Sunday, November 6, 2011
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Radio-frequency (RF) communication saw a progression of innovation throughout the 20th century. In recent years, it has been transformed profoundly by technological advances, both in the capabilities of individual radios and in the design of networks and other systems of radios. This discussion presents some highlights of recent advances;
Digital Signal Processing and Radio Implementation in CMOS
Modern communications technologies and systems, including those that are wireless, are mostly digital. However, all RF communications ultimately involve transmitting and receiving analog signals; Digital signal processing is increasingly used to detect the desired signal and reject other “interfering” signals. This shift has been enabled by several trends:
· Increasing use of complementary metal oxide semiconductor (CMOS) integrated circuits in place of discrete components;
· The application of dense, low-cost digital logic (spawned primarily by the computer and data networking revolutions) for signal processing;
· New algorithms for signal processing;
· Advances in practical implementation of signal processing for antenna arrays; and
· Novel RF filter methods.
The shift relies on an important tradeoff: although the RF performance of analog components on a CMOS chip is worse than that of discrete analog components, more sophisticated computation can compensate for these limitations. Moreover, the capabilities of radios built using CMOS can be expected to continue to improve.
The use of digital logic implies greater programmability. It is likely that radios with a high degree of flexibility in frequency, bandwidth, and modulation will become available, based on highly parallel architectures programmed with special languages and compilers. These software defined radios will use software and an underlying architecture that is quite different from conventional desktop and laptop computers, but they will nonetheless have the ability to be programmed to support new applications.
High degrees of flexibility do come at a cost—both financial and in terms of power consumption and heat dissipation. As a result, the wireless transceiver portion (as opposed to the application software that communicates using that transceiver) of low-cost consumer devices is unlikely to become highly programmable, at least in the near future. On the other hand, there are other applications, such as cellular base stations, where concurrent support of multiple standards and upgradability to new standards make transceiver programmability highly desirable.
Also, the decreasing cost of computation and memory opens up new possibilities for network and application design. The low cost of memory for example, makes practical store-and-forward voice instead of always on voice. This capability creates new opportunities for modest-latency rather than real-time communication and may be of increasing importance to applications such as public safety communications. Digital signal processing of the audio can also, for example, be used to enhance understandability in (acoustically) noisy environments.
Digital Modulation and Coding
Modulation is the process of encoding a digital information signal into the amplitude and/or phase of the transmitted signal. This encoding process defines the bandwidth of the transmitted signal and its robustness to channel impairments. The introduction of the more sophisticated digital modulation schemes in widespread use today—such as CDMA and OFDM, whereby different users using the same frequency band are differentiated using mathematical codes—have further transformed radio communications. Many important advances have also been made in channel coding, which reduces the average probability of a bit error by introducing redundancy in the transmitted bit stream, thus allowing the transmit power to be reduced or the data rate increased for a given signal bandwidth. Although some of the advances come from the ability to utilize ever improving digital processing capacity, others have come from innovative new coding schemes.
Low Cost and Modularity
The low cost and modularity (e.g., WiFi transceivers on a chip) that have resulted from the shift to largely digital radios built using CMOS technology make it cheaper and easier to include wireless capabilities in consumer electronic devices. As a result, developing and deploying novel, low-cost, specialized radios have become much easier, and many more people are capable of doing so. A likely consequence is continued growth in the number of wireless devices and in demand for wireless communications.
Dynamic Exploitation of All Degrees of Freedom
Another important shift in radios has been the ability to use new techniques to permit greater dynamic exploitation of all available degrees of freedom. Theoretic communications capacity is the product of the number of independent channels multiplied by the Shannon limit for a channel. In practice, the capacity (data rate) of an individual channel will be limited by the particular choice of modulation, coding scheme, and transmission power—for any particular profile of background channel noise.
Four independent degrees of freedom can be used to establish independent channels—frequency, time, space, and polarization. In the past, technology and the regulatory schemes that govern it have relied principally on a static separation by frequency and space. Advances in digital signal processing and control make it possible for radios to exploit the available degrees of freedom on a dynamic basis and to coordinate their own use of the various degrees of freedom available so as to coexist with one another and with uncoordinated spectrum occupants. Antenna arrays enable more sophisticated spatial separation through beam forming in all three dimensions. Today’s radio technologies can thus, in principle, take greater advantage of all the degrees of freedom (frequency, time, space, and polarization) to distinguish signals and to do so in a dynamic, fine-grained fashion. An important consequence is that a wider set of parameters (beyond the conventional separation in frequency and space) can be used to introduce new options for allocating usage rights (i.e., defining what a user can do and what the user must tolerate) based on all of these degrees of freedom.
Flexibility and Adaptability
The agility and the flexibility of radios are improving along with advances in the ability to more accurately measure communication channels (sensing), share channels (coordination), and adapt to the operational environment in real time (adaptation). The agility and the flexibility of radios are improving along with advances in the ability to more accurately measure communication channels (sensing), share channels (coordination), and adapt to the operational environment in real time (adaptation). More agile radios can change their operating frequency or modulation or coding scheme, can sense and respond to their environment, and can cooperate to make more dynamic, shared, and independently coordinated use of spectrum. Digital logic advances make it possible for radios to incorporate significant and growing computing power that enables them to coordinate their own use of the various degrees of freedom available so as to coexist with each other and with uncoordinated spectrum occupants. Since much of the processing is performed digitally, the performance improvements popularly associated with Moore’s law that characterize the computer industry are likely to apply to improvements in this type of processing. The result is that radios and systems of radios will be able to operate in an increasingly dynamic and autonomous manner.
Finally, increased flexibility poses both opportunities and challenges for regulators. Although it is much more complex, costly, and power consuming, flexibility makes possible building radios that can better coexist with existing radio systems. Coexistence is sometimes divided into underlay (low-power use intended to have a minimal impact on the primary user) and overlay (agile utilization by a secondary user of “holes” in time and space of use by the primary user). Such overlays and underlays might be introduced by rules requiring such changes or by rules that enable licensees to agree to such sharing in exchange for a market price. Moreover, flexibility allows building radios with operating parameters that can be modified to comply with future policy or rule changes or future service requirements. That is, devices are able to instantiate and operate on specified policies, and the policies (and the devices’ operation) can be modified. Besides providing regulators and system operators with a valuable new tool, this malleability poses new challenges, such as how to assure a radio’s security in the face of potential (possibly malicious) attempts to modify its software. Possible scenarios include rogue software silently placing calls constantly (thus congesting the control channel) or altering the parameters of a cell phone’s transmitter so as to jam transmissions of cellular or other services. Information system security experience from other applications suggests that it will be possible, with significant effort, to provide reasonable security (i.e., against casual efforts to break it) but that it would be quite difficult using today’s state of the art to provide highly robust security against a determined attacker.
Work has been done for many years on antennas that can operate over very wide frequency ranges. In the past decade, an interesting new approach to improved wireless communication began to develop, based on using multiple antennas at both transmitter and receiver. Advances in analog and digital processing have made it possible to individually adjust the amplitude and phase of the signal on each member of an array of antennas. When the approach is used to increase data rates, it is called multiple-input, multiple-output (MIMO), and when it is used to extend range, it is called beam forming.
The most basic form of MIMO is spatial multiplexing, in which a high data- rate signal is split into lower-rate streams and each is broadcast concurrently from a different antenna. (More generally, multiple antennas can be used to obtain the desired degree of enhancement in both data rate and range.) These schemes require significant “baseband” (i.e., digital) processing before transmission and after reception, but are able to provide increased range or data rates without using additional bandwidth or power. They provide link diversity, which improves reliability, and they enable more efficient use of spectrum. This approach is used in a number of commercially deployed technologies including 802.11n (a wireless LAN standard), WiMax (a last-mile wireless local-access technology), and long-term-evolution (LTE; a technology for fourth-generation mobile telephony).
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This post was written by: Alex Wanda