Multi-User MIMO (MU-MIMO) and Single-User MIMO (SU-MIMO) are two antenna technologies used in modern wireless networks, including 4G and 5G mobile networks. While both MU-MIMO and SU-MIMO are part of the overall Multiple Input Multiple Output (MIMO) technology, there are key differences between the two that we will cover in this post.
MU-MIMO uses radio communication layers from multiple cellular network antennas to support multiple mobile devices simultaneously; SU-MIMO uses radio communication layers from multiple antennas to support a single device at a time. Both MU-MIMO and SU-MIMO are used in 4G and 5G mobile networks.
There seems to be a general misconception around Multi-User MIMO as some people consider it just to be a characteristic of the Massive MIMO antenna technology, which is used by 5G New Radio networks. However, multi-User is just a type of MIMO and can exist in any MIMO variant, including Massive MIMO in 5G and the regular MIMO in 4G LTE. For context, multi-user MIMO is also used by other modern wireless networks, including WiFi6.
Different communication layers in MU-MIMO and SU-MIMO
Multiple Input Multiple Output (MIMO) technology in 4G LTE and 5G networks is based on the principles of spatial multiplexing, also known as space-division multiplexing (SDM). SDM consists of a number of antenna elements built into an antenna panel in such a way that they are physically separated in space. With multiple antenna elements built into the antenna panels of the transmitter and receiver, the communication between the base station and the mobile phone takes in various layers. This way, the antenna elements can communicate multiple data streams in parallel between the transmitter and the receiver by efficiently utilising the same time and frequency resources. For example, the original LTE uses a MIMO configuration of 4 x 4 for downlink, which means there are four layers of communication in the downlink, i.e. from the base station to the mobile phone. Since these layers can carry separate streams of data, depending on the data rate required for a particular user device, the network can decide how to utilise the available data rate. For example, suppose the network decides to offer a higher data rate to a single user at a time (e.g. for watching a 4k video). In that case, it may use Single-User MIMO to allocate all the available layers to one user device at a given time. However, if the network decides to accommodate multiple simultaneous data sessions at an instance, it may use Multi-User MIMO to share the available data rate with multiple users.
MU-MIMO and SU-MIMO in 4G and 5G base stations
Like any MIMO technology, multi-user and single-user MIMO need to be built into the network and the user device for the system to use them. On the network side, as part of the mobile radio network, the Multi-User MIMO (MU-MIMO) and Single User MIMO (SU-MIMO) capabilities are built into the cellular base station (cell tower). In 4G LTE networks, the cellular base station is generally the eNodeB or eNB, whereas, in 5G NR networks, the base station is called the gNodeB (gNB). In a 5G network deployment where a 5G core network is used for accommodating both 5G and 4G devices, the base station is called next-generation evolved node B or ng-eNB. The antenna panels within the transmitter and the receiver of eNB, gNB and ng-eNB need to support MU-MIMO and SU-MIMO for the network to accommodate single or multiple simultaneous users. The same is required on the mobile phone side also.
Multi-User MIMO in 5G Massive MIMO
The multi-user aspect of MIMO often becomes a topic of discussion in 5G NR networks that support Massive MIMO. Multi-User support is one of the key characteristics of the Massive MIMO technology that uses a vast number of antenna elements within a single antenna panel. Massive MIMO panels can consist of tens or even hundreds of antenna elements within a single panel for the transmitter and receiver. The principle behind multi-user support in 5G Massive MIMO is to improve network capacity by efficiently using the antenna elements to accommodate a large number of simultaneous users. So instead of creating an overall wide beam of the signal to serve an individual user at any given time, the 5G base station antennas can create multiple narrower signal beams that are targetted at individual users. It is also worth noting that, unlike 4G LTE networks that have a maximum of eight transmission layers (i.e. in LTE Advanced and LTE Advanced Pro MIMO configuration), 5G NR networks can potentially have many layers of communication. However, that doesn’t mean that in a Single-User MIMO scenario in 5G NR networks, all of the massive MIMO antenna elements may be assigned to a single user device. SU-MIMO and MU-MIMO support are required on a user device also for the capability to function. The user device has limitations on the number of layers it can simultaneously support. For example, a 64 x 64 massive MIMO configuration in 5G can theoretically support 64 layers of transmission between a base station and a user device. But in real life, due to practical limitations on a user device’s antenna panel, the 64 antenna elements in a base station transmitter are split across various user devices with a current maximum limit of 16 communication layers for one device. This is aligned with the principle of multi-user support in 5G which is to improve network capacity by supporting multiple simultaneous sessions.
In MU-MIMO (Multi-User MIMO), the multiple layers of communication that result from multiple antenna elements of a MIMO system can simultaneously support multiple user devices. On the other hand, in SU-MIMO (Single User MIMO), the multiple layers of communication that result due to multiple antenna elements in a MIMO system can only support one user device at a time. Both Multi-User MIMO (MU-MIMO) and Single User MIMO (SU-MIMO) are types of antenna technologies employed by 4G and 5G networks. MU-MIMO and SU-MIMO are based on the underlying MIMO technology that has been part of 4G networks from the beginning.
Here are some helpful downloads
Thank you for reading this post. I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need extra support, especially when preparing for a new job, studying a new topic, or buying a new phone. Whatever you are trying to do, here are some downloads that can help you:
Students & fresh graduates: If you are just starting, the complexity of the cellular industry can be a bit overwhelming. But don’t worry, I have created this FREE ebook so you can familiarise yourself with the basics like 3G, 4G etc. As a next step, check out the latest edition of the same ebook with more details on 4G & 5G networks with diagrams. You can then read Mobile Networks Made Easy, which explains the network nodes, e.g., BTS, MSC, GGSN etc.
Professionals: If you are an experienced professional but new to mobile communications, it may seem hard to compete with someone who has a decade of experience in the cellular industry. But not everyone who works in this industry is always up to date on the bigger picture and the challenges considering how quickly the industry evolves. The bigger picture comes from experience, which is why I’ve carefully put together a few slides to get you started in no time. So if you work in sales, marketing, product, project or any other area of business where you need a high-level view, Introduction to Mobile Communications can give you a quick start. Also, here are some templates to help you prepare your own slides on the product overview and product roadmap.