A promising way of realizing ultra-broadband Fixed Wireless services is to deploy networks that exploit the higher frequency bands, speciﬁcally in the millimeter wave (mmWave) spectrum. Access to such spectrum bands enable FWA systems to deliver several hundreds of Mbps to residential and business subscribers, even at the cell edge. 5G, which is based on the 3GPP standard, is a candidate for delivering ultra-broadband FWA, but it is not the only one. There are other purpose built technologies that use mmWave spectrum and are capable of oﬀering similar FWA services with longer cell ranges and less complex core network than 5G. The additional frequency bands that are identiﬁed for the deployment of 5G networks are not exclusively tied to 3GPP. Depending on the country and the relevant regulatory bodies there are cases where spectrum is allocated for both IMT-2020 (5G) use and other ﬁxed wireless technologies. This means that there will be cases of coexistence between 5G and other proprietary FWA networks (incumbent or newcomers). The need for peaceful operation between these networks is a hard requirement and can only be accomplished if interference is kept at a minimum.
To increase ﬂexibility as well as make spectrum usage more eﬃcient, Time Division Duplex (TDD) is becoming increasingly common and important for access applications. TDD uses the same frequency for each duplex direction, with a frame that includes diﬀerent time periods and slots for uplink or downlink communications. By changing the duration of these, network performance can be tailored to meet different needs and help provide the best possible experience.
However, for this to work all TDD networks, either 5G or other proprietary FWA solutions, operating in the same frequency range and within the same area have to be synchronized. Base stations need to transmit at the same ﬁxed time periods and all devices should only transmit in dedicated time periods. Failure to do so creates interference, which has a major impact on performance as well as coverage.
Synchronization is one of the most critical functions of a communication system. Especially for Time Division Duplex (TDD) where both uplink and downlink transmission is on the same frequency, the possibility of interference is much more signiﬁcant.
TDD turns out to be a more attractive option from a spectral eﬃciency point of view because it requires only an unpaired channel for operation, which is beneﬁcial considering the scarcity of frequency resources. While it brings spectral eﬃciency, TDD introduces a critical challenge: Timing and synchronization. Stringent timing restrictions are imposed on a TDD system to avoid interference as both downlink (DL) and uplink UL share the same spectrum.
Besides the common clock/timing reference, TDD deployments also need to use a compatible frame structure between FWA incumbents that use speciﬁc spectrum resources and collocated newcomers (e.g. 5G carriers). If this condition is not met DL (Down Link) spectrum may leak onto the adjacent channels. For FDD, this is acceptable since UL (Up Link) and DL channels are separated by a guard band. For TDD, UL and DL share the same channel. Any DL spectral imperfection may thus create interference in the UL signal of the adjacent operator, especially when the two cells are at the boundaries of each other.
5G NR is using a conﬁgurable slot format which allows the variation of the DL/UL split ratio depending on the type of traﬃc/services that is exchanged between the base station and the UEs (User Equipment). Besides the beneﬁts of a conﬁgurable split ratio it could be challenging if two networks that are oﬀering diﬀerent types of service are located next to each other. Interference can result even though they may be synchronized in time, but their slot formats are not synchronized. The following bullets summarize the synchronization criteria that have to be met in collocated or adjacent TDD networks and also the solution that the operators need to follow so that they will not interfere with one another.
The WiBAS™ G5 PtMP platform is the latest addition to Intracom Telecom’s ultra-broadband FWA product oﬀering. It is operating in the 24.25-29.5 GHz area licensed frequency band, utilizing the advantages and eﬃciency of contiguous TDD spectrum. It can deliver several hundreds of Mbps to subscribers that are located in distances of more than 5km from the base station hub. At the same time, end-to-end SLAs are guaranteed by employing advanced hierarchical Quality of Service mechanism. WiBAS™ G5 is based on a fully purpose-built MAC and Physical Layer, making it thus ﬂexible when it comes to face the challenges that are mentioned in this article.
WiBAS™ G5 can meet all three major challenges. It can deliver various air frame format lengths that match exactly all the available 5G transmission periodicities that result from the numerology agreed between 3GPP operators at a national level. Furthermore, it can also promise common frame start reference by using an external GNSS module achieving, that way, frequency and phase synchronization between all the coexistent base station sites. And ﬁnally, it can split the resources of its air frame in such a way that it can match the various Downlink/Uplink split ratios that are used by the 5G deployments.
The common use of contiguous frequency spectrum in the mmWave band for access networks from diﬀerent available FWA TDD technologies is creating an imminent need for interference free coexistence in collocated or neighboring conﬁgurations. Since 5G is based on the 3GPP standard all other proprietary equipment vendors will need to comply with the challenges set by the new technology. WIBAS™ G5 is able to address all the fundamental synchronization criteria of TDD network coexistence and can guarantee that operators who select this solution for their FWA network will beneﬁt from a simpler core infrastructure and at the same time will not cause interference issues to existing or forthcoming 5G deployments.