Wednesday, October 15, 2014

Link Budget Background, Definitions, and Assumptions

The forward channel is the path from the ground to a remote terminal.

This involves an uplink from the teleport to the satellite, and a downlink from the satellite to the remote terminal.

The return channel is the path from the remote terminal to the ground.

This involves an uplink from the remote terminal to the satellite, and a downlink from the satellite to the teleport.

Forward and Return Channels using an Airborne Remote Terminal (Aircraft Earth Station)

The forward and return channels operate using very different characteristics and limitations.

Ultimately, there are limits the power and spectrum that can be applied.

Scaling spectrum is spectral efficiency, or the data rate (bits per second) achieved from the spectrum committed (Hz).

The spectrum committed or occupied includes the resultant symbol rate (symbols per second) and any guard band between channels.

The modulated symbol rate can be multiplied by a spreading factor.  The spreading factor is simply a means to boost or suppress a signal.

The spectrum committed and the guard band are proportional to the spreading factor times the symbol rate.

Spectral efficiency is impacted inversely by spreading factors.

There are hard limits to spectrum committed due to:
  • available symbol rate (in the modem)
  • available bandwidth (in the transponder)
The guard band is a matter of competitiveness, as its utility does not translate into useful symbol rate.

I am currently modeling 18% (of the symbol rate) guard band, but I have seen implementation with over 30% guard band.

Carrier  to Noise plus interference (dB) is a reflection of the composite signal present, to which I will generically refer to as signal strength.

One converts to Energy per Bit (Eb/No) by applying a modulation factor to signal strength.

Ultimately, the modem is deriving some measure of Eb/No in order to demodulate the bit, and this is the baseline performance goal.

Spectral efficiency can be related to signal strength, and to the best of my knowledge
the Shannon Limit remains the barrier from which we can only approach.

In order to gain enough energy per bit with poor signal strength, less efficient coding and modulations, and even spread spectrum (multiplying) is necessary to dig out a usable signal.

The following chart reflects a fabricated modem that demonstrates how the modulation choices improve with signal, and to appreciate that coding gains vary across the range of a given modulation.

Adaptive Modulation Options for a generic modem
The correlation to a given modulation and coding to signal strength is a matter of propriety and competition (hence my fabrication above, which follows broadly DVB-S2 claims).

Fundamentally, spectral efficiency is related to the highest modulation and coding for a given signal strength.

The ability to adapt as a remote moves across downlink contours (dBW) or uplink contours (G/T) allows for optimized spectral efficiency.

DVB-S2/ACM (adaptive code modulation) is a baseline feature in optimizing the forward channel.

Adaptation on the return channel remains an area of continuing gains, where the baseline operates around static coding and modulation, albeit multiple networks can be combined to allow a remote terminal to move to a more favorable network.

Operating in spot beams introduces more frequent handoffs between transponders (spots) than would be expected in a wide beam.

Enhancements to spot beam handoffs includes an option to include two modems so that the servicing beam and the next beam can be used simultaneously for seamless service transition.  

Spot beam handoffs are a tuning exercise, not a beam steering exercise (as when handing off to a new satellite).

iDirect Evolution 8000 Airborne Router Modem Features  

The very capable iDirect Evolution 8000 Airborne Router Modem has a 45 Msps forward (DVB-S2/ACM) and a 7.5 Msps return (D-TDMA) symbol rate limit.

Forward channel symbol rate limits were not much of a concern until the prospect of HTS with very wide transponders: from 36 MHz to 72 MHz to 125 MHz onto 500 MHz.

Return channels can operate at higher data rates in combination with HTS high uplink G/T, and with modest EIRP, are now bumping into symbol rate limits.

Modem limits, along with their ability to adapt to varying signal conditions, and the complement of modulations and codings are a matter of propriety, discrimination, and continual development.

Information rate is the product of
the symbol rate times the modulation factor divided by the coding gain.
Data rate is information rate times an IP efficiency factor.

  • 90% for the forward channel 
  • 70% for the return channel  

In other words, if the information rate were 1000 bits/second,
then the effective IP throughput would be about

     900 bits/second forward and about 
     700 bits/second return

Internet access for generalized browsing favors the forward channel by ratios of 4-8:1 in comparison to the data rate for the return channel.

Uploading of photos and videos to social media is a driving application for the return channel.

Telemetry applications, such as airplane health monitoring, favor the return channel almost exclusively.

Video streaming entertainment favors the forward channel almost exclusively.

Telephony or video conferencing may provide equal burdens on the forward and return channel.

Transmitted power (or Equivalent Isotropic Radiated Power, EIRP) (dBW) is a combination of the gain of the antenna (dB) and the electrical energy provided at "the antenna flange" (dBW).

The electrical energy is what remains from the output of an amplifier through the interconnecting transmission line.

The amplifier on a remote terminal is constrained by space, power and cooling.

The gain of the antenna in a remote terminal is scaled by the size of the aperture.

The teleport has a very large antenna, and effectively unlimited electrical energy.

The satellite has limits in both antenna gain and available power.

Clearly, High Throughput Satellites (HTS), whether Ku band or Ka band, have much higher gain antennas and also much more available power due frequency reuse offering many times more capacity than a wide beam.

It is these differences that I will explore to characterize their benefits.

Power Spectral Density (PSD) or EIRP Spectral Density (sometimes notated as EIRP, incorrectly in my mind) is effectively EIRP divided by committed spectrum.
In Ku band, we typically use 
       dBW / 4 kHz
 whereas in Ka band, we typically use 
       dBW / 40 kHz
Spectral Flux Density is dBW per MHz per square meter.  It also referred to as Power Flux Density.

Power Equivalent Bandwidth is a measure (in MHz) that indicates the percentage of used Versus available transponder power in comparison the committed and available spectrum for that transponder.

By and large, wide beam satellites operate a forward channel using more PEB than committed spectrum.

Spot beam transponders are more powerful, in part because of the inherent antenna gain benefit, and thus the same forward channel on a spot beam would operate using less PEB than committed spectrum.

The return channel operates using starkly different link budgets to the forward channel.

There is hardly measurable PEB on a return channel.

Depending on the situation, and especially if you are using the same spectrum forward and return, you can rob the PEB budget from the return channel and pay it the the forward PEB deficit, and not result in any real charge for PEB excess.

ASI                      Adjacent Satellite Interference

bps                      bits per second

CDMA                Code Division Multiple Access

C/N                     Carrier/Noise (dB) easily measured Rx signal strength

C/N+I                 Signal strength accounting for noise and interference

dB                      decibel

dBW                  10*log (Power in Watts)

Downconvert     Decrease the carrier frequency by subtracting a fixed frequency offset

Eb/No                Energy per bit over Noise (dB) threshold of demodulation

EIRP                  Equivalent Isotropic Radiated Power (dBW)

ESD                   EIRP Spectral Density (also PSD)

Feeder                Satellite link to teleport

Forward             Received at the airplane (remote terminal)

GSO                  Geo-Stationary Orbit

G/T                   Gain over System Temp Rx Gain – 10 log (Temp deg K)

HP                     High Power

Hz                     Hertz (cycles per second)

IF                      Intermediate Frequency typically 950 – 1450/2150 MHz

IP                      Internet Protocol (network)

LEO                  Low Earth Orbit

L N                   Low Noise

Return              transmitted by the airplane (remote terminal)

RF                    Radio Frequency of carrier

Rx                    Receive

PFD                 Power Flux Density (dBW/MHz/m2) (SFD)

PSD                 Power Spectral Density (EIRP/Hz)

Service            Satellite link to airplane (remote terminal)

SFD                 Spectral Flux Density (dBW/MHz/m2) (PFD)

TDMA            Time Division Multiple Access

Tx                   Transmit

Upconvert       Increase the carrier frequency by adding a fixed frequency offset

Peter Lemme

Copyright 2015
All rights reserved

Check out these related posts

Rain Fade
Skew Angle and Effective Aperture of Airborne Antennas
Spot beams Vs. Wide beams Ku band

Forward Channel Considerations
Transponder Downlink Contours
Forward Channel Downlink Regulations
Ku-band Airborne Antenna Figure of Merit (G/T)
Ka-band Figure of Merit (G/T)

Return Channel Link Budget
Ku-band Return Channel Maximum PSD
Ka-band Return Channel Maximum PSD