GigaBeam turns to III-Vs for ultra-fast broadband radio

Ask any incumbent telecom company to connect two individual gigabit-per-second local-area networks (LANs) and you will be offered 1,000 DSL lines, 647 T1 lines or 500 E1 lines. The copper plant that links users to the communications backbone was never designed to connect modern computers. Gigabit capacity exists in the fiber rings, the LANs, and the computers, but just a few thousandths of this speed is available in the local copper loop.

Fiber can be installed in the last mile, but this occurs infrequently. The 7,500 national and international long-haul fiber rings and the 100,000 local metropolitan rings deployed worldwide have only a few fiber-optic "laterals" connecting them to individual buildings. US businesses use Ethernet networks that operate at 1 Gb/s and computers that run at even higher data rates, and yet 95% of them have no fiber connection.

The lack of provision of fiber services to most buildings arises from the high cost of trenching ($0.2-$0.8 million/km in urban areas), and the 6-12 month wait for installation (caused by the time taken to gain access to private land). The process of laying underground cables has failed to keep pace with the advances in telecommunications and computing. For this reason a market exists for a lower-cost, millimeter-wave radio solution. Radio links can fulfill the demand for high-speed connections from premises to wide-area networks. In the US, 750,000 business locations could benefit from a radio link.

Millimeter-wave radio offering gigabit-per-second data rates can be deployed within a few days, and for a fraction of the trenching costs. These millimeter-wave high-data-rate radios usually cover a 1.6 km range. Although torrential rainfall can impede the propagation of these signals (see "When the rain comes down"), the technology can usually provide connectivity for all but five minutes per year. If customers are prepared to accept this very infrequent disruption, gigabit-per-second radio can serve the 80% of US businesses that are located within 1.5 km of a fiber network.

Today s microwave radios are limited by cost to 100-200 Mb/s data rates, predominantly because the regulatory agencies have authorized only narrow bands. However, because these rates are insufficient for today s needs, GigaBeam has worked with the Federal Communications Commission (FCC) to create rules and authorize the use of spectrum at 71-76 GHz, 81-86 GHz, and 92-95 GHz. The use of these bands was approved in October 2003, and since then GigaBeam has championed the adoption of similar legislation with regulatory agencies throughout the world.

The spectral bands centered at 73.5 GHz and 83.5 GHz are each 5 GHz wide. If low-order modulation formats delivering 1-3 bits/Hz are used, a low-cost 1-10 Gb/s transmission technology is viable. Although the 92-95 GHz spectrum has a 100 MHz band reserved for space-borne radios right in the middle of the band that limits transmission to a few gigabits, this data rate is still valued for next-generation networks.

In commercial environments cost dominates the decision-making process. In general, if you want a lower cost you turn to silicon. Intel has built silicon circuits that operate at 100 GHz, while IBM has used SiGe circuits to produce similarly impressive results. However, practical, longer-range radios cannot be built with silicon-based components, so GigaBeam has chosen GaAs MMICs for its transmitters. We have found that existing GaAs MMICs produced by Velocium, a commercial semiconductor division of Northrop Grumman, have produced excellent results at our frequency range.

The subsystems that form the millimeter-wave circuits include receivers, converters, transmitters, oscillators and modulators/demodulators (modems). The nominal performance parameters required by these parts are outlined in "Requirements".

Among all of the performance parameters of millimeter-wave circuits, one that rarely receives sufficient attention despite its high importance for communication applications is the temperature dependence of the gain per stage. Each stage has a thermally induced gain variation of 0.1 dB per 10°C that depends only slightly on the actual gain of the stage.

GigaBeam has found that the GaAs circuits deliver sufficient gain per stage, unlike their silicon-based counterparts. Velocium s GaAs chips produce a total of a 10 dB variation, so little additional circuitry is required to compensate or deliver additional gain. However, silicon circuits producing a 20 dB gain variation require four to six additional gain stages to overcome the losses, as well as a similar number of circuits that typically use PiN diode actuators for gain compensation.

GigaBeam recently signed an exclusive agreement with Vitesse Semiconductor to use its VIP II HBT technology base for radio applications from 50 GHz to 300 GHz. Vitesse was chosen because its InP manufacturing process can produce transistors with ft and fmax values of 400 GHz and 450 GHz, respectively, as well as deliver high yields for digital circuitry with several thousand gate equivalents. To the best of our knowledge this is the only manufacturing process that produces this number of gates with a high yield on InP, and we believe that it has the potential to deliver high-speed analog-to-digital converters.

GigaBeam will use these circuits for several signal processing functions within the radio, and our main goal is to develop a 4-6 bit A/D converter with a 90 GHz sampling rate. If successful, this A/D converter will enable a new generation of demodulators for 10 or 20 Gb/s links. The target is higher performance and lower cost for the advanced technology base. The first 10 Gb/s radio will probably deploy an analog architecture and operate at 3 bits/Hz, and make use of the entire 5 GHz band. Looking further ahead, even faster data rates are possible by using higher-order modulation schemes featuring 64 or 128 quadrature amplitude modulation processes and a digital architecture.

This year GigaBeam s radio transmitters have enabled the company to win contracts for installation in various locations, including Dartmouth College, Boston University, and the Trump Building in New York City. The development of radio transmitters featuring even faster data rates will only help to generate additional business.

The weather strongly influences millimeter-wave propagation and limits connectivity (figure 1). The proportion of time when communication is lost because of bad weather is defined in terms of "nines". Five nines is a maximum downtime of five minutes per year (99.999% availability), four nines is less than 50 minutes without connectivity per year (99.99% availability), and so on.

At the frequencies used by GigaBeam, rain produces greater signal attenuation than fog or atmospheric gases. However, a rainfall rate of 150 mm per hour occurs for less than five minutes each year ("five nines") across about 80% of continental US and 90% of Europe.

In the past, radio communication operated at frequencies that were largely unaffected by rainfall, and engineers used a rule of thumb that stated that increasing the link system gain by 4 dB doubled the range. Millimeter-wave applications, however, are significantly different.

The rule of thumb can be applied to millimeter-wave communication in clear air, but propagation through heavy rain that corresponds to a "five nines level" availability needs a further 50 dB of gain per mile to overcome the additional attenuation. With today's technology, the weather limits cost-effective, practical, millimeter-wave gigabit-per-second links offering "five nines" availability to a range of 1.6 km.

So, for a 1-mile link using 50 dB gain antennas that are typically 0.6 m high, just 1 μW of power is required for transmission in clear air at 1.25 Gb/s using biphase shift keying modulation, assuming a −60 dBm threshold for 2.5 GHz radio-frequency bandwidth. However, in adverse weather conditions nearly 0.4 W can be needed to deliver the same signal level (figure 2).