8.10.2 CDMA/FDMA And Multicarrier Combinations
In both terrestrial and satellite networks, CDMA is often deployed in combination with FDMA. Early wideband CDMA systems such as IS-95 (cdmaOne) and WCDMA assign users orthogonal or quasi-orthogonal spreading codes within a fixed RF carrier—1.25 MHz in IS-95 and 5 MHz in WCDMA. When additional capacity is required, operators deploy multiple CDMA carriers, each occupying its own FDMA-allocated frequency channel. This carrier partitioning limits mutual interference between code groups and simplifies spectrum planning while preserving the soft-capacity and interference-averaging properties of CDMA.
Related ideas appear in certain broadband and satellite air interfaces through the use of multi-carrier techniques such as multi-carrier CDMA (MC-CDMA) or OFDMA. MC-CDMA spreads each symbol across many subcarriers to achieve frequency diversity, while OFDMA assigns orthogonal subcarrier groups to different users to improve spectral efficiency and robustness to frequency-selective fading. Although OFDMA is not a CDMA scheme, both techniques exploit multi-carrier structure to improve performance in broadband and time-varying channels.
These hybrid structures demonstrate that practical systems often layer multiple-access techniques across dimensions. Frequency, time, and code partitioning may be applied simultaneously within the same architecture.
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