Re: Whose (BLE) RSSI is it anyway?

Subject: Re: Whose (BLE) RSSI is it anyway?
From: Spero Koulouras <email@hidden>
Date: Mon, 14 Jul 2014 16:21:00 -0700

The RSSI reported by CoreBluetooth is exactly what it says: The RECEIVED SIGNAL STRENGTH, measured on the receiving device. The receiver can be a central or a peripheral.

RSSI is MEASURED by the bluetooth chip on the receiving device and reported up through the RSSI property. It is NEVER transmitted over the air, so there is no corresponding service or characteristic. The CoreBluetooth API's simply read the measured value from the chip and make it available in the property.

TXPOWER is a characteristic which is optionally transmitted and can be read. TXPOWER is the signal strength of the transmitting device.

If you read the TXPOWER attribute sent by a device and then measure the RSSI actually received you can get a rough measure of signal loss and can theoretically measure distance. This is what iBeacons do. The accuracy of this calculation is influenced by walls, obstructions, orientation and more so averages over time will be much more meaningful than point readings.

Spero Koulouras (408) 874-6331

On Mon, Jul 14, 2014 at 2:59 PM, Mike Wirth <email@hidden> wrote:

OK folks, I'm probably just confused here (not an unusual situation :-) but the CoreBluetooth docs seem somewhat vague on whose RSSI they're talking about: the central device (iPhone in my case), and the peripheral.

First of all, this is a Received Signal Strength Indicator, so it's clearly measured by the device doing the receiving:
In the case of peripheralDidUpdateRSSI:error: method documented for the CoreBluetooth framework, I infer that we're dealing with the RSSI as measured by the peripheral device and communicated via notification or indication to the central.

Curiously, I just did a scan of specs at bluetooth.org and couldn't find any defined service or characteristic that's used to convey the peripheral's measured RSSI value. Hmmm.... what am I missing? Nearest thing I could find is TX POWER, what the peripheral reports its putting out as transmit power.

Similarly, I infer that subsequent reads of the CBPeripheral objects RSSI property (or value of the getRSSI method on the CBPeripheral object) returns the cached value from the last notification from the peripheral device (or NIL if a notification has never been received)

And just to add to the confusion, the RSSI value that's obtained via the (void)centralManager:(CBCentralManager *)central didDiscoverPeripheral:(CBPeripheral *)peripheral advertisementData:(NSDictionary *)advertisementData RSSI:(NSNumber *)RSSI call must be the RSSI as measured by the iPhone of the received strength of the advertisement packet. (The iPhone hasn't even transmitted anything yet :-)

Is this a correct interpretation? If not, where is it wrong? An earlier comment on this list (more than a year ago) said that the RSSI measured by the iPhone was adjusted based on the peripheral's reported TX POWER. Is this so?

Basically, I'm interested in dynamically measuring both devices' RSSI values dynamically during the connection as they move around. (Hey, we're in a mobile world :-) Any advice on how to accomplish this?

TIA,

Mike
Palo Alto, CA

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Follow-Ups:
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References:
	>Whose (BLE) RSSI is it anyway? (From: Mike Wirth <email@hidden>)

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