The single biggest challenge in radio technology is radio frequency (RF) spectrum utilization. Only a finite amount of RF spectrum is available – it is highly regulated, a lot of it is not usable, and what is available may be full of all kinds of users. Widespread use of wireless connectivity for any intelligent device is driving up demand even more for that limited bandwidth, making the RF spectrum even more crowded. An SDR makes it somewhat easier to maximize available bandwidth, making it possible to select appropriate spectrum based on the mission and environment.
SDR is a radio communication system where components that have been typically implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a computer or embedded system. While the concept of SDR is not new, the rapidly evolving capabilities of digital electronics render practical many processes that used to be only theoretically possible.
SDRs have gained traction in many military and commercial radio applications, especially in applications where many services must use common communication channels or changing mission needs require the communication network to be dynamically reconfigured.
VITA 49 overview
VITA members recognized the limitations of existing SDRs as they related to interoperability between radios and sensors. Several years ago the VITA 49 VITA Radio Transport (VRT) working group was formed to define a framework for SDRs, making them more open and interoperable. Out of that working group emerged two specifications.
ANSI/VITA 49.0, the VRT protocol standard defines a transport-layer protocol or framework designed to promote interoperability between RF receivers and signal processing equipment in a wide range of applications. The VRT framework provides a variety of formatting options that allow the transport layer to be optimized for each application. The framework defines interoperable sensor architectures that are link independent.
Target applications for VITA 49 enabled SDRs include:
- Spectral Monitoring and Scanning
- SIGINT and Tactical Information
- Communications and COMINT
- Radar and EW Countermeasures
The protocol is made up of three components. VRT IF (intermediate frequency) data packets capture payload data, time stamps, channel, and signal ID. The data formats are very flexible to accommodate a wide range of equipment and they support extremely precise time stamping needed for maximum accuracy.
VRT context packets report all operational parameter values of the radio equipment. A standardized methodology allows the protocol to support a wide range of standard and unique parameters implemented in RF receivers.
The third component is the VRT information stream. It contains the first two components of IF data packets and context packets. This enables a RF receiver to associate data and context streams appropriately for different applications.
VITA 49.0 does not support control of hardware or radio transmit operations (see Figure 1). ANSI/VITA 49.1, the VITA Radio Link Layer (VRL) standard, specifies an optional encapsulation protocol for VITA-49.0 VRT packets. There are two motivations behind this encapsulation. The first motivation is to provide link-layer functionality, such as framing and error checking, when it is needed.
The prime example of the need for the VRL protocol is that of a digital recording device that receives a raw bit stream, writes it to disk, and then plays it back later. When playing back the recordings of VRT packets, it is desirable to be able to find packet boundaries from any arbitrary starting point in the recording. Since the VRT protocol alone does not support framing capability, VRL – or some equivalent – is needed.
The second motivation for the VRL standard is the desire to be able to recognize VRT packets when they are carried over a link layer that does not specify the protocol of the payload. For example, a link may carry a mixture of packets, some VRT, and some of other types. The VRT packets might be routed at the receive end to a processing engine by some hardware that routes received packets based on payload type. This application would require unambiguous identification of VRT payloads. Some link protocols provide no method to indicate that a VRT packet is contained. Thus some unique identifier is needed for this purpose. In such cases, the VRL protocol serves to identify the VRT protocol as the payload.
Future enhancements
Originally, the VITA 49.0 VRT protocol was defined for interoperability between radio receivers and signal processing equipment but now it is seeing implementations for digitized signal sample streams for software radio systems with a need to support interoperability between signal processing equipment and radio transmitters. Further efforts have recently begun to take VITA 49 to the next level to support additional use cases.
The VITA 49.2 working group has started working on transmit and control extensions to meet the expanded objectives of VRT (see Figure 2). The original work remains valid but new protocols for complete receive and transmit systems, plus control systems are being proposed and developed.
Functional objectives for VITA 49 are now expanding:
- Precision time stamping for beamforming, antenna array processing
- Synchronization across channels and sites
- Stream tagging for identification, content, format, and operational parameters
- Monitor status of receiver and transmitter equipment
- Control operation of receiver and transmitter equipment
Top of the priority list are a device control packet and spectrum survey control packet. The device control packet would send operational control parameters to radio equipment with acknowledgement. These additions make it possible for active operation of the radio to be interoperable. The proposed packet fields would include:
- Controller/Controlee Unique Identifier
- Message ID
- Control/Ack Indicator
- Control Indicator
- Control Fields
The spectrum survey control packets provide various control fields; center frequency, span, sweep rate, start/stop frequency, and resolution bandwidth critical to controlling a spectrum analyzer. The working group is currently evaluating two different proposals for this packet type.
According to Robert Normoyle at the Johns Hopkins Applied Physics Laboratory (APL), the Office of Naval Research (ONR) initiated a Joint Open Architecture Spectrum Infrastructure (JOASI) effort. Lead by APL, the team consists of 11 prominent organizations that are industry experts on communication, electronic warfare (EW) systems, and RF spectrum standards. This group developed an EW Ontology comprised of an integrated set of spectrum standards that will enable efficient, cooperative, and non-interfering use of the electromagnetic spectrum (EMS) domain. JOASI’s ontology conveys system commands, status, capabilities, observations, and policies that can be used during battle operations.
This effort is introducing additional transmit and control extensions that are under consideration:
- Capabilities Packets that announce configurable assets of each device and parameter ranges
- Transmitter Context Packets that deliver operational status and parameters of transmitters
- Spectrum Packets that deliver limited spectral data for monitoring and scanning
Conclusion
VITA 49 extensions enhance the total SDR system architecture (see Figure 3). The first wave of enhancements help to eliminate stove-pipe architectures that emerge when there are no suitable standards and enhanced interoperability between components when multiple invested parties are involved in the development of a standard specification.
Additional technological enhancements will address standards for multi-channel phase coherent architectures and transport protocols for multi-function SDR architectures.
If you are interested in learning more or would like to participate in the VITA 49 working group, contact VITA for more information: www.vita.com.
References
1. “Software-Defined Radio Handbook,” Rodger Hosking, Pentek, www.pentek.com
2. “Introduction to VRT and Data Packet Structure,” Matt Lamanque, ThinkRF, www.thinkrf.com/blog3/
3. “VPX Optical Interfaces: Standards, Protocols, & Applications,” Embedded Tech Trends presentation by Rodger Hosking, Pentek, www.embeddedtechtrends.com
4. “VITA 49: VITA Radio Transport (VRT), A Spectrum Language for Software Defined Radios,” VITA Standard Organization presentation by Robert Normoyle, Johns Hopkins Applied Physics Laboratory (APL)