In this Tech Topic, we return to consider some of the basic precepts of public safety communications systems while at the same time advocating the potential strengths of software defined radio (SDR) in meeting the demands of public safety communications.
The Bureau recently established a web page elucidating the core concepts in public safety communications.1 These concepts include operability, interoperability, reliability, resiliency, redundancy, scalability, security, and efficiency. This Topic considers the application of software defined radio characteristics within the framework of the core concepts.
- Operability. First and foremost for any communications system, the system must provide the requisite services for which it was designed and installed. So unquestionably, the keynote of any system is its ability to support the needs of the customer; that is to say, the system must provide the requisite services across the spectrum (pun intended) of requirements for public safety. For the public safety community, this means assured communications so the system will always work and provide the capability for which it was designed. Although this is an inherent characteristic of any communications system, certainly it will hold true for software defined systems as well.
- Interoperability. Much has been spoken and written about the failure of communications systems to operate with other communications systems, particularly when the two systems belong to public safety entities. This basic concept is reflected in the ability of disparate systems to operate with each other in a seamless fashion. It is also a particular strength of software defined radios in that dynamic reconfigurability of operating characteristics is a fundamental basis for SDR. It should be noted that the ability of SDRs to reconfigure to alternative radio characteristics spans not only the transmission characteristics at the physical level, but also the information handling characteristics at higher levels including the application level and the human interface mechanisms. Perhaps more than any other characteristic of SDR, reconfigurability via software exchange is a primary strength.
- Reliability. Reliability is a reflection of the confidence that one may have in the assuredness of communications and the relative dependability of the services provided by the system. The IEEE also refers to reliability as, "The ability of a system or component to perform its required functions under stated conditions for a specified period of time."2 In terms of software derived systems, IEEE 982.1-1988 defines Software Reliability Management as "The process of optimizing the reliability of software through a program that emphasizes software error prevention, fault detection and removal, and the use of measurements to maximize reliability in light of project constraints such as resources, schedule and performance."3 Using these definitions, software reliability is comprised of three activities: error prevention, fault detection and removal, and measurements to maximize reliability, specifically measures that support the first two activities. Hence, reliability for SDRs is defined in terms of the assuredness of the software to perform the functions specified for the radio functionality.
- Resiliency. The ability of a communications system to respond to change and/or to recover from mishap is crucial in establishing the resilience of the system. It refers to the system's ability to evolve with technology advancements and changes to operational requirements. Fortunately, this is also a real strength for SDRs. Because all of the functionality of an SDR depends on inherent software, it is exceptionally easy to change, modify, and test the functionality of a radio based on new software as opposed to the development of new hardware. Additionally, it is also easy for SDRs to recover from faults and to change to new demands on the system through simple software modifications.
- Redundancy. This characteristic refers to the functionality of a communications system that is achieved by alternative means. It is also a strength of SDRs since all of the functionality of a radio within a system is exactly the same due to the duplicative nature of software without consideration of any variances in hardware.
- Scalability. Scalability refers to the system's ability to adapt and grow with expanded requirements and users. It is a measure of the flexibility of the system to transparently adjust to changes in operational scope of the system. In this case, SDRs are again very well suited to the task. Since SDRs rely on software for their defining characteristics, it is generally very easy for the systems to morph in the functionality of the system based on changes to the operating software. Hence, version 1.1 of an operating system can recover from errors in version 1.0 as well as expand on the previous functionality. Scalability in this case, is software dependent and easily accomplished as the need may be.
- Security. Security refers to the end-to-end integrity of the transmission process to insure error free and uncompromised exchange of information. It means that verified and authorized transmissions take place and users can be assured of the safe transfer of guarded information. Without question, SDRs are as capable of secure communications as any pure hardware implementation. While additional diligence in control of the inherent software is necessary, nevertheless, secure communications are easily accomplished; in fact, most secure devices rely on software-driven algorithms to provide security measures.
- Efficiency. Efficiency refers to the ability of the communications system to use resources. The characteristic spans the interests of equipment design, operating characteristics such as spectrum and power use, as well as development costs. Again, SDRs exhibit a strong adherence to this concept. While they minimize the costs of hardware development (recall the functionality is essentially in the software), they are also very flexible in development to be able to maximize the results compared to inputs.
In addressing these demanding core concepts for public safety, future networks would benefit from being dynamically adaptable and close to real-time reconfigurable in order to insure interoperability. The implementation and operational costs of public safety radios and networks are high and improvements can be made in interoperability among segmented public safety communities. The US military is facing similar challenges and they have confronted the problems with dynamic new communications technologies that employ the advanced and developing technologies embodied in SDR and Cognitive Radio (CR). We introduce cognitive radio here and will expand our discussion in the next Tech Topic.
In general, SDRs provide software control of the radio frequency operating parameters such as choice of modulation, transmit frequency, bandwidth, and transmit power level. An SDR system can include multiple layers in the OSI hierarchy, but it primarily involves the physical link and often part of the network layer. A CR provides different functionalities such as sensing and monitoring, tracking changes in the RF environment, and autonomously adapting its access to the communications channel. Although the terms SDR and CR are defined slightly differently by different industry sectors, SDR implies implementation aspects using software control, while CR includes extra functionalities other than the basic features of a radio system. Often these two technologies are quoted interchangeably and intermixed in usage with some qualifications in certain cases. For example, a cognitive radio capability is an obvious application to implement a software controllable feature in an SDR.
Supporting adaptive functionalities and reconfigurability in a radio system is quite challenging. SDR and CR are complex paradigms that involve underlying technologies implemented in various types of hardware and software platforms as suggested in our previous Topics. These underlying technologies involve generic hardware implementations of general purpose processors, RF components, digital signal processing (DSP) chips, A/D converters, and smart antennas – among other things – to provide multiple functionalities such as reconfigurability, interoperability, flexibility, upgradeability and cognition capability. SDR also involves new implementations of system and device architectures assembled from RF components, hardware, software and networks. All of these diverse technologies need to be harmonized to implement SDR systems. Because of the magnitude of the diverse and varied technologies involved, progress on their implementation in a practical system is following a path that increases in complexity and functionality for each phase of development. In order to give direction and standardization to the SDR development process, a new IEEE Standards Coordinating Committee (Dynamic Spectrum Access Networks) (IEEE SCC41), was established along with IEEE 1900, a standard for the development of next generation radios4. These standards will provide a technology roadmap for SDR development.
The evolution of multi-band, multi-mode and multi-protocol radios has already contributed to the adoption of SDR technologies in various industry sectors. For example, some functions that are used in SDR/CR technologies now, such as dynamic frequency selection (DFS) and transmit power control (TPC) have already been adopted by FCC rules and implemented in various systems and equipments.
As the precepts mentioned above suggest, the ultimate goal of a public safety network is to provide assured, secure, and seamless communications that are accessible anytime and anywhere with maximum interoperability and adaptability. SDR-based systems are an obvious choice to overcome these challenges in radio design. In the next Tech Topic, the key advances in cognitive radio technologies will be considered along with the potential impact that these technologies may have on the public safety communications environment.
4 "The IEEE P1900 Standards Committee was established in the first quarter 2005 jointly by the IEEE Communications Society (ComSoc) and the IEEE Electromagnetic Compatibility (EMC) Society. The objective of this effort is to develop supporting standards dealing with new technologies and techniques being developed for next generation radio and advanced spectrum management. On March 22, 2007 the IEEE Standards Board approved the reorganization of the IEEE 1900 effort as Standards Coordinating Committee 41 (SCC41), Dynamic Spectrum Access Networks (DySPAN). The IEEE Communications Society and EMC Society are sponsoring societies for this effort, as they were for the IEEE 1900 effort." See http://www.scc41.org/ .