Clearwire has gone into ready mode, slowing its cash burn and cutting its capex in anticipation of the $3.2-billion influx of capital it will receive once its merger with Sprint’s WiMAX division is complete. In the interim, Clearwire is focusing on prepping its initial four WiMAX networks for launch and working with Sprint to develop a joint operational and deployment plan to help ease the burden of integration after the deal closes.

Clearwire reported in its earnings call Thursday that its 31 US markets as a whole had gone cash-flow positive, as Clearwire spent the last quarter focusing on monetizing its current customer base rather than growth. To that end, Clearwire only added 18,000 new subscribers in the second quarter for a total of 227,000 subscribers, but its average revenue per user rose $2.42 from the first quarter to $39.28. Clearwire CEO Ben Wolff said the company was shifting its focus from current growth on the legacy fixed wireless network to the new network it expects to build in conjunction with Sprint next year.

Clearwire managed to cut its capex by $28 million to $62.3 million as it slowed down deployment of new fixed wireless networks using Motorola’s proprietary NextNet gear and shifted its capital spending to its initial four WiMAX networks, which Wolff said should be ready for launch by the end of the year. Portland, Clearwire’s current trial network, has been expanded to cover 1.2 million of the metro area’s population. Clearwire engineers have moved from network testing in Portland to operational testing, the last phase of the trial before commercial launch. Clearwire is also building out infrastructure in its other three launch markets—Atlanta, Las Vegas, and Grand Rapids, Mich.—and expects to have most of the installation done by year end. But Wolff cautioned that actual commercial launches of those networks would be dependent on the joint venture transaction closing.

“If at any point we believe the transaction will take longer to close than expected and we are not able to procure interim financing at terms we find acceptable, we will further moderate our spending,” Wolff said.

Wolff also revealed that Clearwire had begun the network planning necessary to convert its 31 fixed wireless markets to WiMAX in 2009, which could result in a faster rollout of WiMAX than previously expected. Clearwire has 1250 sites either deployed or rolling out in its four WiMAX networks and a total of 10,000 pre-WiMAX and WiMAX sites either already deployed or in development. Converting all of those sites to WiMAX will give Clearwire roughly half the WiMAX footprint it has projected to build by the end of 2010.

Upgrading those sites will likely be far easier and far less expansive than building networks in new markets, said Eric Kainer, analyst for ThinkPamure. “The upgrade will require new antennas and new base-station equipment, but perhaps not much else,” Kainer said in a research note today. While a new WiMAX tower would cost around $50,000, Kainer estimated, an existing fixed wireless site could be upgraded for 50% to 70% of that cost.