SPIRIT1 datasheet(63/94 Pages) STMICROELECTRONICS

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SPIRIT1

Transmission and reception

Doc ID 022758 Rev 3

63/94

9.10.2

Carrier sense

The carrier sense functionality can be used to detect if any signal is being received, the

detection is based on the measured RSSI value. There are 2 operational modes for carrier

sensing: static and dynamic.

When static carrier sensing is used (CS_MODE = 0), the carrier sense signal is asserted

when the measured RSSI is above the value specified in the RSSI_TH register and is de-

asserted when the RSSI falls 3 dB below the same threshold.

When dynamic carrier sense is used (CS_MODE = 1, 2, 3), the carrier sense signal is

asserted if the signal is above the threshold and a fast power increase of 6, 12, or 18 dB is

detected; it is de-asserted if a power fall of the same amplitude is detected.

The carrier sense signal is also used internally for the demodulator to start the AFC and

timing recovery algorithms and for the CSMA procedure (for this use it should be set to

cs_mode = 0).

The carrier sense function is controlled by the following parameters:

RSSI threshold: this parameter sets the minimum signal power above which the carrier

sense signal is asserted (RSSI_TH register).

CS mode: this parameter controls the carrier sense operational modes (RSSI_FLT register,

allowed values 0...3):

●

CS_MODE = 0 static carrier sensing

●

CS_MODE = 1 dynamic carrier sensing with 6 dB dynamic threshold

●

CS_MODE = 2 dynamic carrier sensing with 12 dB dynamic threshold

●

CS_MODE = 3 dynamic carrier sensing with 18 dB dynamic threshold.

9.10.3

LQI

The link quality indicator is a 4-bit value available through the LINK_QUALIF[0] register. Its

value depends on the noise power on the demodulated signal. The lower the value, the

noisier the signal. Be aware that comparing LQI values measured with different modulation

formats or data rate may lead to inconsistent results.

9.10.4

PQI

The preamble quality indicator (PQI) is intended to provide a measurement of the reliability

of the preamble detection phase.

This indicator counts the number of consecutive bit inversions in the received data stream.

The PQI ranges from 0 to 255. It is increased by 1 every time a bit inversion occurs, while it

is decreased by 4 every time a bit repetition occurs.

It is possible to set a preamble quality threshold in such a way that, if PQI is below the

threshold, the packet demodulation is automatically aborted at/after a timeout after the start

of RX.

If the preamble quality indicator check is enabled (field PQI_EN of the QI register set to '1'),

the running peak PQI is compared to a threshold value and the preamble valid IRQ is

asserted as soon as the threshold is passed. The preamble quality threshold is 4×PQI_TH

(PQI_TH = 0...15).

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