From Evernote:

802.11n and MIMO Clipped from: http://www.dsprelated.com/showmessage/98298/1.php

In radio, multiple-input and multiple-output, or MIMO (commonly pronounced my-moh or me-moh), is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology. Note that the terms input and output refer to the radio channel carrying the signal, not to the devices having antennas.

MIMO 示意图，多个发射天线，多个接受天线，每一对之间为一个 path(?)，

这些 pathes 都在同一个 radio frequency channel 上。

In MIMO systems, a transmitter sends multiple streams by multiple transmit antennas. The transmit streams go through a matrix channel which consists of all paths between the transmit antennas at the transmitter and receive antennas at the receiver. Then, the receiver gets the received signal vectors by the multiple receive antennas and decodes the received signal vectors into the original information. A narrowband flat fading MIMO system is modelled as

where and are the receive and transmit vectors, respectively, and and are the channel matrix and the noise vector, respectively.

MIMO 有一项技术是 spatial multiplexing。数据分为 streams，streams 由不同的天线发射。

Spatial multiplexing requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams into (almost) parallel channels. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher signal-to-noise ratios (SNR). The maximum number of spatial streams is limited by the lesser of the number of antennas at the transmitter or receiver. Spatial multiplexing can be used with or without transmit channel knowledge. Spatial multiplexing can also be used for simultaneous transmission to multiple receivers, known as space-division multiple access. The scheduling of receivers with different spatial signatures allows good separability.

802.11n 使用了 MIMO 技术，理论上，使用 40MHz 带宽，每个stream 比特率可以达到 150 Mbps，最多可以有 4 streams. 故理论比特率为 600 Mbps.

目前，3 streams 的路由器已经算是比较新的产品。

The number of simultaneous data streams is limited by the minimum number of antennas in use on both sides of the link. However, the individual radios often further limit the number of spatial streams that may carry unique data. The a x b : c notation helps identify what a given radio is capable of. The first number (a) is the maximum number of transmit antennas or RF chains that can be used by the radio. The second number (b) is the maximum number of receive antennas or RF chains that can be used by the radio. The third number (c) is the maximum number of data spatial streams the radio can use. For example, a radio that can transmit on two antennas and receive on three, but can only send or receive two data streams would be 2 x 3 : 2.

The 802.11n draft allows up to 4 x 4 : 4. Common configurations of 11n devices are 2 x 2 : 2; 2 x 3 : 2; and 3 x 2 : 2. All three configurations have the same maximum throughputs and features, and differ only in the amount of diversity the antenna systems provide. In addition, a fourth configuration, 3 x 3 : 3 is becoming common, which has a higher throughput, due to the additional data stream.^[5]

802.11n is an amendment which improves upon the previous 802.11 standards by adding multiple-input multiple-output antennas (MIMO). 802.11n operates on both the 2.4 GHz and the lesser used 5 GHz bands. It operates at a maximum net data rate from 54 Mbit/s to 600 Mbit/s. The IEEE has approved the amendment and it was published in October 2009.^[14][15] Prior to the final ratification, enterprises were already migrating to 802.11n networks based on the Wi-Fi Alliance's certification of products conforming to a 2007 draft of the 802.11n proposal.

3T2R 怎么传 2 streams

Diversity Coding techniques are used when there is no channel knowledge at the transmitter. In diversity methods, a single stream (unlike multiple streams in spatial multiplexing) is transmitted, but the signal is coded using techniques called space-time coding. The signal is emitted from each of the transmit antennas with full or near orthogonal coding. Diversity coding exploits the independent fading in the multiple antenna links to enhance signal diversity. Because there is no channel knowledge, there is no beamforming or array gain from diversity coding.

Consider a 3x2 MIMO system, 3 antennas in TX and 2 in RX, where 2 spatial streams are transmitted and Alamouti is applied in one of the spatial streams. So at time 0 we transmit for each antenna : [a(0) a(1) b(0)], where a(i) means modulation symbol i from spatial stream 1 and b(i) means modulation symbol i from spatial stream 2. Then at time 2 it is transmitted [-a(1)* a(0)* b(1)]. So Alamouti is being applied to stream 1 and not to stream 2.