No Line by knowledge misunderstanding

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More antennas, better routing?

"The more the antenna coverage, the more the antenna more signal stronger, in short, the more the antenna routing is better"-feel very "common sense" friends can continue to look down the body. For you to understand the relationship between the number of antennas and signal strength!

First of all, we should also note that the old generation of wireless router antenna is certainly not more than one, where the "old generation" refers to the 802.11N protocol before the 802.11a/b/g route, the old 54M products have only one antenna. In this case, 802.11n is clearly a watershed, and since then the antenna is no longer only a lone one (1T1R 150M is an exception), what exactly is that? Here we will mention a 11n protocol to get the specific application of the multiple antenna technology, is also a very important technology in the field of wireless communications--mimo (Multiple-input Multiple-output, more than one)

To take a look at an example, someone said, why I bought a new 3-day cable support 802.11AC protocol wireless router, resulting in signal strength, coverage and even speed did not go up? Not enough antennas? I'm telling you, 300 is useless, check your acceptance terminal support does not support AC Agreement bar. For example, you use the iphone 3, this phone can only support 11a/b/g not even 11n, so even if you give it to remove a few antennas is useless. How to solve it? It is no use to add an AC adapter or switch terminal.

Why do you say that? First of all, Wi-Fi application environment is indoor, our commonly used 802.11 series of protocols are also set against this condition. Because there are all kinds of obstructions between the transmitter and receiver, there is almost no possibility of direct signal when sending and receiving. What about that? This approach is called multipath transmission, also called multipath effect. Multipath, literally also very well understood, is to increase the transmission path.

So the problem is, since it is multipath, there are long and short distances, some may be reflected from the table, and some may be through the wall, which carry the same information but with different phases of the signal to the end together to the receiving end. Modern communication is the packet exchange of storage and forwarding, also called Packet Exchange, the transmission is code (SYMBOL). The ISI (intersymbolinterference) is caused by the different transmission delay of the barrier. To avoid ISI, the bandwidth of communications must be less than the reciprocal of tolerable latency.

For the bandwidth of the 802.11a/b/g 20MHz, the maximum time 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 the protocol also error code retransmission and other means to ensure communication, not to say that there is a little ISI can not work at all. In this case, you will find that for the 802.11a/b/g protocol, even the addition of more antennas does not make any sense. Assuming these antennas work at the same time, the multipath effect can be even worse.

In short, the launch range of the wireless router is the IEEE802.11 protocol, rather than simply looking at the antenna.


Said so much, a single day line by, two-day line by, three-line four lines or even more whether there is no difference? There is, but for the actual use of the process of the impact is not small, this includes signal coverage, signal strength, antenna speed is even more nonsense. Throw away the already rare single antenna, the remaining "multiple antennas" are just the realization of MIMO technology "media" or "tool", the difference is that the use of the architecture is different: the common two-antenna products are mainly used 1t2r or 2T2R, three antenna products are used 2t3r or 3t3r.

Theoretically, increasing the number of antennas will reduce the blind spot of the signal coverage, but we have demonstrated through a lot of evaluation that this difference is completely negligible in the ordinary family environment. Moreover, as the built-in antenna does not output external, the three antennas are not as good as the dual antenna case, in the final analysis of product quality is also an important factor. As for the signal strength and "through the wall" depends on the transmission power, this thing the Ministry of Information has made provisions, not higher than 20dBm (that is, 100mW), "the more the antenna signal stronger" will be self-defeating. The final conclusion is that, as long as the routing uses effective MIMO technology, no need to care about the number of antennas.

On the next page we'll get a closer look at the wonders of MIMO technology.

MIMO Technology

Search various encyclopedia data IEEE802.11 entry, we can read, starting from 802.11n, data transmission rate or the amount of data load has been greatly improved. First of all, 802.11n has a 40MHz mode, but according to the previous theory, its launch range should be reduced by half, but the fact that the data is actually increased by one (70m), what is this?

This is due to the benefits of MIMO technology, we discussed a variety of means to combat the harsh multipath environment, but there is no good side of multipath? In fact, MIMO is also based on Multipath, which we call spatial diversity. Multi-antenna application has many kinds of technical means, here simply introduces two kinds: beamforming (beamforming) and space-time grouping code (mainly introduces 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 can use two antennas to achieve 3dB gain, very good.

The advantage of not needing a plurality of receiving antennas is that not all devices can be fitted with multiple antennas. In order to avoid side lobe radiation (antenna pattern, the maximum radiation beam is called the main lobe, the small beam next to the main lobe is called the sidelobe), satisfying the space sampling theorem, generally to send half of the signal wavelength as the antenna spacing of the entity. Whether it is GSM signal 1.8ghz,1.9ghz or Wi-Fi signal 2.4GHz, we temporarily take 2GHz easy to calculate, half wavelength of 7.5cm. Therefore, we see the router on the antenna on most of the distance is so, it is very difficult for us to install multiple antennas on the phone.

Beamforming (beamforming): By using multiple antennas to produce a directional beam, the energy is concentrated in the direction of the desired transmission, increase the quality of signal transmission, 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 estimate the azimuth of the receiver through the channel, and then to point to the point of launch, to improve the launch power (similar to the spotlight of the flashlight, the smaller the range, the brighter the light). The predecessor of smart antenna technology is beam forming.

Space-time grouping code (space-time block code, i.e. STBC): sending different information at different times on multiple antennas to improve data reliability. 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 information received at the first time is R1=D1H1+D2H2, and then the information received R2=-D2*H1+D1*H2. The received 2-D square is multiplied by the channel, and the D1D2 information can be obtained. It doesn't matter, anyway, it's alamouti. Find a set of orthogonal code rates for the 2x2 matrix, in this way on two antenna emission can not affect each other, you can use an antenna to receive, 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 technologies is actually miso (multiple-input single-output) method, but also want to prove from another aspect, the antenna is too much does not mean they can 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, people used OFDM (orthogonal Frequency Division multiplexing, a technique called Orthogonal frequency Division Multiplexing, multicarrier modulation) to counteract multipath fading due to too much urban or indoor barriers, And now we've started using multipath to improve communication quality. This is a technological leap forward, not simply "take for granted".

PostScript language

MIMO itself is a time-varying, non-stationary, multiple-entry system. The study of MIMO, is a world subject, leaving a lot of questions, the same problem in the academic even will appear different argument. However, for the general consumer can not go into the deep, recognize the beginning of our "misunderstanding", know that the routing antenna is a "tool", ordinary family two antennas enough to see the product specifications, do not be misled by the business.

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