Wireless LAN Power Control in an Automotive Environment

	ABOUT THE AUTHORS Christopher A. Hedges received his MS and BS degrees in electrical engineering from Purdue University in 1995 and 1988, respectively. Hedges has worked for Delphi Corp. since 1989 designing vehicular navigation systems, powertrain controllers, and entertainment systems. He has recently led several short-range, wireless network projects for Delphi's Advanced Engineering group. Timothy D. Bolduc received his MS degree in electrical engineering from Stanford University in 1995 and his BS degree in electrical engineering in 1991 from Marquette University. He has worked for Delphi Corp. since 1991 in various capacities. Currently, Bolduc is serving as a project leader in Delphi's Advanced Engineering group where he is responsible for developing wireless networks and services.

While the engine is running, the WLAN can be operating at full power, since the vehicle alternator is supplying current. However, many network interactions will take place where the power is limited—parked at a public hotspot or at home in the garage. If the battery is excessively drained, the vehicle will not start and the battery will need to be charged.

The WLAN node will likely be comprised of three major components—a SBC (single-board computer), WLAN chipset/module, and mass-storage device. A WLAN chipset requires power to receive and transmit data. Most chipsets have low-power sleep modes, but even in these modes current drain is still on the order of tens of milliamps. An automotive telematics device typically has a limited current budget between ignition cycles. Couple the current needed for the SBC along with the WLAN radio and the result is that the node cannot be active the entire time a vehicle is parked without the engine running.

This article describes actions performed by a vehicle's WLAN node to enable adequate network performance while still ensuring the vehicle battery will have enough energy to start the engine. Figure 1 shows a block diagram of the node and potential hotspots.

Figure 1:  Vehicle wireless node and potential hotspots

A WLAN transmits at varying rates depending on range and interference. However, this data rate is not the actual file transfer rate. On average, the observed file transfer rate is comparable to roughly 60% of the WLAN bit rate. Table 3 takes this into account and shows typical download times for various media files over an 802.11 WLAN.

If all power were available for receiving and storing, the following formula would apply:

Time^{Receive & Store} = Current Budget^Total / Current^{Receive & Store}

In this ideal situation, Time^{Receive & Store} = 1500 mA-hour / 1080 mA = 83 minutes. This is adequate to download an entire DVD. However, in reality, we must use the following formula that takes Listening Mode into consideration:

Time^{Receive & Store} = (Current Budget^Total - (Current^Listening × Time^Listening)) / Current^{Receive & Store}

If Listening Mode lasts one hour, Time^{Receive & Store} = (1500 - (745 × 1)) / 1080 = 42 minutes.

The next section moves away from any specific node design and discusses some general "listening" techniques.

Figure 2:  10-minute timeout

In Point Coordination Function operation, the Timing Synchronization Function will use broadcast beacons to inform the node of impending traffic. The node will poll for incoming traffic. If the node determines that it has traffic, it will begin a negotiation with the hotspot for any pending downloads as seen in Figure 3. The details of this negotiation can vary and are beyond the scope of this discussion. If the result of this negotiation is a download, it can continue past the 10-minute timeout (as long as the 1500 mA-hour current budget is not exceeded).

Figure 3:  Download occurs within 10 minutes

This scheme could also be used at home. However, it is unlikely that a user will arrive home and immediately begin downloading files to the vehicle—thus, this scheme can prove to be cumbersome for the user. The vehicle must be re-awakened to download data. This could be triggered by a user-initiated event, such as unlocking the doors, which is shown in Figure 4.

Figure 4:  Door-unlock event to wake up the WLAN

Figure 5:  Wake-up at a predetermined time

If a hotspot is found and a positive negotiation occurs, queued files will be downloaded. The SBC will download these files and then go back to sleep until the vehicle is started in the morning as shown in Figure 6.

Figure 6:  Wake up and download queued files

Figure 7:  Periodic WLAN wake-up

This mode is the least efficient due to power consumed by repeatedly booting up the node. However, user intervention at the vehicle is not required and download latency is reasonably low.

These methods allow intelligent listening to determine if a node-hotspot transfer should occur. Downloading large video files requires high efficiency. If our node is purely a digital audio player, efficiency may not be as important. If automatic downloads of weather and news are a major feature, the latency associated with Predetermined Wake-up would not be considered a negative factor. A budget lower than 1500 mA-hours may force a User Event to be required for Wake-up. The system's design requirements for efficiency, latency, and ease of use will determine which combination of modes to use.

Stranding a driver at a hotspot is a safety concern. The driver needs to have a fully operational vehicle after all the data has been transferred. We developed the methods discussed in this article to maximize the convenience of a wireless network without jeopardizing vehicle safety.