Nervous System: How to Stuff Cellular Phone Calls in Digital Bubble Wrap

As the use of cellular technology started to spread, CDMA technology enabled genuinely simultaneous users to broadcast on the same frequency at the same time, without risk of interference—like digital bubble wrap. 

Early cellular telephone users had to contend with the fact that the service was very expensive. The enormous capital investments required to develop a mobile network could be offset only from subscriptions by the limited number of users supported by the analog system. Getting more users onto the system, however, involved not only a marketing challenge, but also a technological one. Somehow, multiple users needed to be using the same radio bandwidth channel simultaneously, without interfering with each other.  

This expensive technology was also fairly insecure. To eavesdrop on analog mobile telephone calls, a listener merely needed to tune a receiver to the right channel as one would tune into any radio broadcast.  

Certain privacy laws enacted during the era of early cellular technology reflect this situation. For example, Section 632.7 of the California Penal Code established special privacy protections for communications using cellular or cordless phones because of the comparative ease of intercepting such calls in 1992 when that law was passed.  

The advent of digital cellular service brought numerous advantages, including superior forms of error checking and error-correction technologies that allowed for greater signal fidelity. But arguably chief among the advantages was the opportunity for more sophisticated methods for sharing radio spectrum across simultaneous users.  

Engineers developed three different solutions to the spectrum-sharing problem. 

The first solution would be familiar to any fan of FM radio. Frequency-division multiple access (FDMA) carved the available range of radio spectrum into individual channels. Tuning the radio to different channels shifts it among different communication paths, each of which can be assigned to a different user. The same channel can be assigned to multiple different users, as long as they are geographically spaced far enough apart to minimize interference.  

The second solution, time-division multiple access (TDMA), arose from the recognition that processing voice signals into digital data packets did not occupy the broadcasting channel continuously. The phone selected a chunk of audio, digitized it, sent it, and then processed the next chunk. During the brief interval during which one phone was processing a chunk of data to be sent, a different phone could be using the same bandwidth to send an already-processed chunk. 

TDMA phone networks could accommodate significantly more concurrent users than FDMA systems. But inefficiencies were still baked in. The user’s needs for access and the timing of their turn in the sequence rarely aligned. Real voice calls are unlikely to be uninterrupted walls of continuous sound—many data chunks contain silence. Adhering to a strict timetable of sequential users’ slots meant that phones had to wait their turn simply to send packets of empty air.  

The third solution was both the most advanced yet the most rooted in history. Curiously enough, this clever trick for packing concurrent users onto the same channel emerged from a solution to a completely different problem—how to protect a sensitive transmission over an insecure channel.  

Most technologies for transmitting messages could be jammed on purpose or compromised by accident, not to mention be exposed to eavesdropping. In 1903 Nikola Tesla received a patent for a “Method of Signaling” that he claimed avoided “any danger of the signals or messages begin disturbed, intercepted, interfered with in any way.”  

By 1915 the German military used a version of the same idea in the First World War. In the pitch of the Second World War, actress Hedy Lamarr and colleague George Antheil proposed their own version of the idea, which they developed under the alluring name of “Project X” and patented as the more prosaic “Secret Communication System” in 1942.  

At root, each iteration entailed taking the thin, vulnerable, exposed message at issue and packing it with a sort of electromagnetic bubble wrap of apparent noise. Whereas FDMA and TDMA sought to slice their use of the available bandwidth into ever finer slivers, the idea here was to pad a transmission to fill up the entire frequency band—hence the term “spread spectrum.”  

For analog transmissions, encrypted or plaintext, this approach buried the needle in a haystack. To any eavesdropper, the transmission would appear to be noise. At the receiving end, extracting the true signal from the noise wrapper could be accomplished so long as the recipient knew in advance what the noise would be.  

In the era of digital radio communications, algorithmically generated binary codes could be assigned to each user. Digital cellular handsets using code-division multiple access (CDMA) technology would pad their transmissions to spread across the spectrum with seemingly random (but reproducible) noise derived from their unique codes.  

Instead of limiting the number of users in a region to those few who could share a given spectrum without overlapping, CDMA technology enabled genuinely simultaneous users to broadcast on the same frequency at the same time, without risk of interference.

The views and opinions expressed in this article are those of the author and do not necessarily reflect the opinions, position, or policy of Berkeley Research Group, LLC or its other employees and affiliates.