Samsung’s mobile chief, Dong-jin Koh, has stepped down from running the company’s day-to-day operations. Instead, Tae-moon Roh will take over management of the smartphone business while Koh focuses on the company’s long-term business strategy.

Koh, better known as D.J., has handed over the title of head of Samsung’s mobile communications business to Roh but remains president and CEO of Samsung’s IT & mobile communications division. Roh has overseen development of Samsung’s Galaxy smartphones.

It’s unclear why Samsung decided to shuffle the management of its smartphone business, and the company didn’t immediately respond to CNET’s request for more information.

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Samsung made a similar move in 2015, when it promoted Koh to be head of mobile. Its previous chief of day-to-day smartphone operations, J.K. Shin, retained control of long-term strategy and held onto the mobile CEO title at the time. Two years later, Shin departed the company, and Koh took over as CEO of Samsung’s smartphone operations.

Samsung remains the world’s biggest smartphone company, but it has faced some challenges in recent years. It’s become harder for handset vendors to make huge changes in their devices and differentiate from one another, which is causing consumers in places like the US to wait longer to upgrade. In faster growing markets like China, Samsung has struggled to compete with Huawei and other rivals. And it has struggled to move into new markets like smart speakers.

In the third quarter, the most recent data available, 22% of global smartphone shipments came from Samsung, according to IDC. But Huawei was close behind at 19%, even as it faced bans on using US technology like Google’s Android software.

Koh’s time at the helm of Samsung’s smartphone business has been marked by a variety of highs and lows. He oversaw 2016’s launch — and two subsequent recalls — of the disastrous Note 7, which had battery issues and forced Samsung to kill off the flagship phone. But the following year’s Galaxy S8, which featured a screen that stretched across the entire front of the device, helped Samsung regain customer trust and maintain its position as the world’s biggest smartphone maker.

Under Koh’s tenure, Samsung developed the Bixby voice assistant, but the technology is seen as lagging that of rivals like Amazon’s Alexa. The long-awaited Galaxy Home smart speaker, which would feature Bixby, has never gone on sale. Samsung started talking about the smart speaker in early 2018, and it showed off the device during its Galaxy Note 9 unveiling in August 2018. Koh told CNET in February 2019 that it would hit the market in a couple of months, but it still hasn’t materialized. It now appears Samsung will first launch a smaller version of the smart speaker.

The company has leapt into new businesses like 5G and foldables, but those rollouts has seen some hiccups. Samsung delayed the Galaxy Fold’s launch date by about five months after reviewers found issues with the device’s screen. While Samsung has introduced more 5G phones than any other vendor, analysts believe the super-fast mobile connectivity won’t become mainstream until Apple launches its first 5G phones later this year.

“It may seem counterintuitive that Apple, which currently has no 5G phones in its portfolio will be able to pass current 5G market leaders Samsung and Huawei,” Strategy Analytics analyst Ken Hyers said in November. “But with three new 5G models coming [this] year, Apple merely needs to match its current upgrade rates for newly introduced iPhone models to take the lead [this] year.”

Roh, meanwhile, is Samsung’s youngest-ever president at 51, according to Reuters, which earlier reported the news of the management shuffle. The publication said that Roh “championed Samsung’s shift to outsource more handset production to cut costs and better compete with lower-priced Chinese smartphone makers such as Huawei.”