MU-MIMO: What is Multi-User MIMO in 4G?

MIMO or Multiple Input Multiple Output is an antenna technology used by many modern wireless networks, including 4G LTE, 5G NR and WiFi. The MIMO technology is based on the concept of Space Division Multiple Access (SDMA) that makes use of multiple antennas at the transmitter and the receiver to improve radio communication. In mobile communications, the MIMO technology was initially introduced in the 3G UMTS networks through the HSPA enhancement, however, it is a key building block for the 4G LTE networks. Since the launch of LTE originally in 2009, there have been many enhancements to the MIMO technology. MU-MIMO stands for Multi-User Multiple Input Multiple Output and it is a type of MIMO technology that allows the transmitter to spread the benefits of MIMO to multiple user devices simultaneously. For clarity, a mobile network does not need MIMO to be able to connect to multiple devices because that capability has been there since the first generation (1G) of mobile networks. The value multi-user MIMO brings is that enhancements like improved signal quality and bit rates etc. can be offered to multiple user devices at the same time. If a MIMO deployment is not multi-user, it just means the MIMO capability of the transmitter can only be available to one user device at a time.

Multi-User MIMO (MU-MIMO) in 4G networks is a type of MIMO antenna technology that allows a base station to use multiple layers of data streams to communicate with multiple devices. Multi-User MIMO improves the system capacity by allowing the LTE network to support more users per cell.

Does 4G use MU-MIMO?

4G LE networks use a range of different technologies to improve the data rates for customers. MIMO was introduced in LTE networks as part of their original launch based on 3GPP release 8. Release 8 included support for multi-user MIMO, which was enhanced in the later 3GPP releases. LTE-Advanced and LTE-Advanced Pro, which are based on release 10 and release 13, saw further enhancements to the MIMO configuration and multi-user support. MU-MIMO aims to increase the system capacity in LTE networks that allows them to support more users.

What is the concept of layers in multi-user MIMO?

The term “layer” is often used in MIMO which is a concept linked to the antenna elements on the transmitter and receiver. The layer refers to the communication stream (or data stream) between the transmitter and receiver antenna elements. For example, in LTE Advanced, the downlink channel has an 8×8 MIMO configuration which means eight antenna elements at the transmitter and another eight at the receiver. So in this example, the transmitter can use a maximum of eight antenna elements to establish eight layers of data streams to communicate with the eight antenna elements of the receiver. For multi-user MIMO (MU-MIMO), this simply means that instead of using all these layers to communicate with a single user device, the base station can use each layer to communicate with a different user device.

Difference between SU-MIMO and MU-MIMO in LTE?

Multi-user MIMO and single-user MIMO capabilities are built into the 4G LTE base station – eNodeB. If a base station is capable of Single User MIMO (SU-MIMO), then all the layers of data streams will target a single user device to improve the data rate for that particular device. In SU-MIMO, the layers are at different angles to each other but are all sent in the direction of a single user device so they can reach it. On the other hand, if the base station has Multi-User MIMO capability, the different layers of communication streams are sent simultaneously to different user devices to improve the overall spectral efficiency.

4G networks are based on Orthogonal Frequency Division Multiplexing -OFDM. In addition to carrier aggregation and QAM that collectively increase the carrier bandwidth and efficiency, MIMO provides two additional benefits to the network. It can improve the system capacity to accommodate more users per cell through MU-MIMO, but it can also improve data rates for a device through SU-MIMO. The key capabilities within MIMO that make this happen are spatial multiplexing, beamforming and antenna diversity. Spatial Multiplexing allows a mobile network to send a large amount of data to a mobile phone in multiple streams, each carrying smaller chunks of data. In order for this to happen effectively, both the mobile network and the mobile phone must have the capability to send and receive in this way.

LTE networks use various MIMO configurations. The first LTE launch was based on 3GPP release 8 which adopted a 2×2 MIMO configuration supporting MU-MIMO also. This antenna configuration moved up to 8×8 in downlink and 4 x 4 in the uplink as part of LTE Advanced. LTE-Advanced Pro continued with the same MIMO configuration. The table below summarises this concept.

LTE TechnologyMIMO Configuration
LTEDownlink: 4×4
Uplink: 2×2
LTE AdvancedDownlink: 8×8
Uplink: 4×4
LTE Advanced ProDownlink: 8×8
Uplink: 4×4

MIMO Configuration for LTE, LTE Advanced and LTE Advanced Pro

Conclusion

Multi-User MIMO or MU-MIMO in 4G LTE networks is a type of MIMO (Multiple Input Multiple Output) that allows a base station (eNodeB) to use different layers of data streams to communicate with different devices. The role of MU-MIMO in LTE networks is to improve system capacity by allowing the network to support more users per cell.

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 some extra support, especially when preparing for a new job, studying a new topic, or maybe just 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.

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