To combat road fatalities, the U.S. Federal Communication Commission(FCC) has allocated a 75MHz spectrum at 5.9GHz for vehicle-to-vehicle and vehicle-to-infrastructure communications. The proposed vehicular communication technology, known as the Dedicated Short Range Communication (DSRC),is currently being standardized by the IEEE [1, 2]. Many major car manufacturers have responded positively and are actively working together in bringing this promising technology into reality [3, 4]. Although the primary purpose of DSRC is to enable communication-based automotive safety applications, e.g., cooperative collision warning (CCW), the standard also provisions for a range of non-safety applications, from electronic toll collection to multimedia downloading [3]. The motivation for allowing non-safety over DSRC is to create commercial opportunities thereby making the DSRC technology more cost-effective. Commercial operators wishing to conduct business over DSRC will be expected to acquire the appropriate spectrum licenses.

In a typical vehicular communications network, vehicles communicate with each other as well as with road side infrastructure. For non-safety to co-exist with safety applications over the same DSRC spectrum, it is absolutely necessary to have proper mechanisms in place to protect safety communications from the harmful interference of non-safety data transmissions. To this end, the DSRC standard divides the entire 75MHz spectrum into seven orthogonal 10MHz channels and reserves one of the channels, called control channel (CCH), exclusively for safety communications. The remaining six channels, referred to as service channels (SCHs), are to be used for non-safety applications.

At First, it may appear that a large share of the DSRC (6 out of 7 channels) is available for non-safety communications. However, continuous non-safety communication on a SCH would be possible only if all DSRC devices were capable of simultaneously monitoring the CCH while exchanging non-safety data on SCH. To accommodate single radio DSRC devices that must switch between CCH and SCH to support both safety and non-safety, it is mandatory for all devices, single or dual-radio, to synchronize their safety (and consequently non-safety as well) transmissions. The synchronization requirement leads to the cyclic transmission phenomenon in vehicular networks where safety activities on CCH are followed by non-safety on SCH in a repetitive fashion. In such cyclic transmissions, the share of DSRC available for non-safety is strictly given by the length of the two intervals. For non-safety to avail a large share of DSRC, the safety interval has to be reasonably small compared to the non-safety interval. In other words, the share of DSRC available for non-safety application is limited by the control channel intervals. However, due to the hidden node and power capture effects, larger control channel intervals might be required in high density traffic conditions to meet the reliability and latency requirement of safety applications. As a result, the share of DSRC available for non-safety application can be highly restricted in high density traffic.

Very few studies of the reliability of DSRC safety applications under the IEEE 1609.4 multi-channel operation have been reported. Although, the reliability of one-hop broadcast communications in vehicular environment has been studied through both analytical models [13, 14] and simulation experiments [11, 7]. However, in these studies the reliability was analyzed assuming continuous access to the channel. The DSRC channel synchronization and coordination was not considered.

Some researchers have explored multi-channel coordination schemes that are different than the one specified in IEEE1609.4 standard. In[15], the authors proposed a protocol relying on a road side hot-spot to synchronize the channel coordination. In [4], a protocol named Peercast was proposed so that the channel switching is performed asynchronously. DPC [16], DCA [17], and CHAT [18] multiplex multiple applications over multiple channels. They seek to maximize throughput while preserving fairness. DPC and DCA re-quire each node to have two radios. One radio listens to the control channel (CCH) at all times, and the other radio is used for communicating data on the data channels (DCH). The CCH is used by nodes to reserve DCH access. DCH reservation on the CCH is contention based. The mechanism is very similar to the RTS/CTS handshake. CHAT eliminates the extra control channel, but requires all the nodes to follow a common channel hopping-sequence. Channel reservation on each hop is very similar to RTS/CTS. Once channel reservation succeeds, the sender and receiver(s) remain on the reserved channel for the duration of the data exchange. When the data exchange is done, these nodes synchronize back to the common hopping sequence. Since safety messages are generally useful to vehicles proximate to the sender, the broadcast communication is necessary for efficiency of the safety channel utilization. DPC and DCA only support unicast communication. Though CHAT supports broadcast communication, there is no bound on the latency to all broadcast receivers receiving the message. However, these studies are not compliant to the DSRC standard.

TBA

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http://www-nrd.nhtsa.dot.gov/pdf/nrd-12/1665CAMP3web/images/CAMP3scr.pdf
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