The RF Internet of Things


Over the past several weeks, I have been reading a prominent investor website that offers a mix of free professional and amateur investment analysis.

Perhaps reflecting the PC-centric world we are leaving behind, the general level of knowledge regarding multicore microprocessors, LPDDR4 SDRAM memory, GPUs, audio processors, image processor, video processors, etc. seems to be quite high in this community.  These subjects are constantly debated among the “shorts” and “longs” who debate the merits of investing in companies like Intel, Qualcomm, MediaTek, Broadcom, NXP, Marvell, Spreadtrum et al.  Correspondingly, the general level of knowledge regarding the RF (cellular, Wi-Fi, Bluetooth, NFC, WiGig, ZigBee, 2.4 GHz ISM, etc.) subsystems found in every wireless device is quite low.

There are frequently recurring misconceptions regarding wireless broadband technology I’d like to attempt to correct.  Most of the writers of articles for this particular site seem to be under the impression that the entirety of the RF subsystem is found in the “modem” chip.  In point of fact, today’s RF subsystem comprises several analog and mixed-signal functional elements: the digital baseband processor, the RF transceiver, and the RF front-end.

The digital baseband (DBB) processor is essentially a specialized digital signal processor chip with embedded memory, often with hardware acceleration for forward error corrections algorithm such as Viterbi, convolutional, and turbo codes, as well as low-density parity check.  The DBB pre-distorts the digital signal to maximize RF linearity; rearranges subcarrier sinusoids in the typically used OFDM symbols to minimize peak-to-average ratio statistics; and shifts digital signal sample rates with digital down converters / digital up converters.

The RF transceiver, among other things: converts amplitude and phase modulated discrete-time digital signals back and forth to the continuous-time analog domain using ADCs and DACs; applies anti-alias and anti-image filtering; shifts intermediate frequency analog signals to and from radio frequencies using quadrature mixers and demodulators, and applies digital and analog variable gain.

The RF front-end, often implemented as a packaged multi-die substrate module, has transmit (TX) and receive (RX) signal chains.  The TX signal chain typically include: multi-band, multi-mode power amplifiers with power detectors; envelope tracking power supplies for the PAs; MIMO antenna transmit/receive switches (for TDD systems) and band filter switches; antenna impedance or aperture tuners; SAW/TC-SAW/BAW band filters; antenna diplexers (for FDD systems); and duplexers to enable TX/RF paths to share a single antenna.  The RX signal chain typically includes: low noise amplifiers to maximize receiver sensitivity; RX band filters; and variable gain amplifiers.

Major RF subsystem semiconductor suppliers offer modem ICs, and typically the companion RF transceivers. In most case, the supplier provides a reference design with third-party RF front-end circuits, either discrete or modular.  In a few cases, a single supplier can product the modem, the RF transceiver, and the RF front-end ICs.

The most successful suppliers of broadband RF communications subsystems have years of deployed field experience with full signal chain solutions, from application processor to antenna, have shipped billions of cellular / WiFi and other communications chipsets into every regulatory geography in the world, and possess interoperability experience with the wireless infrastructure equipment of every major wireless services operator on the planet.

The RF Internet of Things is enabled by billions of units per year of RF subsystems of the sort described in this article.  This critical element of our mobile communications world needs to be better understood and appreciated by investors and digital-centric or software-centric technologists alike.

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