From Microwaves to 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.
As ADC technology continues to advance, however, it will relieve pressure on the mixer, opening the way to still cheaper and lower power solutions. With the expiration of Steinbrecher's patent on the paramixer, the business is opening up. Watkins-Johnson has created a tiny mixer device in gallium arsenide the size of your smallest fingernail. So has Mini-Circuits of Brooklyn, New York. "It has 50% less performance than Steinbrecher's, but it costs only 10% as much. Many customers say, 'It's a deal,'" observes former Steinbrecher CEO and president R. Douglas Shute, now contemplating a startup.
AD converters are now edging toward microwave frequencies. Both Analog Devices and Comlinear, a National Semiconductor company, have introduced 40-megasample-per- second products at a resolution of 12 bits. This allows more of the mixing to move into digital multipliers. The first of the digital downconvertor chips came from Harris Corporation of Melbourne, Florida. Harris now has parlayed its expertise in RF and mixers into the creation of a sophisticated programmable machine that demonstrates the management of multiple modulation schemes in one cellular radio. Introduced on the floor of the Fifth Annual Wireless Symposium Exhibition in late February in Santa Clara, California, the Harris smart radio showcases its programmable HSP50214 digital downconvertor chip and is run from a PC. With an array of displays, the machine is designed to allow configuration and testing of smart transceivers from a Windows PC.
With high-powered digital signal processors and leading- edge ADCs, Analog Devices is a paragon of the digital radio paradigm. At the CTIA (Cellular Telecommunications Industry Association) meeting in San Francisco during the first week of March, Analog introduced a wideband smart radio tuned to the cellular band but applicable through the PCS band as well. A reference design to be used by infrastructure manufacturers, it displays an array of new chips from Analog comprising a specialized ADC called the 6600, tunable filters called the 6620 and the 6640 that function as a digital tuner, a SHARC DSP chip that performs the modem and channel-coding role (any advanced DSP will do), and a "sinfully cheap" Watkins-Johnson mixer chip the size of your fingernail. Incorporating an automatic gain control and a received signal strength indicator, the ADC is customized for smart radio applications.
The antenna is from Radio Shack (most any will do). From a Windows PC using Visual Basic, Analog engineers can move from one cellular channel to another and from GSM to CDMA to DECT 1900 to IS-136 to the Japanese Personal Handyphone system (PHS). As manufacturers around the globe converge on a single intermediate frequency of 70 megahertz, the reference radio could adapt to any cellular band, from 850 megahertz on up. All you would have to do is change or retune the mixer. According to Tom Gratzek, Analog Devices's director of base station marketing at the Analog communications center in Greensboro, North Carolina, customers say, "Shazaam!"