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Last year, the mobile industry heralded dynamic spectrum sharing (DSS) as a “game changer” for 5G rollouts, enabling cellular towers to split 4G spectrum into separate 4G and 5G signals to service new and old devices. Now T-Mobile is suggesting that DSS might not be ready for prime time — an issue that could seriously affect some carriers’ 5G rollouts — though larger rival Verizon is contesting its claims.

DSS made headlines by promising to address one of 5G’s biggest challenges: the lack of additional 5G-specific low and mid band spectrum, otherwise known as “sub-6GHz” frequencies. While the FCC slowly reallocates wireless spectrum from other users, such as naval radars and satellite broadcasters, DSS makes existing cellular spectrum simultaneously useful to 4G and 5G devices, dynamically increasing or decreasing the number of 5G connections with actual usage.

During a quarterly financial results conference call with analysts this week, T-Mobile CTO Neville Ray said that unnamed tower hardware vendors have run into real-world problems that may delay DSS’ use in 5G networks, thereby stunting the growth of 5G services. Without naming names, Ray said that one of the top 5G radio vendors is “very late” in delivering DSS functionality, and noted that the company’s tests had revealed early DSS performance to be “pretty corrosive,” eating network capacity “just by rolling out the feature.”

Ray also suggested that while T-Mobile can keep expanding its 5G footprint across the U.S. with previously unused 600MHz spectrum, rivals planning to use DSS for sub-6GHz 5G rollouts would find that “DSS is not going to materially help them.” That shot was aimed directly at Verizon, which has said that it would use DSS to augment its short-distance millimeter wave 5G deployments into a nationwide 5G network.

Verizon spokesperson Kevin King downplayed Ray’s comments today to VentureBeat, saying “Neville Ray and T-Mobile seem pretty preoccupied with our 5G network. It must be because their 5G network can’t even match the performance of our 4G network. We’re confident in our 5G strategy and execution plan, and we’ll keep the commitments we have made to our customers.”

It’s unclear whether the issues Ray highlighted are specific to tower hardware T-Mobile has tested or will actually impact other U.S. carriers. T-Mobile made a significant deal for Nokia 5G gear, and it may be referring to delays in that particular manufacturer’s DSS rollout, while Verizon uses some 5G hardware from Ericsson, which has already launched DSS support. Since each of the carriers has multiple alternatives for sub-6GHz 5G rollouts, including the use of slow low band frequencies and plans to either share or re-farm mid band frequencies, massive disruptions seem unlikely, but delays are possible as hardware makers tweak their software to achieve better 5G and shared 4G-5G network performance.

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