Shulou(Shulou.com)06/01 Report--

This article mainly introduces what are the common RF parameters in the Internet, which can be used for reference. I hope you can learn a lot after reading this article.

1. Rx Sensitivity (reception sensitivity)

Reception sensitivity, which should be one of the most basic concepts, characterizes the lowest signal strength that the receiver can identify without exceeding a certain bit error rate. The bit error rate here is a general name based on the definition of the CS era. In most cases, BER (bit error rate) or PER (packet error rate) is used to examine sensitivity, and in the LTE era, it is simply defined by throughput Throughput-- because LTE simply does not have a circuit-switched voice channel, but this is also a real evolution, because for the first time we no longer use things such as 12.2kbps RMC (reference measurement channel). A "standardized alternative", which actually represents the voice coding of rate 12.2kbps, measures sensitivity, but is defined by the throughput that users can actually feel.

2. SNR (signal to noise ratio)

When talking about sensitivity, we often contact SNR (signal-to-noise ratio, we usually talk about the demodulation signal-to-noise ratio of the receiver). We define the demodulation signal-to-noise ratio as the signal-to-noise ratio threshold that the demodulator can demodulate without exceeding a certain bit error rate (during the interview, someone will often give you a question, give you a string of NF and Gain, and then tell you that the demodulation threshold requires you to push the sensitivity). So where does S and N come from?

S is signal Signal, or useful signal; N is noise Noise, which generally refers to all signals without useful information. The useful signal is generally emitted by the transmitter of the communication system, and the source of the noise is very wide. The most typical one is the famous-174dBm Hzmuri-natural noise bottom. Keep in mind that it is a quantity independent of the type of communication system. In a sense, it is derived from thermodynamics (so it is related to temperature). Another thing to note is that it is actually a noise power density (so there is the dimension of dBm/Hz), and the bandwidth of the signal we receive will be the same as the bandwidth we receive-so the final noise power is obtained by integrating the noise power density with the bandwidth.

3. TxPower (transmit power)

The importance of transmission power is that the signal of the transmitter needs to go through spatial fading before it can reach the receiver, so the higher the transmission power means the farther the communication distance.

So should we pay attention to SNR in our transmitted signal? For example, if our transmitted signal SNR is very poor, is the signal SNR that arrives at the receiver also very poor?

This involves the concept just mentioned, the bottom of natural noise. We assume that spatial fading has the same effect on both signal and noise (in fact, the signal can be coded to resist fading but not noise) and acts like an attenuator, then we assume spatial fading-200dB, transmitted signal bandwidth 1Hz, power 50dBm, signal-to-noise ratio 50dB, what is the SNR of the signal received by the receiver?

The power of the signal received by the receiver is 50-200=-150Bm (bandwidth 1Hz), while the noise of the transmitter 50-50=0dBm passes through spatial fading and the power that reaches the receiver is 0-200=-200dBm (bandwidth 1Hz). At this time, this part of the noise has already been "submerged" under the natural noise bottom of-174dBm/Hz. At this time, we only need to consider the "basic components" of-174dBm/Hz to calculate the noise at the entrance of the receiver. This is applicable in most cases of communication systems.

4 、 ACLR/ACPR

We put these items together because they actually represent part of the "transmitter noise", but the noise is not in the transmission channel, but the part where the transmitter leaks into the adjacent channel, which can be collectively referred to as "adjacent channel leakage".

ACLR and ACPR (actually one thing, but one in terminal testing and the other in base station testing) are both named after "Adjacent Channel", which, as the name implies, describes the interference of the machine to other devices. And they have one thing in common: the power of the interference signal is also calculated on the basis of a channel bandwidth. This measurement method shows that the design purpose of this index is to consider the signal leaked by the transmitter and the interference to the equipment receiver of the same or similar system-the interference signal falls into the receiver band in the same frequency and bandwidth mode. form co-frequency interference to the received signal of the receiver.

In LTE, there are two settings for ACLR testing, EUTRA and UTRA, the former describes the interference of LTE system to LTE system, and the latter considers the interference of LTE system to UMTS system. So we can see that the measurement bandwidth of EUTRAACLR is the bandwidth occupied by LTE RB, and the measurement bandwidth of UTRAACLR is the bandwidth occupied by UMTS signals (FDD system 3.84MHz 1.28MHz). In other words, ACLR/ACPR describes a "peer-to-peer" interference: the interference caused by the leakage of transmitted signals to the same or similar communication system.

This definition is of great practical significance. In the actual network, signals are often leaked from the neighboring areas and nearby areas, so the process of network planning and network optimization is actually the process of capacity maximization and interference minimization. The adjacent channel leakage of the system itself is a typical interference signal for the neighboring community; from the other direction of the system, the mobile phones of users in the crowded crowd may also become sources of interference with each other.

Similarly, in the evolution of the communication system, it has always been the goal of "smooth transition", that is, upgrading on the existing network into the next generation network. Then the coexistence of two or even three generations of systems needs to consider the interference between different systems. The introduction of UTRA by LTE takes into account the radio frequency interference of LTE to the previous generation system in the case of coexistence with UMTS.

5 、 Modulation Spectrum/Switching Spectrum

Back to the GSM system, Modulation Spectrum (modulation spectrum) and Switching Spectrum (switch spectrum, also known as switch spectrum, for the sake of different translation of imports) also play a similar role in adjacent channel leakage. The difference is that their measurement bandwidth is not the bandwidth occupied by GSM signals. By definition, it can be considered that the modulation spectrum measures the interference between synchronous systems, while the switching spectrum measures the interference between non-synchronous systems (in fact, if the signal is not gating, the switching spectrum will certainly drown the modulation spectrum).

This involves another concept: in the GSM system, the cells are out of sync, although it uses TDMA; while, by contrast, TD-SCDMA and the subsequent TD-LTE, the cells are synchronized (the GPS antenna in the shape of a flying saucer or ball is always the shackle of the TDD system).

Because of the non-synchronization between cells, the power leakage of the rising / falling edge of cell A may fall into the payload part of cell B, so we use the switching spectrum to measure the interference of the transmitter to the adjacent channel in this state. However, in the GSM timeslot of the whole 577us, the proportion of rising edge / falling edge is very small, and most of the time the payload parts of two adjacent cells overlap in time. In this case, the modulation spectrum can be used to evaluate the interference of the transmitter to the adjacent channel.

6. SEM (Spectrum Emission Mask)

When talking about SEM, we should first note that it is an "in-band index", which is distinguished from spurious emission, which in a broad sense includes SEM, but focuses on the spectrum leakage outside the working frequency band of the transmitter, and its introduction is more from the perspective of EMC (electromagnetic compatibility).

SEM provides a "spectrum template", and then when measuring the spectrum leakage in the transmitter band, see if there are any points that exceed the template limit. It can be said that it has something to do with ACLR, but it is different: ACLR takes into account the average power leaked into the adjacent channel, so it takes the channel bandwidth as the measurement bandwidth, which reflects the "noise base" of the transmitter in the adjacent channel; SEM reflects the over-standard points in the adjacent frequency band with a smaller measurement bandwidth (often 100kHz to 1MHz), reflecting "stray emission based on the noise floor".

If the spectrometer is used to scan the SEM, we can see that the spurious points on the adjacent channel are generally higher than the ACLR average, so if the ACLR index itself has no margin, the SEM is easy to exceed the standard. On the other hand, excessive SEM does not necessarily mean bad ACLR, there is a common phenomenon is the spurious of LO or a clock and LO modulation component (often very narrow bandwidth, similar to point frequency) concatenated into the transmitter link, even if the ACLR is very good, SEM may also exceed the standard.

7. EVM (error vector)

First of all, EVM is a vector value, that is to say, it has amplitude and angle, and it measures the "error between the actual signal and the ideal signal". This measure can effectively express the "quality" of the transmitted signal-the farther the point of the actual signal is from the ideal signal, the greater the error, the greater the modulus of EVM.

In (1) we have explained why the signal-to-noise ratio of the transmitted signal is not so important for two reasons: the first is that the SNR of the transmitted signal is often much higher than the SNR needed for receiver demodulation Second, when we calculate the reception sensitivity, we refer to the worst case of the receiver, that is, after a large spatial fading, the transmitter noise has long been submerged under the natural noise, and the useful signal is also attenuated to the demodulation threshold of the receiver.

However, the "inherent signal-to-noise ratio" of the transmitter needs to be considered in some cases, such as short-range wireless communication, such as the 802.11 series.

When the 802.11 series evolved to 802.11ac, 256QAM modulation has been introduced. For the receiver, even without considering spatial fading, high-order quadrature modulation signals such as demodulation already need high signal-to-noise ratio. The worse the EVM is, the worse the SNR is, and the more difficult the demodulation is.

Engineers of 802.11 systems often use EVM to measure TX linearity, while engineers of 3GPP systems like to use ACLR/ACPR/Spectrum to measure TX linear performance.

From the point of origin, 3GPP is the evolution of cellular communication. From the very beginning, we have to pay attention to the interference of adjacent channels and inter-channel (adjacent channel, alternative channel). In other words, interference is the first obstacle that affects the rate of cellular communication, so in the process of evolution, 3GPP always aims at "interference minimization": frequency hopping in GSM era, spread spectrum in UMTS era, and the introduction of the concept of RB in LTE era.

The 802.11 system is the evolution of fixed wireless access, which adheres to the spirit of TCP/IP protocol and aims at "service to the maximum capability". In 802.11, there are often time division or frequency hopping means to achieve multi-user coexistence, while the layout of the network is more flexible (after all, mainly local area network), and the channel width is also flexible. Generally speaking, it is not sensitive to interference (or relatively high tolerance).

Generally speaking, the origin of cellular communication is to make phone calls, users who can't get through the phone will go to the telecommunications bureau to smash the place; 802.11 of the origin is the local area network, and the bad network is likely to be patient first, and so on (in fact, the equipment is correcting and retransmitting at this time).

This determines that the 3GPP series must take the performance of "spectrum regeneration" such as ACLR/ACPR as the index, while the 802.11 series can adapt to the network environment at the expense of speed.

Specifically, "adapting to the network environment at the expense of rate" means that in the 802.11 series, different modulation orders are used to deal with the propagation conditions: when the receiver finds a signal difference, it immediately notifies the opposite transmitter to reduce the modulation order, and vice versa. As mentioned earlier, in 802.11 systems, SNR is closely related to EVM, and to a large extent, a reduction in EVM can improve SNR. In this way, we have two ways to improve the receiving performance: one is to reduce the modulation order, so as to reduce the demodulation threshold; the other is to reduce the transmitter EVM to improve the signal SNR.

Because EVM is closely related to the demodulation effect of the receiver, the transmitter performance is measured by EVM in the 802.11 system (similarly, in the cellular system defined by 3GPP, ACPR/ACLR is the main index that affects the network performance); and because the deterioration of EVM by the transmitter is mainly caused by nonlinearity (such as AM-AM distortion of PA), EVM is usually used as a symbol to measure the linear performance of the transmitter.

7.1.The relationship between EVM and ACPR/ACLR

It is difficult to define the quantitative relationship between EVM and ACPR/ACLR. From the point of view of the nonlinearity of the amplifier, EVM and ACPR/ACLR should be positively related: the AM-AM and AM-PM distortion of the amplifier will expand the EVM, and it is also the main source of ACPR/ACLR.

But EVM and ACPR/ACLR are not always positively correlated. We can find a typical example here: Clipping, which is commonly used in digital intermediate frequency, that is, peak clipping. Clipping reduces the peak-to-average power ratio (PAR) of the transmitted signal, and the reduction of peak power helps to reduce the ACPR/ACLR; after passing through the PA, but Clipping will also damage the EVM, because both clipping (windowing) and filter methods will damage the signal waveform, thus increasing the EVM.

7.2.Origin of PAR

PAR (signal peak-to-average ratio) is usually expressed by a statistical function such as CCDF, whose curve represents the power (amplitude) value of the signal and its corresponding occurrence probability. For example, the average power of a signal is 10dBm, and the statistical probability that it exceeds the power of 15dBm is 0.01%. We can think of its PAR as 5dB.

PAR is an important factor affecting the spectrum regeneration of transmitters (such as ACLP/ACPR/Modulation Spectrum) in modern communication systems. The peak power will push the amplifier into the non-linear region to produce distortion, often the higher the peak power, the stronger the nonlinearity.

In the era of GSM, because of the envelope characteristic of GMSK modulation, PAR=0, we often push it to P1dB when designing GSM power amplifier in order to maximize efficiency. After the introduction of EDGE, the 8PSK modulation is no longer a balance envelope, so we tend to push the average output power of the amplifier to about 3dB below P1dB, because the PAR of 8PSK signal is 3.21dB.

In the era of UMTS, the peak ratio of both WCDMA and CDMA was much larger than that of EDGE. The reason is the signal correlation in the code division multiple access system: when the signals of multiple code channels are superimposed in the time domain, the phase may be the same, and the power will peak.

The peak-to-average ratio of LTE is derived from the paroxysm of RB. OFDM modulation is based on the principle of dividing multi-user / multi-service data into blocks in both time domain and frequency domain, so that high power may occur on a certain "time block". SC-FDMA is used for LTE uplink transmission. First, DFT is used to extend the time domain signal to the frequency domain, which equals to "smoothing" the burst in the time domain, thus reducing the PAR.

8. Summary of interference indicators

The "interference index" here refers to the sensitivity test under all kinds of interference in addition to the static sensitivity of the receiver. In fact, it is interesting to study the origin of these test items.

Our common interference indicators, including Blocking,Desense,Channel Selectivity, etc.

8.1. Blocking (blocking)

Blocking is actually a very old RF indicator, which existed as early as the beginning of radar invention. The principle is to inject a large signal into the receiver (usually the first stage LNA suffers most), making the amplifier enter the non-linear region or even saturated. At this time, on the one hand, the gain of the amplifier suddenly becomes smaller, on the other hand, it produces strong nonlinearity, so the amplification function of the useful signal can not work properly.

Another possible Blocking is actually accomplished through the AGC of the receiver: the large signal enters the receiver link, and the receiver AGC therefore produces action to reduce the gain to ensure the dynamic range; but at the same time, the level of the useful signal entering the receiver is very low, and the gain is insufficient, and the amplitude of the useful signal into the demodulator is not enough.

The Blocking index is divided into in-band and out-of-band, mainly because the RF front-end generally has a band filter, which can suppress the out-of-band blocking. However, no matter in-band or out-of-band, Blocking signals are generally point frequency, without modulation. In fact, the point frequency signal without modulation is rare in the real world, and it is only simplified to point frequency in engineering to replace all kinds of narrowband interference signals.

For the solution of Blocking, the main contribution is RF. To put it bluntly, it is to improve the receiver IIP3 and expand the dynamic range. For out-of-band Blocking, the filter suppression system is also very important.

8.2 、 AM Suppression

AM Suppression is a unique index of GSM system. From the description, the interference signal is a TDMA signal similar to the GSM signal, synchronized with the useful signal and has a fixed delay.

This scenario simulates the signal of the adjacent cell in the GSM system. From the point of view that the frequency offset of the interference signal is greater than 6MHz (GSM bandwidth 200kHz), this is a very typical neighboring cell signal configuration. Therefore, we can think that AM Suppression reflects the interference tolerance of the receiver to the neighboring cells in the actual work of the GSM system.

Adjacent (Alternative) Channel Suppression (Selectivity)

Here we collectively call it "adjacent channel selectivity". In the cellular system, we should consider not only the same frequency cell, but also the adjacent frequency cell. The reason can be found in the transmitter index ACLR/ACPR/Modulation Spectrum that we discussed earlier: because the spectrum regeneration of the transmitter will have a strong signal falling into the adjacent frequency (generally speaking, the farther the frequency offset is, the lower the level is, so the adjacent channel is generally the most affected). And this kind of spectrum regeneration is in fact related to the transmitted signal, that is, the synchronous receiver is likely to demodulate this part of the reproduced spectrum for a useful signal, the so-called magpie nest.

For example: if two neighboring cells An and B happen to be adjacent frequency cells (such networking is generally avoided, this is only a limit scenario discussed here), when a terminal registered to District A walks to the junction of two campuses, but the signal strength of the two cells has not yet reached the switching threshold, so the terminal still maintains the A cell connection. The ACPR of the base station transmitter of the B cell is higher, so there is a higher ACPR component of the B cell in the receiving frequency band of the terminal, which overlaps with the useful signal of the A cell in frequency; because the terminal is far away from the base station of the A cell, the strength of the useful signal received from the A cell is also very low, and the ACPR component of the B cell can cause co-frequency interference to the original signal when it enters the terminal receiver.

If we pay attention to the definition of frequency offset of adjacent channel selectivity, we will find that there is a difference between Adjacent and Alternative. Corresponding to the first and second adjacent channels of ACLR/ACPR, it can be seen that "transmitter spectrum leakage (regeneration)" and "receiver adjacent channel selectivity" in communication protocols are actually defined in pairs.

8.3. Co-Channel Suppression (Selectivity)

This describes absolute co-channel interference, which generally refers to the interference mode between two co-channel cells.

According to the networking principle we described earlier, the distance between the two cells with the same frequency should be as far as possible, but no matter how far it is, there will be signals leaking from each other, just the difference in intensity. For the terminal, the signals of both campuses can be regarded as "correct and useful signals" (of course, there is a set of access specifications on the protocol layer to prevent such misaccess), to measure whether the receiver of the terminal can avoid "westerly wind overwhelms the east wind". It depends on its co-frequency selectivity.

8.4 Summary

Blocking is "big signal interference small signal", RF still has room for manoeuvre, while the above indicators such as AM Suppression, Adjacent (Co/Alternative) Channel Suppression (Selectivity) are "small signal interference large signal", the work of pure RF is of little significance, and mainly depends on the physical layer algorithm.

Single-tone Desense is a unique index of CDMA system, it has a characteristic: as an interference signal, single-tone is an in-band signal, and it is very close to the useful signal. In this way, it is possible to produce two kinds of signals that fall into the receiving frequency domain: the first is due to the near-end phase noise of LO, the baseband signal formed by mixing LO with useful signal, and the signal formed by mixing with LO phase noise and interference signal, all fall within the range of receiver baseband filter, the former is useful signal and the latter is interference. The second is due to the nonlinearity of the receiver system, the useful signal (such as the CDMA signal of 1.2288MHz) may produce Intermodulation with the interference signal on the nonlinear device, and the Intermodulation product may also fall within the receiving frequency domain to become interference.

The origin of Single-tone desense is that when North America launched the CDMA system, it adopted the same frequency band as the original analog communication system AMPS, and the two networks coexisted for a long time. As a latecomer, the CDMA system must consider the interference of the AMPS system to itself.

At this point, I think of PHS, which was called "if you don't move, you can't move". Because it has occupied the 1900~1920MHz frequency for a long time, the implementation of China TD-SCDMA/TD-LTE B39 has been in the low 1880~1900MHz of B39 until PHS withdraws from the network.

The textbook explanation of Blocking is relatively simple: when a large signal enters the receiver amplifier, the amplifier enters the nonlinear region, and the actual gain becomes smaller (for useful signals).

But it's hard to explain two scenarios:

Scenario 1: the front-stage LNA linear gain 18dB, when the large signal is injected to P1dB, the gain is 17dB; if no other effects are introduced (the NF of the default LNA has not changed), then the impact on the noise figure of the whole system is actually very limited, except that the denominator of the latter NF becomes a little smaller when it is counted into the total NF, which has little effect on the sensitivity of the whole system.

Scenario 2: the IIP3 of the front-level LNA is very high, so it is not affected, and the second-level gain block is affected (the interference signal makes it near the P1dB), in which case the impact of the whole system NF is even smaller.

I would like to put forward a point of view: the influence of Blocking may be divided into two parts, one is that the Gain is compressed in textbooks, and the other is that the useful signal is distorted in this area after the amplifier enters the non-linear region. This distortion may include two parts, one is the spectrum regeneration (harmonic component) of the useful signal caused by pure amplifier nonlinearity, and the other is the Cross Modulation of small signal modulated by large signal. (understandable)

Therefore, we also put forward another idea: if we want to simplify the Blocking test (3GPP requires frequency sweep, which is very time-consuming), we may be able to select some frequency points, which have the greatest impact on the distortion of the useful signal when the Blocking signal appears.

Intuitively, these frequency points may be: f0Universe N and f0impulse N (f0 is the useful signal frequency, N is the natural number). The former is because the Nth harmonic component generated by the large signal in the nonlinear region itself is superimposed on the useful signal frequency f0 to form direct interference. The latter is superimposed on the N harmonics of the useful signal f0 and then affects the time domain waveform of the output signal f0-- explain: according to Perseval's law, the waveform of the time domain signal is actually the sum of the fundamental frequency signal and the harmonics in the frequency domain. When the power of the Nth harmonic changes in the frequency domain, the corresponding change in the time domain is the envelope change of the time domain signal (distortion occurs).

9. Dynamic range, temperature compensation and power control

Dynamic range, temperature compensation and power control are "invisible" indicators in many cases, showing their impact only when certain limit tests are carried out, but they themselves reflect the most ingenious parts of RF design.

9.1. Dynamic range of transmitter

The dynamic range of the transmitter represents the maximum and minimum transmission power of the transmitter "without harming other emission indicators".

"do not damage other emission indicators" is very broad, if you look at the main influence, it can be understood as: the maximum transmission power does not damage the linearity of the transmitter, while the minimum transmission power maintains the signal-to-noise ratio of the output signal.

At the maximum transmission power, the output of the transmitter is often close to the nonlinear region of all levels of active devices (especially the last stage amplifier). The frequent nonlinear manifestations are spectrum leakage and regeneration (ACLR/ACPR/SEM) and modulation error (PhaseError/EVM). At this time, the biggest disaster is basically the linearity of the transmitter, this part should be easier to understand.

At the minimum transmission power, the useful signal output of the transmitter is close to the bottom of the transmitter noise, and even has the danger of being "submerged" in the transmitter noise. What needs to be guaranteed at this time is the signal-to-noise ratio (SNR) of the output signal, in other words, the lower the transmitter noise at the minimum transmission power, the better.

Something happened in the laboratory: when an engineer was testing the ACLR, he found that the power reduction ACLR was even worse (normally understood that the ACLR should be improved with the decrease of the output power). At that time, the first reaction was that there was something wrong with the instrument, but the test result of another instrument was still the same. Our guidance is to test the EVM at low output power and find that the performance of the EVM is very poor; we judge that the noise bottom at the entrance of the RF link is very high, and the corresponding SNR is obviously very poor, and the main component of the ACLR is no longer the spectrum regeneration of the amplifier, but the baseband noise amplified through the amplifier link.

9.2. Dynamic range of receiver

The dynamic range of the receiver is actually related to the two indicators we talked about before, the first is the reference sensitivity, and the second is the receiver IIP3 (mentioned many times when talking about the interference index).

The reference sensitivity actually represents the minimum signal strength that the receiver can recognize, which will not be repeated here. Let's mainly talk about the maximum receiving level of the receiver.

The maximum receiving level refers to the maximum signal that the receiver can receive without distortion. This distortion can occur at any stage of the receiver, from the pre-stage LNA to the receiver ADC. For the front-stage LNA, the only thing we can do is to improve the IIP3 as much as possible so that it can withstand higher input power; for the latter step-by-step devices, the receiver uses AGC (automatic gain Control) to ensure that the useful signal falls within the input dynamic range of the device. To put it simply, there is a negative feedback loop: detecting the received signal strength (too low / too high)-adjusting the amplifier gain (up / down)-the amplifier output signal ensures that it falls within the input dynamic range of the next device.

Here we will talk about an exception: the front-stage LNA of most mobile phone receivers has AGC function. If you study their datasheet carefully, you will find that the front-stage LNA will provide several variable gain segments, each of which has its corresponding noise figure. Generally speaking, the higher the gain, the lower the noise figure. This is a simplified design, and the design idea is that the goal of the receiver RF link is to keep the useful signal input to the receiver ADC within the dynamic range, and to keep the SNR above the demodulation threshold (it is not demanding that the SNR be as high as possible, but "enough", which is a very smart approach). Therefore, when the input signal is very large, the front-stage LNA reduces the gain, loses the NF, and increases the IIP3; at the same time. When the input signal is small, the front-stage LNA increases the gain, reduces the NF, and reduces the IIP3.

9.3. Temperature compensation

Generally speaking, we only make temperature compensation at the transmitter.

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

The temperature compensation of the transmitter can also be subdivided into two parts: one is to compensate the power accuracy of the transmitted signal, and the other is to compensate the gain of the transmitter with the change of temperature.

Modern communication system transmitters generally have closed-loop power control (except for the slightly "old" GSM system and Bluetooth system), so the power accuracy of the transmitter calibrated by the production process actually depends on the accuracy of the power control loop. Generally speaking, the power control loop is a small signal loop with high temperature stability, 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 usually has two purposes: one is "visible", usually for systems without closed-loop power control (such as GSM and Bluetooth), which usually do not require high accuracy of output power, so the system can apply the temperature compensation curve (function) to keep the RF link gain within a range, so that when the baseband IQ power is fixed and the temperature changes. The output RF power of the system can also be kept within a certain range. The other is "invisible", usually in a system with closed-loop power control, although the RF output power of the antenna mouth is accurately controlled by the closed-loop power control, we need to keep the DAC output signal within a certain range (the most common example is the need for digital predistortion (DPD) of the base station transmitting system). Then we need to accurately control the gain of the entire RF link around a certain value-that's what the temperature compensation is for.

The means of temperature compensation of transmitter generally include variable attenuator or variable amplifier: temperature compensation attenuator is more common in the case of lower early precision and lower precision requirement of low cost; in the case of higher precision, the solution is generally: temperature sensor + numerical control 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. Common functions in 3GPP, such as ILPC, OLPC, CLPC, must be tested in RF design, and the reasons are very complicated. First of all, let's talk about the significance of transmitter power control.

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

First of all, let's talk about power consumption control: in mobile communication, in view of the distance between the two ends and the different interference levels, it is only necessary for the transmitter to maintain the signal strength "enough for the other receiver to demodulate accurately"; if it is too low, the communication quality will be damaged, and if it is too high, the air power consumption will be meaningless. This is especially true for battery-powered terminals such as mobile phones, where you have to haggle every milliampere.

Interference suppression is a more advanced requirement. In CDMA systems, because different users share the same carrier frequency (and can be distinguished by orthogonal user codes), among the signals arriving at the receiver, the signals of any one user are interference covering the same frequency for other users. If the signal power of each user is high or low, then the high power user will drown out the signal of the low power user. Therefore, the CDMA system adopts the way of power control, and sends power control instructions to each terminal for the power of different users who arrive at the receiver (we call it air interface power, referred to as air port power). Finally, the air port power of each user is the same. This kind of power control has two characteristics: first, 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 LTE system, uplink power control also has the function of interference suppression. Because the uplink of LTE is SC-FDMA, multi-users also share carrier frequency and interfere with each other, so it is also necessary to have the same empty port power.

GSM system also has power control, in GSM we use "power level" to characterize the power control step, each level of 1dB, it can be seen that GSM power control is relatively rough.

Interference limited system

Here is a related concept: interference limited system. 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 deinterleaving, then in fact, the capacity of the CDMA system can be infinite, because it can distinguish an infinite number of users with layers of extended user codes on limited frequency resources. But in fact, because the user code can not be completely orthogonal, it is inevitable to introduce noise in multi-user signal demodulation, the more users, the higher the noise until the noise exceeds the demodulation threshold.

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

GSM system is not an interference limited system, it is a system limited in time domain and frequency domain, and its capacity is limited by frequency (200kHz one carrier frequency) and time domain resources (8 TDMA users can be shared on each carrier frequency). Therefore, the power control requirements of the GSM system are not high (rough step size, long cycle).

9.5 transmitter power control and transmitter RF index

After talking about the transmitter power control, we go on to discuss the factors that may affect the transmitter power control in the RF design (I believe many colleagues have encountered the depressed scenario of closed-loop power control test).

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

Because transmitter calibration is actually only carried out at a limited number of frequency points, especially in production testing, the fewer frequency points are done, the better. But in the actual working scene, it is possible for the transmitter to work in any carrier in the frequency band. In the typical production calibration, we will calibrate the high, middle and low frequency points of the transmitter, which means that the transmission power of the high, medium and low frequency points is accurate, so the closed-loop power control is correct at the calibrated frequency points. However, if the transmission power of the transmitter is uneven in the whole frequency band, there is a large deviation between the transmission power of some frequency points and the calibration frequency point, so the closed-loop power control with reference to the calibration frequency point will also have large errors and even errors at these frequency points.

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