Introduction
This article will introduce the main concepts of the Digital Intermediate Frequency Interoperability (DIFI) standard including the new features introduced in version 1.3. With no assumptions of prior knowledge from the reader this should serve as an introduction to the standard but also for those familiar with DIFI raise awareness of the added features of link establishment in new standard version.
Why was the DIFI standard established?
The satellite industry has seen rapid acceleration in the recent years with more and more launch opportunities appearing and cost of payload to orbit decreasing. This paradigm shift is placing pressure in the race to innovation and increasing the satellite share in telecom, IT and GIS markets. The DIFI Consortium has identified a bottleneck in this whole endeavor that needed to be overcome, one specifically in the ground segment, possibly the most overlooked component in these ambitious new developments.
The ground segments have for a long time relied on coaxial cables to connect modems to antennas through analog hardware, such systems are difficult to scale for capacity and complex to operate. The situation is similar as in the telecom markets prior to developing Remote Radio Heads and standard like CPRI some 20 years ago and more recent moves to eCPRI and ORAN for 5G deployments. The satellite industry is now following suite with ideas to digitize and virtualize the infrastructure moving the IF converters as close to the antennas reducing the infrastructure complexities replacing long runs of power hungry coax cables with digital data streams.
The digital interfaces pose a new level of challenge as there are great number of ways to encode samples of the analog signal into a digital data steam. For a true transformation of the ground segment a standardized and interoperable way of encoding this data is required to allow equipment of different origins to work together and prevent vendor lock and allow for economies of scale. The DIFI consortium identified a need for an open and transparent standard adopted by equipment vendors, system operators and end users together. One which would allow for the ground segments to quickly accommodate to the changes in satellite payloads, orbits and constellations.
The DIFI Organizations Mission Statement summarizes it accurately:
“To enable the digital transformation of space, satellite, and related industries by providing a simple, open, interoperable Digital IF/RF standard that replaces the natural interoperability of analog IF signals and helps prevent vendor lock-in.” – DIFI Consortium
What are the key features of DIFI?
DIFI is based on a subset of VITA-49, which is the only wide adopted Digital IF standard by the satellite vertical. eCPRI is a similar framework standard but it was designed around 4G/5G telecom needs, and VITA-49.2 is better suited for satcom and is currently widely adopted in the satellite industry. DIFI draws on the VITA standard for some of its packet types, like signal data packets carrying the data payload as well as context/version data packets that describe the RF signal parameters and command/flow control packets for communicating control, timing and status information.
DIFI defines multiple Information Classes, in a way these are groupings of specific packet classes targeting various protocol capabilities. In its very basic form DIFI Information class 0x0000 defined in the standard version 1.0 consists of Standard Flow Signal Data and Signal Context without any command Packet support. This original Information class was defined for conveying signal data and the corresponding metadata, using Real Time for its fractional timestamp field. In future versions alternative versions of these where introduced with Sample Counters in the timestamp field opposed to real time used. In version 1.2 Command packets where also introduced allowing for flow control mechanisms to be utilized. In the latest version of 1.3 Link Establishment was added to all the existing Information classes further expanding the capabilities.
The newly introduced Link Establishment process allows for setting up streams between devices in a multi device deployment without the need for user intervention. The process involves negotiating the link parameters in band and enabling packet flow. There is a set of packet classes dedicated to exchanging information about device limitations and enabling this process. It involves querying for device capabilities, activating the link, monitoring context for errors and link teardown. To make this possible each device in a deployment is identified with a unique 32bit identifier UUID, since multiple virtual devices can reside under the same IP/MAC address. The Information classes that support link establishment have the additional functions of communicating DIFI sink capabilities to DIFI sources, characterizing link latency/jitter and providing “heartbeat” capabilities, as well as reporting errors and warnings.
Where can we find benefits in deploying DIFI?
The immediate candidate would be satcom gateways, with their constrained deployment architecture, with dozens of antennas spread over several acres, that requires planning for coax cables and central or nearby placement of signal processing. Converting the deployment to DIFI would allow the signal processing to be placed at a different location even in the cloud compute servers, the antennas to be deployed at greater distances or even over multiple separate locations with improved redundancy and resiliency.
Connecting ground stations to modems through terrestrial networks allows the flexibility for the ground station to service multiple downlinks from many satellites that would require different modems. Handling these in software with digitized signals enables the ground station as a service segment to grow rapidly to meet the new demand, something that was not possible prior with matrix switches and hardware modems. Having a global network of interconnected ground stations and modems allow for ground stations to instantly transfer traffic from an overloaded gateway to one with idle capacity, regardless of distance.
Eliminating coax cabling aboard aircraft and in ground stations eliminates multiple engineering challenges. For aircraft these would mainly be weight of the coax cables and power required to drive the signals, as well as shielding collision avoidance systems from L-band frequencies and the installation complexity and costs related to inflexible cable medium. For ground stations overcoming noise associated with the splitters, combiners, and switches in the coax cable path, switching to DIFI will greatly simplify link budgeting and ground station deployments.
Chip Interfaces offers a DIFI IP core, which is a highly scalable and silicon agnostic implementation of the IEEE-ISTO Std 4900-2021: Version 1.3 Digital IF Interoperability Standard targeting ASIC, and FPGA technologies.