Spectral efficiency –the effective data rate that can be achieved from a given slice of spectrum.
If cost=spectrum, then value=data rate.
Spectral efficiency is the value ratio.
You pay for spectrum (the cost). What value achieved is the data rate.
Spectral efficiency lets you sell more Mbps for the same MHz (like a farmer planting crops in their fields).
C/(N+I)
There are two fundamental drivers in determining spectral efficiency– the Carrier and the Noise – and their ratio C/N and
- the Carrier and the Interference - and their ratio C/I.
It gets more complicated, energy per bit, modulation, coding; but we can ignore that for this high level discussion.
Higher C/N = Higher Spectral Efficiency.
The first is the good, the Carrier, the level of the signal received.
The second is the bad, Noise.
The noise can include “warm bodies” and electrical by-products from antenna components.
There are limits (Shannon – physical) that are manifested in the modulation and coding, and that gets to processing power, symbol rates, etc…
The higher the frequency, for the same size aperture, the higher the gain, the smaller the beamwidth.
Discrimination reflects the beamwidth of an antenna, and its ability to suppress adjacent interference sources.
Bigger Antenna or Higher Frequency = Smaller Beamwidth, Greater Discrimination.
Ka antennas the same size as Ku antennas have higher gain and smaller beamwidth.
Smaller beamwidth gives Ka satellites about three times the number of spot beams as an the same satellite operating on Ku. The additional spot beams benefits with higher frequency reuse and improved link margins.
You cannot simply look at G/T, especially for Ka Vs Ku, and draw any conclusions.
In general, if everything were equal, G/T drives C/N which drives spectral efficiency.
Interference is accounted by the term C/I.
C/I degrades C/N if the Interfering Carrier is received anywhere near the power of the Intended Carrier.
A resultant C/N is the combination of the Carrier with each source of Noise and each source of Interference.
A given antenna gain across the geo-stationary arc can be applied to each intersecting adjacent satellite with overlapping beams to compute an interference product, and this can be greatly influence by antenna side-lobes or grating-lobes (areas of significant gain offset from the intended boresight.)
C/I degrades C/N if the Interfering Carrier is received anywhere near the power of the Intended Carrier.
A resultant C/N is the combination of the Carrier with each source of Noise and each source of Interference.
A given antenna gain across the geo-stationary arc can be applied to each intersecting adjacent satellite with overlapping beams to compute an interference product, and this can be greatly influence by antenna side-lobes or grating-lobes (areas of significant gain offset from the intended boresight.)
There are limits (Shannon – physical) that are manifested in the modulation and coding, and that gets to processing power, symbol rates, etc…
Since this is a ratio measured in dB, negative numbers are indicative of when the Noise is greater than the Carrier (the Carrier is below the Noise Floor). DVB-S2X offers a tool box of methods to recover the information, but they take a toll in efficiency.
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| DVB-S2X and DVB-S2 Spectral Efficiency Vs C/N. Note this includes the guard band between carriers (spacing) and reflects information rate. Actual Forward IP data rates are about 90% of Info Rate; Return IP data rates 60%-70% Info Rate. |
Gain and Beamwidth
The bigger an antenna aperture, the higher the gain, the smaller the beamwidth (more focused).The higher the frequency, for the same size aperture, the higher the gain, the smaller the beamwidth.
Discrimination reflects the beamwidth of an antenna, and its ability to suppress adjacent interference sources.
Bigger Antenna or Higher Frequency = Smaller Beamwidth, Greater Discrimination.
Ka antennas the same size as Ku antennas have higher gain and smaller beamwidth.
Smaller beamwidth gives Ka satellites about three times the number of spot beams as an the same satellite operating on Ku. The additional spot beams benefits with higher frequency reuse and improved link margins.
G/T
G/T is the figure of merit in the receiving antenna. It is the beneficial gain.
As you may imagine, simply boosting a signal with an amplifier applies the same gain to the noise, and adds its own noise.
The trick is to get that gain out of the antenna without overcoming it in noise (the noise temp). The antenna components contribute to noise temp.
Thus, G/T is used, not Gain by itself, to scale a receiving antenna (the figure of merit).
Warm Bodies
There are other noise sources incident to the beam itself.
For example, this can be encountered with any beam pattern that intrudes upon the ground. This is a factor in comparing antennas that have different beam patterns for the same gain.
The noise temp varies a bit depending on where you are pointed.
Interference
The antenna beam pattern can cause different levels of interference from adjacent satellites (ASI).
ASI is strongly dependent upon the beamwidth that goes across the geo-stationary orbit plane, and that is where skew angle is a factor.
Different antennas have different beam patterns that manifest different skew effects. Ka antennas of the same size as Ku antennas hold off skew effects a bit better because of higher discrimination.
Rain Fade can contribute to transmission losses, both at the airplane and at the ground station. For a given rain event (mm/hour), Ka is much more impacted than Ku. A rule of thumb is for every rain event that disrupts Ku, there are about 10 times more rain events that effect Ka (for the same locale).
Transmission Losses
Ka transmission has higher losses inherent in the ether than Ku. This can be offset, in part, by the higher gain from the receiving antenna. For the same antennas, the gain is higher for Ka, and this gain benefit is partially offset by the transmission losses.Rain Fade can contribute to transmission losses, both at the airplane and at the ground station. For a given rain event (mm/hour), Ka is much more impacted than Ku. A rule of thumb is for every rain event that disrupts Ku, there are about 10 times more rain events that effect Ka (for the same locale).
Conclusion
You must examine the entire link budget to assess spectral efficiency.You cannot simply look at G/T, especially for Ka Vs Ku, and draw any conclusions.
In general, if everything were equal, G/T drives C/N which drives spectral efficiency.
Peter Lemme
peter @ satcom.guru
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Peter Lemme has been a leader in avionics engineering for 35 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chairs the SAE-ITC AEEC Ku/Ka-band satcom subcommittee, developing ARINC 791 and 792 characteristics and contributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing Project Paper 848, standard for Secure Broadband IP Air/Ground Interface.
Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).
An instrument-rated private pilot, single engine land and sea, Peter has enjoyed perspectives from both operating and designing airplanes. Hundreds of hours of flight test analysis and thousands of hours in simulators have given him an appreciation for the many aspects that drive aviation; whether tandem complexity, policy, human, or technical; and the difficulties and challenges to achieving success.
So does that mean GX (Ka) has more spot beams than Intelsat IS29e (Ku)?
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