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Inventing the Internet Again

In 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 today’s 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, Tellabs’s 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.

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