Dynamic spectrum sharing will enable mobile network operators to expand 5G coverage without having to permanently refarm LTE spectrum or buy 5G spectrum.
Dynamic spectrum sharing (DSS) can bring significant benefits to mobile network operators (MNOs), enabling them to expand 5G coverage without having to permanently refarm Long Term Evolution (LTE) spectrum or buy 5G spectrum. DSS rollout is possible through a software upgrade on existing base stations. Sound too good to be true? Maybe. Here is an overview of the concept, implementation challenges, and possible solutions.
What is DSS?
The 5G vision aims to develop one network able to support multiple and widely-different use cases — enhanced mobile broadband (eMBB), massive machine-type communications (mMTC), and ultra-reliable ultra-low latency communications (URLLC). These use cases require spectrum across the low, mid, and high bands:
However, most 5G networks only use mid and high bands, and 5G services will struggle to reach urban centers and go deep inside buildings without the low bands.
Today, 5G frequency range 1 (FR1) introduction in many markets uses the 3.5 GHz band, although there are deployments using lower bands like 700 MHz. Some operators have been able to close legacy networks and refarm those frequencies to LTE, but many others have kept 2G and 3G running to maintain legacy devices and provide circuit-switched voice support. As a result, many operators do not have spectrum available to deploy 5G on these bands. Refarming LTE to new radio (NR) is not an option because most of the traffic will continue to run on LTE for the next few years. DSS addresses this challenge by enabling operators to introduce 5G on existing 4G bands without refarming carriers, and with minimal impact on existing services.
DSS allows LTE and NR coexistence in the same carrier using spectrum sharing. LTE and NR devices have access to the entire bandwidth. Resources are shared dynamically between the two radios based on traffic demand in the time and frequency domains. Mobile operators can therefore adapt to traffic demand. In addition, they can roll out DSS through a software upgrade. These advantages make DSS a great opportunity for mobile operators, even though it increases scheduling complexity because it requires rapid coordination between the two technologies (Figure 1).
DSS implementation techniques
Backward compatibility is at the core of the DSS concept. Today, there are many LTE devices in use, making it impossible for operators to modify the LTE transmission. The sharing of LTE and NR must be transparent to LTE devices and NR transmission must adapt to coexist with LTE.
LTE transmission uses 15-kHz subcarrier spacing, while NR can use 15- or 30-kHz subcarrier spacing. Initial DSS deployments use 15-kHz subcarrier spacing. NR becomes orthogonal with LTE when using 15-kHz subcarrier spacing because it uses the same time and frequency grid. This is not the case when NR uses 30-kHz subcarrier spacing, though. Yet LTE and NR still share the same time and frequency resources from the network perspective, requiring user equipment (UE) capable of decoding the combined LTE and NR transmission. Legacy LTE devices must decode the LTE signal just as in the traditional LTE network, and NR devices must decode the NR signal. A device that supports both needs the capability to decode both signals simultaneously.
When using 30-kHz subcarrier spacing, NR will occupy twice as much bandwidth in the frequency domain, but half of the duration in the time domain. Mixed numerology causes interference, breaking the orthogonality. Using a guard band to separate assignments in the frequency domain avoids such interference. Time multiplexing also achieves this goal by separating the two transmissions in the time domain (Figure 2).
DSS physical-layer implementation can use two techniques: rate matching and multicast broadcast single frequency network (MBSFN) subframe. The rate-matching technique involves resource elements that carry the LTE always-on signals. Rate matching is the common technique for NR data transmission using 15-kHz subcarrier spacing. Using MBSFN subframes is common for NR synchronization signal block (SSB) transmission and NR data transmission using 30-kHz subcarrier spacing. You can use this technique for other use cases as well, such as transmitting periodic signals.
The rate-matching technique is used for NR physical downlink shared channel (PDSCH) transmission, using the patterns defined in 3GPP technical specifications. The information carried in the pattern is shown in Table 1. The PDSCH demodulation reference signal (DMRS) is not rate-matched in order to guarantee DMRS performance. The rate-matching pattern in 3GPP dictates how the network provides the rate-matching information to a UE. The UE is aware of the resource elements that carry the LTE cell-specific reference signal (CRS) and ignores them when decoding the NR PDSCH.
LTE channel carrier frequency and bandwidth information allow coexistence. The LTE and MBSFN subframe configuration carries information about the LTE subframes configured as MBSFN. This influences the set of orthogonal frequency division multiplexing (OFDM) symbols in which the CRS transmission occurs. The number of LTE CRS antenna ports will impact the set of OFDM symbols on which the CRS transmission occurs and the resource elements in the frequency domain. v-Shift provides the exact frequency domain position of the LTE CRS. The rate-matching pattern in Release 15 is only for single-carrier LTE, and DSS can only be used within a single component carrier, limiting NR bandwidth to 20 MHz.
Table 1 Rate-matching technique using the patterns defined in 3GPP TS 38.214 and TS 38.331
For the NR synchronization signal/physical broadcast channel (SS/PBCH), subcarrier spacing depends on the NR operating band. FR1 bands mostly use 15-kHz subcarrier spacing, but conflicts prevent the use of a normal LTE subframe, requiring MBSFN subframe implementation. You can fit up to two SSBs in the MBSFN region because there is no CRS transmitted in that region. However, not all SSBs fall within a valid MBSFN subframe as SSB locations are fixed in the time domain and need to align with a valid MBSFN subframe. As a result, a mix of both rate-matching and MBSFN-subframe techniques are important for DSS transmission, one for the data transmission and the other for the SSB transmission (Figure 3).
When it comes to the NR physical downlink control channel (PDCCH), it cannot collide with LTE reference signal and control channels. Also, symbol 2 of a subframe is the earliest symbol you can use to transmit NR PDCCH because of the LTE control region, whether you are using normal LTE subframes or MBSFN subframes. However, 5G allows PDCCH transmission on any symbol. You can transmit on any other symbols that do not collide with LTE CRS for more PDCCH.
For the uplink, a half-subcarrier shift is a key consideration for DSS. The LTE uplink has a 7.5-kHz offset to avoid the use of the DC subcarrier, but not NR. The DC subcarrier is used for NR uplink transmission. The 7.5-kHz offset will break the orthogonality of LTE and NR. Adding a 7.5-kHz frequency shift for the uplink addresses this challenge, but NR UEs need to support it (Figure 4).
Figure 4 An NR update for DSS adds a 7.5-kHz frequency shift to the uplink.
DSS RF requirements and validation challenges
One of the key aspects to take into consideration when making measurements for DSS is the synchronization between 4G and 5G systems. They must remain synchronized in the time and frequency domains to prevent resource block misalignment. The other key aspect to pay attention to is the fast coordination rate between the LTE and NR packet schedulers that is essential to handle the dynamic allocation of resources. Allocating the same resources will cause a UE decoding failure.
In addition, keep in mind that DSS also introduces an alternative DMRS location for PDSCH. It moves the additional DMRS location from symbol 11 to symbol 12 to avoid collision with LTE CRS present in symbol 11. The UE will need to notify the network that it supports the use of symbol 12 for DMRS to avoid a high block error rate. From the measurement perspective, LTE and NR systems’ synchronization is essential. Simultaneous capture and parallel LTE and NR measurements are key in a lab environment to validate the implementation before testing with hardware in the field.
When testing a DSS transmitter, verifying that the LTE and the NR signals can be separated from the combined signal is important. You should check for successful synchronization with a high sync correlation. Verifying the functionality of existing LTE devices is also critical because they must remain unaffected. You will also need to check that the SS/PBCH transmission using the MBSFN subframe is successful and that the rate-matching pattern implementation on the NR PDSCH is correct. Checking for low error vector magnitude (EVM) and cyclic redundancy check (CRC) pass/fail will tell you if your physical layer implementations are correct.
Enhanced DSS with 3GPP Release 16
DSS brings new challenges to design and test engineers but is a powerful feature for network operators, enabling them to deploy NR using spectrum that is already available. Backward compatibility with existing LTE devices also ensures LTE subscribers continue to experience the same quality of service. For mobile operators, these benefits are too compelling to pass up and DSS implementation will only increase in the future.
3GPP specifications also continue to evolve. 3GPP Release 16 will introduce improvements for resource efficiency for DSS. NR PDSCH Type B length will increase from a maximum of 7 to 9 or 10 symbols, with the DMRS patterns defined to avoid collision with symbols containing LTE-CRS. The LTE-CRS rate-matching patterns will also support multiple LTE component carriers, enabling a wide-bandwidth 5G NR carrier to overlay across multiple LTE component carriers. These enhancements will make DSS more attractive to operators.
You can learn more about DSS, review underlying concepts, and see examples by viewing this Keysight webinar on the topic, 5G NR Dynamic Spectrum Sharing (DSS): Overview and Test Challenges. For more information, visit Keysight’s page on 5G Dynamic Spectrum Sharing.
Jessy Cavazos is part of Keysight’s Industry Solutions Marketing team.