Explaining the huge gap between fastest and slowest 4G upload speeds in the US

With our summer U.S. report around the corner, we wanted to take the opportunity to highlight a metric that will feature in OpenSignal's U.S. reports going forward. That metric is upload speed, and it's becoming an increasingly important measure of the consumer mobile data experience. When the 4G age dawned, download speed was king as it determined how quickly we could surf the internet and the quality of mobile video. But as social media has boomed, we're no longer a society of content consumers; we've become content creators. And when it comes to sharing the selfies we snap and the videos we record, the ability to send large quantities of data upstream assumes paramount status.

We decided to take a first look at how the four nationwide operators in the U.S. compared in 4G upload speed, drawing on our most recent 90-day test period from March to May. What we found was a bit surprising. In any of our speed metrics we typically see a gap between the fastest and slowest scores, but in the case of U.S. operator 4G upload speed that gap was particularly wide.

T-Mobile users racked up the fastest LTE upload speed results, averaging 7.5 Mbps, which was three times faster than the 2.5 Mbps average we measured for Sprint users. Verizon came in right behind T-Mobile in our measurements with a score of 7 Mbps, while AT&T was 3 Mbps off the leader with an average 4G upload of 4.5 Mbps in our tests.

T-Mobile and Verizon have consistently outpaced AT&T and Sprint in our 4G download metrics by large margins in our last five U.S. reports. Typically when an operator increases its 4G download speeds, its 4G upload speeds grow as well, as they're both linked to a network's total capacity. So we would expect T-Mobile and Verizon's upload speeds to be faster than AT&T and Sprint's. But as our Sprint and AT&T 4G download speed results have been fairly close in our in recent tests, we would expect their 4G upload results to be similar as well. Why are AT&T users seeing upload speeds 125% faster than Sprint users in our measurements?

The answer most likely lies in the nature of Sprint's LTE technology. A large portion of Sprint's 4G network uses a technology called time division duplexing, or TDD, to transmit data to and from devices. Unlike most cellular networks, which use separate dedicated frequency bands for the uplink and downlink, TDD networks use the same spectrum for uplink and downlink. The network essentially splits every two-way transmission into time intervals. One time interval is used to send data from the tower to the device (download) while another time interval is used to send data from the device to the tower (upload). The neat trick about TDD networks is operators can adjust the proportion of time intervals devoted to upload and download as they see fit. You could feasibly configure a network to spend 99% of its time downloading data to devices, but that's hardly feasible. You'd wind up with a service that would be great for streaming video but lousy at sending even the most low-resolution photo.

Sprint has been using this TDD trick for years, and it's definitely given it a competitive advantage. As customer whims and habits shift, Sprint can configure its LTE network to match them. In May 2017, it revealed its most recent LTE tweak, favoring download much more heavily than upload to align with the data usage patterns on its networks (customers were downloading 10 to 12 times more data than they were uploading). We're likely seeing that new configuration reflected in our metrics. While Sprint was at the bottom of our 4G download speed rankings for years, it's making up a lot of ground in the last 12 months, even threatening to overtake AT&T in this metric. But TDD is a balancing act: when download capacity goes up, upload capacity goes down. That's most likely why we're seeing such low scores for Sprint in our 4G upload speed metric.

Sprint may have found the right balance for its current consumer demands, but I'm sure it will need to make further tweaks as mobile app and data trends shift. If Sprint finds its networks suddenly teeming with self-styled videographers sharing their every moment on FaceBook Live, then it may need to start punching up its upload speeds again.

Do you think your operator struck the right balance between upload and download speeds? Let us know in the comments section below.