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[AmpeRose] Down from 10MHZ to 100KHZ

Hello everyone,

Sampling frequency is one the most important factor in AmpeRose, therefore it must be well characterized and well defined. In this post, I will show you how we passed from a 10 MHz frequency down to 100 KHz.

Introduction

AmpeRose is not made to measure current consumption of any existing device. In fact, it targets a very specific range of connected devices. It is only concerned by those which are low consumption. In order to profile an IoT device, it is necessary to study the maximum current required by its modules, typically, the communication module which consumes the majority of current. As examples of communication modules:

  • BLE
  • Lora
  • Wifi
  • Zigbee
  • Etc…

BLE (Bluetooth Low Energy)

Bluetooth Low Energy is an alternative to classic Bluetooth, with reduced cost and power consumption, while maintaining an equivalent range of communication.
BLE seeks to address devices with low computing power, low cost of production and life expectancy is to maximize. The idea is to reduce their activity time compared to devices based on classic Bluetooth. Raw performance such as the beginning of transmission will therefore naturally be affected.

  • Main features
Distance/range 50m
Over the air data rate 1 Mbit/s
throughput 0.27 Mbit/s
Latency(from a non-connected state) ~6 ms
Total time to send data < 3ms
Peak current consumption < 6 ms
  • Smallest activation time

                  According to https://www.eetimes.com/document.asp?doc_id=1278927, the shortest package would have 80 bits that would require 80 us to transmit it.

Lora

LoRa technology is designed for low-power wide area network (LPWAN) applications. It has a range of more than 15 kilometers and a capacity of up to 1 million nodes. The combination of low power consumption and long range limits the maximum data rate to 50 kilobits per second (Kbps).

  • Main features
Distance/range 15 km
Data rate 50 kb/s
Total time to send data (average) < 300ms
Power consumption during transmission < 40 mA
Average current consumption (active) 10 uA
  • Smallest activation time

          According to:

                       – https://www.link-labs.com/hubfs/DOCS.linklabs.com/2017/01/lora-scalability_r254.pdf

                       – http://www.semtech.com/images/datasheet/sx1272.pdf

         the shortest packet would have 16 bits that would require 320 us to transmit it.

Wifi

WiFi is an obvious choice for IoT connectivity because WiFi coverage is now almost ubiquitous, but it’s not always the right choice.

The WiFi standard, while being the obvious choice for IoT, has limits in terms of range and energy efficiency. To meet these requirements, the IEEE has published specifications for standards
–  802.11ah
–  802.11ax

  • Wifi HaLow 802.11ah

WiFi HaLow technology is based on the IEEE 802.11 ah standard ratified in October 2016. It was introduced specifically to address IoT’s range and power concerns. The 802.11ah standard uses the 900 MHz ISM unlicensed band to provide extended reach with low power requirements. The use of power is further optimized by using predefined periods of sleep / slumber and achieves a radius of one kilometer. In addition, it requires a separate infrastructure (compared to the WiFi standard) and specialized clients.

  • HEW (High Efficiency Wirless) 802. 11 ax

This technology also adds a number of user-friendly features for IoTs. It retains targeted wake up time and 802.11ah station pooling capabilities to help customers save energy and avoid collisions. This technology holds many promise for use as a means of communication in IoT.

  • Smallest activation time: 802.11 ah

         According to https://www.researchgate.net/publication/301776005_Throughput_and_Range_Characterization_of_IEEE_80211ah,

        the shortest packet would require (160 + 264 + 560) us = 984 us to transmit it considering delta = transmission delay as zero.

  • Smallest activation time: 802.11 ax

        According to https://www.researchgate.net/publication/313844761_AP-initiated_Multi-User_Transmissions_in_IEEE_80211ax_WLANs,

        the shortest package would require ~ 168 us to transmit it while only considering the physical header and neglecting the other parameters.

 

Zigbee

Zigbee is designed to meet the requirements of communication between devices. It is a specification that is built on top of the IEEE 802.15.4, communication standard. Zigbee is intended for low flow, low power and low cost applications. For this reason, it is much simpler than other protocols such as WiFi (IEEE 802.11).

  • Main features
Distance/range 10 m
Data rate 250 kb/s
Total time to send data (average) < 300ms
Power consumption during transmission < 30 mA
Power consumption during standby 350 uA
  • Smallest activation time

         According to

                  – http://voip.netlab.uky.edu/~fei/teaching/cs687_smartgrid/slides/3.zigbee.pdf

                  – http://www.springer.com/cda/content/document/cda_downloaddocument/9783319478050-c2.pdf?SGWID=0-0-45-1593332-p180328094

         the shortest package would require 352 us to transmit it.

 

Conclusion

According to this study, we can see that BLE technology has the smallest activation time of 80 us. Therefore, in order to detect the current consumption during this activation time, we must sample at F= 1/80us = 12.5 KHZ at least. However, by including a margin error, we will therefore sample at 100 Khz at least (smallest activation time : ~ 20 us and respect of Nyquist criterion Fe >= 2fmax).

 

Bilal ADDAM

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