# Potential of Sub-GHz Wireless for Future IoT Wearables and Design of Compact 915 MHz Antenna
## 2 Comparison of Wireless Performance in the 915 MHz and 2.45 GHz ISM Bands
### DC Power Consumption
#### 2.2.2 Commercial RF Integrated Circuit Radio Transceivers
Equation (1) can be now rewritten as  
\[\delta\ =\ \frac{T_{TX}+T_{RX}}{T_{frame}}\ (1\ +\ PER). \tag{5}\]  
The wireless communications scenario considered is depicted in Figure 1, composed by two nodes, namely the wrist-worn wireless SpO2 sensor and the hub. For the application of interest, we assume that the wrist-worn wireless SpO2 sensor sends a data payload of 7 bytes in total with 3 data bytes for the measured SpO2 information and 4 bytes for the timestamp. The data payload is sent every \(T_{frame}\) s. Once the Hub receives the data payload, it will reply with an acknowledgement (ACK) if the data packet is received with no errors, or else with a non-acknowledgement (NACK) if the received data packet contains errors. Figure 3 illustrates the RFIC transceiver activity in a successful and non-successful data transmission case. In particular, in a successful data transmission case, the wrist-worn wireless SpO2 sensor receives the ACK then it switches to sleep mode and it remains in this status until the new data packet transmission occurs. In a non-successful data transmission case, instead, the wrist-worn wireless SpO2 sensor receives a NACK packet, then it performs a retransmission of the data packet. In Table 4, a comparison among the protocols under evaluation in terms of calculated values for \(T_{TX}\) and \(T_{RX}\) as function of ACK Length, Data Packet Length and Data Rate is reported. The ACK length and Data Packet length values are derived from documents referenced in Protocol column of Table 4. The ACK Length is the number of bits of the ACK packet comprising of the overhead only. As for the Data Packet Length, it considers the sum of overhead plus payload bits. In the fifth column, \(T_{RX}\) is obtained as the ratio of ACK Length to Data Rate. Similarly, in the sixth column \(T_{TX}\) is obtained as the ratio of Data Packet Length to Data Rate. Figure 3: Illustration of the wrist-worn SpO2 RFIC transceiver activity in a successful data transmission case and in a non-successful data transmission case. Finally, from the information in Table 4, the average RFIC current consumption \(I_{avg}\) can be calculated as  
\[I_{avg}=\ \frac{(T_{TX}\ I_{TX}+T_{RX}I_{RX})(1\ +PER)\ +\ T_{slexp}\ I_{sleep}}{T_{frame}}\, \tag{6}\]  
where  
\[T_{slexp}\!\!=\ T_{frame}\ -\ (T_{TX}\ +T_{RX})(1\ +\ PER). \tag{7}\]  
For the calculation of \(I_{avg}\), the lowest possible data rate has been chosen while still \(\geq\) 10 kbps [11]. A considerable difference in terms of Data Rate for ZigBee, BLE and WiFi can be noticed and this is due to the fact that the considered radio transceivers in Table 3 do not allow lowering the Data Rate below the values adopted in Table 4. On the other hand, using a larger Data Rate leads to a consistent reduction of \(\delta\), which determines a predominance of the contribution of the sleep current \(I_{slexp}\) with respect to the contribution of the transmitting current \(I_{TX}\) and receiving current \(I_{RX}\), and hence a lower \(I_{avg}\). As for \(I_{TX}\), \(I_{RX}\) and \(I_{sleep}\), the correspondent values reported in Table 3 have been used in the calculations that are now summarized. Figure 4 shows four histogram graphs of \(I_{avg}\) for each protocol investigated considering four different \(T_{frame}\) values. In Figure 4a, a sampling rate of 1 sample per second with a \(T_{frame}\) of 1 s is considered for the calculation of \(I_{avg}\). In this particular case, it can be noticed that the radio transceiver that implements BLE protocol presents the lowest average current of 0.56 \(\mu\)A. Higher but still comparable values of \(I_{avg}\) are presented by the radio transceivers that implements ZigBee and Z-Star protocol that are respectively 2.79 \(\mu\)A and 3.35 \(\mu\)A. As for the ANT protocol, the corresponding radio transceiver has an \(I_{avg}\) of 27.37 \(\mu\)A, then the LoRa transceiver is characterized by an \(I_{avg}\) of 48.02 \(\mu\)A, the UHF RFID and WiFi transceiver by an \(I_{avg}\) greater than 100 \(\mu\)A.