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The internal and significance of common RF indicators

Click:49 Time:2020-10-25 14:08:23

1. Rx Sensitivity (receiving sensitivity)

Receiving sensitivity, which should be one of the most basic concepts, characterizes the lowest signal strength that the receiver can recognize without exceeding a certain bit error rate. The bit error rate mentioned here is a general term that follows the definition of the CS (circuit switched) era. In most cases, BER (bit error rate) or PER (packet error rate) will be used to examine the sensitivity. In the LTE era, simply use throughput. To be defined by the quantity 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 such as 12.2kbps RMC (reference measurement channel, which actually represents a rate of 12.2kbps The “standardized alternatives” to measure sensitivity are defined by the throughput that users can actually feel.

2. SNR (signal to noise ratio)

When talking about sensitivity, we often refer to SNR (signal-to-noise ratio, we generally talk about the demodulation signal-to-noise ratio of the receiver). We define the demodulation signal-to-noise ratio as the ability of the demodulator to not exceed a certain bit error rate. SNR threshold for demodulation (in interviews, someone will often ask you questions, give you a string of NF and Gain, and then tell you that the demodulation threshold requires you to push the sensitivity). So where do S and N come from?

S means Signal, or useful signal; N means Noise, which generally refers to all signals without useful information. The useful signal is generally emitted by the communication system transmitter, and the source of noise is very wide. The most typical one is the famous -174dBm/Hz-natural noise floor. Remember that it is a quantity independent of the type of communication system. In a sense, it is calculated from thermodynamics (so it is related to temperature); in addition, it should be noted that it is actually a noise power density (so it has the dimension of dBm/Hz), how much bandwidth do we receive? , It will accept the noise of the bandwidth-so the final noise power is obtained by integrating the noise power density over the bandwidth.

3. TxPower (transmission power)

The importance of the transmission power is that the signal from the transmitter needs to pass through the fading of space to reach the receiver, so the higher the transmission power means the longer the communication distance.

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

This involves the concept just mentioned, the natural noise floor. We assume that spatial fading has the same effect on both signal and noise (actually not, the signal can resist fading through coding but noise is not good) and it acts like an attenuator, then we assume spatial fading is -200dB and the transmitted signal bandwidth is 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), and the noise of the transmitter 50-50=0dBm through spatial fading, and the power reaching the receiver is 0-200=-200dBm (bandwidth 1Hz)? At this time, this part of the noise has already been "submerged" under the natural noise floor of -174dBm/Hz. At this time, when we calculate the noise at the receiver entrance, we only need to consider the "basic component" of -174dBm/Hz.

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 transmitting channel, but the part of the transmitter leaking into the adjacent channel, which can be collectively called "Leakage from adjacent channels."

ACLR and ACPR (actually one thing, but one is called in the terminal test, the other is called in the base station test), both are named after "Adjacent Channel", as the name implies, they both describe the machine pair Interference from other equipment. And they have one thing in common, the power calculation of the interference signal is also based on a channel bandwidth. This measurement method shows that the design purpose of this indicator is to consider the signal leaked by the transmitter and the interference to the equipment receiver of the same or similar standard-the interference signal falls into the receiver band in the same frequency and bandwidth mode. Form the same frequency interference to the signal received by the receiver.

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

This definition has very important practical significance. In the actual network, signals often leak from neighboring cells and neighboring cells in the same cell. Therefore, the process of network planning and optimization is actually the process of capacity maximization and interference minimization, and the adjacent channel leakage of the system itself is typical for neighboring cells. From the other side of the system, the mobile phones of users in crowded people may also become a source of interference.

Similarly, in the evolution of communication systems, the goal has always been to "smooth transition", that is, to upgrade and transform existing networks into next-generation networks. So the coexistence of two or even three-generation systems requires consideration of the interference between different systems. The introduction of UTRA in LTE is to consider the radio frequency interference of LTE to the previous-generation system when it coexists with UMTS.

5. Modulation Spectrum/Switching Spectrum

Back to the GSM system, Modulation Spectrum (modulation spectrum) and Switching Spectrum (switching spectrum, also called switching spectrum, due to different translations of foreign products) also play similar roles in adjacent channel leakage. The difference is that their measurement bandwidth is not the occupied bandwidth of the GSM signal. From the point of view of definition, it can be considered that the modulation spectrum is a measure of the interference between synchronous systems, and the switching spectrum is a measure of the interference between asynchronous systems (in fact, if the signal is not gating, the switching spectrum will definitely overwhelm the modulation spectrum. ).

This involves another concept: in the GSM system, the cells are not synchronized, although it uses TDMA; in contrast, TD-SCDMA and later TD-LTE, the cells are synchronized (The GPS antenna with the shape of a flying saucer or a spherical head is always the shackles that the TDD system cannot get rid of).

Because the cells are not synchronized, the power leakage of the rising edge/falling edge of the A cell may fall to the payload part of the B cell, so we use the switching spectrum to measure the interference of the transmitter to the adjacent channel in this state; and in the entire 577us GSM In timeslot, the proportion of rising edge/falling edge is very small after all. Most of the time, the payload of two adjacent cells will overlap in time. In this case, the interference of the transmitter to the adjacent channel can be evaluated by referring to the modulation spectrum.

6. SEM (Spectrum Emission Mask)

When talking about SEM, the first thing to note is that it is an "in-band indicator", which is distinguished from spurious emission. The latter includes SEM in a broad sense, but the focus is actually on the spectrum leakage outside the working frequency band of the transmitter. , Its introduction is more from the perspective of EMC (electromagnetic compatibility).

SEM is to provide 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 is related to ACLR, but it is not the same: ACLR considers the average power leaked into the adjacent channel, so it uses the channel bandwidth as the measurement bandwidth, which reflects the "noise floor" of the transmitter in the adjacent channel; SEM reflects the use of a smaller measurement bandwidth (usually 100kHz to 1MHz) to capture the over-standard points in the adjacent frequency band, which reflects the "noise floor-based spurious emission."

If you scan the SEM with a spectrum analyzer, you can see that the spurious points on the adjacent channels will generally be higher than the ACLR average, so if the ACLR indicator itself has no margin, the SEM will easily exceed the standard. On the other hand, if the SEM exceeds the standard, it does not necessarily mean that the ACLR is bad. A common phenomenon is that there is LO spurious or a certain clock and LO modulation component (often very narrow bandwidth, similar to dot frequency) stringing into the transmitter link. ACLR is good, and SEM may exceed the standard.

7. EVM (error vector)

First of all, EVM is a vector value, which means it has amplitude and angle. It measures "the error between the actual signal and the ideal signal". This measurement can effectively express the "quality" of the transmitted signal-the point distance of the actual signal. The farther the ideal signal is, the greater the error and the greater the modulus of the EVM.

In (1), we have explained why the signal-to-noise ratio of the transmitted signal is not so important. There are two reasons: the first is that the SNR of the transmitted signal is often much higher than the SNR required for demodulation by the receiver; the second is that we calculate The receiver sensitivity refers to the worst case of the receiver, that is, after a large spatial fading, the transmitter noise has already been submerged under the natural noise floor, and the useful signal is also attenuated to near the demodulation threshold of the receiver.

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

When the 802.11 series evolved to 802.11ac, 256QAM modulation was introduced. For the receiver, even if spatial fading is not considered, just demodulating such high-order quadrature modulated signals already requires a high signal-to-noise ratio. The worse the EVM, the worse the SNR, and the higher the demodulation difficulty.

Engineers working on 802.11 systems often use EVM to measure Tx linearity; while engineers working on 3GPP systems, they like to use ACLR/ACPR/Spectrum to measure Tx linearity.

From the origin, 3GPP is the evolution path of cellular communication, and from the very beginning, it has to pay attention to the interference of adjacent channel and alternative channel (adjacent channel, alternative channel). In other words, interference is the number one obstacle that affects cellular communication rates. Therefore, 3GPP always aims to "minimize interference" in the process of evolution: frequency hopping in the GSM era, spread spectrum in the UMTS era, and LTE era. The introduction of the RB concept is the same.

The 802.11 system is an evolution of fixed wireless access. It is based on the spirit of the TCP/IP protocol and aims at "service with the best possible ability". In 802.11, there are often time division or frequency hopping methods to achieve multi-user coexistence. The network layout is more flexible (after all, it is mainly a local area network), and the channel width is also flexible and variable. In general, it is not sensitive to interference (or relatively high tolerance).

In layman's terms, the origin of cellular communication is to make calls, and users who cannot get through the phone will go to the telecommunications bureau to break the market; the origin of 802.11 is the local area network, and the probability of a bad network is to be patient and so on (in fact, the equipment is working at this time. Error correction and retransmission).

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

Specifically, "at the expense of speed to adapt to the network environment" 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 informs the opposite transmitter to reduce the modulation order. vice versa. As mentioned earlier, SNR and EVM in an 802.11 system are highly correlated, and to a large extent, EVM reduction can improve SNR. In this way, we have two ways to improve the receiving performance: one is to reduce the modulation order, thereby reducing the demodulation threshold; the other is to reduce the transmitter EVM, so that the signal SNR is improved.

Because EVM is closely related to the demodulation effect of the receiver, EVM is used to measure transmitter performance in the 802.11 system (similarly, in the cellular system defined by 3GPP, ACPR/ACLR is the index that mainly affects network performance); The deterioration of EVM is mainly caused by non-linearity (for example, PA's AM-AM distortion), so EVM is usually used as a measure of transmitter linear performance.

7.1 The relationship between EVM and ACPR/ACLR

It is difficult to define the quantitative relationship between EVM and ACPR/ACLR. From the non-linearity of the amplifier, EVM and ACPR/ACLR should be positively correlated: the AM-AM and AM-PM distortion of the amplifier will enlarge the EVM, and it is also ACPR/ACLR. Main source.

However, EVM and ACPR/ACLR are not always positively correlated. Here we can find a very typical example: Clipping, which is commonly used in digital intermediate frequency, is peak clipping. Clipping is to reduce the peak-to-average ratio (PAR) of the transmitted signal. The reduction of peak power can help reduce the ACPR/ACLR after passing through the PA; however, Clipping will also damage the EVM, because whether it is clipping (windowing) or using a filter method, Will cause damage to the signal waveform, thus increasing the EVM.


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