6G RAN – key building blocks for new 6G radio access networks

Over the last couple of years, 5G networks have been rapidly deployed across the world, offering unprecedented capabilities. The uptake of 5G has been much faster than the uptake of 4G a decade ago and 5G will continue to evolve for many years to come. Nevertheless, to meet future expectations, work on the next-generation cellular networks – commonly referred to as 6G – is already ongoing, targeting deployments in 2030 and beyond. The 6G networks are expected to manage a significant surge in traffic, stemming from use cases pioneered by 5G, along with new, yet unforeseen use cases.

In this blog post, we will focus on the radio-access network and some design choices that are starting to emerge.

We emphasize that the term “6G” is more than radio-access alone. It also includes core network functionality, exposure functions, automation, and an AI-native design, among other aspects. Let us start by mentioning a couple of general design principles for the 6G RAN to set the scene. For details, please read the recent blog post 6G standardization – an overview of timeline and high-level technology principles. 

• Support key verticals and deployments scenarios such as non-terrestrial access, low-power wide-area (LPWA) IoT, and time-critical communication services from the start.
• Stand-alone design only, for better performance and avoiding market fragmentation.
• 6G RAN should connect to an evolved 5G core network.
• 6G RAN should include open interfaces to support a healthy ecosystem.
• AI-native design to ensure that AI/ML can easily be applied when appropriate, for example, on hard-to-model problems and non-linear effects.
• Energy performance surpassing that of 5G.

With these aspects in mind, we can start the discussion around 6G radio-access technology.

Standardizing a radio access technology (RAT) is a complex task with many decisions to be taken and not only about new use cases. Equally important are the learnings from 5G and the opportunity to improve upon the 5G solutions in several areas when defining 6G. Taken together, these refinements can result in a significant overall performance gain.

Spectrum sharing for 5G-6G networks

Spectrum is a fundamental aspect of any radio-communication system and an important asset for an operator. A wide range of frequency bands are targeted by 6G, described further in our 6G spectrum white paper, including low-band/mid-band as well as mmWave spectrum. In particular the low/midband spectrum is extremely important for wide-area 6G coverage. Unfortunately, there is very little, if any, pristine spectrum in this range and 6G therefore need to share spectrum with the previous generation. This is commonly referred to as multi-RAT spectrum sharing (MRSS). Highly efficient MRSS between 5G and 6G is essential and should be an integrated part of the 6G design from the start. Fortunately, the ultra-lean design of 5G implies that 5G-6G sharing can be made very efficient with an overhead of a few percent at most. This is unlike 4G-5G sharing which is inherently less efficient given the 4G design with cell-specific reference signals. Sharing between 6G and cellular Internet of Things (IoT) technologies LTE Cat-M and Narrowband-IoT is less critical. Such sharing can be done through semi-static split between the RATs as the IoT technologies use a relatively small amount of bandwidth that can be concentrated to one or a few carriers. Sharing between 4G and 6G is not seen as critical; by the time 6G is to be introduced 4G has largely been replaced by 5G.

In addition to low/midband and mmWave spectrum, the centimetric range, particularly 7 – 15 GHz, is a promising band not previously used for cellular communication. This new band can provide additional capacity for 6G and, when combined with massive multiple input and multiple output (MIMO) and beamforming, downlink coverage on par with existing mid-band deployments.

The sub-THz range, 100 – 300 GHz, on the other hand, has challenging propagation conditions and the RF technology is not yet mature enough. Hence, this frequency range is not expected to be part of the first 6G release.

Waveform, coding, and modulation

The choice of waveform often triggered long discussions in earlier generations, with orthogonal frequency division multiplexing (OFDM) being used in both 4G and 5G due to its robustness to time dispersion and ease of exploiting both the time and frequency domains when defining the structure for different channels and signals. We foresee that OFDM (complemented by DFTS-OFDM in the uplink) will continue to be used also in 6G for two main reasons:

• No other waveform showing substantial gains over OFDM motivating a different choice has emerged. Hence, work in 3GPP is better spent on discussing other, more promising features rather than the waveform itself.
• MRSS, an essential feature of any 6G system as discussed above, is significantly simpler if the waveform choice is aligned between 5G and 6G.

To support a larger carrier bandwidth than in 5G, an 8k FFT should be assumed when developing the specifications. Wider carrier bandwidths, where available, are an efficient way of handling bursty data transmissions.

Coding is also expected to be similar to 5G, possibly with some smaller refinements – low-density parity-check code (LDPC) for data and Polar coding for control channels. To enable a high degree of parallel processing in both the network and the device – required to support the highest data rates – mapping of coded bits to OFDM symbols could be refined such that code blocks do not cross OFDM symbol boundaries. Finally, quadrature amplitude modulation (QAM) is a good baseline for modulation.

Multi-antenna transmission

MIMO is, and has been for several years, a key tool to improve spectral efficiency. Two related tracks in the MIMO design can be identified for 6G:

• Very large arrays with even more antenna elements than in 5G to increase performance on the existing grid. For the cmWave bands, antenna units of the same physical size as a 3.5 GHz mid band antenna can fit 4 – 16 times more antenna elements and thereby provide similar downlink coverage as those on the 3.5 GHz band.
• Distributed MIMO where the antenna elements are spread out (in academic literature this is often, somewhat incorrectly, referred to as “cell-free MIMO”). This is primarily of interest for denser deployments. By exploiting transmissions from multiple radio sites, a high degree of reliability can be achieved along with high data rates and interruption-free mobility.

Reciprocity-based beamforming is another area to further evolve in 6G. Examples include reciprocity in frequency division duplex (FDD), for example, to improve capacity for low-band FDD, and improvements to the sounding reference signal (SRS) transmissions (in 5G, the SRS transmission quality is often quite low, negatively impacting the usefulness of reciprocity-based schemes). AI/ML-based beam management, where measurements on wide beams are used to predict narrow beams to lower the amount of beam sweeping and reduce the channel state information reference signals (CSI-RS) overhead, is another example of an area interesting to explore. In the future, when the load in the mmWave range increases, this can help increase the throughput.

Scheduling and data transmission

To efficiently support bursty traffic patterns, which is the typical case for most services, it is important to rapidly exploit a wide transmission bandwidth when needed. This is crucial to efficiently utilize the operator’s highly valuable spectrum assets and speaks in favor of avoiding 5G-like dual connectivity schemes. Instead, the focus should be on the widest carrier bandwidths possible and a fast CA-like scheme to aggregate spectrum in multiple bands. Compared to 5G, a much faster activation of additional carriers is needed. Furthermore, the split into a primary and one or more secondary cells (PCell and SCells) as in 5G should be revisited. Putting the carriers on an equal footing could allow for increased robustness (for example, radio-link failure would not depend on a single carrier only) and a more flexible “mix and match” approach for uplink and downlink spectrum. By partially decoupling uplink and downlink, for which the unified uplink controls signaling is one facilitator as discussed later, a better exploitation of the spectrum assets can be achieved.

Scheduling strategies can also be refined. Ideally, the transmission points and frequency resources in an area should be seen as a single, unified resource over which the scheduler operates with the target of always assigning the best combination of frequencies and transmission points to the transmissions. Although this to a large extent is an implementation aspect, the signaling mechanisms defined for 6G should be designed with this in mind. Avoiding synchronous timing dependencies between the user equipment (UE) and network and a reduction in scheduling constraints are prerequisites for such implementations.

Having one scheduler handling multiple transmission points and carriers also enables various forms of coordination and can better take the interference situation into account. Efficient interference coordination, enabled by a more centralized type of scheduling when feasible, is one of the more promising ways to improve performance on a given site grid. AI/ML is also an interesting tool to investigate in this area.

Instead of the “request-grant-BSR-grant” cycle in 5G preceding the actual user data transmission in many cases, a contention-based mechanism for buffer-status reports can be used. A small amount of resources, suitable for a buffer-status report and possibly some user data, can be accessed in a contention-based manner. Studies show that this is a more efficient scheme for the traffic patterns typically encountered, especially since many data transactions are device initiated and fast access to uplink resources therefore is important. This contention-based mechanism can also be used for small amounts of irregular data, for example, to support LPWA devices as discussed later.

Higher-layer protocols

Efficient data transmission requires not only an efficient physical layer but also streamlined higher-layer protocols. Based on the learnings from 5G, several enhancements can be envisioned, for example in the handling of uplink control signaling, retransmission protocols, and mobility procedures.

In 5G, uplink control signaling uses (sometimes for historical reasons) several mechanisms – L1/L2 signaling on the physical uplink control channel (PUCCH), media access control- control elements (MAC-CEs), and radio resource control (RRC) signaling. Furthermore, reporting on PUCCH is synchronous, that is, the time when the report is sent and the PUCCH format to use is fixed in relation to the request for the information. Unfortunately, this implies that the timing is based on the worst-case situation, despite that the UE in many cases has the information available earlier. The use of multiple signaling mechanisms also complicates the implementation on the UE and network (NW) sides, for example, in the form of undesirable dependencies between uplink and downlink schedulers, as well as scheduling restrictions (for example, a periodic CSI report needs to be requested several slots in advance and in the meantime uplink grants cannot be issued). Furthermore, it is more important for the network to get reliable measurements from the UE once they are available rather than invalid measurements by a hard deadline. In most cases, a reliable measurement result is likely available well before the synchronous deadline. With this in mind, 6G should, whenever possible, aim for a unified and asynchronous reporting mechanism using MAC-CE-like transmissions on the physical uplink shared channel (PUSCH) and omitting the PUCCH. This would overcome the complex timelines in 5G and enable a looser coupling between uplink and downlink and thereby a more efficient utilization of the spectrum resources as already mentioned. Furthermore, it increases the implementation flexibility in the device, results in faster feedback, and will simplify the adoption of future 6G features in later releases.

Transmitting uplink control signaling, specifically the hybrid-ARQ acknowledgements, as in-band MAC-CEs can, in principle, allow for merging the hybrid-ARQ and RLC retransmission protocols into one. Although replacing PUCCH signaling, often carrying only a few bits of hybrid-ARQ feedback, with in-band MAC-CE feedback might seem like an increase in overhead at first sight. However, detailed simulations, taking all protocol layers into account, actually show a slight increase in performance with this simplified scheme. This is one example of the importance of careful analysis, including realistic traffic models, which may show surprising results.

Mobility has evolved over multiple releases, starting with L3-based handover with conditional handover (CHO) and lower-layer triggered mobility (LTM) added in later releases. These mechanisms should be unified in 6G, resulting in a single solution useful for both lower-layer mobility/beam handling as well as “traditional” higher-layer mobility. Mobility-related measurement reports are transmitted using the unified uplink control signaling above.

Last, but not least, the RRC configuration structure of 5G could be simplified in 6G, resulting in a simpler, easier-to-maintain code, and faster implementation of new features. For example, the RRC configuration should make a clear distinction between settings that apply to idle and connected modes and avoid unnecessarily deeply nested structures.

Energy efficiency

Energy efficiency is probably one of the most important areas to address in future networks. Network energy efficiency is to a large extent an implementation aspect, but the standards do set certain boundaries on what can be achieved. In 5G, quite some effort was spent to define an ultra-lean design in the time domain to enable base stations to sleep between transmissions. This mindset should definitely be carried over to 6G, but we should take further steps and extend the ultra-lean design into the spatial and frequency domains. If, on a dynamic basis, a certain carrier or transmission point is not needed for data transmission in the network, it should be possible to put it to sleep and rapidly wake up when needed. This can be achieved by having separate signals for idle and connected mode UEs, unlike the 5G design where basically all transmissions depend on the synchronization signal block (SSB). Rapidly turning off a 5G node is therefore problematic as it would also affect idle-mode UEs relying on SSBs being present continuously. With separate signals for idle and connected mode, a node used only for data transmission would not transmit those SSBs and could hence be turned off without impacting idle mode UEs. It also enables separate optimization of idle and connected mode functionality, which have quite different requirements.

Device energy efficiency is equally important – a long battery lifetime is one of the most important aspects from an end-user perspective and 6G UEs need to be at least as energy efficient as 5G UEs. Wake-up signals is one example of a feature that should be integrated in 6G from the beginning. Especially in idle mode, a wake-up signal can lead to significant energy savings compared to a classical DRX-cycle only, but wake-up signals can also be valuable in connected mode as discussed in the scheduling section if mobility measurements are appropriately addressed. Another important feature example is using a modest reception bandwidth in order to save energy when monitoring for scheduling assignments/grants and reception of small packets and, when needed, dynamically expand to the full carrier bandwidth to receive data at a high rate. The bandwidth-part switching in 5G aimed (among other things) at this scenario, but it became fairly complex and difficult to exploit in practice.

Key scenarios addressed from the start

The design of 6G should support key verticals and deployment scenarios, as we mentioned in the introduction. In earlier generations, support for these scenarios was added at a later stage, sometimes with overly complex solutions, resulting in limited uptake in practice. Consequently, 6G should include basic support for them from the start and avoid over-optimizing for the most extreme scenarios.

In the following, some areas that should be supported by the 6G radio-access technology from the beginning are discussed in more detail.

Low-power wide-area IoT

LPWA IoT services in cellular networks are currently handled by the LTE Cat-M and NB-IoT technologies, developed in the 4G time frame. Given the long replacement cycles of many IoT devices, it is expected that LTE Cat-M and NB-IoT will be around until 2040, thus motivating basic 3GPP spectrum sharing with 6G. Nevertheless, at some point, these devices need to be replaced. Instead of defining a technology fork for IoT devices, as was the case for catM and NB-IoT, it is attractive to have LPWA support integrated into the basic 6G system from the beginning. This can open up for supporting a wide range of devices in the same system. While it is straightforward to add support for new devices with better capabilities than the baseline, it is substantially more difficult to do the opposite in a backwards-compatible manner. This speaks also in favor of considering low-end devices in the early design phase of 6G. Fortunately, many of the features of particular interest for LPWA are equally useful for 6G in general.

From an LPWA perspective, important aspects to consider are:

• Cost; apart from benefiting from high production volumes, cost-reducing features such as support for half duplex, single antenna operation, limited data rates, and a modest device RF bandwidth in the order of 3 – 5 MHz should be part of the first release.
• Coverage; this is mainly a matter of sufficiently low data rates which should be straightforward to support.
• Energy consumption; this is beneficial for devices in general, so long DRX cycles and wake-up signals should be part of the first 6G release. The design should be such that LPWA requirements are also addressed.
• Capacity; the contention-based mechanism for small data packets one example of a feature useful for LPWA. Simplified and lean RRC signaling is another.

In essence, an LPWA device should be a low-end 6G device with support integrated in the 6G radio-access technology from start, not a separate radio-access technology added in a later release.

Fixed wireless access

Fixed wireless access (FWA) is a relatively large part of the overall data volume for many communication service providers. FWA can be supported by the generic 6G RAT. A higher device output power and exploiting the fact that the device is not moving to better exploit multi-user MIMO are two examples of what can be done to further improve the performance.

Non-terrestrial networks

Support for non-terrestrial networks (NTN) was introduced in 5G release 17. Relatively small enhancements were needed, on the lower layers primarily related to handling the Doppler shift and the large roundtrip times. With this in mind, 6G should natively support NTN as a complement to the terrestrial networks. For NTN to materialize and benefit from the 3GPP ecosystem, it is important to minimize the difference between terrestrial networks (TN) and NTN. Technology-wise, it largely boils down to relatively small additions such as sufficiently wide parameter ranges. Solutions that do not depend on external technologies, for example, satellite navigation systems, are interesting to investigate.

In summary, basic NTN support for LEO satellites should be part of the first release of 6G, preferably in a device-transparent way.

Dependable and time-critical communication

Dependable communication is a very important area for 6G with the overall goal of delivering a reliable service of acceptable quality in terms of bounded latency, availability, and security to the end user, resilient to failures. In the past, effort has been spent on defining (sometimes unnecessarily complex) Ultra-Reliable Low-Latency Communications (URLLC) enhancements focusing on the link itself. In 6G, we should take a broader view and focus on the overall system performance – the end user does not care whether the service did not meet the expectations due to a fading dip, a late handover decision, or a hardware fault. Multi-point transmission (a flavor of D-MIMO discussed above), flexible carrier aggregation, and fast measurement and reporting procedures are examples of technical features useful to provide dependable communication. Another example is conditional handover, triggered by the device detecting a network failure.

Observability is also key – without observability it is not possible to control the system to maintain the agreed service quality, nor is it possible to detect if there is a failure in meeting the service-level agreements. To get the required level of observability, there is likely a need to standardize additional UE reports.

Interference and jamming detection are other important aspects to address. Also in this case there could be a need for new UE reports. Together with network measurements, these can be analyzed, possibly using AI/ML, to detect any unexpected interference behavior in the network and trigger the necessary actions.

Positioning and sensing

Positioning has been supported in cellular systems for several years and 6G should build on and evolve these solutions. High-precision indoor positioning for industrial applications is particularly interesting, but more accurate outdoor positioning is also relevant. Both network-assisted and device-assisted positioning should be considered as they complement each other and, when used in combination, can provide added resilience.

In addition, the 6G research community is investigating the possibilities to re-use the communication network for sensing. In essence, this is a radar-like setup to sense and position objects that are not necessarily actively communicating with the network. The specification impact depends on the type of sensing setup to support (NW-only or NW-UE collaboration, mono-static or multi-static) but largely consists of specification of a sensing signal and reporting mechanisms to the node where the radar signal processing is carried out.

Conclusion

In this blog post, we have shared some of our thoughts about a future 6G radio access network. Clearly, there are many more aspects to consider and details to settle. Stay tuned for an exciting technical journey, taking cellular communication into the next decade!

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Read the blog post: 6G straight from the Ericsson labs

Learn more about 6G standardization in this blog post: 6G standardization – an overview of timeline and high-level technology principles