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Inventing the Internet Again
the near future, one wideband radio will suffice. Digital signal processors
ultimately costing a few dollars apiece and draining milliwatts of power
will sort out all the channels, codes, modulation schemes, multipath signals,
and filtering needs. Gone will be the large buildings, the racks of radios,
the arrays of antennas, the specialized hardware processors. Gone will
be the virtual honeycombs towering in the air in time and space with exclusive
spectrum assignments and time slots, and possibly gone will even be the
battalions of lawyers in the communications bar.
All this apparatus can be replaced by a programmable silicon base station
in a briefcase, installed on any lamppost, elevator shaft, office closet,
shopping mall ceiling, rooftop, or even a house. The result, estimated
Don Cox of Stanford, the father of American PCS at Bellcore, could be
a reduction of the capital costs of a wireless customer from an average
of some $5,555 in 1994 to perhaps $14 after the turn of the century. That
is a paradigm cliff of costs.
As smart radios are delivered in the first years of the new century, they
will allow escape from the zoo of conflicting protocols. Base stations
will be programmable in software, able to handle any popular protocols,
including the new technologies that will be emerging. The world of wireless
will escape the bondage of air standards, where if you live in a GSM (global
services mobile) area, you are forced to use GSM, and if you live in a
CDMA (code division multiple access) area, your communications-poor cousins
visiting from Europe will have to give up their GSM phone and demand to
borrow yours (will they ever give it back?). Under the new regime, different
standards mean different software loaded into RAM (random access memory)
in real time. Any cell can accommodate a variety of access standards,
channel assignments, and modulation schemes, and the best ones will win.
From Microwaves Comes Torrential Bits
To get there from here, however, will require heroic achievements in the
technology of radios. Every radio must combine four key components: an
antenna, a tuner, a mixer, and a modem. Easiest is the antenna. Even though
antennas too are converging with computer technology and becoming smart,
for many purposes a shirt hanger will do the trick. It is the other components
that deliver the message to the human ear.
Tuners usually employ the science of resonant circuits to select a specific
carrier frequency or frequency band. The cellular band, for example, comprises
25 megahertz at around 850 megahertz. The PCS band comprises some 30 megahertz
at around 1,950 megahertz. A mixer converts these relatively high microwave
frequencies into an intermediate frequency (IF) or to a baseband frequency,
which can be converted to a digital bitstream.
Familiar in the PC world, a modem is a modulator- demodulator. In transmitting,
it applies an informative wiggle (AM or FM, say) to the carrier frequency.
In receiving, it strips away the carrier, leaving the information.
In the old world of dumb radios, transceivers join all these components
into one analog hardware system. In the new world of smart radios, only
the antenna and the front- end mixer are analog and hardwired. Channels,
frequency bands, modulation schemes, and protocols all can be defined
in software in real time. The radio becomes a programmable microwave eye-a
device that can see whatever colors of RF you want to send it.
The key to digital radio is the analog-to-digital converter. It takes
a radio or intermediate frequency and samples it at least at a rate double
the frequency to translate it into a series of numbers. Imagine a strobe
light illuminating a dancer. The light will have to strobe at least twice
as fast as the dancer moves or you will not be able to detect the dance.
Indeed, in a phenomenon called aliasing, you may see a different, slower
dance, as you see a tire rotating slowly in the wrong direction on a film.
In a similar way, an ADC strobes (samples) the dance of inflected frequencies
on the carrier wave. The resolution of the ADC is measured in bits, setting
how high the number can be that defines the waveform and, in samples per
second, determining how high a frequency the ADC can capture without aliasing.
Ultimately, early in the next century, the advance of analog-to-digital
converters will dispense even with the mixer. Then the all-software radio
will be here. Analog-to- digital converters (ADCs) will be able to translate
microwave frequencies directly from the antenna into a digital bitstream.
Alcatel has already accomplished this feat in the GSM cellular band at
its labs in Marcoussis, France. But so far this almost totally digital
radio is a stunt rather than a product. That will change.
Most of todays ADCs cannot function reliably in real time at microwave
frequencies (above 300 megahertz). Therefore, mixers are vital. Whether
digital or analog, a mixer is essentially a multiplier. As invented by
E. H. Armstrong, the father of FM, mixers are superheterodyne. They use
local oscillators (LOs) to multiply the carrier frequency with a lower
frequency. The key result is a frequency that represents the difference
between the LO frequency and the carrier. This frequency is an intermediate
frequency that holds all the information borne by the carrier but at a
level that can be processed by existing ADCs.
By far the most effective mixer is the paramixer invented by Steinbrecher
Corporation of Burlington, Massachusetts, now owned by Tellabs and renamed
Tellabs Wireless. This device can range gigahertz of frequencies with
a spur-free dynamic range (a range of volumes without spurious crackles
or harmonics) that could capture the sound of a pin dropping at a heavy
metal rock concert. For a fully digital superbroadband radio, a cascade
of these still- costly devices is still the best bet. The pioneer of this
technology since it was conceived a decade ago by MIT professor Donald
Steinbrecher, Tellabss Burlington operation introduced the Steinbrecher
MiniCell in May for wireless local loop and interior cellular applications.
Tellabs has had trouble selling its wideband radios for cellular applications,
for which they may be overdesigned. With the increasing spread of CDMA,
which ordinarily uses only one to three channels, the initial gains from
a broadband radio are small. But for a wireless local loop, with many
thousands of customers in the Third World using all available channels,
a broadband base station could offer large efficiencies. Replacing a large
number of costly custom radios with one programmable device, the MiniCell
may find its niche.