Heavy data use from the proliferation of smartphones and other mobile devices has shifted the focus of cellular network operators from coverage to overall Quality of Experience (QoE), defined primarily by data speeds. More recently, the overall shift in emphasis to in-building QoE is beginning to upend traditional models for delivering wireless service.
Most buildings currently depend on outdoor macro cells or Distributed Antenna Systems (DAS) for coverage, but each has significant downside in terms of quality. Macro cells often have poor penetration into buildings, resulting in sub-optimal performance. Distributed Antenna Systems (DAS) provide better performance, but are not always cost-effective. Small cells have emerged as a third choice to meet the in-building demand while balancing costs with QoE.
Small cell networks need to be designed, tested, and deployed carefully and efficiently. Each small cell provides additional capacity, but overlapping coverage areas result in interference that reduces QoE. Passive intermodulation (PIM) interference, generated by passive network components such as antennas and cables, may also reduce data speeds.
An integrated, multi-phase approach, as illustrated in Figure 1, can maximize the usage of available spectrum. To ensure timely deployments, this approach should include thorough testing, PIM-rated antennas, and on-site planning and design tools.
Figure 1. Multi-Phase Approach to QoE Assurance for In-Building Small Cell Networks
Network Planning involves gathering inputs for model tuning, including a baseline wireless site survey and CW testing.
Network Design includes propagation model optimization, small cell RF network design, and equipment selection.
Network Build is when small cells are installed and commissioned.
Network Integration involves a network benchmarking walk test to determine whether performance has met acceptance criteria and to create a system performance baseline. Handoff optimization, including neighbor list optimization, may also be conducted if the first benchmarking walk test shows that the network does not meet performance criteria.
Network Optimization comes once the integration phase is complete and implements regular benchmarking, as well as troubleshooting and mitigating interference issues as they arise.
Following these steps can minimize the cost of small cell deployments and optimize the user experience. Let’s examine each one of these phases in detail to provide a complete understanding of the process.
Figure 2. Baseline Survey Results Showing Macro Cell Penetration
Wireless Site Survey—The wireless site survey, also known as a baseline survey, provides the foundation for small cell network design. It maps the current RF landscape for deep analysis (see Figure 2). Site surveys must be conducted at multiple points of entry and on multiple floors of the building. The purpose of these surveys is to determine macro cell signal penetration, which can vary from floor to floor. The small cell network must overcome this signal in order to meet coverage and QoE goals.
A perimeter walk test conducted as part of the site survey provides further useful data for network planning. The perimeter walk test helps to pinpoint which macro cells might be problematic to the small cell. It also identifies neighbor list candidates in the macro cell network for the small cell network.
High performance scanning receivers are the best tools for baseline surveys because their high dynamic range allows them to accurately identify in-building penetration. Scanning receivers can also eliminate repetitive walk tests by testing multiple operators simultaneously.
Once the walk test is completed, the site survey is uploaded to an in-building or planning design tool (such as iBwave) where the building dimensioning is added. Site survey data provides the macro cell coverage parameters for the design tool, enabling a comprehensive and proper design of the small cell network.
Pre-Deployment Continuous Wave (CW) Test—The wireless site survey is followed by the pre-deployment CW test, which employs a CW transmitter. The CW test determines actual RF propagation characteristics of the venue (including propagation from one floor to another). The perimeter walk test also helps to determine attenuation values of exterior walls and possible leakage of internal small cells outdoors (see Figure 3).
The pre-deployment CW test identifies the complete propagation profile for the intended small cells that will be used in the actual deployment. Engineers place antennas to simulate small cell RF propagation. This test also requires accurate scanning measurements with high dynamic range and a CW transmitter. Figure 3 illustrates the RF propagation inside the building. The results will be used for model tuning in an in-building design tool during the Network Design phase. Consequently, the preliminary CW test is performance critical for understanding the RF propagation characteristics which ultimately affect network performance.
Figure 3. CW Test Results Showing Transmitter Location and Signal Propagation
Propagation Model Optimization and RF Network Design—Network Design makes use of all the intelligence gathered during Network Planning (Phase 1). To maximize efficiency, both propagation model optimization (PMO) and RF network design should be conducted on-site, at the same time as Phase 1. PMO and network design are performed using advanced planning tools, preferably with support for real-time integration of site survey and CW test data. Optimal small cell antenna count is determined in this phase via a calibrated propagation model to ensure appropriate signal quality. Using this optimized propagation model as its basis, an effective design tool will predict dominance over the macro cell (see Figure 4), ensure compliance to key performance Indicators (KPIs), and confirm design parameters are met on all floors and extensions of the building.
Equipment Selection—Equipment selection is also a key part of this process. High quality mechanical and electrical performance is essential to ensure that the equipment performs as predicted by the design tool. Small cells should come from a manufacturer that uses antennas with consistent and predictable radiation patterns. This will mitigate coverage issues from under-propagation or interference issues from over-propagation. Equipment selection is also crucial for minimizing PIM interference. Small cells should be built and tested to minimize PIM interference. They should include antennas that were designed by a trusted manufacturer to ensure compliance with PIM standards.
Figure 4. iBwave Mobile Planner Small Cell Dominance Over Macro Network Report Indicating Need for Second Small Cell to Achieve Coverage Goals
Installation and Commissioning—Quick installation is important for delivering projects on time and within budget, but careful procedures during installation are also imperative. A deliberate installation process can avoid problems that might be difficult to diagnose and solve later in the process.
Once the small cells are installed and powered up, and parameters have been set based on the small cell design, system performance can be measured via a walk test for final commissioning of the network. As part of the commissioning process, scanners with high dynamic range are typically utilized to validate the performance of the network.
Testing the entire small cell network on each technology and frequency is required to ensure proper performance. Final cable routes and small cell positions may change. Capturing and tracking these changes is critical, as they affect many elements of the network. It is essential to quickly identify malfunctioning components or misconnected cables during the site walk. The relevant site information is captured in preparation to submit an updated closeout package that reflects what was installed. The results of the walk test during commissioning are shown in Figure 5.
Figure 5. Walk Test Results of the Small Cell Area
Network Benchmarking—Once small cell commissioning is complete, the final network benchmarking walk test is performed. During this phase, which may also be referred to as the “final integration test” or “acceptance test,” actual cellular test traffic is used. If parameter adjustments are not necessary, the commissioning walk test done in Phase 3 may suffice for the final benchmarking report. The network benchmarking walk test verifies system performance to design specifications, including KPIs for coverage, quality, neighbor lists, soft hand-off percentages, throughput (estimated or actual), and MIMO paths when applicable.
Modifications that aid the integration of small cells include adjusting hysteresis values, handoff parameters, power levels such as CPICH or RSRP out of each access point, and controlling the small cell to interact with only the small cell system. In addition, this test verifies any macro cell power adjustments that were required based on the initial wireless site survey.
Acceptance test reports can include an array of signal and KPI measurements. All technologies are tested in a small cell network, which can include 3G and 4G technologies, possibly across multiple operators. The small cell network often requires more stringent testing in this phase compared to a DAS.
Additional testing may include an outside perimeter walk test to ensure that pedestrian traffic or traffic from neighboring businesses are not unintentionally handed off to the small cell network being deployed in the venue being tested.
Figure 6. LTE Measurement Report
Handoff Optimization—If the network benchmarking test reveals problems with handoff, whether within the building or around the perimeter, engineers conduct handoff optimization. Parameters for small cells or even macro cells may be adjusted. This process includes both neighbor list optimization and RF optimization. Once optimization is complete, another network benchmarking test is performed. This process is repeated until the network meets KPI requirements as per design specifications. Various results are shown in Figure 6.
Network Benchmarking, Network Troubleshooting, and Interference Management—Annual maintenance should be performed on a small cell network to ensure peak performance and a high QoE. Network benchmarking via a walk test is performed to verify all small cell locations are performing properly. This provides a comparison to the initial benchmark performed during the Network Integration phase. Compliance and small cell dominance criteria are checked at this time.
Interference management should be an ongoing process involving continuous monitoring of the network. Analytic tools that organize and evaluate network KPIs can help isolate interference sources. Interference hunting tools may be required if an outside interferer is suspected after the network was initially deployed. If another carrier added a small cell network in the venue or an outside macro cell was added or re-tuned, the small cell settings may require re-adjustment.
In order to maximize the efficiencies of the annual retest, the RF engineer should have access to spare parts to make necessary repairs. The RF engineer should also acquire reliable site documentation for reference before going on site. This documentation should be in a form which can be easily updated during or immediately after each visit.
In summary, small cell networks require a multi-phased approach using the proper design and engineering tools for successful deployment. Minimizing design, testing, and commissioning costs is essential as small cell networks are deployed in venues that have very cost-sensitive business models.
Small cell network owners who follow the multi-phased approach will both maximize the performance of their in-building cellular networks and minimize their own infrastructure investment. A high-performance network will provide a clear competitive advantage to the owners or operators of the building, whether they are interested in leasing floor space or ensuring high QoE for the building’s users.
