In the context of the LTE Remote Unit (RU) design shown in the diagram, the maximum power output is specified as 46 dBm (which translates to approximately 40 watts). The RU does not require a Power Amplifier (PA) capable of handling peaks reaching 1000 watts (60 dBm) primarily due to a technique called Crest Factor Reduction (CFR) and the characteristics of the Digital Pre-Distortion (DPD).

The signal level of -17 dBFS is derived from:

• -3 dBFS: DAC full scale (includes a 3 dB back-off for buffer)
• -6 dB: DPD expansion (adding 6 dB to manage linearity in the PA)
• -8 dB: CFR threshold (reducing the peak-to-average ratio to control excessive peaks)

So, the calculation aligns as follows:

-17 dBFS = -3 dBFS (DAC) - 6 dB (DPD) - 8 dB (CFR)

This ensures that the overall signal is controlled for efficient amplification while maintaining linearity and preventing peak distortions.

Here's more explanation:

1. Peak-to-Average Power Ratio (PAPR) Management: The RU operates with a maximum average power level of 46 dBm. However, due to high PAPR in LTE signals, the peak power levels can be significantly higher than the average power. The CFR block in the signal chain reduces these peaks, thereby limiting the need for the PA to handle extremely high peak powers like 1000 watts (60 dBm). The CFR's threshold is set at 8 dB by following 3GPP standard in PAR for LTE (8.9) and EVM testing, effectively reducing the peak-to-average ratio.
2. DPD and Signal Expansion: The DPD is designed to linearize the PA by compensating for its non-linear behavior, which leads to an expansion of around 6 dB. The result is that while the average signal power may be maintained, the peaks are managed in such a way that they do not require a significant increase in PA capacity.
3. Power Output vs. Peak Handling: The design aims to achieve an average output power of 46 dBm without requiring the PA to handle full peaks of up to 1000 watts (which would translate to around 60 dBm). The DPD ensures the PA works effectively within its operating range, and the CFR ensures that the peaks are not excessive, remaining around 8 dB above the average power (meaning peaks can reach around 54 dBm, which is approximately 250 watts).
4. Maximum Output Power and PA Capability: The PA is designed to handle peak power efficiently without being over-dimensioned for extremely rare peaks like 1000 watts. The peaks are controlled within a more manageable range (8 dB above average), so a PA designed for up to 400 watts peak (around 56 dBm) is sufficient.

CFR and Peak-to-Average Power Ratio (PAPR):

The diagram shows how the CFR (Crest Factor Reduction) operates within the signal chain. The primary function of the CFR is to reduce the PAPR, which is the ratio of peak power to average power. It trims the extreme peaks in the signal before it reaches the PA, preventing excessive peak power levels that would otherwise exceed the PA's linear operating range.

• PAPR Control: The histogram in the image illustrates that the CFR reduces the input signal's peaks so that they are within a manageable range. After the CFR, the average signal power remains constant, but the peaks are limited. This results in a signal that has reduced peaks relative to its average level, ensuring that the PA does not have to handle unnecessarily high peaks.
• Effect on Signal Levels: The typical input level is around -15 dBFS, and the CFR maintains the peaks to a specific level (PAPR dBFS). After DPD expansion (which adds about 6 dB), the signal level remains within the PA's manageable range, ensuring efficient amplification without distortion.

The image provides specific details about the PA used in the RU, which has a Doherty design, allowing efficient amplification of signals with high PAPR.

• Doherty PA Efficiency: The Doherty amplifier design uses a "main" and "peak" amplifier, where the main amplifier handles the average power, and the peak amplifier activates only when needed for higher power levels. This architecture is highly efficient for signals like LTE that have high PAPR, as it allows the PA to handle peaks effectively without requiring constant high power consumption.
• Power Levels: The PA is designed to handle an average output power of around 47.5 dBm (approximately 56 watts), while the peak power is specified as around 55.5 dBm (approximately 355 watts). This is consistent with the PAPR control provided by the CFR, which reduces the need for handling peaks up to 1000 watts.
• PA Gain and Efficiency: The graph in the PA specification shows that the gain remains stable across various power levels, and the efficiency increases as the average output power rises. This is crucial for maintaining power-efficient amplification within the specified power output range.

Summary:

The combination of the CFR and DPD ensures that the signal's peak levels are reduced and maintained within a manageable range relative to the average power. The CFR controls the PAPR to avoid extreme peaks, while the DPD further optimizes the signal for the PA. The PA, with its Doherty design, efficiently handles the average power and occasional peaks up to around 355 watts (55.5 dBm) without needing to handle extreme peaks (like 1000 watts).

In conclusion, the RU's design effectively manages signal peaks through PAPR reduction and DPD expansion, allowing the PA to handle up to 400 watts (peak power) efficiently, without requiring a larger PA designed for 1000 watts, which would be inefficient and unnecessary for the LTE signal characteristics.