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.
Antennas
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).
