* Investment would mark U.S. re-entry after Helio withdrawal (Adds backgroud)

SEOUL, Sept 20 (Reuters) - South Korea's top mobile carrierSK Telecom Co Ltd said on Monday that it was in talks to investin U.S. upstart wireless operator LightSquared, a venture backedby hedge fund Harbinger Capital Partners.

"We are in talks with Harbinger to decide investment terms,"an SK Telecom spokeswoman said.

The possible investment would mark the operator's reentryinto the U.S. market in two years.

SK Telecom exited the U.S. market in 2008, selling itsstruggling mobile unit Helio to Virgin Mobile USA as part of adivestment of foreign assets. SK Telecom sold back its stake inChina Unicom for $1.3 billion in 2009.

LightSquared plans to build an open wireless broadbandnetwork using Long Term Evolution (LTE) technology, afourth-generation protocol that established operators VerizonWireless, AT&T Inc and MetroPCS Communications Inc also plan tosupport.

The venture said in July that it planned to kick off networkconstruction with $1.75 billion of financing from financialinvestors, and expected to raise a second round of financing inthe next two years.

Nokia Siemens Networks (NSN) will deliver equipment toLightSquared this year so it can test network services inBaltimore, Phoenix, Las Vegas and Denver in the first half of2011. (Reporting by Hyunjoo Jin; Editing by Chris Lewis)