5.7 Sub-GHz physical layer modulations

The sub-GHz ISM band has two physical layer modulation techniques: CSS and GFSK. The modulation process consists of two stages: digital modulation and analog modulation. During digital modulation, the data payload (link layer frame) is attached to preamble and the CRC is placed at the end. The frame layout is defined by IEEE in 802.15.4 [1]. For more information, see section 5.1.4. The analog part of modulation is applied afterwards (CSS or FSK respectively). During this process, data bits are assigned to symbols according to a modulation scheme. The result is analog signal in baseband. For more information on CSS modulation, see section 5.7.2; for more information on FSK modulation, see section 5.7.3.

The next step is to upconvert the baseband (BB) signal to either the working RF channel (in case of single channel mode SubG-CSS) or to the scheduled channel allocated (for SubG-FSK). For more information on frequency hopping, see section 5.7.1.

Finally, the RF-modulated signal is amplified and filtered to achieve the required RF power spectrum. The RF signal must be compliant with normative requirements for bandwidth and power density. The Normative requirements are defined by relevant administrative bodies: ETSI for ITU Region 1, FCC for ITU Region 2., and others for ITU Region 3. Available channel lists with modulation parameters and duty cycles are shown in Table 5.5 and Table 5.6 for ITU Region 2.

Table 5.5 SubG-CSS Channel allocation for Region 2

Channel spacing for SubG-CSS is 800 kHz.

Channel 26 is the default working channel.

Table 5.6 FSK Channel allocation for Region 2

Channel spacing for SubG-FSK is 200 kHz. Sixty-nine channels are used in Amazon Sidewalk.

Half-duplex (HDX) transmission modes are implemented in both SubG-CSS and SubG-FSK due to the requirement for multi access on a single channel and limitations for radiated power. Both modulation methods can use any of the available channels available in the regional channel grid. As examples in ITU Region 2. 915 MHz band is available.

5.7.1 Frequency hopping

There are two reasons for developing a frequency hopping system. Firstly, the system becomes more immune against fades and single channel interferences. Secondly, the system fulfills requirements for channel-radiated power density normative limits. Slow frequency hopping is implemented in the Amazon Sidewalk protocol. There is a precalculated hopping pattern based on random seed and table-based seq_id for each entity. An algorithm implements Beacon channel collision avoidance. Details are described in section 5.2.4.

5.7.2 LoRa modulation for SubG-CSS

LoRa modulation is a kind of spread spectrum modulation where a frequency chirp is implemented as a constant envelope frequency sweep within the working channel. Symbol rate (SR) is defined as:

SR = BW / (2^SF)

where BW is signal bandwidth and SF is spreading factor. SF 11 is used in Amazon Sidewalk rel. 1.10.

Bit rate (Rb) is calculated as:

Rb = (SF * CR * BW) / (2^SF) [bits/s]

where CR code rate 4/5 … 4/8.

Higher SF provides better processing gain per symbol and hence requires less SNR. Furthermore, higher SF provides more immunity to co-channel interference.

Frequency tolerance: < +/-90.25kHz @500kHz BW; RX sensitivity: ≤ -128 dBm @ SF11/500kHz BW. [2]

5.7.3 GFSK modulation for SubG-FSK

In GFSK modulation, a Gaussian filter (pulse shaper with predefined bandwidth) is applied after the FSK modulator. This module reduces the occupied bandwidth of the modulated signal. NOTE: Lower BT increases inter-symbol interference. Occupied bandwidth (OBW) can be estimated as:

OBW = Rb + 2 * Fdev

where Rb is the PHY bitrate, and Fdev is the frequency deviation corresponding to the chosen modulation index. Rb equals symbol rate for GFSK modulation.

TX frequency clock accuracy ≤ +/- 40PPM [1],[4]; RX sensitivity: ≤ -107 dBm [3];

Blocking and selectivity in accordance with local requirements [5].

References:

1. IEEE 802.15.4g (GFSK).
2. Semtech SX 1272/73 application note.
3. TI CC1070 application note.
4. TI CC1200 application note.
5. FCC CFR 47.

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