# An Enhancement for IEEE 802.11p to Provision Quality of Service with Context Aware Channel Access for the Forward Collision Avoidance Application in Vehicular Ad Hoc Network

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## Abstract

**:**

## 1. Introduction

#### 1.1. Motivation and Objectives

#### 1.2. Novelty and Contributions

- The protocol improves IEEE 802.11p performance for supporting forward collision warning applications in VANET. The scheme improves the delivery of emergency (event) messages by prioritizing them over the non-emergency messages by using an adaptive contention window.
- The proposed scheme provides a service differentiation among the emergency and non-emergency message in the safety category of IEEE 802.11p by implementing a dynamic contention window for the emergency packets based on the priority of the packet from the node at the instant. The non-emergency message, the routine safety message in IEEE 802.11p, follows the same strategy that was implemented in IEEE 802.11p.
- The performance of the broadcast with service differentiation for emergency messages over non-emergency message in the safety category of IEEE 802.11p with an analytical model is studied.

#### 1.3. Paper Organization

## 2. Related Work

## 3. Proposed Method: Priority-Based Cooperative MAC (PCMAC)

#### 3.1. Relevance of the Proposal

#### 3.2. Scenario

#### 3.3. Priority Calculation

Algorithm 1: Algorithm to find the accident zone |

1: procedure ACCIDENT ZONE (x_{s}(t), y_{s}(t), R, x_{r}, y_{r}) |

// location of vehicle in accident, R-Range and Range coordinates |

2: ds(t) = $\sqrt{{\left(({x}_{r}-{x}_{s}\left(t\right)\right)}^{2}+{\left({y}_{r}-{y}_{s}\left(t\right)\right)}^{2})}$ |

3: while ds(t) ⇐ R/2 do |

4: Read Accident Zone from $[({x}_{s}\left(t\right),{y}_{s}\left(t\right),\text{}\left({x}_{s}\left(t\right)-\frac{R}{2},{y}_{s}\left(t\right)-\frac{R}{2}\right)]$ |

5: while ds(t) > R/2 do |

6: B(t) = R − ds(t); |

7: Get ${B}_{x}\left(t\right)$ and ${B}_{y}\left(t\right)$; |

8: Read Accident Zone from $\left[\left({x}_{s}\left(t\right),{y}_{s}\left(t\right),{B}_{x}\left(t\right),{B}_{y}\left(t\right)\right)\right]$ |

- Impact
_{i}(t): This defines the severity of the event. This paper mainly concentrates on the forward collision warning application. The forward collision happens when the vehicles behind are not able to control their speed as the vehicle in front stops abruptly. This can be avoided by quickly alerting the vehicles behind the broken vehicle so that the drivers can control the speed of their vehicle to avoid successive collision. The maximum number of collisions that can occur can be equated to the number of vehicles behind the vehicle that has entered a collision. Impact_{i}(t) measures the severity of the accident. It is calculated as a function of the factor of the number of nodes that enter into the accident probable zone with the total number of nodes in the platoon. The vehicles close to the broken node have more chances to collide. Therefore, we consider a probable zone of accident occurrence. This zone is defined based on the location of the accident. If the vehicle in the accident has a distance to cover the range that is less than half the range, it will consider vehicles between its point and half of the range. Otherwise, it will look for the vehicles between the maximum range and its point. Algorithm 2 shows the steps to determine the accident probable zone. Equation (1) defines Impact_{i}(t) of a vehicle at an instant t.$$Impac{t}_{i}\left(t\right)=\frac{Number\text{}of\text{}vehicles\text{}behind\text{}the\text{}vehicl{e}_{i}\text{}at\text{}instant\text{}t}{Total\text{}number\text{}of\text{}vehicles\text{}in\text{}the\text{}platoon}$$ - AD
_{i}(t): The average relative distance of nodes with respect to node_{i}at instant t. The average relative distance of the vehicles with respect to the vehicle involved in the accident in the probable area of accident occurrence is considered. This can be represented as Equation (2). The nodes with a less than average relative distance will be more highly affected. The lesser the average relative distance, the higher the chance to have a forward collision.$$A{D}_{i}\left(t\right)=\frac{1}{\left(N-1\right)}{\displaystyle \sum}_{i=1}^{N}{D}_{ij}\left(t\right)$$ - AV(t): The average velocity of the platoon at instant t. This can be represented as Equation (3). The platoon with a high average velocity will be affected more.$$AV\left(t\right)=\frac{1}{N}{\displaystyle \sum}_{i=1}^{N}{V}_{i}\left(t\right)\text{}$$

Algorithm 2: Priority or an event message at a node for an instant t |

1: Read input: Time (t), Number or nodes (N(t)), position(x(t), y(t)). Type of traffic (e(t)), velocity (v(t)) |

2: Calculate $Impac{t}_{i}\left(t\right)$ // as shown in Equation (1) |

3: Calculate $A{D}_{i}\left(t\right)$ // as shown in Equation (2) |

4: Calculate $AV\left(t\right)$ // as shown in Equation (3) |

5: procedure PRIORITY(Impact_{i}(t),AD_{i}(t),AV(t)) |

6: Calculate Priority_{i}(t) // as shown in Equation (4) |

#### 3.4. Priority Mapping to Contention Window

_{max}[i] and CW

_{min}[i] is taken as (3) and (7), respectively. The access parameter contention window of the voice is taken for the calculation of the contention window for emergency messages.

_{min}as (3) and CW

_{max}as (7) is taken as access parameters for non-emergency messages in the simulation.

Algorithm 3: This procedure calculates the Back-off value for a node |

1: procedure BACK·OFF(traffic_{i}(t), Priority_{i}(t), CW_{max}, CW_{min}) |

2: while traffic_{i}(t) == emergency do |

3: Calculate CW_{newmax}(t) // as shown in Equation (5) |

4: Back-off_{i}(t) = Random([0,CW_{newmax}(t)]) |

5: end while |

6: while traffic_{i}(t) == non-emergency do |

7: Back-off_{i}(t) = Random([CW_{min},CW_{max}]) |

8: end while |

9: end procedure |

## 4. Mathematical Modeling

_{b}represents the chance of the medium to be busy and P

_{f}= (1 − P

_{b}) represents the likelihood of the medium to be free. The values of P

_{b}and P

_{f}are defined in terms of CW, which is randomly chosen from the interval of 0 to (W−1). Vacant state of the buffer is represented as state E. After the transmission of a message, the packet will enter an empty state, so the transition probability to move from state 0 to state E is 1. The transition probabilities as per Figure 8 are as follows:

**b**(

**t**) represent the stochastic processes for the back-off timer, and ${\mathit{b}}_{\mathit{j}}=\underset{\mathit{t}\to \infty}{\mathbf{lim}}\mathit{P}\left\{\mathit{b}\left(\mathit{t}\right)=\mathit{j},\text{}\mathit{j}\in \left[\mathbf{0},\mathit{W}-\mathbf{1}\right]\right\}$ can have the probability values of CW being k.

**W**).

**W**is mapped to the priority of the node at an instant. Priority is calculated as a function of Impact

_{i}(t). The average relative distance of nodes is with respect to node

_{i}at instant t and Average velocity of the platoon at instant t. Considering the range as 100 m and the maximum velocity of the vehicle as 100 Km/h, the maximum possible value of priority depends on the Impact

_{i}(t). Impact is measured as shown in Equation (1), where the maximum number of nodes possible behind a node is (N−1).

**n**be the cardinality of the low priority node set and

_{0}**n**be the cardinality of the high priority node set. The collision probability of the beacon message is studied by analyzing the events that happen in time slots. In a time slot, the following events can happen:

_{1}- The probability of the medium to be idle, i.e., this condition happens when no node tries to occupy the channel.$${\mathit{P}}_{\mathit{f}}={\left(\mathbf{1}-{\mathit{\tau}}_{\mathbf{0}}\right)}^{{\mathit{n}}_{\mathbf{0}}}{\left(\mathbf{1}-{\mathit{\tau}}_{\mathbf{1}}\right)}^{{\mathit{n}}_{\mathbf{1}}}$$
- Probability of the channel to be busy, i.e., this condition happens when at least one node in the network tries to occupy the channel.$${\mathit{P}}_{\mathit{b}}=\mathbf{1}-[{\left(\mathbf{1}-{\mathit{\tau}}_{\mathbf{0}}\right)}^{{\mathit{n}}_{\mathbf{0}}}{\left(\mathbf{1}-{\mathit{\tau}}_{\mathbf{1}}\right)}^{{\mathit{n}}_{\mathbf{1}}}]$$
- Probability of success, i.e., this condition happens when only one node selects the channel for transmission.$${\mathit{P}}_{\mathit{s}}={\mathit{n}}_{\mathbf{0}}{\mathit{\tau}}_{\mathbf{0}}{\left(\mathbf{1}-{\mathit{\tau}}_{\mathbf{0}}\right)}^{{\mathit{n}}_{\mathbf{0}}-\mathbf{1}}{\left(\mathbf{1}-{\mathit{\tau}}_{\mathbf{1}}\right)}^{{\mathit{n}}_{\mathbf{1}}}+{\mathit{n}}_{\mathbf{1}}{\mathit{\tau}}_{\mathbf{1}}{\left(\mathbf{1}-{\mathit{\tau}}_{\mathbf{1}}\right)}^{{\mathit{n}}_{\mathbf{1}}-\mathbf{1}}{\left(\mathbf{1}-{\mathit{\tau}}_{\mathbf{0}}\right)}^{{\mathit{n}}_{\mathbf{0}}}$$

**E**[

**L**] is the average length of the packet. Delay is calculated as the sum of time for back-off and transmission time.

## 5. Results and Discussion

#### 5.1. Simulation Environment

#### 5.2. Communication Configuration

_{h}) 50 bytes and the data rate is taken as 6 Mbps. In CSMA/CA protocol, the nodes sense the channel for a distributed inter frame space (DIFS) period before trying to attempt channel access. The time for sensing and transmission is divided into slots. Each slot is 13 µs. The value of DIFS is taken as 64 µs, as in IEEE 802.11p. To compare the performance of the proposed protocol and to demonstrate the need for considering the severity of data generation for improving the performance of IEEE 802.11p for forward collision warning, the following protocols with similar scenarios are studied.

- IEEE 802.11p: Simulations are performed to compare the performance of the fixed contention window with respect to the dynamic contention window for broadcast messages.
- R-MAC [27] is an adaptive MAC protocol for cooperative collision avoidance applications that consider the probability of an accident to happen. Time frame of R-MAC has three segments: the RSU broadcast, the TDMA for beacons, and the CSMA part for transmitting event messages. The number of CSMA slots depends on the probability of vehicle collisions that can happen in a platoon.
- Priority-based direction-aware MAC (PDMAC) [26] protocol is a cluster based V2V MAC with the objective of prioritizing warning message delivery in VANET. Protocol works with the TDMA approach considering the severity level, direction, and message type to prioritize the warning message. The scheme decreases the end-to-end delays and increases the network throughput.
- The following parameters are used to study the performance of the protocol.
- Delivery delay of emergency packet is the average time taken from the generation of a packet to its delivery.
- Packet delivery ratio (PDR) is defined as a fraction of the number of packets transmitted by the total number of packets generated during simulation.
- The relevance of location and velocity with respect to the event is studied with the parameter of how many nodes are affected if an accident occurs at a node. This parameter in turn shows the risk of having an accident in the area. It talks about the importance of making the message available to the nodes behind the vehicle in the accident.

#### 5.3. Simulation Results

_{i}(t), average relative distance of nodes with respect to node

_{i}at instant t, and average velocity of the platoon at instant t. Impact

_{i}(t) is defined as a factor of the number of nodes behind the node involved in the accident and the node density in the coverage. This gives an importance to the location of accident. Therefore, the importance of location is measured in terms of vehicles that follow the vehicles behind the node in the accident. The accident at a certain vehicle must be transmitted to the vehicles behind without much delay so as to avoid successive collisions. By receiving the message, the drivers can control the vehicle, avoiding successive collision. The priority of the packet from the node depends on the number of trailing vehicles of the node in the accident, the node’s relative velocity with respect to other nodes, and the average velocity of the platoon. This value is mapped to the contention window. The result displays that, as the number of vehicles increases, the risk in the probable accident zone also increases. The location of the accident zone in an RSU can be represented as below.

- Location 0: Starting location of the zone to R/2 where R is the range of the zone
- Location 1: Between R/2 to the end of the zone.

## 6. Conclusions and Future Work

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Conflicts of Interest

## References

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**Figure 3.**Synchronization interval [5].

**Table 1.**Parameter value settings for access categories in IEEE 802.11p [4].

Access Category | VO | VI | BE | BK |
---|---|---|---|---|

AIFSN | 2 | 3 | 6 | 9 |

CW_{min} | 3 | 7 | 15 | 15 |

CW_{max} | 7 | 15 | 1023 | 1023 |

Features | Method | Purpose | Disadvantages |
---|---|---|---|

Based on network statistics and density | [13,14,15,16,17,18,28] Adjust the contention window based on either the node density, transmission power or transmission probability. | Improves throughput, average packet delivery delay. | The mobility of the nodes is not taken into account. The nodes may leave the range if it has a higher velocity. |

Based on mobility of nodes | [20,21,22,25] Adjust contention window based on the impact of velocity of the node. | Fair channel access, QoS. | It considers the velocity of the node, but the node density is another parameter that has a great impact on the quality of service of an application. |

Based on the severity level of the message | [26,27,29,30,31] Nodes calculate priority of the message based on the severity of the message. The severity of the message is calculated based on the direction, location, access type, etc. | Improving QoS considering co-operative collision warning applications. | These schemes follow a hybrid approach, including both TDMA and CSMA schemes, to provide importance for both beacon and emergency schemes. Therefore, there is a need for time synchronization. A method for time synchronization is shown in [28]. The schemes follow a clustered approach. |

Notations | Description |
---|---|

T | Every instant of time |

v_{i}(t) | Vehicle speed of node_{i} at instant t |

e_{i}(t) | Access category of node_{i} at instant t |

i,j, etc. | ID |

(x_{r},y_{r}) | Range coordinates |

(x_{s}(t),y_{s}(t)) | Position coordinates of vehicle in accident |

d_{s}(t) | Distance to cover the range from the accident point |

R_{i} | Transmission range of node_{i} |

N | Number of nodes in the range |

n_{0} | Cardinality of the set of low priority nodes (non-emergency) |

n_{1} | Cardinality of the set of high priority nodes (emergency) |

P_{f} | Probability of channel to be free |

P_{b} | Probability of channel to be busy |

P_{s} | Probability of transmission to be successful |

P_{c} | Probability of collision |

Impact_{i}(t) | Importance of access category at node_{i} in the region at tth instant |

AD_{i}(t) | Average relative distance of nodes to node_{i} at instant t |

AV_{i}(t) | Average relative velocity of nodes to node_{i} at instant t |

$\overline{V}$ | Average velocity of the platoon |

Priority(t) | Priority of nodei at instant t |

T_{transmit} | Time for transmitting a packet |

CW_{newmax}(t) | Upper limit of contention window for nodei at instant t. This is mapped to slots for defining a backoff value for node_{i} at instant t. |

Backoff_{i}(t) | Backoff value taken by nodei for communication at instant t. This is a random value between 0 and the maximum slots defined by UpBound_{i}(t) of node_{i} at instant t |

CW_{max}[i] | Maximum contention window of the access category i |

CW_{min}[i] | Minimum contention window of the access category i |

Parameter | Values |
---|---|

Number of vehicles | 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 |

Speed range | 10–100 km/h |

simulation time | 1000 s |

Communication range | 100 m |

Emergency message size (L_{p}) | 100 bytes |

Non-emergency size | 100 bytes |

MAC header (L_{h}) | 50 bytes |

DIFS | 64 µs |

Slot value | 13 µs |

Data rate (data_rate) | 6 Mbps |

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**MDPI and ACS Style**

C, T.; M G, J.
An Enhancement for IEEE 802.11p to Provision Quality of Service with Context Aware Channel Access for the Forward Collision Avoidance Application in Vehicular Ad Hoc Network. *Sensors* **2021**, *21*, 6937.
https://doi.org/10.3390/s21206937

**AMA Style**

C T, M G J.
An Enhancement for IEEE 802.11p to Provision Quality of Service with Context Aware Channel Access for the Forward Collision Avoidance Application in Vehicular Ad Hoc Network. *Sensors*. 2021; 21(20):6937.
https://doi.org/10.3390/s21206937

**Chicago/Turabian Style**

C, Tripti, and Jibukumar M G.
2021. "An Enhancement for IEEE 802.11p to Provision Quality of Service with Context Aware Channel Access for the Forward Collision Avoidance Application in Vehicular Ad Hoc Network" *Sensors* 21, no. 20: 6937.
https://doi.org/10.3390/s21206937