5G NR deployments at the 3.5 GHz frequency band and eventually at the 26 GHz frequency spectrum will bring many challenges for indoor coverage, currently provided by outdoor base stations. In the long term, customers will expect 5G indoor coverage almost everywhere. The expectations in terms of 5G data rates are high, especially with the expected 5G network densification and the creation of hot spots leading to the increased demand for dedicated indoor solutions. These indoor scenarios are very diverse, and there is no one-fits-all solution. Therefore, multiple strategies have to be considered, and respective solutions are currently being proposed.
The main topics to be addressed are mainly network criteria such as coverage, capacity, latency, and high availability. For example, one needs to decide on which services have to be supported and if there is an existing installation, how to upgrade to 5G NR. In other words, there needs to be a decision whether to currently use the 3.6 GHz frequency band or directly expand to new frequency bands in the millimeter-wave region of 26/28 GHz bands and beyond. This, of course, will depend on the ease of new deployments and the flexible expansion of existing ones. Moreover, the final decision will also depend on the actual existing installation and whether there is a single or a multi-operator solution.
Providing 5G NR indoor coverage
The appropriate selection of a 5G NR solution depends on the use case to be supported. For example, for frequency bands below 3 GHz, the existing indoor solution can be reused. Under the assumption that these existing systems support 5G NR, DSS LTE/NR could be used. Therefore, the same conditions as for outdoor deployments could apply, such as the need for an anchor band in an NSA scenario. If a new band, for example, 700 MHz, should be added, the support of this band and the inter-modulation situation should be checked.
The 3.6 GHz frequency band can be the primary candidate for use cases that demand higher data rates and need to support multi-gigabit connectivity. However, this band has an increased path loss of about 6 dB compared to 2 GHz bands, which is why different options should be considered depending on each specific scenario and use case.
For the frequency band of 26 GHz and above, a new deployment is also needed. Spectrum might not yet be available in all countries, but it will definitely be in the near future. Again high path loss will be present, especially in locations of access points where high data is needed. One thing to also consider is whether this might become a mainstream solution in the future, and dedicated fibers should be deployed for easy upgrades. In this case scenario, there will be no homogenous coverage.
Distributed Antenna Systems
Operators will need to choose between implementing a Passive and/or Active DAS deployment with 5G. Each system has its own pros and cons for different deployment scenarios. With Passive DAS, there can be a quick launch of FDD 5G with DSS on top of the existing 2G/3G/4G deployment components that support currently used legacy bands. Updated passive components are also available for new bands with various RFQs by Operators around the globe already ongoing. This is also an easy solution in the case of operator sharing. Finally, reusing, even partially, the existing installation and legacy investment is also possible.
A disadvantage of this solution is that there is no higher-grade MIMO feasibility, for example, above 4×4. Existing solutions are mainly single-input single-output (SISO) radio systems where there are no multiple antennas in the transmitter or receiver side. There are also a few 2×2 multiple-input and multiple-output MIMO examples where multiple antennas are used both in the transmitter and receiver to improve communication performance. However, there is a limited throughput performance due to the narrow spectrum and MIMO. The upgrade or sectoring can be difficult or expensive to implement since it requires system re-building. Furthermore, this cannot be a really futureproof system since additional capacity or performance requirement is more complicated. Finally, there is a higher copper consumption and cost, while there are also limitations in the RAN, such as the missing functionality of 1T1R.
Active DAS
On the other hand, with active DAS systems, 5G performance is better than the legacy 4G networks. This solution is flexible, easy, and cheaper for scaling and upgrading. There is full control of the active elements, and both 2×2 or 4×4 MIMO can be supported. Using active DAS is an overall more futureproof solution, with the ability to flexibly deal with additional capacity and performance requirements in the near future.
However, there are also some disadvantages, especially taking into account the higher initial cost of investment. Operator sharing can also prove challenging, if at all possible, depending on each individual case. Furthermore, new and additional fiber and cabling installations are also part of deploying new DAS systems. Moreover, although active DAS systems have better scaling and performance, existing passive DAS with DSS can be a fast alternative solution for cases with limited throughput required.
5G Upgrade Options
Upgrading existing systems to 5G and choosing between legacy and new frequency bands may become quite challenging for any Operator. Existing Passive DAS means operating on existing bands such as the 800, 900, 1800, 2100, and 2600MHz FDD spectrum. Using these bands also for 5G deployment is a solution that has advantages since legacy bands are supported by existing components and DSS functionality is again possible.
This legacy band solution can provide an immediate 5G coverage, with operator sharing possible. Disadvantages include, as mentioned above, the SISO system and a minimal MIMO upgrade path towards 5G systems. In other words, the 5G logo may be available, but with limited performance. Moreover, the additional power needed for these systems will result in a higher risk of passive intermodulation (PIM). It becomes evident that using existing legacy bands will offer an overall 4G-like performance, with no visible differentiation of 5G to 4G services.
Passive DAS and 5G on new bands
Another alternative is to use existing Passive DAS systems with 5G on new bands, for example, the 700, 1500, and 3600MHz frequency spectrum. This kind of deployment will benefit from the fact that a passive component portfolio is already available and that for 700 and 1500MHz solutions, cabling can be untouched, while antenna replacement can be relatively quick and come at a low-cost. Operator sharing will again be possible.
Disadvantages include the higher feeder attenuation for the 3400-3800MHz case. This means that all passive elements have to be replaced to support new bands, and the upgrade path is relatively complex and expensive to perform. Operators should, therefore, re-design their network, of course, with the additional cost.
5G on existing Active DAS
Finally, 5G can be deployed on existing Active DAS systems using legacy bands such as 800, 900, 1800, 2100, 2600MHz FDD, and new bands such as 700, 1500, and 3600MHz. The advantages are that these systems already offer flexibility, easy scaling, and complete control of active elements. These solutions can support 4×4 MIMO and are again more futureproof since they provide a flexible encounter with additional capacity and performance requirements in the future. Once again, the disadvantages include a higher degree of difficulty for Operator sharing and new and extra fiber and cabling installations.
As mentioned earlier, there is no one-fits-all solution. Designing the proper architecture to support the new 5G use cases and the 5G networks of the future can sometimes be a demanding task, but if done properly, it can provide the greatest of benefits at the lowest possible cost network complexity.
