CDMA - Code Division Multiple Access

Code Division Multiple Access (CDMA) is a channelized resource sharing technology renowned for its complexity and popularity in latest generation mobile networks. Unlike FDMA and TDMA all users are served by a single wideband carrier pair. The deployment of additional wideband carriers in a CDMA system is only required for capacity reasons. The carrier bandwidth selection of 1.25MHz, about six times the bandwidth of a GSM FDMA carrier, is the key to understanding of CDMA.

Wideband channels have the advantage of being noise resistant and resilient towards narrowband in-channel interferences. The latter is practical in applications designed to avoid signal jamming. A good example to help us understand the noise resiliency of wideband channels would be a person listening to an orchestra performance on his HI-FI at home. The Hi-Fi sound perception is comparable to the wideband channel defined by our audition sense. If while listening to the orchestra the phone suddnely starts ringing at a reasonable level of loudness, representative here for the narrowband in-channel interference, this may temporarily degrade but will not disrupt the listeners's pleasant sound perception.

The differentiation of the users inside the wideband carrier pair is achieved with the help of carefully selected pseudo-random codes. These are mathematical functions which are orthogonal to one another, meaning that the product between each two of these functions is equal to zero. Each user gets assigned a pseudo-random code for the duration of a call which also spreads the user's data stream uniformly across the whole channel. That way all the active users with assigned pseudo-random codes can co-exist provided that their signal spread is done at similar power levels. The desired user is selected by matching his unique pseudo-random code while all other users, because of their codes being orthogonal to the one of the desired user, will be considered as noise.

Maybe the best example for easier understanding of CDMA is the airport shuttle bus. Imagine a crowded bus full with passengers from different nations, for the sake of the example, including from Japan. All are talking in high spirits about their landing. They chat in groups in their mother languages, the noise level in the bus is quit high but uniformly spread among the groups. If a late comer, a Japanese, gets on that bus, he'll automatically be locked into the conversation of the Japanese group (corresponding to his own pseudo-random code) and will ignore all others as they'll be to him no different than an ambient noise.

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