Breakthrough Ideas (continued)
The Puzzling Problem of Reuse
When multiple powerful transmitters propagate radio waves of similar frequencies these waves may overlap, creating cross talk, cancellation, and interference. In trying to avoid such interference, a carrier might transmit widely separated frequencies to prevent overlap, but then the operator would be limited to too few customers. This conundrum presented by radio wave physics creates a fundamental business problem that a wireless operator must solve: how can our mobile wireless system enable enough customers to reuse our limited spectrum so we can make a profit?
After the Second World War, the first radiotelephones filled an automobile’s trunk because it required a transmitter powered by large batteries to reach the city’s central antenna tower, with its own powerful transmitter, perhaps 40-50 miles away. Although it was a mobile car radiophone, it was bulky, expensive, unreliable, limited in capacity, and lacked privacy. All new technologies begin in market niches because their limited features and high costs attract only those customers who have special needs.
In the 1980’s, a first-generation (1G) cellular system using analog technology divided a market area into a grid of cells, each with its own low power handsets and Base Station Transceiver Subsystem (BTS). By handing off control from one cell’s BTS to the next within this grid, the carrier offered greater coverage with more capacity at a lower cost. This division of the service area into a cellular grid reduced the power needed for two-way transmissions. Reducing power reduced interference if, and only if, FM radio frequencies were divided into a precise seven-cell pattern that reduced worst-case interference from adjacent cells. Thus, each cell was assigned one-seventh of the voice channels, with the other six adjacent cells assigned the remaining frequencies to form a precise hexagonal pattern that filled the grid. Of course, the neat hexagons were an idealization of its actual amoeba-shaped cells corresponding to the terrain and its demands for service.
Frequency Division Multiple Access (FDMA) provides for multiple users by division of its access mode: frequency. Because each cell used low-power transmitters in the terminals and base station, the same frequencies could be reused, but only in non-adjacent cells. Interference remained problematic near borders where radio signals from the cells overlapped, and fading signals at the margins dropped many calls. Nonetheless, consumers responded, eventually filling systems to capacity.
In the 1990s, the second-generation (2G ) era introduced wireless communication to the ongoing digital revolution. Another breakthrough idea, the digital revolution became possible with the invention of a solid-state switch, called a transistor. When scientists and engineers devised ways to combine many transistors into an integrated circuit (IC), the digital revolution gained the momentum that Gordon Moore characterized in his eponymous law. Combining increasing performance generated by decreasing line-widths within an IC with ever-lower costs coming from more dies produced by increasing wafer size, the integrated circuit spurred many advances in digital communication beyond usage of its binary zeros or ones, advances such as encoding, decoding, compressing, multiplexing, encrypting, detecting errors, and reducing signal distortion.
Two digital access modes competed in a 2G standards war: the front-runners, TDMA/GSM, and the latecomer, CDMA. A mode is defined by shared similarities in its air interface¾by the requisite architectural features in its Radio Access Network (RAN). Time Division Multiple Access (TDMA, which is also the access mode of GSM) divides a specified frequency into 3-10 time-slots within a narrowband radio spectrum (30-khz). The BTS receives signals from multiple users because each user is allocated a specific, rapidly repeating time-slot within a specific radio frequency. In comparison to FDMA, this frequency-by-time division into multiple time-slots-per-frequency increased system capacity. Thus, after digitizing the analog signal, TDMA/GSM divided FDMA’s frequency division by its time division to increase somewhat reuse of a limited spectrum.
Code Division Multiple Access (CDMA) is a digital wireless technology commercialized by Qualcomm, whose air interface spreads a uniquely encoded signal across 1.25 MHz of spectrum. Why use coding for an access mode? Because a mode that creates a unique code to identify each user permits universal frequency reuse. Why spread the spectrum? Because responding specifically to the coded signal and treating all other frequencies in the spread spectrum as noise fundamentally alters the RF system’s sensitivity to interference. Thus, the use of spread spectrum solved the problem of frequency reuse.
The First Breakthrough Idea
Perplexing problems transformed into solutions create breakthrough business opportunities. Atop such solutions, strategies stand as stepping-stones to success.
Breakthrough idea #1¾advanced power control of an encoded spread spectrum signal generated a revolutionary advance by solving the problem of reuse by introducing a discontinuous architecture, CDMA, as its air interface.
Skeptical for many years, critics saw no way for Qualcomm to prevent a form of co-channel interference described as the near-far problem: stronger signals from mobiles near the BTS necessarily interfere with weaker signals from mobiles far from the BTS, drowning them out. Beyond its propaganda value, when critics suggested that CDMA violated the laws of physics, they probably had in mind this near-far problem.
However, their conception of the near-far problem contained an unrecognized assumption that created a perceived conundrum: critics assumed that the mobiles must transmit at equal power because they always had. Whereas, the solution to building a high capacity CDMA system required this fresh idea: control the power used to transmit signals from all mobile users to make the power of received signals equivalent.
Compared to CDMA’s universal frequency reuse of one, FDMA and TDMA necessarily limit their frequency reuse. TDMA/GSM engineers must assume worst-case interference from adjacent cells; whereas, CDMA engineers are free to assume average interference from a summed noise-like carrier. Thus, Qualcomm’s engineers exploited Claude Shannon’s principle: once a bit-signal is amplified just enough to be detected, using higher power only increases interference and degrades the signal to noise ratio.
According to Qualcomm co-founder Dr. Andrew Viterbi the technical reasons can be:
…summarized by three attributes in which CDMA excels: channel measurement, control, and interference suppression. Briefly, measurement accuracy increases with the bandwidth occupied, and spread spectrum helps particularly in identifying, isolating, and mitigating multipath propagation. Control is facilitated by the sharing of bandwidth by all users, with an important corollary being the ability to perform soft handover between base stations. Interference suppression is the hallmark of CDMA, which not only permits more users per base station, but also avoids the efficiency reduction in FDMA and TDMA necessitated by the requirement to assign different frequency allocations to neighboring base stations to avoid mutual interference, and even to antenna sectors within the same base station. gte.com
Critics of Qualcomm’s CEO, Dr. Irwin M. Jacobs, had underestimated his understanding of the theoretical problems, Qualcomm’s ingenuity in devising solutions, and the power of Moore’s law, which permitted the complex algorithms required for power control. Using superior engineering, Qualcomm transformed its generative breakthrough idea into software-as-codified-knowledge, creating revolutionary competitive advantage. |
