MIMO and Smart Antennas for 3G and 4G Wireless Systems ____________________________ Practical Aspects and Deployment Considerations September 2012 Release 2 Page 1/138 4G Americas – MIMO and Smart Antennas for Mobile Broadband Systems - October 2012 - All rights reserved. CONTENTS Summary .................................................................................................................................................. 4 INTRODUCTION ...................................................................................................................................... 4 1 Antenna Fundamentals ..................................................................................................................... 6 1.1 Base Station Antenna Types and Evolution .............................................................................. 8 1.2 Reconfigurable Beam Antenna ................................................................................................ 11 1.3 Integrated Radio/Antenna ........................................................................................................ 13 1.4 Active Antenna System (AAS) Technology ............................................................................. 17 1.5 High Gain Antennas for Wide Area ......................................................................................... 22 1.6 Commercial Antenna Types Supplied – One Vendor .............................................................. 27 2 MIMO with LTE ............................................................................................................................... 30 2.1 LTE Downlink MIMO Basics .................................................................................................... 31 2.2 Antennas for MIMO .................................................................................................................. 38 2.3 Antenna Array Calibration ........................................................................................................ 47 3 Reconfigurable Beam Antennas ..................................................................................................... 52 3.1 How Reconfigurable Beam Antennas Work ............................................................................ 52 3.2 Use Cases of Reconfigurable Beam Antennas in 3G Networks ............................................. 53 3.3 Comparison of RET, 2D, and 3D Reconfigurable Beam Antennas ......................................... 55 3.4 Measurement of Coverage, Interference, and Load Balancing with Reconfigurable Beam Antennas ............................................................................................................................................ 55 3.5 Network Optimization Versus Load Balancing ........................................................................ 57 3.6 Antenna Beam width Distribution ............................................................................................ 58 3.7 Reconfigurable Beam Antennas – Cyclical Traffic Pattern Management ............................... 59 3.8 Reconfigurable Beam Antenna Summary ............................................................................... 60 4 Deployment Scenarios .................................................................................................................... 61 4.1 Typical Cell Site Architecture ................................................................................................... 61 4.2 Current Deployments ............................................................................................................... 65 2 4G Americas MIMO and Smart Antennas for Mobile Systems – October 2012 – All Rights Reserved 5 Small Cells and HetNet Deployment .............................................................................................. 71 6 Miscellaneous Commercial and Deployment Issues ...................................................................... 75 6.1 Constraints on the Antenna Deployments due to Commercial Considerations ...................... 75 6.2 Electrical and Mechanical Tilting of Antennas ......................................................................... 76 6.3 Passive Intermodulation (PIM) Site Considerations ................................................................ 90 6.4 Independent Antenna Tilt Optimization by Air Interface .......................................................... 91 6.5 Remote Radio Heads for MIMO .............................................................................................. 92 6.6 Cable Tradeoffs for Remote Radio Heads .............................................................................. 96 6.7 Co-Siting of Multiple Base Stations and Technologies .......................................................... 103 6.8 Indoor Distributed Antenna System — MIMO Coverage ....................................................... 115 7 Terminal Antennas ........................................................................................................................ 117 7.1 Prospects and Characteristics of Multiple Antennas in Terminals ........................................ 117 7.2 UE Performance at 750 MHz with MIMO .............................................................................. 121 7.3 Current Status of terminal antennas ...................................................................................... 125 Definitions and Acronymns .................................................................................................................. 128 References ........................................................................................................................................... 133 Acknowledgements…………………………………………………………………………………………138 3 4G Americas MIMO and Smart Antennas for Mobile Systems – October 2012 – All Rights Reserved SUMMARY In 2010 this organization, then called 3GAmericas, published the first version of this whitepaper on antennas. At the time, only simulations and very early trials of LTE were available. Since then, along with the terrific growth in smart phones and wireless data traffic, the industry has seen important changes to the types and methods of deploying wireless systems. Wireless coverage is no longer the driving force for deployment; capacity is. The need to double capacity each year has resulted in the major push for LTE, the deployment of 3G Small cells, HET-NETs with WiFi, and residential cellular base stations. Some operators have deployed 4 branch antenna systems and Remote Radio Heads/Units have become common. Increasingly, operators are integrating more active electronics on tower tops and even inside the antenna radomes. Active antenna arrays capable of vertical sectorization are in trials, and LTE-Advanced standards have been finalized and it has been in lab trials with deployments being planned. In short, the wireless landscape has changed in the past two years and it is time for a refresh of this document. INTRODUCTION The extraordinary growth in wireless data traffic is putting immense strain on the operator’s network. To address this demand and increase capacity, operators have five primary tools at their disposal: 1) Adding Cell Sites is an effective but expensive approach to adding capacity. In general adding new real estate is time consuming and increasingly prohibitive. With median inter- site distances dropping from 5km to 2km and recently to less than 200m in dense urban areas, the operator has less choice in selecting affordable property. Doubling the number of cell sites approximately doubles the network capacity and the throughput per user (assuming the user density stays constant), and greatly improves the peak user and the aggregate throughput per km2. 2) Adding sectors, such as changing from 3 sectors to 6 sectors, is a useful way to approximate the introduction of new cells. However, this does not quite double the capacity as the “petals” of 6 sector coverage do not interleave as well as 3 sector coverage, and the fractional overlap of 6 sectors is greater. This also challenges handoff processing when near highways. This is a common approach in dense urban areas where rooftops are available. There is about a 70% increase in capacity in moving from 3 to 6 sectors in an environment with low angle spread (where the base station is located above the clutter). 3) Adding Carriers (or more accurately, bandwidth) directly adds to capacity. The LTE standard is particularly adept at utilizing increased bandwidth. In addition, in the USA, the FCC permits increasing radiated power with the bandwidth in the PCS, AWS, and lower 700 MHz bands providing improved penetration and coverage. Doubling bandwidth at least doubles throughput. 4) Improved air interface capabilities, such as in evolving from R99 UMTS to Release 5 HSDPA, provided well over 4 times the aggregate downlink capacity for example. However, in moving from, say, Release 6 HSPA (1x2) to Release 7 (1x2) with 64QAM and 2x2 MIMO we see a more modest ~10-20% improvement in the aggregate throughput. As has been observed before, with improvements in air interface (while leaving everything else the same such as bandwidth and antenna configuration) we are seeing diminishing returns on improvements. It is clear that something more than simply increasing modulation and coding rates is needed. 4 4G Americas MIMO and Smart Antennas for Mobile Systems – October 2012 – All Rights Reserved 5) Smart antennas provide the next substantial increase in throughput. The peak data rates tend to be proportional to the number of send and receive antennas, so 4x4 MIMO is capable of serving twice the peak data rates as 2x2 MIMO systems. We’ve witnessed an important trend in the nearly 3 decades of wireless system growth, shown in Figure 1 below. On a log plot we see the rather steady growth of the number of macrocell sites in the United States.* We see a very steady growth of about 30% year over year from 1986 through about 2002, but since then the growth rate has decreased to about 2% since 2005. We interpret these trends as corresponding to a coverage growth phase and a more recent phase where the number of sites has not grown much but the capacity of the sites have been greatly expanded. More carriers, more bands, and more capable air interfaces have been deployed at these more intensely active base stations. Figure 1 – The number of macrocells deployed in the United States. Source of data: CTIA Semi-Annual Reports.1 This “intensification” of the existing cell sites has involved deploying multi-band antennas and remote radio heads adjacent to the antennas, as well as having more carriers with their associated power * These are reported by the CTIA as “cells” referring to the number of independent base station sites deployed by the various operators. A single tower may have as many cells as there are operators collocated at the tower. Our understanding is that these sites do not include femtocells or indoor sites that are not inventoried by the operators. 5 4G Americas MIMO and Smart Antennas for Mobile Systems – October 2012 – All Rights Reserved supplies and baseband processing facilities located at these sites. In general, the sites have evolved to more capable and “smarter” equipment, including smarter antennas. By “smart antennas” we refer here to adaptive antennas such as those with electrical tilt, beam width, and azimuth control which can follow relatively slow-varying traffic patterns, as well as so called intelligent antennas that can form beams aimed at particular users or steer nulls to reduce interference and Multiple-Input-Multiple Output (MIMO) antenna schemes. Finally, we also consider adaptive antenna arrays with the ability to apply separate signals to antenna elements in both the vertical and horizontal axes to form beams or sectors in the vertical plane, as well as implement MIMO and receive diversity with elements on the other axis. A goal of this paper is to focus attention on the practical aspects of deploying smart antenna systems in Radio Access Systems (RAS). Additionally, networks are increasingly using small cells and base stations deployed indoors or below the clutter where the experience gained from decades of tower mounted antennas do not apply. This paper also addresses the practical aspects of deploying these modern base stations with their increasingly capable antennas. A substantial body of theoretical and field experience is able to provide reliable guidance in the tradeoffs of various antenna configurations. Operational experiences with commercial LTE wireless systems have demonstrated many of these gains and their practical deployment issues. Several previous papers describe the theoretical capabilities of smart antennas and the mechanisms that provide for their support in the standards. The reader is referred to surveys such as those in various recent publications 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 Section 2.2 of this paper gives the basics of LTE downlink MIMO schemes. Section 1.1 covers the evolution of the base station antenna. Section 2.2 describes MIMO antennas and their operation. Section 1.4 gives an overview of Active Antenna Systems and summarizes current performance expectations. A good portion of this paper comes from a 4G Americas Whitepaper on the general subject of MIMO and Smart Antennas published in May 2010 that the reader is referred to for further background. 1 ANTENNA FUNDAMENTALS Antennas are critical to all wireless communications and significant advances in their capabilities have been made in the past several decades. Figure 2 below shows the inside of a modern antenna, where we are reminded that what we refer to as an antenna consists of a number of individual antenna elements all contained within a single radomes. The antenna shown below has four coaxial DIN† connectors serving two frequency bands each with two polarizations. The coaxial connections feed a distribution network that connects the 4 separate signals to the radiating elements. In one case, the coaxial connector feeds the +45° polarization of the 5 higher frequency band radiating elements (mounted on the circular plates) while another coaxial connection feeds the +45° polarization radiating elements in the 4 lower frequency band radiating elements. The feed network includes a variable phase shifter shown in Figure 3 that introduces a larger transmission delay to the lower elements so that the electromagnetic waves radiating from the elements will be in phase at an angle tilted down † Coaxial RF connectors standardized by the Deutsches Institut für Normung (DIN). 6 4G Americas MIMO and Smart Antennas for Mobile Systems – October 2012 – All Rights Reserved toward the ground where the mobile users are located. The tilt angle may be adjusted with a manual tilt rod or a motorized actuator controlled remotely over the AISG connection. Figure 2 – Internals of a typical modern antenna structure for mobile wireless applications. This has four electrical ports. We see in this structure a total of 18 radiating antenna elements; 5 high band at +45° and at -45°, and 4 low band of each polarization. When packaged in a common radome we refer to this overall structure as a single antenna even though there are these 18 antenna elements inside. We refer to this as a single cross polarized column with two frequency bands interspersed. Also, even though the tilt actuator is motorized, we refer to this as a passive antenna because there are no active elements in the signal paths. (Active electronics use DC power to amplify or transform signals.) We will see in section 1.3 that there are emerging new Active Antennas (AA) with active electronics in the radome as well. Generally, the taller an antenna is, and the more elements there are in a column, the more resolution we have in shaping the vertical characteristics of the radiated pattern. That is to say, doubling the height allows us to about halve the vertical beam width and about double the antenna gain. This is tied to the wavelength so as the frequency doubles with a fixed height radome, we also tend to be able to double the antenna gain and halve the vertical beam width. Consequently, in many installations where the antennas are limited to a fixed height such as 6 feet for esthetic and zoning reasons, we see that the higher frequency bands can have twice the antenna gain (3dB) as the lower frequency bands. 7 4G Americas MIMO and Smart Antennas for Mobile Systems – October 2012 – All Rights Reserved Likewise, the antenna width impacts the horizontal beam width. This is why a six sectored installation requires antennas that are about double the width of three sectored installations. More detailed definitions and acronyms concerning antennas are listed in the Appendix. Figure 3 – View of the back of a typical modern antenna showing the tilt mechanisms. Base station antenna technology has progressed in response to industry requirements and trends. The key drivers have been the continuing addition of cellular frequency bands, the integration of more functionality into single radome housing, and antenna techniques that contribute additional capacity to cellular networks. The following figures concisely describe the development of the base station antenna including advanced antenna technologies in use and emerging today. 1.1 BASE STATION ANTENNA TYPES AND EVOLUTION The early days of commercial cellular communications was deployed primarily through omni- directional antennas. These “Omni” antennas are typically linear cylinders, resembling a pipe. They generally radiate in all azimuth directions, hence the name “omni-directional”. Omni antennas provide low capacity when compared to more current technologies. For receive spatial diversity, the antennas are normally spaced 10λ apart. Refer to Figure 4. The second season of base station antenna technology introduced the first “panel” antennas. These antennas were packaged in wider housings, supported with brackets on the back surface. They offered some beam control and were commonly vertically polarized. Azimuth beamwidth could now be controlled, providing cell sector handoff capability. With defined elevation beamwidth, site coverage 8 4G Americas MIMO and Smart Antennas for Mobile Systems – October 2012 – All Rights Reserved could then be tailored using mechanical downtilting through the mounting brackets. These are also shown in Figure 4. The next evolution step was the inclusion of log periodic dipole antenna elements. These radiating element arrays offered improved directivity. This improved beam control resulting in better sector handoffs and reduced interference. Again, refer to Figure 4. Two very significant advancements came next in base station antenna technology. Dual-polarized antenna arrays were invented. In this advancement, a combined element array using polarization diversity provided two ports to be combined in a single antenna. The elements used 45° slant polarization, with the second polarization rotated 90°. These dual-polarized antennas replaced two spatial diversity vertical polarized antennas. The second key development in this phase was electrical tilt of the antenna beam. Phase controlled tilt utilized new technology to tilt the beam, providing some coverage control without the distortion effects found with mechanical tilting. See Figure 5. Figure 4 - Early base station antenna technology (courtesy of CommScope, used with permission.) Next came variable tilt antennas and Remote Electrical Tilt (known as RET). This technology utilized phase shifting devices inside the antenna to more precisely control the beamtilt. With this design, the phase shifters could be coupled to motorized actuation systems with remote control capabilities. This allowed downtilt control and cell coverage optimization from remote sites without climbing towers. See Figure 5. The next significant development was dual-band antennas. These antennas combined two different frequency band antennas into a single housing. This again reduced the required antennas by 50%, 9 4G Americas MIMO and Smart Antennas for Mobile Systems – October 2012 – All Rights Reserved reducing leasing costs and increasing tower space efficiency. Each frequency band had capability for independent RET controls. A further advancement included three frequency bands in a single housing. See Figure 5. Figure 5 – Typically deployed existing antenna technology (Courtesy of CommScope, used with permission.) A more recent development is a line of antennas intended for 6-sector cell site configurations. These antennas feature azimuth beamwidths of 33 or 45 degrees. The 3-sector arrangements typically use 65, 85, or 90 degree beamwidths. The 6-sector arrangements offer increased antenna ports and therefore increased capacity capability. These are shown in Figure 6. A further refinement of 6-sector antenna solutions incorporates multiple beams in a single housing. A common twin-beam version incorporates two 38 degree beams in a single housing. This is another development to increase coverage and capacity without additional antenna housings on the tower. Refer to Figure 6. Another antenna trend is concealment or integrated housings. In these designs, the antennas may be housed in a structure which is disguised. Many of these include 3-sectors and may also include tower mounted amplifiers, remote radio units, or other peripheral devices inside the housing. One example is shown in FIgure 6. 10 4G Americas MIMO and Smart Antennas for Mobile Systems – October 2012 – All Rights Reserved