AT&T and the parent company of T-Mobile USA, Deutsche Telekom, are considering a joint venture that would pool network assets from the two wireless carriers in case the $39 billion merger doesn't go through, sources close to the situation tell the Wall Street Journal.

Specifics of the joint venture are unclear, as the companies still plan to forge ahead with the merger. However, opposition to the merger is mounting. The Federal Communications Commission released a 109-page report Tuesday criticizing the merger, saying it would be bad for consumers.

We conclude that [AT&T and T-Mobile] have failed to meet their burden of proof to show that the proposed transaction is in the public interest, the FCC's staff analysis said. Harm to competition could take the form of an increase in price (or a reduction in the rate of price decline), a reduction in output or service quality, or a reduction in the rate of new product development or other innovation.

The U.S. Department of Justice filed suit against the merger in August. The trial is scheduled for Feb. 13. Similarly, Sprint Nextel and C-Spire wireless have filed their own separate lawsuits.

It is also unclear whether AT&T would have to pay Deutsche Telekom a $4 billion breakup fee if the deal ends in a joint venture rather than a merger.

Shares of AT&T rose 0.92 percent to $28.98 at market close.