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

MU-MIMO or Multi-User MIMO or Multi-User Multiple Input Multiple Output is a type of antenna technology that spreads the benefits of MIMO to multiple simultaneous users. While MIMO (Multiple Input Multiple Output) has existed in the mobile communications industry since HSPA+ (HSPA Evolution), MU-MIMO is part of 4G LTE and is also used in 5G networks.

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 enabling the LTE network to support more users per cell.

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) which uses 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 as part of Evolved High-Speed Packet Access or HSPA Evolution (HSPA+). However, it is a key building block of 4G LTE and 5G NR networks. Multi-User MIMO is a type of MIMO technology that allows the antenna elements to support multiple users simultaneously.

Is MU-MIMO used in 4G LTE networks?

4G LTE networks supported basic Multi-User MIMO (MU-MIMO) from the beginning when LTE was introduced as per 3GPP release 8; however, the primary focus was Single-User MIMO (SU-MIMO). MU-MIMO support was enhanced in the later releases of LTE, including LTE-Advanced and LTE-Advanced Pro.

The value Multi-User MIMO (MU-MIMO) brings to LTE networks is that the core benefits of MIMO, including improved signal quality and bit rates, can be extended to multiple user devices simultaneously. MU-MIMO aims to increase the system capacity in LTE networks allowing them to support more users. 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, i.e. Single-User MIMO (SU-MIMO).

4G LTE networks were introduced as per the specifications in 3GPP release 8. In the original LTE release, the Multiple Input Multiple Output (MIMO) configuration is 4 x 4 for the downlink (DL) and 2 x 2 for the uplink (UL). This essentially means that there are four layers of transmission in the downlink (from the base station to the mobile phone) and two layers of transmission in the uplink (from the mobile phone to the base station). The type of MIMO in this release was mainly Single User MIMO (SU-MIMO), but it also included basic Multi-User MIMO capabilities. As per 3GPP release 8, the uplink MIMO in LTE is Multi-User MIMO. It means that multi-user MIMO support was already possible from the first LTE release, but it received enhancements in the following releases. LTE-Advanced (3GPP release 10) and LTE-Advanced Pro (3GPP release 13) include further improvements to the MIMO configuration and multi-user support. Please have a look at the table below that summarises the MIMO configuration for LTE, LTE Advanced and LTE-Advanced Pro and outlines which MIMO types they support.

LTELTE AdvancedLTE Advanced Pro
Downlink MIMO: 4 x 4Downlink MIMO: 8 x 8Downlink MIMO: 8 x 8
Uplink MIMO: 2 x 2Uplink MIMO: 4 x 4Uplink MIMO: 4 x 4
Supports both SU-MIMO and MU-MIMOSupports both SU-MIMO and MU-MIMOSupports both SU-MIMO and MU-MIMO
—Table summarising MIMO Configurations for LTE, LTE Advanced and LTE Advanced Pro—

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

In MU-MIMO (Multi-User MIMO) and SU-MIMO (Single-User MIMO), the number of communication layers is created by an equivalent number of antennas at the transmitter and the receiver end. In MU-MIMO, each communication layer can be used to communicate with a separate user device.

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 x 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 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?

In Single-User MIMO (SU-MIMO), all communication layers are assigned to a single user device to improve data rates and signal quality for an individual user. In Multi-User MIMO (MU-MIMO), each communication layer can be assigned to a separate user device in order to support more simultaneous users.

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 various 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 enhance 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. For this to happen effectively, both the mobile network and the mobile phone must have the capability to send and receive in this way.

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 various 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 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.

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