A New Adapted Back-off Scheme for Broadcasting on IEEE 1609.4 Control Channel in VANET

A. AHMAD1, 2, M.DOUGHAN2, I. MOUGHARBEL2, and M. MAROT1 1 SAMOVAR

Institut Telecom, Telecom Sud Paris Evry, France

e-mail: {Abdel_mehsen.Ahmad, Michel.Marot} @telecom-sudparis.eu

2RITCH Lebanese University

Beirut, Lebanon e-mail: {imadmoug,mdoughan}@ul.edu.lb

Abstract—In this paper, we propose a novel MAC scheme for broadcasting in IEEE 802.11p/WAVE standard during the CCH periods. We address two problems which lead to the same undesired result: the degradation in control channel performance. The first problem is related to the congestion control and the challenge is viewed in the random backoff mechanism to be adapted for broadcasting messages. It has been evident that the exponential back-off is not efficient in broadcast and where there are beacons. The problem becomes more challenging when these beacons have an expiry time and they are dropped once this time is reached before transmission. When the vehicle density is high, the probability to have two or more vehicles having the same random back-off increases and results in an increase of collision probability. One solution is in incrementing the Contention Window (CW) but if the CW is increased, the waiting time before transmitting increases and it results in the rise of the packet drop rate due to expiry time. The absence of acknowledgement for broadcast messages makes the CW adaptation very difficult. Our proposed scheme addresses this problem.

This paper also covers the issue of channel congestion phenomenon following a channel switch (from SCH to CCH) mentioned in Annex B of the IEEE Std 1609.4-2010. Just after a channel switch, a large number of WAVE devices are found in a period of above-average channel congestion resulting in an unexpectedly high collision rate. Our scheme presents a MAC layer approach used by the devices to avoid the “start-of-interval contention”, especially in a high vehicle density.

Our simulation results show that using a modified backoff procedure after spreading the density of transmissions on the CCH interval is an effective solution to decrease the collision probability and increases the packet reception rate. The spread is based on geographical positions. Moreover, the modified backoff, instead of freezing the counter when the channel is occupied, it still decreases it.

Keywords-component; CCH; Back-off; VANET; Broadcast; Collision; IEEE 802.11p/1609.4;

I. INTRODUCTION The IEEE 1609.4 standard is the multichannel specification

for the IEEE 802.11p/WAVE vehicular network (VANETs) systems. It uses seven channels, one of which being a control channel (CCH) that devices will tune to in a periodic manner, and the other six channels are used as service channels (SCHs). It also defines a division of time into alternating CCH intervals and SCH intervals. The synchronization between both intervals supposes that all devices have access to the Universal Coordinated Time (UTC), e.g., from a GPS signal. The CCH is used primarily for two types of messages: (i) Safety messages from one vehicle to another, and (ii) WAVE

service advertisements (WSAs) used to announce the availability of one or more WAVE services on the SCHs during the next SCH interval.

One of the challenging problems in this kind of systems is the dissemination of the broadcast messages either for safety or signaling issues. There are two reasons. On the first hand, a channel congestion phenomenon follows a channel switch: as explained in [1], “A WAVE device might queue an MPDU at a time when the intended channel is not available. For example, an MPDU intended to be sent on the CCH during the CCH interval might be placed in its queue during the SCH interval. In an even worse scenario, MAC-layer queues can build up in the device if there are multiple MPDUs to be sent. Subclause 5.2 states that an MPDU that is at the head-of-line of a queue at the end of a guard interval will undergo a random back-off before attempting to access the channel. This helps avoid, but does not prevent, collisions between MPDUs in different devices. In particular, if two such MPDUs in neighboring devices choose the same back-off length they are likely to collide. If a large number of neighboring devices have packets queued at the end of a guard interval, the probability of collisions can be much higher than normal; in the case where there are multiple MPDUs in the MAC-layer queue, the duration of the packet collision phenomenon is even longer. These collisions can lead to a serious degradation in application performance”.

On the other hand, when the vehicle density is high, the probability to have two or more vehicles having the same random back-off increases and then the same for the collision probability. Normally, incrementing the Contention Window (CW) results in decreasing the collision probability. But if CW increases, the waiting time before transmitting increases and it results in the rise of the packet drop rate due to expiry time. The absence of Acks for broadcast messages makes the CW adaptation very difficult. The scheme proposed in the IEEE 802.11 MAC (Random backoff time and Distributed coordination function access procedure) to minimize collisions during such periods of high load, is not effective for broadcast MPDUs as it is for unicast. This is due to the fact that the unicast adaptation mechanism relies on acknowledgments which are not allowed for broadcast messages. “The

2012 The 11th Annual Mediterranean Ad Hoc Networking Workshop (Med-Hoc-Net)

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transparency of MPDUs collisions to the applications that use their payloads, due to the lack of error recovery mechanisms, will impact application performance with a greater likelihood” (cf. [1]).

When the message generation rate is high, the number of messages waiting in the queue increases resulting in an increase in the collision probability, especially if the density of devices that are switching for the same CCH interval is large. In this paper, we present an approach helping devices to avoid the “start-of-interval contention”, especially in a high vehicle density. Our scheme spreads the starting of transmission times between vehicles to give them the opportunity to broadcast their messages with a less pressure and a wide free-air during a fixed time while avoiding the expiry time suppression. We also show that the current backoff procedure used is not suitable for broadcast messages on the CCH. Finally we show that using a minimum initial CW in a backoff procedure a little bit modified and after minimizing the density of transmissions on the CCH interval with our spreading of transmission times over the whole CCH interval is an effective solution to decrease the probability of collision and increase the packet reception rate.

In the next section, the related work is reviewed. The third section presents our approach, the performance of which is evaluated in the fourth section. The last section concludes this paper.

II. RELATED WORK A common assumption, like in [3], is to assume that, at the

beginning of every CCH interval, each vehicle has a packet (beacon or WSA) ready to be transmitted and the expiry time of beacon and WSA frames is bounded by one CCH interval. After this interval, non transmitted packets are dropped. In general, a packet transmission could fail to: (i) collisions with other nodes, which have the same back-off counters; or (ii) outdated dropping packets due to a delayed back-off counter that is still not zero at the CCH interval end.

The question of finding the right value for the initial CW of the backoff is discussed in the literature. It appears that it depends of the type of traffic: for unicast traffic the goal is to maximize the throughput while it is mainly to minimize the collision probability for broadcast traffic. Increasing the initial CW may be good for unicast traffic while being bad for broadcast. The authors in [4] concludes that “the initial CW value of 15 proposed in the IEEE 802.11p standard performs significantly well under almost all studied circumstances”. Now, most of the papers dealing with backoff issues in VANETs mention that the CWmin proposed in the current version of the standard (between 5 and 15) is far from optimal [2,3]. Researchers try to find a dynamic way to adapt this value. In [2], the authors focus on traffic safety applications which rely on delivering a non big amount of data to extend the knowledge of the driver has about the vehicular environment. To limit the data losses due to the expiry time, they propose to decrease the initial contention window after each dropped packet due to expiry time.

Some other papers (e.g. [6, 7]) try to adjust the CWmin in function of the vehicle density by assuming that the estimation of the duration of the collision periods can be somehow done. In [11], the authors adapt the back-off time as a function of the vehicle density, which is estimated using a function calculating the distance between vehicles in a given transmission range. Like mentioned in [2], this assumption is very hard for broadcast messages because there is no efficient real-time method to separate between the collision and the radio propagation error (there is no acknowledgement for broadcast traffic). Estimating the vehicle density is also used in [12], but the authors propose to adapt the carrier sensing threshold for increasing the reception probability. When the density is low (resp. high), the transmitter selects a low (resp. high) threshold by which a large (resp. few) number of nodes are listened and therefore the probability to listen the channel occupied is high (resp. low). This would result a slow (resp. rapid) decrease of the back-off counter. This mechanism results in adapting the back-off time depending on the vehicle density. Relying on the vehicle density, regardless of its estimation accuracy, does not solve totally the problem in the broadcasting scheme of VANETs since even for medium density there may be an important data load.

The important part in IEEE 1609.4 is the channel switching scheme. However, one of the negative attributes of this scheme is the bandwidth wasted. Having two intervals one for CCH and the other for SCH results in reducing the CCH capacity to the half. All the safety communications should be transmitted during the CCH interval. In the standard, the sync interval is defined as 100 ms. A guard interval of 4 ms is located at the beginning of each channel interval, so each of these lasts becomes 46 ms. Thus, the load may reach often the capacity. For example, with a 6Mbps link, a 46ms CCH interval duration and a 400 byte message length, only 86 messages can be sent, which corresponds to 43 vehicles per lane on a two lane road if each one is willing to send a message in the CCH, that is 43 veh/lane/km (the communication range for each vehicle can reach 1000m). However, the average vehicle density is between 25 and 31 veh./km/lane while the jam density is usually more than 115 veh./km/lane (cf. [13]). Of course the message frequency must be considered but some applications require very high message density and in case of emergency situations like accidents burst of messages occur while the jams appear.

This capacity limitation makes more important to use a more efficient scheme to minimize the probability of collisions between vehicles.

III. OUR PROPOSED SCHEME

A. Sowing and Sensing Scheme for Broadcasting in VANETs 1) Approach using the standard backoff mechanism

We propose a two-pronged scheme. Firstly the nodes, before the beginning of each CCH interval, are timely sowed (disseminated) over the CCH interval. Each node is equipped with a sowing calendar containing a mapping between geographic sectors and the corresponding slot numbers over

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the CCH interval (Fig. 1). The method for generating the sowing calendar is described in the next section. Note here that this scheme does not allocate slots for vehicles, but it only spreads the transmission starting times of nodes. It helps solving the problem of congestion phenomenon following a channel switch and helps devices to avoid “start-of-interval contention”. In the random backoff procedure, vehicles can have the same selected counters. In contrast, our scheme tends to fully minimize the probability to have two or more vehicles selecting the same back-off counter. Its goal is to minimize the probability of collisions that can be occurred during the CCH interval.

Figure 1. Nodes are sowed randomly using the prefixed sowing calendar.

During the CCH interval, each node tries to transmit its packet at its proper slot number. In this phase, the back-off procedure is used to minimize the collisions number between several broadcast messages. Each vehicle, before transmitting its message, invokes the backoff procedure with the minimal contention window (CWmin). The vehicle starts a backoff timer which is drawn from a uniform distribution over the interval [0,CWmin]. This counter is decremented only if the channel was idle for one slot, otherwise, the counter is frozen. When the counter reaches zero, the vehicle broadcasts its message. This phase aims to more reduce the probability of collision in the case where two or more vehicles have the same sector number and therefore they have to wait for the same time. The minimal CW is used to maximize the successful packet transmission rate and at the same time decrease the packet drop rate due to expiry time. The size of CWmin in slots (between 5 and 15) is static because there is no feedback from broadcast messages to indicate if a failing transmission was occurred and therefore changing the value of CW.

Note that this sensing phase is very important in the way that it minimizes the collision probability in the case where two or more vehicles can be located in the same geographic sector. That is caused by a large sector size in which many vehicles can be located, or also by the GPS position errors that can make two or more vehicles being in the same sector.

2) Approach with a new backoff procedure However, the standard backoff procedure, that obliges the

node to freeze the counter if the channel is busy, results in a

wasted time that is not used by this vehicle to broadcast its message. This effect is very intensive in the CCH which already, as seen before, has a problem with saturation even in normal road traffic. To deal with this problem, we propose a simple scheme to adapt the backoff procedure. Each vehicle starts a counter uniformly drawn from the interval [0, CWmin]. This counter is decremented after each slot, even it is idle or not. When it reaches zero, the vehicle listens for one slot time and if the channel is idle, then it can transmit its packet. Otherwise, the vehicle reruns the same procedure and starts a new counter time.

Figure 2. Standard and Proposed back-off procedures

This effect can be shown in the Fig. 2. The node that uses the standard backoff has to wait a long duration (Ta) before it can transmit its packet. This long duration is no more justified if the packet transmission times are quite well spread over the whole CCH interval. The inter-packets time due to the backoff counter is considered to be a lost time and results in an increase of the contention between the non-transmitted packets. This time is minimized in our proposed scheme. During the time Tb, the node may have starting several counter times. Finally, it can immediately transmit its packet when it listens that the channel is idle after one slot. In short, in our scheme, the vehicle has to walk during the CCH interval with small and convergent steps to arrive at a free slot where it can broadcast its message.

3) Sowing calendar Each vehicle is equipped with position acquisition devices,

such as GPS and can acquire real-time position information. The sow calendar is defined as the function that assigns each sector a unique time slot. This mapping function reuse time slot assignments subject to the constraint that no two sectors within a predefined range of each other be mapped to the same time slot. This range is equal to the sum of two transmission range to face against the hidden terminal problem.

The surface, like in [15], is discretized into N numbered sectors, each one of them containing at most one vehicle. N slot numbers are mapped to these sectors one-by-one. The sowing calendar, which is the way of disseminating the slots numbers over the sectors, is important to make a fair geographic distribution. A simple method is to generate N random sequence of numbers between 1 and N and to map each number to a sector (cf. Fig. 3).

Each geographic sector has its own slot number. Before the beginning of the CCH interval, each node calculates its

CCH interval (time)

Each node selects its starting slot regarding to its geographic position

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geographic position using the GPS data. Possessing the sowing calendar, it can find out in which sector it is on and therefore find out in which slot time it can begin its transmission.

Figure 3. An example of a random mapping of transmission slot numbers to

geographic sector numbers.

With an accurate GPS receiver, each vehicle receives a calculated geographic position which is different from its neighbors. However, accurate GPS are expensive and it is necessary to deal with the GPS errors. In the next section we take into account the effect of position errors caused by the GPS receivers. B. GPS Errors

In VANET, to assure the localization, each vehicle is equipped easily with a GPS receiver. However, GPS receivers display some undesirable problems such as not always being available and not being robust enough for some applications. There are some factors that affect the quality of the GPS signal and cause calculation errors [9]:

• Ionosphere and troposphere disturbances; • Signal reflection; • Ephemeris errors; • Clock errors; • Visibility of Satellites; • Satellite Shading; • Intentional degradation.

Also, typical GPS receivers have a localization error of ±10m (cf. [16]). One positive aspect of these errors is that they are correlated by the fact that nearby GPS receivers tend to have the same localization error oriented in the same direction (cf. [9], [14]).

Generating GPS Error Vectors (Ei)

As in [10,14], the correlation between two vectors is expressed using a standardized form of covariance between

iE and jE : the correlation coefficients ( ijρ ). While constructing eC , we assume that the same all the correlation coefficients are equal for all vehicle pairs:

1 ij

i ji j

ρρ

≠⎧= ⎨ =⎩

Once the correlation coefficients are determined, the covariance matrix eC for a given standard deviation σ can be constructed as:

( )2 2

2 2 1eCσ ρσρσ σ

⎛ ⎞⎜ ⎟= ⎜ ⎟⎜ ⎟⎝ ⎠

The iE vectors satisfying eC given in equation 1 can be generated as follows:

1) For K vehicles, generate K independent vectors from the normal distribution ( )0,1N and insert them to rows of

matrix 1 2[ ... ]TkX X X X= . Since these vectors are generated

independently, the correlation coefficient between them is 0. Moreover, if the vectors are long enough, their variances become approximately equal to 1. As a result, the covariance matrix xC between the rows of X becomes:

1 0 00 1 00 0 1xC I

⎛ ⎞⎜ ⎟⎜ ⎟= =⎜ ⎟⎜ ⎟⎝ ⎠

2) In multivariate normal random variables, the linear transformation of original random variables produce a new set of variables whose covariance matrix 'C becomes

' TxC LC L= where L is the linear transformation matrix in

operation E LX= . We use this property of multivariate normal random variables to generate a set of error vectors. Since the covariance matrix xC in step 1 is I , the result of linear transformation gives.

( )' 2T TC LIL LL= =

3) Using equation 2, we can find the appropriate transformation matrix L which transforms xC to eC

( )TeC LL= . Note that eC is our target covariance matrix and

it can be decomposed into a lower ( L ) and upper triangular (TL ) matrices using Cholesky’s factorization if it is positive

semi-definite.

4) Once the linear transformation matrix L is obtained in step 3, multiply matrix X with L to obtain the correlated GPS error vectors ( iE ).

1 11 1 12 2 13 3

2 21 1 22 2 23 3

3 31 1 32 2 33 3

E L X L X L XE L X L X L XE L X L X L X

+ +⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟+ +⎜ ⎟ ⎜ ⎟=⎜ ⎟ ⎜ ⎟+ +⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

We assume that in the general case a vehicle in sector (0,0) can receive an erroneous GPS data which locates it in its real sector or in one of the four neighbor sectors (see Fig. 4). Fig. 5 shows the correlation effect of three error vectors, by plotting each vector with respect of the two others vector, in three- dimensional mode. It is obvious that if these vectors are not correlated, the results of this figure will be independent scattered points.

IV. SIMULATION RESULTS For the performance evaluation, we compare three

mechanisms: the standard backoff, our scheme where an initial slot is chosen during the CCH based on the geographical

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position together with our new backoff mechanism proposed section III.2 and a third scheme where the initial slot is no more chosen based on the geographical position but uniformly on the whole CCH together with our new backoff mechanism. All the simulations are implemented using MATLAB. The MAC parameters are set according to the IEEE 802.11p standard, and the simulation parameters are shown in Table II.

Figure 4. Fifteen neighborhood sectors in 3-lane road. The dotted zones represent the possible calculated sectors for the (0,0) sector.

TABLE I. Example of possible interfering sectors (for sector N°0)

Sector Possible Interfered

Sectors with (0,0)

(0,0) (0,0) (0,1) (0,-1) (-1,0) (1,0)

(-1,1) (0,1) (-1,0)

(-1,0) (0,0) (-1,0)

(-1,-1) (0,-1) (-1,0)

(-2,0) (-1,0)

(0,1) (0,1) (0,0)

(0,-1) (0,-1) (0,0)

(1,1) (0,1) (1,0)

(1,0) (0,0) (1,0)

(1,-1) (0,-1) (1,0)

(2,0) (1,0)

To show the performance of our scheme, we first studied the effect of GPS errors. It appears that, even by taking into account the GPS errors, the collision probability between the slots at which the backoff starts, is less in the case of our geographical position based scheme, than in the case of a uniformly drawn slot selection. To study the effect of GPS errors, we generate error correlated vectors. Then we calculate the probability to have one or more vehicles from different sectors having the same corresponding sector (calculated using GPS and the sowing calendar) with the vehicle of sector (0, 0) (cf. TABLE I).

The first thing we study is the effect of having each vehicle selecting its slot at the beginning of the CCH interval. It is evident that this additional step would decrease the pressure on the first slot of CCH. However, the way to assign each slot is an important issue. We compare our approach with the one assigning a random slot number, independent of the geographical location. In both approaches, the selected slots are between 0 and S < Smax which is the maximum number of slots presented in a CCH interval. As can be seen in Fig. 6, the coincidence probability between vehicles, when using the

geographic position, is much better than the one achieved by the random scheme.

This is expected since in the random scheme one or more vehicles can select the same slot number. This collision can be also observed in our scheme but only between the neighbors, but with a very weak effect. It is due to GPS errors. This weak effect is due to the fact that GPS’s error vectors are correlated. Moreover, it is evident that when the GPS accuracy is high, the probability of interference between vehicles decreases.

Figure 5. Three correlated error vectors

TABLE II SIMULATION PARAMETERS

Road type Highway

Number of lanes/road 3

Sector size 10mx5m

Standard deviation 15

Correlation coefficient (ρ) 0.95

Position error [-10m ,10m]

Transmission range up to 1000 m

Figure 6. Coincidence ratio vs. number of vehicles

The next step is to apply the whole scheme in a vehicular environment with different traffic densities. Our goal is to study the effect of our approach on the back-off procedure. We compare the packet loss probability between the standard backoff and our new proposed backoff when using our proposed position-based (noted as ‘Geo’) transmission slot

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time affectation. We also check the performance in the case where the slot at which the backoff starts is chosen with a uniform random assignment instead of our position-based scheme.

The performance results are plotted in Fig. 7 for different values of CW with different nodes numbers. Let us recall that in the case of broadcast there is no feedback and, in this case, the contention window is not changed and always equal to CWmin. As seen in section II, most of the recent works show that the CWmin used by the standard is not optimal. As shown by this figure, our scheme outperforms the standard and also the mechanism where the starting of the backoff is assigned uniformly on the whole CCH. Moreover, in our case, the minimal contention window used as default value for beaconing is the most adequate value. As mentioned before, increasing the value of CW increases the waiting delay which results in an increase in the packet loss probability due to expiry time.

Figure 7. Packet loss probability for different values of CW and

contending nodes

Figure 8. Packet reception ratio vs. contending nodes.

In Fig. 8, the performance of our mechanism in terms of packet reception ratio when varying the number of contending vehicles is presented. The optimal CW values are taken for

each of the three mechanisms. As seen figure 8, our proposed new backoff shows a better performance compared to the standard backoff.

V. CONCLUSION In this paper, we propose a new approach to ease the

pressure of the high vehicular traffic density at the CCH interval. It is mainly designed for broadcast traffic with a fixed expiry time. The nodes start their backoff at a time depending on their geographical position. We compare it with the standard and with the case where it is chosen uniformly on the whole CCH. Moreover, we propose a new backoff procedure by which a node can broadcast its packet as soon as it can after a small uniform random delay. The simulation results indicate that our approach has the highest performance in terms of channel capacity and collision probability. We also show that the standard minimal value of the contention window in the backoff mechanism is the most adequate one for broadcasting messages on CCH with expiry time. Finally, our scheme solves an important issue in the CCH capacity with simplicity and without requiring any information exchange between vehicles.

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[5] IEEE Std 802.11™-2007 (Revision of IEEE Std 802.11-1999). [6] D. Deng, H. Chen, H. Chao, Y. Huang. “A Collision Alleviation Scheme

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[8] John B. Kenney. “Standards and Regulations”, chap. 10, H. Hartenstein, K. P Laberteaux, “VANET: Vehicular Applications and Inter-Networking Technologies”, JohnWiley & Sons Ltd, 2010.

[9] S.OLARIU, M.WEIGLE. “Vehicular Networks from theory to practice”, Taylor & Francis Group, LLC,2009.

[10] Martin Haugh. “The Monte Carlo Framework, Examples from Finance and Generating Correlated Random Variables”, Monte Carlo Simulation: IEOR E4703 c 2010.

[11] A. Barbosa and al. “An adaptive Mechanism for Access Control in VANETs”, ICN 2011.

[12] R. Stanica, E. Chaput, A.-L. Beylot. “Physical Carrier Sense in Vehicular Ad-Hoc Networks” , in Eighth IEEE International Conference on Mobile Ad-Hoc and Sensor Systems, 2011.

[13] Transportation Research Board. Highway Capacity Manual 2000. National Research Council, Washington, D.C.

[14] Gokhan Korkmaz and Eylem Ekici. "Effects of Location Uncertainty on Position-Based Broadcast Protocols in Inter-Vehicle Communication Systems," Proceedings of the Fifth Annual Mediterranean Ad Hoc Networking Workshop (Med-Hoc-Net 2006), Lipary, Italy, June 2006.

[15] Jeremy J. Blum and Azim Eskandarian. “A reliable Link-Layer Protocol for robust and scalable intervehicle communications”, IEEE

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[16] Data from the GPS Operations Center. https://gps.afspc.af.mil/gpsoc/PerformanceReports.aspx

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