WO2015066906A1 - 一种自适应退避算法中竞争窗的确定方法及系统 - Google Patents
一种自适应退避算法中竞争窗的确定方法及系统 Download PDFInfo
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W74/00—Wireless channel access
- H04W74/08—Non-scheduled access, e.g. ALOHA
- H04W74/0808—Non-scheduled access, e.g. ALOHA using carrier sensing, e.g. carrier sense multiple access [CSMA]
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- the present invention belongs to the field of electrical communication technologies, and in particular, to a method and system for determining a contention window in an adaptive backoff algorithm.
- Wireless LANs wireless local area networks
- Wireless ad hoc networks Wireless ad hoc networks
- Wireless mesh networks Networks Wireless mesh networks Networks
- MAC Medium access control
- the Media Access Control Protocol is primarily responsible for the scheduling and data transmission and reception of wireless nodes, while the most widely used MAC protocol is IEEE 802.11.
- IEEE 802.11MAC The protocol defines two channel access methods: DCF (Distributed Coordination Function) and Freely Selectable PCF ( Point coordinate function, centralized coordination function).
- DCF Distributed Coordination Function
- PCF Point coordinate function, centralized coordination function
- the DCF protocol is also known as CSMA/CA (Carrier Sense Multiple). Access with Collision Avoidance, Carrier Sense Multiple Access with Collision Avoidance).
- CSMA/CA Carrier Sense Multiple Access with Collision Avoidance
- DCF when multiple nodes compete for channels
- the protocol uses physical monitoring and virtual carrier monitoring mechanism to detect channel status. If the channel is busy, the random backoff mechanism is used to delay the access of the node, so as to effectively reduce the collision probability. Therefore, the choice of the backoff mechanism directly affects the performance of the entire network. Bad.
- the most widely used backoff algorithm in the 802.11 protocol is the BEB (Binary exponential backoff) algorithm.
- BEB Binary exponential backoff
- a node When a node wants to access a channel, it first detects the channel state. If the channel is busy, the node needs to delay access until it detects the channel idle DIFS (DCF interframe space) time slot and then accesses again. After the channel idle DIFS time slot, the node randomly generates a backoff time before the next transmission, and counts down the backoff time until the backoff time reaches zero, then the node sends data.
- DIFS DCF interframe space
- the backoff time is Where CW is the competition window, an integer greater than 0, Random(0,CW) means randomly taking an integer between 0-CW, aSlotTime means a slot time, which is the time of the system default one slot, CW min It is determined by the physical layer characteristics and is the initialization value.
- the backoff rules are as follows:
- the BEB algorithm is easy to implement, so as 802.11 Standard backoff algorithm. However, when the node successfully transmits once, its contention window is reset to the minimum value, so that the node still has a greater chance of contention success in the next time the channel is contending, and when the number of nodes is large, it is easy to cause serious unfairness.
- the CW replication mechanism is adopted - an area is added to the data packet to record the CW value.
- the neighbor node replicates and uses the CW of the source node to ensure that the neighbor nodes have the same CW.
- MILD algorithm effectively solves BEB The unfair problem, but the replication mechanism requires adding another space storage contention window to the package, which increases the complexity of the algorithm. At the same time, when nodes in different regions use different contention window values, it is difficult to determine which region's contention window to use in the intermediate nodes of adjacent regions, which is the migration problem.
- the status history of the channel needs to be further checked, and the CW is adjusted based on this history.
- the HBAB algorithm is simpler to implement and incorporates the historical channel state; however, its parameters It is based on the empirical value allocation, so it can not effectively reflect the actual channel state, and is not suitable for channels that change faster in real time.
- the traditional backoff algorithm some need to dynamically adjust the CW according to the estimated number of nodes, the CW obtained by such an algorithm can effectively reflect the theoretical environment, but the calculation of the estimated number of nodes requires a large number of calculations, and if the estimation is wrong, it may be the entire algorithm. The effect has a negative effect.
- Some algorithms simply consider changing the CW by linear or multiplicative rules, such as BEB and HBAB above. The CW is simply multiplied by 2 or initialized to CW min .
- the improvement of HBAB compared to BEB can be based on Channel history state to change CW, but its change factor According to the direct distribution of empirical values, the two algorithms are easier to implement, but they cannot effectively reflect the real-time channel environment.
- the first technical problem to be solved by the present invention is to provide a method for determining a contention window in an adaptive backoff algorithm. It aims to adaptively adjust the CW change rules according to the past real-time channel state, thereby increasing throughput, reducing delay, and improving network performance.
- the present invention is implemented in such a manner that a method for determining a contention window in an adaptive backoff algorithm includes the following steps:
- Step A Initialize the system and initialize the value of the contention window CW to the system default value
- Step B Record the channel status and combine the historical channel status in step A. Updating the value of the contention window CW based on the system default value;
- Step C Obtaining a backoff time slot by the value of the updated contention window CW, obtaining a channel utilization rate during the backoff time slot, and further obtaining a transmission probability according to the channel utilization rate;
- Step D acquiring a contention window change factor, and then determining a contention window CW according to the contention window change factor and the transmission probability .
- the contention window change factor in the competitive window determination method provided by the present invention
- the value is not fixed, but is obtained by considering adding new variable channel utilization.
- Optimized value dynamically adjusted based on the proportion of conflicts in the past when retreating Value, thus obtained
- the value is changed and is obtained according to the actual situation. Therefore, the size of the competition window is also dynamically adjusted, which can effectively reduce the unfairness of the entire network, increase system throughput, and reduce system delay.
- FIG. 1 is a flowchart of an implementation of a method for determining a contention window in an adaptive backoff algorithm according to an embodiment of the present invention
- FIG. 2 is a comparison diagram of a linear change of a competition window variation factor and a channel utilization ratio and a curve change according to an embodiment of the present invention
- FIG. 3 and FIG. 4 are schematic diagrams showing comparison of system throughput and time delay of the simulation software provided by the embodiment of the present invention in different node numbers;
- FIG. 5 and FIG. 6 are diagrams showing the simulation software provided by the embodiment of the present invention when the number of fixed nodes is 50. , a comparison of system throughput and delay in the case of different maximum number of retransmissions;
- FIG. 7 is a schematic structural diagram of a system for determining a contention window in an adaptive backoff algorithm according to an embodiment of the present invention.
- Figure 8 is a schematic diagram of channel utilization provided by an embodiment of the present invention.
- FIG. 9 is a table of rules of an HBAB algorithm according to an embodiment of the present invention.
- FIG. 10 is a simulation result of an algorithm for performing simulation using OPNET 14.5 simulation software according to an embodiment of the present invention.
- the invention is obtained by considering adding a new variable channel utilization rate Optimized value, dynamically adjusted based on the proportion of conflicts in the past when retreating Value, and finally achieve dynamic adjustment of the competition window.
- FIG. 1 is a flowchart showing an implementation process of a method for determining a contention window in an adaptive backoff algorithm provided by the present invention, which is described in detail below.
- step A the system is initialized and the value of the contention window CW is initialized to the system default.
- contention window CW min having a minimum contention window CW CW min and a maximum value CW max
- the default value of contention window CW min is located between this minimum value and a maximum value CW max.
- step B record the channel status and combine the historical channel status in step A.
- the value of the contention window CW is updated based on the system default value.
- the present invention is implemented based on the HBAB algorithm, and the HBAB algorithm rules are shown in the table of FIG. 0 in the table indicates a transmission failure or conflict, and 1 indicates that the transmission was successful. Represents the contention window factor. It can be known that when the current channel state is busy, the contention window CW is updated according to the rule to When the current channel status is at leisure, in addition to the past history of the state to channel 00 (represented in the past have suffered defeat twice sent or conflict) situation, the competition window directly into a minimum contention window CW min, as in the past sent two experienced The conflict, the size of the competition window becomes .
- step C The backoff time slot is obtained by the value of the updated contention window CW, and the channel utilization rate is obtained during the backoff time slot, and the transmission probability is further obtained according to the channel utilization rate.
- An integer is randomly selected between the CW values of the competition window and multiplied by the number of slots specified by the system, aSlotTime, to obtain the required backoff slot value.
- the channel utilization is obtained by the following formula:
- Busy_slots is the number of collisions experienced during the backoff time slot, that is, during the backoff, if the collision is stopped, the principle of stopping the backoff is counted, and once stopped, one is added.
- Figure 8 shows a schematic diagram of channel utilization. Slot time indicates the backoff time slot. There is a shadow padding part with a conflicting part, which is marked as slot time (busy). If there is no padding part, it means no conflict. ( dile ).
- the transmission probability can be obtained in two ways on the basis of the above channel utilization.
- the transmission probability p can be expressed as: , . Since mode one is linear, it may result in The change is too fast, so the maximum number of retransmissions m is added to the second method to obtain a new one. value: ,
- the maximum number of retransmissions of the parameter indicates the maximum number of retransmissions of the contention channel of the node.
- Figure 2 shows the comparison of the competition window variation factor in the linear variation of mode one and the curve variation of mode two. It can be seen that when linear variation is adopted, The value of the linear increase between 1 and 2 - increases as the channel utilization increases, decreases and decreases, but considers the case in the more extreme case, that is, when the number of nodes is large and small, When the number of nodes is large, the channel utilization rate is high and the conflict is large. The value may be close to 2, when the number of nodes is small, the channel utilization is low, and the conflict is small. Value may be close to 1, which may result in The value of the shock.
- step D And obtaining a contention window change factor, and then determining a contention window CW according to the contention window variation factor and the transmission probability.
- the new competitive window change factor For mode one, the new competitive window change factor : , .
- the new competitive window change factor For mode two, the new competitive window change factor : .
- Competition window change factor After determining, the competition window CW is determined according to the rules defined in Table 1 above.
- the above algorithm is simple to implement and adopts an adaptive backoff algorithm. Compared with the existing algorithm, it can effectively reduce the unfairness of the entire network, increase system throughput, and reduce system delay.
- Specific use of OPNET 14.5 The simulation software is used to simulate and the simulation parameters are in accordance with the IEEE 802.11 standard, as shown in the table shown in Figure 10.
- the simulation scenario is an area of 300 ⁇ 300 square meters.
- the network nodes are randomly distributed.
- the communication transmission range is 300 meters
- the simulation time is 300 seconds
- the basic access mode ie, no RTS/CTS mode is used.
- the simulation considers two situations:
- the 802.11 protocol has been widely used in real life, such as campus networks, office networks, urban building communications, and military communications networks. And the present invention is IEEE Improvements in the HBAB algorithm in the 802.11 protocol, if combined with hardware device improvements, will better facilitate the development of wireless communications, especially mobile communications.
- FIG. 7 shows the structural principle of the determination system of the contention window in the adaptive backoff algorithm provided by the present invention. For the convenience of description, only the parts related to the present invention are shown.
- the determination system of the contention window in the adaptive backoff algorithm includes a contention window value update module 71, a channel utilization and transmission probability calculation module 72, and a contention window value determination module 73.
- the competition window value update module 71 initializes the value of the contention window CW to the system default value after initializing the system, and updates the contention window CW based on the system default value based on the detected current channel state of the system and the recorded historical channel state. value.
- the channel utilization and transmission probability calculation module 72 obtains the backoff time slot by competing the value of the contention window CW updated by the window value update module 71, and obtains the channel utilization rate during the backoff time slot, and further obtains the transmission probability according to the channel utilization rate.
- the competition window value determination module 73 obtains the contention window change factor, and then determines the competition CW based on the competition window variation factor and the transmission probability calculated by the channel utilization and transmission probability calculation module 72.
- the channel utilization and transmission probability calculation module 72 obtains the channel utilization rate q according to the following formula: Where Total_slots is the backoff time slot, Busy_slots is the number of collisions experienced during the backoff time slot, and the transmission probability p can be expressed as: , .
- the contention window value determination module 73 determines a new contention window change factor based on the following formula. : , .
- the channel utilization and transmission probability calculation module 72 obtains the channel utilization rate q according to the following formula:
- Total_slots is the backoff time slot
- Busy_slots is the number of collisions experienced during the backoff time slot
- the transmission probability p can be expressed as: , where m is the maximum number of retransmissions of the parameter.
- the contention window value determination module 73 determines a new contention window change factor based on the following formula. :
- the HBAB algorithm actually considers the channel state in consideration of the BEB algorithm, and further adjusts the competition window.
- the biggest change is in the change rule, when the current state is idle. And when the past historical state is two busy, it does not directly set the competition window to the competition window minimum, but the value is the competition window divided by
- the algorithm author recommended The value is between 1-2, which is twice as large as the BEB. It is equivalent to adding the idea of considering slow backoff to effectively avoid excessive change of the contention window.
- the disadvantage of the existing HBAB is that The fixed value of the value, the algorithm author did not give The optimal value is directly distributed between 1-2 according to the empirical value, and the new algorithm provided by the present invention focuses on solving Optimization by considering adding new variable channel utilization Optimized value, dynamically adjusted based on the proportion of conflicts in the past when retreating Value, thus obtained The value is obtained according to the actual situation and is a change value, so the size of the competition window is also dynamically adjusted.
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Abstract
本发明适用于电通信技术领域，提供了一种自适应退避算法中竞争窗的确定方法及系统。方法包括下述步骤： A ：初始化系统，并将竞争窗CW的值初始化为系统默认值CWmin；步骤 B ：记录信道状态，并结合历史信道状态，在步骤 A 系统默认值的基础上更新竞争窗CW的值；步骤 C ：通过更新后的竞争窗CW的值获得退避时隙，并在此退避时隙期间获得信道利用率，再进一步根据信道利用率获得传输概率；步骤 D ：获取竞争窗变化因子，然后根据竞争窗变化因子以及传输概率确定竞争窗CW 。本发明通过考虑加入新的变量信道利用率来获得竞争窗变化因子的优化值，通过过去退避时候发生冲突的比例大小来动态调整，这样获得的变化因子是变化的，竞争窗也是动态调整的。
Description
本发明属于电通信技术领域，尤其涉及一种自适应退避算法中竞争窗的确定方法及系统。
在过去几十年，随着无线终端技术的发展，分布式网络正经历着一个高速发展的阶段，如无线局域网（
Wireless LANs ），无线自组织网（ Wireless ad hoc networks ）以及无线 mesh 网（ Wireless mesh
networks ）。在这些网络中，由于其移动性，低成本，灵活性等优点，分布式网络已经得到迅速的普及和推广，逐渐产业化，走进日常生活。
在无线分布式网络中， MAC （ Medium access control
媒体接入控制）协议主要负责无线节点的调度和数据的收发，而使用最广泛的 MAC 协议则是 IEEE 802.11 。 IEEE 802.11MAC
协议定义了两种信道接入方式： DCF （ Distributed coordinate function ，分布式协调功能）和可自由选择的 PCF （
Point coordinate function ，集中式协调功能）。
DCF 协议也就是大家所熟知的 CSMA/CA （ Carrier Sense Multiple
Access with Collision Avoidance ，带冲突避免的载波侦听多址接入）。当多个节点竞争信道时， DCF
协议采用物理监控和虚拟载波监控机制，检测信道状态，如信道忙则采用随机退避机制使得节点延迟接入，以此来有效减小冲突概率，所以退避机制的选择直接影响到整个网络性能的好坏。
802.11 协议中使用最广泛的退避算法是 BEB （ Binary exponential
backoff ，二进制指数退避）算法。当节点想接入信道时，先检测信道状态，如果信道繁忙，则节点需延迟接入，直到监测到信道空闲 DIFS （ DCF
interframe space ）个时隙后再次接入。在信道空闲 DIFS
时隙后，节点则在下次传输前随机产生一个退避时间，并对退避时间作倒计时操作，直到退避时间到达零，则节点发送数据。退避时间为
其中， CW为竞争窗，一个大于 0 的整数，Random(0,CW) 表示在 0- CW 之间随机取一个整数，
aSlotTime指 a slot time ，也就是系统默认的一个时隙的时间，
CWmin是由物理层特性决定的，为初始化值。退避规则如下：
BEB 算法实现容易，故作为 802.11
的标准退避算法。但是当节点成功发送一次后，其竞争窗重置为最小值，这样该节点在下次争用信道时仍有更大机会争用成功，在节点数较多时，容易导致严重的不公平问题。
为了改进
BEB
的不公平问题，
MILD
（
Multiplicative Increase Leaner Decrease
，乘性增加线性减小）算法在经历冲突时，将 CW 乘以一个因子
；在成功发送后，则减去一个因子
，同时采用 CW 复制机制 -
在数据包中加入一个区域，记录CW值，发送数据包时，邻节点复制并采用源节点的CW,以此来保证邻节点都有相同的CW 。其规则如下：
经历冲突
成功发送
MILD 算法有效解决了 BEB
中的不公平问题，但是复制机制要求在包中加入另外的空间存储竞争窗，这样增加了算法的复杂性。同时，当在不同区域的节点使用不同的竞争窗值时，在相邻区域的中间节点很难决定使用哪个区域的竞争窗，这就是迁移问题。
后面又有学着提出HBAB（History Based Adaptive
Backoff,基于历史记录的自适应退避）算法，该算法将过去的信道状态纳入考虑范围，以此来反映网络状态，据此来决定是否该增加CW 还是减小
CW,当传输失败时，CW 乘以一个参数
；传输成功时，则需要进一步检查信道的状态历史，基于这个历史来调整CW。
HBAB 算法实现较为简单，且加入考虑历史信道状态；但是其参数
是根据经验值分配，故不能有效反映实际信道状态，也不大适合实时变化较快的信道。
总之，传统的退避算法，有些要根据估计节点数来动态调整
CW,此类算法获得的CW能有效反映理论环境，但是计算估计节点数需要大量的计算，同时如果估计错误，则可能对整个算法的效果产生不良的影响。而有些算法则简单的考虑通过线性或者乘性的规则来改变CW，如上文中的
BEB 和 HBAB ，其CW都是简单的乘 2 或者初始化为CWmin ， HBAB 的相比 BEB
的改进在其能根据信道历史状态来变化CW，但是其变化因子
根据经验值直接分配，此两种算法实现较为容易，但是不能有效反应实时的信道环境。
本发明所要解决的第一个技术问题在于提供一种 自适应退避算法中竞争窗的确定方法
，旨在根据过去实时的信道状态，自适应调整CW的改变规则，以此增加吞吐量，减小延迟，提高网络性能。
本发明是这样实现的，一种 自适应退避算法中竞争窗的确定方法，包括下述步骤：
步骤 A ：初始化系统，并将竞争窗CW的值初始化为系统默认值；
步骤 B ：记录信道状态，并结合历史信道状态，在步骤 A
系统默认值的基础上更新竞争窗CW的值；
步骤 C
：通过更新后的竞争窗CW的值获得退避时隙，并在此退避时隙期间获得信道利用率，再进一步根据信道利用率获得传输概率；
步骤 D ：获取竞争窗变化因子，然后根据所述竞争窗变化因子以及所述传输概率确定竞争窗CW
。
与传统的 HBAB 算法相比，本发明提供的竞争窗确定方法中竞争窗变化因子
的取值不固定，而是通过考虑加入新的变量信道利用率来获得
的优化值，根据过去退避时候发生冲突的比例大小来动态调整
值，这样获得的
值是变化的，是根据实际情况来获得的，故竞争窗大小也是动态调整的，可有效减小整个网络的不公平性，同时增加系统吞吐量，减小系统时延。
图 1 是本发明 实施例提供的 自适应退避算法中竞争窗的确定方法的实现流程图；
图 2 是本发明 实施例提供的 竞争窗变化因子 随信道利用率的线性变化和曲线变化对比图；
图３、图４是本发明实施例提供的仿真软件在不同节点数情况下，系统吞吐量和时延的对比示意图；
图５、图６是本发明实施例提供的仿真软件当固定节点数为 50
，在不同最大重传次数情况下，系统吞吐量和时延的对比示意图；
图７是本发明实施例提供的自适应退避算法中竞争窗的确定系统的结构原理图；
图 8 本发明实施例提供的信道利用率的示意图；
图9是本发明实施例提供的HBAB 算法规则表；
图10是本发明实施例提供的算法采用OPNET 14.5 仿真软件来进行仿真的仿真结果 。
为了使本发明的目的、技术方案及优点更加清楚明白，以下结合附图及实施例，对本发明进行进一步详细说明。应当理解，此处所描述的具体实施例仅仅用以解释本发明，并不用于限定本发明。
本发明通过考虑加入新的变量信道利用率来获得
的优化值，根据过去退避时候发生冲突的比例大小来动态调整
值，最终实现竞争窗的动态调整。
图 1 示出了本发明提供的自适应退避算法中竞争窗的确定方法的实现流程，详述如下。
在步骤 A 中，初始化系统，并将竞争窗CW的值初始化为系统默认值。
本发明中，竞争窗CW具有一最小值CWmin和最大值CWmax，系统默认的竞争窗值位于此最小值CWmin和最大值CWmax
之间。
在步骤 B 中，记录信道状态，并结合历史信道状态，在步骤 A
系统默认值的基础上更新竞争窗CW的值。
本发明基于 HBAB 算法实现， HBAB 算法规则如图9的表所示。 该表中 0
表示发送失败或冲突， 1 表示发送成功，
表示竞争窗因子。可知当当前信道状态为忙时，竞争窗CW根据规则更新为
，当当前信道状态为闲时，除了过去信道历史状态为 00
（表示过去两次发送都遭遇失败或冲突）的情况，竞争窗直接变为竞争窗最小值CWmin ，如过去两次发送都经历了冲突，则竞争窗大小变为
。
在步骤 C
中，通过更新后的竞争窗CW的值获得退避时隙，并在此退避时隙期间获得信道利用率，再进一步根据信道利用率获得传输概率。
获得退避时隙的公式为
，即从 0
至竞争窗CW值之间随机选择一个整数乘以系统规定好的时隙数aSlotTime,以此来获得所需退避的时隙值。
在退避时隙期间，通过下述公式获得信道利用率：
其中，Total_slots为所述退避时隙，
Busy_slots为在所述退避时隙期间经历的冲突数，即，在退避期间，如果碰到冲突则停止退避的原理来计数的，停止一次则加一。图 8 示出了信道利用率的示意图，
slot time 表示退避时隙，有有阴影填充部分为有冲突部分，记为 slot time （ busy ），无填充部分则表示没有冲突，记为 slot time
（ dile ）。
本发明中，在上述信道利用率基础上具体可通过两种方式获得传输概率，方式一，传输概率p则可以表示为：
,
。由于方式一为线性变化，可能导致
变化过快，故方式二中加入参数最大重传次数m，获得新的
值：
,
,本发明中，参数最大重传次数表示的是节点竞争信道的最大重传次数，当节点竞争信道失败时，会进入重传阶段，做下一次竞争，但是系统不会无休止的让节点多次进入竞争，以免影响系统容量，当节点的重传次数超过m的时候，则节点会采取丢包的措施，即将发送的数据包丢弃，所以m值对系统容量的影响也很大。
图2示出了方式一的线性变化和方式二的曲线变化中竞争窗变化因子的对比，可见，当采用线性变化时，
值在1和2之间做线性增加的变化-随着信道利用率的增加而增加，减小而减小，但是考虑在比较极端的情况下，即节点数较多和较少时的情况，节点数较多时，信道利用率高，冲突大，
值可能就取到接近2，当节点数较小时，信道利用率低，冲突小，
值可能取到接近于1，这样可能造成
取值的震荡。如果采用曲线形式，一方面是基于慢慢变化的考虑，即减小
取值的震荡，同时加入考虑m取值，可以发现，当节点重传次数很多的时候，如果我们采用较小的
变化，则其接入信道机会更大，这样能增加整个系统的公平性。比如说在信道利用率为0.9的情况，这时冲突很大，在线性变化时，
取值一直为1.9，而对于重传次数较大的节点，采用曲线变化则可以变成小于1.9的值，这样能给节点更小的CW，这样可以使得重传次数多的节点更高优先级下次争用信道，以此来增加系统的公平性。
在步骤 D
中，获取竞争窗变化因子，然后根据所述竞争窗变化因子以及所述传输概率确定竞争窗CW。
对于方式一，新的竞争窗变化因子
:
,
。 对于方式二，新的竞争窗变化因子
：
。 竞争窗变化因子
确定后，再根据上表一定义的规则来确定竞争窗CW 。
上述算法实现简单，且采用自适应的退避算法，相比现有算法，能有效减小整个网络的不公平性，同时增加系统吞吐量，减小系统时延。具体可采用 OPNET 14.5
仿真软件来仿真，仿真参数按照 IEEE 802.11 标准，详见图10所示的表。
仿真场景为300×300平方米的区域，网络节点随机分布，通信传输范围为300米，仿真时间为300秒，采用基本的接入方式（即不使用RTS/CTS模式）仿真。仿真考虑两种情况：
（1） 不同节点数情况下，比较系统吞吐量和时延，见图３和图４；
（2）固定节点数为50，在不同最大重传次数情况下，比较系统吞吐量和时延，见图５和图６。
IEEE
802.11协议目前已广泛应用于实际生活中，如校园网、办公室网络、城市建筑通信以及军事通信网等。而本发明对IEEE
802.11协议中ＨＢＡＢ算法的改进，如能结合硬件设备改进，将能更好的促进无线通信尤其是移动通信的发展。
图７示出了本发明提供的自适应退避算法中竞争窗的确定系统的结构原理，为了便于描述，仅示出了与本发明相关的部分。
参照图７，本自适应退避算法中竞争窗的确定系统包括竞争窗值更新模块７１、信道利用率及传输概率计算模块７２和竞争窗值确定模块７３。竞争窗值更新模块７１在初始化系统后将竞争窗CW的值初始化为系统默认值，并根据检测到的系统当前信道状态和记录的历史信道状态，在系统默认值的基础上更新竞争窗CW的值。信道利用率及传输概率计算模块７２通过竞争窗值更新模块７１更新后的竞争窗CW的值获得退避时隙，并在此退避时隙期间获得信道利用率，再进一步根据信道利用率获得传输概率。竞争窗值确定模块７３获取竞争窗变化因子，然后根据竞争窗变化因子以及所述信道利用率及传输概率计算模块７２计算的传输概率确定竞CW。
作为本发明的一个实施例，信道利用率及传输概率计算模块７２具体根据下述公式获得信道利用率q：
其中，Total_slots为所述退避时隙，Busy_slots为在所述退避时隙期间经历的冲突数，传输概率p则可以表示为：
,
。然后竞争窗值确定模块７３根据下述公式确定新的竞争窗变化因子
:
,
。
作为本发明的另一实施例，信道利用率及传输概率计算模块７２具体根据下述公式获得信道利用率 q:
其中，
Total_slots为所述退避时隙，Busy_slots为在所述退避时隙期间经历的冲突数；
传输概率 p 则可以表示为：
，其中， m 为参数最大重传次数 。 然后 竞 争窗值确定模块７３根据下述公式确定新的竞争窗变化因子
:
。
综上所述，通过比较 HBAB 算法和 BEB 算法可知， HBAB 算法相比 BEB
算法，其实就是加入考虑了信道状态，进而进一步的调整竞争窗，最大的变化在于变化规则中，当现在的状态为空闲，而过去历史状态为两个繁忙时，其不是直接将竞争窗置为竞争窗最小值，而是取值为竞争窗除以
，另外，算法作者推荐
取值在 1-2 之间，这样相比 BEB 直接为两倍变化，也相当于加入考虑慢退避的思想，有效避免竞争窗变化过大；但是现有的 HBAB
的缺点在于
取值的固定性，算法作者并没有给出
的最优值，而是依照经验值直接分配在 1-2 之间，而本发明提供的新算法则重点解决
的优化，通过考虑加入新的变量信道利用率来获得
的优化值，根据过去退避时候发生冲突的比例大小来动态调整
值，这样获得的
值是根据实际情况来获得的，是一变化值，故竞争窗大小也是动态调整的。
以上所述仅为本发明的较佳实施例而已，并不用以限制本发明，凡在本发明的精神和原则之内所作的任何修改、等同替换和改进等，均应包含在本发明的保护范围之内。
Claims (10)
- 一种 自适应退避算法中竞争窗的确定方法，其特征在于，包括下述步骤：步骤 A ：初始化系统，并将竞争窗CW的值初始化为系统默认值CWmin；步骤 B ：记录信道状态，并结合历史信道状态，在步骤 A 系统默认值的基础上更新竞争窗CW的值；步骤 C ：通过更新后的竞争窗CW的值获得退避时隙，并在此退避时隙期间获得信道利用率，再进一步根据信道利用率获得传输概率；步骤 D ：获取竞争窗变化因子，然后根据所述竞争窗变化因子以及所述传输概率确定竞争窗CW。
- 如权利要求1所述的自适应退避算法中竞争窗的确定方法，其特征在于，所述步骤C中具体根据下述公式获得信道利用率q:
其中， Total_slots 所述退避时隙， Busy_slots为在所述退避时隙期间经历的冲突数，传输概率 p则可以表示为：
- 如权利要求2所述的自适应退避算法中竞争窗的确定方法，其特征在于，所述步骤D根据下述公式确定新的竞争窗变化因子
- 如权利要求1所述的自适应退避算法中竞争窗的确定方法，其特征在于，所述步骤C中具体根据下述公式获得信道利用率q：
其中，Total_slots为所述退避时隙，Busy_slots为在所述退避时隙期间经历的冲突数；传输概率p则可以表示为：
- 如权利要求4所述的自适应退避算法中竞争窗的确定方法，其特征在于，所述步骤D根据下述公式确定新的竞争窗变化因子
- 一种自适应退避算法中竞争窗的确定系统，其特征在于，包括：竞争窗值更新模块，用于在初始化系统后将竞争窗CW的值初始化为系统默认值，并根据检测到的系统当前信道状态和记录的历史信道状态，在系统默认值的基础上更新竞争窗CW的值；信道利用率及传输概率计算模块，用于通过所述竞争窗值更新模块更新后的竞争窗CW的值获得退避时隙，并在此退避时隙期间获得信道利用率，再进一步根据信道利用率获得传输概率；竞争窗值确定模块，用于获取竞争窗变化因子，然后根据所述竞争窗变化因子以及所述信道利用率及传输概率计算模块计算的传输概率确定竞争窗CW。
- 权利要求6所述的自适应退避算法中竞争窗的确定系统，其特征在于，所述信道利用率及传输概率计算模块具体根据下述公式获得信道利用率q：
其中，Total_slots为所述退避时隙，Busy_slots为在所述退避时隙期间经历的冲突数，传输概率p则可以表示为：
- 权利要求7所述的自适应退避算法中竞争窗的确定系统，其特征在于，所述竞争窗值确定模块根据下述公式确定新的竞争窗变化因子
- 如权利要求6所述的自适应退避算法中竞争窗的确定系统，其特征在于，所述信道利用率及传输概率计算模块具体根据下述公式获得信道利用率q：
其中，Total_slots为所述退避时隙，Busy_slots为在所述退避时隙期间经历的冲突数；传输概率p则可以表示为：
- 权利要求9所述的自适应退避算法中竞争窗的确定系统，其特征在于，所述竞争窗值确定模块根据下述公式确定新的竞争窗变化因子
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