Baseband Processor

A baseband processor (also known as baseband radio processor, BP, or BBP) is a device (a chip or part of a chip) in a network interface that manages all the radio functions (all functions that require an antenna); however, this term is generally not used in reference to Wi-Fi and Bluetooth radios. A baseband processor typically uses its own RAM and firmware.

Baseband processors typically run a real-time operating system (RTOS) as their firmware, such as ENEA's OSE, Nucleus RTOS (iPhone 3G/3GS/iPad), ThreadX (iPhone 4), and VRTX. There are more than a few significant manufacturers of baseband processors, including Broadcom, Icera, Intel Mobile Communications (former Infineon wireless division), MediaTek, Qualcomm, Spreadtrum, and ST-Ericsson.

The rationale of separating the baseband processor from the main processor (known as the AP or Application Processor) is threefold:

Since the software which runs on baseband processors is usually proprietary, it is impossible to perform an independent code audit. By reverse engineering some of the baseband chips, researchers have found security vulnerabilities that could be used to access and modify data on the phone remotely. In March 2014, makers of the free Android derivative Replicant announced they have found a backdoor in the baseband software of Samsung Galaxy phones that allows remote access to the user data stored on the phone.

...
Wikipedia