Interesting Feature: ITA

The EBEMs (Enhanced Bandwidth Efficiency Modem) that are located in the SWANv3/VSAT-L serve as the FDMA (Frequency Division Multiple Access) modems within the system.  Among other interesting features, these modems have a feature known as ITA (Information Throughput Adaptation).  According to the EBEM Operator’s Manual, the purpose of the feature is to “mitigate downlink rain-fades by means of throughput adaptation (changing data rate, modulation and coding in any combination) while maintaining a constant transmit symbol rate”.  Simply put, this as an auto-negotiation mechanism for your satellite link.

During one of my recent ops, I was able to put ITA into action and it made a huge difference for us.  In our case, we were unable to get the shot to run clean enough to run consistently at our assigned modulation (16-APSK, 16-level Amplitude Phase Shift Key).  After some research, we discovered ITA, convinced the distant end to turn it on for troubleshooting purposes, and the shot came up immediately (after negotiating down to QPSK, Quadrature Phase Shift Key). Because changing the modulation would normally require a change to the SAA (Satellite Access Authorization), ITA would be a much less cumbersome way to accomplish this switch.

If this is something you are looking at employing, there are some considerations you will want to know and understand:

• ITA must be enabled on both ends.
• Changing the modulation, coding and data rate will affect your bandwidth.
• ITA does NOT alter your frequencies or Tx power.
• This is not a feature that is regularly employed so you will need to explain in your SAR (Satellite Access Request) why you want to use it.

Here’s a quick example to illustrate:

• A link negotiated with modulation of 16-APSK and a code rate of .95 will yield 3.8 bits per symbol.
• A link negotiated with modulation of QPSK and a code rate of .95 will yield 1.9 bits per symbol.

The example above illustrates that you can achieve significant bandwidth increases when advanced modulation and code rates are used.  It is not always as simple as just requesting advanced modulation, because weather and other conditions can stop your shot from working altogether (like what happened to me when we were assigned a higher modulation than we requested).  ITA gives us the best of both worlds: bandwidth savings when conditions allow, and negotiating settings down to preserve the link under challenging conditions.

I hope this has given you a better idea of what ITA is, along with and some practical reasons for employment.  If you want to use ITA, make sure you work with your planners to explain why and specifically request the use in the comments section of your SAR.  If you have any experience with ITA, positive or negative, please leave a comment.  I’m interested to hear other experiences and use cases.