MIMO in LTE: What is Multiple-Input-Multiple-Output in 4G?

The MIMO antenna technology has been a critical part of 4G LTE (Long Term Evolution) networks. 4G networks have seen multiple enhancements to the MIMO configuration to improve data rates and signal quality.

MIMO or Multiple Input Multiple Output is an antenna technology in 4G LTE that uses multiple antenna elements at the transmitter and the receiver to improve signal quality and bit rates. MIMO employs spatial multiplexing, diversity and beamforming techniques to improve link quality and data rates.

What is MIMO?

MIMO stands for Multiple Input Multiple Output, and it is an antenna technology that improves the quality of service you receive on your mobile phone. However, MIMO implementation is complex in mobile networks, and it poses additional hardware requirements both on the transmitter and the receiver sides.

Multiple Input: Multiple antennas at the transmitter

The multiple input part of MIMO is about the number of antennas at the transmitter side. Rather than using only one antenna at the transmitter, with MIMO, it is possible to use 2, 4, 8 or even more antennas. The use of additional antennas allows sending multiple streams of the same signal to the receiver (e.g. your 4G phone or 4G router) to improve the overall signal performance.

Multiple Output: Multiple antennas at the Receiver

The multiple output part of MIMO is about the number of antennas at the receiver side. Instead of using just one antenna to receive the various streams of the signal coming from the transmitter, your receiver (e.g. phone) can use multiple antennas to catch the different streams of the signal. For example, it is possible to have 2, 4, 8 or even more antennas in your phone to receive the multiple signal streams.

MIMO antenna Configuration

The addition of antennas and the inter-working of multiple transmitter and receiver antennas is complex. Depending on the aim of the connectivity technology, the decision about the number of antennas for the transmitter and the receiver is made carefully. This is where the antenna configuration for MIMO comes in. If the transmitter has four antennas and the receiver has two antennas, then the MIMO configuration will be 4 x 2. There are two communication links in mobile networks, the uplink and the downlink. MIMO is handled separately in the uplink and the downlink. For example, it is possible to have 4 x 2 in the downlink (for downloads) and 2 x 2 in the uplink (for uploads). An inter-related technology to MIMO is SISO which uses single antennas at the transmitter and the receiver.

How does MIMO work in 4G LTE?

The original 4G LTE networks use MIMO configurations of 4×4 in the downlink and 2×2 in the uplink as per 3GPP Release 8; LTE Advanced and LTE Advanced Pro enhancements use 8×8 MIMO configuration in the downlink and 4×4 in the uplink.

LTE Technology3GPP ReleaseAntenna Configuration (MIMO)
LTERelease 84 x 4 Downlink
2 x 2 Uplink
LTE AdvancedRelease 108 x 8 Downlink
4 x 4 Uplink
LTE Advanced ProRelease 138 x 8 Downlink
4 x 4 Uplink
Table showing MIMO configurations for LTE, LTE Advanced and LTE Advanced Pro

4G LTE and LTE Advanced networks use many techniques to improve the channel bandwidth and bit rates. MIMO is one such technology that enhances the transmission layers by deploying multiple antennas both for transmission and reception. The original LTE networks launched as part of 3GPP Release 8 in 2009 employed a 4 x 4 MIMO configuration for the downlink transmission (network to the phone) and 2 x 2 for the uplink transmission (phone to the network). This configuration means that four antennas can be used at the cellular base station to transmit the signal and four antennas at the receiver, e.g. a phone to receive the signal. Have a look at the simplified network diagram below that shows a 2 x 2 MIMO for downlink and 2 x 2 for uplink.

An illustration of 2 x 2 MIMO uplink and downlink - Multiple Input Multiple Output 4G LTE
An illustration of 2 x 2 MIMO uplink and downlink

The communication in the other direction, mobile phone to the base station, utilises two antennas at the transmitter (e.g. phone’s transmitter) and the receiver (base station receiver). With LTE Advanced, in Release 10, the MIMO configuration went up to 8 x 8 for the downlink and 4 x 4 for the uplink. LTE Advanced Pro networks in Release 13 maintained the same MIMO configuration as LTE Advanced.

Why 4G LTE networks use MIMO?

MIMO in 4G LTE networks is primarily used due to spatial multiplexing that improves data rates by using multiple antenna elements that are physically separated in space on a transmitter or receiver. MIMO also offers other benefits through spatial diversity and beamforming.

1 – Spatial Multiplexing

Spatial Multiplexing improves data rates by transmitting the data payload in separate streams through spatially separated antennas carrying bits and pieces of the overall data. At the receiver end, another set of spatially separated antennas combines the separate streams into a single data stream.

Spatial Multiplexing is also referred to as Space Division Multiplexing, and it allows spatially separated antennas to transmit various data streams of the signal, which are transmitted through a radio channel. At the receiver end, the data streams are received by another set of spatially separated antennas to put the various data streams back together as a single stream. This allows the available frequency and time resources to transmit multiple signal streams in parallel to improve the overall efficiency and hence the resulting bit rates for the user.

2 – Spatial Diversity

Spatial diversity is about improving the quality of the radio signal by using multiple transmissions to address the negative impact of multipath fading. In MIMO, multiple copies of the same signal are communicated between the transmitter and the receiver.

One of the benefits of using MIMO is antenna diversity which is a capability that allows the signal to be diversified or split either at the transmitter end or the receiver end. Now you may be wondering why someone would want to do that? Let’s look at it from a transmitter viewpoint first. For example, suppose a cellular tower transmits a signal in a single stream, and that stream has to travel long distances with many obstacles in the way, e.g. buildings. In that case, that signal stream can potentially become very weak by the time it reaches your phone’s receiver. But if the same cellular tower breaks the signal stream into four sub-streams and then transmit through four separate antennas, the overall risk of signal fading can be mitigated.

If we were to use MIMO at the receiver end, e.g. your 4G phone’s receiver, then the likelihood of the signal streams being received by the antennas increases if your phone has multiple antennas. This is because if one of the signal streams fades and can’t make it to one of the receiving antennas, there is a chance that another receiving antenna can pick up that signal.

3 – Beamforming

In beamforming, multiple antennas transmit the same signal towards specific directions to extend the range of the signal and improve bit rates. Beamforming allows the receivers in those locations to receive a more robust signal because the transmission power is targeted in a particular direction.

Beamforming is one of the key benefits of MIMO and is also a key technique used by 5G New Radio (NR) networks. The directional approach is not only for the transmitter but also for the receiver. The receiving antennas can use beamforming to focus on the transmitted signal coming from a particular cell of a base station (cell tower). The multiple antenna approach plays a crucial role in 5G networks, especially for the higher frequency bands, and therefore, MIMO is one of the key building blocks of 5G.


In summary, MIMO – Multiple Input Multiple Output is an antenna technology that employs multiple antennas at the transmitter and receiver ends to improve the signal quality and bit rates for 4G LTE and other modern networks. It creates multiple paths for the transmission and reception of the radio signal. MIMO achieves signal and bit rate enhancements through spatial multiplexing, antenna diversity and beamforming.

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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