Learn about the antenna of the wireless router[Copy link]
First of all, you should have noticed that the old generation of wireless routers will definitely not have more than one antenna. The "old generation" here refers to the 802.11a/b/g routers before the 802.11n protocol. The old 54M products only have one antenna. In this case, 802.11n has obviously become a watershed, and it is also from then on that the antenna is no longer just a single one (1t1r's 150M is an exception). So what is going on? Here we have to mention a multi-antenna technology that was only put into practical use after the 11n protocol, and is also a very important technology in the field of wireless communications - MIMO (Multiple-Input Multiple-Output). Let's take a look at an example. Someone said, why did I buy a new model of wireless router with 3 antennas supporting the 802.11ac protocol, but the signal strength, coverage and even speed did not improve? Not enough antennas? Let me tell you, 300 antennas are useless. Check whether the receiving terminal you are using supports the AC protocol. For example, the iPhone 3 you are using only supports 11a/b/g, not even 11n. So even if you disassemble it and add a few antennas, it will be useless. How to solve it? Install an AC network card or change the terminal. In short, don't fight with the antenna. Why do we say so? First of all, the application environment of Wi-Fi is indoors, and the 802.11 series protocols we commonly use are also established for this condition. Since there are various obstacles between the transmitter and the receiver, there is almost no possibility of direct signal when sending and receiving. What should we do? We call this method multipath transmission, also known as multipath effect. Multipath, literally, is to increase the transmission path.
Then the question is, since it is multipath, the transmission distance may be long or short, some may be reflected from the table, and some may pass through the wall. These signals carrying the same information but with different phases are finally gathered together at the receiving end. Modern communications use storage and forwarding packet switching, also known as packet switching, which transmits codes (Symbols). Because obstacles produce different transmission delays, it causes inter-symbol interference (ISI). In order to avoid ISI, the communication bandwidth must be less than the inverse of the tolerable delay.
For 802.11a/b/g 20MHz bandwidth, the maximum delay is 50ns, and the transmission radius without ISI under multipath conditions is 15m. In the IEEE802.11 protocol, we can see that the maximum range of this value is 35m. This is because there are various means such as error retransmission in the protocol to ensure communication. It does not mean that a little ISI will not work at all. In this case, you will find that for the 802.11a/b/g protocol, even if more antennas are installed, it is meaningless. Assuming that these antennas can work at the same time, it will make the multipath effect worse. It doesn't matter if you can't read the following. In short, the transmission range of the wireless router is determined by the IEEE802.11 protocol, not just the antenna. Is there any difference between single-antenna routing, dual-antenna routing, three-wire, four-wire or even more? Yes, but it doesn't have much impact on actual use, including signal coverage and signal strength. It's even more nonsense to say that more antennas mean faster speeds. Aside from the rare single antenna, the remaining "multiple antennas" are just "mediums" or "tools" for implementing MIMO technology. The difference lies in the different architectures used: common dual-antenna products mainly use 1T2R or 2T2R, while three-antenna products use 2T3R or 3T3R. In theory, increasing the number of antennas will reduce signal coverage blind spots, but we have confirmed through a large number of evaluations that this difference can be completely ignored in an ordinary home environment. Moreover, just as the built-in antenna is not inferior to the external one, the situation that the coverage of three antennas is not as good as that of two antennas is not an isolated case. In the final analysis, product quality is also an important factor. As for the signal strength and "wall penetration", it depends on the transmission power. The Ministry of Industry and Information Technology has stipulated that it must not exceed 20dBm (i.e. 100mW). The "more antennas, the stronger the signal" is self-defeating. The final conclusion is that as long as the router uses effective MIMO technology, there is no need to care about the number of antennas.
In the next page, we will further understand the magic of MIMO technology. The content may be a little jerky. If you are interested, you can take a look. MIMO technology Searching various encyclopedias for IEEE802.11 entries, we can read that since 802.11n, the data transmission rate or the amount of data carried has been greatly improved. First of all, 802.11n has a 40MHz mode. However, according to previous theories, its transmission range should be reduced by half, but in fact the data has doubled (70m). Why is this?
This is due to MIMO technology. All the methods we discussed just now are to combat the harsh multipath environment, but does multipath have a good side? In fact, MIMO is also based on multipath, which we call spatial diversity. There are many technical methods for the application of multiple antennas. Here are two of them: beamforming and space-time block coding (mainly Alamouti's code). The advantage of these two technologies is that they do not require multiple receiving antennas. Especially Alamouti code, even channel information is not needed, and only mathematical operations can be used to achieve 3dB gain using two antennas. It's great, right? The advantage of not needing multiple receiving antennas is that not all devices can be equipped with multiple antennas. In order to avoid sidelobe radiation (on the antenna pattern, the largest radiation beam is called the main lobe, and the small beam next to the main lobe is called the side lobe) and meet the spatial sampling theorem, the physical antenna spacing is generally taken as one-half wavelength of the transmitted signal. Whether it is a GSM signal of 1.8GHz, 1.9GHz or a Wi-Fi signal of 2.4GHz, we will temporarily take 2GHz for ease of calculation, and the half wavelength is 7.5cm. Therefore, the distance between antennas on routers we see is mostly like this, and it is for this reason that it is difficult for us to install multiple antennas on mobile phones. Beamforming: Multiple antennas are used to generate a directional beam, which concentrates energy in the direction of desired transmission, increases signal transmission quality, and reduces interference with other users. We can simply and generally understand the directivity of the antenna in this way: assuming that the power of an omnidirectional antenna is 1, the power of a directional antenna with a range of only 180 degrees can reach 2. So we can use 4 90-degree antennas to theoretically increase the power by 4 times. Another mode of beamforming is to estimate the direction of the receiving end through the channel, and then transmit in a directionally targeted manner to increase the transmission power (similar to a focused flashlight, the smaller the range, the brighter the light). The predecessor of smart antenna technology is beamforming.
Space-Time Block Code (STBC): Send different information at different times on multiple antennas to improve data reliability. Alamouti code is the simplest type of space-time block code. In order to transmit the two codes d1d2, d1,-d2* and d2,d1* are sent on the two antennas 1 and 2 respectively. Due to multipath, we assume that the channels of the two antennas are h1h2, so the information received by the receiver at the first moment is r1=d1h1+d2h2, and the information received afterwards is r2=-d2*h1+d1*h2. The received 2D square matrix can be multiplied by the channel to get the information d1d2. It doesn't matter if you don't understand. In short, Alamouti found a set of orthogonal code rate 2 2 matrices. In this way, the transmission on two antennas can not affect each other; it can be received by one antenna, and the transmission information can be obtained after mathematical operations.
As for other MIMO, it may be easier to understand in concept. For example, two transmitting antennas t1t2 transmit to two receiving antennas r1r2 respectively, which is equivalent to two groups of people working at the same time, and the speed is increased by 2 times. However, in actual implementation, on the one hand, multiple receiving antennas are required in hardware, and on the other hand, communication algorithms such as channel estimation are required, which are very complex and time-consuming and hardware-consuming calculations. The above two technologies are actually MISO (Multiple-Input Single-Output) methods, which also want to prove from another aspect that more antennas do not mean that they can work together. 100 years ago, people knew that the more antennas, the better, and the bigger the better, but the genius Alamouti code was not proposed until 1998, and the 802.11n protocol with multi-antenna technology was not applied until 2009.
20 years ago, people used OFDM (Orthogonal Frequency Division Multiplexing, a multi-carrier modulation technology) to combat multipath fading caused by too many obstacles between cities or indoors, and now we have begun to use multipath to improve communication quality. This is a rapid development in technology, and it is not something that can be achieved simply by "taking it for granted". MIMO itself is a time-varying, non-stationary multiple-input and multiple-output system. The research on MIMO is a global topic, and there are still many questions left. The same problem may even have different opinions in academia. However, for ordinary consumers, there is no need to delve into this. Just recognize the "misunderstanding" we talked about on the first page and know that the router antenna is a "tool". Dual antennas are enough for ordinary households. When purchasing, read the product specifications carefully and don't be misled by the merchants.
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