SDRRCS: Software Defined Radio for Regenerative Communications Satellites
The primary objective targeted the development of a software defined radio prototype platform applied to regenerative satellite communications. Such a platform must be capable of upgrading and reconfiguring itself to adapt to current and emergent signalling standards.
Additional goals include:
The SDRRCS prototype is designed to offer a reconfigurable and upgradeable wireless communication system. The goal is to perform as much signal processing as is possible in reprogrammable devices such as FPGAs or GPPs. Figure 1 shows the general layout of the receiver and which hardware components perform what functionality.
Figure 1. Signal Processing Blocks Required at the Receiver, as well as Types of Devices Which Perform the Processing
In developing the architecture, the goal was to develop a standardized OnBoard Regenerative Processor (OBRP) for the deployment of the DVB-S and DVB-S2 waveforms using COTS elements. It was also deemed desirable to perform as much of the signal processing as possible on general purpose processors, while still retaining the use of the Application Specific Integrated Circuits (ASICs) where bandwidth requirements dictated. The prototype is designed using Commercial Off The Shelf (COTS) components. Essentially four COTS components are used in its construction:
COTS components enable small-to-medium (SME) companies to rapidly prototype generic SDR hardware platforms as well as perform the system integration. Designs become modular and can easily be upgraded. For example, as individual components are improved upon such as faster GPPs, then these components can easily replace the existing processors. The net result is a system that now offers greater functionality than before.
The project plan is to develop an integrated hardware and software SDR solution that can be used as a regenerative transponder for satellite communications. The prototype transponder will show compatibility for the SCA-CF, which is a set of operating standards dictating how SDR should internally function. The prototype will be constructed using COTS components, both hardware and software that are (or can be made to be) SCA compliant. This will then guarantee that they will function properly when incorporated into the prototypes' architecture.
Since the SDR paradigm (including the use of COTS for product development) is still new and unproven. There are many issues and concerns that must be overcome in order for it to achieve wide scale acceptance and success.
For the SDRRCS development project the two most important issues are:
An SDR, SCA complaint radio was built using COTS equipment. In addition SCA complaint versions of the DVB-S and DVB-S2 waveforms were also built. The radio successfully demonstrated the required traits of upgradeability and reconfigurability by uploading, and correctly deploying variants of the DVB-S and DVB-S2 waveforms.
However, when the waveforms were started and their throughput evaluated, the performance was below estimated values (approximately half target values).
Initial targets were for 1Mbps for the DVB-S2 waveform. It was apriori estimated that the bottlenecks in performance would be the matched filtering for estimating the phase error and the iterative process of decoding required by the LDPC codes. Benchmarking of the two individual processes showed that both were capable of exceeding a target rate of 1 Mbps; however, the PC (represented under the SCA as a single node with one executable device - the GPP) was overwhelmed when simultaneously running all signal processing components of the waveform.
Analysis revealed the aggregate real-time performance of the radio platform was based on four components:
The most significant limiting factor was the performance of the PC host platform itself. The PC was responsible for phase and all baseband waveform processing. Additionally, the PC must provide resources for the radio management (the SCA, the Linux OS, and control of the Pentek card). These results indicate that although COTS components and in particular PC processors have rapidly developed, dedicated signal processing is still best left to FGPAs and DSPs.
The main benefit offered by SDR technology (and is validated by this project) is that the prototype, constructed using COTS components was made reconfigurable and upgradeable to different waveforms. These two abilities allow SDR devices to ?future-proof? themselves against changing standards, something conventional satellite transponders systems cannot do.
Current Status (dated: 04 Apr 2007)
Perspective on Future SDR Development
Developing SDR technology is clearly an iterative ongoing process, but initial steps such as this development of a prototype OBRP system is a necessary first step. It should be emphasized that SDR is more of a concept than a physical entity such as a piece of software or hardware. The benefit of SDR is the gestalt benefit that is realized from the confluence of these components. There is currently no universally accepted definition of precisely what is an SDR device; only that after a device possesses enough SDR traits (upgradeability, reconfigurability, etc.) may it be considered as an SDR device.
The individual components comprising SDR devices are rapidly maturing - if only because they had so far to go. These are the FPGAs, the software development tools, the reference architectures for the SCA, etc. Nevertheless, complete SDR devices are still in the technology development / demonstration stage (January 2007).
SDR must address three primary areas of risk which focus around the SDR core technology itself and its relation to the current wireless value chain.