Coverage, Capacity, Sectorisation, and MIMO for 5G-Ready In-Building Coverage

In my first installment, I spoke about the purpose and importance of In-Building Coverage, specifically through the implementation of Distributed Antenna System (DAS). I mentioned how the Mobile Network Operators (MNOs), via the Mobile Carrier Forum (MCF), have established technical specifications for DAS design, installation and performance to ensure facilities are suitable for Carrier connection. I also touched on the evolution of DAS to support Mobile technology generations (2G, 3G, 4G) and more pointedly an increased number of frequency bands especially as a lead up to the MCF 2021 DAS specifications which introduce requirements for 5G. Critical to the discussion is the understanding that an In-Building Coverage (IBC) solution requires on-site connectivity to a MNO’s Radio Access Network (RAN). The RAN facilities not only power the DAS but are a critical enabler of the end user experience. Today I want to explore the key features of the DAS and RAN that determine Coverage and Capacity especially as provided for in MCF 2021 (Draft).

Above all, the DAS provides the Mobile ‘Air Interface’ to users/devices by deployment an array of small form antennas that enable RF Coverage and connectivity throughout the building. There are a few different technology types when it comes to DAS, which I’ll talk about in a future instalment, but the majority of installations today are Passive systems. With this, a MNO’s Base Station (BTS) equipment connects to the DAS, at a defined interface point in the building. As the Active element, the BTS provides the RF power for the passive DAS.

The IBC/DAS arrangement is effectively the same as for a Macro BTS where power from Radio Units drive large panel antennas. In the case of DAS the BTS radio power is distributed across all the antennas with the DAS delivering the optimal amount of power to each antenna, providing not only a consistent service level but also ensuring EME levels are not exceeded, noting that RF emissions from DAS antenna are, by regulation, very low.

Achieving Coverage for DAS

I have mentioned previously the requirements and KPI provisions for Coverage in the MCF specification. The governing principle for these requirements is maintained in MCF 2021 (Draft), however the provisions for the 5G-exclusive 3500MHz band (3400-3700MHz) are introduced. The practical design implication for DAS from MCF 2021 (Draft) is that 3500MHz becomes the highest frequency band and therefore sets the benchmark for the Coverage capability. By inference, if the Coverage KPI at 3500MHz can be achieved, then all other bands will operate successfully. This KPI supersedes the previous benchmark of 2600MHz which aligns to 4G.

The MCF DAS specifications also include quality markers and metrics for Coverage. This is critical as service performance relates more to radio signal quality, where the effects of noise and signal interference are considered, rather than just power level alone. This consideration comes into focus particularly regarding achievement of signal dominance, where the signal level from the DAS must achieve at least 10 times greater power than signals originating from network external to the building which are typically on the same frequency band.

In consideration of the total IBC solution (i.e. the combination of BTS and DAS), assuming the MNO BTS can supply a signal of sufficient level and quality (which is valid assumption), then the Coverage performance and achievement of KPI’s resides in the design and installation quality of the DAS.

Achieving Capacity for DAS and Sectorisation

The MCF DAS specifications also include quality markers and metrics for Coverage. This is critical as service performance relates more to radio signal quality, where the effects of noise and signal interference are considered, rather than just power level alone. This consideration comes into focus particularly regarding achievement of signal dominance, where the signal level from the DAS must achieve at least 10 times greater power than signals originating from network external to the building which are typically on the same frequency band.

In consideration of the total IBC solution (i.e. the combination of BTS and DAS), assuming the MNO BTS can supply a signal of sufficient level and quality (which is valid assumption), then the Coverage performance and achievement of KPI’s resides in the design and installation quality of the DAS.

For a large-scale DAS implementation, a single BTS (cell) may not be able to provide sufficient power (and/or Capacity). Hence multiple cells or sectors may be required.

While sectorisation may assist to provide sufficient power to a large-scale DAS, the primary role of sectorisation is to cater for sufficient Capacity. Practically the BTS power for a given sector can usually sustain the Capacity requirements without exhausting resources. Capacity requirement is determined as the bandwidth (via spectrum resources) to support the number of users to be served under the Coverage area at the specified service performance level such that users experience a persistent quality of service at all times (i.e. enough Mbps per person or device in catering for peak demand).

To determine the number of sectors, the total occupancy of the building is required, in view of permanent tenants or residents, visitors, and the distribution of people across the building profile (i.e. for each building level or area). The occupancy of buildings and profile vary quite significantly across public, private and semi-private domains. Cases where the building occupation has high ephemeral peaks must also be supported such as for sports stadiums and convention centres

Further Capacity Boost for DAS with MIMO

Quite differently to Coverage, in an IBC Solution, almost all of the resources for Capacity are in the domain of the RAN, and the MNO’s BTS provisions. The RAN determines the technologies (3G, 4G, 5G), the specific spectrum bands and carriers applied to the DAS, and the bandwidth for each. Also appearing more and more in the Mobile arena are provisions for MIMO, which enables multiple radio paths or streams using specialised antennas. MIMO provides significant Capacity uplift and is an inherent part of 5G technology. Now that 5G is entering the IBC domain, DAS must be able to support MIMO requirements. Notwithstanding all of this, the DAS and its components play a role of compliancy and support rather than performance when it comes to Capacity attributes. It’s a case of ensuring all possible frequency bands are supported, appropriate partitioning for sectorisation is present, and the antennas and antenna line equipment is in accordance with SISO or MIMO requirements (e.g. 2×2, 4×4).

MIMO is a very effective technology for increasing Capacity, with each MIMO stream acting as a Capacity multiplier. Hence 2×2 doubles the Capacity of the single stream SISO system, and 4×4 quadruples it. To provide a quantitative sense on Capacity, the application of 20MHz of bandwidth supports a theoretical peak data speed of 75Mbps. Applying 2×2 MIMO increases this to 150Mbps.

However, particularly for Passive DAS, its implementation adds significantly more infrastructure in league with each stream. The additional infrastructure and specialised antennas required weighs heavily on providing a cost effective DAS and the benefits are not justified in every case.

Capacity is a Key Focus for MCF 2021

MCF 2021 (Draft) places significantly more focus on Capacity, sectorisation and MIMO requirements than the existing MCF 2018 specification. It specifies in great details parameters on users / devices per sector, and the maximum area for sectors in line with different building types such as airports, shopping malls, commercial buildings. And then, within these buildings defined functional areas including lobbies, lifts, public areas, car parks. The MCF 2021 (Draft) mandates the heavy use of MIMO and identifies the specific MIMO configuration (2×2 or 4×4) required (per band) in line with the building types and areas mentioned above.

Spectrum Resources for IBC

I’d like to finish off this installment by talking about spectrum resources in the context of IBC/DAS.

There are currently eight low/ mid frequency bands in operation for mobile networks in Australia. The MNO RAN spectrum comprises frequency bands acquired from spectrum releases in line with 2G, 3G, 4G, and most recently 5G-technology introductions. Also, as older technologies are retired the frequency bands are re-deployed or re-farmed to new technologies.

There are three low-bands (frequency range <1GHz) and five mid-bands (1GHz< frequencies <6GHz) in operation.

Generally speaking, low-bands are good for providing Coverage but have limited bandwidth, in other words, low Capacity.

Mid-bands are not strong in providing Coverage and penetrating into buildings but their Capacity stocks are favourable, generally increasing with higher frequency spectrum bands.

The illustration below provides a sense of the Coverage and Capacity value of each band. The current main technology usage(s) for each band are also identified. Note that unlike previous technologies that aligned to specific band releases, 5G is band agnostic.

The MCF Specification mandates the DAS must support all the frequency bands listed such that any and all MNOs are afforded the opportunity to provide service. However, the bands implemented are at the discretion of each participating MNO.

In the early history of Mobile support of 2G and 3G was the focus. Hence 850MHz, 900MHz, 1800MHz, and 2100MHz were the preferred bands and these bands are still in operation today on most DAS. With the advent of 4G, 700MHz, 2300MHz and 2600MHz entered the market. Now given the higher bands have most Capacity, and by this stage the DAS specifications mandated that all bands achieve the required Coverage KPI, the reach advantage of the low bands was nullified for IBC. Higher frequency bands providing at least 20MHz per operator were the preferred choice.

Looking to the future, bands implemented by MNOs will likely be in the range 1800MHz and higher to support Capacity drivers especially with the advent of 5G. For Passive DAS, given component bandwidths have a range from around 600MHz to around 6000MHz, support for the higher bands is achievable. However Passive DAS is not favourable with MIMO systems due to the need for multiple feeder infrastructure in line with MIMO stream count. (2-folds or even 4-folds in feeder infrastructure).

Active and Digital DAS are also capable of supporting higher bands but (as for all bands) do so in a modular fashion with equipment tailored to each band. The use of optical fibre transport and Radio Units with in-built antennas requires less infrastructure and facilitates MIMO in a more elegant and streamlined manner largely avoiding infrastructure multiplication. Mandates from MNOs that DAS installations must support all bands, whether ultimately used or not, are somewhat counterproductive in respect of maintaining small equipment form factors and in support of cost-effective solutions.

Ultimately, both Passive and Active DAS types have their place however the provision of options tailored to addressing a customer’s Capacity needs is missing from the current process. This would include a more considered and selective approach to band implementation and if a SISO or MIMO system is required instead of adopting a ‘one size fits all’ approach.

I have not included mmWave in the scope of IBC/DAS spectrum discussion. Whilst mmWave band (26-28GHz) is now available, the value for IBC/DAS is still the subject of investigation. In any case should mmWave technologies prove valuable they will be almost certainly be deployed standalone from low/mid band DAS.

My next installment will talk in more detail about newer DAS technologies which involve Active components and Digital technology. These DAS types have the potential to significantly reduce the complication and cost of IBC solutions.