- System Description
- Historical Perspective
- Systems Engineering and the Role of the Systems Engineer
A hundred years ago, a radio "system" was a transmitter, a receiver, and a path that could be successfully traversed by the miracle of radio waves. Even then there were broader issues to resolve—trade-offs that could be made between one element of the configuration and another. A more powerful transmitter or a more sensitive receiver; a higher mast or a directive antenna—these were some of the potential design improvements that could extend the range of the system when needed. Which of these to adopt became an important question, affecting cost and having performance implications in other dimensions. Was the power demand excessive? Was range being limited by circuit noise within the receiver or by external environmental noise? Was it limited by a physical obstruction over which one might radiate?
Radio had evolved from the design of general-purpose transmitters and receivers to a variety of "systems" with specific applications. Broadcast systems created the wildly popular phenomena of radio and television entertainment, by creating a way to deliver that entertainment inexpensively to a mass market. The trade-offs shifted again; base transmitters could be extremely powerful and expensive, sited on tall buildings or hills, using tall masts and elaborate gain antennas, but the millions of home receivers had to be low-cost consumer products.
"Propagation engineers" now had a more difficult problem; rather than designing a single path from one radio to another, they were concerned with an "area of coverage" in which signal quality was likely (but not guaranteed) to be acceptable. Moreover, the demand for channels required channels to be reused in nearby areas, so that interference needed to be predicted and controlled in the service areas of systems. Probability and statistics had joined the sciences that contributed to system design.
The first mobile telephone systems emerged in the 1940s, quickly became popular, and introduced a number of new trade-offs. The mobile equipment, carried in the trunks of cars and powered from the car battery, needed to be smaller and lower in power (as well as cheaper) than the base station equipment; but coverage areas needed to be large, since cars would need to operate throughout large urban areas. A single high-powered base station could serve an entire urban area of more than a thousand square miles, but the lower-powered return paths from the vehicles could not, and satellite base station receivers became necessary. The higher cost of the (relatively few) satellite base stations could now be traded off for the (smaller) savings in power in the more numerous mobile units. This trade-off of expensive base equipment against more numerous mobile radios is characteristic of such systems.
In the major urban areas, a mobile telephone system would now consist of several radio channels, serving several hundred customers. This aggregation of radios and customers led to the incorporation of telephone traffic-handling probabilities into mobile system design—designers would now calculate the probability that an idle channel would be available. Because of the shortage of channels, however, service was very poor before the days of cellular systems. In the 1950s mobile operators who set up calls were replaced by equipment to automatically select idle channels, allowing the dialing of telephone calls in both directions. Signaling had been added to voice communication on radio channels, together with the first steps toward complex logic.
As early as the 1940s, when the first crude mobile telephone systems were going into service, AT&T had begun to propose a new concept in mobile radio system design. Rather than using a single high-powered base station to cover an entire urban area, they proposed to create a service area from a grid of smaller coverage areas, called "cells." This had several important advantages. It allowed both base and mobile radios to operate at lower power, which would reduce radio costs. It also allowed larger service areas, since additional small coverage areas could always be added around the periphery to expand the system. Most importantly, although nearby cells required different channels to prevent interference, farther cells could reuse the same channels. In this way each channel could handle tens or even hundreds of calls in the same urban area, overcoming the limitations on capacity that were a result of spectrum shortages. These new systems would require a few hundred channels to get started, however, and the needs of the broadcasters were more persuasive in that period.
In 1968 the FCC finally opened the inquiry that ultimately led to cellular systems in the 1980s. For the advantages they provided, however, these systems demanded a new level of complexity. This time, the major complexity was not in the radio design, which saw few radical changes. With the introduction of small cells, calls could cross many cells, requiring mobile locating, channel switching during calls, and the simultaneous switching of wireline connections from one cell to another. Mobiles had to identify systems and find the channels on which calls could be received or originated, which required the introduction of microcomputers in mobile radios and made the technology of telephone switching machines an important element of radio system design. Moreover, with the introduction of so many disciplines in the design of a single system, and a variety of new trade-offs to be made, it was no longer practical for the many engineers to mediate these trade-offs, and the practice of systems engineering became a new and important discipline.
The introduction of cellular systems also marked the continuation of a long-term trend, in which spectrum shortages drove system designs to higher and higher frequencies. Frequencies such as 900 MHz (and later, 2 GHz) were used for such applications for the first time, and it became necessary to understand the propagation characteristics at these frequencies in real-world environments. Moreover, the old methods of the propagation engineer, in which terrain elevations were plotted to determine coverage, were no longer practical for hundreds of cells in a single system, and statistical coverage methods were developed to assure an acceptable quality of coverage. This trend has reversed once again more recently, as computers have allowed detailed terrain studies to be carried out for many cells.
Even as the first analog systems such as the Advanced Mobile Phone Service (AMPS) were being deployed in the early 1980s, efforts were under way to provide significant performance and capacity enhancements enabled by digital communications, advancements in digital signal-processing technology, and speech encoding. The Global System for Mobile Communications (GSM) was a cooperative effort of European countries to define an evolutionary system that provided increased capacity (or equivalently improved spectral efficiency), improved the quality of service, and allowed seamless roaming and coverage across the continent, and eventually around the world. The GSM standard was the first to encompass both the radio elements and the interconnection of serving areas to provide a holistic approach to ubiquitous service. As the first commercial digital cellular system, GSM demonstrated the power of digital signal processing in providing spectrally efficient, high-quality communications. GSM systems began deployment in the early 1990s.
By the mid-nineties, digital spread-spectrum systems were being introduced in North America under standard IS-95. Introduced by Qualcomm, Inc., a U.S.-based company, this system allows all cells to use the same frequency. Each channel is distinguished not by a distinct frequency or time slot but by a spreading code. The fundamental basis for this system is a technique called code-division multiple access (CDMA), a technique that has become the universal architecture for third-generation systems and beyond. CDMA systems have provided another technological leap in complexity, bringing additional enhancements to capacity, information bandwidth, quality of service, and variety of services that can be provided.
Each generation of wireless systems builds upon the technological advances of the prior generation. For each step in this evolution, the classical tools of the engineer remain, but they are honed and reshaped by each subsequent generation. The importance of system design and the role of systems engineering have grown substantially with each new technological generation. The continuing demand for new services and increased capacity, interacting with ongoing technological advancement, leads to new opportunities for system design, new problems to solve, and even the development of new engineering disciplines.