NR LTE coexistence – Dynamic Spectrum Sharing (DSS)
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) 4×4, 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].
Due to the occupation of its FDD-based spectrum assets, service providers are forced to choose between the acquisition of new spectrum or refarm spectrum that’s already in use. Both options are costly. Therefore, the 5G NR standard offers the possibility of adapting it to existing LTE deployments and sharing the spectrum that is today exclusively used by LTE. The enabling feature is called Dynamic Spectrum Sharing (DSS) which is part of the overall mechanism that allows NR and LTE to coexist while utilizing the same spectrum assets. Another feature that enables NR LTE coexistence is the decoupling of downlink and uplink transmissions, necessary due to coverage issues in uplink direction at the FR1 mid-range frequency bands of 3.5 GHz and beyond. This feature results in a function called supplemental uplink (SUL) that allows a device to switch its uplink transmission from the midrange spectrum to low band spectrum, based on received signal quality measurements. However, this feature will not be further discussed in this article, as the industry is right now focusing on DSS.
In the long term, DSS enables network operators to provide a coverage layer for 5G using the lower frequency bands, typically targeting frequencies below 1 GHz. DSS needs infrastructure updates, advertised as software-only updates, but also requires the second generation of 5G chipsets and handsets taking advantage of these new modems. Roll out of DSS is expected for early to mid-2020.
The awakening of MBSFN
DSS has an impact on both standards, LTE and 5G NR. The effect on LTE is marginal, as it is hard to change a successfully deployed technology to enable its successor. A 5G NR device needs to detect the synchronization signal blocks (SSB) to access the network. To maintain synchronization in time and frequency, these SSBs need to be sent periodically by the network. To transmit the SSB on an already occupied frequency channel that is used by LTE, there needs to be a gap defined to send them. The ideal feature to allow this gap in a continuous LTE transmission is to use Multimedia Broadcast Single Frequency Network (MBSFN) subframes. Defined initially to enable broadcast via an LTE network and therefore make the transfer of content more efficient than unicast transmissions, this feature is part of the evolved multimedia broadcast multicast services (eMBMS) functionality. eMBMS is part of 3GPP’s Release 9 set of technical specifications.
Six out of ten subframes forming the LTE radio frame can be configured by the network to become MBSFN subframes. Based on the standard, this could be subframes #1, #2, #3 and #6, #7, and #8. To minimize the impact on the performance of LTE, typically, only one subframe out of the six possible subframes is 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. However, the 5G NR part of the handset, scanning the targeted frequency band for sharing, will detect the transmitted SSB within that freed up LTE subframe on the desired frequency channel. As DSS is intended to provide a coverage layer for 5G, typically frequency bands below 1 GHz are shared between LTE and 5G NR. Initially, a subcarrier spacing of 15 kHz is used for 5G NR to avoid interference between both technologies, resulting in the same number of subframes used in LTE and 5G NR. Based on the mapping principles for SSB for 15 kHz, defined as Case A, the targeted frequency band of below 1 GHz leads to a maximum of four SSB’s transmitted by the 5G network. Typically, the SSBs are transmitted by the network in the first half-frame (5 ms) of a radio frame. As it is not possible in LTE to configure subframe #0 for MBSFN, subframe #1 is required to be configured as the MBSFN subframe so it can carry SSBs. However, an MBSFN subframe is never entirely empty. There is a non-MBSFN region defined that can be one or two OFDM symbols long, dependent on overall signal bandwidth. This region is intended to carry the control channels for LTE, such as physical hybrid ARQ channel (PHICH), physical control format indicator channel (PCFICH), and the physical downlink control channel (PDCCH). Therefore, any NR transmission can only start at OFDM symbols #2 or #3 within an MBSFN subframe. The requirement of transmitting control information for NR, to schedule the reception of NR’s physical data shared channel (PDSCH), and the necessity to map a demodulation reference signal for the data channel to the beginning of the subframes lead to the configuration illustrated in Figure 1.
Out of the total 14 OFDM symbols forming a subframe of 1 ms duration, corresponding for SCS of 15 kHz to one slot in 5G NR, only 12 OFDM symbols are available for NR transmission. For proper demodulation of the PDSCH and to enable mobility, a second symbol carrying DMRS for the data channel is required. According to [3], this additional symbol is symbol #9, as depicted in Figure 1.
Increasing capacity and improving 5G NR while DSS is configured
With just one subframe available for 5G NR, the technology operates under its potential. Therefore, DSS additionally enables the use of subframes that are dedicated to LTE and not configured for MBSFN via two distinct features. Dependent on the used MIMO mode (2×2, 4×4), 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 domain. Assuming a simple scenario that the scheduler in the LTE base station based on the current load situation is not mapping any data into such a subframe, the LTE CRS would still be present, maintained, and transmitted. To enable NR to use these subframes, rate matching around LTE CRS is adopted by the standard. Several factors impact the rate matching algorithm. The first parameter is required to align the subcarrier positioning for 5G NR related to LTE. The value corresponds to the offset to Point A divided by 15 kHz. Second, the bandwidth and number of antenna ports used by LTE need to be known as the MIMO mode dictates the mapping of cell-specific reference signals per antenna port. As mentioned earlier, DSS targets frequencies below 1 GHz, therefore typically two antenna ports (MIMO 2×2) is used. Lastly, the factor vshift represents the impact of the physical cell identity (PCI; vshift = PCI mod 6) which defines the starting point (subcarrier) for the mapping of LTE’s sequence used for generating CRS. Figure 2 displays all required parameters to configure the LTE CRS rate matching algorithm on the R&S®SMW200A vector signal generator, intended to perform physical layer testing of the 5G modem.
In a real network, this information is signaled to the device via the dedicated RRC connection. As NSA mode is the initial deployment mode that DSS is tested with, this information is sent over the established LTE connection to the device. In SA mode it would be sent over the NR RRC connection.
The second required feature is the support of an additional position for the mapping of the PDSCH Demodulation Reference Signal (DMRS). Based on a standard LTE subframe, with LTE control channel and CRS present, assuming the scheduler does not schedule any PDSCH, the remainder of the subframes is available to 5G NR and therefore the CORESET, and the NR PDSCH with rate matching active, including DMRS is mapped on the available resource element as shown in as an example in Figure 3. Due to CORESET occupying OFDM symbol #2, the first PDSCH DMRS is assigned to symbol #3. The position information (l0) of the OFDM symbol (2 or 3) that carries the first DMRS is indicated with the master information block (MIB) carried by the physical broadcast channel (PBCH) as part of the SSB. To support mobility, proper channel estimation is a prerequisite that can only be guaranteed if at least two symbols within a slot carry PDSCH DMRS. According to [3], for the provided example, this additional symbol to carry DMRS would be OFDM symbol #11. However, from an LTE perspective, cell-specific reference signals are still present and transmitted in this symbol. Therefore the additional position of the DMRS needs to move from symbol #11 to symbol #12 [Figure 3]. This feature is a device capability, which means the device signals its support of this functionality to the network during the initial registration process.
In general, this change is only applicable if three conditions are fulfilled. First, the device needs to have submitted the support of the capability to the network. Second, the network has configured the device with the rate matching parameter for LTE CRS via RRC, and third, the first position of the PDSCH DMRS is set to l0 = 3.
Where is the dynamic sharing element?
Up to now, we have more or less discussed a semi-static configuration of both LTE and NR to enable the use of specific subframes for NR when LTE is not present at all, or mechanisms that allow NR to transmit in LTE subframes that are not used by LTE but where essential LTE signals components are still sent in. The question remains, is there a way for LTE and NR to share a subframe, and for both to transmit control information (PDCCH and CORESET) and data (PDSCH)? The answer is yes.
So far we have discussed fundamental mapping of the data channel and corresponding demodulation reference signal for 5G NR based on the so-called PDSCH mapping type A. But the 3GPP standard defines additionally the PDSCH mapping type B. The difference between these two is that type A defines the mapping relative from the slot start, whereas type B represents the mapping relative from the beginning of the PDSCH within the slot. PDSCH mapping type A allows, according to the standard, a maximum symbol offset of 2 which, under certain conditions, is not favorable when applying dynamic spectrum sharing between LTE and 5G NR. PDSCH mapping type B overcomes this drawback. Figure 4 shows a configuration example for LTE with 5 MHz channel bandwidth, where NR is transmitted on the upper six out of 25 resource blocks.
Coordination, coordination, and again… coordination
All described physical layer features require coordination among both radio access networks as in a dual connectivity approach, both (LTE and NR) are using two independent schedulers. The resulting E-UTRA – NR cell resource coordination procedure [Figure 5] that can be triggered by both nodes over the Xn interface is defined in [4].
The process allows the coordination of scheduling resources in the frequency domain and time domain at the medium access control (MAC) layer in both base stations. When initially testing DSS in NSA mode, the coordination process is triggered by the eNB. In its request message, the eNB sends the data traffic resource indication towards the gNB. This information element contains the information if the sharing is in uplink only or for uplink and downlink. For the latter case, two individual bitmaps are provided with the message that contains a bitmap between 6 and 17600 bits long. Each position in this bitstring stands for a physical resource block pair (PRB) that is reserved for E-UTRA if it is set to ‘1’ and it is not used for E-UTRA when it is set to ‘0’. The bit string may span across multiple contiguous subframes. The first position of the data traffic resources information element corresponds to the receiving node’s subframe 0. The length of the bitmap is an integer multiple of the bandwidth. Assuming the signal bandwidth to be 5 MHz (25 PRB), a total of 704 subframes are addressed through the bitmap. The bitmap length doesn’t provide the flexibility to address all subframes within radio frames, which makes it necessary to provide information with which system frame number (SFN) the bitmap become active Figure 6 gives an example of the bitmap. Based on this configuration, resource blocks 15 to 24 in subframe #0 (light blue), resource blocks 5 to 24 in subframe #1 (green), resource blocks 3 to 17 (brown) and resource blocks 8 to 22 (purple) in subframe #703 can be used for 5G NR transmission and reception.
Besides, other resources in LTE are protected through the exchange of the Protected E-UTRA Resource Indication message between the eNB and the gNB during the setup of the X2 interface, which is a separate but mandatory prerequisite. As explained earlier, LTE configures MBSFN subframes to allow the transmission of SSBs for NR. These subframes are protected from the above configuration by additional information provided by the data traffic resource indication as a reserved subframe pattern.
Testing for dynamic spectrum sharing
DSS is a powerful feature, but also requires extensive testing for several reasons. This applies to lab-based LTE and 5G user equipment testing as well as for carrying out network performance measurements using scanners (sensitive receivers) and devices to estimate coverage and end-to-end (E2E) performance.
First of all, the activation of DSS within the network should not create any interference for the exiting LTE deployment. Second, it needs to be ensured that also LTE-only devices don’t suffer any interference or impact when, i.e., configuring MBSFN subframes within the network. Further, with MBSFN active, end-to-end throughput tests are required to ensure minimal impact on LTE performance. While 5G NR, including synchronization signal blocks, are transmitted within MSBFN subframes, receiver sensitivity tests for LTE devices are favorable, to ensure sensitivity requirements are still met by the device when 5G NR is present within the channel at defined subframes.
A 5G NR capable devices need to be able to synchronize in time and frequency to the 5G radio access network (RAN) when SSBs are transmitted within MBSFN configured subframes. When 5G NR being sent in Non-MBSFN subframes using LTE CRS rate matching pattern for NR’s PDSCH, then a data throughput test is adequate to verify correct implementation of the stack features. Advanced device testing would include dynamic scheduling procedures that follow the mimic the described E-UTRA NR resource coordination procedure, where data is scheduled according to data traffic resource indication, including the validation of PDSCH mapping types A and B. The new R&S CMX500 mobile radio tester platform from Rohde & Schwarz, in combination with the R&S CMW500 wideband radio communication tester, is the right tool to carry out these extensive test scenarios on mobile devices supporting LTE and 5G NR.
DSS has a further impact on mobile network testing when performing coverage measurements and network optimization for LTE and 5G NR. The toolset used by network equipment providers during initial rollout and network operators during optimization and maintenance phase is based on a network scanner, a sensitive receiver, and a drive test software, collecting the measurements from this passive probe. A mobile device complements the setup that is used to perform end-to-end (E2E) performance tests in the network, e.g. file download and upload or video streaming. In this setup, a scanner performs the same signal quality measurement on the transmitted downlink signal as the mobile device; thus, the results can be correlated and validated. In terms of DSS, the scanner needs now the capabilities to not just identify LTE and NR signals, but moreover have the capabilities to identify subframes, that are configured as MBSFN and carry NR signals.
Outlook – DSS enhancements in 3GPP Release 16
As depicted in Figure 4, PDSCH Mapping Type B, as specified in 3GPP Release 15, has the limitation that at maximum, seven OFDM symbols can be allocated when this mapping type is configured. In 3GPP Release 16 PDSCH mapping type B will be extended so that nine and ten symbols can be assigned to use the slot efficiently.
Another change is that with Release 16, the definition of multiple LTE CRS rate matching patterns is supported for one device. The reason is that a network operator may have deployed multiple LTE carriers within a frequency band, i.e. three, as shown in Figure 7. These three carriers can use different bandwidths and MIMO schemes (e.g. ten vs 20 MHz, MIMO 2×2 vs 4×4). Even if the carriers have the same bandwidths and MIMO schemes (i.e. carrier #2 and #3), they have different physical cell identities (PCI) that dictate which resource elements carry the cell-specific reference signals and therefore impacts the rate matching algorithm. 5G NR supports wider bandwidths, e.g. 50 MHz, and could, therefore, use the entire channel. The definition of multiple LTE CRS rate matching algorithms to be used by the device supports now this specific deployment scenario and enables the use of dynamic spectrum sharing functionality.
Summary and conclusion
Dynamic Spectrum Sharing is the hot buzz word in the wireless industry right now. It provides a service provider with the possibility to enable a coverage layer for 5G NR at low band frequencies. Besides, it allows the carrier to smoothly transition the LTE subscriber base towards 5G and rollout standalone mode much faster by avoiding high cost for spectrum refarming or acquiring new spectrum licenses.
From a technology standpoint, DSS combines several sophisticated features to enable coexistence between LTE and 5G NR using the same spectrum band. Therefore, it is essential to verify all the features and functionalities described above, not only in the lab by testing DSS-capable 5G handsets, but also in the network itself when performing drive tests for network optimization. Rohde & Schwarz, as a premium supplier of test solutions to the wireless industry, has the right toolset in its portfolio to carry out these tasks.
References
[1] 3GPP TS 38.101 V15.6.0 (2019-06), User Equipment (UE) radio transmission and reception; Part 1: Range 1 Standalone
[2] Meik Kottkamp, Anil Pandey, Daniela Raddino, Andreas Roessler, Reiner Stuhlfauth; 5G New Radio – Fundamentals, procedures, testing aspects; 1st edition, 2019
[3] 3GPP TS 38.211 V15.6.0 (2019-06), Physical channels and modulation (Release 15)
[4] 3GPP TS 38.423 V15.4.0 (2019-07), NG-RAN; Xn application protocol (XnAP) (Release 15)
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