On Distributed Communications Series
VI. Mini-Cost Microwave
This Memorandum considers the feasibility of building microwave equipment designed solely for the transmission of digital signals within the Distributed Adaptive Message Block Network proposed in this series. Restricting the system to digital modulation permits a significant reduction in the signal-to-noise ratio over that required for analog modulation and thus allows use of microwave transmitters radiating only 32 milliwatts. The low-voltage power requirements for transmission fall within the present state-of-the-art of crystal-controlled, solid-state, microwave generators--even at Ku-band frequencies. Two transmitter designs have been considered: the first uses small receiving tube klystrons, and the second uses a chain of solid-state varactor multipliers--with the latter being the preferable choice. In both cases, the low-output power requirements result in very low equipment costs.
The low primary power required by either klystrons or a varactor chain can be supplied by an inexpensive thermoelectric generator.
The base of the antenna tower proposed would contain a tank holding 200 gallons of L-P fuel. As each entire relay--tower site would consume only 50 watts of electricity, this local supply of fuel for emergencies would last for at least three months after failure of the normal primary power source. By keeping the relay spacing short--on the order of about 20 miles--relatively short and inexpensive towers are shown to suffice. The transmitter/receiver units will be about the size of a shoe-box and can be built using only semiconductor components. In alternative klystron version, a total of three tubes per relay station would be required. A pre-tuned transmitter/receiver package plugs into the center of a molded foam plastic antenna reflector.
Error rates are computed for single "hops" and for 200-mile spans. The spans are composed of about ten "hops." It is shown that the use of a 25-db fading margin plus a 15-db normal signal-to-noise ratio, or a 40-db total signal-to-noise ratio, should produce a block error rate ten times lower than that needed to maintain the established Distributed Adaptive Message Block Network system user-to-user error rate of less than l0-8.
The fading margin chosen is such that each span may be expected to be above the fade margin 99.3 per cent of the time. During the short periods of fade, the instantaneous alternative routing capability of the network is utilized to prevent loss of transmitted information.
The equipment design makes extensive use of stripline or "tri-plate" construction. All units are mounted in replaceable pre-tuned packages. In principle, the all solid-state version should prove highly reliable. Preliminary price estimates for the system offer hopes of better than a decimal order of magnitude lower cost than present equipment designed along more traditional lines. Costs on the order of $300 per airline mile, at a data rate of 4.5-million bits per second, appear feasible, provided the links are both manufactured and sited on a mass production basis and use is made of existing government rights-of-way alongside roads for antenna sites.