BLE 4.1 data compression (20 bytes)

Using Arduino Project Guidance
1

Hello everyone,

I am working on a project using TinyCircuit parts and trying to send data from 2 9DOF sensors over BLE.
Unfortunately, the BLE module acquired is based on the BlueNRG-MS chipset, which specifies that the ATT_MTU is maxed at 20 bytes. So I can only send 20 bytes per message.

The problem is that I have 18 double to send, plus a counter for the sample and an identifier for the sensor. So I came up with something like:
999S0-1.23-4.567.89-1.23-4.567.89-1.23-4.567.89S1-1.23-4.567.89-1.23-4.567.89-1.23-4.567.89#

Which concatenates the counter with two sensors [sensor number, acc XYZ, gyr XYZ, mag XYZ] and # to identify the end of a message. To send this string, I have to send 5~6 messages of 20 bytes each. I got safe messages sent by around 50ms, so I would have a sample per ~300ms, which means only 3~4 samples per second.

I am trying to search over ways to compress this string, but I don't know if it's worth it, since the compression itself might not generate good results for this string schema, and it might take longer to compress than to just send another message (rate is 20bytes per 50ms).

Do any of you guys have experience with this? What is the recommended approach to send data fast through ble? Is there a recommended way to increase the throughput or compress this string before sending it?

Thank you all in advance.

2

Why are you trying so hard to compress?
Why not simply send multiple messages?

3

I am already sending multiple messages, however, I am trying to find ways to increase the throughput. Since a sample is around 95 bytes, I can only get a sample after 5 messages, which means roughly 250~300ms.

This puts me at 3~4 samples per second, the amount of samples per second is kinda low for what I am aiming for.

4

Start off with a long message, then just transmit deltas?
Occasional re-syncs, should increase the average data rate.

Even raw, twenty bytes is only five floats, or two and a half doubles.

5

Oh, that's really smart. Might be worth a shot.

Merci beaucoup mec.

6

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