Potential for Intra-Vehicle

Wireless Automotive

Sensor NetworksPresented by:

Kiana Karimpoor

Wired Network• This paper looks into replacing the wires that carry data between

the vehicle's controllers and the sensors/switches with wireless links.

• The wiring harness is the heaviest, most complex, bulky and expensive electrical component in a vehicle and it can contribute up to 50 kg to the vehicle mass

• It is desirable to investigate other alternatives, such as WSNs. Intra-vehicle WSNs have the potential to solve this problem but can they deliver the same level of performance and reliability offered by wired subsystems?

Sensor Requirements:• In a typical modern vehicle, the most demanding sensor will

require a latency of approximately less than 1 msec with throughput of 12 kbps and the network will need to support about 15 sensors with this requirement.

• The least demanding sensor will require a latency of approximately 50 msec with data throughput rate of 5 bps and will need to support about 20 of sensors.

• A current production vehicle can have greater than 150 sensors and switches. This number is expected to grow as additional features are added to vehicles.

WASN Differences with WSN:

• 1. Heterogeneity: it is difficult to find sensors from same nature. there is a heterogeneous network of sensors, making a one-size-fits-all solution.

• 2. Wireless Channel: Fading and path loss characteristics of the intra-vehicle wireless channel will greatly impact the design choices.

• 3. Modulation waveform: The choice of the waveform has implications on error performance and energy consumption. Since WASNs need to employ ultra power-efficient devices, the choice of modulation waveform and type is significant.

• 4. Wireless networking: The WASN proposed in this paper needs to work in harmony with the existing data buses that connect the various electronic controllers on a vehicle. The connectivity of the wireless sensors/switches to these controllers are determined as a result of various trade-off relationships including factors such as cost, power consumption and latency.

• 5. Medium access: The radio resources are shared between all wireless sensors/switches within the WASN. The way the common resource is shared determines the resulting latency in packet delivery.

Network Model and Assumptions

It is importance for the control software onboard each electronic control unit (ECU) to get its input on time, execute the control algorithm and deliver its monitor/control output on time.

• 1.A wireless subsystem is modeled as a star network as shown in Figure 1. Even though some nodes might not be able to reach the ECU over a single-hop due to NLOS. First, we focus on single-hop stars in this first step of the study.

Network Model and Assumptions

• 2. This paper focus on contention-free multiple access schemes due to its collision-free nature. Therefore, we focus on slotted scheduled MAC schemes where each sensor reading fits exactly in a single time slot.

• 3. This paper assumes N uplink nodes who wish to communicate their readings to the ECU over the shared wireless medium.

• 4. This paper assumes that the underlying PHY layer is reliable (i.e. error-free) so a transmission taking place in a contention-free manner in any slot is successfully received with probability 1.(by the low data rate)

• 5. This paper focus on the MAC and interference issues within a single in-vehicle subsystem rather than interference between different subsystems.

• 6. This paper focus on replacing signal wires connecting switches and low-current sensors to the ECU with wireless links.

Network Model and Assumptions

(Low-current sensors and switches span a diverse range of requirements, depending on their impact on safety, comfort/convenience features and vehicle controls.)• 7. This paper assumes that uplink and downlink communications

are separated, in a manner similar to cellular systems.• 8. This paper assumes that each sensor node has a Transmission

Buffer that stores packets for waiting transmission. We also assume that packets to be transmitted arrive at the buffer of any sensor node periodically, i.e.

• The traffic arrival pattern at each node is deterministic.• The packet arrival process at each node is deterministic • The packet transmission time is deterministic

Sensor-to-ECU Latency Requirements

Analysis• The "uplink nodes", typically sensors and switches, communicate their

sample measurements for processing by the ECU software modules. These modules generate a decision/reading that is fed to the "downlink nodes", typically actuators and dashboard displays.

• The overall latency to communicate a reading from uplink node Ui to its corresponding downlink node Di, is given by:

Overall Latency(i) = UL Latency(i) + ECU Processing Latency +DL Latency(i) Eq1

• UL Latency(i) = the latency involved in communicating a sample measurement from node Ui to the ECU.

• DL Latency(i) = the latency incurred for communicating the ECU output to node Di.

• The ECU_Processing_Latency = the time required for the ECU software modules to process the input data.

• Typically, processing delays are orders of magnitude less than communication delays and, hence, can be safely ignored.

Uplink LatencyIn this analysis, we focus primarily on the uplink latency due to i) The latency requirements cause it to be upper bounded in invehicle subsystems, called Required-Latency, and defined as the deadline by which the sensor sample should be available for the ECU software module ii) It has direct impact on the design of MAC schemes.

“UL Latency(i) < Required Latency(i) ' i”If we consider N uplink nodes What are the upper and lower bounds on the Slot-Duration? (1) “UL_Latency(i) < Required_Latency(i) i = 1,2,..., N”

Number of uplink nodes=N

Uplink Latency “UL Latency(i) = QLatency(i) + Tx_Time(i) + Propagation Delay”

• QLatency(i) is the time spent by the packet queued in the buffer of node i.

• Tx Time(i) is the time needed for transmitting the packet over the air and is given by

• The propagation delay can be ignored due to the short distance (typically few meters) of wave propagation inside the vehicle.

“QLatency(i) <Required Latency(i) - Tx_Time(i) ” Eq2

• Each uplink node is assigned one slot per frame and, hence, the number of slots per frame becomes S = N.

Required Latency• Under the best-case scenario, a packet arrives at node i within the slot

immediately preceding the slot assigned to i. hence, 0 < Min_QLatency < 1 slot.• On the other hand, N-1 < Max_QLatency < N slot when a packet arrives at

node i within the slot assigned to i.All nodes experience the same performance under the homogenous node assumption, we will drop the node index i. “Max QLatency < Required Latency- Tx Time” “N * Slot-Duration < Required Latency - Tx-Time” Slot-Duration < • as the Required Latency becomes more smaller slot duration is essential to

support the same number of uplink nodes. Slot-Duration = is a MAC layer parameter

Required Latency• However, there is a limit on how short a slot could be that is

dictated by the transmission time and guard bands used for synchronization.

Slot Duration <

• As the Required Latency becomes more smaller (<10 msec), the upper bound reaches the lower bound and the Tx Time becomes the limit. At this point, the Link Bit Rate needs to be increased in order to maintain a reasonable range for the Slot-Duration degree of freedom.

Conclusion

• For a single in-vehicle subsystem, modeled as a star network, we established bounds on the slot duration of a TDMA scheme as a function of the sensor latency requirements among other parameters.

• Explored the potential of using intra-vehicle WSNs for the purposes of monitoring, control and communication between components and subsystems.

Bibliography• ElBatt, T.; Saraydar, C.; Ames, M.; Talty, T., "Potential for Intra-

Vehicle Wireless Automotive Sensor Networks," Sarnoff Symposium, 2006 IEEE , vol., no., pp.1,4, 27-28 March 2006.

Questions? Do you have any Idea about calculating Latency by different way?