the surface, is quantified to address antenna coverage requirements.
• The energy radiated by the satellite antenna is concentrated; the antenna gain g(t) is inversely proportional to the solid angle.
• Link performance improves as p(t) increases and t(s) decreases.
• The antenna characteristics are dependent with a fixed coverage area, offering no frequency advantage.
• Higher frequency is required in a small coverage area due to antenna size limitations on orbiting vehicles.
• Can make similar conclusion for an uplink with fixed terrestrial service.

Uplink

• Basic uplink performance is evaluated with the consideration of more parameters.
• Uplink parameters are represented by subscript 'U' when mentioned in equations.
• Uplink is specified by power flux density requirement at the satellite receiver antenna, which supports output transmit power.
• The final power amplifier in the satellite—TWTA or SSPA—is critical in defining the flux density requirement.

Power Backoff

• Power Backoff is a power level below the saturation point which allows the amplifier to operate in the linear region even when the input power level slightly increases.
• Power amplifiers are generally operated close to their saturation point to keep efficiency maximal.
• Reducing the power level from the maximum efficiency point ensures operation in the linear region, even with a small increase in power.
• The power level is lowered, with the amount called Power Backoff.

Intermodulation

• A multi-tone signal in a power amplifier creates unwanted intermodulation (IM) products.

• Intermodulation in the linear region does not interfere with main signals due to frequency differences.

• Non-linear behavior can cause interference that reduces quality or linearity.

• Applications that require linear should have the level lower than the saturation point to avoid operational interference.

• Single Carrier Operation

• S, increasing uplink, improves frequency performance and link losses.

• The resulting link equation is independent from the decreases in losses.

• Multiple Carrier Operation

• The TWTA or SSPA exhibits different non-linear response.

• Has to be backed off to the linear to reduce the distortion effects.

• The carrier operation point is also quantified as output power.

• The general performance measures is provided to maintain TWTA-output.

Downlink

• Equations can represent basic performance (when carrier is saturated).
• When backoff is involved, appropriate equations must be included for analysis.
• eirp is downlinked; an result of output BO, and given, the downlink eirp, that is the eirp from the satellite, resulting from operation output backoff is considered to maintain this.

Percent of Time Performance Specifications

• Communications links are specified under statistical parameters.
• Based on time parameters or percent of time a specific value is equaled.
• Including things like carrier to noise ratio, atmospheric noise (rain attenuation), video signal to noise, and carrier to interference ratio.
• Time is specified with time: annual time and worst month.
  • Annual time is around 8769 hrs.
  • Worst month is around : 730hr.
• Predication models on service requirements: Worst month or annual statistics
• It is necessary to see what models apply to service because annual may be only statistics.
• Table 5.1 shows the annual and monthly outage number which is associated with 100-p.
• The ITU-R created a design-tool for the design of radios, it transfers stats and provides relationships.
• The value has a rain, annual, and worst case.