nRF52833
A powerful new nrf52 series of micro-controller has been announced with CTE capabilities . CTE is the constant tone extension is a section of consecutive 1's without whitening, which is effectively a +250kHz wave on top of the carrier wave. In the Bluetooth Core Specification Version 5.1, both periodic advertising packets and connection packets can contain a constant tone extension(CTE) after the CRC. The CTE can only be sent using uncoded PHY.
The process of converting a CTE to AoA consists of three steps. 1. collect phase information by sampling the I/Q data using several antennas spaced out with known distances. 2. calculate the phase difference between all the antennas. 3. Finally convert the phase difference into Angle of Arrival.
From the nrf52833 datasheet are the key features to enable CTE AoA calucatations using the new nrf52833 and nrf52811 series chips.
"The RADIO implements the Angle-of-Arrival (AoA) and Angle-of-Departure (AoD) Bluetooth Low Energy feature, which can be used to determine the direction of a peer device. The feature is available for the BLE 1 Mbps and BLE 2 Mbps modes. When using this feature, the transmitter sends a packet with a continuous tone extension (CTE) appended to the packet, after the CRC. During the CTE, the receiver can take IQ samples of the incoming signal. An antenna array is employed at the transmitter (AoD) or at the receiver (AoA). The AoD transmitter, or AoA receiver, switches between the antennas, in order to collect IQ samples from the different antenna pairs. The IQ samples can be used to calculate the relative path lengths between the antenna pairs, which can be used to estimate the direction of the transmitter."
"The CTE is from 16 µs to 160 µs and consists of an unwhitened sequence of 1's, equivalent to a continuous tone nominally offset from the carrier by +250 kHz for the 1 Mbps PHY and +500 kHz for the 2 Mbps BLE PHYs. Antenna switching is performed during switch slots and the guard period. The AoA/AoD feature requires that one IQ sample is taken for each microsecond within the reference period, and once for each sample slot. Oversampling is possible by changing the sample spacing. The switch slot and sample slot durations are either 1 or 2 µs, but must be equal. The format of the CTE and switching and sampling procedures may be configured prior to, or during, packet transmission and reception. Alternatively, during packet reception, these operations can be configured by reading specific fields of the packet contents. "
"The eight antenna selection signals are mapped to physical pins according to the pin numbers specified in the PSEL.DFEGPIO[n] registers. Only pins that have the PSEL.DFEGPIO[n].CONNECTED field set to Connected will be controlled by the RADIO. Pins that are Disconnected will be controlled by GPIO. During transmission in AoD TX mode or reception in AoA RX mode, the RADIO automatically acquires the pins as needed. At times when the RADIO does not use the pin, the pin is released to its default state"
"The values of the GPIOs while switching during the CTE are configured by writing successively to the SWITCHPATTERN register. The first write to SWITCHPATTERN is the GPIO pattern applied from the call of TASKS_TXEN or TASKS_RXEN until the first antenna switch is triggered. The second write sets the pattern for the reference period and is applied at the start of the guard period. The following writes set the pattern for the remaining switch slots and are applied at the start of each switch slot. If writing beyond the total number of antenna slots, the pattern will wrap to SWITCHPATTERN[2] and start over again. During operation, when the end of the SWITCHPATTERN buffer is reached, the RADIO cycles back to SWITCHPATTERN[2]. At the end of the AoA/AoD procedure, SWITCHPATTERN[0] is applied to DFECTRL1.TSWITCHSPACING after the previous antenna switch. The SWITCHPATTERN buffer can be erased/cleared using CLEARPATTERN. "
"The RADIO uses DMA to write IQ samples recorded during the CTE to RAM. Alternatively, the magnitude and phase of the samples can be recorded using the DFECTRL1.SAMPLETYPE field. The samples are written to the location in RAM specified by DFEPACKET.PTR. The maximum number of samples to transfer are specified by DFEPACKET.MAXCNT and the number of samples transferred are given in DFEPACKET.AMOUNT. The IQ samples are recorded with respect to the RX carrier frequency. "
References
Bluetooth Direction Finding A Technical Overview