JP2017028582A - Wireless communication system and wireless communication method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To adjust vertical throughput ratio, while improving the throughput characteristics of RoF-WLAN, in a configuration where existing WLAN and RoF-WLAN coexist.SOLUTION: A wireless communication system coexisting with existing wireless communication system performing access control by CSMA/CA system, and performing access control without performing carrier sense includes means for acquiring a NAV period, where data transmission and reception is performed, from an access control signal for performing data transmission and reception in the existing wireless communication system for setting, and transmission control means waiting for transmission with a predetermined transmission standby probability α, when there is transmission data and transmitting the access control signal, after a time Tw of less than the DIFS time in the CSMA/CA system and 0 or more, with the internal delay time before the finish time of the NAV period as an origin.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a wireless communication system and a wireless communication method for performing access control by a CSMA / CA (Carrier Sense Multiple Access / Collision Avoidance) method.

  In order to cope with many wireless LAN devices of various standards, a wireless LAN system (RoF-WLAN) using RoF (Radio over Fiber) has been studied.

FIG. 3 shows a coexistence configuration example of RoF-WLAN and existing WLAN.
In FIG. 3, an existing AP (Access Point) 11 and an existing STA (Station) 12 of an existing WLAN, a RoF-AP RF processing unit 21 and a RoF-STA 24 of a RoF-WLAN exist in a home or office. Here, the function of the RoF-AP is divided into an RF processing unit 21 connected via an optical fiber 23 and an AP processing unit 22 in the office. The RF processing unit 21 of the RoF-AP is configured to perform conversion between an optical signal and a radio wave. The AP processing unit 22 of the RoF-AP performs AP processing of the PHY (Physical) layer and the MAC (Medium Access Control) layer. Do. Therefore, a propagation delay occurs due to communication via the optical fiber 23 between the RF processing unit 21 and the AP processing unit 22, and frame transmission / reception and carrier detection are delayed as RoF-AP.

  Therefore, in the configuration where existing WLAN and RoF-WLAN coexist, when access control is performed by the CSMA / CA method, collision occurs due to delay in frame transmission / reception and carrier detection of RoF-WLAN, and the throughput characteristics of RoF-WLAN are reduced. to degrade. FIG. 4 shows the characteristics of the ratio of the downlink throughput to the uplink throughput (upper and lower throughput ratio) corresponding to the propagation delay (0 to 50 μs) of RoF-WLAN, and the degradation of the downlink throughput is large as shown in the conventional method.

  In Non-Patent Document 1, in order to improve the downlink throughput characteristics of RoF-WLAN in a coexistence configuration of RoF-WLAN and existing WLAN, there is a method of giving a transmission right preferentially to RoF-AP accompanying the communication of existing WLAN. Proposed. As shown in FIG. 4 as the proposed method of Non-Patent Document 1, the effect of improving the downlink throughput characteristic is remarkable, and the vertical throughput ratio greatly exceeds 1, as shown below.

FIG. 5 shows an example of RTS frame transmission control in RoF-WLAN shown in Non-Patent Document 1.
In FIG. 5, the RoF-AP RF processing unit 21, AP processing unit 22, and RoF-STA 24 receive an existing WLAN RTS / CTS frame, acquire a NAV (Network Allocation Vector) period, and transmit the NAV period. Refrain from. However, in RoF-AP, the set NAV period is also delayed due to the delay of the RTS / CTS frame. Therefore, when there is transmission data in RoF-AP, transmission control is started at a timing before the round-trip propagation delay (RTT) time from the end time of the NAV period, and a random backoff period is not provided after Tw time less than DIFS time. RTS frame is transmitted. Thereby, the transmission of the RTS frame from the RoF-AP to the RoF-STA is successful, and it becomes possible to improve the downlink throughput in the RoF-WLAN. Even when Tw = DIFS, transmission of the RTS frame from the RoF-AP to the RoF-STA is successful, except when the existing WLAN finishes data transmission / reception and transmits an RTS frame with a random backoff period 0 after the DIFS. To do.

  Further, uplink transmission start control in RoF-STA is performed according to a normal RTS / CTS access procedure by performing carrier sense based on CSMA / CA with an existing WLAN.

Funabiki, Nishio, Morikura, Yamamoto, Murayama, Sugiyama, "Proposal of coexistence method of RoF wireless LAN and existing wireless LAN", IEICE Technical Report, vol.114, no.492, SRW2014-50, pp.19-24 March 2015

  By the way, in the RTS transmission control of the RoF-AP shown in FIG. 5, the transmission right is given to the RoF-AP preferentially along with the existing WLAN communication, so that the downlink throughput is higher than the uplink throughput as shown in FIG. (Upper and lower throughput ratio greatly exceeds 1). In particular, when communication of existing WLAN is frequently performed, there is a problem that the downlink throughput becomes excessive in the RoF-WLAN. Also, the vertical throughput ratio varies greatly depending on the propagation delay time of RoF-WLAN.

  Further, in the non-patent document 1, the RoF-AP transmits the RTS frame to the problem that the downlink throughput becomes excessive in the RoF-WLAN when the RoF-AP RTS transmission control shown in FIG. 5 is performed. Previously, a method for probability controlling whether or not to transmit an RTS frame has been proposed. That is, the transmission of the RTS frame is made to wait with a predetermined transmission waiting probability α so that the downlink throughput does not become excessive. When waiting for the transmission of the RTS frame, the CTS frame of the existing WLAN is received, and after the NAV period, the next transmission operation is started according to the normal RTS / CTS access procedure.

  In order to set the vertical throughput ratio in RoF-WLAN to the target value, it is necessary to appropriately adjust the transmission standby probability α according to the propagation delay time and the communication status of the existing WLAN. However, Non-Patent Document 1 specifically shows how to adjust the transmission standby probability α in order to set the vertical throughput ratio to a target value or to make the vertical throughput fair. Absent.

  An object of the present invention is to provide a radio communication system and a radio communication method capable of adjusting a vertical throughput ratio while improving throughput characteristics in a RoF-WLAN in a configuration in which an existing WLAN and a RoF-WLAN coexist. And

  A first aspect of the present invention is a wireless communication system in which access control is performed without performing carrier sense coexisting with an existing wireless communication system that performs access control according to the CSMA / CA scheme. Means for acquiring and setting the NAV period in which data transmission / reception is performed from the access control signal of the CSMA / CA system, the time Tw being equal to or less than the DIFS time in the CSMA / CA system, starting from the end of the NAV period and before the internal delay time Later, when there is transmission data, transmission control means for waiting for transmission with a predetermined transmission standby probability α when transmitting an access control signal.

  In the wireless communication system of the first invention, the transmission control means is configured to measure the actual throughput as the transmission standby probability α and set the actual throughput to a value close to the target throughput.

  In the wireless communication system of the first invention, the transmission control means is configured to set the transmission standby probability α to a value that approximates the target throughput and the actual throughput based on the number of coexisting terminals in the active state.

  In the wireless communication system of the first invention, the transmission control means is configured to set the transmission standby probability α to a value that approximates the target throughput and the actual throughput based on the time occupancy rate of the channel to be used.

  A second aspect of the present invention is a wireless communication method in which access control is performed without performing carrier sense coexisting with an existing wireless communication system that performs access control according to the CSMA / CA scheme. A step of acquiring and setting a NAV period in which data transmission / reception is performed from the access control signal of the CSMA / CA system, and a time Tw which is less than or equal to DIFS time in the CSMA / CA system starting from the end time of the NAV period and before the internal delay time And a step of waiting for transmission with a predetermined transmission standby probability α when transmitting an access control signal when there is transmission data.

  In the wireless communication method of the second invention, the transmission standby probability α is set to a value in which the actual throughput approaches the target throughput by measuring the actual throughput.

  In the wireless communication method of the second invention, the transmission standby probability α is set to a value that approximates the target throughput and the actual throughput based on the number of coexisting terminals in the active state.

  In the wireless communication method of the second invention, the transmission standby probability α is set to a value that approximates the target throughput and the actual throughput based on the time occupancy rate of the channel to be used.

  In the RoF-AP of RoF-WLAN coexisting with an existing WLAN that performs CSMA / CA control, the present invention controls transmission of RTS frames and transmission standby based on the calculated transmission standby probability α, thereby providing a carrier in real time. Even when sensing is not possible, data transmission can be performed efficiently.

  Further, when the actual throughput is measured and the transmission standby probability α is set to a value that approximates the target throughput, the adaptive control of the throughput becomes possible. It is also possible. For example, as shown in FIG. 4, when performing normal CSMA-CA control in RoF-AP with a transmission delay of 30 μs, or when performing RTS transmission control of RoF-AP shown in FIG. Although the ratio greatly deviates from 1, RoF-WLAN can coexist while protecting the communication quality of the existing WLAN by using the transmission standby probability α by the calculation method 1.

  In addition, when the transmission standby probability α is set to a value that approximates the target throughput and the actual throughput based on the number of coexisting terminals in the active state, control according to the number of coexisting terminals becomes possible, so coexistence When the number of terminals is large, the transmission standby probability α can be increased to protect the communication quality of other terminals.

  In addition, when the transmission standby probability α is set to a value that approximates the target throughput and actual throughput based on the time occupancy rate of the channel to be used, it becomes possible to control according to the radio resource congestion, so that coexisting terminals When communication is frequently performed (including systems other than WLAN), the transmission standby probability α can be increased to protect the communication quality of other terminals including systems other than WLAN.

It is a figure which shows the structural example of RoF-AP in this invention. It is a flowchart which shows the RTS transmission procedure example of RoF-AP in this invention. It is a figure which shows the example of a coexistence structure of RoF-WLAN and the existing WLAN. It is a figure which shows a vertical throughput ratio characteristic. It is a time chart which shows the example of RTS frame transmission control of RoF-AP.

FIG. 1 shows a configuration example of RoF-AP in the present invention.
In FIG. 1, RoF-AP includes an RF processing unit 21 and an AP processing unit 22 that are connected via an optical fiber 23, and the RF processing unit 21 has a function of converting optical signals and radio waves. The AP processing unit 22 includes a NAV setting unit 221, an RTS transmission control unit 222, a CTS transmission control unit 223, a data transmission / reception unit 224, and an ACK transmission unit 225 as control units having an AP function. An optical communication unit 226 that performs optical communication between them is provided.

  The existing AP, the existing STA, and the RoF-STA have a carrier sense function unit, and perform data transmission if the channel is idle by carrier sense in the DIFS and the random backoff period, or an RTS / CTS frame for data transmission. Starts transmission / reception processing. However, in the case of RoF-AP, even if carrier sensing is performed by the RF processing unit 21, the AP processing unit 22 cannot perform processing corresponding to carrier sensing in real time, so the AP processing unit 22 does not include a carrier sensing function unit. It is configured.

  As described above, the present invention is generally applicable to a communication apparatus (AP or STA) that has a large internal delay time and cannot perform carrier sense processing in real time. In the case of application, the operation of each unit will be described with reference to the RTS transmission procedure example of RoF-AP shown in FIG. The basic operation is the same as in the RTS frame transmission control example shown in FIG.

  When the NAV setting unit 221 receives an RTS frame or CTS frame that is not addressed to itself (S1), the NAV setting unit 221 acquires and sets the NAV period during which data transmission / reception is performed (S2). The RTS transmission control unit 222 determines whether or not there is transmission data in the data transmission / reception unit 224 when the round-trip propagation delay (RTT) time comes before the end time of the NAV period (S3: Yes) (S3: Yes). S4). Note that the round-trip propagation delay (RTT) time in this RoF-AP generally corresponds to the internal delay time of the communication apparatus. If there is transmission data, a transmission standby probability α for refraining from transmitting the RTS frame is calculated (S5). If there is no transmission data, the process returns to the RTS frame or CTS frame reception operation in step S1.

  Here, a feature of the present invention is that a transmission standby probability α for adjusting the vertical throughput ratio is calculated by a method described later, and RTS frame transmission determination is performed based on the transmission standby probability α (S6). That is, the RTS frame is transmitted with a probability α, and the RTS frame is transmitted with a probability (1−α). For example, a random number x (0 ≦ x ≦ 1) is generated. If x <α, transmission is suspended, and if x ≧ α, the process proceeds to a process of transmitting an RTS frame.

  When an RTS frame is to be transmitted (S6: Yes), the system waits for a time Tw (0 ≦ Tw ≦ DIFS) from the end of the NAV period before the round-trip propagation delay (RTT) time (S7). RTS frame is transmitted without providing a random back-off period (S8).

  Further, the RTS transmission control unit 222 performs control to transmit data from the data transmission / reception unit 224 when a CTS frame corresponding to the RTS frame is received from the RoF-STA. At this time, as shown in FIG. 5, a propagation delay occurs in the RTS / CTS frame, the data signal, and the ACK signal transmitted / received between the RoF-AP and the RoF-STA, so that delay (RTT × 2) A NAV period in consideration of the above is set, and transmission suppression control for the existing WLAN is performed.

  The CTS transmission control unit 223 receives the RTS frame addressed to the RoF-AP from the RoF-STA and transmits the CTS frame to the RoF-STA if it is in a receivable state, and the data transmission / reception unit 224 waits for data reception. When the data transmission / reception unit 224 normally receives data, the ACK transmission unit 225 transmits an ACK signal.

  Hereinafter, with respect to the calculation method of the transmission standby probability α in step S5, the calculation method 1 based on the actual throughput, the calculation method 2 based on the number of coexisting terminals in the active state, and the time occupancy rate (number of busy detections) of the channel to be used. The calculation method 3 based on this will be described.

(Calculation method 1 of transmission standby probability α)
The calculation method 1 is calculated based on the actual throughput. Let α _k be the transmission standby probability calculated at a certain time. Let T _{k be} the actual throughput of the local station measured when α _k is calculated. The method for measuring the actual throughput T _k is not limited, but for example, it may be an average throughput during a fixed time t from the calculation time point, or may be a weighted average.

The transmission standby probability α is feedback-controlled so that the actual throughput T _k approaches the target throughput T _T.

First, an example of a method of calculating the target throughput T _T corresponding to the number of terminals N existing WLAN coexist formula (1-1) to Formula (1-3).
N> N _th1 : T _T = T _T1 (1-1)
N _th1 ≧ N ≧ N _th2 : T _T = T _T1 + T _t2 × N (1-2)
N _th2 > N: T _T = T _T3 (1-3)

Here, T _T1 is a minimum desired throughput and T _T3 is a maximum restricted bandwidth. In this example, T _T1 = 1 Mbps and T _T3 = 51 Mbps. The range N _th1 ≧ N ≧ N _th2 of the assumed number N of terminals is determined according to the installation environment. For example, when it is assumed that about 1 to 50 terminals coexist in an indoor office environment, N _th1 = 51 and N _th2 = 1. At this time, T _T2 is obtained by equation (2).
T _T2 = (T _T3 -T _T1 ) / (N _th1 -N _th2 ) = (51-1) / (51-1) = 1 Mbps (2)

Next, based on the difference e _k = (T _T −T _k ) between the target throughput T _T and the actual throughput T _k , the transmission standby probability α _k is _obtained by, for example, Expression (3) or Expression (4).
α _k = f (α _k−1 , e _k ) (3)
α _k = g (α _k−1 , e _k , e _k−1 , e _k−2 ) (4)

α ₀ may be given an arbitrary value. Functions f (), g () is assumed to have a function to approximate e _k to zero. For example, Formula (3) can use Formula (5-1) to Formula (5-3), and Formula (4) can use Formula (6). Here, Δα ′, Δα ″, Th 1, Th 2, a, b, and c are predetermined constants. These calculation methods are not particularly limited, but are empirically obtained by performing multiple times with different values. However, it is desirable that Δα ′> 0, Δα ″> 0, Th1 ≧ 0, Th2 ≧ 0, Th1> Th2, a ≧ 0, b ≧ 0, and c ≧ 0.
Equation (3):
e _k > Th1: α _k = α _k−1 −Δα ′ (5-1)
Th1 ≧ e _k ≧ Th2: α _k = 0 (5-2)
Th2 ≧ e _k : α _k = α _k-1 + Δα "(5-3)
Equation (4):
α _k = α _k−1 −a × e _k −b (e _k −e _k−1 ) −c ((e _k −e _k−1 ) − (e _k−1 −e _k−2 ))
… (6)

(Calculation method 2 of transmission standby probability α)
In the calculation method 2, the transmission standby probability α is calculated based on the number of coexisting terminals in the active state. The number of coexisting terminals at the time of obtaining the transmission standby probability α _k is N _k . N _k may be the number of STAs and APs that have performed communication in a predetermined past t _N seconds. It can be calculated by decoding the MAC address or IP address or other identifier.

The transmission standby probability α _k is obtained using, for example, Expression (7). Here, d and e are predetermined constants. These appropriate values are easily determined empirically. It is desirable that d ≧ 0.
α _k = d × N _k + e (7)

Here, an example of the calculation method of d and e is shown. Assume that about 50 to 100 terminals coexist in an office environment. The target throughput when the number of coexisting terminals N _k is 1 and 50 is obtained using equation (1). They shall be 51 and 1 Mbps, respectively. When the number of terminals coexists, the value of α (α _51M , α _1M ) is obtained that makes the calculated target throughput and actual throughput close to each other. Specifically, a value that minimizes the difference from the target value is searched for while changing the value of α under the circumstances. D and e can be obtained by solving the equation (8) from the obtained (α _51M , α _1M ).
α _51M = d + e
α _1M = 50 × d + e (8)
For example, if (α _51M , α _1M ) = (0.1, 0.99), d = 0.018 and e = 0.081.

(Calculation method 3 of transmission standby probability α)
The calculation method 3 is calculated based on the time occupation rate of the channel to be used. The time occupancy rate is the ratio of the busy period to the measurement period. An example of how to calculate the time occupancy rate is shown below. For example, periodically performs carrier sense in RoF-STA, of the last n _a single carrier sense enforcement, when detected a n _b times busy, the time occupancy r _b, represented by the formula (9) .
r _b = n _b / n _a (9)

The transmission standby probability α _k is obtained using, for example, Expression (10). Here, f and g are predetermined constants. These appropriate values are easily determined empirically. It is desirable that f ≧ 0.
α _k = f × r _b + g (10)

Here, an example of the calculation method of f and g is shown. The value of r _b of the time and off-time congestion is composed of each 1 and 0. 1Mbps throughput to be secured at a minimum, a maximum limit throughput and 51Mbps, by changing the alpha _k when r _b is 1 and 0, respectively 1Mbps, such that the value closest to 51Mbps (α _51M, α _1M) Search for. By solving Equation (11) from the obtained (α _51M , α _1M ), f and g can be obtained.
α _51M = 0 + g
α _1M = f + g (11)
For example, if (α _51M , α _1M ) = (0.1, 0.99), f = 0.89 and g = 0.1.

11 Existing WLAN AP
12 STA of existing WLAN
21 RoF-AP RF processing unit 22 RoF-AP AP processing unit 23 Optical fiber 24 RoF-STA
221 NAV setting unit 222 RTS transmission control unit 223 CTS transmission control unit 224 Data transmission / reception unit 225 ACK transmission unit 226 Optical communication unit

Claims (8)

In a wireless communication system that coexists with an existing wireless communication system that performs access control by CSMA / CA (Carrier Sense Multiple Access / Collision Avoidance) method and performs access control without performing carrier sense.
Means for acquiring and setting a NAV period during which data transmission / reception is performed from an access control signal for performing data transmission / reception in the existing wireless communication system;
When transmitting the access control signal when there is transmission data after a time Tw that is equal to or less than the DIFS time in the CSMA / CA scheme and is equal to or greater than 0, starting from the internal delay time before the end time of the NAV period, A wireless communication system comprising: a transmission control unit that waits for transmission with a transmission standby probability α. The wireless communication system according to claim 1, wherein
The radio communication system, wherein the transmission control means is configured to measure an actual throughput as the transmission standby probability α and set the actual throughput to a value approaching a target throughput. The wireless communication system according to claim 1, wherein
The wireless communication system, wherein the transmission control means is configured to set the transmission standby probability α to a value that approximates the target throughput and the actual throughput based on the number of coexisting terminals in the active state. The wireless communication system according to claim 1, wherein
The radio communication system, wherein the transmission control unit is configured to set the transmission standby probability α to a value that approximates a target throughput and an actual throughput based on a time occupation ratio of a channel to be used. In a wireless communication method that coexists with an existing wireless communication system that performs access control by CSMA / CA (Carrier Sense Multiple Access / Collision Avoidance) method and performs access control without performing carrier sense,
Acquiring and setting a NAV period in which data transmission / reception is performed from an access control signal for performing data transmission / reception in the existing wireless communication system;
When transmitting the access control signal when there is transmission data after a time Tw that is equal to or less than the DIFS time in the CSMA / CA scheme and is equal to or greater than 0, starting from the internal delay time before the end time of the NAV period, And a step of waiting for transmission with a transmission standby probability α. The wireless communication method according to claim 5, wherein
The transmission standby probability α is set to a value in which the actual throughput is measured and the actual throughput approaches the target throughput. The wireless communication method according to claim 5, wherein
The transmission standby probability α is set to a value that approximates the target throughput and the actual throughput based on the number of coexisting terminals in the active state. The wireless communication method according to claim 5, wherein
The transmission standby probability α is set to a value that approximates the target throughput and the actual throughput based on the time occupancy rate of the channel to be used.

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