Introduction to Dynamic Spectrum Sharing
Dynamic Spectrum Sharing (DSS) is a groundbreaking technology that allows both 4G and 5G networks to operate simultaneously within the same spectrum band. This innovation is pivotal in the smooth transition from 4G to 5G, providing enhanced network capabilities without the need for additional spectrum allocation. This article delves into the intricacies of DSS, exploring its benefits, technical implementation, challenges, and future prospects.
Table of Contents
Understanding Dynamic Spectrum Sharing
Dynamic Spectrum Sharing allows both 4G and 5G to simultaneously operate within the same spectrum, i.e. the set of Resource Elements is shared between the two technologies. Dynamic Spectrum Sharing is applicable to Frequency Range 1 because existing 4G operating bands arc below 6 GHz.
Dynamic Spectrum Sharing allows spectrum to be re-farmed without a step change in its utilisation, i.e. it allows a smooth evolution from one technology to another technology. Figure illustrates an example Base Station which is initially configured with two 4G earners
The first option for re-farming involves the complete replacement of one 4G carrier with a 5G carrier. This approach can lead to a step change in the spectrum utilisation if the re-farming is completed at a time when the penetration of 5G devices is low. The original 4G carrier was heavily loaded whereas the new 5G carrier is unloaded. In addition, the remaining 4G carriers become more congested due to the reduced quantity of 4G spectrum.
The second option for re-fanning is based upon Dynamic Spectrum Sharing. In this case, the 4G system continues to use both carriers but one carrier can also be used by the 5G system. The shared carrier is able to support both 4G and 5G devices. This requires co-ordination of the resources used by each system.
The reforming of 4G spectrum is unlikely to provide 5G with the wide channel bandwidths required to achieve very hight throughputs but it can be used to provide 5G with spectrum within the lower operating bands which offers improved coverage due to the lower air interface attenuation.
The Benefits of Dynamic Spectrum Sharing
Dynamic Spectrum Sharing requires the 4G and 5G systems to be synchronised in both the time and frequency domains. In the downlink direction, it is relatively simple to achieve radio frame synchronisation between the two technologies. Both sets of transmissions can share the same downlink timing reference at the Base Station, e.g. derived from GPS or Timing over Packet. In the uplink direction it is necessary to align the Timing Advance applied by the population of 4G and 5G lJE
A UE adds a fixed Timing Advance Offset to the Timing Advance commands provided by the Base Station. In the case of 4G, the fixed Timing Advance Offset is '0' for FDD and '624 x Ts' for TDD. 5G uses a time unit of Ts rather than Tc , where Ts = 64 x Tc. This means that a 5G UE must be configured to use an offset of 0 when sharing spectrum with 4G FDD, and an offset of 64 x 624 = 39936 x Tc when sharing spectrum with 4G TDD.
The Base Station instructs the UE to use a specific fixed Timing Advance Offset to the Timing Advance Offset information element shown in Table 364. The value of25600 is the normal value fur 5G when using Frequency Range 1 without Dynamic Spectrum Sharing (applicable to both FDD and TDD). In the case of Frequency Range 2, Dynamic Spectrum Sharing is not applicable and a fixed value of 13792 is used.
How Dynamic Spectrum Sharing Works
Achieving frequency synchronisation between 4G and 5G is relatively simple in the downlink direction. A common frequency reference can be used at the Base Station and both technologies generate a baseband signal which is centered around 0 Hz. However, it should be noted that 4G does not transmit data on the DC subcarrier. When the 4G channel bandwidth includes an odd number of Resource Blocks, this leads to the central Resource Block occupying 13 subcarriers rather than 12 sub carriers. When the 4G channel bandwidth includes an even number ofResource Blocks, this leads to an additional subcarrier between the central pair ofResource Blocks. In both cases, it means that the 4G and 5G Resource Blocks will not be aligned both sides of the central subcarrier. Figure illustrates the case for an odd number of 4G Resource Blocks. The 4G and 5G Packet Schedulers arc responsible for ensuring that Resource Block allocations do not overlap.
The 4G transmission will use a 15kHz subcarrier spacing, while the 5G transmission can use either a 15kHz or 30kHz subcarrier spacing. If using the 30 kHz subcarrier spacing, a 5G Resource Element will occupy twice as much bandwidth in the frequency domain but half of the duration in the time domain. Nevertheless, the 4G and 5G transmissions remain orthogonal.
How Dynamic Spectrum Sharing Works
Achieving frequency synchronisation in the uplink direction requires an adjustment to the default 5G waveform. The SC-FDMA waveform used by 4G has a 7.5 kHz offset applied to avoid having a DC subcarrier. By default, the CP-OFDM and DFT-S-OFDM waveforms used by 5G do not include this 7.5 kHz offset and the DC subcarrier can be used to transfer data.
In the case of Dynamic Spectrum Sharing, the Base Station can instruct the 5G UE to apply a 7.5 kHz offset using the frequency Shift 7p 5kHz information element presented in the Table . This aligns the 4G and 5G uplink transmissions and allows them to coexist within the same Resource Element grid.
5G transmissions must operate around the 4G Cell specific Reference Signal (CRS). Figure 489 illustrates an example ofthe 4G CRS when using 2 antenna ports. One antenna port transmits the CRS using the Resource Elements labelled as 'CRS', while the other antenna port transmits the CRS using the Resource Elements labelled as 'DTX', i.e. each antenna port applies DTX to the Resource Elements used by the other antenna port.
This 4G transmission pattern creates a challenge for the 5G SS/PBCH Blocks. An SS/PBCH Block occupies 4 symbols, whereas Figure below illustrates that there is a maximum of 3 empty symbols between the CRS transmissions. (this reduces to a maximum of 2 empty symbols when 4G uses 4 antenna ports for CRS transmission). Thus, it is not possible to time multiplex an SS/PBCH Block with the 4G CRS when the SS/PBCH Block uses a 15 kHz subcarrier spacing. It indicates that a large proportion of the operating bands within Frequency Range I use a 15 kHz subcarrier spacing for the SS/PBCH. Thus, an alternative solution is required.
Case Study: Dynamic Spectrum Sharing in Action
The 4G Base Station can use MBSFN subframes to reduce the quantity of symbols used by the CRS. An MBSFN subframe restricts the CRS to the first symbol. The remaining 13 symbols are then available for the 5G Base Station to transmit either one or two SS/PBCH Blocks. In the case of FDD, MBSFN subframes can be configured during subframes I, 2, 3, 6, 7 and 8, whereas in the case of TDD, MBSFN subframes can be configured for use during subframes 3, 4, 7, 8 and 9. A slot offset can be applied to delay the timing of 5G radio frames relative to 4G radio frames. When using FDD, this offset can be used to allow 5G slot O to coincide with 4G slot 1, i.e. 5G slot 0 coincides with an MBSFN subframe and the SS/PBCH Block transmissions can then start from 5G slot 0
A drawback associated with using MBSFN subframes as a solution for time multiplexing the 5GSS/PBCH Blocks is that they can create an overhead towards the transfer of 4G data. Older 4G devices will not be able to receive downlink data during MBSFN subframes. 3GPP release 10, and newer devices are able to receive downlink data when using Transmission Modes 9 or I 0
When 5G uses the 30kHz subcarrier spacing, the SS/PBCH Block occupy only 2LTE symbols. In this case, the SS/PBCH Blocks can be time multiplexed between the CRS transmissions. Figure illustrates an SS/PBCH Block which is time multiplexed between CRS transmissions when assuming 4 CRS antenna ports. The SS/PBCH Block starts during symbol 4 when using the 30 kHz symbol numbering. This corresponds to an SS/PBCH Block transmission belonging to the first 30 kHz option.
In terms of 5G PDSCH transmission, there are two general options which depend upon the subcarrier spacing:
When using the 1 S kHz sub carrier spacing. the SG PDSCH can be time and frequency multiplexed around the 4G CRS, i.e. the 4G CRS punctures the SG PDSCH transmission, reducing the number of Resource Elements available for PDSCH transmission. In this case, 5G PDSCH resource allocations can occupy a relatively large number of symbols.
When using the 30 kHz subcarrier spacing, it is not possible to frequency multiplex the 4G CRS and the 5G POSCH (due to the wider bandwidth of each 5G subcarrier). In this case, only time multiplexing is permitted. When using 4 antenna ports to transmit the CRS, the 5G PDSCH resource allocation duration is limited to a maximum of 4 symbols.
Future Prospects of Dynamic Spectrum Sharing
Table below presents the parameter structure which can be provided to the UE when 5G is using the 15kHz subcarrier spacing and the 5G PDSCH is frequency multiplexed with the CRS. It is used to provide the UE with information regarding the location of the CRS, so the UE can then ignore the CRS Resource Elements when decoding the 5G PDSCH.
5G transmissions must also account for the 4G Synchronisation Signals and the 4G PBCH. The Base Station can use the RateMatchPattern parameter structure to configure Reserved Resources which occupy specific combinations of Resource Blocks and symbols. A first set of Reserved Resources can be configured for the Synchronisation Signals using a period of 5 ms, whereas a second set of Reserved Resources can be configured for the PBCH using a period of IO ms. The Reserved Resources indicate Resource Block/ symbol combinations within which the UE should not attempt to receive the 5G PDSCH.
The 5G Demodulation Reference Signal (DMRS) for the PDSCH can be impacted by Dynamic Spectrum Sharing with LTE. There ir an impact if a single additional DMRS position is configured for a PDSCH resource allocation duration of 13 or 14 symbols. The additional DMRS transmission usually occupies symbol 11, but this symbol coincides with a CRS symbol when using Dynamic Spectrum Sharing with a 15 kHZ subcarrier spacing. In this case, the additional DMRS symbol is moved from symbol 11 to symbol 12.
In addition to the above, the 5G Packet Scheduler must coordinate it's resource allocations with the 4G Packet Scheduler. Co-ordination ensures that the two Packet Schedulers do not allocate the same resources. The rate at which the co-ordination function operates will determine the responsiveness of the system. A rapid co--ordination rate allows the Packet Schedulers to frequently change the resources they are permitted to allocate.
UE declare their support for some aspects of Dynamic Spectrum Sharing within their UE capability infonnation. The raleMalching LTE-CRS information element is used to indicate within which bands the UTI supports rate matching around the LTE Cell specific Reference Signal (CRS). The additional DMRS-DL-Alt information element is used to indicate within which bands the UE supports the use of symbol 12 rather than symbol 11 for the additional OMRS symbol described above.
Conclusion
Dynamic Spectrum Sharing is a transformative technology that plays a crucial role in the evolution from 4G to 5G. By enabling the simultaneous operation of both technologies within the same spectrum, DSS optimizes spectrum utilization, enhances network performance, and provides a cost-effective solution for operators. As the telecom industry continues to innovate, DSS will undoubtedly be a cornerstone in the future of mobile communications.
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