Design Considerations for Public Safety DAS in the LTE EraAbstract

This white paper discusses the convergence of Land Mobile Radio (LMR) communication systems for emergency services personnel to 4G and 5G wireless technologies. It identifies best practices for designing in-building distributed antenna systems (DAS) for optimal coverage and capacity during the transition period between these technologies.

What is the difference between LMR and LTE?

LMR networks are a mix of digital and analog, trunked and conventional push-to-talk (PTT) radio networks. In-building Emergency Responder Radio Communication Systems (ERRCS), also known as public safety DAS, are required by fire (NFPA 1221 & IFC) and building codes to extend wide area dispatch networks throughout 99% of a building. These systems provide emergency personnel the coverage needed during an event inside of a building. Other parts of the codes require hardiness of the systems against power outages, fire damage, and water from hoses or sprinklers. In general, LMR runs on frequencies below 1GHz, and they transmit voice and low bandwidth data like messaging. Additionally, many LMR systems like analog FM, TETRA, and Digital P25 are incompatible with one another. The image below shows a typical public safety DAS topology. A BDA-Booster filters and amplifies the wide area public safety services for ERRCS within a building.

Design Considerations for Public Safety DAS in the LTE EraCommercial 4G LTE systems are designed for high levels of capacity and coverage for both voice communication and data transmissions. They are not mandated by code to be installed at all, nor have requirements for coverage or resilience to damage. In the US, LTE bands span from 600 MHz up to 6 GHz, and they can be used for broadband, high throughput applications like HD video. Yet emergency service personnel have many use cases for that level of data throughput which LMR cannot support. These include real-time video, criminal database queries, and location awareness data. LTE systems are also compatible with one another because they are all built to the same international standards set by 3GPP. These are key reasons why public safety LMR is converging with LTE.

How will LMR transition to LTE?

FirstNet, the First Responder Network Authority, was created by the US federal government to establish, operate, and maintain an interoperable public safety broadband network. In 2017 FirstNet formed a public-private partnership with AT&T to build the network which includes 20MHz of dedicated spectrum in LTE Band 14. AT&T was awarded $100 billion over 25 years for meeting specific deadlines of technology deployment. Part of the contract allows public safety members to access any frequency band of the AT&T network with pre-emption over other users. All 50 US states, Washington DC, and two territories have opted into FirstNet.

Design Considerations for Public Safety DAS in the LTE Era

Just because the states have opted into it, does not mean everyone will convert immediately. Eventually LMR will evolve to 4G LTE and ultimately 5G, but they will co-exist and complement each other for many years because not every local fire or police department has the funds to buy brand new equipment. Also, FirstNet is only around 50% deployed, and it does not yet support mission-critical voice. The 5G network is only starting to be deployed, and it will take up to a decade to be ubiquitous nationwide. Therefore, these networks are each still evolving in parallel. In-building communication systems will need to support both LMR and LTE for years to come. Existing commercial cellular systems will require additional coverage to meet mission critical ERRCS building and fire code requirements.

Design Considerations for Public Safety DAS in the LTE Era
LMR/Cellular Convergence of Devices

How to design for both LMR and LTE

To build and deploy a DAS that enables legacy LMR, 4G LTE, and future 5G, there are several factors that designers must take into consideration. In some cases, it may not be possible to completely future proof at a reasonable cost, so the trade-offs should be evaluated. The major factors to consider are the supported frequency bands, the coverage area, and the quality of service. LMR communications can be operated as low as 138 MHz and as high as 960 MHz. Meanwhile LTE is currently used all the way up to 5925MHz with LTE-LAA. To design a system that covers 80 to 5925MHz would be extremely expensive. Signals at low frequencies propagate farther than higher frequencies when transmitted at the same power level and building materials will cause different losses at different frequencies. Therefore, the link budget at the low end and high end of the spectrum dictate a different antenna density. As mentioned previously, commercial wireless coverage is not mandated. For a commercial building to be deemed safe for occupancy, for sale, rent, or lease, it must be inspected and given a Certificate of Occupancy (CO). CO’s for commercial properties typically require the installation of an ERRCS Fire Service, however building owners may opt to strictly adhere to code and not include support for commercial cellular coverage. In this case, designers would use components that support 138-960MHz that support both the low and high end of that range, including LTE bands, such as FirstNet’s Band 14, for future deployment.

Design Considerations for Public Safety DAS in the LTE Era
FirstNet LTE Band 14 Spectrum

In a commercial LTE system, the heavily populated areas of a building (hallways, hotel rooms, conference rooms, etc.) receive the best signal coverage and capacity. In a Public Safety DAS, all areas need to have close to 100% coverage because emergency services may need to access areas such as fire equipment rooms, utility areas, stairwells, and elevator shafts from the basement to the rooftop. For Public Safety, coverage is more important than capacity at present, but that will change over time. Designers will have to consider how to meet the coverage requirements of public safety while ensuring the system can meet capacity needs in the future.

In LMR, the system is measured by “Delivered Audio Quality” (DAQ) because that is the primary use. With the evolution to LTE for both voice and broadband, coverage and capacity are linked, and the system designer must ensure both are optimized using key performance indicators (KPI).

Design Considerations for Public Safety DAS in the LTE Era
LTE Signal Strength and Quality

To design coverage for efficient signal propagation Reference Signal Received Power (RSRP) is used. The in-building LTE system must have a higher RSRP throughout the venue than the nearby macro sites to overcome the near-far effect. Failing to account for near-far can cause poor performance in the venue and to macro users as well.

The performance of the DAS system is not only related to the absolute signal level but also the quality of the signal. Signal-to-Interferenceplus- Noise Ratio (SINR) is a measure of signal quality that quantifies the relationship between RF conditions and throughput. A high SINR is critical to realizing the benefits of multiple-input multiple-output (MIMO). The modulation scheme used in LTE networks is directly related to SINR, as high SINR makes possible high-order modulation such as 256-QAM which increases the throughput.

Design Considerations for Public Safety DAS in the LTE Era
Signal to Noise Ratio vs Capacity in SISO and MIMO Systems
Design Considerations for Public Safety DAS in the LTE Era
PIM Signals Generated from Mixing Multiple Carriers

Interference that degrades SINR can come from internal sources of the DAS. Passive Intermodulation (PIM) from system components causes a rise in the noise floor which limits capacity in LTE systems.

A typical receiver noise floor of a 10MHz LTE radio is -102dBm. PIM can cause interference within a single band or within adjacent bands. Designers must be aware that it is important to select components with PIM ratings that are at least 10dB lower (-112dBm) so as not to increase the noise and reduce SINR. The designer should select parts that are below the -112dBm threshold, so they have margin in their design. SMA and QMA connectors are not recommended for PIM. Type N and 4.3-10 are the most widely used today, but to support smaller form factors some commercial deployments are moving to NEX10™. It is almost the same size as SMA, but it has low PIM. The trade-off is that only 1/4in cable can be used, which has higher loss than ½ in.

Design Considerations for Public Safety DAS in the LTE Era

With the different design factors to consider and associated equipment requirements, it is essential to select an equipment provider that offers components covering both LMR and 4G/5G frequencies and has the experience to assist with assessing design trade-offs.

For more than 70 years, Microlab has been trusted to provide components and consultation for commercial LTE and LMR DAS deployments. Microlab has a wide assortment of products supporting frequencies from DC to 6GHz with guaranteed PIM ratings, dedicated LMR products for bands under 1GHz, and fusion products that cover both. In addition, Microlab’s leading edge SMART passives line monitors a building’s DAS for cabling and antenna integrity. Microlab is ready to assist with custom and off the shelf mission critical solutions for your ERRCS applications.