Is MIMO different from spatial diversity?

MIMO stands for Multiple Input Multiple Output, and it is a key antenna technology used by modern wireless networks including 4GLTE, 5G NR and WiFi6. Even though it is an advanced antenna technology, the fundamental concept of MIMO is not brand new. On the other hand, antenna diversity is an older concept that has existed in mobile communications for a long time. This post aims to outline the differences between the two terminologies to ensure that the distinction between the two is clear.

Spatial diversity is a technique used in mobile networks to improve radio signal reliability through multiple antennas either at the transmitter or the receiver; MIMO is an antenna technology in 4G and 5G that uses spatial diversity by employing multiple antennas at the transmitter and the receiver.

Spatial Diversity improves signal quality

Antenna diversity or spatial diversity is a technique used in mobile communications and other wireless systems. It employs multiple antennas separated physically in space, either at the transmitter or the receiver side, to improve the radio link quality. Antenna diversity has historically been used in mobile communications at the base station, where multiple antennas are deployed to pick up the signals coming from the mobile phones. The basic design of antenna diversity is based on the principle of signal multipath fading. When a radio signal travels in the air, it can take several routes to reach the receiving antenna depending on the obstructions in the way. For example, if you are sitting in an office building trying to make a phone call through your mobile phone, your phone will communicate with the nearest possible cellular base station. However, your mobile signal may take various routes due to the thick walls, glass windows, reflecting surfaces and many other obstructions to travel from your phone’s transmitter to the base station’s receiver. By the time it gets to the receiver, the signal may or may not have faded considerably. Therefore, mobile operators use multiple receiving antennas at the base station to improve the likelihood of successfully receiving the signal. The primary idea is that each receiving antenna gets a replica of the signal. Therefore, if the antennas are spatially separated, the signal is more likely to be received without too much fading. At the receiving end, the signal is reconstructed by combining the output of each of the antennas.

Typically diversity is either applied at the transmitter end or the receiver end. It means we can either have multiple antennas at the transmitter end or the receiver end. In MIMO, diversity is applied at both the receiver and the transmitter ends because multiple antennas are on both sides.

MIMO improves signal quality and data rates

MIMO- Multiple Input Multiple Output is an antenna technology that uses multiple antenna elements at the transmitter and receiver sides to improve radio signal quality and data rates. In MIMO, the transmitter antenna has multiple antenna elements, e.g. 2, 4, 8 or more, which transmit multiple signal data streams to the receiver. The receiver antenna also has various antenna elements to receiver the data streams. The configuration of the MIMO system is defined by the number of antenna elements at the transmitter and receiver. For example, 4G LTE-Advanced networks have a configuration of 8×8 for the downlink which means eight antenna elements at the transmitter and another eight at the receiver side when sending the signal from the base station to the mobile phone. LTE-Advanced networks use a lower configuration for the uplink with four antenna elements at the transmitter and four at the receiver (4×4) when sending the signal from the mobile phone back to the base station.

Antenna diversity is only one of the building blocks of MIMO systems where MIMO benefits from spatial diversity at both the transmitter and the receiver sides. The key reason why MIMO is used in 4G LTE and 5G NR is due to a capability called spatial multiplexing. Unlike antenna diversity, spatial multiplexing can improve the overall network capacity and possible data rates for the customer. Spatial multiplexing is based on the concept of Space Division Multiple Access or SDMA. With spatial multiplexing, the multiple antenna elements that are part of one transmitter antenna send numerous data streams, each carrying bits and pieces of the overall data (e.g. a WhatsApp message) aimed towards the target user device (e.g. a 4G phone). The receiver antenna elements of the user device pick up the various data streams to put the overall data back together as a single data stream. This way, the mobile network can utilise the available radio network resources (i.e. frequency and time-slots) more efficiently by transmitting multiple data streams in parallel to improve the data rates for a user. Another feature of MIMO is beamforming which directs the signal by pointing the antenna beams towards the target user device. Beamforming improves the range of the signal.

Conclusion

Antenna diversity or spatial diversity is one of the most fundamental techniques used in mobile communications to improve radio signal reliability by using multiple antennas either at the transmitter or the receiver. MIMO employs various antenna elements both at the transmitter and the receiver to take advantage of antenna diversity and spatial multiplexing 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, 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.

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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