JP5591844B2 - Wireless access system, access control method, and base station apparatus - Google Patents

Description

  The present invention relates to a radio access system, an access control method, and a base station apparatus in which a plurality of terminals under the control of a base station perform radio communication with a base station by random access.

  Currently, a wide area ubiquitous network suitable for collecting and controlling various devices is being developed, which has a wider range of radio waves from a base station than a mobile phone communication network. In a wide area ubiquitous network, in order to construct a wide area cell, a low transmission rate wireless line having a low transmission capacity but a low required C / N is used. Moreover, since it is a wide area cell, the base station accommodates more terminals than the conventional wireless communication system. Generally, a wide area ubiquitous network accommodates about tens of thousands of terminals. Furthermore, since terminals accommodated in a wide area ubiquitous network are devices similar to sensors, it is common to transmit short data of 100 bytes or less.

The access control system of the wireless access system used as the access system of the wired network includes a centralized control system in which the base station manages the transmission timing of all terminals, and distributed control in which each terminal independently determines the transmission timing and performs transmission. It can be roughly divided into two methods.
The distributed control method has a simpler base station configuration than the centralized control method. In addition, since there is no need to control the terminal from the base station, a control signal is not required, and high throughput can be realized. Therefore, the transmission rate per terminal is low as in a wide area ubiquitous network, and one base station is enormous. When accommodating a large number of terminals, a distributed control method is used.

  In the distributed control method, each terminal individually determines the transmission timing, so there is a possibility that transmission packets may collide when multiple terminals start transmission at the same time, and packet collisions frequently occur in a congested state. Reduce throughput. In particular, in a wide area ubiquitous network, since the number of terminals accommodated per base station is enormous, there is a possibility that a very strong congestion state may continue for a long period of time when the terminals start transmission all at once.

  As a method for detecting such congestion and maintaining the throughput, several methods have been proposed so far.

  In Patent Document 1, it is determined that the base station is in a congested state when the number of successful receptions of random access channel signals per unit time is equal to or greater than a specified value. In the case of congestion, a downlink channel is transmitted from the base station to the terminal. A method of suppressing the transmission of the terminal by notifying the fact of congestion through the network is shown.

  In Patent Document 2, the reception power of each random access slot is measured at a base station, and when reception fails when the reception power exceeds a specified value, it is determined that there is a collision, and congestion is determined based on the level of the collision rate. In the case of congestion, a method is shown in which the base station reduces the traffic by reducing the antenna directivity to improve the throughput.

  In Patent Literature 3, similar to Patent Literature 2, the transmission success time rate and the transmission failure time rate are calculated from the reception power level and the reception success / failure, and the congestion judgment is made by comparing each value with the simulation result obtained in advance. Is going.

JP 2010-021942 A JP 2007-036442 A Japanese Patent Laid-Open No. 08-336177

  Conventional random access congestion determination methods can be broadly classified into two types: (method 1) congestion detection using the number of successful receptions of random access signals per unit time, and (method 2) congestion detection using the received power level. .

  Method 1 is a method in which congestion is determined when the number of successful receptions of random access signals is greater than a predetermined threshold. However, when a large number of terminals start transmission at the same time, a random access signal cannot be detected due to a collision, and thus method 1 cannot detect congestion. In particular, in a wide area ubiquitous network, the number of accommodated terminals is much larger than that of a conventional wireless communication system, and therefore it is frequently encountered that the method 1 cannot detect congestion.

  Method 2 uses the received power at the base station for terminal transmission determination, but when the transmission power of the terminal is very small in a wide-area cell, there are frequent cases where the terminal is transmitting but the base station cannot detect the transmission. May occur and congestion may not be detected.

  An object of the present invention is to provide a radio access system, an access control method, and a base station apparatus that can easily detect congestion in a wide area ubiquitous network in which congestion may not be detected by a conventional method.

  In a wireless access system in which a plurality of terminals under the control of a base station perform wireless communication with the base station using random access, the plurality of terminals can perform random access in an uplink frame transmitted from the terminal to the base station. A random access area to be used and a shared access area to be used for transmission of a signal for estimating the number of terminals having transmission data are set. When the terminal has transmission data, a signal is transmitted using a shared access slot of the shared access area. And a means for transmitting a signal composed of a common modulation scheme and a data string common to all terminals with a shared access slot transmission probability of transmitting the base station, and the base station transmits signals in the shared access slot in a plurality of uplink frames. The number of successful receptions is measured, the shared access slot transmission probability, the total number of shared access slots, and the shared access slots. Based on the received number of successes in Tsu preparative comprises means for estimating the number of terminals having transmission data.

  In the wireless access system of the first invention, the base station includes means for notifying the terminal of the shared access slot transmission probability for each one or a plurality of MAC frames.

  In the radio access system of the first invention, the base station determines a shared access slot transmission probability to be used for the next uplink frame or a plurality of subsequent uplink frames according to the estimated number of terminals having transmission data. And means for notifying the terminal.

  In the radio access system of the first invention, the base station adjusts the shared access slot transmission probability so that the ratio of the number of received signals and the number of non-received signals in the shared access slot is constant.

  In the radio access system of the first invention, the base station repeatedly estimates the number of terminals having transmission data for each of a plurality of uplink frames, and sets the average value as the estimated number of terminals.

  Also, if the reception succeeds in all shared access slots of a plurality of uplink frames, or if no reception succeeds in all the shared access slots, or if the situation continues, it is estimated in the past The average number may not be used by clearing the number of terminals.

  According to a second aspect of the present invention, in the access control method for a radio access system in which a plurality of terminals under the base station perform radio communication with the base station by random access, the plurality of terminals are randomly transmitted in an uplink frame transmitted from the terminal to the base station. A random access area used for access and a shared access area used for transmitting signals for estimating the number of terminals having transmission data are set. When a terminal has transmission data, the terminal uses a shared access slot of the shared access area. The base station transmits a signal in a shared access slot with a plurality of uplink frames with a transmission scheme with a shared access slot transmission probability that is common to all terminals and with a data sequence that is common to all terminals. The number of successes is measured, the shared access slot transmission probability, the total number of shared access slots, and Based on the received number of success in the slot, to estimate the number of terminals with transmission data.

  According to a third aspect of the present invention, in a base station apparatus of a radio access system in which a plurality of terminals under the base station perform radio communication with the base station by random access, the plurality of terminals are randomly transmitted in an uplink frame transmitted from the terminal to the base station. Set a random access area used for access and a shared access area used to transmit signals to estimate the number of terminals with transmission data, and terminals with transmission data transmit signals using the shared access slot in the shared access area When transmitting a signal consisting of a common modulation scheme and a common data string in all terminals with the shared access slot transmission probability to measure the number of successful reception of signals in the shared access slot in a plurality of uplink frames, Shared access slot transmission probability, total number of shared access slots and the number of shared access slots Based on the signal success count comprises means for estimating the number of terminals having transmission data.

  In the present invention, a terminal having transmission data transmits a signal with a shared access slot transmission probability using a shared access slot of the shared access area, and the base station measures the number of successful receptions of the signal received in the shared access slot, By estimating the number of terminals having transmission data based on the shared access slot transmission probability, the number of access slots, and the number of successful receptions in the slot, the number of terminals performing random access can be estimated with high accuracy. Thereby, it becomes possible to detect the congestion state in random access.

It is a figure which shows the structural example of the radio | wireless access system of this invention. It is a figure which shows the structural example of the MAC frame used by this invention. It is a figure which shows the structural example of the base station and terminal in this invention. It is a figure which shows the estimation sequence of the transmission state terminal number estimation part 11 of the base station 10. FIG. It is a flowchart which shows the example of a calculation procedure of the shared access slot transmission probability p.

FIG. 1 shows a configuration example of a radio access system of the present invention.
In FIG. 1, a plurality of terminals 20 are connected to one base station 10 connected to a wired network 31 via a wireless line, and each terminal 20 sends a control signal and a data signal to the base station 10 by random access. Send.

  FIG. 2 shows a configuration example of a MAC frame used in the present invention. TDMA is used as the multiple access method, and TDD is used as the duplex method.

  In FIG. 2, the MAC frame length is fixed, the first half is a downstream frame from the base station to the terminal, and the second half is an upstream frame from the terminal to the base station. In the downlink frame, in addition to a data signal carrying user data, a control signal carrying a control command for performing radio channel control or the like is transmitted from the base station to the terminal. The uplink frame includes a shared access area used for transmission of a signal for estimating the number of terminals having transmission data, a random access area used for random access by a plurality of terminals, and other areas. One or a plurality of access slots in the shared access area are provided.

  When the terminal has data to be transmitted, the terminal transmits a bandwidth allocation request by random access. When transmitting a bandwidth allocation request by random access, the terminal performs Exponential-backoff to avoid congestion. In back-off, random numbers are calculated within a predetermined window size range before transmission, and transmission is performed after back-off in random access slots equal to the random number. If the transmission fails, the window size is multiplied by X, the random number is calculated again, and the retransmission is performed after backoff. The retransmission of the terminal is repeated until the transmission is successful or the maximum number of retransmissions is reached. The base station that has received the bandwidth allocation request allocates a bandwidth for data transmission according to the request. A terminal to which a band for data transmission is assigned transmits data using the band.

  In the present invention, a terminal performing random access including back-off performs a transmission operation in a shared access area separately from random access. When a transmission slot (bandwidth) is allocated in the shared access area, the terminal performs transmission with a certain probability. The probability of transmission is reported from the base station as a shared access slot transmission probability. According to this probability, the terminal transmits in the access slot of the shared access area.

FIG. 3 shows a configuration example of a base station and a terminal in the present invention.
In FIG. 3, the base station 10 and the terminal 20 are connected via a wireless line. The base station 10 includes a transmission state terminal number estimation unit 11, a shared access slot transmission probability calculation unit 12, a random access parameter calculation unit 13, a control signal generation unit 14, a radio transmission unit 15, and a radio reception unit 16. The terminal 20 includes a shared access signal generation unit 21, a random access signal generation unit 22, a wireless transmission unit 23, and a wireless reception unit 24.

  The transmission state terminal number estimation unit 11 of the base station 10 estimates the number of terminals in transmission state (terminals having data to be transmitted) from the number of successful receptions of packets received in the shared access area.

  The shared access slot transmission probability calculation unit 12 calculates the probability that the terminal transmits in the shared access area based on the number of terminals estimated by the transmission state terminal number estimation unit 11. Here, a transmission probability that improves the estimation accuracy in the next estimation of the number of terminals is calculated.

  Based on the number of terminals in the transmission state estimated by the transmission state terminal number estimation unit 11, the random access parameter calculation unit 13 calculates a random access parameter that can be expected to improve the throughput most.

  The control signal generator 14 generates a signal for notifying the terminal of the transmission probability of the shared access area calculated by the shared access slot transmission probability calculator 12 and the parameter calculated by the random access parameter calculator 13.

The wireless transmission unit 15 transmits the signal generated by the control signal generation unit 14 to the terminal 20 via a wireless line.
The wireless reception unit 16 receives signals transmitted from the terminal 20 via the wireless line in the random access area and the shared access area.

  The shared access signal generation unit 21 of the terminal 20 generates a shared access signal according to the transmission probability of the shared access area notified from the base station 10.

  The random access signal generation unit 22 performs random access according to the random access parameter notified from the base station 10 and transmits transmission data.

The wireless transmission unit 23 transmits the signals generated by the random access signal generation unit 22 and the shared access signal generation unit 21 to the base station 10 via a wireless line.
The radio reception unit 24 receives a radio signal transmitted from the base station 10, notifies the transmission probability of the shared access area to the shared access signal generation unit 21, and notifies the random access parameter generation unit 22 of the random access parameter.

FIG. 4 shows an estimation sequence in the transmission state terminal number estimation unit 11 of the base station 10.
In FIG. 4, the number of transmission state terminals is estimated for each super frame composed of a plurality of (four in FIG. 4) MAC frames. In the example of the figure, the shared access slot transmission probability instructed from the base station 10 to the terminals 20-1 to 20-4 is not changed in one superframe, and a fixed value is notified to each terminal. When the shared access slot transmission probability is not changed during the super frame, the shared access slot transmission probability may be notified only once in the top frame of the super frame.

  In the super frame # 1, the base station 10 notifies the terminals 20-1 to 20-4 of the shared access slot transmission probability p0 for each MAC frame. The terminal 20 having the transmission data, according to the notified shared access slot transmission probability, transmits a packet of a fixed pattern (a data string common to all terminals) in the access slot of the shared access area to a modulation method common to all terminals. Send with. As a result, in the shared access area, even if signals transmitted from a plurality of terminals collide, it can be received without failure.

  However, only terminals having data to be transmitted are transmitted, and terminals having no data do not transmit. For example, when the notified shared access slot transmission probability p0 is 0.5, a terminal having data transmits once every two slots on average. At this time, it is desirable that the terminal selects a transmission slot, for example, randomly or according to a fixed pattern such as a pseudo random number.

  The base station 10 holds the access history of the shared access area slot. The history records the number of shared access area slots allocated in one superframe and the number of shared access area slots successfully received. In the example shown in the figure, at time t1, the number of successful reception slots 3 is recorded with respect to the number 4 of shared access area slots. Based on the reception status of such a previous period, the shared access slot transmission probability used in the next superframe is recalculated and notified.

(Estimation of the number of terminals in transmission status)
A method for estimating the number of terminals in transmission state (terminals having data to be transmitted) in the transmission state terminal number estimation unit 11 of the base station 10 will be described.

  The number of transmission state terminals is estimated for each superframe composed of a predetermined number (N) of MAC frames. Since the shared access slot transmission probability and random access parameter are calculated based on the estimated number of terminals, both the shared access slot transmission probability and random access parameter are updated once every N frames. The transmission state terminal number estimation unit 11 records the number of successful reception slots, which is the number of times a packet is received without error, with respect to the number of shared access area slots in the past N frames.

When h access slots (h is an integer equal to or greater than 1) are allocated to the shared access area for each MAC frame, the number of shared access area slots in the past N frame periods from the estimation timing is h × N. The number k of successful reception slots is measured. Assuming that the shared access slot transmission probability in the past N frames from the estimated timing is p, the estimated number of terminals x is expressed by the following equation.
x = (log ((hN-k) / hN)) / (log (1-p)) (1)

  When reception is successful in all access slots (k = hN), or when reception is not possible in all access slots (k = 0), the number of terminals in the transmission state cannot be estimated, and will be described below. Thus, the shared access slot transmission probability p is recalculated and notified to each terminal, and the same processing is performed for the next N frames.

  Moreover, even if the estimated number of terminals x is obtained, an error is included in one estimation, and therefore an average value of a plurality of estimated values is set as the estimated number of terminals. When taking an average, the estimated number of terminals of the past m superframes is stored, and an average value of the values is calculated.

  In addition, when the average value is the estimated number of terminals, when reception is successful in all slots (k = hN), or when the situation continues, the number of terminals in the transmission state increases in a short time. Since the past estimated number is meaningless, the past number of terminals is cleared and the average value is not used. Furthermore, it is also effective to perform the same operation even when none of the slots has been successfully received (k = 0) or when the situation continues.

(Calculation of shared access slot transmission probability)
A method for calculating the shared access slot transmission probability in the shared access slot transmission probability calculation unit 12 of the base station 10 will be described.

For each superframe, the number of successful reception slots k corresponding to the shared access slot transmission probability p is measured. If the shared access slot transmission probability p is high, the number of successful reception slots k is large, and if the common access slot transmission probability p is low, the number of successful reception slots k is small. Therefore, the shared access slot transmission probability p is adjusted so that the probability that the target number of successful reception slots among y slots is y (0 <y <Z) is high. The shared access slot transmission probability p in which the number of successful reception slots among the Z slots is y is calculated by the following equation.
p = 1- (1-y / Z) ^{1 / x} (2)
However, x is the estimated number of terminals obtained by equation (1).

Here, when reception is successful in all access slots (k = hN), or when reception is not possible in all access slots (k = 0), the estimated number of terminals x cannot be calculated using equation (1). The shared access slot transmission probability p set in the next superframe cannot be calculated using the equation (1). Therefore, when a reception success in all access slots for it can be estimated that transmission state terminal unexpectedly increased, to 1 shared access slot transmission probability p from the current value of k _d min. On the other hand, if the number of successful receptions becomes 0, it is increased by k _i times.

FIG. 5 shows an example of a procedure for calculating the shared access slot transmission probability. Here, even if an estimation error occurs in the estimation of the number of transmission state terminals, y is set to Z / 2 and k _d = k in order to avoid a situation where no transmission is performed or a case where transmission is performed in all slots. _i = 10.

In FIG. 5, it is determined whether or not reception is successful in all access slots (k = hN) (S1). If reception is successful in all access slots, the shared access slot transmission probability p is set to p / 10 (S2). ). If reception is not successful in all access slots, it is determined whether or not reception is performed in all access slots (k = 0) (S3). If there is no reception in all access slots, the shared access slot transmission probability p is determined. P × 10 (S4). When the number of successful receptions is 0 <k <hN, the shared access slot transmission probability p set in the next superframe according to the estimated number of terminals x obtained by the equation (1) is
p = 1- (1/2) ^{1 / x}
(S5).

(Access parameter calculation)
When the random access parameter used for random access in the random access area is determined according to the estimated number of terminals x obtained by equation (1), the random access parameter is updated for each superframe. Access parameters differ depending on the random access method. Here, (1) random access without exponential backoff,
(2) Explain how to change access parameters for random access when exponential backoff is performed.

(1) When Exponential Backoff is not Performed The only access parameter for random access when exponential backoff is not performed is the window size W. When transmitting a packet, the terminal selects a transmission slot in the random access area according to the window size and transmits the packet. For example, when the window size W is 10, a transmission slot is randomly selected from the 0th to 9th slots. In changing the access parameter, the window size W is set to the same value as the estimated number of terminals x.

(2) When Exponential Backoff is Performed Access parameters for random access when exponential backoff is performed are the initial window size W ₀ and the persistent factor PF. In this method, the window size W for random access is set to n as the number of transmissions of the same data.
W = W ₀ × PF ^n-1
Determined by The transmission method using the window size W is the same as in the case of (1). In updating the access parameter, the initial window size W ₀ is set to the same value as the estimated number of terminals x.

DESCRIPTION OF SYMBOLS 10 Base station 20 Terminal 11 Transmission state terminal number estimation part 12 Shared access slot transmission probability calculation part 13 Random access parameter calculation part 14 Control signal generation part 15 Wireless transmission part 16 Wireless reception part 21 Shared access signal generation part 22 Random access signal generation Unit 23 wireless transmission unit 24 wireless reception unit

Claims (8)

In a wireless access system in which a plurality of terminals under the base station perform wireless communication with the base station using random access,
In the uplink frame transmitted from the terminal to the base station, a random access area used by the plurality of terminals for random access and a shared access area used for transmitting a signal for estimating the number of terminals having transmission data are set.
When the terminal has transmission data, the terminal uses a shared access slot transmission probability of transmitting the signal using a shared access slot of the shared access area, from a modulation scheme common to all terminals and from a data string common to all terminals. Means for transmitting said signal comprising:
The base station measures the number of successful receptions of the signal in the shared access slot in the plurality of uplink frames, and determines the transmission probability of the shared access slot, the total number of shared access slots, and the number of successful receptions in the shared access slot. A wireless access system comprising means for estimating the number of terminals having the transmission data based on the transmission data. The radio access system according to claim 1,
The base station includes means for notifying the terminal of the shared access slot transmission probability for every one or a plurality of MAC frames. The radio access system according to claim 1,
The base station determines the transmission probability of the shared access slot to be used for the next uplink frame or a plurality of subsequent uplink frames according to an estimated value of the number of terminals having the transmission data and notifies the terminal A wireless access system comprising: means. The radio access system according to claim 1,
The base station adjusts the transmission probability of the shared access slot so that the ratio of the number of received signals and the number of non-received signals in the shared access slot is constant. The radio access system according to claim 1,
The base station repeats estimation of the number of terminals having the transmission data for each of the plurality of uplink frames, and an average value thereof is set as the estimated number of terminals. The radio access system according to claim 5,
Estimated in the past when reception is successful in all shared access slots of the plurality of upstream frames, or when reception is not successful in all shared access slots, or when those situations are continuous. A radio access system characterized by clearing the number of terminals and not using an average value. In a radio access system access control method in which a plurality of terminals under the base station perform radio communication with the base station by random access,
In the uplink frame transmitted from the terminal to the base station, a random access area used by the plurality of terminals for random access and a shared access area used for transmitting a signal for estimating the number of terminals having transmission data are set.
When the terminal has transmission data, the terminal uses a shared access slot transmission probability of transmitting the signal using a shared access slot of the shared access area, from a modulation scheme common to all terminals and from a data string common to all terminals. Sending said signal
The base station measures the number of successful receptions of the signal in the shared access slot in the plurality of uplink frames, and determines the transmission probability of the shared access slot, the total number of shared access slots, and the number of successful receptions in the shared access slot. An access control method characterized by estimating the number of terminals having the transmission data. In a base station apparatus of a radio access system in which a plurality of terminals under the base station perform radio communication with the base station by random access,
In the uplink frame transmitted from the terminal to the base station, a random access area used by the plurality of terminals for random access and a shared access area used for transmitting a signal for estimating the number of terminals having transmission data are set.
The terminal having transmission data transmits the signal using a shared access slot of the shared access area, and the signal composed of a modulation scheme common to all terminals and a data string common to all terminals When transmitting, the number of successful receptions of the signal in the shared access slot is measured in a plurality of the uplink frames, and the transmission probability of the shared access slot, the total number of shared access slots, and the number of successful receptions in the shared access slot are measured. A base station apparatus comprising: means for estimating the number of terminals having the transmission data based on the transmission data.

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