How many modems can the code words and spreading algorithm of synchronous code division multiple access (S-CDMA) enable to transmit simultaneously on the same RF return channel?

In synchronous code division multiple access (S-CDMA), each modem is assigned a unique spreading code that allows multiple signals to occupy the same frequency channel without interference. The number of modems that can transmit simultaneously is determined by the unique codes available in the system.

For S-CDMA, the standard might typically allow for a larger number of codes because each user needs a unique orthogonal code. The maximum theoretical capacity often cited for such systems can be as high as 128 simultaneous users. Therefore, 128 is a practical limit for many modern implementations.

While the other options (64, 256, and 512) may represent various configurations or systems, 128 aligns well with common standards, making it a favored choice in many practical applications of S-CDMA. This reflects a balance between maintaining the integrity of signals and the number of simultaneous transmissions possible within a given frequency band.