Sprint is triple-downing on prepaid. On Friday, Sprint introduced Sprint Prepaid, a revamped take on an existing program that includes less expensive plans — with caveats — and new options for smartphones, including a “Spark” device able to access higher LTE speeds. Sprint Prepaid represents yet another push by the carrier to reinforce its position in the prepaid market, complementing its two other brands, Virgin Mobile and Boost Mobile, both of which attack slightly different demographics of consumers. The launch comes at a time when its business is under siege by T-Mobile, which offers its own expanding prepaid brand in MetroPCS, and when AT&T is preparing to take a bigger step into prepaid with its just-completed acquisition of Leap Wireless. Sprint Prepaid is designed for consumers who want a no-contract offer without credit checks, but aren’t necessarily comfortable with a prepaid brand, and would prefer to stick with a national carrier like Sprint, the company said.