# Frame rate at 6.8 Mbit/s

I assume that DWN1000 module support also data communication, without ranging.

At 6.8 Mbit/s how many frames per millisecond is possible to acheive, assuming 64 us preample and 8 Byte of data?

Which is the typical LOS at 6.8 Mbit/s?

Documentation with this information is appreciated

As with everything the answer is probably that it depends what you are doing.

With the settings you gave the packet length will be around 110 us which would imply 9 frames per second maximum. But realistically thatâ€™s not going to happen.
If all the packets are being sent by one device then it may be possible to send 7 or 8 packets in 1 ms.

If multiple devices are sending packets then you need to time synchronize between them in some way, thatâ€™s going to cost you time.
I have had a system where 4 packets were sent every 1 ms, 2 each way between a pair of devices. The timings on that were getting fairly tight.

Range - it depends a lot on the environment but in theory 30 m or so should be possible. But it again depends on who is doing the transmitting. The standard sets a maximum average power over 1 ms, but thatâ€™s per device. So if 4 units each send 1 packet over a 1 ms time period they can use a higher transmit power and so get more range than if a single unit was to send all 4 packets.

Sounds perfect. I assume your answer is based on 8 Byte of data in each frame.

What is the min, max number of bytes in a frame?
What is the number of bytes/bits overhead in a frame still 64 us preample?

regards Kent

To calculate the length of a frame for any given settings you can download the spreadsheet linked to from here: DW1000 Power Calculator not working
If you go to the TWR Exchange tab and set the packet settings/length on the left it will calculate the frame length.

Minimum frame size if 3 bytes. Maximum is 127 bytes sticking to the IEEE standard or 1023 using the decawave only settings.

The frame overhead is dominated by the preamble length, there are other overheads but that is by far the largest component.

Realistically, about 8 frames per 1 ms can be achieved, but it takes significant effort to achieve this.

The way the regulations work, it is advantageous to only send one packet per 1 ms window from any one given node. The reason is that your spectral density limits, typically -41.3 dBm/MHz, are averaged over 1 ms of time. So if you send more packets in that time, your power has to be reduced, which limits range.

It is okay to have 8 nodes each send in that 1 ms of time as the regulations only apply per node.

It isnâ€™t efficient to have one node emit 8 packets in a row carrying only 8 bytes. You wasted all your air time on preamble and sent very few real bytes. Better to gather the 64 bytes into one packet and send just that.

Using 64 preamble length, 64 MHz PRF, 6.81 Mbs bit rate, 8 byte payload, maximum legal regulatory power, we routinely achieve 50+ meters range between nodes with a properly designed non directional antenna. If we use LNA (low noise amplifier), we can double that to 100+ meters. This is the shortest, fastest packet one can make in a DW1000.

If you use a directional antenna, your transmit power is still limited to the same peak output in the best direction (no range improvement for any given receiver), but you get a gain on the receive side. There is no upper bound to the range you can achieve, it is limited only by the antenna you can build. This one would give you ridiculous range:

This is the Green Bank Telescope (GBT, aka â€śgreat big thingâ€ť) in WV, USA. The prime surface is 100 meters diameter, and it cost ~100M \$US in 2001.

Mike Ciholas, President, Ciholas, Inc
3700 Bell Road, Newburgh, IN 47630 USA
mikec@ciholas.com
+1 812 962 9408

Hey, sorry to let this post come up again, but I have a question for mciholas.
You said that the FCC regulations are designed to not exceed -41.3dBm per 1MHz for an average of 1ms. But doesnâ€™t the DW1000 IC automatically ensure that the -41.3dBm is not exceeded by the internal maximum transmit power? I can see no difference if I send a packet with 1ms duration or e.g. 5 packets with 0.2ms duration. Or does the IC always adjust the optimal or maximum transmission power based on selected parameters, so that the FCC regulations are met?

As long as SmartTX in the DW1000 is disabled one should be able to sent as may frames as possible per ms? Or am I neglecting any assumption here?

Best regards,
Bjorn

No, the DW1000 doesnâ€™t automatically ensure the transmit power meets the regulatory limit. That is something that has to be tested and then the specific transmit power set so as to not violate the regulations.

Sort of. See section 7.2.31.2 â€śSmart Transmit Power Controlâ€ť in the user manual for an automatic transmit power adjustment feature for packets shorter than 1 ms. If enabled, the adjustment is quantized to 4 settings for 1 ms or greater, 500 us to 1 ms, 250 us to 500 us, and under 250 us. You still need to load the proper transmit power settings for each packet size class that was determined in regulatory testing.

In our designs, we donâ€™t use the Smart TX feature for a variety of reasons, the primary one being it is too quantized with only 4 sizes, so we look up the absolutely optimum transmit power for each length we can use. The other reason is if we ever need to transmit two packets closer than 1 ms apart, the Smart TX feature doesnâ€™t handle that case.

Correct, as long as the power is set so that a 1 ms packet is within limits, then you can send as many smaller packets as you want. In this case, you are reducing your range over a system that optimizes each packet power to just stay under the limit.

It is extremely rare that any node would need to transmit more than 1 packet in 1 ms window. If that is occurring, then I question the design of the over the air protocol.

Mike Ciholas, President, Ciholas, Inc
3700 Bell Road, Newburgh, IN 47630 USA
mikec@ciholas.com
+1 812 962 9408

1 Like