Nokia Siemens Networks said Monday that it had agreed to buy the wireless-network equipment division of Motorola for $1.2 billion in cash.

Nokia Siemens, which makes telecommunications equipment, is a joint venture of Nokia of Finland and Siemens of Germany. Nokia, one of the world’s largest mobile phone makers, has struggled to maintain its footing in the cellphone industry as rivals like Apple and HTC of Taiwan have gained market share.

The Motorola unit that Nokia Siemens is buying supplies wireless carriers with the equipment needed to build and maintain cellular networks, including infrastructure for the fourth-generation mobile technologies known as LTE and WiMax.

Nokia Siemens said the sale would be instrumental in helping it improve its position in the United States and Japan, as well as bolster diminishing profits.