New MIPI RF front end spec takes 5G beyond mobile

The MIPI Alliance, an industry organization which develops interface specifications for mobile systems, has released version 3.0 of its RF front end control interface (RFFE) which delivers tighter timing precision and reduced latencies. This will enable new capabilities in the specification to help expand 5G deployment beyond mobile for use in automotive, industrial and internet of things (IoT) applications.

The first MIPI interface, initially released 10 years ago, provided connectivity between application processors and cameras and displays, enabling the first camera phones. Today, virtually all 4G smartphones – along with many tablets, connected vehicles and IoT devices – use at least one MIPI interface. According to the MIPI Alliance chairman, Joel Huloux, development of the MIPI RFFE v3.0 was “laser focused” on satisfying the unprecedented requirements for tight timing precision and low latency in the 3GPP 5G standard. “In this way, the specification helps ensure that manufacturers’ 5G devices deliver the high-performance RF capabilities necessary to enable critical consumer and business features in emerging 5G application spaces.”

MIPI RFFE simplifies the design, configuration and integration of the increasingly complex RF front end—which encompasses the power amplifiers, antenna tuners, filters, low-noise amplifiers (LNAs) and switches—connecting with the modem baseband and/or RF integrated circuit (RFIC) transceiver. As the number of RF bands involved in both uplink and downlink communications has exploded in the rollout of 5G, the subcarrier spacing (SCS) windows among RF packets have narrowed. MIPI RFFE v3.0 addresses the decreased reconfiguration windows and lower-latency switching among various bands and band combinations demanded in the 3GPP 5G standard by delivering enhanced triggering features and functionality, which results in fast, agile, semi-automated and comprehensive control of individual RFFE subsystems.

MIPI RFFE v3.0 utilizes multiple, complementary triggers to synchronize and schedule changes in register settings, either within a slave device or across multiple devices:

  • Timed triggers — allows for tighter, synchronized timing control of multiple carrier aggregation configurations.
  • Mappable triggers — enables groups of control functions to be remapped to other triggers quickly and easily.
  • Extended triggers — boosts the number of unique triggers available in the RF control system and accommodates increasingly complex radio architectures.

With the enhanced triggering functions, MIPI RFFE v3.0 improves throughput efficiencies and reduces packet latency, while also improving the precision in trigger placement. For back-to-back triggering operations, for example, the specification delivers a 20x improvement in timing precision.

A typical RF front-end system configuration (Image: MIPI Alliance)

In addition, the flexibility conveyed in v3.0 allows a system to be designed with a reduced number of RFFE buses on an RF board. The interface’s mappable triggers enable the dynamic remapping to trigger assignments to address the increased number of uplink and downlink carrier aggregation combinations with the ever-increasing number of possible RF bands. The channel is optimized, increasing the utilization of devices. Also, with all of the control combined within one bus, pins are conserved on the master baseband transceiver. It is possible that software may also be simplified, with a fewer number of disparate RFFE buses to coordinate across. Cost savings are realized across the board.

Because MIPI RFFE v3.0 is backward compatible with prior generations of the specification, original equipment manufacturers and device vendors can migrate to 5G systems more quickly and easily, without changes to the physical layer of the control interface. The RFFE specification was initially released in 2010 and has seen numerous revisions since that time. Each release has provided additional functionality for developers to aid in the demand for newer RF front-end features.

With MIPI RFFE v3.0, the specification has been streamlined and optimized to deliver the specific capabilities required in today’s 5G rollout across the Frequency Range 1 (FR1) of traditional sub-6 GHz cellular bands. According to Jim Ross, MIPI RF Front End Control working group chair. “The working group is always looking to refine the specification to continue differentiating and benefitting our user community, and we welcome engagement in requirements gathering for Frequency Range 2 (FR2) and the ongoing evolution of the next-generation MIPI RFFE for the subsequent stages of 5G deployment.”

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