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low power design and wifi

I am not a WiFi expert, but is that 100 seconds typical ?

Yes, but it depends on many other factors as well. Since Dot 11 (802.11) requires full handshake before transmission (think TCP), You have to keep the radio active for the entire period. OTOH, Dot 15 can send independent packet (think UDP) in half duplex. ZagAnt (#1) use 10 seconds turn around full duplex, for applications that needs bi-directional communications. For comparisons:

Mode System Power RF Power Minimum Time
Dot 11 F 500mW 100mW 100s
Dot 11 H N/A N/A N/A
Dot 15 F 80mW 10mW All (Server/Gateway mode)
Dot 15 F 80mW 1mW 10s (Client Bi-directional)
Dot 15 H 80mW 1mW 1s (Client Uni-directional)

Note #1:
Someone already claimed ZigAnt. So, we are switching to ZagAnt. Ant for Asymetric Network Transmission protocol.
I am sure, but I'll ask, does distance require more power or will the
router not accept the connection if power is too low ?
( or does it slow down )

It will either drop the data rate or more retries (at 500ms intervals).

I have read many papers and more than happy to show your more numbers.
 
J

josephkk

Jan 1, 1970
0
Hi guys.
I need a suggestion regarding one project.
Well, the project should be one micro controller battery powered device
with wifi connection. Of course, the main problem is wifi and power
consumption.

Base processor will be ARM cortex M0 and the device should be powered
with 3,6V non rechargeable battery ( it's is not a must ).
WIFI range should be around 50 meter and it would be nice if i can hook
up external antenna.

Data will de send in half minute package and device should work 24/7.
Minimal work time of the device : 2 years.


I have found few ready made solutions like
- spark core ( over project budget )
- cc3000 ( TX power consumption 275mA )
- ultra low wifi ( small range )

Size of the device should be in 15x10 cm range so i can use some kind of
battery array but then again, this will be over project budget.

The main thing is to pass the data to Internet so some other solution
can be used.

Rather than WiFi i think i would try 902 to 928 MHz spread spectrum (DSSS)
ISM radio. Of course for both cases bigger buffers help rather than hurt,
there is sync-up time even for DSSS or FHSS. GMSK might be a better idea.

?-)
 
Data will de send in half minute package and device should work 24/7.
Minimal work time of the device : 2 years.

What is the transmitter duty cycle, i.e. how many bytes do you need to
send every 30 s ?

The higher the peak transmission rate, the higher received power
requirement (look at the receiver sensitivity specs for various data
rates) and hence more transmission power is needed (typically limited
to 100 mW or +20 dBm by standards).

This also requires a stiff power supply in order to deliver the peak
power without too much voltage sag. A huge capacitor might be needed
in parallel with a small battery. If the mobile device also needs a
receiver, the power consumption can be considerable, especially if the
receiver needs to be kept on all the time. If it is possible to keep
the Rx power on for only during some short mutually agreed periods,
the average consumption can be reduced.

What about sending the data at a lower rate constantly. Assuming some
ISM device with 9600 bit/s data rate, it can transfer about 1
kilobyte/s or 30 kB in 30 s. Is that enough ?

This has the advantage of reducing the peak current and hence a
battery without a storage capacitor should be enough.

It should be noted that in a free space environment, the path loss
(actually loss of receiver antenna capture area) is increased by 6 dB,
each time the RF frequency is doubled. Thus, using omnidirectional
antennas, the received signal power at 433 MHz ISM band is 15 dB
stronger compared to 2.45 GHz with the same transmitter power.

This advantage could be used to lower the Tx power on increase the
range 5-6 times in free space. The greater diffraction at lower
frequencies will help to propagate the signal around the obstacles.

For that 50 m range required, a few mW of Tx power should be enough.

Thus, you should make a careful comparison between burst mode WiFi and
continuous ISM band operation.
 
What is the transmitter duty cycle, i.e. how many bytes do you need to
send every 30 s ?

The higher the peak transmission rate, the higher received power
requirement (look at the receiver sensitivity specs for various data
rates) and hence more transmission power is needed (typically limited
to 100 mW or +20 dBm by standards).

This also requires a stiff power supply in order to deliver the peak
power without too much voltage sag. A huge capacitor might be needed
in parallel with a small battery. If the mobile device also needs a
receiver, the power consumption can be considerable, especially if the
receiver needs to be kept on all the time. If it is possible to keep
the Rx power on for only during some short mutually agreed periods,
the average consumption can be reduced.

This is our setup, similar to what the OP needs:

The biggest cap we have is a 47uF 0805 SMD chip.

Data: 10 bytes max.
Refresh rate: 5 seconds min.
Transmission packet: 64 bytes max.
Range: 20 meters

Carrier: 2.4GHz
Data rate: 256Kb/s

Transmission window: 64 * 8 / 256,000 = 2 msec
Transmitter ram-up delay = 5 msec

Transmitter system power = 200mW
Transmitter RF power = 1mW

Transmitter battery = 800mAHr
Battery cycle = 72 Hours

// receiver is main connected, power does not matter much
Receiver system power = 300mW
Receiver RF power = 10mW
 
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