The importance of phase noise


Phase noise is a key component of any satellite link. But what is it exactly? What are the major factors that contribute to it? And why is it so significant now? This post explores answers to these questions and more.

Improvements to GEO satellites and modems

Satellite bandwidth is still a scarce and valuable resource. Until LEO constellations open the floodgates to massive and affordable bandwidth, we’ll all continue to primarily use GEO systems. But these systems are changing. 

Bigger, more powerful payloads, higher frequency bands (Ka, and future V and Q), spotbeam, beamforming and other new antenna technologies are transforming GEO satellites. And on the ground, modem technology is evolving with more efficient modulation and coding schemes (MODCODs) that improve the capacity and efficiency of satellite communication links.

As data rates and MODCODs push forward, ground station operators will need to revisit their RF equipment. There is a very real possibility that an older BUC or LNB that was carrying QPSK or 8PSK traffic won’t be good enough to provide a quality link for higher modulations (16APSK and above). Why? Because of the phase noise they add to the link.

What is phase noise?

Phase noise is the noise produced by rapid, short-term fluctuations in a satellite signal. The fluctuations spread the power of a signal to adjacent frequencies, resulting in “noise sidebands.” Phase noise reduces signal quality and increases error rates in communication links. Although there is no such thing as zero phase noise, the less you have, the better.

Phase noise is particularly important to SATCOM system engineers. They are acutely aware of the impact of any noise source and design networks accordingly.

Sources of phase noise

A variety of factors in a satellite link will contribute to and affect phase noise:

  • Phase lock reference for BUC/LNB
  • PLL LNB design
  • Loop bandwidth
  • Power supply noise
  • Grounding (electronics, cables and antenna structure)
  • 20 Log(n)
Some of these factors can be minimized through proper selection of subsystem equipment (stated/tested specs from the BUC and LNB manufacturers). Some can be mitigated through good engineering practices and careful site installation (proper grounding in particular). One of the most important contributing factors is the stability of the oscillator that is generating the timing reference for any equipment with a frequency conversion stage (BUC or LNB).

Effects of phase noise on signal demodulation

Any noise introduced into a satellite link makes proper demodulation and decoding more difficult. The demodulated constellation of received phases gets distorted by the added noise. The pair of constellation diagrams below illustrate the case for a simple modulation scheme, QPSK:

image source

The plot on the left is a relatively clean diagram, with the demodulated signal suffering only from random Gaussian noise (AWGN). The plot on the right shows the presence of phase noise and how it starts to spread the received constellation. With additional phase noise, the diagram will degrade into a ring of scatter points, making demodulation and decoding impossible (and producing a very high bit error rate).

Watch the degradation of the constellation as additional phase noise is added to the link:

As modems and satellite links push for higher modulation schemes, these constellation diagrams become ever more sensitive to the effects of phase noise.

The relevance of phase noise standards

Phase noise is typically expressed in units of dBc/Hz, and it represents the noise power relative to the carrier contained in a 1 Hz bandwidth centered at certain offsets from the carrier. For example, a certain signal may have a phase noise of -80 dBc/Hz at an offset of 10 kHz, and -95 dBc/Hz at an offset of 100 kHz.

Satellite operators, particularly those that allow access to their fleet by a wide variety of user configurations and equipment vendors, require a minimum set of standards for phase noise. A very good example of this is the U.S. DoD’s Wideband Global SATCOM (WGS) system – a nine satellite constellation operating in X-band and military Ka-band. To get authorization to operate on this fleet, earth station equipment configurations must undergo testing (ARSTRAT certification) and meet the MIL-STD-188-164B standard for phase noise. The “phase noise mask” that must be met for a satellite terminal is as follows:

Figure from MIL-STD-188-164B: Terminal Phase Noise

Testing is rigorous, time-consuming and expensive. You definitely want to pass the first time.

The advantage of ultra-low phase noise

Phase noise standards are implemented for good reason. By meeting or exceeding their specifications, you can optimize and maximize usage of the satellite resource. Well-designed earth stations with ultra-low phase noise have the advantages of:

  • Improved availability (less susceptibility to other noise sources, even atmospheric/rain)
  • Increased effective coverage area
  • Reduced operating costs
  • Reduced antenna sizeLower civils/capital costs
  • Higher throughput (combination of data rate, modulation and coding rate)

These types of earth stations require modern LNBs with the lowest phase noise possible. If you’re looking for an LNB like this – we can help. Here’s an example of an actual Orbital Research Ka-band LNB, tested and plotted against the MIL-STD mask:
Ka-band-LNB-testClearly the unit passed, exceeding the standard anywhere from 5dB to 30dB. Outperforming the spec not only ensures ease of certification, it provides additional improvements to the overall satellite link. This is important to consider, whether your application requires formal certification or not.

Lastly, a noisy reference source can undermine the performance of your LNB – no matter how good it is. You’ll get better specs by coupling your LNB with a high-stability, low phase noise oscillator. Orbital also offers a series of oscillator modules that meet this need, even on fast-moving vehicles or aircraft.

I hope this post has given you a better understanding of phase noise – including what it is, why it’s important, and how the right LNBs and oscillators can help minimize it. Want to know how your own system can be improved by better phase noise?

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