Sigfox

2.1. Modulation

The D-BPSK modulation required by the sigfox network is not intrinsic to the radio part of the bc68f2150 but with a simultaneous access of the amplitude and frequency register it is possible to emulate it. With this D-BPSK emulation it becomes possible to pass the class 0 spectrum template of sigfox radio specification. This D-BPSK emulation consists of playing a amplitude LUT and process a frequency step when the amplitude LUT is at its lowest power level.

• Frequency step must be played during a fixed time with a fixed frequency to accumulate a phase change of π (PSK like). Moreover his frequency step is alternatively positif and negative to balance spectrum template.

• Amplitude values was empirically define to limit spread spectrum.

• LUT element number and LUT element playing period are carefully chosen to be compliant with the MCU real time.

For easier implementation several parameters are linked together. The frequency step fixed time and LUT element playing period are the same, as well as the LUT element number and LUT element playing period are linked with the aim that total duration of the LUT lasts a full sigfox symbol (at 100bps or 600bps)

The table below shows all value selected for these différents parameter and how they are selected.

Parameter	Value(s) 100bps	Details 100bps	Value(s) 600bps	Details 600bps
Amplitude LUT	List of output power	Empirically determined value.
Frequency step	10Khz	10Khz is the minium of chipset datasheet To accumulate a phase change of π : $T = 1/(2*Frequency Step)$
LUT element playing period	50µs	$T = 1/(2*Frequency Step)$ $=1/(2 * 10)=50µs$
LUT element	200	$LUT element=Symbole Time/LUT element Playing Period$ $=10ms/50µs=200$

2.2. Library

This implementation example integrate the sigfox ep library through a git submodule. As the bc68f2150 is an low cost transmitter, a few specific code optimisation flag of the sigfox ep library are available to define/undefine in the project. These optimisation flags allows to reduced the memory footprint (RAM and ROM) in order to keep more place for your future application. In this example all flags are defined but can be modified according to your custom application.

The table below shows the different flags value supported with by this implementation example and the settled value in the project ( firmware_detail section show where this value are set in project ).

Flag name	Value	Project settled value
RCx	undefined / defined	RC1
APPLICATION_MESSAGES	undefined / defined	defined
CONTROL_KEEP_ALIVE_MESSAGE	undefined	undefined
BIDIRECTIONAL	undefined	undefined
ASYNCHRONOUS	undefined	undefined
LOW_LEVEL_OPEN_CLOSE	undefined	undefined
REGULATORY	undefined	undefined
LATENCY_COMPENSATION	undefined / defined	undefined
SINGLE_FRAME	undefined / defined	undefined
UL_BIT_RATE_BPS	undefined / 100 / 600	100
TX_POWER_DBM_EIRP	undefined / <tx_power_dbm_eirp>	undefined
T_IFU_MS	undefined / <t_ifu_ms>	undefined
T_CONF_MS	undefined	undefined
UL_PAYLOAD_SIZE	undefined / <ul_payload_size>	undefined
CRC_HW	undefined / defined	undefined
MESSAGE_COUNTER_ROLLOVER	128 / 256 / 512 / 1024 / 2048 / 4096	4096
PARAMETERS_CHECK	undefined / defined	undefined
CERTIFICATION	undefined / defined	undefined
PUBLIC_KEY_CAPABLE	undefined / defined	undefined
VERBOSE	undefined / defined	undefined
ERROR_CODES	undefined / defined	undefined
ERROR_STACK	undefined	undefined