CN104144518A - Self-adaption access method for car networking control channel on basis of time delay optimization - Google Patents

Abstract

The invention provides a self-adaption access method for a car networking control channel (CCH) on basis of time delay optimization, and belongs to the technical field of car networking communication. Access delays of the control channel on the basis of car networking time slot alternation comprises freeze time delays in service channel intervals (SCHIs) and retreat time delays in control channel intervals (CCHIs). In the previous SCHI of a current time slot, the number N of requests for having access to the CCH is counted, and in alternate guard intervals of the time slot, contention windows of access requests in the SCHIs and retreat values are set according to the number N; according to an earliest deadline first (EDF) mechanism, access requests with the shortest remaining valid time are selected and accessed; contention windows which continue to retreat the access requests and retreat values are set according to the remaining valid time of the access requests, and an access contention of the next round continues to be carried out. The access contention of the control channel can ensure that access requests of all priorities have access to the control channel in time, and access performance of 820.11p is improved.

Description

A kind of car networking control channel self-adapting cut-in method based on time delay optimization

Technical field

The invention belongs to car connected network communication technical field, specifically, refer to a kind of car networking control channel self-adapting cut-in method based on time delay optimization.

Background technology

The control channel (CCH) of car networking mainly transmits information that traffic safety is relevant or the control information of car networking.During car networking intercommunication, need to avoid phase mutual interference, when traffic congestion this interference more serious, therefore need effective access strategy.And the information of transmitting in CCH is and the closely-related delay sensitive business of traffic safety therefore the access delay of access strategy to be had to strict demand.In existing car networking, the access strategy of CCH has adopted the enhanced distributed channel access (Enhanced Distributed Channel Access is called for short EDCA) of IEEE 802.11e.The self adaptation of contention window size and the division of service priority are mainly paid close attention in correlative study, so that EDCA adapts to car networking better.

But do not consider the alternately impact on time delay of time slot under car networking multi-channel operation at existing access strategy.In addition, existing research is mainly paid close attention to cycle information as beacon message etc., and in real-time applied business in CCH, as bend, accelerate Event triggered information or the emergence messages such as warning, emergency brake indication, anti-collision warning, lane change prompting, be all information aperiodic that can file a request at any time.

In the research of existing relevant access strategy, do not consider business aperiodic of Event triggered, under time slot replaces, dock the impact into delay performance, as: 1. in same time slot, the access delay difference of access request is very not large in the same time; 2. access strategy does not consider that low priority access consumes under effective time delay and becomes the short urgent access of residual time delay at SCHI (Service Channel Interval, business time-slot), should have the demand of high priority access order; 3.CCHI (Control Channel Interval, control slot) starts to have the access request accumulating in SCHI, causes accessing the increase access delay that blocks up.

Summary of the invention

The object of the invention is to cannot guarantee problem for control channel access delay under car networking multi-channel operation, a kind of car networking control channel self-adapting cut-in method based on time delay optimization is disclosed, under the prerequisite replacing at time slot of the method, the derived expression formula of control channel access delay, by utilization, remain effective time delay and control the home window that accesses in access, self-adapted service time slot, preferential overdue conflict processing method the earliest, improve the access delay performance of control channel.

First, the access delay that the invention provides the alternately lower control channel of a kind of car networking time slot is determined method, access delay T _{d, i}comprise and freeze time delay T in SCHI _{f, i}with in CCHI, keep out of the way time delay T _{b, i}, be expressed as: T _{d, i}=T _{f, i}+ T _{b, i}.

Access delay based on provided is determined method, a kind of car networking control channel self-adapting cut-in method based on time delay optimization provided by the invention, carries out following steps 1～step 6 at n time slot, after n time slot finishes, execution step 7, specifically each step is as follows:

Step 1: in the last business time-slot SCHI of n time slot, the number N of statistics access control channel CCH request;

Step 2: being replaced in the protection interval of control slot CCHI by SCHI, determine the initial value of the competition window of access request in SCHI, and the backoff value of each access request;

Wherein, the initial value CW of the competition window of access request _initial=τ N, τ is the factor of influence of SCHI access request number N to initial window size, by emulation matching, obtains; The backoff value of access request is from span [0, the CW of competition window _initial] in random selection obtain.

Step 3: in CCHI, carry out the access contention based on keeping out of the way, when depositing competition conflict, execution step 4; Otherwise, execution step 5.

Step 4: to all access requests that meet access conditions, select the shortest access request residue effective time according to the override mechanism that expires the earliest, then carry out step 5 and carry out; Remaining access request is proceeded to keep out of the way, and enters step 6 and carries out.

The access request that meets access conditions comprises two kinds: the new access request that current time proposes; Current time backoff counter is zero access request.

Residue T effective time of access request _e=T _b-T _dfor real-time value, wherein, T _bfor effective time delay of access request, T _dfor the access delay that constantly starts timing from proposing access request.

Step 5: access request access CCH, if access successfully, completes the access of this access request; If access failure, continues this access request to keep out of the way, enter step 6 and carry out.

Step 6: to the access request that continues to keep out of the way, determine the residue effective time of access request, and reset competition window and the backoff value of access request, then go to step 3 execution.

If the priority level of certain access request is i, the competition window of the j time competition of this access request is CW _{i, j}, after competing unsuccessfully the j time, competition window increases as CW _{i, j+1}=min[CW _{i, j}* MF ⁿ, CW _{i, max}], min represents to get minimum value.

MF wherein ⁿfor growth factor, computing formula is as follows:

${\mathrm{MF}}^n=\mathrm{\α}\·\frac{T_e}{T_b}+(1-\mathrm{\α})\·\frac{N_{\mathrm{collision}}^{n-1}}{N_{\mathrm{data}\_\mathrm{sent}}^{n-1}}$

with be illustrated in and in n-1 time slot, clash and successfully access the number of times of channel, its ratio is the average collision rate of time slot, for judging the tight situation of current control channel; α is weight factor, for determining the influence degree of the effective time delay of residue when determining growth factor.

Priority level be i access request the j+1 time competition backoff value from [0, CW _{i, j+1}] in select at random.

Step 7: after n time slot finishes, add up and clash in n time slot and successfully access the number of times of channel, determine the average collision rate of this time slot, continue the access contention in next time slot.

Advantage of the present invention and good effect are: the present invention take that to optimize delay performance be object, set up model analysis and affected the factor of access delay, by the cut-in method of delay sensitive, make can guarantee in the access contention of control channel the timely access control channel of access request of each priority, improve the access performance of 820.11p.

Accompanying drawing explanation

Fig. 1 is the two-dimentional Markov chain model schematic diagram in an AC;

Fig. 2 is the overall flow figure of car networking control channel self-adapting cut-in method of the present invention;

Fig. 3 is optimum CW _initialthe schematic diagram changing with access request number in CCHI;

Fig. 4 is the schematic diagram that access failure rate changes with access request number;

Fig. 5 is the schematic diagram that average access delay changes with access request number.

Embodiment

Below in conjunction with drawings and Examples, the present invention is described in further detail.

Model of the present invention time-delay analysis model analysis in car networking CCHI and SCHI replace the access delay of CCH under scene, and consider the wherein feature of business real-time, a kind of car networking control channel self-adapting cut-in method based on time delay optimization has been proposed on this basis, utilize the effective time delay of residue to control the access in CCH, adopt the principle that expires the earliest to process access interference, reduced average access delay.Car networking control channel self-adapting cut-in method based on time delay optimization of the present invention, has reduced the alternately impact on access delay of time slot, and both can also can be used for for cycle information the access control of Event triggered information.

First time-delay analysis model is described.

In car networking CCH, transmission, except cycle information, can propose at any time in addition the Event triggered information of access, so in the present invention, the proposition of access request is different from the lasting access based on website in 802.11e, but is more suitable for the non-lasting access of CCH feature.Time slot alternately under, at SCHI, can not meet the request of access CCH, the backoff counter of access request is in frozen state at SCHI always, so the access delay of CCH is subject to the impact of SCHI, different from the time-delay analysis in EDCA.The present invention according to structure of time slot access delay T _{d, i}be divided into two parts: in SCHI, freeze time delay T _{f, i}with in CCHI, keep out of the way time delay T _{b, i}, be expressed as:

T _D，i＝T _F，i+T _B，i (1)

Wherein, i represents that access priority is not the access request of i (being called for short i class access request below), and in the embodiment of the present invention, i value is less represents that priority is higher, and i is natural number.Can not access in the same time the T of CCH according to structure of time slot analysis _{f, i}, the position that its size and when access are engraved in synchronization slot is relevant, has nothing to do with the priority of access request.T _{b, i}relevant with the backoff procedure in access contention with the priority i of access request, can analyze with Morkov chain model.For example, in CCHI, propose access request and successfully access therein CCH, T _{f, i}=0 is that access delay equals T _{b, i}, all the other access situations are as the access request that proposes or keep out of the way SCHI in SCHI, T _{f, i}non-vanishing.

Secondly, take 100ms as time delay restriction, analyze the alternately impact on time delay of time slot.Time delay restriction: if access delay is greater than time delay restriction, this request surpasses its effective time, the failure of access channel.In the embodiment of the present invention, slot length is 100ms, and wherein front 50ms is control slot CCHI, and rear 50ms is business time-slot SCHI.Note t (0≤t≤100) be access request in the turn-on time of a synchronization slot, and represent T with X _{b, i}.Then divide following two class situations that n the access delay in time slot is discussed.

First kind situation: the access request proposing at n time slot CCHI, i.e. 0≤t≤50.

CCHI at n time slot has the time of (50-t) ms for its access channel.There are two kinds of situations: if 1. this access request accesses successfully at the CCHI of n time slot, T _{f, i}=0, access delay equals T _{b, i}; 2., if this access request is at n time slot access failure, will keep out of the way n+1 time slot, because the timer of keeping out of the way of this access request keeps freezing at whole SCHI, so T _{f, i}perseverance is 50ms.In the second situation, this access request only successfully accessed channel and just can satisfy the demands before the t of n+1 time slot, and now (50-t<X≤50); Otherwise access delay will surpass 100ms and access failure, now X>50.Known, the time delay that proposes access request at n time slot CCHI can represent suc as formula (2).The access request of keeping out of the way n+1 time slot should be considered as urgent access, need in competition, have higher access priority.

$T_{D,i}=\left\{\begin{array}{cc}X& \mathrm{if\; X}<50-t\\ X+50& \mathrm{if\; X}>50-t\end{array}\right.---\left(2\right)$

Equations of The Second Kind situation: the access request proposing at n time slot SCHI, i.e. 50<t≤100.

Now, the backoff counter of access request will freeze always until the CCHI of n+1 time slot, T _{f, i}=100-t.The time of supposing access request is uniformly distributed in SCHI, T _{f, i}be uniformly distributed in [0,50] ms, and T _{f, i}average E (T _{f, i})=25ms.If this access request successfully accesses channel before the time slot of n+1 time slot replaces, in the control slot of n+1 time slot, access, now access delay can satisfy the demands, and accesses successfully.Otherwise if access request is kept out of the way the SCHI of n+1 time slot, access delay is necessarily greater than 100ms and surpasses its effective time, access delay can not satisfy the demands, access failure.Therefore, the access delay of the access request of n time slot SCHI proposition is suc as formula shown in (3).In this case, access strategy should be according to T _{f, i}value control access to reduce the alternately impact on time delay of time slot.

$T_{D,i}=\left\{\begin{array}{cc}100-t+X& \mathrm{if}0\&le;X\&le;50\\ 100-t+X+50& \mathrm{if\; X}>50\end{array}\right.---\left(3\right)$

From above, time and competition process that success access channel is proposed by access request determine jointly, and this is the result of being freezed to cause at SCHI by the backoff counter of access request.T _{f, i}time Yanzhong accounted for very large proportion, and because fixing time slot replaces T _{f, i}can not reduce.For strict guarantee time delay demand, unique method is according to T _{f, i}reduce T _{b, i}.

In lower surface analysis competition process, keep out of the way time delay.Set up the Markov chain model of a kind of priority access style (access category, AC) backoff procedure in non-lasting business, be used for analyzing and keep out of the way time delay T _{b, i}, as shown in Figure 1.Wherein, state idle represents do not have packet to transmit in node, and the value that state (j, c) is illustrated in backoff counter in the j time competition is c, P _arepresent to complete in channel the probability that still has new data packets to arrive in queue after the transmission of a packet.

Backoff procedure in access contention is as follows, and as busy in channel during the j time access, backoff counter is at random from [0, W _{i, j}-1] in, select an integer value to keep out of the way, as shown in Figure 1, W wherein _{i, j}=CW _{i, j}+ 1.W _{i, j}be the value number of the competition window of i class access request in the j time competition, CW _{i, j}it is the size of the competition window of i class access request in the j time competition.To i class access request, the excursion that competition window is set is [CW _{i, min}, CW _{i, max}].If channel idle detected in backoff procedure, backoff counter subtracts 1, otherwise backoff counter is freezed and its probability P _{i, b}represent.When backoff counter is decremented to zero, access attempts can be proposed, if there are a plurality of access attempts to cause access interference, probability P _irepresent, select the access channel that priority is high.If i class access request accesses channel failure, competition window CW when the j time competition _{i, j}being multiplied by growth factor MF increases as CW _{i, j+1}, the backoff procedure of continuation next round.If it is M that the competition window of i class access request rises to the number of times of maximized window, establish f for the competition window number before the minimum competition window of i class access request, maximum competition number of times of i class access request are M+f.

Backoff counter will propose transmission attempt while being decremented to zero, therefore the transmission probability τ of i class access request _ifor

${\mathrm{\&tau;}}_i=\underset{j=0}{\overset{M+f}{\mathrm{\&Sigma;}}}{b}_{j,0}=\frac{{2P}_a\&CenterDot;(1-P_{i,b})}{1-P_i}\&CenterDot;{\left(\underset{j=0}{\overset{M+f}{\mathrm{\&Sigma;}}}{\left(P_i\right)}^j(W_{i,j}+1)\right)}^{-1}---\left(4\right)$

B _{j, 0}the probability that proposes transmission request while representing the j time competition, from formula (4), obtains τ _ialso need P _i, P _{i, b}.For simplifying the analysis, in the embodiment of the present invention, only consider the situation of two kinds of priority, i value be 0 and 1, AC0 be high priority, AC1 is low priority, n _irepresent the number of i class access request.

The value of backoff counter is kept to Probability p zero but that need to continue to keep out of the way _ifor:

$P_i=\left\{\begin{array}{cc}1-{(1-{\mathrm{\&tau;}}_0)}^{n_0-1}& i=0\\ 1-{(1-{\mathrm{\&tau;}}_0)}^{n_0}{(1-{\mathrm{\&tau;}}_1)}^{n_1-1}& i=1\end{array}\right.---\left(5\right)$

The probability P that backoff counter is freezed _{i, b}be:

$P_{i,b}=1-{(1-{\mathrm{\&tau;}}_0)}^{n_0}{(1-{\mathrm{\&tau;}}_1)}^{n_1}---\left(6\right)$

Composite type (4), (5), (6), can obtain τ by numerical method _i.

With the τ obtaining _isubstitution formula (5) is tried to achieve P _i, what be used for calculating access CCH on average keeps out of the way time delay.I class access request accesses successful Probability p in the k time competition _{i, k}be

p _i，k＝(P _i) ^k-1(1-P _i) (7)

The time delay D of the k time competition of i class access request _{i, k}ask for as follows: establish F _{i, j}in the j time competitive stage, often to keep out of the way the mean value of the freeze-off time of a step, w _{i, k}at [0, W _{i, k}-1] equally distributed stochastic variable, δ is the size of a time slot, time delay D _{i, k}for:

$D_{i,k}={\mathrm{\&Sigma;}}_{j=1}^{W_{i,k}}{F}_{i,j}+w_{i,k}\mathrm{\&delta;}---\left(8\right)$

Derive known:

$\begin{array}{c}E\left(T_{B,i}\right)=E\left(\underset{k=0}{\overset{M+f}{\mathrm{\&Sigma;}}}\left(p_{i,k}\underset{h=1}{\overset{k}{\mathrm{\&Sigma;}}}{D}_{i,h}\right)\right)\\ =\frac{\mathrm{\&delta;}+T[1-{(1-{\mathrm{\&tau;}}_0)}^{n_0}{(1-{\mathrm{\&tau;}}_1)}^{n_1}]}{2}\underset{k=0}{\overset{M+f}{\mathrm{\&Sigma;}}}[{\left(P_i\right)}^k-{\left(P_i\right)}^{M+f+1}]W_{i,k}\end{array}---\left(9\right)$

Wherein, P _{i, k}the probability that while being the k time competition of i class access request, backoff counter is freezed, D _{i, h}be the time delay of the h time competition of i class access request, T representative is processed once to conflict or be successfully completed and is once transmitted needed time, P _iit is the probability of i class access request generation access interference.E (x) represents to ask for the expectation of x.

Therefore, the expectation E (T of access delay _{d, i}) be:

E(T _D，i)＝E(T _F，i)+E(T _B，i) (10)

Known by upper surface analysis, the time delay that access request is freezed at SCHI can be determined constantly by its access.Access request have time delay restriction and SCHI freeze time delay on the condition of the impact of access delay under, the present invention utilize the effective time-delay analysis of the residue of access request not in the same time access request to accessing successful urgency, and change the growth rate of competition window with it, remain the competition window growth rate that effective time delay is little slower, remain the competition window growth rate that effective time delay is large very fast.From formula (8), the value of competition window diminishes, and can reduce the time delay D of keeping out of the way of competition each time _{i, k}, and then reduce total time delay of keeping out of the way.Distinguish different access priorities, it is CW that the window of each priority arranges scope simultaneously _minand CW _maxremain unchanged.The method that the present invention increases with Competition adjustment window, makes to remain the short access of effective time delay and accesses as early as possible CCH, reduces and is subject to T _faffect the access delay of access request, and then reduce average access delay E (T _{d, i}).

The request of access CCH is freezed SCHI's, can cause the conflict that access request time delay is large and CCHI starts that runs into SCHI in access contention, and T _fvalue cause that access delay difference is not large in the same time, the present invention controls access, the access self-adaptive initial window of SCHI and the prioritization criteria three aspects: that expires the earliest by the effective time delay of residue respectively and improves the impact of docking into delay performance, has proposed a kind of car networking control channel self-adapting cut-in method based on time delay optimization.

First aspect, the strategy that remains effective time delay control access is as follows.

Access delay is divided into the time delay of keeping out of the way of freezing Markov chain in time delay and CCHI in SCHI, different accesses access requests constantly, and the chance and the access delay difference that successfully access channel are very large.From upper surface analysis, SCHI freezes time delay and accounts for access request access delay greatly in SCHI, can reduce the residue effective time of a bag.If continue access strategy and window growth pattern before using, access in SCHI and next time slot have access and the higher access contention access channel of priority of initial contention window mouth, the chance that success accesses is especially little, and access delay is very large, affect 802.11p at the average access performance of CCH.

$T_d=T_{F,i}+T_{B,i}=T_{F,i}+\underset{j=0}{\overset{k}{\mathrm{\&Sigma;}}}{D}_{i,j}---\left(11\right)$

T _e＝T _b-T _d (12)

Wherein, T _dthe access delay that expression starts timing constantly from proposing access request, D _{i, j}the time delay that represents the j+1 time competition of i class access request, k+1 represents the competition number of times of access request, T _bfor effective time delay of access request, represent effective time limit turn-on time of an access request.T _bdeduct the access delay T that constantly starts timing from proposing access request _dremain exactly effective time delay T _e, as shown in formula (12).When backoff counter is decremented to zero, but channel is used real-time T when still busy _evalue decide the size of competition window growth factor.

The calculating of competition window growth factor is suc as formula shown in (14), with represent respectively the access number that produces conflict and transmission success in n-1 time slot, its ratio represents n the collision rate in time slot.MF ⁿbe illustrated in the growth factor of competition window in n time slot, visible, growth factor is directly proportional to the effective time delay of residue and collision rate, and access control can remain effective time delay according to the utilization of channel jam situation and adjust growth factor, reaches the object that reduces access delay.

${\mathrm{MF}}^n=\mathrm{\&alpha;}\&CenterDot;\frac{T_e}{T_b}+(1-\mathrm{\&alpha;})\&CenterDot;\frac{N_{\mathrm{collision}}^{n-1}}{N_{\mathrm{data}\_\mathrm{sent}}^{n-1}}---\left(13\right)$

α is weight factor, and for determining the influence degree of the effective time delay of residue when determining growth factor, weight factor span is [0,0.5], and the channel weight factor of more blocking up is less.

In addition, the feature of and business real-time relevant according to car networking CCH access request safety, the classification of priority is by all information, the influence degree of traffic safety and its urgency level to be determined according to the actual requirements.Known in car networked environment with the closely-related information of traffic safety or emergence message, normally time delay restriction is smaller and need to have higher access priority, as the anti-collision warning mentioning before only has the maximum delay of 20ms.Therefore, use effective time delay T of access request _bdistinguish different access priorities, with residue T effective time of access request _econtrol the access in CCH, make the access mechanism in CCH more adapt to car networking.

Second aspect, the setting of the access self-adaptive initial window of SCHI.

The access request proposing in SCHI, backoff counter can not reduce before CCHI.If therefore continue to use the access strategy in 802.11p, all initial window size that propose access CCH request in SCHI are all CW _min.When access number is larger, at CCHI, start, due to less competition window and a lot of access requests, be equivalent to arrival rate p now _avery large, can produce serious access interference, and then affect access performance.

The conflict starting for CCHI, the big or small CW of initial contention window mouth in definition SCHI access _initialas follows:

CW _initial＝τN (14)

The size of initial contention window mouth is relevant to the access number of last SCHI, and access request number represents with N, and τ is the factor of influence of SCHI access number N to initial window size.Number at SCHI statistics access CCH, is used for determining at the protection interval of next CCHI the home window value accessing in SCHI, carries out backoff procedure.The present invention determines factor of influence τ by emulation, is calculated the initial contention window mouth size of access request in SCHI by formula (14).

The third aspect, applies the prioritization criteria that expires the earliest and processes access interference.

Consider that in CCH, transmission information is to improving the importance of traffic safety, access strategy of the present invention is not to be limited with M+f access chance, but with its effective time delay T _bbe as the criterion, only have access delay to surpass effective time delay T of access request _bability packet loss.When access interference occurs, consider fixedly access priority and access in real time conflicting between priority demand, because time slot replaces low priority, accessing the situation that emergence message should priority access in access contention that becomes.Application preferentially (Earliest Deadline First, the EDF) mechanism that expires is the earliest processed access interference, selects the effective time delay T of residue _ethe shortest access request access CCH.EDF is an optimum scheduling strategy: in a series of independently tasks, each has a time of advent and execution requirements deadline, exists a kind of strategy to guarantee that all tasks all complete within deadline.

Can scheduling function as follows:

$U=\underset{i=1}{\overset{m}{\mathrm{\&Sigma;}}}\frac{C_i}{T_i}\&le;1---\left(15\right)$

Wherein, { C _ibe m the time that process is required, { T _icycle of arriving of the process that represents.Therefore can be not more than under the condition of its access capacity at the access request number of CCH, meeting above-mentioned schedulable condition.In the car networking real-time Transmission of Event triggered and free restriction access, EDF is a kind of method of effective raising access delay performance.

Car networking control channel self-adapting cut-in method based on time delay optimization provided by the invention as shown in Figure 2, carries out following steps 1～step 6, until this time slot finishes in a time slot.After this time slot finishes, execution step 7.Below each step is specifically described.

Step 1: in the last SCHI of n time slot, the number N of statistics access CCH request;

Step 2: being replaced in the protection interval of CCHI by SCHI, by the N obtaining, determine the initial value of the competition window of access request in SCHI according to formula (14), and determine the backoff value of each access; The backoff value of access request is at random from span [0, the CW of competition window _initial] in select to obtain;

Other access request of different priorities arranges different competition window excursions, and the competition window excursion of i class access request is [CW _{i, min}, CW _{i, max}].The resulting CW of this step _initialbe exactly CW _{i, min}, and CW _{i, max}remain unchanged, by CW is set _initialguaranteeing on the basis that existing priority is divided, reducing the access that time slot alternately brings and blocked up.The minimum competition window CW of the competition window of all kinds of access requests _{i, min}with maximum contention window CW _{i, max}, minimum competition window and the maximum contention window in general corresponding 802.11e mechanism, for different priorities, do not set.

Step 3: in CCHI, carry out the access contention based on keeping out of the way, when depositing competition conflict, execution step 4; Otherwise, execution step 5;

Step 4: to all access requests that meet access conditions, according to the prioritization criteria that expires the earliest, select the shortest access request residue effective time, enter step 5 and carry out, remaining access request is proceeded backoff procedure, enters step 6 and carries out.

The access request that meets access conditions comprises: 1. the new access request this moment proposing; 2. backoff counter is zero access request this moment.Utilize the override mechanism that expires the earliest to select residue effective time the shortest access request to access, and other access requests are proceeded backoff procedure.

The residue of access request is determined according to formula (11) and (12) effective time.

Step 5: if access successfully, access request completes access, if access failure enters step 6 by access request and carries out.

Step 6: to the access request that continues to keep out of the way, determine the residue effective time of access request, and reset competition window and the backoff value of access request, then go to step 3 execution.

Keep out of the way after process through one, if channel still busy needs continue to keep out of the way, calculate residue effective time, with the MF in formula (13) ⁿreset the size of window, enter next round backoff procedure.

Priority level is the access request of i, and the competition window of the j time competition is CW _{i, j}, after competing unsuccessfully, at the competition window CW of the j+1 time _{i, j+1}=min[CW _{i, j}* MF ⁿ, CW _{i, max}], min represents to get minimum value wherein; MF ⁿfor growth factor is determined according to formula (13).

Priority level be i access request the j+1 time competition backoff value from [0, CW _{i, j+1}] in select at random.

Step 7: after n time slot finishes, add up and clash in n time slot and successfully access the number of times of channel, determine the average collision rate of this time slot, continue the access contention in next time slot.

By emulation, obtain, in SCHI, access optimum initial window size along with access number in SCHI increases and increases, and be approximated to linear relationship, as shown in Figure 3.By curve, obtain, the value of factor of influence τ is 1.3.Simulation result in this part is all by its mean value, to be obtained from the emulation of 100s.

By Fig. 4, can be obtained, when high priority access is blocked up, total access number approaches access capacity, and the access failure rate of the inventive method (representing with DAAS in figure) is higher than 802.11p, and the inventive method is better than 802.11p when blocking up when low priority.But the access failure rate difference of the inventive method in different priorities is less, be more suitable in car networking control channel safety and control information to mortality requirement.Along with the increase access failure number of access number sharply rises, but the inventive method under jam situation by the required average access rate of Fig. 4, Performance Ratio 802.11p is good.

From average access delay Fig. 5, the inventive method contrasts 802.11p access delay when low priority very large improvement, particularly access number a hour access delay reduced a lot, along with the increase gain of access number diminishes, it is 12% that minimal time delay gains.During high priority, the inventive method time delay is close with 802.11p performance, is because the inventive method can improve delay performance, but meanwhile the inventive method preferential selection when conflict remains the time delay that the less access channel of effective time delay has increased high priority.The gain of access delay is because the inventive method is introduced variation and the control access order that residue decides window effective time, when guaranteeing high priority access, reduce low priority access slot and replaced the access delay in situation, and then reduced average access delay.In the situation that blocking up, the access delay of the inventive method is still little than 802.11p as expected.

Claims (3)

1. the car networking control channel self-adapting cut-in method based on time delay optimization, is characterized in that, in n time slot, carries out following steps 1～step 6, after n time slot finishes, and execution step 7, specifically each step is:

Step 1: in the last business time-slot SCHI of n time slot, the number N of statistics access control channel CCH request;

Step 2: being replaced in the protection interval of control slot CCHI by SCHI, determine the initial value of the competition window of access request in SCHI, and the backoff value of each access request;

Wherein, the initial value CW of the competition window of access request _initial=τ N, τ is the factor of influence of SCHI access request number N to initial window size; The backoff value of access request is from span [0, the CW of competition window _initial] in random selection obtain;

Step 3: in CCHI, carry out the access contention based on keeping out of the way, when depositing competition conflict, execution step 4; Otherwise, execution step 5;

Step 4: to all access requests that meet access conditions, select the shortest access request residue effective time according to the override mechanism that expires the earliest (EDF), then carry out step 5 and carry out; Remaining access request continues to keep out of the way, and enters step 6 and carries out;

The access request that meets access conditions comprises two kinds: the new access request that current time proposes; Current time backoff counter is zero access request;

Residue T effective time of access request _e=T _b-T _d, T _bfor effective time delay of access request, T _dfor the access delay that constantly starts timing from proposing access request;

Step 5: access request access CCH, if access successfully, completes the access of this access request; If access failure, this access request continues to keep out of the way, and enters step 6 and carries out;

Step 6: to the access request that continues to keep out of the way, determine the residue effective time of access request, and reset competition window and the backoff value of access request, continue to go to step 3 execution;

If the priority level of certain access request is i, the competition window of the j time competition of this access request is CW _{i, j}, after competing unsuccessfully the j time, competition window increases as CW _{i, j+1}=min[CW _{i, j}* MF ⁿ, CW _{i, max}], CW _{i, max}the maximum contention window that represents the access request that priority level is i; MF ⁿfor growth factor, computing formula is as follows:

${\mathrm{MF}}^n=\mathrm{\&alpha;}\&CenterDot;\frac{T_e}{T_b}+(1-\mathrm{\&alpha;})\&CenterDot;\frac{N_{\mathrm{collision}}^{n-1}}{N_{\mathrm{data}\_\mathrm{sent}}^{n-1}}$

with be illustrated in and in n-1 time slot, clash and successfully access the number of times of channel, the average collision rate that its ratio is n-1 time slot; α is weight factor, for determining the influence degree of the effective time delay of residue when determining growth factor;

Priority level be i access request the j+1 time competition backoff value from [0, CW _{i, j+1}] in select at random;

Step 7: add up and clash in n time slot and successfully access the number of times of channel, determine the average collision rate of this time slot.

2. a kind of car networking control channel self-adapting cut-in method based on time delay optimization according to claim 1, is characterized in that the T described in step 4 _daccording to following formula, obtain:

$T_d=T_{F,i}+T_{B,i}=T_{F,i}+\underset{j=0}{\overset{k}{\mathrm{\&Sigma;}}}{D}_{i,j}$

Wherein, T _{f, i}for the time delay of freezing in SCHI, T _{b, i}for the time delay of keeping out of the way in CCHI, D _{i, j}the time delay that represents the j+1 time competition of i class access request, k+1 represents the competition number of times of access request.

3. the access delay of the alternately lower control channel of car networking time slot is determined a method, it is characterized in that, according to following formula, determines

T _D，i＝T _F，i+T _B，i

Wherein, T _{d, i}represent access delay, T _{f, i}represent to freeze time delay, T in business time-slot SCHI _{b, i}represent the time delay of keeping out of the way in control slot CCHI.