Federal Communications Commission Chairman Tom Wheeler is sticking with his plan to limit the amount of spectrum the biggest carriers can buy at the next big auction, refusing to grant a T-Mobile US request to impose even stricter limits.

More than a year ago, the FCC tentatively decided that the planned auction of broadcast TV airwaves to wireless carriers will set aside up to 30MHz in each geographic area for carriers that don't have significant amounts of low-band spectrum in the region. In most cases, this means that AT&T and Verizon Wireless will not be able to bid on that 30MHz. (The reserve could be lower than 30MHz if it turns out the total amount of available spectrum is less than 70MHz.)

T-Mobile has argued that the FCC should make 40MHz available exclusively to the smaller carriers, but Wheeler today said he's sticking with his 30MHz plan. His latest proposal, which would be voted on July 16, "concludes that the current reserve size of 30 megahertz balances the desire to make low-band spectrum available to parties with limited holdings while facilitating competitive bidding for all auction participants," Wheeler wrote on the FCC blog today.

Smaller carriers could still buy 40MHz or more—they'd just have to pay the market rate for anything beyond 30MHz.

Low-band spectrum like the 600MHz airwaves that are expected to be auctioned in early 2016 is important for allowing cellular networks to cover great distances and indoor locations. Wheeler wrote that smaller carriers need help getting more of these airwaves because "more than 70 percent of low-band spectrum [is] in the hands of just two providers."

While the FCC is giving T-Mobile 75 percent of what it's asked for, the company said in a recent filing to the FCC that "30 megahertz is simply insufficient to promote the competitive environment that our industry so desperately needs. Largely as a result of spectrum giveaways from the government before the Commission received auction authority, the two dominant incumbents have roughly 50 megahertz of low-band spectrum each."

There are significant technical advantages to having 20MHz blocks of spectrum, T-Mobile noted. A 30MHz set-aside will only allow one smaller carrier to get at least 20MHz at below-market rates.

"Adopting a spectrum reserve of at least 40 megahertz will help ensure multiple carriers have an opportunity to acquire the low-band spectrum necessary to improve network reliability, extend coverage deeper inside buildings, and bring more meaningful consumer choice to rural and underserved areas throughout the United States," T-Mobile wrote.

T-Mobile isn't completely without low-band spectrum. For example, it bought more than $3 billion worth of 700MHz licenses from Verizon last year.

Verizon argues that bidding restrictions "rob taxpayers who won't get paid what the spectrum is actually worth," and that T-Mobile and Sprint are capable of paying market rates because they "are owned by large multinational corporations (Deutsche Telekom and Softbank, respectively)."

In a previous fight between the carriers, the FCC in December granted a T-Mobile petition that could force AT&T and Verizon to charge lower prices for data roaming.

In another proposal described by Wheeler today, the FCC will impose new limits on "small business" discounts that Dish, a very large business, used to get $3.3 billion worth of discounts in another recent spectrum auction. In the upcoming broadcast TV spectrum auction, the proposal would impose a $150 million cap on small business discounts and restrictions on certain types of joint bidding arrangements. Dish got its discounts by placing bids through subsidiaries.

The new rules wouldn't apply retroactively to Dish, but the FCC is conducting a review of Dish's bids to determine whether it should force the company to pay full price.