Is spatial multiplexing different from spatial diversity?

Spatial multiplexing and spatial diversity are two radio communication techniques used in modern antenna systems in 4G LTE and 5G NR networks. Both these techniques play essential but separate roles in the MIMO (Multiple Input Multiple Output) antenna systems.

Spatial diversity is a technique in MIMO that reduces signal fading by sending multiple copies of the same radio signal through multiple antennas; spatial multiplexing is a technique in MIMO that boosts data rates by sending the data payload in separate streams through spatially separated antennas.

The MIMO antenna technology has been a key part of mobile communications since the 3G era. MIMO was first introduced during the evolution of HSPA-High Speed Packet Access to improve the achievable data rates in 3G UMTS networks considerably. The more recent mobile networks, including 4G LTE and 5G NR, rely heavily on the enhanced variants of MIMO technology, including Massive MIMO. Spatial multiplexing and diversity are the building blocks of MIMO systems, and this post aims to look at the roles they both play in 4G and 5G mobile networks.

Radio link quality vs achievable data rates

While the MIMO technology is capable of improving radio link quality and data rates, 4G LTE and 5G NR networks employ MIMO primarily to enhance the data rates. In spatial multiplexing, the overall data payload to be sent to the end-user device is transmitted in multiple parallel data streams. Each data stream can carry portions of the overall data, allowing the network to communicate with the device more efficiently. As a result, the mobile network utilises the available frequency spectrum more efficiently, which improves the overall network capacity. The other benefit of MIMO systems in 4G and 5G networks is spatial diversity which occurs due to the use of spatially separated antennas at the transmitter and the receiver. Spatial diversity helps improve the radio signal link quality by addressing the negative impact of radio signal fading.

How spatial multiplexing improves data rates

Spatial multiplexing improves data rates by allowing the overall data payload to be communicated to a user device in the form of multiple data streams that carry small portions of the overall information. The data streams can be targeted at a single user device or multiple user devices.

Spatial multiplexing or Space Division Multiplexing (SDM) is a multiplexing technique employed by MIMO antenna systems. It is an essential feature of MIMO and is the primary reason for introducing MIMO in 4G LTE and 5G NR networks. In spatial multiplexing, a transmitter or receiver can use several antennas separated in space by their angular direction. These antennas can send and receive multiple data streams using the same frequency and time resources and act as individual channels to communicate the information (e.g. a WhatsApp message) between the transmitter and receiver. The multiple data streams within a MIMO system can target a single user device or multiple user devices for simultaneous communication. When the data payload is sent towards a user device in the form of multiple concurrent streams, the data rate for the user device goes up. 5G NR networks use an enhanced version of MIMO, called Massive MIMO which consists of tens or even hundreds of antenna elements within a single antenna panel. Due to the sheer volume of antenna elements and the multi-user support capability, Massive MIMO can simultaneously offer higher data rates to several user devices.

How spatial diversity improves radio signal link quality

Spatial diversity improves radio signal link quality by employing multiple antennas at the transmitter or receiver to communicate numerous copies of the same signal. That allows the antennas to overcome the negative impact of multipath fading by using the copies of the signal to reconstruct it.

Diversity is not a new concept in mobile communications and has been used for years to address the negative impact of signal fading. When a radio signal (e.g. a mobile signal) travels from the cellular base station to the receiver of a mobile phone, it can take many routes depending on the obstructions in its way. Obstructions can be things like buildings, trees, poles, mountains etc. When the signal encounters any obstructions, it can get scattered and become weak or “fade” by the time it reaches the receiver. Diversity in radio communications is the ability of an antenna system to create redundant network resources for the signal to minimise the overall impact of signal fading. In plain English, it means creating additional copies of the signal so that bits and pieces of the scattered signal can be picked up to reconstruct the signal. At a theoretical level, at least three types of diversity solutions are available, including frequency, time, and space diversity. Frequency diversity requires multiple frequency channels, each communicating a version or copy of the same signal. Time diversity does the same thing but uses different time-slots instead so that different copies of the signal are communicated at different time intervals. But the diversity type employed by MIMO antenna systems is space diversity, also known as spatial diversity. In MIMO systems, spatial diversity is achieved by multiple antennas at the transmitter and the receiver that communicate (transmit or receive) a different version of the same signal. These versions are essentially a replica of the original signal. If used at the receiver end, the receiver can collect all the different versions of the signal to reconstruct it to overcome the negative impact of signal fading. In 4G LTE and 5G NR networks, spatial diversity is a critical part of MIMO systems that use multiple antennas at the transmitter and the receiver.

Conclusion

MIMO systems in 4G LTE and 5G NR networks use both spatial multiplexing and spatial diversity to improve data rates whilst improving signal quality. In MIMO, spatial diversity is a technique that provides the ability to overcome the negative impact of multipath signal fading by communicating separate versions or copies of the same signal through multiple antennas. On the other hand, spatial multiplexing is a technique that improves the achievable data rates for end-users by transmitting and receiving multiple streams of data through various spatially-separated antennas.

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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