CN103326974B - A kind of Adaptive Transmission model selection system and method thereof of vehicle communication Access Network - Google Patents

Abstract

The invention discloses an adaptive transmission mode selection system of a vehicle communication access network and a method thereof. The system comprises: a vehicle information service flow generating module at an I/B, a vehicle information service flow classification and mapping module, a packet delivery module, Adaptive transmission mode selection module, OFDMA wireless resource configuration module, frame sending/receiving module, access channel/driving state/geographic area information detection module, and vehicle information service flow generation module at SV, vehicle information service flow classification Mapping module, packet submission module, adaptive transmission mode selection module, OFDMA wireless resource configuration module, frame sending/receiving module, access channel/driving status/location information feedback module. The present invention introduces OFDMA and AMC technology, and can make a trade-off between the effectiveness of ITS information service flow transmission and system transmission efficiency, so as to obtain the highest possible average effective data of the system while meeting the QoS requirements of various ITS information service flows Transmission rate.

Description

An adaptive transmission mode selection system and method for vehicle communication access network

technical field

The invention belongs to the technical field of vehicle communication, and relates to a vehicle communication method, in particular to a vehicle communication access network adaptive transmission mode selection system and its method.

Background technique

As one of the important subsystems of the Intelligent Transportation System (ITS), the vehicle communication access network is to ensure the smooth communication channel between the information service platform and the ITS vehicle terminal, and to realize the safety warning and efficient guidance of the ITS for the supervised vehicles, and provide services for passengers. The key to providing professional multimedia and mobile Internet application services. The target packet error rate reflects the reliability requirements of the ITS transmission information service flow, and is a key parameter to measure the quality of service (Quality of Service, QoS) of the vehicle communication access network. ITS information service flow can be divided into three categories according to application types, namely: ①Safety early warning: to effectively avoid and reduce traffic accidents through active early warning, it is necessary to quickly and reliably send such information service flow to the vehicle terminal of the supervision vehicle ; ②Driving Guidance: Assist driving through active guidance or independent inquiry to improve road traffic efficiency; ③Multimedia application: Provide various multimedia application services to the vehicle terminal on demand to enrich the user's driving experience. Table 1 lists the application examples of various ITS information service flows and the corresponding target packet error rate requirements. It can be seen that safety warning information services that are closely related to life and property safety are usually extremely important, and the vehicle communication access network needs to ensure It has high transmission reliability, and multimedia application information service, as an additional service during vehicle driving, usually allows high packet error rate during transmission.

Table 1: Application examples of ITS information service flow and corresponding target packet error rate

The current vehicle communication access network mostly uses GPRS or WiFi to provide many ITS user vehicles (Subscriber Vehicle, SV) with the "last mile" wireless access service to the infrastructure/base station (Infrastructure/BaseStation, I/B). The communication method does not consider the transmission reliability requirements of different application-level information service streams, and usually uses a fixed modulation and coding mode for data transmission, so the access rate is not high, and it cannot meet the growing diversification of information services that prioritize safety for passengers appeal. In addition, the ITS vehicle-mounted terminal usually integrates a high-precision Global Positioning System/Geographic Information System (GPS/GIS) module, which can obtain a wealth of external information, such as driving speed, driving direction, own position information, where Regional geographic information, etc. However, the current vehicle communication access network does not consider using this information to help improve the QoS performance of various information service flows. Therefore, the diversity of ITS information service flow application types and the specificity of the vehicle communication access network industry have brought challenges and opportunities to the design of the next generation vehicle communication access network.

Contents of the invention

The purpose of the present invention is to overcome the defects of the prior art and provide an adaptive transmission mode selection system and method for vehicle communication access network, which combines Orthogonal Frequency Division Multiple Access (OFDMA) and adaptive modulation Coding (Adaptive Modulation and Coding, AMC) technology is introduced into the physical layer (PHY layer) of the vehicle communication access network. The switching of the transmission mode depends not only on the status information of the access channel, but also on the status information of the vehicle, the geographical area information and various ITS information. The target packet error rate limit of the service flow is closely related, which can obtain a higher average effective data transmission rate of the system while ensuring the transmission quality of various ITS information service flows.

The purpose of the present invention is solved by the following technical solutions:

The adaptive transmission mode selection system of the vehicle communication access network includes: a vehicle information service flow generation module at the I/B, a vehicle information service flow classification mapping module, a packet delivery module, an adaptive transmission mode selection module, an OFDMA wireless Resource configuration module, frame sending/receiving module, access channel/driving state/location information detection module, and vehicle information service flow generation module at SV, vehicle information service flow classification mapping module, packet delivery module, self-adaptive Transmission mode selection module, OFDMA wireless resource configuration module, frame sending/receiving module, access channel/driving state/geographical area information feedback module; access channel/driving state/geographical area at the I/B The information detection module periodically broadcasts the comprehensive information request signaling INF-REQ to all SVs in the I/B service area requesting to obtain the ITS information service flow, and the access channel/driving status/geographical area information feedback module located at each SV After receiving the signaling, it immediately returns the integrated information response signaling INF-RSP, and follows the random back-off mechanism to feed back the detected integrated information to the I/B from the integrated information response signaling INF-ACK, and then the integrated information The message signaling INF-MES is delivered to the adaptive transmission mode selection module; the vehicle information service flow generation module and packet delivery module at I/B periodically send TPER-MES signaling and DE-MES signaling to the adaptive transmission mode selection module , the TPER-MES signaling carries the target packet error rate limit information of the application-level information service flow to be transmitted, and the DE-MES signaling carries the type information of the submitted packet; the adaptive transmission mode selection module at the I/B is based on the INF- The information carried on the MES, TPER-MES and DE-MES signaling generates the modulation and coding mode selection signaling MC-MES, which carries the adaptive transmission mode selection information that the SV should adopt when transmitting data streams on each OFDMA sub-channel, The information will be periodically sent to the OFDMA wireless resource configuration module at I/B through signaling MC-MES; various ITS data flow packets are submitted to the frame sending/receiving module through the OFDMA wireless resource configuration module at I/B, together with the internal The adaptive transmission mode selection information embedded in the control field is encapsulated into a frame, and then sent to the SV; the adaptive transmission mode control module located at the SV extracts the corresponding adaptive transmission mode selection information from the frame sending/receiving module, And by generating corresponding MC-MES signaling to assist the OFDMA wireless resource configuration module at the SV to demodulate and decode the data stream, and finally complete the delivery of the data stream to the application-level information service stream.

The above comprehensive information is access channel information, driving status information and geographical area information.

The present invention also proposes a method for selecting an adaptive transmission mode of a vehicle communication access network based on the above system, comprising the following steps:

1) Determination of switching threshold of multi-level modulation coding transmission mode

Taking the signal-to-noise ratio as a parameter index to measure the status of the SV access channel, the entire signal-to-noise ratio interval is divided into non-repetitive and continuous modulation, coding, and transmission mode switching intervals. The corresponding modulation, coding, and transmission mode switching thresholds are:

Among them, t is the application type of ITS information service flow, l is the modulation and coding transmission mode level, and Table 2 shows the optional 6-level modulation and coding transmission mode for the vehicle communication access network;

Table 2: 6-level modulation and coding transmission mode

Modulation coding transmission mode level l Modulation Coding rate _Rc Data transfer rate R _l (bit/symbol) 1 BPSK 1/2 0.5 2 QPSK 1/2 1.0 3 QPSK 3/4 1.5 4 16QAM 9/16 2.25 5 16QAM 3/4 3.0 6 64QAM 3/4 4.5

The I/B determines the switching threshold of the multi-level modulation and coding transmission mode according to the target packet error rate information carried on the TPER-MES signaling;

2) Initialization of modulation coding transmission mode level L

When the system transmits the ITS information service flow with the application type t, the I/B initializes the application type t according to the SV initial instantaneous access channel state information γ′ carried on the INF-ACK signaling and the multi-level modulation and coding transmission mode switching threshold The ITS information service flow modulation and coding transmission mode level L, let L=l′, satisfy

3) Determine whether to start the adaptive transmission mode selection process

I/B follows the random backoff mechanism with a backoff time interval of (0, C _W -1), and starts the adaptive transmission mode according to the geographical area information of the SV and the SV driving state information carried on the INF-ACK feedback signaling Judgment process, if the SV is in a special geographical area, or the driving speed of the SV exceeds 60km/h, it indicates that the system is not suitable for starting the adaptive transmission mode selection process, and the current modulation and coding transmission mode level will remain unchanged; otherwise, start the adaptive transmission mode selection process and go to step 4);

4) Determination of the level of adaptive transmission mode

I/B judges according to the OFDMA frame cycle according to the SV instantaneous access channel state information γ carried on the INF-ACK feedback signaling, if Then update the modulation and coding transmission mode level L used to transmit the information service flow, let L=l'';

5) Repeat step 3) and step 4) until all data packets of the ITS information service flow are sent.

Further, in the above step 3), the special geographical area refers to an area with severe weather environment, complex road environment or bridge tunnel environment.

Further, in the above step 3), the multiplication rule in the random back-off mechanism satisfies C _W,new =2 ^m C _W,old , where C _W,new is the new value of the time window after implementing the random back-off mechanism once , C _W,old is the old value of the time window before the random backoff mechanism is implemented.

Further, in the above step 4), when When the channel experiences deep fading, I/B will no longer send the data packets of this ITS information service flow.

Further, in the above step 1), the I/B determines the multi-level modulation and coding transmission mode switching threshold according to the target packet error rate information carried on the TPER-MES signaling, specifically:

When the application type of ITS information service flow is t, the level-l modulation and coding transmission mode switching threshold It can be obtained by the following formula:

$\∀t\∈\left\{\mathrm{1,2,3}\right\}$ and $\∀l\∈\left\{\mathrm{1,2,3,4,5,6}\right\}$

Among them, a _l and g _l are curve fitting coefficients, ζ _TPER,t represents the target packet error rate required by the ITS information service flow of application type t. Assume that the system meets the following conditions:

The power of I/B transmission of various ITS information service streams is constant; the SV's access channel/driving status/geographical area information feedback module can obtain error-free comprehensive information, and feed it back to I without error and delay /B's access channel/driving status/at the geographical area information detection module; the packet error rate detection based on the cyclic redundancy check is perfect, ignoring the redundancy caused by the packet sequence number and CRC check, and not considering MAC layer ARQ technology, and ignore its impact on the packet error rate; SV access channel is a slow-varying flat fading channel, the channel status remains unchanged within one frame, but can change between two frames, encapsulated in one frame The application types of the ITS information service flow in the network are the same, and the selection of the multi-level modulation and coding transmission mode is carried out on the basis of each frame; the Nakagami-m model is selected to model the slow-varying flat fading channel, and the instantaneous SNRγ of the SV access channel condition The probability density function of the ITS obeys the Gamma distribution, then when the I/B uses the l-level modulation and coding transmission mode to send the ITS information service flow of type t, the packet error rate of the information service flow satisfies the following formula:

$\∀t\∈\left\{\mathrm{1,2,3}\right\}$ and $\∀l\∈\left\{\mathrm{1,2,3,4,5,6}\right\}$

Then the curve fitting coefficients a _l , g _l and γ _l are taken according to Table 3:

Table 3: Packet Error Rate Curve Fitting Parameters for 6-level Modulation and Coding Transmission Mode

Compared with the prior art, the present invention has the following beneficial effects:

The present invention proposes an adaptive transmission mode selection system and its method suitable for the next generation vehicle communication access network. Compared with the existing system and its method, the present invention makes the selection of the adaptive transmission mode not only depend on the Channel status information is also closely related to vehicle driving status information, geographical area information of the vehicle, and target packet error rate limit information of various ITS information service flows, which can be used in various ITS information service flow transmission effectiveness and system transmission efficiency Make a good trade-off between them, so as to obtain the highest possible average effective data transmission rate of the system while meeting the QoS requirements of various ITS information service flows.

The present invention can adaptively adjust the corresponding SNR switching threshold according to the application type of the ITS information service flow, while the prior art can only use a predetermined and constant SNR switching threshold. Assume that there are three types of ITS information service flows from I/B to SV, and the target packet error rates are ζ _TPER,1 ＝10 ^-6 , ζ _TPER,2 ＝10 ^-5 and ξ _TPER,3 ＝10 ^-2 , and the generation probabilities are is 1/3. According to Table 4, it can be seen that: using the technology provided by the present invention, the SNR switching thresholds of the three types of ITS information service flows are all different; using the existing technology, the SNR switching threshold of the first group can be obtained by the curve fitting parameter γ _l , and the SNR of the second group The switching threshold is the same as the SNR switching threshold of the first type of ITS information service flow in the present invention, and aims to make the transmission quality of all ITS information service flows meet the strictest reliability limit.

Table 4: ITS information service stream modulation and coding mode switching threshold (dB)

Description of drawings

Fig. 1 is a system basic implementation architecture diagram of an adaptive transmission mode selection method;

Fig. 2 is a process diagram of encapsulation and framing of ITS information service flow;

Fig. 3 is a diagram of a method for selecting an adaptive transmission mode of a vehicle communication access network;

Fig. 4 is a performance comparison diagram of the packet error rate of the first class ITS information service flow;

Fig. 5 is a performance comparison diagram of the packet error rate of the second type ITS information service flow;

Fig. 6 is a performance comparison diagram of the packet error rate of the third type ITS information service flow;

Figure 7 is a performance comparison diagram of the average effective data transmission rate of the system.

detailed description

The present invention is described in further detail below in conjunction with accompanying drawing:

Referring to Fig. 1, the present invention first proposes an adaptive transmission mode selection system of a vehicle communication access network, the system includes: a vehicle information service flow generation module at the I/B place, a vehicle information service flow classification mapping module, and a packet delivery module , Adaptive transmission mode selection module, OFDMA wireless resource configuration module, frame sending/receiving module, access channel/driving state/geographic area information detection module, and vehicle information service flow generation module at SV, vehicle information service flow Classification mapping module, packet submission module, adaptive transmission mode selection module, OFDMA wireless resource configuration module, frame sending/receiving module, access channel/driving status/location information feedback module.

Taking the downlink (I/B to SV) as an example, the adaptive transmission mode selection system is supported by the signaling bearer information listed in Table 5, and the operation process is as follows: access channel at I/B/driving state/ The geographical area information detection module periodically broadcasts the comprehensive information request signaling INF-REQ to all SVs in the I/B service area requesting to obtain ITS information service flow, and the access channel/driving state/geographic location located at each SV After the regional information feedback module receives the signaling, it immediately returns the comprehensive information response signaling INF-RSP, and follows the random back-off mechanism to feed back the detected comprehensive information from the comprehensive information response signaling INF-ACK to the I/B. Then the comprehensive information message signaling INF-MES is delivered to the adaptive transmission mode selection module; the vehicle information service flow generation module and packet delivery module at I/B periodically send TPER-MES signaling and DE to the adaptive transmission mode selection module - MES signaling, TPER-MES signaling carries the target packet error rate limit information of the application-level information service flow to be transmitted, and DE-MES signaling carries the type information of the delivered packet; adaptive transmission mode selection at I/B According to the predefined adaptive transmission mode selection algorithm, the module generates the modulation and coding mode selection signaling MC-MES according to the information carried on the INF-MES, TPER-MES and DE-MES signaling, which carries the SV in each OFDMA sub-signal The adaptive transmission mode selection information that should be adopted when the channel transmits data streams, the information will be periodically sent to the OFDMA wireless resource configuration module at I/B through signaling MC-MES; various ITS data streams are grouped through OFDMA at I/B After the wireless resource configuration module submits it to the frame sending/receiving module, it is encapsulated into a frame together with the adaptive transmission mode selection information embedded in the control field, and then sent to the SV; the adaptive transmission mode control module located at the SV transmits from the frame The /receiving module extracts the corresponding adaptive transmission mode selection information, and assists the OFDMA wireless resource configuration module at the SV to demodulate and decode the data stream by generating corresponding MC-MES signaling, and finally completes the data stream to the application level Delivery of information service streams. The specific data stream encapsulation and framing process is shown in Figure 2.

Table 5: Signaling description

Signaling name Signaling description INF-REQ Integrated Information Request Signaling INF-RSP Integrated Information Response Signaling INF-ACK Integrated Information Response Signaling INF-MES Comprehensive (access channel/driving status/geographical area) information message signaling TPER-MES Target packet error rate message signaling DE-MES Commit Packet Type Message Signaling MC-MES Modulation and coding mode selection signaling

The adaptive transmission mode selection algorithm implemented by the adaptive transmission mode selection method is open, and can be selected according to actual application scenarios. The existing adaptive transmission mode selection algorithm adopts the system’s pre-defined transmission mode switching interval based on channel conditions, without considering the reliability requirements of different ITS information service flows, and without considering the impact of external auxiliary information obtained by ITS vehicle terminals on the system average. The contribution of the effective data transmission rate, so the present invention proposes a method for selecting an adaptive transmission mode of the vehicle communication access network based on the basic implementation framework of the above-mentioned system, as shown in Figure 3, specifically including the following steps:

1) Determination of switching threshold of multi-level modulation coding transmission mode

The I/B determines the switching threshold of the multi-level modulation and coding transmission mode according to the target packet error rate information carried on the TPER-MES signaling. Taking the signal-to-noise ratio as a parameter index to measure the status of the SV access channel, the entire signal-to-noise ratio interval is divided into non-repetitive and continuous modulation, coding, and transmission mode switching intervals. The corresponding modulation, coding, and transmission mode switching thresholds are:

Among them, t is the application type of ITS information service flow, l is the modulation and coding transmission mode level, and Table 2 shows the optional 6-level modulation and coding transmission mode for the vehicle communication access network;

2) Initialization of modulation coding transmission mode level L

When the system transmits the ITS information service flow with the application type t, the I/B initializes the application type t according to the SV initial instantaneous access channel state information γ′ carried on the INF-ACK signaling and the multi-level modulation and coding transmission mode switching threshold The ITS information service flow modulation and coding transmission mode level L, let L=l′, satisfy

3) Determine whether to start the adaptive transmission mode selection process

I/B follows the random backoff mechanism with a backoff time interval of (0, C _W -1), and starts the adaptive transmission mode according to the geographical area information of the SV and the SV driving state information carried on the INF-ACK feedback signaling Judgment process, if the SV is in a special geographical area, such as severe weather environment, complex road environment, bridge tunnel environment, etc., or the driving speed of the SV exceeds 60km/h, it indicates that the system is not suitable for starting the adaptive transmission mode selection process, and the current modulation will be maintained The coding transmission mode level remains unchanged, so as to avoid the parameter adjustment speed unable to keep up with the channel change speed due to the channel condition changing too fast; otherwise, start the adaptive transmission mode selection process and enter step 4); among them, the random backoff mechanism multiplies The rule satisfies C _W,new = 2 ^m C _W,old , where C _W,new is the new value of the time window after implementing a random back-off mechanism, and C _W,old is the time window before this random back-off mechanism is implemented old value;

4) Determination of the level of adaptive transmission mode

I/B judges according to the OFDMA frame cycle according to the SV instantaneous access channel state information γ carried on the INF-ACK feedback signaling, if Then update the modulation and coding transmission mode level L used to transmit the information service flow, let L=l''; it is worth noting that when When , the channel experiences deep fading, at this time, I/B will not send ITS information service flow;

5) Repeat step 3) and step 4) until all data packets of the ITS information service flow are sent.

The process of determining the multi-level modulation and coding transmission mode switching threshold in step 3) of the adaptive transmission mode selection method provided by the present invention is given below:

It is assumed that the system satisfies the following conditions: the power of I/B transmission of various ITS information service streams is constant; the information feedback module of SV’s access channel/driving status/geographical area can obtain perfect comprehensive information, and send it error-free and error-free The delay is fed back to the I/B access channel/driving status/geographical area information detection module; the packet error rate detection based on the cyclic redundancy check is perfect, ignoring the packet sequence number and CRC check The redundancy of the MAC layer, regardless of the ARQ technology of the MAC layer, and its impact on the packet error rate; the SV access channel is a slow-varying flat fading channel, and the channel status remains unchanged within one frame, but can occur between two frames Change, the application type of the ITS information service flow encapsulated in one frame is the same, and the selection of the multi-level modulation and coding transmission mode is carried out on the basis of each frame. The Nakagami-m model is selected to model the slow-varying flat fading channel, then the probability density function of the instantaneous SNRγ of the SV access channel condition obeys the gamma distribution, and satisfies the formula (1):

${\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&gamma;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; m</span>}}^{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}^{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}}^{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; \&Gamma;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span>}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; m\&gamma;</span>}}{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; \&gamma;</span>}<span\; class="notranslate">\; \&Greater\; Equal;</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

in, is the average SNR, Γ(m) is the gamma function and has the following definition:

$\mathrm{<span\; class="notranslate">\; \&Gamma;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{<span\; class="notranslate">\; \&infin;</span>}{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; t</span>}}{\mathrm{<span\; class="notranslate">\; t</span>}}^{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; dt</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

According to the different values of the fading parameter m, the Nakagami-m model can represent multiple types of multipath channels: when m=1, it is a Rayleigh fading channel; when m>1, there is a one-to-one mapping relationship between the Rice factor K and m, It can be approximated as a Rician fading channel.

When the I/B uses the l-level modulation and coding transmission mode to send the ITS information service flow of type t, the packet error rate of the information service flow satisfies the formula (3):

$\&ForAll;t\&Element;\left\{\mathrm{1,2,3}\right\}$ and $\&ForAll;l\&Element;\left\{\mathrm{1,2,3,4,5,6}\right\}$ (3) Among them, a _l , g _l and γ _l are curve fitting coefficients, see Table 3.

When the application type of ITS information service flow is t, the level-l modulation and coding transmission mode switching threshold It can be obtained by transforming (3):

$\&ForAll;t\&Element;\left\{\mathrm{1,2,3}\right\}$ and $\&ForAll;l\&Element;\left\{\mathrm{1,2,3,4,5,6}\right\}$ (4) Among them, ζ _TPER,t represents the target packet error rate required by the ITS information service flow of application type t. The target packet error rate ζ _TPER,t is a key parameter to ensure the QoS of ITS information service flow, and t is different, and the ζ _TPER,t representing its reliability limit is also different, as shown in Table 1.

Carry out performance analysis to the self-adaptive transmission mode selection method that the present invention provides, I/B adopts the 1st order modulation coding transmission mode to send to SV the probability that application type is the ITS information service stream of t:

$p\left(l\right|t)={\&Integral;}_{{\mathrm{\&gamma;}}_{t,l}}^{{\mathrm{\&gamma;}}_{t,l+1}}{p}_{\mathrm{\&gamma;}}\left(\mathrm{\&gamma;}\right)\mathrm{d\&gamma;}={\&Integral;}_{{\mathrm{\&gamma;}}_{t,l}}^{{\mathrm{\&gamma;}}_{t,l+1}}\frac{m^m{\mathrm{\&gamma;}}^{m-1}}{{\stackrel{\&OverBar;}{\mathrm{\&gamma;}}}^m\mathrm{\&Gamma;}\left(m\right)}\exp (-\frac{\mathrm{m\&gamma;}}{\stackrel{\&OverBar;}{\mathrm{\&gamma;}}})\mathrm{d\&gamma;}$ (5)

$<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; m\&gamma;</span>}}_{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}}{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; m\&gamma;</span>}}_{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}}}{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span>$

Among them, P(x,m) is an incomplete gamma function, defined by the following formula:

$\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Gamma;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span>}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; x</span>}}{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; t</span>}}{\mathrm{<span\; class="notranslate">\; t</span>}}^{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; dt</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

The average packet error rate of I/B using the l-level modulation and coding transmission mode to send the ITS information service flow with the application type t to the SV It can be obtained according to (1), (3) and (4):

${\stackrel{\&OverBar;}{\mathrm{\&zeta;}}}_{t,l}={\&Integral;}_{{\mathrm{\&gamma;}}_{t,l}}^{{\mathrm{\&gamma;}}_{t,l+1}}{\mathrm{\&zeta;}}_{t,l}\left(\mathrm{\&gamma;}\right)p_{\mathrm{\&gamma;}}\left(\mathrm{\&gamma;}\right)\mathrm{d\&gamma;}={\&Integral;}_{{\mathrm{\&gamma;}}_{t,l}}^{{\mathrm{\&gamma;}}_{t,l+1}}{a}_l\exp (-g_l\mathrm{\&gamma;})\frac{m^m{\mathrm{\&gamma;}}^{m-1}}{{\stackrel{\&OverBar;}{\mathrm{\&gamma;}}}^m\mathrm{\&Gamma;}\left(m\right)}\exp (-\frac{\mathrm{m\&gamma;}}{\stackrel{\&OverBar;}{\mathrm{\&gamma;}}})\mathrm{d\&gamma;}$ (7)

$<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}}{{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; b</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; m</span>}}}{<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; m</span>}}{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; b</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}{\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}_{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; b</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}{\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}_{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>$

in, $b_l=m/\stackrel{\&OverBar;}{\mathrm{\&gamma;}}+g_l.$

The average effective data transmission rate of I/B using the l-level modulation and coding transmission mode to transmit the ITS information service flow of application type t to SV (bit/symbol) is:

${\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}}_{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; \&zeta;</span>}}}_{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; )</span>$

Among them, R ₁ is the data transmission rate using the l-level modulation and coding transmission mode, as shown in Table 2:

According to formulas (5), (7) and (8), the average effective data transmission rate of the selected multi-level modulation and coding transmission mode of the system can be obtained (bit/symbol) is:

${\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}}_{\mathrm{<span\; class="notranslate">\; EFF</span>}}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\underset{\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 6</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; )</span>{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}}_{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\underset{\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 6</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; \&zeta;</span>}}}_{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 9</span>}<span\; class="notranslate">\; )</span>$

System Average Packet Error Rate for:

${\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; \&zeta;</span>}}}_{\mathrm{<span\; class="notranslate">\; EFF</span>}}<span\; class="notranslate">\; =</span>\frac{\underset{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\underset{\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 6</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; \&zeta;</span>}}}_{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}}}{\underset{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\underset{\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 6</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}}<span\; class="notranslate">\; =</span>\frac{\underset{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\underset{\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 6</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}{\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; \&zeta;</span>}}}_{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}}}{\underset{\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\underset{\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 6</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; |</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; )</span>$

Among them, p(t,l) represents the probability of ITS information service flow modulation and coding transmission mode level l and application type t, p(t) is the probability of I/B transmitting ITS information service flow of application type t to SV .

Assume that there are three types of ITS information service flows from I/B to SV, and the target packet error rates are ζ _TPER,1 ＝10 ^-6 , ζ _TPER,2 ＝10 ^-5 and ξ _TPER,3 ＝10 ^-2 , and the generation probabilities are for 13. According to Table 4, it can be seen that: using the technology provided by the present invention, the SNR switching thresholds of the three types of ITS information service flows are all different; using the existing technology, the SNR switching threshold of the first group can be obtained by the curve fitting parameter γ _l , and the SNR of the second group The switching threshold is the same as the SNR switching threshold of the first type of ITS information service flow in the present invention, and aims to make the transmission quality of all ITS information service flows meet the strictest reliability limit.

Figure 4, Figure 5, and Figure 6 respectively compare the average packet error rate performance of the three types of ITS information service flows using the prior art and the technology provided by the present invention under slow-varying flat fading channels. It can be seen that when using the adaptive SNR threshold switching interval of the technology provided by the present invention and the second fixed SNR threshold switching interval of the prior art, the three types of ITS information service flows can all meet the respective target packet error rate restrictions, and When using the first fixed SNR threshold switching interval in the prior art, it is difficult for the three types of ITS information service flows to meet the corresponding QoS requirements. This goal can only be achieved when the channel condition is very good. For example, when m=1, When the signal condition is greater than 28dB, the Class 3 ITS information service flow with the lowest reliability requirement can meet the limit value of the target packet error rate of less than 10 ^-2 when the channel condition is greater than 28dB. Fig. 7 compares the average effective data transmission rate of the system when using the existing technology and the technology provided by the present invention under the slow flat fading channel. It can be seen that the average effective data transmission rate of the system increases with the increase of the m value. Compared with the second fixed SNR threshold switching interval of the prior art, when the adaptive SNR threshold switching interval of the technology provided by the present invention is adopted, The system will achieve a higher average effective data transfer rate.

In summary, the present invention introduces Orthogonal Frequency Division Multiple Access (OFDMA) and Adaptive Modulation and Coding (Adaptive Modulation and Coding, AMC) technologies, which represent the future communication development direction, into the physical layer (PHY layer) of the vehicle communication access network. , can greatly optimize system transmission performance and obtain higher data transmission rate. AMC is an adaptive wireless transmission technology that can dynamically adjust the modulation mode and coding mode according to channel conditions. When the channel condition is good, the system uses high-order modulation and high coding rate to achieve a higher peak rate of data transmission, such as , 64QAM, 5/6 code rate; when the channel condition is poor, the system uses low-order modulation and low coding rate to ensure the reliability of data transmission, for example, BPSK, 1/2 code rate. OFDMA is a multiple access/multiplexing technology that combines Orthogonal Frequency Division Multiplexing (OFDM) with frequency division multiple access. Scenarios where supervised vehicles move at high speed, such as highways, sub-channels are formed by arranging subcarriers in a pseudo-random manner, which can obtain frequency diversity gain and help eliminate inter-cell interference; continuous mode is suitable for supervised vehicles that stop or move at a low speed Scenarios, such as urban streets, sub-channels are formed by dividing continuous sub-carriers. Based on continuous sub-channels, AMC technology can be used to obtain the highest possible system throughput while meeting the packet error rate limit.

Compared with the existing adaptive transmission mode selection method, the present invention makes the selection of the adaptive transmission mode not only depend on the status information of the vehicle access channel, but also on the vehicle driving status information, the geographical area information of the vehicle and various ITS information services The target packet error rate limit information of the flow is closely related, so that a good balance can be made between the effectiveness of various ITS information service flow transmission and the system transmission efficiency, and the QoS requirements of various ITS information service flows can be obtained while meeting the QoS requirements of various ITS information service flows. Potentially high system average effective data transfer rate.

Claims (8)

1. an adaptive transmission mode selection system of vehicle communication access network, it is characterized in that, comprises: the vehicle information service flow generation module at I/B place, vehicle information service flow classification mapping module, packet delivery module, adaptive transmission Mode selection module, OFDMA wireless resource configuration module, frame sending/receiving module, access channel/driving state/location information detection module, vehicle information service flow generation module at SV, vehicle information service flow classification and mapping module, Packet submission module, adaptive transmission mode selection module, OFDMA wireless resource configuration module, frame sending/receiving module, access channel/driving status/location information feedback module; The access channel/driving state/geographical area information detection module at the I/B periodically broadcasts the comprehensive information request signaling INF-REQ to all SVs in the I/B service area requesting to obtain the ITS information service flow, located at After receiving the signaling, the access channel/driving state/geographical area information feedback module at each SV immediately returns the comprehensive information response signaling INF-RSP, and follows the random back-off mechanism to transfer the detected comprehensive information from the comprehensive The information response signaling INF-ACK is fed back to I/B, and then the integrated information message signaling INF-MES is delivered to the adaptive transmission mode selection module at I/B; the vehicle information service flow generation module at I/B sends I/B The adaptive transmission mode selection module at /B periodically sends TPER-MES signaling, and the packet delivery module at I/B periodically sends DE-MES signaling to the adaptive transmission mode selection module at I/B, and TPER- The MES signaling carries the target packet error rate limit information of the application-level information service flow to be transmitted, and the DE-MES signaling carries the type information of the submitted packet; the adaptive transmission mode selection module at the I/B is based on INF-MES, TPER - The information carried on the MES and DE-MES signaling generates the modulation and coding mode selection signaling MC-MES, and the MC-MES carries the adaptive transmission mode selection information that the SV should adopt when transmitting data streams in each sub-channel of OFDMA , the adaptive transmission mode selection information will be periodically sent to the OFDMA wireless resource configuration module at I/B through signaling MC-MES; various ITS data flow packets will be submitted to I/B through the OFDMA wireless resource configuration module at I/B After the frame sending/receiving module at the SV, together with the adaptive transmission mode selection information embedded in the control field, it is encapsulated into a frame, and then sent to the SV; the adaptive transmission mode selection module at the SV sends/ The receiving module extracts the corresponding adaptive transmission mode selection information, and assists the OFDMA wireless resource configuration module at the SV to demodulate and decode the data stream by generating corresponding MC-MES signaling, and finally completes the data stream to the application level information Delivery of service flow; the I/B is the infrastructure/base station, the SV is the user vehicle, and the ITS is the intelligent transportation system. 2 . The adaptive transmission mode selection system of the vehicle communication access network according to claim 1 , wherein the comprehensive information is access channel state information, driving state information and geographical area information. 3 . 3. An adaptive transmission mode selection method based on the vehicle communication access network of the system according to claim 1, characterized in that, comprising the following steps: 1) Determination of the switching threshold of multi-level modulation and coding transmission mode Taking the signal-to-noise ratio as a parameter index to measure the status of the SV access channel, the entire signal-to-noise ratio interval is divided into non-repetitive and continuous modulation, coding, and transmission mode switching intervals. The corresponding modulation, coding, and transmission mode switching thresholds are: And l∈{1,2,3,4,5,6}} Among them, t is the application type of ITS information service flow, l is the level of modulation and coding transmission mode, is when the application type of the ITS information service flow is t, the first-level modulation and coding transmission mode switching threshold; I/B determines the multi-level modulation and coding transmission mode switching threshold according to the target packet error rate limit information carried on the TPER-MES signaling ; 2) Initialization of modulation coding transmission mode level L When the system transmits the ITS information service flow with the application type t, the I/B initializes the application type t according to the SV initial instantaneous access channel state information γ′ carried on the INF-ACK signaling and the multi-level modulation and coding transmission mode switching threshold The ITS information service flow modulation and coding transmission mode level L, let L=l′, satisfy When the application type of the ITS information service flow is t, the switching threshold of the l′-level modulation and coding transmission mode; When the application type of the ITS information service flow is t, the l′+1th level modulation and coding transmission mode switching threshold; 3) Whether to start the judgment of the adaptive transmission mode selection process I/B follows a random backoff mechanism with a backoff time interval of (0, C _W -1), where C _W is a time window. According to the geographical area information of the SV carried in the INF-ACK feedback signaling and the SV travel The status information starts the adaptive transmission mode determination process. If the SV is in a special geographical area, or the SV travel speed exceeds 60km/h, it indicates that the system is not suitable for starting the adaptive transmission mode selection process, and the current modulation and coding transmission mode level will remain unchanged; otherwise , start the adaptive transmission mode selection process and enter step 4); 4) Determination of the level of adaptive transmission mode I/B judges according to the OFDMA frame cycle according to the SV instantaneous access channel state information γ carried on the INF-ACK feedback signaling, if Then update the modulation and coding transmission mode level L adopted to transmit the information service flow, so that L=1"; 5) Step 3) and step 4) are repeated continuously until all data packets of the ITS information service flow are sent. 4. The adaptive transmission mode selection method of the vehicle communication access network according to claim 3, characterized in that, in step 3), the special geographical area refers to: a bad weather environment, a complex road environment or a bridge tunnel area of the environment. 5. The adaptive transmission mode selection method of the vehicle communication access network according to claim 3, characterized in that, in step 3), the multiplication rule in the random back-off mechanism satisfies C _W,new =2 ^m C _W,old , where C _W,new is the new value of the time window C _W after a random backoff mechanism is implemented, C _W,old is the old value of the time window before the random backoff mechanism is implemented, and m is the fading parameter . 6. The adaptive transmission mode selection method of the vehicle communication access network according to claim 3, characterized in that, in step 4), when When the channel experiences deep fading, I/B will no longer send the data packets of this ITS information service flow. 7. the adaptive transmission mode selection method of vehicle communication access network according to claim 3, it is characterized in that, in step 1), I/B limits information according to the target packet error rate carried on the TPER-MES signaling, Determine the switching threshold of the multi-level modulation coding transmission mode, specifically: When the application type of ITS information service flow is t, the level-l modulation and coding transmission mode switching threshold It can be obtained by the following formula: $\&ForAll;t\&Element;\{1,2,3\}$ and $\&ForAll;l\&Element;\{1,2,3,4,5,6\}$ Among them, a _l and g _l are curve fitting coefficients, ζ _TPER,t represents the target packet error rate required by the ITS information service flow of application type t. 8. The adaptive transmission mode selection method of the vehicle communication access network according to claim 7, characterized in that, it is assumed that the system satisfies the following conditions: The power of I/B transmission of various ITS information service streams is constant; the SV's access channel/driving status/geographical area information feedback module can obtain error-free comprehensive information, and feed it back to I without error and delay /B's access channel/driving status/at the geographical area information detection module; the packet error rate detection based on the cyclic redundancy check is perfect, ignoring the redundancy caused by the packet sequence number and CRC check, and not considering MAC layer ARQ technology, and ignore its impact on the packet error rate; SV access channel is a slow-varying flat fading channel, the channel status remains unchanged within one frame, but can change between two frames, encapsulated in one frame The application types of the ITS information service flow in the network are the same, and the selection of the multi-level modulation and coding transmission mode is carried out on the basis of each frame; the Nakagami-m model is selected to model the slow-varying flat fading channel, and the instantaneous SNRγ of the SV access channel condition The probability density function of the ITS obeys the Gamma distribution, then when the I/B uses the l-level modulation and coding transmission mode to send the ITS information service flow of type t, the packet error rate ζ _t,l (γ) of the information service flow satisfies the following Mode: $\&ForAll;t\&Element;\{1,2,3\}$ and $\&ForAll;l\&Element;\{1,2,3,4,5,6\}$ Then the curve fitting coefficients a _l , g _l and γ _l are taken according to the following table: Table 6 Modulation coding transmission mode packet error rate curve fitting parameters Modulation coding transmission mode level l a _l g _l γ _l (dB) 1 274.7229 7.9932 -1.5331 2 90.2514 3.4998 1.0942 3 67.6181 1.6883 3.9722 4 50.1222 0.6644 7.7021 5 53.3987 0.3756 10.2488 6 35.3508 0.0900 15.9784