Adaptive Coding and Modulation Now Available for TDMA Return Channels

New JUPITER System Feature Offers Greater Efficiency and Simplicity

Most people in the satellite industry agree that the incorporation of adaptive coding and modulation (ACM) into the DVB-S standards was instrumental in driving efficiencies for the forward channel. With ACM, the combination of modulation and coding (or “MODCOD”) is optimized by individual remote terminal, resulting in significant bandwidth savings. Today, in fact, all major VSAT (very small aperture terminal) systems support ACM for the forward channel.

At Hughes, we’re taking the benefits of ACM even farther with ACM for the TDMA (time division multiple access) return channels.

With the latest JUPITER™ System Release 7.4, Hughes enables ACM for TDMA return channels. Hughes remote terminals, including the HT2010, HT2300, HT2500 and HT2600, which utilize linear radios and support QPSK, 8PSK and 16APSK modulation, can dynamically change transmitted MODCOD based on real-time feedback from the JUPITER gateway station demodulators. This feedback tells the remote terminal the optimal MODCOD combination, providing maximum bits per Hz (efficiency) while maintaining link availability. To support this, JUPITER System gateway demodulators can receive and process different MODCOD bursts within the same TDMA frame on a given channel.

There are multiple benefits associated with return channel ACM. This first is the increased efficiency that results from not having to set aside low efficiency MODCOD inroutes to support rain fade. We see efficiency gains up to 30% — with the largest gains for smaller networks.

In addition to greatly enhancing the user experience and simplifying network operations, there are other advantages of using ACM on the return channel, including:

• ACM provides up to 11 dB of rain fade margin (16APSK R9/10 to QPSK R1/2) on a given channel.
• When channel symbol rates are appropriately provisioned, there is minimal need for a VSAT to change channels to mitigate fade conditions, eliminating the inefficiencies of requesting bandwidth and awaiting assignment to lower MODCOD channels.
• With ACM, the operator does not need to provision separate QPSK, 8PSK, and 16APSK channels – simplifying the network tremendously.
• If the network is designed properly, there should be minimal need for channels to be provisioned exclusively for rain fade.
• Depending upon network type (e.g. Consumer Network), the need to actively “down-speed” channels (for example from a 4 Msps channel to 2 x 2 Msps channels) is greatly reduced during significant rain fade conditions.  This reduces operational disruption to the network.

Many of our competitors talk about “adaptive inroutes.” Don’t be fooled; they are not referencing anything like the ACM being implemented by Hughes. One competitor’s interpretation of “adaptive inroute” is to change the MODCOD of the return channel. That might be great for the one remote experiencing fade, however every other remote on the inroute will perform less efficiently.

Another interpretation of adaptive inroutes is to have remotes assigned to different inroutes to support the right MODCOD. Unfortunately, that means the operator must create and maintain multiple return channels to accommodate rain fades – which wastes bandwidth.

Supporting true ACM for the return TDMA channels gives JUPITER System operators the ability to achieve the greatest possible efficiency while also simplifying their network operations.