Active Antenna Systems have been "in field trials" for almost 20 years in the mobile market, and each time the idea gets a little closer to commercial success. Back in the 1990s, companies such as Metawave, Arraycomm, Andrew and even Ericsson tested prototype antenna arrays with progressive customers such as Vodafone and AT&T. A few of these systems had limited commercial success (such as the Arraycomm technology in PHS and wireless local loop applications), but at that time the market decided that AAS technology was too expensive for the capacity benefits that were produced, and in fact a move from 3G to LTE has produced bigger capacity benefits at lower cost.
Recently, every one of the top base station OEMs has renewed their programs for AAS. The new approaches take advantage of cost reductions in signal processing, as well as some new sophistication in semiconductor integration. Having seen dozens of AAS demos over the past 20 years, I believe that we're getting closer now to a solution which will succeed commercially. There are technical reasons why AAS is becoming more important, and there are business dynamics which are converging to make AAS more viable.
Technical stuff: First of all, the TD-SCDMA network deployed by China Mobile over the past several years has relied on active antenna steering in order to avoid self-interference from one base station to another. Although the acronym "TD-SCDMA" includes the word "synchronous", in fact this scheme only synchronizes the uplink signals but leaves flexibility in assignment of uplink vs. downlink traffic, which leaves the system open to interference from one base station's downlink to other base station's uplink. China Mobile found that active antenna steering was essential, to avoid the base station self-interference problem.
This technical issue carries into TD-LTE as well. To get the greatest flexibility in a TDD network, ideally the operator will want to assign time slots to uplink or downlink, based on the need at each base station. If the operator takes advantage of this flexibility, there will be cases where one base station is transmitting in the downlink during a neighbor cell's uplink reception. AAS is a necessary component to making TD-LTE work, and the huge China Mobile TD-LTE deployment uses an 8-way antenna array to steer the beams and reduce interference.
The Cost/Benefit Tradeoff: In the TD-LTE world, AAS will get started because the benefits of TDD depend on it. But what about FDD? When will AAS reach a "tipping point" where the benefits outweigh the costs? Here are some considerations:
- The primary benefit of AAS in the FDD case will be capacity enhancement. Features such as active beamsteering and vertical sectorization will enable capacity improvement in the range of 75 percent or more, at a lower cost than small-cell solutions or new towers in an urban environment.
- The progression from 2G to 3G and finally to LTE has pushed wireless performance close to some fundamental limits. The laws of physics dictate that any new "5G" air interface will not improve greatly on LTE's spectral efficiency. Instead, the entire industry needs to turn toward spatial efficiency, using directional antennas, small cells, and other techniques to re-use spectrum more wisely.
- TD-LTE production in China will create economy of scale for AAS semiconductors and software solutions. This is an important step, where the initial production related to TD-SCDMA will be followed by TD-LTE ASIC solutions which are cost-optimized to run AAS algorithms.
- Simplified architectures under investigation by major OEMs will bring down cost as well. The start-up companies dominating the AAS research area for the past 20 years have always tried to promote very complex solutions, for dramatic benefits. Major OEMs do not need to swing for a home run...and will be able to implement more modest solutions with a better cost/benefit ratio. The Ericsson AIR 32 product is a good example of simple antenna beam control at a low cost.
- Nowadays, a 50 percent improvement in capacity is a lot more meaningful than it was in the past. Mobile operators pull in more revenue for each base station sector today, making a $20,000 investment in an AAS upgrade a better ROI than it was just five years ago.
Limitations: Of course, taking the AAS approach in an FDD base station will only pay off in cases where the tower reaches a lot of users. Urban centers with high-rise buildings are one example where the traffic density is high, and in fact the traffic can be broken into vertical sectors within a short distance of the tower. In this specific case, augmenting the tower with a simple Active Antenna System can be less expensive than outfitting all of the surrounding buildings with small cells.
On the other hand, in applications with lower traffic density (suburban areas), small cells may be a more practical solution because long-distance coverage from a tower--with penetration of the walls--will still result in low spectral efficiency and lower throughput.
In summary, we see the "tipping point" coming. China Mobile is driving the industry to massive deployment of AAS solutions, and in crowded, urban cases we will see simple AAS solutions in the FDD world as well. The Mobile Expert's forecast predicts 40 percent average market growth for AAS during the next five years.
Joe Madden is Principal Analyst at Mobile Experts LLC. Mobile Experts is a network of market and technology experts that provide market analysis on the mobile infrastructure and mobile handset markets. He provides market forecasts for handset, DAS, small cell, and base station markets, with in-depth research down to the nitty gritty details of frequency bands and power levels. Mr. Madden graduated, cum laude, from UCLA in 1989 and is a Silicon Valley veteran. He has survived IPOs, LBOs, divestitures, acquistions, and mergers during his 24 years in mobile communications.