RF Front End Control Interface (RFFE) in smartphones

What is RFFE (RF Front End Control Inferface)?

RFFE is a two-wire interface for controlling radio frequency front-end subsystems that uses unclosed single-ended CMOS I / O for lower performance. The interface can contain up to 19 components (amplifiers, LNAs, antenna tuners, filters and switches) per bus instance including their power supply.

Where is an RF frontend used in mobile devices?

In current mobile devices, there is a sophisticated circuit that is responsible for converting information from the near-zero frequency band signals that are used to convey information and data into radio signals that can be received or sent over the air. The RF frontend takes on these tasks.

What tasks does RFFE take on in mobile devices?

The RF Front End (RFEE) has to process the right data at the right time with the right information and transmit on the right band at the right power level. In a modern smartphone there are several antennas for sending and receiving signals. Depending on the technology, there may be 6 or more antennas (3G, 4G, 5G, WIFI, GPS ...)! For each antenna, there must be a transmit or receive path that carries the signal from the antenna to the model. These paths are called RF signal pads or RF chains. The more antennas that are added to the device, the more RF chains you will need. Every RF chain requires a number of components to process the signal, and all of these components, as well as how these components interact with antennas and with the modem, need to be understood in detail.

How does an RF front end work in a conventional radio receiver?

Let's take a closer look at the RF front end of a radio receiver circuit to see which tasks the front end takes on. The RF front end is the generic name for all circuits between a receiver antenna input and the mixer. For most architectures, the RF front end consists of:

An RF filter (which is actually a band pass filter) receives the electromagnetic signal from the antenna. Its job is to remove the image frequency and prevent strong out-of-band signals from saturating the input stages.
An RF amplifier used to amplify weak signals without noise
A local oscillator that generates a stable high frequency signal close to the input signal
A mixer that creates a signal at a certain desired frequency by “mixing” the EM wave signal from the antenna with the local oscillator signal.

How do RF frontends work in cell phones?

In cell phones, the frequency generated by the mixer (also called intermediate frequency) is digitized, sampled and converted into a binary digital form. The rest of the processing is done through digital filters, which are smaller, use less power, and can have more selectivity.

What are the main challenges in developing such RF front ends for cell phones?

Every RF chain requires a number of components that process the signal. Cell phones and mobile devices have many challenges due to the number of antennas:

The various signals that must not interfere with one another
Choosing or designing the right components for each part of the RF chains
Designers need to think about how to design systems that support the communication architecture of the future!

Which manufacturers offer RFFE solutions for mobile devices?

Numerous manufacturers offer suitable solutions for smartphones and co. including Qualcomm, NXP, etc. An example of this is NXP's Advanced RF WLAN11ax portfolio.

Advanced 5G devices place high demands on performance, integration, size and Wi-Fi 6 capability. NXP's RFFE solution is highly integrated and tightly packed in a 3mm x 4mm package. It is designed with Wi-Fi 6 capability to support advanced portable computing devices including top-notch 5G smartphones and enable 2x2 MIMO functions with the highest performance. NXP's compact, high-performance RFFE solution can reduce design time and significantly reduce time-to-market for original equipment manufacturers (OEMs) NXP's powerful WLAN11ax portfolio helps customers meet the ever-growing need for more bandwidth by adding both the 2.4 -GHz- as well as the 5-GHz band, which correspond to the 802.11ax Wi-Fi 6 standard. NXP offers a flexible portfolio that extends beyond these specifications. NXP offers MIMO (2x2 Multiple Input Multiple Output) support for IEEE802.11a / n / ac / ax applications.

Development and future of RFFE Interface

With 5G in mind, which will bring more and more antennas to the device, it is important that RFFE evolves! The current version, MIPI RFFE v3.0, was designed to provide the more accurate timing precision and reduced latencies required to drive the 5G rollout worldwide. The new version optimizes and optimizes the interface to enable faster and more dynamic configuration changes within and between RFFE subsystems and to provide the specific functions required for frequency range 1 (FR1) of conventional sub-6 GHz cellular bands. In recent years, numerous new technologies have been introduced that promise a technological revolution.

This also includes the new 5G radio standard, which specifies significantly higher data rates, transmission capacities and very low latencies. 5G should work in many more frequency bands than its predecessors. The RF subsystem is correspondingly complex. The new RFFE Version 3.0 published by MIPI offers optimized trigger functions and a simplified front-end control, which should improve the performance of the 5G radio interface and make it easier to implement 5G in your own projects.

To ensure that the components and function blocks within the high-frequency front-end module (HF-FEM) work together better, the MIPI Alliance has extensively revised the "Radio Frequency Front End Control Interface" specification - MIPI RFFE for short. In future, the entire control of the HF-FEM is to be carried out via a single bus. This should enable a more flexible structure of the front end. The radio interface is a core component of every mobile radio technology.