Improving Metro Cell Performance with Electrical Downtilt and Upper Sidelobe Suppression

Transcription

1 White Paper Improving Metro Cell Performance with Electrical Downtilt and Upper Sidelobe Suppression Steve Kemp Metro Cell SME September, 214

2 Contents Executive Summary 3 Small cells deployed outdoors as heterogeneous networks grow 3 Increased interference challenges 4 Macro cell experience helps improve metro cell performance 5 Antenna selection 6 Simulation results confirm hypothesis 8 Effects of enhanced inter-cell interference control (eicic) 1 Analyzing total cost of ownership 1 Recommendations for selecting and deploying metro cell antennas 11 Summary 12 References 12 2

3 Executive summary In their 212 report 1, Small Cell Market Status, Informa Telecoms and Media outlined the top five concerns multi-network operators (MNOs) had regarding the deployment of small cells outdoors. 1. Deployment issues (placement, power, environmental) 2. Backhaul 3. Cost 4. Newness of equipment 5. Their distributive nature At a glance, this list is indicative of the typical concerns MNOs would have regarding most emerging wireless technologies namely, the logistical issues of implementing and integrating new radio equipment within an existing environment. Should Informa choose to do a follow-up survey in a few years, however, it is quite likely that concerns over outdoor small cell deployment will also include issues involving overall network performance, especially interference management. The outdoor wireless landscape is changing, and small cells are playing an increasingly larger role. As MNOs look to increase capacity in urban hot spots and plug coverage holes or not spots in their networks, they are turning to specialized outdoor small cells known as metro cells. Small enough to be deployed just about anywhere more network capacity and coverage are needed, metro cells are typically layered onto the macro network to create a relatively new kind of heterogeneous network. While the use of metro cells will help MNOs satisfy increasing traffic demand in areas where macro base stations are being pushed to their limits, they will also pose new challenges one of the biggest being how to adequately control the RF footprints of the macro and metro cell sites in order to minimize interference and maximize the operating potential and profit of these networks. CommScope, a leader in the design and optimization of base station antennas, is a pioneer in the use of electrical downtilt and upper sidelobe suppression (USLS) to control antenna patterns in macro networks. Today, the use of electrical downtilt is a standard method for controlling interference. In developing a new family of metro cell antennas, CommScope engineers have discovered that the same principles used in macro cell interference management can be used to control interference in metro cells. To confirm and quantify these findings, CommScope commissioned third-party testing using industry-accepted simulation tools. The results revealed that MNOs can increase overall network capacity gains by 4 to 5 percent while decreasing CapEx and OpEx 25 to 35 percent. This paper details the processes and highlights some of the outcomes of the exercise. Small cells deployed outdoors as heterogeneous networks grow Initially, the growth in small cells was driven by indoor applications primarily the use of femtocells in wireless home networks, as well as enterprise cells, including distributed antenna systems. When deployed indoors, these small cells are far less likely to create interference with, or be interfered by, signals from surrounding macro sites. High-power signals from the macro site are substantially reduced as they attempt to pass through the building s bricks, steel and glass. This allows the indoor user equipment (UE) to communicate easily with the lower power small cell. Likewise, the small cell s emitted signal is kept from interfering with the macro cell. 3

4 Recently, however, small cells are being used outdoors with greater frequency as MNOs seek to meet additional coverage and capacity demands. This is especially true in dense urban areas where hot spots strain system capacity and end users can be frustrated by the lack of service in not spots frequently found in urban canyons. In areas such as these, metro cells are layered on top of the macro network and, as illustrated in Figure 1, can either share the same frequency band as the macro sites or use a unique band. Figure 1: Single-band and dual-band heterogeneous network designs are shown. Increased interference challenges As outdoor deployment of metro cells becomes more pervasive, the likelihood and cost of interference with the macro network increases significantly. This will be especially true in LTE networks where macro and metro cells operate using the same set of frequencies. When small cells are deployed indoors, MNOs can effectively manage interference using power control and attenuation. When metro cells are used outdoors, and become more fully integrated with the macro network, metro cell interference becomes more problematic. Figure 2: In a heterogeneous LTE network, macro cells and metro cells may interfere with each other. In an ideal world, the edge of one metro cell would stop nearly where another cell began, minimizing the overlap and potential for interference. This interference is most likely to affect user equipment (UE) located close to the cells edges. But, as Figure 2 illustrates, the blanket coverage within a macro cell s radiation pattern often spills into the metro cell s designated coverage area. 4

5 Macro cell experience helps improve metro cell performance In 1995, CommScope engineers pioneered the first remote electrical downtilt (RET) antennas. The company s continued efforts in optimizing network performance through electrical downtilt have provided unique insight into other applications for the technology. Today, the use of RET to control interference in macro networks is common practice. Base station antennas are selected, in large part, for their capability to provide electrical downtilt and excellent upper sidelobe suppression (USLS). Precise electrical downtilt enables the operator to control signal transmission at the cell s edge while well-designed USLS sharpens the cell s edge to minimize interference as the main beam is tilted down. It should be noted that the degree of effective downtilt and the operator s ability to control it vary by frequency. Assuming the number of antenna elements remains the same, the vertical beamwidth increases as frequency decreases. In order to decrease the vertical beamwidth within the lower bands, more elements must be added, increasing the overall antenna length. Due to zoning restrictions that often limit the height of metro cell antennas, achieving effective downtilt at frequencies of 9 MHz or lower is, in most cases, impossible. In 212, CommScope engineers theorized that the same techniques used to control interference in macro cell deployment specifically, downtilt and USLS could be used to great effect in improving the performance of heterogeneous metro cell networks. CommScope s preliminary research, conducted in partnership with the University of Texas Austin and documented in an upcoming paper 2, indicated that electrical downtilt and good USLS could indeed be used to combat interference in metro cells. To confirm and further quantify this, CommScope commissioned Telecom Technology Services, Inc. (TTS) to conduct a series of simulations involving the use of electrical downtilt and USLS with metro cells to minimize interference. The simulations were carried out using the Atoll radio-planning tool from Forsk and ray tracing with an application plug-in from WinProp. Engineers began by defining the physical environments to be studied. The team modeled four scenarios, shown in Table 1. Network scenario A. Dense urban infill using single band for macro and metro cells B. Dense urban infill and capacity using different bands for macro and metro cells Bands 7 MHz, 19 MHz, 26 MHz 19 MHz, 26 MHz Metro cell power options 1W, 5W 1W, 5W C. Residential coverage using single band for macro and metro cells 19 MHz 5W D. Dense urban hot spot using different bands for macro and metro cells 7 MHz, 19 MHz 1W, 5W, 1W Table 1: The use of electrical downtilt and USLS were tested in simulations of these heterogeneous environments. Scenarios A and B are modeled on areas of Lower Manhattan in New York City. Scenario A assumes the metro cells are used for infill coverage and that the macro and metro cells operate on the same frequency. Scenario B assumes the metro cells are used for both coverage and capacity and that they operate on a frequency distinct from the macro network. 5

6 FTP download properties Uplink (UL) Downlink (DL) Avg. demand (kbps) 1,24 5,12 Highest bearer Lowest bearer 1 1 Max demand kbps) 1,24 1,24 Min demand (kbps) Band properties Channel width Adjacent channel suppression Sampling frequency Number of resource blocks LTE duplexing method Antenna height 5 MHz db 7.68 MHz 25 RB FDD 2 feet (6 meters) Table 2: These were the assigned download and band properties. Monte Carlo simulations Max DL traffic load 8% Max UL traffic load 8% Max number iterations 1 DL traffic load convergence threshold 5% UL traffic load convergence threshold 5% UL noise rise convergence threshold UL power control 1 db Off Table 3: This shows the Monte Carlo simulation variables. Scenario C is based on the suburbs of Livermore, California, with metro cells being used to supply additional coverage. Scenario D reflects a singular dense, urban hot spot situated in Madison Square and Union Square in New York City. Engineers then assumed the FTP download and frequency band properties as indicated in Table 2, and the traffic and power settings in Table 3. Antenna selection Once the simulation variables had been defined and included, the engineers factored in the antennas to be tested. For the control group, engineers chose a typical 8-inch (2 mm) omnidirectional metro cell antenna. The single-element antenna had a half-power vertical beamwidth (HPBWv) of 5 78 degrees and no electrical downtilt. To assess the effects of electrical downtilt and well-designed USLS, engineers used the V65S-1XR, a 24-inch (6 mm) metro cell antenna from CommScope. It features an HPBWv of degrees and an electrical downtilt range of 2 degrees. The antenna can be deployed in a variety of configurations. The Atoll simulations included one V65S-1XR modeled as a quasi omnidirectional antenna and a second V65S-1XR configured as a sectorized antenna, as well as the typical 8-inch (2 mm) omnidirectional antenna. The azimuth patterns for all three antennas are illustrated in Figure 3 while the elevation patterns are shown in Figure 4. The blue patterns indicate antenna performance at 19 MHz, while the red pattern illustrates performance at 26 MHz for the CommScope V65S-1XR antenna and at 21 MHz for the typical 8-inch (2 mm) omnidirectional antenna. 6

7 V65S-1XR, quasi-omni V65S-1XR tri-sector Typical 8 omnidirectional Figure 3: Azimuth patterns for antennas used in simulation are shown V65S-1XR, quasi-omni V65S-1XR tri-sector Typical 8 omnidirectional Figure 4: Elevation patterns for antennas used in simulation are shown. In order to assess the impact of antenna electrical downtilt and USLS performance on the various scenarios, engineers were interested in the following results: Reference signal received power (RSRP) for given points in the network Signal-to-interference-plus-noise ratio (SINR) for given points in the network Total number of metro and macro sites required per scenario Total number of cells and sectors required per scenario Peak load per site Total users supported per site and across the network Average spectral efficiency (ASE); ASE = peak network /Hertz/square mile Average per cell rise in uplink noise floor Average downlink per user within the radio link control (RLC) layer Peak downlink per cell, site, user and network within the RLC layer 7

8 Simulation results confirm hypothesis In order to establish a baseline, researchers first modeled the effects of adding metro cells to an existing macro cell network while observing the impact to downlink (DL) at the RLC layer. The network simulation used a 19 MHz operating frequency and assumed one-to-one frequency re-use. The macro site radios were set to 4W while the metro cells were simulated at 1W and featured a typical 8-inch (2 mm) omnidirectional antenna with a 5-degree HPBWv. Table 4 illustrates the overall positive effects of the metro cells. Site Cell Connected users Peak RLC user Peak RLC site Peak RLC network Average spectral efficiency (Bits/Hz/km 2 ) 4W macro network, 1 downtilt W metro cell network, 8" (2mm) omnidirectional antennas, downtilt Table 4: The impact of metro cells on network efficiency is shown. As demonstrated in Table 4, the addition of metro cells to the macro base station network can raise the ASE and network capacity approximately 127 percent. This capacity gain can be used to either satisfy more subscribers or provide higher per subscriber. In addition, the peak network increases 13 percent while the peak user more than doubles. Next, the team ran simulations in order to measure the effect of good USLS and electrical downtilt. Table 5 shows how network performance improves using CommScope s quasi omnidirectional metro cell antennas set to 12 degrees of downtilt. As indicated in Table 5, the USLS performance and electrical downtilt of the CommScope antennas produced: 31 percent increase in peak site 31 percent increase in peak network 32 percent gain in average spectral efficiency Overall, the simulations indicated that MNOs could achieve a 32 percent gain in network capacity through better RF path control via electrical downtilt and better USLS. Note that these increases are above and beyond the 127 percent gain in network capacity realized by deploying the metro cells as opposed to the homogeneous macro cell network. The spectrum efficiency gained can be used to further offset CapEx and OpEx expense, as well as the cost of spectrum. Site Cell Connected users Peak RLC user Peak RLC site Peak RLC network Average spectral efficiency (Bits/Hz/km 2 ) 4W macro network, 1 downtilt Heterogeneous network, 8" omnidirectional antenna, no downtilt Heterogeneous network, 24" quasi omnidirectional antenna, 12 downtilt Table 5: The impact of improved heterogeneous network operation using electrical downtilt and USLS is shown. 8

9 The gains from downtilt and USLS become even more pronounced as frequency increases. Table 6 shows the results when the same antenna configurations are used at 26 MHz. Compared to the typical 8-inch (2 mm) omnidirectional antenna with no downtilt, the CommScope 24-inch (6 mm), quasi omnidirectional antenna with 12 degrees of downtilt increases aggregate network capacity 72 percent and peak user 54 percent. Average spectral efficiency rises by 5 percent. Site Cell Connected users Peak RLC user Peak RLC site Peak RLC network Average spectral efficiency (Bits/Hz/km 2 ) 4W macro network, 1 downtilt Heterogeneous network, 8" omnidirectional antenna, no downtilt Heterogeneous network, 24" quasi omnidirectional antenna, 12 downtilt , Table 6: The impact of the control over USLS and use of electrical downtilt at 26 MHz is shown. To assess the effects of metro cell sectorization, the research team measured the same performance characteristics across the three different antennas. The simulations assumed the metro cell networks were on a unique band from the macro cells and were operating at 19 MHz. The metro cell network used a contiguous design within a dense urban area, similar to a downtown hot zone, and a power setting of 5W. The results of these simulations are shown in Table 7. Site Cell Connected users Peak RLC user Peak RLC site Peak RLC network Average spectral efficiency (Bits/Hz/km 2 ) 8" omnidirectional antenna, downtilt , " quasi omnidirectional antenna, 12 downtilt , " tri-sector antenna, 12 downtilt , Table 7: The impact of metro cell sectorization is shown. As indicated in Table 7, the combined benefits of high USLS, electrical downtilt and cell sectorization deliver a best-case scenario and, when compared to the typical 8-inch (2 mm) omnidirectional antenna, both CommScope antennas significantly improve network performance. 27 to 62 percent increase in aggregate network capacity 24 to 6 percent gain in peak user 27 to 62 percent increase in average spectral efficiency 28 percent reduction in site Comparing the two CommScope antennas to each other allows us to isolate the advantage of sectorization. User grows by 3 percent Network rises by 27 percent Average spectral efficiency increases 27 percent 9

10 Naturally, achieving the and capacity gains outlined above requires multi-sector antennas and additional radio capacity (hence, the increase in cell ). The actual number of radios required per site may be higher in any event, however, as the addition of a second band for a second technology (e.g., 3G) can force this situation. Effects of enhanced inter-cell interference control (eicic) In addition to greater RF path control is the potential application of enhanced inter-cell interference control (eicic). However, the use of eicic comes at the expense of more complex X2 network interconnectivity and spectrum, as almost blank sub-frames (ABS) are employed to partition the spectrum of macro enodeb and metro cells to prevent interference. The following table provides a simulation contrasting configurations both with and without eicic. Site Cell Connected users Peak RLC user Peak RLC site Peak RLC network Average spectral efficiency (Bits/Hz/km 2 ) Baseline macro network without eicic Dedicated metro cell network, omnidirectional antennas, proportional fair scheduling and eicic Dedicated metro cell network, quasi omnidirectional antennas, 12 downtilt, proportional fair scheduling and eicic Dedicated metro cell network, quasi omnidirectional antennas, 12 downtilt, maximum C/I , , , Table 8: The effect of downtilt with eicic using 5W metro cells is shown. As illustrated in Table 8, while there may be benefits to employing eicic for better cell edge performance and a more consistent user experience, it is not without a loss in spectral efficiency and an increase in network complexity. Nevertheless, the use of quasi omnidirectional antennas having electrical downtilt and better USLS in an eicic network can deliver an additional 18 percent gain over use of a typical omnidirectional antenna and eicic alone. Analyzing total cost of ownership Using a network cost estimating tool developed by CommScope, the team ran a series of calculations designed to accurately estimate CapEx and OpEx within the context of the network and application variables. Third-party solution provider, EdgeConneX, and industry analyst Mobile Experts, helped form and clarify the assumptions that were used. The estimates were based on the total number of sites and radios required to achieve the best spectral utilization within the 7, 19 and 26 MHz bands. As with the RF performance simulations, the costing analysis aced for several morphologies and site parameters, including a dedicated band overlay or same band infill operating at 1W and 2W in a dense urban area; a 5W overlay in a suburban network; and isolated 1W, 5W, and 1W metro cells in Madison Square and Union Square in New York City. 1

11 The individual cost variables included equipment, power, construction, site visits, leases, permits, and the typical elements included in site construction. Simulated backhaul methods included dark fiber, Ethernet-MetroE, Ethernet-GPON, Ethernet-DSL and mmwave/microwave. Highlights of the cost analysis are shown in Table 9. It offers a summarized expense comparison between metro cells equipped with the three tested metro cell antennas. TOTAL COST OF OWNERSHIP 2x 5W with MIMO outdoor Metro Cell Overlay Network at 19MHz Using Single Element Omnidirectional Antennas Using 5-Element Quasi Omnidirectional Antennas Using 5-element Tri-Sector Antennas with Additional Radios CAPEX $941,6 $758,45 $89,2 NON-RECURRING COSTS $1,428,45 $1,27,95 $1,27,95 ANNUAL OPEX $1,347,74 $969,843 $969,843 SPECTRUM $2,46, $2,46, $2,46, Cost per MHz per POP $4. $4. $4. POPs per km 2 41, 41, 41, Cost per MHz per km 2 $1,64, $1,64, $1,64, Cost of Spectrum 1.5 km 2 $2,46, $2,46, $2,46, Unit Cost per Paired Mb/s Network Wide $1,579 $1, $ YEAR GRAND TOTAL INCL. SPECTRUM (TCO) $ 6,177,754 $5,216,243 $5,347,993 1-YEAR METRO CELLS ONLY (TCO) $3,717,754 $2,756,243 $2,887,993 1-YEAR COST PER METRO CELL SITE $34,745 $35,795 $37,56 Table 9: A TCO analysis for networks using the tested metro cell antennas is shown. Recommendations for selecting and deploying metro cell antennas Choose an antenna with USLS of 1 db or better, up to 45 degrees over the horizon. Maintain an antenna height of 2 feet (6 meters) or higher over the surrounding area. This will enable you to use the antenna height to optimize the cell footprint, while taking maximum advantage of the sharp vertical beam roll-off of the USLS. Maximize electrical downtilt when positioning the antenna at its minimum height (e.g., 2 feet, or 6 meters), unless the footprint becomes too small. Try to keep the antenna height below 4 feet to prevent the main lobe from growing too large. Avoid pointing antennas directly at buildings that are over 16 feet (49 meters) tall and within 3 feet (9 meters). Antenna height Tilt Outer measure at 3 db 2 ft. (6 m) 2 deg. 13 ft. (4 m) 2 ft. (6 m) 16 deg. 175 ft. (53 m) 4 ft. (12 m) 16 deg. 35 ft. (17 m) Table 1: The typical range of antenna main beam at given heights is shown. Refer to Table 1 to see the impact tilt has at a given antenna height. 11

12 Summary Mobile operators are under increasing pressure to boost network capacity and coverage while reducing their cost per Mb/s delivered. Repeated analysis performed independently during the past few years has quantified the gains in spectral efficiency that can be achieved using metro cells. However, as this paper indicates, whether they are used in heterogeneous networks or as a stand-alone underlayment, extracting maximum value from metro cells requires explicit control over the RF path. Through simulation, CommScope has demonstrated that operators can apply macro cell principles to metro cell network design to minimize metro cell interference and optimize spectral efficiency. Improved spectral utilization enables operators to increase user (upwards of 3 percent) or satisfy more users. In either case, the overall cost per Mb/s per population (POP) served is reduced substantially, resulting in CapEx/OpEx savings on the order of 25 to 35 percent. References 1 Small Cell Market Status; Informa Telecom & Media report; June Impact of metro cell antenna pattern and downtilt in heterogeneous networks ; unpublished. 214; Xiao Li, University of Texas-Austin and Southeast University (Nanjing, China); Robert W. Heath, Jr., University of Texas-Austin; Kevin Linehan and Ray Butler, CommScope Visit our website or contact your local CommScope representative for more information. 214 CommScope, Inc. All rights reserved. All trademarks identified by or are registered trademarks or trademarks, respectively, of CommScope, Inc. This document is for planning purposes only and is not intended to modify or supplement any specifications or warranties relating to CommScope products or services. WP EN (9/14) 12