The Limitations of the Legacy Distributed Antenna Systems in the 5G Era

In the 5G era, many legacy Distributed Antenna Systems (DAS) are facing several limitations to incorporate 5G and increase the overall capacity of the system. These limitations are mainly associated with two factors: the new frequency bands in which 5G typically operates and the major infrastructure works needed to increase the capacity of the system. Some of these limitations can be described as follows:

• The frequency range of legacy passive components span from 700 MHz to 3 GHz. Thus, if an upgrade to incorporate 5G in 3.5 GHz is required, all passive components for main and Multiple Input, Multiple Output (MIMO) channels will require replacement to comply with the new frequency range.
• A redesign and additional installation work will be required in order to provide blanket coverage in the 3.5 GHz band, since propagation conditions are very different compared to lower frequencies.
• Major upgrade works involve new active equipment, passive components, cabling infrastructure, and antenna swap. In addition, a dedicated link budget analysis will typically need to be carried out so that main and MIMO channels are well balanced.

According to recent research findings, Mobile Network Operators (MNOs) have started to deploy 5G indoors in high-density venues such as stadiums and music venues. For example, Deutsche Telekom in Germany deployed 5G in Munich's Allianz Arena. The upgrade consisted of the installation of 11 5G radios mounted on the roof of the arena and on a parking garage to provide indoor and outdoor coverage. Similarly, Verizon and AT&T in the United States are adding 5G to sports venues (mainly NFL and AT&T stadiums). Many of these upgrades involve new DAS infrastructure covering the 5G frequency range. Different strategies are being adopted by MNOs to incorporate 5G and leverage legacy DAS infrastructure. These strategies are mainly spectrum re-farming and the overlay of Distributed Radio Systems (DRS) like Ericson DOT, Nokia AirScale, and Huawei LampSite.

The spectrum re-farming technique allows MNOs to easily deploy 5G in legacy DAS systems as long as the platform is able to support the new frequency bands. For instance, in Hong Kong, China, there are MNOs that re-farmed a 3G 10 MHz at 2.1GHz for 5G. The operator used the 2.1 GHz band in legacy DAS for 5G and overlaid a new DRS solution to introduce 50 MHz of 5G spectrum in the 3.5 GHz frequency band. In this upgrade, the 2.1 GHz band was used for signal continuity and the DRS solution was used to meet the capacity needs in the more congested areas. This approach, however, will not provide huge benefits to end users in terms of higher throughputs since there is limited spectrum bandwidth in the sub-3 GHz frequency bands. A further example can be AT&T in the United States, which has reframed 5 MHz of spectrum in the low-band 850 MHz, although there is no evidence that AT&T has deployed this frequency band indoors yet. Regardless, there is evidence that MNOs are re-farming their spectrum to favor the deployment of 5G. Thus, ABI Research envisages that this technique will be used by MNOs in order to easily upgrade legacy DAS systems to incorporate 5G.

The system overlay technique has also been adopted by several MNOs. With this approach, legacy DAS systems have been left in place without any major changes for 3G/4G. Instead, the legacy 4G infrastructure has been used to work as an anchor band for the deployment of 5G in Non-Standalone (NSA) configuration. In Figure 1, there is an example of how the overlaid upgrade could be carried out by deploying DRS systems. Firstly, a new baseband module capable of supporting 4G and 5G technologies needs to be deployed, thus replacing the legacy 4G baseband unit. Secondly, the new DRS infrastructure is deployed to provide 5G coverage in the target areas.

DRS systems mainly encompass the baseband unit, which is linked through fiber to an aggregation switch or radio controller. Then the radio controller connects to multiple radios, which are like Wi-Fi Access Points (APs), by means of fiber or CAT6 cable depending on the vendor and the total length between the radio unit and the aggregation switch.

DRS systems typically have a more simplified architecture compared to legacy DAS solutions. DRS systems give MNOs the ability to immediately increase MIMO capabilities through the distributed MIMO features of the solution. Many DRS systems available in the market are capable of supporting 4x4 MIMO and even 8x8 MIMO capabilities, further increasing capacity in the venue using the same radio unit, CAT6 cable, and fiber link without the need for additional infrastructure, hence representing an alternative for MNOs to effectively deploy more advanced features without significant cost.

As explained in the sections above, legacy DAS systems are facing challenges to upgrade to 5G. However, there are ways for these systems to support 5G in a cost-effective way and avoid rip and replace scenarios. ABI Research does not expect major upgrade works to be carried out in legacy DAS systems in order to incorporate 5G, as these could easily become expensive and time-consuming. Instead, ABI Research recommends that the architecture and capabilities of legacy DAS be left in place without modification to support legacy cellular technologies (3G/4G) and that the new generation of digital DAS solutions, like DRS, are overlaid. With this approach, the end user will benefit from higher data throughput and additional capacity since all the 5G bandwidth will be deployed in the overlaid system. Naturally there are drawbacks to this option, especially since building owners and DAS operators will have to operate two largely independent systems. However, until DRS systems become fully mature and existing DAS installations need to be replaced, these options represent a graceful compromise.

Flexible solutions with advanced features and capabilities like DRS systems are disrupting the market. These solutions change the way traditional DAS systems are designed and installed due to their simplified and future-proofed architecture. ABI Research expects that the market opportunity for DRS systems will grow in the following years, during which the solution is not only going to be adopted to address 5G upgrades for legacy DAS systems but also as the main indoor wireless solution for all venue sizes.

Furthermore, spectrum re-farming techniques could leverage legacy DAS infrastructure. If this technique is used to complement overlaid DRS solutions, the overall upgrade to 5G could work with carrier aggregation further increasing user data throughputs and system capacity. It is worth noting that spectrum re-farming is a technique that many MNOs are adopting in order to favor the deployment of 4G and 5G. Therefore, ABI Research foresees the adoption of spectrum re-farming and overlaid 5G systems as a technique to boost 5G coverage in venues of all sizes where there are legacy DAS systems.