Model, Analysis, and Improvements for Inter-Vehicle Communication Using One-Hop Periodic Broadcasting Based on the 802.11p Protocol

Abstract

Many future vehicle safety applications will rely on one-hop periodic broadcast communication (oPBC). The key technology for supporting this communication system is the new standard IEEE 802.11p which employs the carrier sense multiple access/collision avoidance (CSMA/CA) mechanism to resolve channel access competition. In this work, we first aim at understanding the behavior of such oPBC under varying load conditions by considering three important quality aspects of vehicle safety applications: reliability, fairness, and delay. Second, we investigate possible improvements of these quality aspects. We start with a clear mathematical model which gives the foundation for making an accurate simulation model as well as for defining new appropriate metrics to judge the aforementioned quality aspects. We evaluate oPBC with a strictly periodic broadcasting scheme, i.e., each vehicle broadcasts messages in a strictly periodic manner. The evaluation reveals that the hidden terminal, or Hidden Node (HN), problem is the main cause of various quality degradations especially when the network is unsaturated. To be more specific, the HN problem reduces the message reception ratio (i.e., reliability degradation) and causes unfair message reception ratios for vehicles (i.e., fairness degradation). Moreover, it causes long-lasting consecutive message losses (i.e., delay degradation) between vehicles while they are encountering each other, i.e., entering their Communication Ranges (CRs). In some serious cases, a certain vehicle could not successfully deliver any of its messages to a particularly destination vehicle throughout an entire encounter interval of these two vehicles. We propose three simple, but effective broadcasting schemes, to alleviate the impact of the HN problem. Though these solutions do not affect the message reception ratio (i.e., reliability) of the entire network, they do improve the fairness and delay aspects. These solutions are fully compatible with the IEEE 802.11p standard, i.e., they are application-level solutions and can be easily introduced in practice.

Appendices

Appendices

A CSMA/CA

This section explains how CSMA/CA operates in periodic broadcast mode. Figure 21 shows a basic flowchart and Fig. 22 gives a corresponding state machine diagram. According to CSMA/CA, when a station becomes ready to broadcast (Ready To Transmit (RTT)), the station must first check the channel for a duration of AIFS. If the channel has been idle for longer than AIFS, the station starts its transmission immediately. If the channel is busy or becomes busy during AIFS, the station must wait for the channel to become idle. 802.11 refers to this wait as Access Deferral. If access is deferred, the station first waits for the channel to become idle for AIFS again. If the channel is idle, the station must perform Bf procedure by starting a Bf timer which is set to a random number drawn from an interval of {0,1,…,CW}. The timer has the granularity of a slot time and is decremented every time when the channel is sensed to be idle for a slot time. The timer is stopped in case the channel becomes busy and the decrementing process is resumed when the channel becomes idle again (i.e., idle for a duration of AIFS). The station is allowed to transmit its message when the Bf timer reaches zero. Depending on the channel condition, a station may experience multiple AIFS plus access deferrals. Note that the Bf counting is at most done once. If a new message arrives from the upper layer, then the current message must be dropped and the new message transmission will start.

Fig. 21

CSMA/CA procedure in periodic broadcast mode

Fig. 22

CSMA/CA state machine

As shown in the state machine diagram in Fig. 22, each station is in one of the five states: IDLE, WAIT_AIFS, DEFERRING, Bf COUNTING, and XMIT. The station is in the IDLE state when it is neither in transmission nor RTT. In this diagram, we use several variables and constants to show the state transition conditions and timing changes.

• The “n” holds the current time.

• The “a” holds the current value of the AIFS counting.

• The “c” holds the current value of the Bf counting.

• The “do_c” is a boolean variable to indicate whether the station should perform the Bf counting.

• The “busyTime” is the duration of the channel being busy.

• The “msgDelay” is the duration of one message transmission.

• The “Next Message Time (NMT)” is the time at which the station becomes ready to transmit its next message.

• The “busy” to indicate the busy channel.

• The “AIFS” is the number of slots for the AIFS waiting and it is given by the standard.

• The “CW” is the number of slots for the Bf and it is given by the standard.

• The “a Slot Time (ST)” is the duration of one slot time and it is given by the standard.

B Signal Reception Model

There are generally two methods used for physical layer (or PHY) modeling in network simulations, namely SINR threshold based and Bit Error Rate (BER) based [22]. Under the former method, the receiver accepts the message when the computed SINR value is above the SINR threshold for a particular modulation scheme. The method based on BER decides whether or not a message is received successfully based on the message length and bit error rate deduced by the pre-computed BER versus SINR curve for every modulation scheme at the receiver.

In our case, the signal reception model is taken from the NS-2 implementation of 802.11p [18], where the signal reception decision is based on SINR ratio. In this model, three basic signal threshold concepts play a role. The first one is a Power Sense Threshold, PsTh.⁵ If any receiving signal is equal to or greater than PsTh, the PHY will try to decode the signal. In addition, any signal equal to or greater than PsTh is considered to be strong enough to interfere with any other signal. Therefore, such signal is added into the cumulative signal level of the receiver. The second threshold is the SINR threshold (or receiving threshold), SrTh. To receive a message successfully, the preamble must be received successfully. While a station is not transmitting nor receiving any signal, namely the PHY is constantly searching for a preamble and if a new signal arrives with a signal strength that is equal to or greater than SrTh with respect to all other interfering signals plus noise, then the station will start receiving the signal as the preamble. If this SINR ratio holds for the entire duration of the preamble length, the PHY can finish the preamble reception successfully. Once the preamble is received successfully, the PHY will inform the MAC layer that the PHY is receiving a message and will continue until the complete payload has been received regardless of whether the signal level is greater or less than SrTh, unless the frame capturing feature is enabled. Note, the preamble includes information about payload (the length of payload, modulation scheme etc.). In addition, if the receiving signal strength becomes less than SrTh during the payload reception, PHY layer puts an error in the payload such that when the MAC layer checks the CRC of the received message it will not send the message to the application layer.

The third threshold is a carrier sense threshold, CsTh. If the cumulative power level of the receiver is equal to or greater than CsTh, then the PHY layer will inform the MAC layer that the channel is busy. From this, we can see that the MAC layer is informed about the channel status in two different ways. First, no matter what if the cumulative power is equal to or greater than CsTh, then the MAC is informed the channel is busy. Second, if the PHY layer successfully receives a preamble viz., start receiving a payload, the MAC layer is informed the channel is busy.