What assigns a unique code to each channel while allowing all channels to share the same frequency band simultaneously?

The identification of code-division multiplexing (CDM) as the correct answer is based on its ability to assign unique codes to each channel, which allows multiple channels to operate simultaneously over the same frequency band. In CDM, each user is assigned a unique spreading code that modulates their data signal. This modulation spreads the signal over a wider bandwidth than the minimum bandwidth required, allowing multiple signals to coexist in the same frequency space without interference. The receivers use the unique codes to isolate and decode the intended signals from the mixtures.

This multiple access technique is particularly effective in wireless communications, as it enables multiple users to make efficient use of the available spectrum without the need for time slots or separate frequencies. In contrast, other multiplexing methods like time division multiplexing (TDM) allocate specific time slots to each channel, frequency division multiplexing (FDM) divides the frequency spectrum into separate bands for each signal, and wavelength division multiplexing (WDM) is used specifically for optical fibers, assigning different wavelengths (or channels) for simultaneous transmission. These methods do not allow for simultaneous sharing of the same frequency band in the same manner as CDM.