Özet:
Vehicular ad hoc networks (VANETs) have recently attracted interest for automation
and intelligent transportation system (ITS). VANETs increase transportation
efficiency and road safety. VANETs comprise inter-vehicle communication (V2V,
vehicle-to-vehicle) as well as communication between vehicle and infrastructure (V2I,
vehicle-to-infrastructure). VANETs facilitate numerous applications which can be
categorized as safety messages (sm) and non-safety data (nsd). However, to have
useful applications the communication between vehicles and with existing networking
infrastructures should be efficient. For any ad hoc network, medium access control
(MAC) protocol is one of the most significant parts, because efficient and reliable data
transmission directly depends on MAC protocol.
The IEEE 802.11p standard is outlined by the institute of electrical and electronics
engineers (IEEE) to support wireless access in vehicular environments (WAVE) and to
provide MAC and Physical (PHY) Layer specifications for VANETs. Later IEEE 802.11
standard incorporated IEEE 802.11p. In IEEE 802.11p and IEEE 802.11, distributed
coordination function (DCF) is the fundamental mechanism to access the medium
and enhanced distributed channel access function (EDCAF) is sketched to support
contention-based prioritized Quality of Service (QoS) at MAC layer. In this thesis, the
performance of DCF and EDCAF for both IEEE 802.11p and IEEE 802.11 is modeled
and analyzed. An analytical analysis based on Markov chain model is presented. The
derived performance model is justified by numerical results. Then we investigated the
xix
influence of channel fading and capture effect. Nakagami, Rayleigh, and Rician faded
channels are considered. However, both IEEE 802.11p and IEEE 802.11 can not fulfill
the performance criteria of VANETs.
In VANETs, high mobility and relative mobility among vehicles can result in rapid
topology changes with frequent link breakage and unstable communications which
cause collision and packet loss. Alternatively, clustering VANETs into small groups
which limits channel contention and controls the network topology efficiently. In this
thesis, a novel cluster-based MAC (CB-MAC) protocol is proposed for VANETs. The
cluster formation process is defined. Moreover, cluster head (CH) election and cluster
merging processes are described for efficient communication in the cluster as well
as out of the cluster. The mechanism defined in IEEE 802.11 is specially designed
for only direct communications and is not suitable for cluster-based communications.
Therefore, new control packets are introduced and the existing control packet format is
modified to support cluster-based communications. For effective MAC protocol design,
the request to send (RTS)/ clear to send (CTS) mechanism is not used for sm which
are of broadcast nature. On the other hand, the RTS/CTS mechanism is used for
nsd delivery to eliminate hidden node problem. Markov chain model based analytical
model is presented to explore the performance of proposed CB-MAC protocol. The
proposed protocol is validated by numerical studies. The numerical results exhibit
that the proposed CB-MAC protocol improves system performance and satisfies the
delay constraint of 100 ms for sm.
Another way, cooperative transmission can improve the communication reliability and
can enhance communication rate with lower delay by alleviating wireless channel
impairments caused by the mobility in VANETs. In this thesis, a novel reliable and
efficient cooperative MAC protocol for VANETs (RECV-MAC) is also proposed which is
designed both for sm and nsd transmission. RECV-MAC ensures reliable and efficient
transmission with the help of helpers which have good channel condition to both the
sender and the receiver. New control messages are introduced to support cooperative
communication. The mechanism is defined to choose the suitable transmission mode
as well as to select the optimal helper. To investigate the performance of the proposed
RECV-MAC protocol, Markov chain model based analytical analysis is provided. The
proposed RECV-MAC protocol is validated by numerical studies. The performance of
the RECV-MAC protocol is compared with the traditional MAC model which is based
on the IEEE 802.11p and a quantitative comparison with existing cooperative MAC
schemes is presented. It is obvious from the comparison that the RECV-MAC performs
better than IEEE 802.11p and existing cooperative MAC schemes. The numerical
results demonstrate that the RECV-MAC protocol improves the performance with
higher throughput, enhances reliability of communication by decreasing PDR, and
xx
decreases delay, in particular, satisfies the delay constraint of 100 ms for sm.
The performance of IEEE 802.11p, IEEE 802.11, and CB-MAC is optimized.
Performance optimization mechanism is presented. The comparison between IEEE
802.11p and IEEE 802.11is provided. The microscopic mobility model is generated in
SUMO for practical scenario. Monte Carlo simulation results are presented which
verify analytical analysis. Complexity analysis is presented. Comparison between
complexity analysis is illustrated.