 Hi, I'm Mr. T. with RVelement. In this video, we'll continue with the topic of MIMO, multiple input and multiple output systems. If you haven't seen the introductory part, make sure to check it out first. In the intro to MIMO, we talked about single-user MIMO, where the pre- and post-processing happens on both sides of a link, the access point and the CPE. SU-MIMO systems work poorly in line-of-sight scenarios. To operate well, they need a scattering-rich environment, so the signal is reflected, scattered and diffracted from the surroundings, ensuring that the signals arriving at different antennas are not the same. In a MU-MIMO system, the pre- and post-processing only happens on the side of the access point, which communicates with multiple independent CPEs at the same time. There are two major differences between the single and multi-user MIMO on system level. First is the channel state information, or sounding. Before the link with each CPE can be established, access point needs to know how the propagation channel to each CPE looks like, so the access point sends the test data to every client station based on the predetermined knowledge of the test data. The client stations can calculate the channel response and send it back to the access point, which then uses it during the download for correct operation. Second, CPEs need to be synchronized for correct upload function. During the upload, the signals from all client stations must arrive at the access point at the same time. Since they are typically in different physical locations, the client stations need to know what delay to introduce such that the signals arrive at the access point simultaneously. Once the MU-MIMO system is set up, the links can be established. MU-MIMO systems typically use antenna arrays to maximize signal-to-noise ratio of signal each CPE receives. If you want to know more about antenna arrays, you can check our previous video through the link in the corner or in the video description. Depending on the number of the client stations, there is a corresponding number of independent data streams the access point works with. For each data stream, the antenna array is configured such that the main beam of the radiation pattern is aimed at the corresponding client station. So the individual setup of the antenna array is different for each client station. Since all antenna array setups for each CPE are known, the final radiation pattern is a sum of the individual patterns. Because of that, there is a limit to how many CPEs can be served at the same time. Minimum beam with size limits how close the served client stations can be. If the stations are too close, they are covered by the same beam and therefore cannot be separated from each other in a logical sense. To be able to do so, the beam width would have to shrink even more, which is possible by increasing the size of the antenna array. There are two major ways to determine which client stations can be grouped and served at the same time. First is to search for the maximum achievable SNR among all the client stations and picking those that have the best SNR available. Second is to search for the opposite, to find directions to which the nulls are steered to aim them at the CPEs that are not part of the served MU MIMO group. It is important to understand that the transmission of the signals to all CPEs happens at the same time and at the same frequency channel. During the download, each CPE receives all the signals containing all the data streams involved. Thanks to the signal processing techniques and the knowledge of the propagation channel characteristics between the access point and the client station, each of them can pick the data stream that is meant for them. During the upload, synchronized CPEs send their data to the access point, which again, thanks to the knowledge of the propagation channel characteristics associated with each client station and the signal processing techniques, then sorts through the signals received and separates them into individual data streams. If you find that video useful, consider subscribing to our channel, like or let us know in the comments what is your opinion on the maturity of MU MIMO technology in today's world.