Implementation is done with different sets of parameters. Each group will be evaluated for the three scenario as discussed in section 5.3. The setup with the flesh and medical implant is shown in the appendix.
5.4.1
Implant location in flesh for different scenarios
The scenarios explained in section 5.3.2 is exhibited as shown in the figures in Appendix A. The sensor encapsulated in a paraffin coat is placed in the subcutaneous layer. Power source is placed outside the flesh. The effect of metal and electrical interference to RF link is reduced by placing all the power source outside the flesh. The UART connection is made in order to send and receive the RSSI values. Once the physical layer validation is set and the optimum is chosen, the network parameters will be logged locally on the flash memory. The flash is read out once the timespan for respective experiment is completed.
5.4.2
Collective evaluation of PHY parameters
The evaluation parameters of PHY are interdependent. In order to find an optimum, their inter-dependency should be studied in detail. The choice of evaluation parameters are done based on the linearity based statistical approach. The main aim of the choice of parameters is to find and verify the linear dependencies between each parameter. Once the parameters from
CHAPTER 5. CHARACTERISATION OF PHY LAYER OF AN IMPLANATABLE SENSOR NODE
Set of parameters Evaluating parameters Set of physical parameters Transmission power (dbm)
Transmission distance (cm) Antenna orientation (degrees) Set of network parameters Operating frequency (Hz)
Data rate (Kbps) Packet length (bytes) Packet delivery ratio (%)
Table 5.1: Evaluation of hardware with two different sets of physical and network parameters.
set of physical parameters are evaluated, an optimum setting is chosen from the results, for the evaluation of set of network parameters. Out of the results, the optimum is chosen for the evaluation of network performance in Chapter6. In order to simplify the measurement pro- cess, a UART interpreter is developed. A command with different PHY parameters is sent and the corresponding results is read out using the same USRT interpreter. The implementation of UART interpreter is shown in appendix. The radio has to be reset for every change in the PHY parameters. The resetting of radio will put the radio in sleep mode. Thus for every change, the node has to initialize before sending any data. A flowchart for the evaluation of physical layer parameter is shown in Fig. 5.4.2.
The evaluation is carried out in groups to find the optimum parameters chronologically. A set of basic parameters including transmission power and distance with antenna orientation is car- ried out. Secondly, the set focusing on the transmission rate, frequency of operation and packet length is carried out. Modulation is not an important consideration as the chosen parameters will not greatly affect from modulation format.
Optimum definition The optimum value is defined as the value which is neither the best case nor the worst case for the given evaluation. By doing so, the further results are not influ- enced by the previous results. Also, in reality the optimum value will make will be chosen as a close resemblance to the real-world operation.
initialize CC430 RADIO UART Apply radio settings with PHY parameters Read the RSSI value to UART- buffer Clear radio settings Check if UART in- terpreter has a valid command stop yes no
Figure 5.7: Flowchart for PHY evaluation
5.4.3
Set of physical parameters
The first set of parameters to be evaluated in the in-body scenario is the transmission power and the received signal strength for different transmission distance, orientation of the antennas and power of transmission. As explained previously, the nodes are evaluated at different scenarios SC1-SC3. The application may vary for different configurations, but the need of the evaluating the physical layer parameters is required to design the upper layers of the OSI model. Table 5.2 shows the different configurations of first set of experiments for evaluation with scenario 1. This model is defined as the communication between the node within the body. The nodes are placed in the distance of maximum 16 cms, which is large enough for accommodating two different sensors within the torso. Perhaps, most of the application such as drug delivery devices, pace- maker, and neural stimulators are placed in the torso along with sensors such as blood pressure, glucose, heart rate and movement. Further more, evaluating this Scenario will help to study the effect of flesh in the radio communication and the design dependency for upper layer protocols.
Scenario 2 Consecutively, Table 5.3 shows different parameter settings for the Scenario 2. In SC2 the sensor node is placed inside the body and communicates to the sensor node outside the body. The maximum distance chosen for this evaluation is 160 cms, which covers the body of an
CHAPTER 5. CHARACTERISATION OF PHY LAYER OF AN IMPLANATABLE SENSOR NODE
Antenna orientation Evaluating parameters
0◦ Transmission power (dbm) - 5,0,-10,-15,-30 Transmission distance (cm) - 0,2,4,8,16 90◦ Transmission power (dbm) - 5,0,-10,-15,-30 Transmission distance (cm) - 0,2,4,8,16 180◦ Transmission power (dbm) - 5,0,-10,-15,-30 Transmission distance (cm) - 0,2,4,8,16
Table 5.2: Set of physical parameters for SC1.
average human being. In this situation, a sensor may be placed in a part of the body such as the thigh or hip. In more than 80 % of the medical cases, the sensor is not placed in a highly mobile human part such as leg. This is because the noise in the measured signal level gets distorted and the behavior of the sensor values contradicts to the normal measurement. However, even if the sensor node is placed in mobile part of the human, the values chosen will be within the range of the average human body.
Antenna orientation Evaluating parameters
0◦ Transmission power (dbm) - 5,0,-10,-15,-30 Transmission distance (cm) - 0,20,40,80,160 90◦ Transmission power (dbm) - 5,0,-10,-15,-30 Transmission distance (cm) - 0,20,40,80,160 180◦ Transmission power (dbm) - 5,0,-10,-15,-30 Transmission distance (cm) - 0,20,40,80,160
Table 5.3: Set of physical parameters for SC2.
Scenario 3 Scenario 3 is evaluated with larger distances, where the nodes are placed outside the body, and communicate with each other. In this case, the application scenario would be to have two different controllers such as drug-delivery device and central data logger. The medical sensor data could be logged to a device placed externally to the human body. It can also be the case that the medical device need to communicate to the logger for a intelligent decision. In such situation the device may be in need to communicate to the device worn outside the body. The maximum distance evaluated in this case is 300 cms. Another interesting evaluation would be to find the impact of the flesh if placed in the surface. In this case, the node is place on the skin of the animal flesh and the same experiments are repeated.
5.4.4
Set of network parameters
The set of network parameters used for the evaluation are data rate, packet length, frequency of operation. The transmission power, distance and antenna orientation are chosen from the
Antenna orientation Evaluating parameters 0◦ Transmission power (dbm) - 5,0,-10,-15,-30 Transmission distance (cm) - 0,25,50,75,100,150,225,300 90◦ Transmission power (dbm) - 5,0,-10,-15,-30 Transmission distance (cm) - 0,25,50,75,100,150,225,300 180◦ Transmission power (dbm) - 5,0,-10,-15,-30 Transmission distance (cm) - 0,25,50,75,100,150,225,300
Table 5.4: Set of physical parameters for SC3.
previous evaluation. From this set of network parameters, the values are varied and the packet delivery ratio is measured. This will help in finding the optimum settings for measurement of network performance. This chronological evaluation will help to find the network performance in a reliable setting of the PHY parameters. The values of different parameters and its corres- ponding values for characterizing are shown in table 5.5. Upon the selection of parameters, an implementation is done in the implant using UART interpreter. A command is sent from the PC, and based on the input given through UART. Different results are read back from the interpreter through UART and stored in the PC. These data are later analyzed in PC with MATLAB. The results from data analysis are discussed in section 5.5.
CHAPTER 5. CHARACTERISATION OF PHY LAYER OF AN IMPLANATABLE SENSOR NODE
Packet length Evaluating parameters
5bytes Data rate(Kbps) - 2,20,40,60,80,100,150
Packet delivery ratio PDR (%)
20bytes Data rate(Kbps) - 2,20,40,60,80,100,150
PDR (%)
40bytes Data rate(Kbps) - 2,20,40,60,80,100,150
PDR (%)
60bytes Data rate(Kbps) - 2,20,40,60,80,100,150
PDR (%)
80bytes Data rate(Kbps) - 2,20,40,60,80,100,150
PDR (%)
100bytes Data rate(Kbps) - 2,20,40,60,80,100,150
PDR (%)
Table 5.5: Set of network parameters. Repeated for SC1, SC2, SC3