9.3, temperature compensation

Generally speaking, we only do temperature compensation in the transmitter.

Of course, receiver performance is also affected by temperature: the receiver link gain decreases at high temperatures and NF increases; at low temperature receiver link gains increase, and NF decreases. However, due to the small signal characteristics of the receiver, both the gain and NF are within the system redundancy range.

For transmitter temperature compensation, it can also be subdivided into two parts: one part is the compensation for the power accuracy of the transmitted signal, and the other part is the compensation for the transmitter gain with temperature changes.

Transmitters of modern communication systems generally perform closed-loop power control (except for the slightly "old" GSM system and Bluetooth system). Therefore, the power accuracy of transmitters calibrated through production procedures actually depends on the accuracy of the power control loop. Generally speaking, the power control loop is a small signal loop, and the temperature stability is very high, so the demand for temperature compensation is not high, unless there are temperature-sensitive devices (such as amplifiers) on the power control loop.

Temperature compensation for transmitter gain is more common. This kind of temperature compensation has two common purposes: one is "visible", usually for systems without closed-loop power control (such as the aforementioned GSM and Bluetooth), which usually do not require high output power accuracy. Therefore, the system can apply a temperature compensation curve (function) to keep the RF link gain within an interval, so that when the baseband IQ power is fixed and the temperature changes, the RF power output by the system can also be kept within a certain range; One is "invisible", usually in a system with closed-loop power control. Although the RF output power of the antenna port is precisely controlled by the closed-loop power control, we need to keep the DAC output signal within a certain range (the most common An example is the need for digital predistortion (DPD) of the base station transmission system), then we need to control the gain of the entire RF link more accurately around a certain value-this is the purpose of temperature compensation.

Transmitter temperature compensation methods generally include variable attenuators or variable amplifiers: in the early stage of low accuracy and low cost accuracy requirements, temperature compensation attenuators are more common; in the case of higher accuracy requirements, the solution Generally: temperature sensor + digital attenuator/amplifier + production calibration.

9.4 Transmitter power control

After talking about dynamic range and temperature compensation, let's talk about a related and very important concept: power control.

Transmitter power control is a necessary function in most communication systems. Commonly used in 3GPP, such as ILPC, OLPC, and CLPC, must be tested in RF design, often causing problems and complicated reasons. Let's first talk about the meaning of transmitter power control.

All transmitter power control purposes include two points: power consumption control and interference suppression.

Let us first talk about power consumption control: In mobile communications, in view of the changes in the distance between the two ends and the different levels of interference, for the transmitter, it is only necessary to maintain the signal strength "enough for the receiver to demodulate accurately"; If it is low, the communication quality is impaired, and if it is too high, the empty power consumption is meaningless. This is especially true for battery-powered terminals like mobile phones, where every milliampere of current needs to be measured.

Interference suppression is a more advanced requirement. In a CDMA system, since different users share the same carrier frequency (differentiated by orthogonal user codes), in the signal arriving at the receiver, the signal of any user is covered on the same frequency for other users. If the signal power of each user is high or low, the high-power user will drown out the low-power user’s signal; therefore, the CDMA system adopts a power control method to control the power of different users reaching the receiver (we call it It is the air interface power, referred to as air interface power, and sends power control commands to each terminal to make the air interface power of each user the same. This kind of power control has two characteristics: the first is that the power control accuracy is very high (the interference tolerance is very low), and the second is that the power control cycle is very short (the channel may change quickly).

In the LTE system, uplink power control also has the effect of interference suppression. Because LTE uplink is SC-FDMA, and multiple users also share carrier frequencies, which also interfere with each other, so the same air interface power is also necessary.

The GSM system also has power control. In GSM, we use "power level" to characterize the power control step length, each level is 1dB, which shows that GSM power control is relatively rough.

Interference limited system

Here is a related concept: interference limited system. The CDMA system is a typical interference limited system. In theory, if each user code is completely orthogonal and can be completely distinguished by interleaving and de-interleaving, then the capacity of the CDMA system can be infinite in fact, because it can be used on limited frequency resources. Layers of extended user codes distinguish an infinite number of users. But in fact, because the user codes cannot be completely orthogonal, noise is inevitably introduced during multi-user signal demodulation. The more users there are, the higher the noise will be until the noise exceeds the demodulation threshold.

In other words, the capacity of the CDMA system is limited by interference (noise).

The GSM system is not an interference limited system, it is a time domain and frequency domain limited system, its capacity is limited by frequency (200kHz one carrier frequency) and time domain resources (each carrier frequency can share 8 TDMA user). Therefore, the power control requirements of the GSM system are not high (the step length is coarser and the period is longer).

9.5 Transmitter power control and transmitter RF specifications

After talking about the power control of the transmitter, let's discuss the factors that may affect the power control of the transmitter in the RF design (I believe that many colleagues have encountered depressed scenes that cannot be tested in closed-loop power control).

For RF, if the power detection (feedback) loop design is correct, then we can do not much for the transmitter closed-loop power control (most of the work is done by the physical layer protocol algorithm), the most important It is the flatness in the transmitter band.

Because the transmitter calibration can only be carried out on a limited number of frequency points, especially in the production test, the less frequency points the better. However, in actual working scenarios, the transmitter is completely possible to work on any carrier in the frequency band. In a typical production calibration, we will calibrate the transmitter's high, middle and low frequency points, which means that the transmission power of the high, middle and low frequency points is accurate, so the closed-loop power control is correct at the calibrated frequency points. However, if the transmitter's transmit power is not flat in the entire frequency band, the transmit power of certain frequency points deviates greatly from the calibration frequency point. Therefore, the closed-loop power control with the calibration frequency point as a reference will also occur at these frequency points. Errors and even mistakes.

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