Clearwire is preparing to upgrade its 2.5GHz WiMAX network to support a variant of LTE, TD-LTE, that it's co-developing with China Mobile — and the broadband wholesaler has just announced that it'll kick off that upgrade in 31 "hot zones" where wireless service is most crunched. Most of those markets have yet to be announced, but it has teased five today: New York, Los Angeles, Chicago, San Francisco, and Seattle, all of which are expected to start going live in early 2013; "remaining markets" will come online by the middle of the year.

Sprint, which owns a significant chunk of Clearwire, is underwriting a portion of the company's LTE upgrade costs and is expected to take advantage of the service once it's available. In the meantime, it's using both 1900MHz and freed 800MHz iDEN spectrum for its initial LTE deployment (which is using the more common FDD flavor shared by Verizon, AT&T, and others).

Building a network takes money — a lot of it

The news comes as Clearwire has announced its Q1 2012 earnings which tell a mixed tale — it wasn't long ago that the company nearly missed making interest payments on its debt, and it's got a long way to go to become a profitable organization. Revenue is up 36 percent year over year to $322.6 million from 11 million subscribers, up from 6.1 million a year prior; that's good, but revenue is actually down about $39 million from Q4 2011. Overall, it chalked up an operating loss of some $422.9 million, slightly better than last quarter's $433.1 million. Importantly, cash on hand is on an upward trend: it's now at $1.433 billion between cash, cash equivalents, and investments, likely thanks in no small part to a capital infusion from Sprint. Building a network takes money — a lot of it — and Clearwire's balance sheet tells the tale quarter after quarter.