Frame capture in IEEE 802.11P vehicular networks: a simulation-based approach

This thesis describes the occurrence of Frame Capture in the IEEE 802.11p physical layer. Frame Capture occurs when two nodes simultaneously transmit a message (a PLCP frame) that spatially and temporally overlaps at a certain receiver, creating a collision. Collisions can occur because of hidden terminals, or because of simultaneously ending backoff counters between coordinated nodes. If both frames are equal in signal strength, they both interfere in such a degree with each other that the receiver will not be able to decode either one of them. However, if one frame is received with a signficantly higher power level than the other, the receiver can (under some conditions) be able to suppress the weaker frame and correctly receive the stronger one. We studied the available literature on Frame Capture and concluded that the capture behavior is different among various chipsets and depends largely on the arrival time difference (in the order of microseconds) between both frames and on whether both frames interfere in each other's PLCP preamble or not. The literature compares Atheros and Prism chipsets, and demonstrates that if the stronger frame arrives before the weaker frame, both chipset types are able to capture the frame if the signal to noise ratio is high enough. If the stronger frame arrives later, the required SNR is significantly higher because the receiver must be able to clearly hear the stronger frame's preamble, even in the presence of interference, to lock onto the new frame correctly. We implemented this capture behavior in our wireless network simulator, OMNeT++ with the MiXiM module, with the use of capture thresholds. Depending on the moment of arrival and the current state of the receiver, a certain threshold must be exceeded for the receiver to capture the frame. These capture thresholds are based on the results in the literature, but could be refined later on with experiments of our own. Note that capturing the frame (which happens after the preamble) does not guarantee correct reception: if for example during the frame more interference arrives, the frame can still be lost. We also enhanced the bit error calculation & prediction formulas in MiXiM, which where only suited for 802.11b, a completely different physical layer. We now approximate bit errors with theoretical formulas derived for an AWGN channel (i.e. where white noise is the only interference). This is not perfect yet but a significant improvement over the current implementation, and it will help further increase the channel model accuracy in the future. iii After the implementation work was complete, we performed simulations of vehicular networks with which we demonstrate that Frame Capture is especially important in vehicular networks with broadcast traffic. This is because the hidden terminal problem becomes more prevalent when all frames are broadcast, because the standard RTS/CTS mechanism can not be used to warn hidden terminals about ongoing transmissions. We demonstrate that both Atheros and Prism chipsets show a performance gain of at least 20% over (reference) chipsets which do not perform any kind of Frame Capture. By artificially disabling the hidden terminal problem (i.e. silencing all hidden terminals during a transmission) we show that Frame Capture is mostly important when resolving hidden terminal collisions. Collisions between coordinated nodes are more likely to have an equal power level and can usually not be resolved with Frame Capture. Because nodes do not update their MAC contention window if broadcast traffic is the only type of traffic in the network, occurrence of collisions between coordinated nodes increases with increased node density.