Obtain bluetooth signal strength on Raspberry Pi of BT device without pairing

Question

I like to create a kind of indoor-tracking-system for my already existing home automation system. I thought of using BLE. I already successfully set up hcitool on my Raspberry Pi and I can connect to my iPhone without any problems. But how can I obtain the signal strength between my Raspberry Pi and my iPhone without connecting them. I already tried to use sudo hcitool cc [BTADDRESS] to connect to my iPhone without authentication, but it looks like the iPhone don't allow those connection to stay open. I think that must be a way to get the signal strength without connecting both devices. I want to use it to determine the distance from my Raspberry Pi to my iPhone. May I am able to calculate the distance from the time I need to discover my iPhone?

Answer 1

There are two ways to go, and by now I have been able to get both work reliably only on Android devices.

1. Exploiting the Bluetooth friendly name of the smartphone and set the discoverability to infinite. I have done this writing a simple app. Works in background, also after that the app has been killed, since the discoverability setting is preserved. At the best of my knowledge, this is not possible in iOS.
2. Advertising a UUID in a BLE packet from the phone. This can be done by both Android and iOS devices. However, while in background, iPhones switch the advertising to a shrinked mode that makes the packet unidentifiable. The problem of identifying an advertising iOS devices in background is still open.

On the raspberry, I used PyBluez to scan and looking for the presence of smartphones running (1) or (2). I report a code example:

import bluetooth
import bluetooth._bluetooth as bluez
import struct, socket, sys, select
def hci_enable_le_scan(sock):
    hci_toggle_le_scan(sock, 0x01)

#Discover name and RSS of enabled BLE devices
class MyDiscoverer(bluetooth.DeviceDiscoverer):

    def pre_inquiry(self):
        self.done = False

    def device_discovered(self, address, device_class, rssi, name):
        discovery_logger.info("Discovered %s" % (address, ))
        if name == "YOUR-DEVICE-FRIENDLY_NAME":
            #Use the RSS for your detection / localization system

    def inquiry_complete(self):
        self.done = True

#Performs inquiry for name request
def async_inquiry():
    d = MyDiscoverer()
    while True:
        d.find_devices(lookup_names = True)
        readfiles = [ d, ]
        while True:
            rfds = select.select( readfiles, [], [] )[0]
            if d in rfds:
                d.process_event()
            if d.done:
                break
        time.sleep(DISCOVERY_INTERVAL)

#Parse received advertising packets
def parse_events(sock):
# save current filter
old_filter = sock.getsockopt( bluez.SOL_HCI, bluez.HCI_FILTER, 14)

flt = bluez.hci_filter_new()
bluez.hci_filter_all_events(flt)
bluez.hci_filter_set_ptype(flt, bluez.HCI_EVENT_PKT)
sock.setsockopt( bluez.SOL_HCI, bluez.HCI_FILTER, flt )
while True:
    pkt = sock.recv(255)
    ptype, event, plen = struct.unpack("BBB", pkt[:3])
    if event == LE_META_EVENT:
        subevent, = struct.unpack("B", pkt[3])
        pkt = pkt[4:]
        if subevent == EVT_LE_CONN_COMPLETE:
            le_handle_connection_complete(pkt)
        elif subevent == EVT_LE_ADVERTISING_REPORT:
            #Check if the advertisement is the one we are searching for
            if getASCII(pkt[start:end]) == "YOUR-UUID"
                report_pkt_offset = 0
                report_data_length, = struct.unpack("B", pkt[report_pkt_offset + 9])
                # each report is 2 (event type, bdaddr type) + 6 (the address)
                #    + 1 (data length field) + report_data length + 1 (rssi)
                report_pkt_offset = report_pkt_offset +  10 + report_data_length + 1
                rssi, = struct.unpack("b", pkt[report_pkt_offset -1])
                #Now you have the RSS indicator, use it for monitoring / localization

sock.setsockopt( bluez.SOL_HCI, bluez.HCI_FILTER, old_filter )
dev_id = 0
try:
    sock = bluez.hci_open_dev(dev_id)
except:
    print "error accessing bluetooth device..."
    sys.exit(1)

p = threading.Thread(group=None, target=parse_events, name='parsing', args=(sock, ))
d = threading.Thread(group=None, target=async_inquiry, name='async_inquiry', args=())
try:
    p.start()
except:
    print "Error: unable to start parsing thread"

try:
    d.start()
except:
    print "Error: unable to start asynchronous discovery thread"