Targeting Cat-NB1 instructions delivers power savings

If one wireless IoT technology fit every possible use case, we would have one specification. Many tradeoffs – battery life, mobility, indoor coverage, licensed versus unlicensed spectrum, and more – have made for many potential solutions. A heated discussion right now is over the future of LPWAN technologies, with LoRA, SIGFOX, Ingenu, and Weightless in the mix, versus the potential for the evolution of cellular-based technologies to handle IoT needs.

3GPP has been working very hard on the latter. Thankfully, the once-passionate “war” of M2M versus IoT has come to an end. With mobile revenue flattening and the use cases starting to overlap, the carrier community has embraced the IoT in hopes of reigniting growth beyond what M2M solutions can provide. That in turn drives a need for an entirely new class of chips, ones able to handle a sophisticated protocol stack at low power consumption.

In their June 2016 update “The Evolution to Narrow Band Internet of Things”, the GSA (Global mobile Suppliers Association) discusses the trends in NB-IoT at length. Acceptance of LTE Cat-1 is global, and almost all major carriers are already deployed or in trials. With most carriers skipping Cat-0, interest accelerated in NB-IoT standardization. 3GPP Release-13 now formalizes definitions for Cat-M1 and Cat-NB1, and operators are already chasing pre-commercial trials and targeting full commercial rollout by mid-2017.

The traditional approach to wireless sensor networks was to grab a microcontroller-class core and add a hardware radio. Increasingly, that approach is becoming uncompetitive, burning more area and power than a more optimized IoT solution. This is especially true for Cat-NB1, where the baseband workload in layer 2 with encryption and compression is substantial.

Looking at the diagram above explains in part why CEVA has been pursuing its new strategy with the CEVA-X framework. The same architecture spans the range of LTE requirements by changing the number of scalar execution units. To get to Cat-NB1, CEVA has moved to one scalar execution unit in the new CEVA-X1, its third CEVA-X family member and smallest so far.

“Notice we don’t say DSP here.” When Emmanuel Gresset said that during our briefing, it was telling. LTE requires both efficient control processing and DSP elements. That goes beyond simply adding a fast multiplier to a microcontroller engine, and reaches more into efficient addressing and pipelining made for both control and signal processing needs.

CEVA has taken its basic CEVA-X ISA combining CPU and DSP processing in a single scalar unit, and added “less than 10 specific instructions” for Cat-NB1 processing. The CEVA-X1 delivers a CoreMark/MHz of 3.3, nearly equal to that of an ARM Cortex-M4 core, while handling the DSP capability and instruction acceleration for a full software modem implementation. Gresset added that unlike bigger LTE SoC implementations that benefit from complete hardware accelerator units, Cat-NB1 performance is improved by targeting particular instructions. In the case of the CEVA-X1, another 30% power savings come from using these dedicated Cat-NB1 instructions.

Cache is optional in the CEVA-X1; Gresset explains that many implementations focus on tightly-coupled memory (TCM) instead. Also, the core interfaces via either AHB or AXI, making it more flexible for integration. The CEVA-X1 can also handle processing for other wireless needs beyond Cat-NB1. For example, it can also perform GNSS processing, which is an interesting use case since unlike mobile phones doing precise mapping while streaming data, a Cat-NB1 device probably wouldn’t need to do positioning and data transmitting simultaneously.

CEVA’s slides indicated RTOS support for this core, and of course that usually means FreeRTOS. Gresset is seeing the same thing I am, however – many requests for Apache Zephyr. That’s a noteworthy trend particularly given the ability to extend the instruction set.

Richard Kingston of CEVA indicates that over 50% of CEVA’s current revenue is coming from China. Both China Mobile and China Unicom are betting heavily on NB-IoT technology. CEVA’s wireless expertise combined with a growing market and what may be some backlash against the ARM acquisition puts them in a good position.

For more on the CEVA-X1, here’s the press release:

CEVA Introduces Lightweight Multi-Purpose Processor for the Massive Internet of Things

The bigger question is does NB-IoT win out over the LPWAN solutions? I can see niche cases where those LPWANs, particularly LoRA and Ingenu, may hold up. As a wise engineer once told me, “Never bet against Ethernet”, simply because as new versions of the specification appear, advantages of the alternatives are nullified. I see NB-IoT fitting the same pattern; if all the major carriers support it and get the business model right, evolution will eventually win on the larger playing field.