8.3.5 Implementation Variants

Although the fundamental principle of FDMA is simple—assign each user a separate frequency band—practical implementations vary depending on how user signals are organized and transported. Two broad architectural approaches are commonly encountered:

Single Channel Per Carrier (SCPC). In a single-channel-per-carrier system, each carrier transports one user signal (or one logical channel). Each transmitter modulates its own carrier independently, and the set of carriers occupies the shared frequency band simultaneously. SCPC is conceptually the purest form of FDMA, since each user is assigned a dedicated frequency allocation, carriers may be either fixed or dynamically assigned, and both power and bandwidth are allocated on a per-user basis. Historically, SCPC has been used in voice circuits, point-to-point radio links, trunked radio systems, and satellite earth-station links. In modern digital systems, SCPC may carry a single compressed voice stream, a data circuit, or a broadband link. Because each carrier is independently generated, SCPC systems are particularly sensitive to frequency stability, power balance among users, and intermodulation effects in shared amplifiers. Dynamic frequency assignment schemes may allocate SCPC carriers from a common pool when traffic demand arises and release them when idle. This improves spectrum utilization but introduces the need for control signaling and coordination.
Multiple Channels Per Carrier (MCPC). In a multiple-channels-per-carrier system, several user streams are first multiplexed at baseband before modulating a single carrier. That carrier then occupies one frequency allocation within the FDMA plan such that fewer carriers may be required to support the same total traffic volume. Historically, analog systems used frequency-division multiplexing at baseband, followed by FM modulation of a carrier. Digital systems typically employ time-division multiplexing or packet aggregation before phase or amplitude modulation. MCPC reduces the total number of RF carriers in the composite signal, which may ease intermodulation constraints. However, it shifts complexity to the baseband multiplexer and requires higher per-carrier data rates.
Comparative Considerations. The choice between SCPC and MCPC depends on system objectives. SCPC provides flexibility and fine-grained allocation of bandwidth and power to individual users, making it well suited to variable or geographically distributed traffic. MCPC, by contrast, provides spectral consolidation and can offer improved amplifier efficiency by reducing the total number of RF carriers, making it well suited to aggregated backbone or trunk links. In modern broadband systems, aggregation is often dynamic, and the distinction between SCPC and MCPC may blur. For example, packet-based systems may aggregate traffic statistically before modulation while still operating within an overall FDMA framework. Regardless of implementation, the underlying physical constraints discussed earlier—adjacent-channel interference, nonlinear distortion, and power scaling—continue to govern system performance.

Regardless of implementation, the underlying physical constraints discussed earlier—adjacent-channel interference, nonlinear distortion, and power scaling—continue to govern system performance.