5G NR (New Radio) wireless mobile communications will bring higher data rates, reduced latency and greater system capacity. The first implementation of 5G NR will utilize existing 4G LTE infrastructure in a non-standalone (NSA) mode, while a full standalone (SA) mode that does not rely on LTE will follow later. The introduction of dynamic spectrum sharing (DSS) will permit 5G NR and 4G LTE to coexist, allowing network operators a smooth transition from LTE to 5G NR.
Initial 5G Deployment
The first 5G NR networks are on-air using NSA mode, where an LTE anchor is required to exchange control and signaling information. The anchor is also required to configure, add, modify and release the connection to the 5G NR radio access network (RAN). In this setup, the LTE base station takes on the role of the master cell group and the 5G base station becomes the secondary cell group. Both RANs connect to the existing LTE core network.
Since the majority of frequency bands worldwide are frequency-division duplex (FDD) and are used by 4G LTE, the first 5G NR network deployments took advantage of the underutilized time-division duplex (TDD) frequency bands, including 3.5 GHz in Frequency Range 1 (FR1, the sub-6 GHz frequency bands). The first generation of 5G modems and mobile devices only support the TDD mode for FR1. Today’s 5G deployments typically combine multiple LTE carriers with one 5G NR carrier.
The next phase of 5G NR deployments will be based on FDD in the paired spectrum, as almost 90 percent of the spectrum below 8 GHz is organized as paired frequency bands, where downlink and uplink use different frequencies.
Not all service providers own spectrum licenses within a TDD band. To take advantage of 5G with its optimized quality of service, and to further address the new market verticals (e.g., automotive and industrial), a network operator must transition to standalone (SA) mode, in which the 5G RAN is connected to the 5G core network. There are several intermediate steps leading to a standalone deployment, and each operator can follow the path defined by its 5G strategy. For a detailed description of these options, please refer to Kottkamp et al.1
Due to the occupation of their FDD-based spectrum assets, service providers are forced to choose between two costly options:
- Acquiring new spectrum
- Refarming spectrum already in use
However, the 5G NR standard offers the possibility of adapting to existing LTE deployments and sharing the spectrum used exclusively by LTE today. The enabling mechanism is DSS, which allows 5G NR and 4G LTE to coexist while using the same spectrum. In the long term, DSS enables network operators to provide a coverage layer for 5G using the lower frequency bands. Some network operators already take advantage of DSS, and a large scale deployment is expected late 2020 to early 2021.
Impact on LTE and 5G NR
The impact of DSS on LTE is marginal, as it is difficult to change a successfully deployed technology to enable its successor. A 5G NR device needs to detect synchronization signal blocks (SSB) to access the network. To maintain synchronization in time and frequency, SSBs must be sent periodically by the network, with a gap defined to transmit the SSB on an already occupied frequency channel used by LTE. The ideal feature to allow this gap in a continuous LTE transmission is to use multimedia broadcast single frequency network (MBSFN) subframes.
To minimize the impact on LTE performance, typically only three out of 40 subframes are configured to be an MBSFN subframe. The applied configuration is broadcast by the LTE network with system information block type 2 (SIB2). This is the same SIB that informs a 5G-capable terminal that the LTE serving cell can connect the handset to the 5G RAN. A standard LTE terminal would read in the MBSFN configuration from SIB2 and ignore the subframes configured for broadcast.
Initially, DSS is tested based on NSA mode; thus, the 5G handset would have two radios active, LTE and 5G NR. The LTE portion will follow the same principles as an LTE-only device. The 5G NR part, as it scans the targeted frequency band for sharing, will detect the transmitted SSB within the open LTE subframe on the desired frequency channel.
With just three subframes available for 5G NR, the technology operates under its potential. DSS additionally enables the use of subframes that are dedicated to LTE and not configured for MBSFN via two distinct features:
- Depending on the MIMO mode, standard LTE subframes include cell-specific reference signals (CRS) mapped to certain resource elements in the time-frequency grid. An LTE terminal uses CRS for channel estimation and to maintain full synchronization in time and frequency. To enable NR to use these subframes, rate-matching around the LTE CRS has been adopted.
- An alternative additional position for the mapping of the physical data shared channel (PDSCH) demodulation reference signal (DMRS) is supported, again to avoid collision with LTE CRS. This feature is a device capability; the device signals its support of this functionality to the network during the initial registration process.
This discussion has focused on a semi-static configuration, enabling the use of specific subframes for NR when LTE is not present or when mechanisms that allow NR to transmit in LTE subframes not used by LTE but where essential LTE signals components are still sent. It is possible for LTE and NR to share a subframe and for both to transmit control information and data, when the mapping is defined relative to the beginning of the PDSCH within the slot.
Extensive testing is required for DSS implementation. This includes lab-based LTE and 5G user equipment testing as well as network performance measurements using scanners (sensitive receivers) and devices to estimate coverage and end-to-end (E2E) performance.
The activation of DSS within the network should not create any interference for the existing LTE deployment. LTE-only devices must not suffer any impact when configuring MBSFN subframes. With MBSFN active, E2E throughput tests are required to ensure minimal impact on LTE performance. While 5G NR, including SSB, is transmitted within MBSFN subframes, receiver sensitivity tests for LTE devices must be favorable, to ensure requirements are still met by the device when 5G NR is present.
A 5G NR capable device must be able to synchronize in time and frequency with the 5G RAN when SSBs are transmitted within MBSFN configured subframes. When 5G NR is sent in non-MBSFN subframes using an LTE CRS rate-matching pattern for NR’s PDSCH, a data throughput test is adequate to verify correct implementation of the stack features. Advanced device testing includes dynamic scheduling procedures that mimic the E-UTRA NR resource coordination procedure.
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