Microsoft said that Motorola's Android phones infringe nine patents, including some that would appear to threaten most smartphone platforms. Android is the open source OS built by Microsoft rival Google.
The patents appear to include some related to Microsoft Exchange ActiveSync, which syncs e-mail, calendar and contacts between a mobile phone and a software-based computer program, according to a blog post written by Horacio Gutierrez, general counsel at Microsoft.
Other patents involve technology that displays signal strength and battery power on phones.
While Google licenses Microsoft's ActiveSync for use in Android, Microsoft may plan to argue that handset makers that add their own technologies to Android also need a license for ActiveSync, said Chris Hazelton, an analyst with The 451 Group. Motorola does add its own enhancements to Android phones.
"Motorola, of the major Android supporters, is the only one that doesn't license ActiveSync themselves," Hazelton said. HTC, Samsung, Sony Ericsson, Dell and others all license ActiveSync, he said.
Microsoft could also be using the lawsuit as a way to pressure Motorola into building phones using Windows Phone 7, Microsoft's new phone operating system, Hazelton said. Though Motorola made phones using Microsoft's operating systems in the past, it now says it is committed to Android.
Motorola may have been unwilling to license ActiveSync because it can be expensive, Hazelton said. "This is not a small amount of money. We're talking hundreds of millions of dollars," he said. Motorola, which is about to split up into several companies, has been struggling over the past few years.
Microsoft may have other motives for filing the lawsuit beyond potential revenue. "They're trying to slow down Android," said Jack Gold, an analyst with J. Gold Associates. "That's for good reason. Look at the charts: Android's momentum is killer."
Gartner expects Android to become the second-largest smartphone platform by 2012, behind only Symbian.
"That scares the traditional players who have invested in this market for years," Hazelton said. "That is compelling Microsoft to say, 'How do we combat this? We'll make it costly and risky to people who are considering going with Android.'"
Microsoft said it was acting to protect its intellectual property investments, and noted that Nokia and other vendors have also filed lawsuits over smartphone technologies. Microsoft has "a responsibility to our customers, partners, and shareholders to safeguard the billions of dollars we invest each year in bringing innovative software products and services to market," the company said.
The suits shouldn't come as a surprise. Earlier this year, HTC announced it had licensed Microsoft patents relevant to its Android phones. The companies did not disclose what kinds of technologies the patents covered. At the time, Microsoft said it was in discussions with other phone makers using Android.
Microsoft isn't the only company posing a threat to Android. Apple has sued HTC over its Android phone, and Oracle has sued Google over the way it handles Java in Android.
Microsoft even referred to those cases in its blog post about the suit. "Our action today merely seeks to ensure respect for our intellectual property rights infringed by Android devices; and judging by the recent actions by Apple and Oracle, we are not alone in this respect," Gutierrez wrote.
The suits "are dark clouds over Android," said Florian Mueller, who founded the NoSoftwarePatents campaign in Europe. "Google must now act constructively and try to work out amicable arrangements with those right holders. Otherwise I'm afraid that third-party application developers investing their money, creativity and hard work in the Android platform will be harmed because of an irresponsible approach to intellectual property in a market in which patents have always played an essential role."
In a statement, Motorola said it had not received a copy of the complaint. "Motorola has a leading intellectual property portfolio, one of the strongest in the industry. The company will vigorously defend itself in this matter," it said.
The first phones running Microsoft's revamped mobile phone software are scheduled to appear this month. The company has said that one benefit of using its Windows Phone 7 software, compared to some of the free OSes like Android or Symbian, is that it has a broad patent portfolio and can protect handset makers from threats.
"Microsoft indemnifies its Windows Phone 7 licensees against patent infringement claims," the company said recently. "We stand behind our product, and step up to our responsibility to clear the necessary IP rights."
Gold questioned the timing of the Motorola suit. "If you're going out and suing everyone and you're about to release a new OS, what's going to grab the headlines? You want your new OS to, not the lawsuit," he said.
