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. While the term MIMO may sound a bit theoretical at first, the concept is very practical and easy to understand. However, the implementation of MIMO is complex in mobile networks, and it poses additional hardware requirements both on the transmitter and the receiver sides.
Breaking down MIMO
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.
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.
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. In mobile networks, there are two communication links, 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).
How does MIMO work in 4G LTE?
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 through the deployment of multiple antennas both for transmission and reception. The original LTE networks launched as part of 3GPP Release 8 in 2009 employed 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.
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.
|LTE Technology||Antenna Configuration (MIMO)|
|LTE||4 x 4 Downlink|
2 x 2 Uplink
|LTE Advanced||8 x 8 Downlink|
4 x 4 Uplink
|LTE Advanced Pro||8 x 8 Downlink|
4 x 4 Uplink
Why 4G LTE networks use MIMO?
MIMO in 4G LTE networks is primarily used for spatial multiplexing capability. However, it does offer other benefits to a mobile user too. Spatial multiplexing is when multiple antennas are placed on a transmitter or receiver and separated physically in space. 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.
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.
Finally, another key benefit of MIMO is beamforming. Beamforming is when multiple antennas transmit the same signal towards specific directions or locations. As a result, the receivers in those locations can receive a more robust signal because the transmission power is targeted in a particular direction. This results in a more extended range of the signal and improved bit rates. The directional approach can also be used on the receiver side, where the receiving antennas focus on the transmitted signal coming from the cellular tower. The multiple antenna approach also plays a crucial role in 5G New Radio (5G NR) networks, and MIMO is one of the key enabling technologies in 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 some extra support especially when preparing for a new job, or 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 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 product overview and product roadmap.