"The more antennas you cover, the more antennas you use, the more signals you have, and the more antennas you have, the better."
——Friends who feel that they are “common sense” can continue to read down the text. They feel that this kind of topic is weak, and Xiao Bian’s estimate of what will not come in. Still in that sentence, most of our dry cargo posts are for literacy. You are welcome to the gods to supplement and correct them...
First of all, we should also note that the antenna of the old generation wireless router will certainly not exceed one. The "older generation" here refers to the 802.11a/b/g route before the 802.11n protocol. The old 54M product has only one antenna.
In this case, 802.11n clearly became a watershed, and from then on, the antenna is no longer a solitary one (the 150M of the 1T1R is an exception).
What the hell is it all about?
Here we need to mention a multi-antenna technology that is applied after an 802.11n protocol, and it is also a very important technology in the field of wireless communications - MIMO (Multiple-Input Multiple-Output).
Let's look at an example first. Some people say why did I buy a latest 3-antenna wireless router that supports the 802.11ac protocol, and the result is that the signal strength, coverage, and even speed are not up? Not enough antenna?
Tell you, 300 is useless, check that you accept the terminal support does not support 802.11ac agreement.
For example, if you use the iPhone 3, this phone can only support 11a/b/g even 802.11n can not talk about, then even if you add a few antennas to this cargo is useless. How to deal with it? Add an AC network card or change the terminal, in short, do not compete with the antenna.
why would you say so? First of all, the Wi-Fi application environment is indoors, and our commonly used 802.11 series protocols are also built for this condition.
Since there are various obstacles between the transmitter and the receiver, there is almost no possibility of direct signal transmission and reception. What should I do? We control this method called multipath transmission, also called multipath effect.
Multipath, which is also well understood literally, is to increase the transmission path.
Then the question arises. Since it is multipath, the length of the journey may vary, some may be reflected from the table, some may be through the wall, and these signals carrying the same information but having different phases will eventually turn together. Collected on the receiver.
Modern communication uses packet forwarding for store-and-forward, also called packet switching, and transmits symbols. Because of the different transmission delays caused by the obstacles, ISI (Inter Symbol Interference) is caused. To avoid ISI, the bandwidth of the communication must be less than the reciprocal 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 IEEE 802.11 protocol, we can see that the maximum range of this value is 35m. This means that there are error codes and retransmissions in the protocol to ensure communication. It does not mean that ISI will not work at all.
In this case, you will find that even if you add more antennas to the 802.11a/b/g protocol, it does not make any sense. Assuming that these antennas can work simultaneously, it will make the multipath effect worse.
It does not matter that everyone behind you can't see it. In short, the wireless router's transmission range is determined by this IEEE 802.11 protocol, rather than simply looking at the antenna.
summary
Having said so much, is there a difference between single-antenna routing, dual-antenna routing, three-wire, four-wire, and even more?
Yes, but the impact on the actual use of the process is not large, including signal coverage, signal strength, antenna speed is even more nonsense.
Leaving aside a single antenna that has rarely been seen, the remaining “multi-antennas” are just “media” or “tools” for MIMO technology. The difference lies in the architecture used: Common dual-antenna products use 1T2R or 2T2R, 3 antenna products use 2T3R or 3T3R.
Theoretically, increasing the number of antennas will reduce blind spots in signal coverage, but we have confirmed through extensive evaluation that this difference is completely negligible in an ordinary home environment. Moreover, just as the built-in antenna does not lose its external position, three-antenna coverage is not as good as double The situation of the antenna is no exception. After all, product quality is also an important factor.
As for the signal strength and “through the wall” it depends on the transmission power. This Ministry of Industry and Information Technology of the East has provided regulations that must not be higher than 20dBm (ie, 100mW). “The more signals the antenna has, the stronger the signal” will not break.
The final conclusion is that as long as the routing uses effective MIMO technology, there is no need to care about the number of antennas.
Next, we will further understand the miracle of MIMO technology. The content may be somewhat jerky. If you are interested, you can look again.
MIMO technology
Searching various encyclopedias of IEEE802.11 entry, we can read that starting from 802.11n, the data transmission rate or the amount of data carried has greatly improved.
First of all, 802.11n has a 40MHz model, but according to the previous theory, its launch range should be reduced by half, but in reality the data is doubled (70m). What is this all about?
This will benefit from MIMO technology. The various methods we discussed just now are to fight against the harsh multipath environment. But is there a good side to multipath?
In fact, MIMO is also based on multipath, which we call spatial diversity. The application of multiple antennas has many technical means. Here are two simple introductions: beamforming and spatio-temporal block codes (mainly introducing Alamouti's code).
The advantage of these two technologies is that they do not require multiple receiving antennas, especially the Alamouti code. Even if channel information is not used, only two antennas can be used to achieve a gain of 3dB using mathematical operations.
The advantage of not requiring multiple receiving antennas is that not all devices can be equipped with multiple antennas. In order to avoid side-lobe radiation (in the antenna pattern, the maximum radiation beam is called the main lobe, and the small beam next to the main lobe is called the side lobe), and satisfies the spatial sampling theorem, one half-wavelength of the transmitted signal is generally used as the physical antenna spacing.
Whether it is GSM signal 1.8 GHz, 1.9 GHz or 2.4 GHz, 5 GHz of Wi-Fi signal, we temporarily take 2 GHz to calculate, the half wavelength is 7.5 cm.
So, we see most of the distance of the antenna on the router, and it is precisely because of this, it is very difficult to install multiple antennas on the mobile phone.
Beamforming
By using multiple antennas to generate a directional beam, the energy is concentrated in the direction that you want to transmit, increase the signal transmission quality, and reduce the interference with other users.
We can understand the directivity of the antenna in a simple and general way: Assuming that the omnidirectional antenna power is 1, the directional antenna power with a range of only 180 degrees can reach 2, so we can theoretically increase the quadruple antenna with four 90-degree antennas by a factor of four. The power.
Another mode of beamforming is to estimate the receiver's position through the channel, then directional to that point to increase the transmit power (similar to the spotlight flashlight, the smaller the range, the brighter the light). The predecessor of smart antenna technology is beamforming.
Space-Time Block Code (STBC)
Different information is sent at different times on multiple antennas to improve data reliability. The Alamouti code is the simplest type of space-time block code.
In order to transmit the d1d2 two codes, d1, -d2 and d2, d1 are transmitted on the two antennas 1, 2 respectively. Due to multipath, we assume that the channels of the two antennas are h1h2 respectively, so the information received by the receiving end at the first moment is r1=d1h1+d2h2, and then the received information r2=-d2h1+d1h2. The received 2-D square matrix is multiplied by the channel to obtain the d1d2 information.
Do not understand it does not matter, in short, Alamouti is to find a set of orthogonal code rate of 2 × 2 matrix, in this way the two antennas can transmit without affecting each other; can use an antenna to receive, after mathematical operations to get The method of transmitting information.
Other MIMO may be conceptually better understood. For example, two transmit antennas t1t2 transmit to two receive antennas r1r2, respectively, which is equivalent to two people working at the same time, the speed is increased by 2 times, and so on, but it is actually realized. In terms of hardware, multiple receive antennas are required on the one hand, and communication algorithms such as channel estimation are required on the other hand, which are all very complex and time-consuming hardware-consuming calculations.
The above two technologies are actually MISO (Multiple-Input Single-Output) methods, and they also want to prove from another aspect that more antennas do not mean that they can work together.
One hundred years ago, people knew that the more antennas the better, the bigger the better. However, the genius Alamouti code was only introduced in 1998. The 802.11n protocol of multi-antenna technology only began to be applied in 2009.
Twenty years ago, people used OFDM (Orthogonal Frequency Division Multiplexing, a technology of multi-carrier modulation) to combat multipath fading caused by too many obstacles between cities or rooms, and now we have begun to use more Way to improve communication quality. This is a swift advancement in technology, not a simple "takes it for granted".
Write last
MIMO itself is a time-varying and unstable multi-input multi-output system. The research on MIMO is a worldwide issue. There are still many questions left, and even the same questions may have different arguments in academics.
However, for the average consumer, I don’t need to go deeper and understand the “misunderstandings” we mentioned on the first page. We know that the routing antenna is a “tool” and that ordinary home dual antennas are sufficient. When seeing the product specifications when purchasing, do not be misled by the business.
