8.4.3 Throughput And Power Scaling
In TDMA, users transmit in bursts rather than continuously. This burst-mode operation has important implications for instantaneous bit rate, required transmit power, and overall system throughput.
8.4.3.1 Burst Rate And Average Rate
Let the average user data rate be Rb. If the user is assigned a burst duration TB within a frame of period TF , then the instantaneous burst rate R must satisfy Equation (8.3). Thus, the instantaneous transmission rate increases in inverse proportion to the duty cycle TB / TF. As the number of users sharing the frame increases, each user’s duty cycle decreases, and the required burst rate rises correspondingly.
8.4.3.2 Power Scaling
To achieve a specified bit-error probability, a certain energy per bit Eb relative to noise spectral density N0 is required. The corresponding carrier-to-noise density ratio is
Substituting the burst-rate expression:
This relationship shows that as the duty cycle decreases (i.e., TB / TF becomes smaller), the required instantaneous carrier power must increase proportionally to maintain the same error performance.
In practical terms, shorter bursts require higher peak transmit power, the peak-to-average power ratio increases, and power amplifiers must accommodate higher instantaneous output levels. Unlike FDMA, where multiple carriers coexist simultaneously and intermodulation constrains amplifier operation, TDMA allows a single carrier to occupy the channel at any instant. As a result, amplifiers may operate closer to saturation during bursts without generating multi-carrier intermodulation products. However, the higher instantaneous power requirement can impose hardware constraints, particularly in power-limited terminals.
8.4.3.3 Guard And Control Overhead
Net throughput is reduced by guard intervals between bursts, preamble and synchronization fields, and control and scheduling information. If TG represents total guard time per frame and TC represents control overhead, the effective data duty cycle becomes
As the number of users increases, total guard overhead may increase proportionally, reducing efficiency.
8.4.3.4 Throughput Scaling Behavior
In contrast to FDMA, where throughput eventually plateaus due to intermodulation and composite power constraints, TDMA throughput scales more predictably with the number of users, provided that frame overhead remains moderate, peak power requirements are satisfied, and synchronization remains stable. Capacity is limited primarily by the total available bandwidth, the acceptable burst rate, the extent of control and guard overhead, and the required signal-to-noise performance.
Thus, TDMA is often power-limited rather than distortion-limited, whereas FDMA is typically distortion-limited in multi-carrier environments.
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