The fifth generation (5G) communications technologies target diverse applications ranging from automotive, industrial communications, smart health, agriculture, entertainment, public safety and even tele-surgery. These applications have diverse requirement that includes very high data rates (in order of multi-gigabit per second), ultra- high reliability (99.99999% availability) and extremely low latency (as low as 1 ms). To fulfill these requirements, numerous new candidate technologies such as the use of millimeter wave frequency bands, licensed assisted access (LAA) in unlicensed bands, non-orthogonal multiple access (NOMA), cloud radio access (C-RAN) and massive multiple input multiple output (massive-MIMO) have emerged. It is evident that many 5G applications will be served by multiple radio access technologies in tandem to fulfill the diverse application requirements while ensuing the most efficient use of radio resources. For example, a remote-surgery application would require (i) low-latency radios to transport haptic feedback, (ii) very high data rates to transfer 360 immersive videos, and, finally, (iii) ultra-high reliable links. This aggregate set of performance targets can be met by opportunistically activating multiple radio components, at different frequency bands, to match the specific communications requirements.
Introduction: With the ratification of the 1st phase of the 3GPP’s 5G New Radio (NR) standardization, it is clear that 5G air-interface would employ multiple frequency bands ranging from sub-6 GHz to mmWave frequency bands. Owing to the heterogeneous requirements of 5G applications, efficient interplay among different air-interfaces is naturally desired to exploit the full potential of available licensed and unlicensed spectrum resources. Generally, the interworking of air-interfaces can mainly be facilitated by three means: (i) Offloading, (ii) fallback, and (iii) aggregation. Offloading is the procedure of vertical handover among different radio technologies which is primarily aimed at de-congesting the licensed frequency bands. Fallback is also achieved by triggering vertical handovers, but it is generally initiated when a particular frequency band is affected by adverse channel conditions. This is particularly the case with mmWave bands when bad channel conditions arise due to blockages or beam misalignments, and the support of sub-6 GHz band is invoked to maintain the connectivity. On the other hand, aggregation aims to combine all the available frequency bands to boost the data rate by providing simultaneous multi-band connectivity. It is obvious that these operations should intelligently happen without the end user noticing any degradation in quality of service and experience.
RAT interworking in 5G and beyond networks: As a starting point for multi-radio interworking, the 1st phase of 3GPP NR provides multi-RAT dual connectivity (MR-DC). In MR-DC, a user equipment (UE) is configured to utilize radio resources provided by LTE E- UTRA (Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access) and NR access. Although the 3GPP NR provides specifications for the inter-RAT handovers between LTE and NR at the core network (CN) level, multiple possibilities for standardization remains open among 3GPP RATs and non-3GPP RATs. For example, the high data rate unlicensed access in 60 GHz band provided by IEEE 802.11ad or IEEE 802.11ay needs to be integrated with the 3GPP NR. This is needed for efficient offloading to mmWave unlicensed band, fallback to sub-6 GHz band and to facilitate the aggregation of multiple licensed and unlicensed mmWave bands.
In 4G communications similar offloading in the sub-6 GHz band is facilitated between WiFi (unlicensed) and 3G/4G access (licensed) networks using the Access Network Query protocol (ANQP) provided by 3GPP and the Access Network Discovery Function (ANDSF) provided by the Hotspot Alliance. This offloading solution works at the core-network (CN) and is referred as loose coupling solution as it is outside the operator’s control. Hence it is best suited for the best effort traffic offloading to WiFi band. Since handover decisions in 4G offloading solutions are taken at the CN level, a long handover delay is expected. This delay would further increase if similar solutions would be used in case of 5G and beyond networks. The primary reason is that the mmWave band Access Point (AP) or Base Station (BS) would have a small coverage area resulting in frequent handover triggers. Furthermore, the use of mmWave beamforming would add extra delay as compared with the sub-6 GHz communication where beamforming is not mandatory.
Thus the 5G networks require new approaches for integrating licensed and unlicensed mmWave RATs with each other and with the sub-6 GHz bands. In particular, radio access network (RAN) level coupling between different air-interfaces (3GPP and non-3GPP) so that the handover process can be expedited, would be highly beneficial. In 3GPP, this technique is known as harmonization, i.e., the ability to split the connection within the CN, at a specific layer of the stack. Further, the use of directional antennas makes it difficult for mmWave BS/AP to initiate the handovers requiring new approaches at RAN layer to facilitate efficient fallback when needed. Furthermore, the differing propagation characteristics of sub-6 GHz and mmWave signals will require new solutions for efficient carrier aggregation over such a wide frequency range. We enlist the following tasks where efficient interworking solutions are required:
- Interworking (aggregation) of licensed sub-6 GHz and mmWave frequency bands.
- Interworking (fallback) of licensed mmWave to licensed sub-6 GHz band.
- Interworking (fallback) of unlicensed mmWave to unlicensed sub-6 GHz band.
- Interworking (offloading) of licensed sub-6 GHz/mmWave to unlicensed mmWave GHz bands.
Interworking solutions are not only important to decongest the licensed bands (as it was the case with 3G/4G networks), but are also highly desired to fulfill the heterogeneity in service requirements. For example, a high data rate (multi-Gbps) application requiring low latency and ultra-high reliability would require mmWave access for high data rate, NR access for low latency and air-interface (frequency, BS, etc. diversity to ensure reliability. The conventional offloading, handovers and aggregation mechanisms would not be sufficient due to the large range of frequencies (sub-6 GHz and mmWave bands) and diversity of applications targeted by 5G and beyond networks. Specifically, the rapid fluctuation in mmWave channel conditions requires new approaches to enable fast switching among different RATs. This warrants immediate standardization efforts to establish the much needed symbiosis of multiple licensed and unlicensed RATs envisioned for 5G and beyond networks.
R. Venkatesha Prasad∗
† NYU Tandon School of Engineering, Brooklyn, US Email: firstname.lastname@example.org
‡ Alexander TEI of Thessaloniki and Bournemouth University Email: email@example.com