The encyclopedia · Software & IT · Technical decision · 1985-1993
Qualcomm let many callers share one frequency by spreading each into a code
Qualcomm's CDMA gave every call its own spread-spectrum code so a wide channel carried many users, up to 10x analog capacity.
Qualcomm
the move
In 1985 Irwin Jacobs and a group of engineers founded Qualcomm with a contrarian idea: reuse space that the cellular world had been writing off. They believed a channel's real capacity came from interference, not from clocking users into distinct frequencies or slots.
Their technique, code division multiple access, spreads each caller's bits across one wide band using a pseudorandom code. The receiver inverts only its own code, which turns everyone else's transmission into a low-level floor of noise. Many callers share the same spectrum.
In the spring of 1989 Qualcomm and PacTel Cellular staged a live demonstration for fifty industry leaders. In July 1993 the Telecommunications Industry Association adopted CDMA as a standard, and the first commercial system went live in Hong Kong in 1995.
why it works
- A shared band with orthogonal codes uses spectrum that fixed-slot schemes waste at the guardbands.
- Capacity interpolates as a signal-to-noise tradeoff, so it scales past a hard number of channels.
- Spreading makes the signal robust to narrowband interference, improving voice and data quality.
what transfers
When a scarce resource is carved up physically, adding users is zero-sum. Carve it up with mathematics, so the limit becomes an adjustable signal-to-noise bound, not fixed slots.
what came after
CDMA became the basis for IS-95 and the 3G family, licensing out of Qualcomm's patent pool to phone makers and carriers. The spread-spectrum design it proved out set up the cellular standard wars that followed.
references
- Spread spectrum multiple access communication system using satellite or terrestrial repeaters (US4901307)
- Qualcomm [company history]
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