First of all, a misunderstanding is: the more the antenna coverage, the more the antenna signal stronger
MIMO (Multiple-entry) is the technology of multiple antennas is from the 802.11N protocol only after the 802.11a,b,g have not. That is to say, the first generation of routers (802.11n ago) will never have more than one antenna. And you bought a new 3-day line to support 802.11AC (the latest protocol) of the router, if your device is an old product, such as only support 802.11a,b,g Iphone3, then it is regrettable that so many antennas do not make any sense to you. If you want to shoot multiple antennas at the same time, it will not have a good effect.
Why do you say that? The first WiFi application environment is indoors. The 802.11-series protocols we use are also built on this condition. That is because there are many buildings or obstacles, there is almost no direct signal between the transmitter and receiver. We call this multipath transmission. Since it is multipath, then the transmission of the distance is long and short, some may be reflected from the table, and some may be through the wall. So these signals that carry the same information but have different phases converge together on the receiver. We know that modern communication uses packet switching, which transmits code (symbol). Due to the different delays mentioned above, the ISI (intersymbolinterference) is caused by inter code interference. To avoid ISI, the bandwidth of communications must be less than the reciprocal of tolerable latency.
For the bandwidth of 802.11a,b,g20mhz, the maximum time delay is 50ns, and the transmission radius of ISI is 15m under multipath conditions. In the IEEE802.11 agreement we can see that its maximum range is 35m, which is also the protocol in the error code retransmission and other means to ensure communication, not to say that there is a little ISI can not work at all.
In other words, the launch range of the router is in fact the protocol decision. For 802.11a,b,g, an increase in antenna has no meaning. Assuming these antennas work at the same time, the multipath effect can be even worse.
In the Wikipedia link (IEEE802.11) we can find that from the 802.11n, the data has been greatly improved, first 802.11n has 40MHz mode, according to the previous theory, his launch range should be reduced by half, and the data is increased by one-fold (70m), Why, then?
This is mainly due to the multi-antenna technology, we discussed a variety of means to combat the harsh multipath environment, but there is no good side of multipath? In fact, multi-antenna technology is also based on Multipath, which we call spatial diversity. Multi-antenna applications have a variety of technical means, here a brief introduction to 2 kinds: beamforming (beamforming) and space-time block code (mainly introduced Alamouti ' Scode). The advantage of both techniques is that there is no need for multiple receive antennas. In particular, Alamouti code, not even the channel information, using only mathematical operations with two antennas to achieve the gain of 3dB, all teachers are full of praise!
The advantage of not needing multiple receiving antennas is that not all devices can be fitted with multiple antennas. In order to avoid sidelobe radiation and satisfy the spatial sampling theorem, it is generally used to transmit half wavelength of the signal as the antenna spacing of the entity. Whether it is GSM signal 1.8ghz,1.9ghz or WiFi signal 2.4GHz, we temporarily take 2GHz for easy calculation, half wavelength of 7.5cm. So we see most of the distance on the router's antenna. For this reason, it is difficult to install multiple antennas on the phone (don't mention the 7-inch Samsung phone thanks).
1, Beam forming (beamforming): By a number of antennas to produce a directional beam, the energy concentration in the direction of transmission, increase signal quality, and reduce interference with other users. We can simply generalize this way to understand the antenna's directivity: Assuming that the omni-directional antenna power is 1, the range of only 180-degree directional antenna power can reach 2. So we can theoretically increase the power by 4 times times with 4 90-degree antennas. Another mode of beamforming is to judge the azimuth of the receiver by channel estimation, and then to launch the point to improve the transmitting power. (similar to a spotlight flashlight, the smaller the range, the brighter the light). But it's not clear which protocol the model is applied to.
2, Space-time block code STBC (space-timeblockcode) is to send different information at different times on multiple antennas to improve the reliability of the data. Alamouti code is the simplest one in space-time block codes. In order to transmit d1d2 two yards, d1,-d2* and d2,d1* are sent on two antennas 1,2 respectively. Because of multipath, we assume that the channel of two antennas is h1h2, so the first time the receiver received the information R1=D1H1+D2H2, then the information received R2=-D2*H1+D1*H2. The received 2-D square is multiplied by the channel, and the D1D2 information is available ... Well, it seems that there is no explanation, no way to take notes is not around, searched a circle also did not find the right material. All in all, it's alamouti. Find a set of orthogonal rate of a 2x2 matrix, in this way, two antennas can be emitted from each other, can be used to receive a single antenna, after the mathematical operation of the method of transmitting information.
Other MIMO, in terms of the concept may be better understood, such as 2 transmitting antenna t1t2 respectively to two receiving antenna R1R2 launch, then the equivalent of two people working at the same time, speed up twice times and so on. However, it is very complicated and time-consuming to compute the hardware, on the one hand, we need multiple receiving antennas on the hardware, and channel estimation and other communication algorithms.
Speaking of the above two actually is the Miso method also wants to prove from another aspect, the antenna is many does not represent them to work together. 100 years ago, people knew that the more the antenna the better the better, but the genius of the Alamouti code 1998 years before the multi-antenna technology 802.11n protocol began to apply for 2009 years.
20 years ago, when people used OFDM to combat multipath decline due to too many urban or indoor barriers, we have now started to use Multipath to improve communication quality. This is a technological leap forward, not simply "take for granted". Because the class notes are not around, always feel some not too big grasp of the place. For a three-day router that is supposed to work in 802.11a,b,gsiso mode, can deny a larger gain for 3 antennas? " And "is the device between two bridging mode routers downloading data from both routers at the same time, how do I sync?" Have doubts. After all, there is still a distance from the knowledge of the book to the practical application, so if there is something wrong, please correct me.