CN113518111B - Fountain code driven multi-channel side unit cooperative caching method - Google Patents
Fountain code driven multi-channel side unit cooperative caching method Download PDFInfo
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- CN113518111B CN113518111B CN202110496190.0A CN202110496190A CN113518111B CN 113518111 B CN113518111 B CN 113518111B CN 202110496190 A CN202110496190 A CN 202110496190A CN 113518111 B CN113518111 B CN 113518111B
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L67/00—Network arrangements or protocols for supporting network services or applications
- H04L67/50—Network services
- H04L67/56—Provisioning of proxy services
- H04L67/568—Storing data temporarily at an intermediate stage, e.g. caching
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
- H04L1/0056—Systems characterized by the type of code used
- H04L1/0057—Block codes
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L67/00—Network arrangements or protocols for supporting network services or applications
- H04L67/01—Protocols
- H04L67/12—Protocols specially adapted for proprietary or special-purpose networking environments, e.g. medical networks, sensor networks, networks in vehicles or remote metering networks
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/16—Performing reselection for specific purposes
- H04W36/18—Performing reselection for specific purposes for allowing seamless reselection, e.g. soft reselection
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W4/00—Services specially adapted for wireless communication networks; Facilities therefor
- H04W4/30—Services specially adapted for particular environments, situations or purposes
- H04W4/40—Services specially adapted for particular environments, situations or purposes for vehicles, e.g. vehicle-to-pedestrians [V2P]
Abstract
The invention discloses a fountain code driven multi-channel side unit cooperative caching method, which comprises the following steps: determining a file to be cached, and dividing the file to be cached into a plurality of data packets; storing each data packet in different road side units, wherein the data packets stored in any two road side units are overlapped; when the roadside units receive a request of a vehicle for a file to be cached, each roadside unit performs fountain coding on a data packet stored by the roadside unit, the data packets are selected inequality during coding to form a coding packet, and then the coding packet is sent; the vehicle decodes the received encoding packet sent by the road side unit to recover the cache file, and completes the fountain code-driven multi-path side unit cooperative cache.
Description
Technical Field
The invention belongs to the technical field of wireless communication, and relates to a fountain code driven multi-channel side unit cooperative caching method.
Background
In recent years, with the increasing maturity of 5G technology and the further deepening of 5G application, the car networking technology has received extensive attention from academic and industrial circles. In the face of many challenges such as various multimedia vehicle-mounted services with huge data volume, automatic driving application with high safety and high stability requirements, the vehicle-mounted network must adopt a more effective communication technology to meet the increasingly expanded vehicle-mounted network capacity requirement so as to meet higher service quality requirements. Researches show that the network bandwidth pressure can be effectively relieved, network congestion is reduced, and the service quality of users is improved by a caching technology of deploying files required by the users into a network in advance. By sinking the vehicle-mounted network cloud to a high-degree distributed macro base station, a small base station and even a road side unit, the vehicle-mounted network cloud provides the capabilities of communication, calculation, storage, control, management and the like close to the user side for the vehicle-mounted network.
The fountain code serving as a coding scheme without code rate has strong flexibility, and supports reliable data transmission under the scenes of high-speed movement of users and limited road side unit coverage capacity in an Internet of vehicles system. Therefore, the existing buffering scheme in the car networking widely uses fountain codes for content coding. However, for the fountain code-based encoding and caching technology in the car networking, the existing research mainly performs strategic optimization on caching positions and caching contents, and allocates the caching contents through specified objective functions, such as minimizing the time delay for users to acquire the contents, maximizing the unloading rate of the contents, and the like. The method for researching the fountain code-driven multi-path side unit cooperative caching has important significance in view of the problems.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a fountain code driven multi-channel side unit cooperative caching method, which can relieve the caching pressure of a side unit and improve the transmission efficiency of cached contents and the file recovery rate.
In order to achieve the above object, the method for buffering the fountain code driven multi-path side unit cooperation includes the following steps:
determining a file to be cached, and dividing the file to be cached into a plurality of data packets;
storing each data packet in different road side units, wherein the partial data packets stored in each road side unit are the same, and the content placement is finished;
when the roadside unit receives a request of a vehicle for a file to be cached, the service roadside unit of the vehicle performs fountain coding on a data packet stored by the roadside unit to form a coding packet, then sends the coding packet to the vehicle, and completes content distribution;
and the vehicle decodes the received encoded packet sent by the road side unit to restore a cache file, and completes the cooperative cache of the fountain code driven multi-path side unit.
And (4) dividing the file f to be cached into K data packets with equal length.
A roadside unit cluster is provided and comprises N roadside units which are continuously deployed. The data packets stored by each road side unit in the road side unit cluster have correlation, namely a plurality of identical data packets are stored in any two road side units in the road side unit cluster, and the number of the identical data packets is equal. There is no specificity between all the same packets stored on one rsu as the other rsus. And recording the same data packet as a superposition part data packet, and recording the rest data packets as non-superposition part data packets.
For the non-coincident data packets, starting from the first road side unit providing service for the vehicle, k is distributed to the road side units in sequence along the driving direction of the vehicle non And each data packet is independent, and the data packets are repeated for N times until the placement of the non-overlapped data packets on the N road side units is completed.
For coincident data packets, all coincident data packets are divided into
Portions, wherein each portion comprises k each_over Randomly selecting two RSUs from the N RSUs as a RSU combination, distributing a group of overlapped data packets for the RSU combination, and repeating the selection and distribution process
And thirdly, until all the coincident data packets are distributed. At the moment, N-1 parts of overlapped data packets are stored on each road side unit, and the total number of the overlapped data packets is k over ＝(N-1)k each_over 。
The road side unit selects different probability of each part of data packet when generating the coding packet in the content distribution stage. The generation process of the coding packet comprises the following steps: according to the robust soliton degree distribution function, the degree of generating a coding packet is d, and the physical meaning of the coding packet degree d is as follows: the encoded packet consists of d data packets. D non-duplicate data packets are selected from the storage space of the road side unit and subjected to exclusive-or operation according to bits. When the first selection data packet is selected, the data packet of the residual superposition part in the current road side unitNumber of is
The number of remaining non-overlapped part packets is
The probability of each packet in the overlap being selected is
The probability of each packet in the non-overlapping portion being selected is
A packet is selected according to a given probability. If a packet in the overlap is selected,
if the data packet in the non-overlapped part is selected, then
Wherein
The condition of the number of data packets of each part in the road side unit when the data packets are selected for the first time is shown; p over ，P non To initially select a probability, satisfy
In the process of generating the current encoding packet, the data packet which is already selected is not selected any more. And updating l = l +1, and repeating the data packet selection process until d data packets are selected. And carrying out bitwise XOR operation on the d data packets, and combining the bitwise XOR operation with the coding information to form a fountain code coding packet.
And after the vehicle receives the coded packet, decoding the coded packet through a belief propagation algorithm.
And after the vehicle recovers all the original data packets of the cache file, sending a feedback signal to the road side unit which provides service currently.
The invention has the following beneficial effects:
according to the fountain code driven multi-path side unit cooperative caching method, during specific operation, the caching files are stored in the multiple road side units after being divided, the caching pressure of the road side units is relieved through a content distribution mode of fountain code coding, the feedback retransmission burden caused by packet loss of a user is avoided, and the problem of transmission discontinuity caused by switching of the road side units in the vehicle driving process is solved. Compared with the existing fountain code coding content caching scheme, the vehicle can acquire the required file with higher probability within limited time, and the caching benefit is improved.
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The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the invention and not to limit the invention. In the drawings:
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a diagram of the placement of packets when the number of RSUs in a cluster of RSUs is 3;
FIG. 3 is a Markov state transition diagram of the state of data packets stored in the roadside unit during the formation of an encoded packet;
FIG. 4 is a comparison graph of the performance simulation of the file acquired by the receiving end in two different schemes.
Detailed Description
The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings. It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict.
The following detailed description is exemplary in nature and is intended to provide further details of the invention. Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the invention.
Referring to fig. 1, the method for buffering the fountain code driven multi-channel side unit cooperation according to the present invention includes the following steps:
1) Determining a file f to be cached, dividing the file f to be cached into K data packets with equal length, and considering that the K data packets are dispersedly stored in a target road side unit.
Defining a roadside unit cluster, wherein a cluster of roadside units includes N continuously deployed roadside units, that is, a set of roadside units {1,2, …, N }, data packets stored by each roadside unit in one roadside unit cluster have correlation, that is, data packets stored in any two roadside units in the roadside unit cluster are partially identical, the identical data packets are called overlapped data packets, the correlation between the data packets stored by each roadside unit is identical, that is, the number of overlapped data packets between any two roadside units is identical, and fig. 3 is a placement condition of the data packets when N = 3.
According to the equation
The combined solution obtains the number N of road side units in a road side unit cluster and the number of data packets stored on a road side unit i
Number of non-overlapped partial data packets stored on road side unit i
The number of packets that coincide between RSU i and RSU j
Wherein, R is the road side unit coverage area, v is the vehicle running speed, and D is the number of the downlink coding packets of the road side unit in unit time after normalization. If there is no specificity among the road side units, there are
The same data stored on one rsu as the other rsusThe packets are collectively referred to as coincident portion data packets. At this time, the total number of the data packets of the overlapped part on one road side unit is k over ＝(N-1)k each_over The non-overlapping part contains k data non 。
2) In order to improve the probability of recovering the file by the user, the process of placing the file f to be cached on the road side unit clusters is designed, and the process of placing the file f to be cached on the two clusters of road side units is completed.
The method comprises the following steps of taking the placement process of the content on a cluster of road side units as follows:
21 Placement of non-coincident data packets
Starting from the first road side unit serving the vehicle, k is assigned to the road side units in the vehicle travel direction non And each data packet is independent from each other and is repeated for N times until the placement of the non-overlapped data packets on the N road side units is completed.
22 ) coincident packet placement
Dividing all coincident data packets into
Portions, wherein each portion comprises k each_over When the quantity of the data packets is insufficient in the distribution process, randomly selecting and supplementing the distributed non-overlapped data packets, randomly selecting two road side units from the N road side units as a road side unit combination, distributing a group of overlapped quantity packet groups for the road side unit combination, and repeating the selection and distribution process
And secondly, the selected RSU combination cannot be superposed with the distributed RSU combination until all superposed data packets are distributed completely, and (N-1) k is stored on each RSU in the RSU cluster at the moment each_over A coincident data packet and k non A non-coincident data packet.
3) The fountain code coding-based content distribution can effectively cope with channel packet loss, a vehicle does not need to perform packet loss information feedback, when a roadside unit receives a request of the vehicle for a file f to be cached, the roadside unit performs fountain code coding on a data packet stored by the roadside unit to form a coding packet, then transmits the coding packet, and when the coding packet is generated, the probability of selecting each part of the data packet is different;
the generation process of the coding packet comprises the following steps: according to the robust soliton degree distribution function, the degree of generating a coding packet is d, and the physical meaning of the coding packet degree d is as follows: the encoded packet consists of d data packets. D non-duplicate data packets are selected from the storage space of the road side unit and subjected to exclusive-or operation according to bits. When the first selected data packet is selected, the number of the remaining overlapped part data packets in the current road side unit is equal to
The number of remaining non-coincident data packets is
The probability of each packet in the overlap being selected is
The probability of each packet in the non-overlapping portion being selected is
A packet is selected according to a given probability. If a packet in the overlap is selected,
if the data packet in the non-overlapped part is selected, then
Wherein
The condition of the number of data packets of each part in the road side unit when the data packets are selected for the first time is shown; p over ，P non Initially selecting a probability for the packet, which satisfies
In the process of generating the current encoding packet, the data packet which is already selected is not selected any more. And updating l = l +1, and repeating the data packet selection process until d data packets are selected. And carrying out bitwise XOR operation on the d data packets, and combining the bitwise XOR operation with the coding information to form a fountain code coding packet.
And the road side unit continuously sends the coded packets to a target user in a content distribution stage until the user leaves the coverage range of the target user or receives feedback information that the user successfully recovers the file.
4) When the content request vehicle receives the encoding packet of the road side unit, decoding is carried out through a belief propagation algorithm until the required file is recovered, and then a feedback signal is sent to the road side unit providing the service.
The following explains the packet unequal probability selection algorithm in step 3), taking N =3 as an example:
the generation process of the coding packet is disassembled, when the coding packet is generated for one time, each data packet stored on the road side unit can be selected only once, and the data packet is stored in the road side unit 1
Each data packet overlapped with the road side unit 2 is marked as an overlapped part 1, and the road side unit 1 stores the data packet
The packets overlapped with the roadside unit 3 are referred to as overlapped portions 2,k non Unique data packets are designated as non-overlapping portions. Because the number of the data packets overlapped between any two road side units is the same, the overlapped parts are equivalent, the data packets of the overlapped parts are regarded as a whole and are recorded as the overlapped parts again, and the total number of the data packets of the overlapped parts is k over Satisfy the following requirements
CounterweightThe selection of each packet in the pool portion is perfectly equally probable. In the process of generating a coded packet, when a data packet is selected for the first time, the probability that each data packet in the overlapped part is selected is P over Wherein the probability of each data packet being selected is P non . The state change process of the packet can be modeled as a binary markov process. By doublet (n) α ,n β ) Describing the state of the data packet, where n α 、n β Respectively representing the number of the selected data packets in the overlapped part and the number of the selected data packet groups in the non-overlapped part after selecting i (i is more than or equal to 0 and less than or equal to d) data packets, and simultaneously satisfying n α ≤k over ,n β ≤k non ,n α +n β And (i) = i. Similarly, let (n) α +1,n β ) Indicating that a coincident data packet is selected next time a data packet is selected, (n) α ,n β + 1) indicates that a non-coincident packet was selected.
The state of the data packet is represented by (n) α ,n β ) Transfer to (n) α +1,n β ) Has a probability of (k) over -n α )P over /[(k over -n α )P over +(k non -n β )P non ]Is transferred to (n) α ,n β A probability of + 1) is (k) non -n β )P non /[(k over -n α )P over +(k non -n β )P non ]。
According to the above analysis, a one-step state transition probability matrix P of the Markov process can be written, and the d-th power of the matrix P is calculated to obtain a d-step transition probability matrix P thereof d 。
Assuming that the Markov chain has M states, the state sequence is {1,2, …, M }, and a state transition probability matrix is represented as:
wherein the element representation of the ith row and j column is transferred from the state i through one stepProbability of state j. Calculating a d-step state transition matrix P through a one-step state transition probability matrix d ，P d The ith row and j column elements in (b) represent the probability of transitioning from state i to state j through step d.
In addition, the present invention only concerns the matrix P d The last few elements in the first row, i.e., the probability of selecting all possible combinations of d packets from the packets stored by the rsu. When an encoded packet is generated, the packet state is transferred from (0,0) to (n, d-n), where n ∈ [ max {0,d-k non },min{d,k over }]. Therefore, when the packet state is known as (n, d-n), the probability that a packet is selected in the overlapped part of packets is all
The probability that a packet in the non-overlapping part is selected is
When the degree of acquisition is d, the probability that one data packet in the overlapped part data packets stored in the road side unit is selected is as follows:
similarly, when the degree of the encoded packet is d, the probability that one data packet in the non-overlapped part of data packets stored in the road side unit is selected is as follows:
the degree distribution function used in the fountain code encoding process is μ (d), so that in the fountain code generation process, the average probability of selecting the overlapped part of the data packets stored in one road side unit is as follows:
similarly, the average probability of the non-overlapped part of the data packets being selected is as follows:
since the data packets of the overlapped part are cached twice, the probability of the non-overlapped data packets stored in one road side unit appearing at the receiving end is 2 times of the probability of the overlapped data packets appearing at the receiving end, so that the probability of the original data packets appearing at the receiving end is equal, and the effectiveness and the high efficiency of decoding are ensured.
The following system of equations is thus obtained:
obtaining P approximately satisfying the equation set by Monte Carlo simulation method over And P non The probability solution is substituted into step 3 to select a data packet.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and although the present invention is described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications and equivalents may be made to the embodiments of the invention without departing from the spirit and scope of the invention, which is to be covered by the claims.
Claims (3)
1. A fountain code driven multi-path side unit cooperative caching method is characterized by comprising the following steps:
determining a file to be cached, and dividing the file to be cached into a plurality of data packets;
storing the data packets in different road side units in a scattered manner;
when the road side unit receives a request of a vehicle for a file to be cached, the service road side unit of the vehicle performs fountain coding on a data packet stored by the service road side unit of the vehicle to form a coding packet, and then sends the coding packet to the vehicle;
the vehicle decodes the received encoding packet sent by the road side unit to recover the cache file;
dividing a file f to be cached into K data packets with equal length;
setting a road side unit cluster, wherein data packets stored by each road side unit in the road side unit cluster have correlation, namely, a plurality of identical data packets are stored in any two road side units in the road side unit cluster, the identical data packets are recorded as overlapped part data packets, the rest data packets are recorded as non-overlapped part data packets, and the data packets are recorded according to an equation
The combined solution obtains the number N of road side units in a road side unit cluster and the number of data packets stored on a road side unit i
Number of non-overlapping partial data packets stored on RSU i
The number of packets that coincide between RSU i and RSU j
Wherein R is the road side unit coverage, v is the vehicle running speed, D is the number of the downlink coding packets of the road side unit in unit time after normalization, and if each road side unit has no specificity, the number of the downlink coding packets of the road side unit in unit time after normalization has the
For non-coincident data packets, the vehicle is provided with the clothes from the firstStarting from the road side unit of the service, each road side unit is assigned k in turn in the direction of travel of the vehicle non Each data packet is independent, and the data packets are repeated for N times until the non-overlapped data packets on the N road side units are placed;
for coincident data packets, all coincident data packets are divided into
Portions, wherein each portion comprises k each_over Randomly selecting two RSUs from the N RSUs as a RSU combination, distributing a group of overlapped data packets for the RSU combination, and repeating the selection and distribution process
Thirdly, until all the coincident data packets are distributed, the total number of the coincident data packets stored in one road side unit is k over ＝(N-1)k each_over ；
The generation process of one code packet is as follows:
according to a robust soliton degree distribution function, the degree of generating a coding packet is d, the coding packet degree d is equal to the number of data packets forming the coding packet, d non-repeated data packets are selected from a storage space of a road side unit and are subjected to exclusive OR operation according to bits, and when the first selected data packet is selected, the number of residual coincident data packets in the current road side unit is d
The number of non-coincident data packets is
The probability that each coincident data packet is selected is
The probability of each non-coincident data packet being selected is
When a coincident data packet is selected, then
When a non-coincident data packet is selected, then
Wherein the content of the first and second substances,
P over and P non In order to initially select the probability(s),
and updating l = l +1, repeating the selection process of the data packets until the d data packets are all selected, performing bitwise XOR operation on the d data packets, and combining the bitwise XOR operation with the coding information to form the fountain code coding packet.
2. The fountain code driven multiple side unit cooperative buffering method according to claim 1, wherein the vehicle decodes the received encoded packets by a belief propagation algorithm.
3. The fountain code driven multi-channel side unit cooperative buffering method according to claim 1, wherein after the vehicle recovers the buffered file, a feedback signal is sent to the currently serving side unit.
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