Tutorial: Software-Defined Radio Technology Mark Cummings enVia Technology Partners, Inc.
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Todor Cooklev San Francisco State Univ.
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Abstract software. However, a fundamental barrier was found. Due to modulation and encoding evolution, the complexity of the air interfaces of wireless systems was also increasing. This increase results from the increased baseband bandwidth, and increased spectrum efficiency (increasing the number of bits per Hz), of modern wireless systems. Discrete digital logic components could not be replaced by a DSP with software, but they could be combined into a single chip. This combination of discrete digital logic into a single chip came to be called an Application Specific Integrated Circuit (ASIC). It achieved the cost, size and power consumption advantages inherent in integrated circuits, but it didn’t have the flexibility inherent in software driven general purpose processors. We call radios built with these components “hardware Radios”. ASIC/DSP combo radios were the industry standard for high performance applications such as cellular by the late 1980’s and early 1990’s. A variety of military and commercial markets for wireless communications systems have been experiencing rapid growth since the beginning of 1990-ties. In addition to the cellular market, in recent years other markets experiencing fast growth include Wireless Local Area Networks (WLAN’s), Wireless Personal Area Networks (WPAN’s) and Metropolitan Area Networks (WMAN’s). Demand for universal wireless communications began to appear. At the same time rapid evolution of technology for implementing modulation/demodulation and encoding/decoding has resulted in a proliferation of air interface standards for each of these technologies. The number of air interface standards is large and growing. None of them could by itself provide the ‘anywhere, anytime’ service desired. The situation is very similar to that the computing industry faced in the mid-1970s, where each application required a specially built dedicated ‘intelligent terminal’, which resulted in limited usability, deployment and management problems. The subsequent advent of the PC, with its standard hardware platform, which could be suited to a wide variety of tasks by application of the appropriate software, dramatically changed the computer industry. By the early 1990s, the wireless communication
Software Defined Radio (SDR) is one of the most important emerging disruptive technologies that will shape the future of the wireless communication and mobile computing industries. It is the result of a long process of technology evolution. Since wireless communications systems first began to make significant appearances in the 1890’s, evolution has progressed along two axes: modulation and encoding schemes, and technology for implementing modulation/demodulation and encoding/decoding. The superheterodyne architecture was invented in 1915. It was developed to overcome the noise problems inherent in the Direct Conversion or Homodyne Architecture (sometimes called Zero IF) developed in the 1890’s. By the end of the Second World War, wireless communication systems had evolved to the point where they could be broken down into Human Interface (HI), Local Control (LC), Protocol Stack (PS), Low Speed Signal Processing (LS SP), High Speed Signal Processing (HS SP), RF front-end (FE), and Antenna. In these early systems, each function was implemented with discrete analog technology. This resulted in relatively large, expensive, high power consuming systems which were difficult to design, manufacture, and manage/maintain in the field. The desire to lower cost size and power consumption while making devices easier to manage in the field has driven the technology evolution path we are still on today. As digital technology arrived and entered the beginning of its period of rapid evolution, a pattern developed. First a function previously performed in discrete analog circuitry was implemented with discrete digital components, which subsequently was implemented on a single integrated circuit. In order to do this, Analog to Digital Converters (ADC) and Digital to Analog Converters (DAC) were required. The expectation was that the same process described above would continue and that the High Speed Signal Processing would be implemented by some kind of a microprocessor and software. The dream was that this process would continue to the point where an Rx subsystem would consist of an A/D converter at the antenna and everything else would be done in
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industry began to search for an analogous standard hardware and software platform. For a variety of reasons this process still continues. In the early 1990s, some solutions to the high speed signal-processing requirements that offered software driven flexibility and the ability to change AIS baseband subsystems to support different AISs through software began to appear. These initial solutions can be characterized as being based on reconfigurable logic. With the advent of these wireless systems based on reconfigurable logic, the term Software Defined Radio (SDR) was coined. This evolutionary process is still underway. Having a reconfigurable Base Band processor is necessary for SDR and provides a lot of benefit, but it is not sufficient for a fully realized SDR implementation. In front of the Base Band processor is the Transceiver, the amplifiers, filters, switches and antennas. Early SDR’s used multiple chains of discrete analog components; one for each different AIS anticipated. Transceiver and filter architectures are most closely tied to the use of old large discrete analog components. Evolution to bring these subsystems into small, inexpensive, power efficient packages has proceeded along lines similar to that of Base Band semiconductor evolution. Antenna technology is proceeding along a different evolutionary line. It is not so concerned with semiconductor fabrication processes. Rather, as the size of the other subsystems shrink and the range of frequencies and modulation schemes a single system must support grows, the fundamental requirements antennas face are changing. The “ideal” software radio consists of a wideband antenna, wideband ADC and DAC, and a programmable processor. The key difference between digital radios and software radios is the ability to upgrade the hardware or software of the radio device. The software upgrade can be performed directly over the wireless medium. Software Defined Radio alters traditional hardware radio designs and approaches the ideal in three distinct and complementary ways: it (1) Moves analog/digital (A/D) conversion as close to the receiving antenna as possible; (2) Substitutes software for hardware processing; and, (3) Facilitates a transition from dedicated to general-purpose hardware. Each of these changes has important implications. Very high sampling rates and a high number of bits per second are required at the same time from the ADC. Second, it is difficult to design linear amplifiers that can amplify the wideband signal at the antenna without distortion. Third, substituting software for hardware increases flexibility. That is, replacing software – especially if this can be done remotely – is faster (reducing Time to Market) and lower-cost than replacing hardware. Fourth, SDR facilitates the
transition from specialized to general-purpose hardware, although performance generally declines in the shift from dedicated to general-purpose hardware. From a different perspective, in a more general sense the term SDR includes all the enabling technologies that are required to realize the ideal architecture. These technologies include hardware technologies such as wideband antennas, smart antennas, wideband ADCs, powerful DSPs, ASICs, FPGAs, hardware components such as MEMS, etc. Other technology enablers are advanced algorithms for digital signal processing. Software technologies such as middleware are also important technology enablers. Physical layer and medium-access protocols that are tailored for software radio systems are yet another technology enabler. The full value of software-defined radios will be achieved when they can fully meet the goals and objectives of all vendors in the wireless value chain. Note that it is important that this be done in a way that is not perceived as being biased by any of the various players. In order to achieve this, it is necessary to have a language that provides an interface to each of these groups on one side and to the radio systems (hardware and software) on the other side. This language, called a Metalanguage, contains information about hardware / software functionality / configuration, Air Interface Standards, the information being exchanged, and end users. At the same time, there is a lot of attention being focused on what is referred to as “cognitive radio.” The basic idea is to make radio receivers and transmitters more intelligent (incorporating Artificial Intelligence software) and adaptive so that they can respond to changes in their local environment. These may include adapting to changing interference or congestion conditions, or adapting to facilitate interoperability among diverse devices, or adapting to accommodate the requirements of changing applications (e.g., from wireless email to video to voice). Cognitive radios would be self-configuring. The increased adaptability to local conditions would greatly expand the range of services that could be offered and the range of congestion management (i.e., to address spectrum scarcity concerns) strategies that might be employed. We also briefly discuss the standardization process, the relevant government regulations, and the various products and services that have emerged.
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