Dallas-based cell-phone service provider MetroPCS Communications has raised $1.15 billion in its initial public offering, the largest U.S. IPO of the year.

The offering priced at $23 per share, above the forecast range of $19 to $21 a share. Only 14 percent of the company’s shares were floated, and the IPO gives the company a market capitalization of almost $8 billion.

MetroPCS offers flat monthly rates for unlimited local calls.

The company’s earliest venture backers — Accel and Battery Ventures — stand to make great money from this, because they hung in there for a decade even as the company’s fortunes waxed and waned. They and other less fortunate investors — some who arrived later and paid a higher price for the company’s shares while it was private, and others who came early, but abandoned the company — invested more than $420 million. Other investors included Columbia Capital, Flagship Ventures, Madison Dearborn and TA Associates.

It had a net income of $29 million last year.