8.3.4 Throughput And Power Scaling
In an ideal linear system, adding additional frequency channels would increase total system capacity proportionally. If each carrier occupies bandwidth, B, and delivers data rate, R, then N carriers would yield total throughput NR. In practice, the presence of intermodulation fundamentally limits the number of carriers that can be accommodated within a shared amplifier chain. As more carriers are added, total composite power increases, nonlinear distortion rises, and usable capacity does not scale linearly.
When multiple carriers are transmitted simultaneously through a shared amplifier or medium, two constraints arise: total composite power increases with the number of carriers, and nonlinear distortion increases as operation approaches amplifier saturation. If the amplifier is driven near saturation to maximize efficiency, intermodulation products rise rapidly and degrade signal quality. To maintain acceptable error performance, the amplifier must therefore be operated with output backoff. As the number of carriers increases, greater backoff is typically required to preserve linearity.
Let the total available amplifier output power Ptotal. If N carriers share this power equally, each carrier ideally receives Pcarrier = Ptotal / N. However, because additional backoff is required as increases, the usable per-carrier power decreases more rapidly than 1/N. At some point, further increases in carrier count yield diminishing improvements in total usable throughput. The total system throughput therefore initially increases with the number of carriers but eventually plateaus or even declines as nonlinear distortion and noise dominate.
The practical implications are that there exists an optimal number of carriers for a given amplifier and bandwidth, increasing the carrier count indefinitely reduces the per-carrier signal-to-noise ratio, and overall system capacity becomes interference-limited rather than purely bandwidth-limited. This behaviour contrasts with time-division systems, where only one carrier is present at any instant and amplifiers can operate closer to saturation without generating intermodulation between users.
Thus, in FDMA systems, total capacity is governed jointly by available bandwidth, available linear power, the acceptable signal-to-interference ratio, and the required bit-error performance. The throughput limitations imposed by nonlinear distortion are fundamental to all multi-carrier systems and remain relevant in modern broadband and wireless networks.
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