Sprint's stock took a hit yesterday after a prominent research firm downgraded the operator on a "very legitimate risk" of bankruptcy, sending the operator's shares down 4.5 percent after a recent rally.

Bernstein Research laid out two possible outcomes for the company, which is juggling a major overhaul of its network and the hugely expensive iPhone with a spectrum shortage and a significant amount of debt.  

In the first scenario, Sprint successfully upgrades its network, gets Clearwire on stable ground and is able to offer LTE service on par with its competitors.  

In the second, "some combination of its gargantuan take-or-pay contract with Apple, a hobbled 4G offering, and a stupendous debt burden bring the company to its knees." 

"To be clear, we are not predicting a Sprint bankruptcy. We are merely acknowledging that it is a very legitimate risk," Bernstein Research analyst Craig Moffet, author of the report, said in a research note. "And notwithstanding a recent rally in Sprint shares, we believe that risk is rising." 

A Sprint spokesman declined to comment on the report. 

Sprint has less spectrum to deploy LTE than its competitors, but has repeatedly insisted its speeds will be comparable to those of AT&T and Verizon Wireless thanks in part to a capacity boost from Clearwire’s yet-unbuilt TD-LTE network. 

But Moffet expressed doubt about Sprint's plan to use Clearwire's TD-LTE network to supplement capacity. Clearwire's TD-LTE deployment will be limited, and combining Sprint's FDD-LTE and Clearwire's TD-LTE could make it harder to secure devices that support both flavors of the technology. 

"We expect Sprint's competitiveness to begin to backslide when LTE becomes the nation's de facto standard," Moffet. 

Sprint's LTE network and iPhone gamble need to pay off so it can make the billions in debt payments coming due over the next three years.  

The operator should be able to handle debt maturities totaling $3.2 billion through 2014, Moffet said, but "thereafter the company faces a sustained multi-year barrage of large maturities that will need to be addressed."  

Clearwire, in which Sprint holds the largest stake, has $3 billion in debt due in 2015, the same year Sprint has another $2.6 billion due, Moffet said.  

"If Sprint's performance is not substantially improved from current levels by that time, capital may not be made available for the refinancings," he said.  

The possible result: bankruptcy. 

Sprint could also find itself at a significant disadvantage if Apple - with whom it's paying $15.5 billion for the iPhone - comes out with an LTE version of the hugely popular device, since Sprint's LTE deployment is behind competitors AT&T and Verizon Wireless.  

"We believe an LTE iPhone will likely be badly disadvantaged on Sprint's network, potentially impairing sales… at a time when Sprint is subject to a punishing take-or-pay deal with Apple," Moffet said. Sprint has previously said it expects iPhone sales to surpass the amount of devices it is committed to buy under its contract with Apple, but its iPhone sales have been weaker than at Verizon and AT&T.

Sprint plans to have its LTE service up and running in 10 markets before the middle of this year, but won’t start selling devices that can get a boost in speed from Clearwire’s network until sometime in 2013. Meanwhile, Verizon’s LTE footprint covers more than 200 cities and AT&T’s network will soon be live in about 40 markets.