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JJD: What devices are most critical to the wireless-infrastructure industry?
PS: It is challenging to identify one or two pieces of equipment that are the most critical. Every device in the radio-frequency (RF) path can have a big impact on network performance. After spending billions of dollars on spectrum and network infrastructure, the last thing wireless operators want is a loose or poorly made connection that degrades network performance and quality of service (QoS). But that is exactly what can happen in extra-sensitive LTE networks.
PS: LTE is an interference-limited technology, meaning the most significant barrier to optimal performance is signal interference. One of the most disruptive causes of interference is passive intermodulation (PIM). PIM is a performance-degrading phenomenon that can result in degraded voice quality, dropped calls, and reduced data throughput. The effects of PIM can be significant—for example, just a 1-dB drop in uplink sensitivity due to PIM can reduce wireless coverage by 11%.
Network operators are advised to take a multi-prong approach to preventing PIM. Numerous resources are available to help avoid PIM-related performance issues. These resources include an online certification program for installers and field engineers, system calculators for network designers, and PIM-limiting, performance-assuring infrastructure solutions. In addition, there are passive components designed specifically to operate as low-PIM alternatives, such as splitters, couplers, and other RF components. They enable operators to achieve and maintain effective network performance while supporting cost-effective business practices in distributed-antenna-system (DAS) applications.
JJD: What are the leading semiconductor technologies for the wireless-infrastructure industry?
PS: The main goals for RF low-power active devices are high integration and low power consumption with DC voltage lower than 3.3 V. Currently, Analog Devices and PMC Sierra—among others—offer single-chip, software-defined-radio (SDR) multicarrier RF transmitter and receiver chains. They are JESD204B digital-interface-compliant and meet LTE macro-base-station standard requirements. They consume a total of about 5 W.
For RF power amplifier (PA) applications, gallium-nitride (GaN) technology could revolutionize the wireless market in terms of power efficiency—even if today, its cost is still too high with respect to LDMOS. GaN technology is attractive due to its lower parasitic output capacitance [in particular, increasing the direct-current (DC) drain voltage at 50 V].
JJD: How are these technologies implemented and where are the greatest design challenges?
PS: The greatest challenge is the increase of power efficiency for RF active power devices. Currently, GaN semiconductors are characterized by efficiency with 4% to 5% more than LDMOS (considering the same linearity figure and the same operating conditions).
JJD: How are power amplifiers optimized for this application?
PS: The PA optimized structure has to take some parameters from the cellular-system requirements into account. In detail, PA efficiency for a certain linearity value versus average output power represents the most important figure of merit for the PA. This figure is impacted by the peak-to-average ratio of the signal, according to specific standard requirements and the number of transmitted RF carriers.
A Doherty-amplifier architecture leverages the tradeoff between linear operation and efficiency. It reduces the amplifier’s saturated power level when the signal level is low. When signal peaks demand it, however, it quickly ramps to full power capability.
Typically, the Doherty architecture is used for the final amplifier stage, which contributes significantly to the thermal dissipation of the overall PA. Classic Doherty amplifiers use two active devices with the same saturated output power. Yet an asymmetric Doherty architecture is preferred when a higher peak-to-average-ratio signal needs to be transmitted according to LTE requirements.
In any case, a Doherty architecture is needed to keep an eye on the operating bandwidth—considering that all of these architectures are inherently narrowband circuits.
Keep in mind that an increase in efficiency can be obtained by using GaN devices instead of LDMOS, even though GaN technology still costs more than MOS or bipolar options. The tradeoff between linearity and efficiency can be further increased by considering more efficient classes for the active devices, such as Class-F.
JJD: What current trends are happening around DAS adoption?
PS: The indoor space is the last frontier for wireless. About 80% of mobile sessions originate inside buildings, but only about 2% of the world’s business enterprises have a dedicated cellular system. Wireless operators have already deployed DASs in many of the largest public venues, such as airports and stadiums. Now, the move is to medium-sized office buildings, high-rises, college campuses, and healthcare systems.
Although distributed antenna systems are well-proven solutions for adding coverage and capacity, innovations for simplifying the solution have been slow. The lack of simplification has meant longer deployment time, expensive engineering skill-level requirements, and notoriously difficult optimization. Every step in traditional DAS deployment can bring extra expense and delayed return on investment. Operators continue to use DAS, however, because of the pressing need to offload traffic from the macro-network and deliver a superior customer experience.
JJD: What are the latest achievements in DAS technology for open environments like stadiums or airports?
PS: Most companies with DAS expertise are focused on the practical, logistical side of DAS with innovations that make it easier to implement. Many of the latest technologies and techniques have stripped the complexity from DAS for larger venues with a unified indoor-outdoor and low/high power approach. A few of the latest systems include embedded intelligence, which guides the design, planning, installation, setup, commissioning, and optimization with virtually foolproof simplicity. These systems may also have remote configuration tools that enable operators to re-sectorize as well as auto-level. In addition, there are more built-in monitoring features for measuring network quality, monitoring PIM and other interference, and conducting detailed spectrum analysis. Some of these systems have troubleshooting smart alarms and automatic documentation for reducing errors. Overall, the goal of these systems is to enable network operators to design, deploy, and optimize a DAS more quickly and efficiently and at a lower total cost of ownership.
JJD: How are DASs geared toward office environments?
PS: There are distributed antenna systems designed specifically for the enterprise market. They provide a unified wireless infrastructure designed to be integrated more easily in enterprise settings. Often, these systems utilize the structured cabling that is common to office buildings, making the system friendly to both wireless operators and business enterprises. Some of these systems even bring licensed wireless and power together with Gigabit Ethernet into one unified wireless network that can scale to building size. Several systems are designed for flexible use. They implement spectrum and adaptive features like multi-band, multi-operator, and multi-technology capabilities.
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Network Challenges
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JJD: What are some significant challenges for network operators? Are their solutions on the horizon?
PS: The mobile-data surge is putting extraordinary pressure on wireless networks worldwide. Mobile subscribers are consuming many more data services—especially streaming video—thanks to the rising numbers of smartphones and tablets in use. The network capacity for meeting this increased data consumption is the biggest challenge faced by today’s mobile network operators.
Small-cell solutions are gaining traction with operators all over the world as a way of solving the capacity challenge. Operators are interested in small cells because they know that the macro network—which typically broadcasts cellular signals over a large area—no longer can provide enough capacity for all of the data traffic. Nor does the macro network provide the flexibility to quickly add or shift capacity where it’s needed. This feature is needed to keep pace with users who are constantly on the go and always consuming great amounts of bandwidth.
JJD: What is the difference between a small-cell, macro-cell, pico-cell, femto-cell, etc.?
PS: Like many in the industry, CommScope defines a small cell as anything that is not a macro cellular site. The original small cell is a DAS, which takes a donor feed from the macro cell and distributes it over fiber throughout a building or outside space. In addition to enabling new traffic in previously non-covered areas, the DAS offloads traffic from the macro network. A DAS is an operator-, frequency-, and network-protocol-agnostic solution.
Concealed, integrated metrocells are mini-macro sites that can be installed more quickly in metropolitan areas to boost network capacity. Metro cells address all issues relating to macro-cell site deployment in dense urban areas. In other words, they ease regional-zoning and site-acquisition issues. They can be easily fitted to street furniture, such as lamp posts. In addition, they are aesthetically pleasing and even flexible and scalable.
Pico cells, femto cells, and mini-remote radio heads are targeted at adding capacity in medium to large buildings for one operator only. They are essentially low-power radios of various sizes, which are deployed to add localized capacity and coverage. Most people are familiar with Wi-Fi, which offloads data traffic onto a wireline network for transmitting over the internet. For the last number of years, many wireless operators have relied on Wi-Fi to offload data traffic from their overburdened networks. Wi-Fi serves a similar role as other small-cell solutions, but it ultimately is taking traffic that could be monetized off of the operators’ network.
To accomplish the objective of small cells—providing more capacity in heavy-traffic areas—network operators should also look at solutions for the macro site. Sectors in a macro site can be split to add capacity—much like a small cell would. By splitting a sector in two using a twin beam antenna, for example, an operator can nearly double capacity. Each sector is smaller, but there are more sectors for the entire cell, supporting more customers and data traffic.
This file type includes high resolution graphics and schematics when applicable.