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Microsoft s share of the U.S. smartphone market continued to decline through May, according to research firm comScore.

For the three-month period between the end of February and the end of May, comScore estimated Microsoft s U.S. share dipping from 7.7 percent to 5.8 percent. That comes despite the marketing push behind the Windows Phone platform.

During the same period, adoption of Google s Android platform rose from 33 percent to 38.1 percent, while Apple enjoyed a slight uptick from 25.2 percent to 26.6 percent. Research In Motion continued its market slide, declining from 28.9 percent to 24.7 percent.

The top mobile OEMs, in descending order of market share, included Samsung, LG Electronics, Motorola, Apple and Research In Motion.

Despite the softness in Windows Phone s share, Microsoft is actively seeking another way to profit off the smartphone market: extracting royalties from Android device manufacturers. According to a July 6 Reuters report, Microsoft is demanding that Samsung pay $15 in royalties for every Android-based smartphone the latter produces.

Microsoft insists that the Android platform infringes on a number of its patents, and has used its considerable legal resources to pursue manufacturers of Android smartphones and tablets. Some of those manufacturers, including HTC, have agreed to pay Microsoft royalties. Over the past 10 days, Microsoft has entered into a series of patent-licensing agreements with four other Android device manufacturers, including Wistron Corp., Onkyo Corp., Velocity Micro and General Dynamics Itronix.

But some Microsoft targets have been more willing to fight back. Motorola retaliated to a Microsoft patent-infringement suit with an intellectual-property complaint of its own. And Barnes & Noble, whose Nook e-reader uses Android, filed a counter-suit against Microsoft after the latter sued it for patent infringement.

In the meantime, Microsoft can only hope that its upcoming Windows Phone Mango update will increase the platform s appeal to consumers and businesspeople. As Microsoft executives demonstrated for a small group of media and analysts during a May press event in New York City, Mango s new features include a redesigned Xbox Live Hub, home-screen tiles capable of displaying up-to-the-minute information, the ability to consolidate friends and colleagues into groups, and visual voicemail more than 500 new elements in all, if Microsoft is to be believed.

Mango is due for release sometime this fall. Samsung, HTC, LG Electronics and Nokia have all committed to building new Windows Phone devices preloaded with Mango. Meanwhile, Acer, Fujitsu and ZTE have apparently agreed to produce Windows Phone units for the first time.

However, chances are good that Windows Phones released during the fall timeframe will run head-on into Apple s next iPhone, which current rumors suggest will launch sometime in September. At the same time, various Android manufacturers and Research In Motion are making concerted efforts to push their own devices into the ecosystem. To say this is a crowded marketplace is an understatement.

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