Over the past several years, numerous specifications have been developed defining the interface between the RF front-end and the baseband processing sub-systems in advanced wireless products. These interfaces include the Reference Point 3 specification developed under the open base station architecture initiative (OBSAI), the DigRF interface specification under development by the mobile industry processor interface (MIPI) alliance, the VITA radio transport draft standard developed by the VITA standards organization as VITA 49, and the common public radio interface (CPRI). The reason these specifications were developed is simple: standardization of these interfaces allows an ecosystem of vendors to form interoperable RF and baseband processing modules. As more and more vendors enter the market with these standards-based modular products, the modules will become commoditized, allowing wireless original equipment manufacturers (OEMs) to choose equivalent modules from a range of competing vendors, reducing their overall development and production costs.

The ongoing proliferation of software-defined radio (SDR) technologies in advanced wireless systems, allowing some or all of the radio's physical layer (PHY) processing to be defined in software and HDL code, has created a need for a further level of standardization in the interface between these two subsystems — standardization of an application-programming interface (API) that would provide a layer of hardware abstraction from the underlying RF front-end and baseband processing connections, including support for both signal transport and control. Toward that end, the SDR Forum has been working with other organizations to define a model for a transceiver API that, when standardized, will provide significant benefit to the wireless community by facilitating the insertion of new or updated physical-layer waveform applications and air interface standards while the radio is in operation and, at the same time, improving the portability of these applications and standards from radio to radio. This article introduces the work of the SDR Forum Transceiver Subsystem Interface Task Force in defining this “Transceiver Facility,” discussing a process they are following in developing the API, presenting the features of the API in detail, and discussing the next steps toward final standardization.

The term “transceiver” is used here to encapsulate the entire set of hardware and software components within a radio set necessary to convert a low-power RF signal to digital baseband on the receive side, and reciprocally to convert digital baseband signals to low-power RF on the transmit side. Any management and control features necessary to support the transceiver usage are also considered as part of the transceiver sub-system (Figure 1). The transceiver subsystem provides intermediary processing between the radio's baseband or modem-processing subsystem, and the other subsystems contained with the “RF front-end” such as the power amplifier, the co-site mitigation, and the antennas.

While numerous specifications and standards have been developed over the past several years defining the lower level interfaces between transceiver subsystem and the rest of the radio set, there has been a recognized need within the reconfigurable radio community to pursue the definition of an API to support this interface at the system level. The reason for this interface is that this software-level frontier should match the industrial frontier already identified as one key reference point between baseband and transceiver stages. Setting the frontier “closer to the RF” presents a number of drawbacks, including introducing dependencies on implementation-dependant choices in the radio set design, which lowers the portability of the developed software. Setting the frontier at some higher level also presents drawbacks in that it prevents significant parts of the waveform application software from taking advantage of the platform abstraction.

The initial work on the transceiver API was done within the scope of the French Department of Defense (DoD) “Software Radio Study,” (ended 2006) where a first level of definition was issued in order to define a common transceiver abstraction. This study used two transceiver platforms that were manufactured following two entirely different hardware designs by THALES and Raytheon. However both platforms supported the same transceiver API, and this level of transceiver abstraction was flexible enough to support the needs of two different waveforms: FM3TR and STANAG 4285. When implemented on the two platforms, these waveforms were demonstrated to be interoperable, across themselves and with legacy radios.

The work on the transceiver API was then progressed within the European End-to-End Reconfigurability Phase II (E²R II) Project.^[1,2] Under the auspices of the European Union “6th” Framework Program, this program had as a goal to define standardized system architectures that “provide common platforms and associated execution environments for multiple air interfaces, protocols and applications.” The E²R contribution has been to leverage the previous program results in order to meet the specific industrial requirements of the transceiver abstraction endeavour, in satisfying the needs of multiple waveforms and highly different possible platforms. The key breakthrough to square of the circle has been the introduction of the transceiver facility concept, which yielded to the creation of the task force at SDR Forum level.

This work is now being refined in the SDR Forum under the Transceiver Subsystem Interface Task Force. The objective of this group is to mature the specification to ensure that it supports the needs of a broad base of radio architectures in the commercial, civil and defense communications domains. A key objective of the group is to evaluate the proposed transceiver facility against the various lower-level transceiver interface specifications and standards that have been developed for the commercial and defense wireless markets over the past several years to ensure that they can be supported by the resulting API. This evaluation is expected to include the Reference Point 3 specification developed under the OBSAI, the DigRF Interface specification under development by the MIPI Alliance, the VITA Radio Transport draft standard developed by the VITA Standards Organization as VITA 49, and the CPRI. Several additional developments are also envisioned, including providing extensions to better support spectrum-aware cognitive radio or smart radio systems.

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