NR LTE coexistence – Dynamic Spectrum Sharing (DSS)

November 26, 2020 // By Andreas Roessler, Technology Management North America, Rohde & Schwarz
NR LTE coexistence – Dynamic Spectrum Sharing (DSS)
As of now, all 5G NR FR1 deployments are based on time division duplex (TDD) and therefore use unpaired frequency bands typically at the 3.5 GHz frequency range. The second phase of 5G NR network deployments, anticipated for 2020, will utilize frequency division duplex (FDD) mode as almost 90% of all spectrum below 8 GHz is organized as paired frequency bands, where downlink and uplink use different frequencies. However, all the targeted frequency bands are already in use by 4G LTE. While adding spectrum sharing capabilities to the 5G NR standard, this spectrum can be accessed while in use, and coexistence between 4G LTE and 5G NR is enabled. The sharing of the spectrum allows network operators a smooth transition from LTE to 5G without the need for spectrum refarming. This article will discuss the required feature sets, summarized as dynamic spectrum sharing (DSS), and analyze related test and measurement challenges from a network and device testing point of view.

The first 5G NR networks are on-air using network deployment option 3X and utilizing E-UTRA New Radio Dual Connectivity (EN-DC) with a split bearer setup. In this deployment scenario, the so-called non-standalone (NSA) mode, an LTE anchor is required to exchange control and signaling information. In addition to LTE signaling, the anchor is also required in order to be able to configure, add, modify, and release the connection towards the 5G NR Radio Access Network (RAN). In this setup, the LTE base station (eNB) takes on the role of the master cell group (MCG), where the 5G base station (gNB) becomes the secondary cell group. Both RANs connect to the existing LTE core network (Evolved Packet Core, EPC). According to the 3GPP standard, for each cell group, carrier aggregation can be activated. However, today's 5G deployments at sub 8 GHz frequencies, also called frequency range 1 (FR1, 410 MHz to 7.125 GHz), combine multiple LTE carriers with typically one NR carrier. A vast majority of these networks worldwide use 3.5 GHz, a carrier bandwidth of up to 100 MHz, applying multiple input multiple output (MIMO) 4x4, and utilizing TDD mode. Due to varying local regulatory requirements, the 3.5 GHz band is covered by three different frequency bands from a standardization perspective. These bands are n77 (Asia), n78 (Europe), and n48 (USA) [1]. As the majority of frequency bands worldwide are FDD based and used by LTE, the first 5G NR network deployments took advantage of the underutilized TDD frequency bands, including 3.5 GHz. The first generation of 5G modems and subsequently, the first generation of 5G mobile devices do only support TDD mode for FR1. FR1 FDD is something that the industry is still working on to commercialize.


The need for dynamic spectrum sharing

Not all service providers own spectrum licenses within a TDD band. To take full advantage of 5G with an optimized quality of service flow, to lower latencies and to further address the new market verticals (i.e., Automotive and Industry 4.0), a network operator has to transition to standalone (SA) mode, in which the 5G RAN is connected to the 5G core network (5G-CN, Option 2). There are several intermediate steps (Option 4, 5, and 7) defined that lead towards a standalone deployment. Which path an operator follows is up to their 5G deployment strategy. For a detailed description of any of these options and other fundamental aspects of the fifth generation of wireless communication, please refer to [2].

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