# ICARUS--Very Low Power Satellite-Based IoT
## 3 Physical Layer and Doppler Compensation  
### PHY Overview  
ICARUS signal transmission is based on single CDMA (code division multiple access) bursts which are emitted during the ISS overflight. The burst structure is depicted in Figure 3. The preamble is used for time and frequency synchronization. It is explicitly designed to jointly estimate exact burst start timing under high-Doppler shifts and low SNR [9].  
After the synchronization preamble, the signaling preamble (green part in Figure 3) is transmitted which encodes the spreading code ID used for direct-sequence spreading of payload and pilot symbols. The subsequent burst structure then uses an interleaved payload-pilot pattern whereas the pilot symbols are BPSK (binary phase shift keying) encoded, while the payload symbols are QPSK (quadrature phase shift keying). Both payload and pilot symbols are spread with the same spreading code of length 1023. The interleaved pilots are explicitly made for keeping track of the time-varying radio channel impulse response which exhibits volatile multi-path [7]. Channel impulse response (CIR) estimation is performed using the inserted pilot sequences to constantly track the Rake receiver [10] which is used to equalize and to de-spread the received CDMA chips. Figure 4 shows the architecture of the channel estimation circuitry used in the ISS spaceborne receiver:  
Figure 3: ICARUS uplink bust structure.  
Figure 2: Tag wake-up procedure during ISS overflight: hibernation phase (1), satellite search phase (2), downlink beacon reception (3), uplink window (4), downlink tag command reception (5), orbit propagation for next overflight and go back to hibernation (6).  
The principle of the ICARUS Rake receiver is depicted in Figure 5 for the case where two so called 'Rake fingers' are used. Each Rake finger corresponds to one single channel tap, all of which are equalized by a maximum-ratio-combiner. Figure 5 shows the schematics of the implemented Rake receiver for two rake fingers, whereas ten Rake fingers are used in the uplink space-borne receiver, and five fingers are used in the ICARUS tag to equalize the downlink signal. The * sign in Figure 5 denotes conjugate complex signal operation.  
The rake receiver performs constructive signal combining by means of a maximum ratio combining operation [10]. It exploits the good auto-correlation properties of the spreading code which nearly cancels out time-shifted versions of the received signal. As an important side effect, the Rake receiver is allowed to keep signal orthogonality of different tag signals even after equalization in low SNR (\(<\)-20 dB). It is allowed to operate a random access (RA) channel even under difficult channel properties and low SNR. These important properties lead to the definition of the new class of space IoT systems: RA-vLPWAN.  
The downlink signal structure (see Figure 6) is similar to the uplink burst structure, whereas the DL signal forms a continuous stream of frames, realizing a continuous carrier. The DL beacon has a similar structure as the UL synchronization preamble. It is made for time and frequency synchronization under high-Doppler shift and low SNR. Regular pilot insertions allow it to track the time-varying channel impulse response used for Rake reception. The first pilot field is different from other pilot fields since it implements the frame start delimiter (FSD) which marks the start of a new LDPC FEC frame.  
Figure 4: Correlation-based channel estimation.  
Figure 5: Rake receiver with two rake fingers for channel equalization.