TOKYO--While arguing his case that he can upend the U.S. wireless industry as he has done successfully in Japan, SoftBank Corp. (9984.TO) Chief Executive Masayoshi Son lists off the parallels between the two markets -- high revenue per user, scarcity of pre-paid users, and consumer thirst for smartphones.

But the difference in network speeds on either side of the Pacific Ocean -- Japanese smartphone connection speeds are 30% to 40% faster than in the U.S. -- presents the fiercely ambitious Japanese billionaire with what he says is an opportunity that he can capitalize on to turn around Sprint Nextel Corp. (S), the number-three U.S. carrier.

"Right now, every time I come to the States, I say "Wow! this is so slow, what is this? It's unbearable,'" he said in October when he announced SoftBank's initial agreement with Sprint, pledging to speed up Sprint's network speeds.

After weeks of suspense due to a rival bid from cable network operator Dish Network Corp. (DISH), Sprint shareholders are expected to approve SoftBank's $21.6 billion deal to acquire 78% of the U.S. wireless carrier. The vote, along with expected approval from the U.S. Federal Communications Commission, clears the way for the deal to close in early July, pitting the Internet billionaire against AT&T Inc. (T) and Verizon Wireless, the two titans of the U.S. wireless industry.

Armed with Sprint's access to Clearwire's rich spectrum holdings -- rights to use swaths of airwaves to transmit phone calls, Internet traffic and other data -- Mr. Son said he plans to replicate his success in Japan in building a network supporting what he says is the world's fastest and smoothest smartphone connectivity.

"We created a network that can withstand the traffic to be expected in the age of smartphones without "packet freezes" and data disruptions. SoftBank's network is superior in terms of both speed and quality," Mr. Son said on Friday at SoftBank's shareholder meeting when he laid out just about grand his ambitions are: to be the No. 1 company in the world by sales, profit, and market capitalization "at any cost."

Sprint's turnaround won't be easy. Developing networks in the U.S. is difficult, where huge areas need to be covered and upgraded unlike the more densely populated Japan. According to data from Cisco Visual Networking Index, the average smartphone connection speed in Japan was 2.1 megabytes per second in 2012 -- nearly 40% faster than the average 1.5 megabytes per second in the U.S.

While Mr. Son said Sprint is already showing signs of turnaround, the U.S. carrier narrowed its losses in the first quarter partly because it gained fewer customers, meaning lower upfront costs to cover subsidies for new handsets. Its postpaid net additions totaled 12,000 subscribers in January-March, with its total end of period subscribers standing at 55.2 million subscribers at the end of March, down from 56.1 million from a year ago.

SoftBank's acquisition would inject $5 billion directly into Sprint to develop its high-speed network. The money also could help Sprint continue to offer new flat-rate, unlimited data plan, seen as key for it to compete against AT&T and Verizon Wireless, both of which have restrictions on their most recent smartphone data plans. Sprint will also need such a plan to stay ahead of No. 4 U.S. carrier T-Mobile, which offers unlimited data plans.

SoftBank and Sprint together have 97 million users, and that scale would help Sprint win more than $2 billion of cost cuts every year from handset and network equipment makers, which could mean a more economical expansion of Sprint's network, Mr. Son said.

In a May interview, Mr. Son said that the U.S. market is overdue for disruption. Verizon and AT&T together hold two-thirds of the U.S. market and customers and have felt no need to aggressively compete in pricing or services, he said. "There are all kinds of ways to excite the people with segmentation, with packaging or new campaigns or introduction of new innovative service or innovative products."

When SoftBank bet the house to acquire the loss-making Japanese arm of U.K.-based Vodafone Group PLC (VOD) for Y1.75 trillion in 2006, it took a no-holds-barred approach to make waves in a market 80% controlled by the top two players: NTT DoCoMo Inc. (9437.TO) and KDDI Corp. (9433.TO). To win customers, SoftBank slashed monthly payment charges to about one-quarter of the competition's. Eventually, rivals caved and matched SoftBank's low-cost pricing. He also unveiled an aggressive marketing blitz that trumpeted the savings that SoftBank can offer consumers.

"I would expect Mr. Son to advertise greater value for money -- bigger bang for buck and more data speeds for a lower price than Verizon and AT&T," said Richard Guppy, head of mobile strategy at consultancy Strategy Analytics. "But I don't think he needs to repeat the kind of scorched earth tactics it took in Japan. Sprint's pricing plans are already low."

SoftBank's mobile-phone business, a distant No. 3 when Mr. Son bought Vodafone Japan, has now led its two bigger rivals in winning new net subscribers -- or new users minus cancellations -- for 17 straight months and it won more than half of net new users in the year ended in March. Its market share in Japan has grown from 16% in 2006 to almost 25%, while the number of its subscribers has doubled.

Part of the gain in subscribers has been the result of a steady improvement in the performance of SoftBank's mobile network. After often being derided for years as being inferior to that of its rivals, SoftBank said its network is now on par with the competition in terms of network speed, coverage area and connection stability.

"You cannot dismiss him as a normal businessman who just talks big. He is a logic and data machine," said Yusuke Tsunoda, analyst at Tokai Toyo Research Center. "When he says he will be the world's No. 1 firm, that's what his calculations are telling him, period."

Write to Mayumi Negishi at mayumi.negishi@dowjones.com and Spencer E. Ante at Spencer.Ante@wsj.com

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