CN102076106B - Time frequency two-dimensional hybrid MAC layer access method - Google Patents

Abstract

The invention discloses a time-frequency two-dimensional hybrid MAC layer access method. In the two-dimensional directions of the frequency domain and the time domain, multiple users can jointly compete for available resources, and the available resources include various sub-channels and different time slots. The back-off mechanism is used to avoid conflicts, and users access the wireless channel for data transmission by competing for available resources and combining with the reservation mechanism. The present invention utilizes frequency resources and time resources at the same time, and combines competition and reservation mechanisms to increase system channel utilization and avoid conflicts, thereby effectively improving the throughput of the entire system.

Description

Time-Frequency Two-Dimensional Hybrid MAC Layer Access Method

technical field

The present invention relates to the field of communication technology, in particular to a time-frequency two-dimensional mixed media access control (Media Access Control, MAC) layer access method based on CSMA/CA and OFDMA.

Background technique

With the rapid development of mobile communication technology, Internet technology and computer technology, mobile communication is no longer limited to pure voice communication, and high data rate services have attracted more and more attention. In the future, mobile communication users will continue to improve their expectations in terms of types and applications of the communication services that can be realized. Wireless local area network is the product of the combination of computer network and wireless communication technology. As a typical form of broadband wireless IP access, wireless local area network has the advantages of mobile computing, flexible and fast networking, low maintenance costs and good scalability. It is widely used in many areas, such as airports, schools, enterprises, hotels, etc. It has its unique advantages in providing occasions where broadband high-speed wireless access is required and the mobile speed is not high. The International Telecommunication Union (ITU) pointed out that in the future, users expect a dynamic and continuous service, and can seamlessly realize the required service across the network through the same terminal device (same number or address). The all-IP network composed of wireless local area network and Internet is a powerful supplement to the wide-area cellular mobile communication network.

The most important role of the MAC layer access protocol is to coordinate the efficient sharing and use or access of wireless channel resources by STAs (short for Stations). In addition, the functions of the MAC layer protocol also include ensuring the quality of service (Quality of Service, QoS) of the system, reducing the redundancy of data stream transmission as much as possible, etc. An efficient MAC layer protocol can bring higher efficiency and system throughput to WLAN (Wireless Local Area Networks, WLAN). The next generation of WLAN systems will require higher transmission rates with wider transmission bandwidths. The MAC layer access protocol has become one of the key issues in the next generation WLAN system. The IEEE802.11 standard defines the MAC layer access protocol of WLAN, which adopts the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance, Carrier Sense Multiple Access with Collision Avoidance) mechanism with binary exponential backoff, called DCF (Distributed Coordination Function, distributed coordination function). Due to the high collision probability of the existing simple CSMA/CA mechanism MAC layer protocol, especially in the case of a large number of STAs and a large amount of data, the impact of collisions reduces the efficiency of the MAC layer and limits the entire system. throughput.

For the next-generation broadband wireless communication system, OFDMA (Orthogonal Frequency Division Multiple Access) is a key access technology, which can divide the entire frequency band into multiple narrowband sub-channels, enabling different sites to use it simultaneously Different sub-channels transmit data at the same time without interference.

Contents of the invention

(1) Technical problems to be solved

The technical problem to be solved by the invention is: how to improve the channel utilization rate of the system, avoid conflicts and improve the throughput of the system at the same time.

(2) Technical solutions

In order to solve the above technical problems, the present invention provides a time-frequency two-dimensional hybrid MAC layer access method, comprising the following steps:

S1. When the user has data to be sent, the user periodically detects the current usage of each sub-channel. When an idle sub-channel is detected, select the sub-channel and enter step S2. Otherwise, the user continues to detect the current usage of each sub-channel. usage;

S2. The user enters the back-off phase: set the value of the back-off counter, and the back-off counter starts to subtract one from this value at the end of each time slot, and enters step S3 until the value is zero;

S3. The user reserves a time-frequency resource to the access point. After the reservation is successful, the time-frequency resource is used to send data. If the reservation is not successful, re-enter step S1, that is, re-detect the idle sub-channel, and re-detect the idle sub-channel. Set the value of the backoff counter;

S4. After the user successfully receives the confirmation frame sent back by the access point, the value of the backoff counter is reset to zero; otherwise, return to step S1, and the user resends the lost data.

Wherein, the sub-channel is formed in the following manner: multiple OFDM sub-carriers form a sub-channel, and the entire channel is divided into multiple sub-channels, and each sub-channel is orthogonal to each other. This ensures that there is no mutual interference when sending data on each sub-channel, and the channel is efficiently utilized in both the time domain and the frequency domain.

Wherein, in the step S2, if the user detects that the currently selected subchannel is occupied by other users prior to the current time slot, then it is detected whether there are other idle subchannels, and if there is, the backoff counter is decremented by one, if the current If all subchannels are occupied, the current value of the backoff counter is kept unchanged, and when the user detects an idle subchannel again, the backoff counter starts to decrement from the current value. Therefore, users can flexibly access idle sub-channels, shorten waiting time, ensure efficient use of channels, and improve channel utilization.

Wherein, the value of the backoff counter is calculated according to the value of the backoff period.

Wherein, when the user finishes sending a piece of data and has the next piece of data to be sent immediately, the currently occupied sub-channel is first released, and the backoff phase starts again. In this way, the fairness of channels occupied by each user can be guaranteed.

Among them, the backoff criterion adopted in the backoff phase is the binary exponential backoff criterion.

Wherein, the time-frequency resources include subchannels and time slots.

Wherein, if the value of the backoff counter is BackoffCounter _n , then:

BackoffCounter _n = INT(CW×Random()×T _slot )

In the above formula, INT() means rounding operation, Random() means random number operation between [0, 1], T _slot means a time slot, CW=2 ⁱ CW _min , i∈[0, m] Indicates the back-off period, where m indicates the maximum value of the back-off period, and CW _min indicates the minimum value of the contention window defined in the carrier sense multi-point access/collision avoidance mechanism.

The handshake frame used in step S3 includes a request to send frame and a clear to send frame, the request to send frame and the clear to send frame all contain the length information of the data sent by the user, when other users detect the length information, Update the local network allocation vector set and no longer send handshake information to reserve the currently occupied sub-channel.

(3) Beneficial effects

The present invention adopts time-frequency two-dimensional access, and each station arbitrarily detects idle sub-channels or idle time slots to realize data transmission, thus increasing the chance of random access, that is, increasing available resources, thereby improving resource utilization and making The system can accommodate more users, reduce the probability of collision, and improve transmission efficiency, thereby increasing the amount of data that may be transmitted per unit time, that is, improving system throughput.

Description of drawings

Fig. 1 is method flowchart of the present invention;

Figure 2 is a time-frequency two-dimensional access sequence diagram;

Fig. 3 is a flow chart of STA data frame status;

Fig. 4 is the conflict probability performance curve obtained by simulating the method of the present invention and the traditional method;

FIG. 5 is a normalized saturated throughput performance curve obtained by simulating the method of the present invention and the traditional method.

Detailed ways

The specific implementation manners of the present invention will be described in further detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

The design idea of the present invention is: in the frequency domain dimension, use OFDMA technology for access: a group of subcarriers forms a subchannel, the entire bandwidth is divided into multiple different subchannels, and different users occupy different subchannels. The sub-channels are orthogonal to each other without interference. In the time dimension, the use of CSMA/CA technology enables different users (hereinafter referred to as stations) to compete to reserve different time slots, and each user occupies different time slots to send data, avoiding interference. Thus, two-dimensional access in the time domain and frequency domain is realized by combining OFDMA and CSMA/CA technologies.

Referring to Figure 1, assuming that there are N stations (STAs) in the system, the channel is divided into M sub-channels and N≥M, the state when the channel is occupied is called Busy state, and the state when the channel is not occupied is called Idle (Idle) state, DIFS (Distributed Interframe Space) means distributed interframe space, SIFS (Short Interframe Space) means short interframe space, and the SIFS duration is less than the DIFS duration, that is, T _SIFS < T _DIFS .

First execute step S1: when any station n (STAn) has data to send, detect the busy/idle status of all M sub-channels, if it detects that the sub-channels are in the Idle state for the continuous DIFS duration, then STAn adopts any backoff criterion , for example the binary exponential backoff criterion (which is prior art) enters the backoff phase (step S2):

First STAn calculates the value of its backoff counter, BackoffCounter _n :

BackoffCounter _n = INT(CW×Random()×T _slot ) (1)

Among them, INT () represents the rounding operation, Random () represents the random number operation between [0, 1], T _slot represents a time slot (its value is determined by the physical layer), CW=2 ⁱ CW _min , i ∈[0,m] represents the backoff period, where m represents the maximum value of the backoff period. CW _min represents the minimum value of the contention window (see the definition in the CSMA/CA mechanism), which is usually set according to system parameters, and can take the value 32 defined in the 802.11 standard.

The initial value of the backoff period i is 0, and as long as the STAn detects that there is an Idle subchannel in the current time slot, its backoff counter will decrease by one at the end of each time slot. If STAn detects that the currently selected sub-channel is occupied by other STAs in the current time slot, STAn will detect other sub-channels again, and if it finds that there are other idle sub-channels, STAn will continue to decrement the backoff counter at the end of each time slot. One, if it is found that all subchannels in the current time slot are in the Busy state, STAn keeps the value of the current backoff counter unchanged and waits to continue detecting available Idle subchannels.

When BackoffCounter _n reaches 0, STAn enters the stage of competing for reserved time-frequency resources (step S3):

STAn sends an RTS (Request to Send) frame to the AP (Access Point, request to send) to reserve time-frequency resources, that is, to reserve the currently detected available subchannels and time slots. When the AP receives the RTS frame, it waits for the SIFS duration Then send a CTS (Clear to Send) frame to STAn. When STAn receives the CTS, it indicates that it has successfully reserved resources and can use the current time slot and sub-channel to start sending data. Then STAn starts sending data frames immediately after waiting for the SIFS duration . When STAn does not receive a CTS frame from the AP within the CTS frame waiting timeout time (CTS_Timeout), STAn considers that RTS has collided or RTS/CTS is lost. At this time, the backoff period i is increased by one, and the backoff counter will be based on the new CW=2 ⁱ CW _min is recalculated according to formula (1) until the CW reaches the maximum value CW _max , usually CW _max = 2 ^m CW _min .

S4: After the AP receives the data frame from STAn, it sends an acknowledgment frame (ACK (Acknowledge) frame) immediately after the SIFS duration. After STAn receives the ACK, it indicates that the data has been successfully received by the AP. This sending process is completed, and the backoff counter value is reset to zero. If STAn has not received an ACK frame from the AP after the ACK Timeout period has elapsed, STAn judges that the data transmission has failed, then STAn re-detects the available time-frequency resources, and re-reserves time-frequency resources for retransmission data.

It can be seen that users reserve available resources through competition, and the AP does not perform centralized allocation and scheduling, and users do not need to wait for the allocation signal from the AP, which can ensure that frequency and time resources are fully utilized, and can ensure the fairness of channels occupied by each user. In addition, the above-mentioned RTS, CTS, and ACK frames and data packets can all be sent on each sub-channel, that is, the system does not distinguish between the control frame channel and the data packet channel, which can avoid causing conflicts between the control frames of each user on the control channel cause inefficiency. Moreover, various handshake frames, including RTS and CTS frames, contain the length information of the data packet sent by the user. When other users detect this length information, they will automatically update the local network allocation vector (Network Allocation Vector, NAV) set The handshake information will not be sent to reserve the currently reserved sub-channel, which improves the efficiency.

The following example illustrates.

FIG. 2 shows a sequence diagram of time-frequency two-dimensional access. In the frequency domain, a channel is divided into multiple sub-channels, and each sub-channel is formed by multiple sub-carriers. As shown in Figure 2, STA1, STA2, and STA4 respectively occupy different sub-channels to transmit data at the same time, and STA3 waits for STA2 to complete sending, then reserves resources and starts sending. It can be seen from FIG. 2 that the STAs do not interfere with each other, and resources can be efficiently used.

Fig. 3 shows the data frame state transition diagram of each STA. The circles in the figure represent the current states, and E1...E7 represent various events that trigger state transitions. The meanings are as follows:

E1 indicates initialization and there is data to be sent; E2 indicates that there is an idle subchannel available in the current time slot; E3 indicates that all subchannels in the current time slot are occupied, and the backoff counter has not reached 0; E4 indicates that the current time slot is detected to be available subchannel, and the backoff counter is not 0; E5 indicates that the backoff counter is 0, and at least one subchannel is idle in the current time slot; E6 indicates that no CTS frame is received from the AP within the CTS Timeout; E7 indicates that a CTS frame is received Start sending data; E8 means that the ACK from the AP has not been received within the ACK Timeout; E9 means that the data is sent successfully and needs to return to the initial state.

Initialization (INIT) indicates the initialization state; detection (SENSING) indicates detection of an idle subchannel in the current time slot; backoff (BACKOFF) indicates that the backoff counter starts to work; freezing (FREEZE) indicates that the backoff counter is frozen (that is, the value of the backoff counter remains unchanged) And detect available resources; reserve (RESERVE) means reserve available resources; send (TRANSMIT) means send data frame.

Use the form of event-triggered state transfer shown in Figure 3 to complete the work of the two-dimensional access mechanism. The specific process is as follows:

1) In the INIT state, when there is data to be sent, it is triggered by the event E1 to enter the SENSING state;

2) In the SENSING state, the STA detects the available subchannels of the current time slot. When an available resource is detected, it is triggered by the event E2 to enter the BACKOFF state to enter the backoff phase;

3) In the BACKOFF state, the STA calculates the value of BackoffCounter and automatically decreases it by one at the end of each time slot. When all available resources are occupied before the counter reaches 0, it is triggered by event E3 to enter the FREEZE state;

4) In the FREEZE state, the STA saves the current BackoffCounter value, and continues to detect available resources, once the available resources are detected, it will be triggered by the event E4 to return to the BACKOFF state;

5) In the RESERVE state, the STA reserves idle resources and enters the TRANSMIT state triggered by event E7 after the reservation is successful. If the appointment is not successful, it will be triggered by event E6 to return to the SENSING state;

6) In the TRANSMIT state, the STA starts to send data frames. When the transmission is successful, the event E9 triggers to return to the INIT state. If no confirmation response from the AP is received, the event E8 triggers to return to the SENSING state.

Figure 4 shows the performance comparison curve of the probability of collision (Probability of collision) obtained by simulating the time-frequency two-dimensional hybrid access mechanism and the traditional CSMA/CA mechanism. The simulation is carried out by using different numbers of STA stations, and M represents the number of sub-channels. M>1 means the method of the present invention, and M=1 means the traditional CSMA/CA mechanism. As shown in Figure 4, when multiple sub-channels are used, that is, when M>1, the collision probability of the system is significantly lower than the traditional CSMA/CA mechanism, so it can be seen that the method of the present invention significantly reduces the collision probability under the same number of STAs.

Figure 5 shows the normalized saturated throughput (Saturated Throughput) performance curve obtained by the policy time-frequency two-dimensional hybrid access mechanism and the traditional CSMA/CA mechanism, M represents the number of sub-channels, and M>1 represents the situation of the present invention, M=1 means the traditional CSMA/CA mechanism. As shown in Figure 5, when multiple sub-channels are used, that is, when M>1, the system saturation throughput is significantly higher than that of the traditional CSMA/CA mechanism, so it can be seen that the method of the present invention can bring about improvement of system throughput.

It can be seen from the above embodiments that since the present invention adopts time-frequency two-dimensional access, each STA arbitrarily detects idle subchannels or idle time slots to realize data transmission, thus increasing the chance of random access, that is, increasing the available resources , thus improving resource utilization, enabling the system to accommodate more users, reducing the probability of collision, improving transmission efficiency, thereby increasing the amount of data that may be transmitted per unit time, that is, improving system throughput. The invention is especially suitable for the next generation super high speed WLAN system.

The above embodiments are only used to illustrate the present invention, but not to limit the present invention. Those of ordinary skill in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all Equivalent technical solutions also belong to the category of the present invention, and the scope of patent protection of the present invention should be defined by the claims.

Claims (7)

1. A time-frequency two-dimensional hybrid MAC layer access method, characterized in that, comprising the following steps: S1. When the user has data to be sent, the user periodically detects the current use of each sub-channel. When an idle sub-channel is detected, select the sub-channel and enter step S2. Otherwise, the user continues to detect the current use of each sub-channel. Condition; S2. The user enters the back-off phase: set the value of the back-off counter, and the back-off counter starts to subtract one from this value at the end of each time slot, and enters step S3 until the value is zero; S3. The user reserves a time-frequency resource to the access point. After the reservation is successful, the time-frequency resource is used to send data. If the reservation is not successful, the idle sub-channel is re-detected, and the value of the backoff counter is reset after the idle sub-channel is detected again; S4. After the user successfully receives the confirmation frame sent back by the access point, the value of the backoff counter is reset to zero; otherwise, return to step S1, and the user resends the lost data; The handshake frame used in step S3 includes a request to send frame and a clear to send frame, the request to send frame and the clear to send frame all contain the length information of the data sent by the user, when other users detect the length information, Update the local network allocation vector set and no longer send handshake information to reserve the currently occupied subchannel; the request to send frame, clear to send frame, confirmation frame and data can be sent on each subchannel; The sub-channel is formed in the following manner: multiple OFDM sub-carriers form a sub-channel, and the entire channel is divided into multiple sub-channels, and each sub-channel is orthogonal to each other. 2. The time-frequency two-dimensional hybrid MAC layer access method according to claim 1, wherein in said step S2, if the user detects that the currently selected subchannel is occupied by other users prior to the current time slot , then detect whether there are other idle sub-channels, and if so, decrement the back-off counter by one, if all the current sub-channels are occupied, keep the current value of the back-off counter unchanged, when the user detects an idle sub-channel again, back off A counter is decremented from said current value. 3. The time-frequency two-dimensional hybrid MAC layer access method according to claim 1, wherein the value of the back-off counter is calculated according to the value of the back-off period. 4. The time-frequency two-dimensional hybrid MAC layer access method as claimed in claim 1, characterized in that, when the user has sent a data and immediately has the next data to be sent, first release the currently occupied sub-channel, and re- Start detection of idle subchannels and backoff phase. 5. The time-frequency two-dimensional hybrid MAC layer access method according to claim 1, wherein the backoff criterion adopted in the backoff phase is a binary exponential backoff criterion. 6 . The time-frequency two-dimensional hybrid MAC layer access method according to claim 1 , wherein the time-frequency resources include subchannels and time slots. 7. The time-frequency two-dimensional hybrid MAC layer access method as claimed in claim 3, wherein, if the value of the backoff counter is BackoffCounter _n , then: BackoffCounter _n =INT(CW×Random()×T _slot ) Among them, INT() means rounding operation, Random() means random number operation between [0,1], T _slot means a time slot, CW=2 ⁱ CW _min , i∈[0,m] means backoff period, where m represents the maximum value of the backoff period, and CW _min represents the minimum value of the contention window defined in the carrier sense multi-point access/collision avoidance mechanism.

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