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NF: What trends in the mobile industry are most affecting antenna design? Are small cells and heterogeneous networks (HetNets) causing the biggest headaches?

BG: Most significantly, we’re seeing an increase in the number of antennas per handset. Traditionally, phones have had one main cellular antenna and a receive diversity antenna. In order to meet multiband requirements and have a single phone model support multiple regions, however, original equipment manufacturers (OEMs) are now using two or more main antennas as well as a diversity antenna. In addition, LTE-Advanced (LTE-A) adds the option for a transmit diversity antenna, which can be utilized in a 2x2 multiple-input multiple-output (MIMO) configuration. LTE-A also supports higher levels of MIMO, such as 3x3 and 4x4. That’s even more antennas to pack into a very small and thin housing. Plus, many OEMs are trying to create new and exciting designs through the use of materials that are not necessarily “antenna-friendly.”

WiFi also is transitioning from a single antenna to MIMO along with the addition of 5-GHz-band support. WiFi devices need additional antennas for Bluetooth, GPS, and near-field communications (NFC). More antennas can deliver better performance across the 700-to-2900-MHz bands. But they create design challenges in terms of space and cost. Multiple antennas also present performance challenges. Devices must have sufficient isolation so that there is low correlation in the far field to support the higher data rates.

Bob Giometti

Small cells and HetNets also present challenges. Mobile operators are eager to provide the next level of performance supported by LTE-Advanced. Small cells are being used to deliver LTE-A cell service in a small geographic space, such as sports stadiums. The challenge is not that it’s particularly hard to design a femtocell antenna. Rather, it is difficult for the smartphone to know how to interact with HetNets or how to execute fast, smooth handovers between networks.

NF: We also are seeing an increase in antennas for WiFi systems and, of course, a rise in distributed antenna systems (DASs). Can you share some numbers and insight into those trends?

BG: LTE adoption has accelerated the consumer demand for global data services. This demand has the potential to overwhelm service capacity, but many providers mitigate this issue by offloading data onto WiFi networks. HetNets enable service providers to absorb demand. Service providers also are generating additional revenue streams by offering new WiFi-based services.

According to a report issued by online research firm ReportsnReports.com, small cells and carrier WiFi deployments are expected to carry more than 60% of all mobile network data traffic by 2020—accounting for $352 billion in mobile data service revenue. The report also notes that small cells, carrier WiFi, DAS, and cloud RAN infrastructure investments will account for a $42-billion HetNet ecosystem by 2020.

NF: What design challenges have been created by the growth of WiFi systems in particular?

BG: As I mentioned, the growing number of antennas creates design challenges. In addition, passive intermodulation (PIM) can create interference in WiFi signals from sources that designers may not anticipate. Designing antennas with low PIM is one of the challenges. WiFi access points (APs) also are integrating LTE capability, so that adds even more antennas.

NF: Is wireless broadband currently driving the most growth in terms of antenna-design market numbers?

BG: We don’t think this is the case. It’s possible for someone to have a 4G phone without a data plan. We see growth coming from smartphones with data plans. As such, mobile data should fuel significant growth. It’s important to keep in mind, however, that core mobile voice services continue to grow in much of the world. Mobile broadband data is still in its early stages worldwide. Wireless broadband is a driver of antenna growth, but not the biggest driver.

NF: For antenna designers, what does wireless broadband mean in terms of tradeoffs they must accomplish, constraints they must adhere to, requirements that must be satisfied, etc.?

BG: Near-term requirements call for utilization of a single mobile band. But the industry is soon moving beyond that, which will add new design constraints and requirements. A key feature of LTE-A is carrier aggregation, which uses multiple LTE bands/channels simultaneously to achieve higher data rates. Carrier aggregation combines up to five 20-MHz channels to create one ultra-fast, reliable channel. In doing so, it enables new levels of services, such as concurrent voice and downloading of video. Though appealing, leveraging multiple bands in parallel over multiple antennas without interference and with low correlation is often challenging for system-level designers.

NF: Are material developments keeping pace with today’s antenna developments?

BG: Yes, materials are evolving along with mobile handsets. Manufacturers are using new enclosure materials ranging from plastics to metals. They’re also developing rugged phones that are more water- and shock-resistant and working to eliminate cracked screens via flexible phones with displays based on newer organic-light-emitting-diode (OLED) plastics.

As these designs become reality, antenna design also is shifting. For example, new laser-direct-structure (LDS) designs allow antennas to be more easily integrated while using less space within a phone. And there are new techniques for making flexible, foldable antennas using flexible printed-circuit boards (FPCBs). New methods also are being used to combine multiple antennas together on a common structure. These structures are more spatially efficient and can be placed on the back of a phone or around the edges, enabling innovative form factors.

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