What is DSS-Dynamic Spectrum Sharing in 5G NR networks?

Whenever a new cellular technology is introduced, the need for the frequency spectrum is a crucial consideration. 5G is a flexible technology that can operate at multiple frequency bands. It can use different frequency bands starting from below 1 GHz (e.g. 700 MHz) going all the way up to tens of GHz (e.g. 28 GHz). The 5G spectrum consideration is primarily for the higher frequency bands, including the millimetre frequencies that have not been used in mobile communications before. DSS or Dynamic Spectrum Sharing is a technology that is relevant for situations when a lower frequency band is used for 5G cellular services. The majority of the lower frequency bands in mobile communications today are used for existing 4G LTE services. DSS makes it possible to use lower frequency bands for both 4G and 5G services.

Dynamic Spectrum Sharing (DSS) is a technology that allows 5G networks to use the 4G LTE frequency spectrum by enabling a cell tower to dynamically assign bandwidth to either 4G or 5G based on customer need. With DSS, a mobile operator can use a lower frequency band, e.g. 700 MHz for both 4G and 5G.

Why do we need Dynamic Sepctrum Sharing?

Procuring the frequency spectrum is one of the most fundamental and expensive considerations for a mobile network operator when launching a new cellular technology. The prices for the frequency spectrum can be in hundreds of millions of dollars in some countries, and it is one of the most scarce resources for an operator. So it is only fair if mobile operators try to find methodologies that help them efficiently use their allocated frequency spectrum. DSS or Dynamic Spectrum Sharing is a technology that allows mobile operators to launch 5G cellular services by utilising the existing 4G LTE frequencies. With DSS, 4G LTE and 5G NR can co-exist and complement each other by using the same frequency carrier for both 4G and 5G. DSS saves mobile operators a lot of time and money by allowing them to enter the 5G market quicker.

Why 5G needs 4G frequencies?

Even though the 5G NR networks can operate at various frequencies, the lower frequencies allow them to successfully provide wide area coverage. 5G NR and LTE Advanced can work together to address the capacity and coverage needs for a range of use cases for an average user. The difficulty with higher frequencies is the range because higher frequencies incur higher losses when they travel through the air, which results in the signal getting weaker quickly due to fading. Lower frequencies, on the other hand, incur fewer losses and can therefore travel longer distances more easily. Even though 5G employs Massive MIMO which can potentially extend the coverage range through beamforming in a better way than the regular MIMO, it is more efficient to cover wider areas by using lower frequencies. The challenge, however, is that the majority of the lower frequency spectrum is currently used by the LTE networks. Since LTE is the primary technology that currently fulfils the majority of the traffic demand, it is not a great trade-off for an operator to launch 5G services by limiting their LTE coverage.

Can the spectrum sharing be static?

There are multiple ways in which 4G LTE and 5G NR networks can share the same frequency spectrum. If a mobile operator wants to use their existing LTE frequencies for 5G, the most straightforward option is to allocate some part of the LTE spectrum to 5G NR. The downside however is that with that approach they would have less spectrum available for the 4G traffic. For example, suppose a mobile operator wants to use their 700 MHz frequency band for 4G LTE and 5G NR. They can split an available 20 MHz carrier within that band into two, allocating 10 MHz to 4G and 5G each. This technique is called static frequency domain sharing, which essentially splits an existing 4G frequency carrier into two to free up 10 MHz from the LTE carrier to use that for 5G services. That is a lose-lose situation for both 4G LTE and 5G NR because the available bandwidth (after the split) may not be sufficient to ensure high data rates through either of these two technologies. The static frequency domain sharing technique is particularly unsuitable for legacy 4G devices such as 4G phones that do not have the dual connectivity capability to connect to two base stations simultaneously. The 5G devices have dual connectivity capability and are able to connect to both 4G and 5G simultaneously.

How does Dynamic Spectrum Sharing work?

Dynamic Spectrum Sharing allows a cell to allocate its full carrier bandwidth to 4G or 5G by dynamically assigning the bandwidth to either of these technologies based on the customer demand. Once DSS is implemented in a cell, a 4G device can access the full carrier bandwidth through 4G connectivity and a 5G device can access the full carrier bandwidth through 5G connectivity. That way, the frequency spectrum and the carrier bandwidth is fully utilised to ensure peak data rates for the user through the most appropriate technology required to address the customer demand.

Dynamic Spectrum Sharing - Visual representation through an example
Dynamic Spectrum Sharing – Visual representation through an example

The most fundamental capability within DSS is the dynamic scheduling of the bandwidth for both LTE and NR. This dynamic scheduling allows for the dynamic assignment of the bandwidth to 4G or 5G depending on which device a customer has and what they are trying to do. For example, if the customer has a 5G device, this approach ensures that they get the full bandwidth, e.g. 20 MHz, through the 5G NR technology. On the other hand, if the device is 4G, the entire 20 MHz bandwidth shall be allocated to the device through LTE technology. Furthermore, just like LTE, 5G NR is also based on OFDMA – Orthogonal Frequency Divison Multiple Access (OFDMA). The 15 kHz sub-channel structure in 5G NR supports the use of the same carrier as LTE.

Conclusion

Dynamic Spectrum Sharing is a technology used in 5G New Radio (NR) networks that allows them to utilise the frequency spectrum that is already used for 4G LTE networks. It allows mobile operators to introduce 5G services without investing heavily in a lot of new spectrum. 5G NR is a flexible technology that can operate at high and low-frequency bands. The lower frequency bands are primarily providing 4G LTE services today. With DSS, a mobile operator can use a lower frequency band, e.g. 700 MHz for both 4G LTE and 5G NR. This way, 4G and 5G can co-exist and complement each other.

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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