In satellite communications engineering, which technique is specifically used to allocate fixed portions of a transponder's bandwidth and power when a channel plan follows fixed-assignment frequency-division multiplexing (FDM)?

Introduction:
Satellite transponders support multiple simultaneous carriers. When a system planner decides to partition the transponder's spectrum into preassigned slices for different earth stations or services, the multiple-access method must match that fixed frequency plan. This question asks which technique corresponds to fixed-assignment FDM capacity allocation.

Given Data / Assumptions:

• A satellite transponder provides a continuous RF bandwidth and finite power.
• Capacity is allocated using fixed-assignment frequency-division multiplexing (FDM).
• Multiple earth stations may transmit concurrently on different sub-bands.
• We are choosing the multiple-access method, not the modulation format of each carrier.

Concept / Approach:
Fixed-assignment FDM divides the available spectrum into non-overlapping channels. Each user (or carrier) occupies a distinct frequency slot. The corresponding multiple-access method is Frequency-Division Multiple Access (FDMA). FDMA describes how many users share the transponder simultaneously; their individual waveforms can use various modulations (AM, FM, FSK, QPSK, etc.) inside their assigned slices, but the sharing principle is frequency-division.

Step-by-Step Solution:

Verification / Alternative check:
In classic analog telephony over satellite, large FDM/FM carriers used FDMA at the transponder. In digital systems, multiple QPSK/8PSK carriers can also share a transponder via FDMA if frequency slots are preassigned. This confirms that FDMA is the multiple-access label for fixed FDM planning.

Why Other Options Are Wrong:

• Amplitude modulation: waveform modulation type, not an access method.
• Frequency modulation: modulation method, not the sharing strategy.
• Frequency-shift keying: digital modulation, not a multiuser access technique.
• None of the above: invalid because FDMA matches fixed FDM allocation.

Common Pitfalls:
Confusing multiple-access schemes (FDMA/TDMA/CDMA) with modulation formats (AM/FM/FSK/QAM). Another pitfall is assuming FDM itself is the access method; FDM is the multiplexing plan, while FDMA names the multiuser access approach that uses that plan.

Final Answer:
Frequency-division multiple access.