Isochronous Channel Parameters For Adjusting Throughput and Range

[tim.fort]

Hello,

I'm interested in increasing the range I can get out of an Isochronous connection between two nrf52832 while maintaining about the same throughput. I'm not concerned about data loss. The top level ACL connection is established with the base parameters from BT_LE_CONN_PARAM_DEFAULT. The CIG connection is established with:

    struct bt_iso_cig_param param = {
        .cis_channels = channels,
        .num_cis = ARRAY_SIZE(channels),
        .sca = BT_GAP_SCA_UNKNOWN,
        .packing = BT_ISO_PACKING_SEQUENTIAL,
        .framing = BT_ISO_FRAMING_UNFRAMED,
        .c_to_p_latency = BT_ISO_LATENCY_MIN,   /* ms */
        .p_to_c_latency = BT_ISO_LATENCY_MIN,   /* ms */
        .c_to_p_interval = 5000, /* us */
        .p_to_c_interval = 5000, /* us */
    };

The channel qos are established with:

static struct bt_iso_chan_io_qos iso_tx = {
    .sdu = 247,
    .phy = BT_GAP_LE_PHY_2M,
    .rtn = 0,
    .path = NULL,
};

static struct bt_iso_chan_io_qos iso_rx = {
    .sdu = 247,
    .phy = BT_GAP_LE_PHY_2M,
    .rtn = 0,
    .path = NULL,
};

static struct bt_iso_chan_qos iso_qos = {
    .tx = &iso_tx,
    .rx = &iso_rx,
};

I don't really have any interesting configurations in the prj.conf relating to this. Just the items from the examples to enable iso central and peripheral in BLE.

[Asbjørn]

Hello Tim,

Increase in range I assume you mean you want to maximize the distance between the two devices. 

• You're not worried about data loss, but how about data rate? I just wanted to ask, even though I see you've selected 2M BLE mode. If data rate isn't an issue either, the lower data rate coded 250k would be a solid range extended obviously. Don't feel it applied, but wanted to bring it to your attention.
• Designing a board with a straight quarter wave antenna in as close to free space as possible, is a good way to make sure the range is as good as possible.
• Maximum output power to be used.
• Have you considered a front end module with LNA/PA? https://www.nordicsemi.com/Products/nRF21540

  nRF21540 Range Extender - Nordic Semiconductor nRF21540 Range Extender www.nordicsemi.com

  

Best regards

Asbjørn

[tim.fort]

Hello,

Thanks for the suggestions. nrf52832 doesn't support coded but I switched to 1M PHY for some minor gains. I'd like to keep this focused on the ble parameters and how they affect range. we are already incorporating an LNA at max power with an external antenna, etc.

I optimized things a bit and lowered our SDU from 247 to 103 and adjusted the interval from 5ms to 10ms. This had a notable increase in our ability to maintain a connection at range. I'm not sure how to explain that. Does the isochronous channel starve management of the ACL connection?

[Kenneth]

Hi,

Just pitching in here.

A problem with coded phy is the time on air. In the 2.4GHz space all communication is packet based, so the longer a packet is, the greater the risk of collision, interference etc. So even though you in theory can get much better range, that is mainly the case if you are outdoor with no WiFi or other BT devices.

Also, I would claim using a PA is much better in this situation, because a PA will increase the SNR, while an LNA will increase the power from any nearby WiFi or other BT devices (thereby you don't improve the SNR).

A theoretical time of a 247byte payload at coded phy with ACK:

Tpre and post processing: 1.25ms I think
TIFS startup/switching: 150us
Time on air: 247byte * 8bit/byte / 125kbit/s = 15ms
TIFS startup/switching: 150us
TACK: (Dont remember exactly the time for that one, but think) 400us

So I would expect if you want to send 247byte payloads with coded phy you likely should not use a connection interval lower than 20ms to have some margin.

Kenneth

[tim.fort]

Kenneth Are those numbers around the timing documented somewhere?

[Kenneth]

These were just approx numbers from my head.

If you need accurate numbers please use chatgpt to calculate them for you or refer to bluetooth core specification, or I guess there are also a lot of guides on the topic.

Kenneth