JP2012191592A - Cognitive radio communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cognitive radio communication system that can save traffic to a control station and bands used even when secondary systems increase.SOLUTION: A cognitive radio communication system includes: a control station 30 for controlling the entire system; a primary system 10 for preferentially establishing radio communication; and a plurality of secondary systems 20 for detecting the presence of the primary system 10 and notifying the detection result to the control station 30 to establish such radio communications so as not to interfere with the radio communication by the primary system 10. The control station 30 identifies when necessary a secondary system 20 that is incapable of cooperative detection in detecting the presence of the primary system 10, and rejects the detection result notification therefrom.

Description

  The present invention relates to a cognitive radio communication system, and more particularly to a cognitive radio communication system capable of saving a communication amount to a control station and a band to be used even when the number of secondary systems increases.

  With the shift from analog TV to digital TV, some frequencies are released and those frequency bands are used for mobile communications. In particular, in recent years when new communication services and various applications are put into practical use one after another, the allocation of the radio band is subdivided and complicated. While the communication services allocated in this way are mixed in the radio band, it is gradually becoming difficult to allocate new communication services to the radio band.

  In recent years, a cognitive wireless communication technique has attracted attention as a method of performing wireless communication while effectively using frequencies. Cognitive radio communication is a technology that uses frequency resources more efficiently by reusing limited frequency resources. That is, according to cognitive radio communication, devices belonging to the secondary network recognize frequency resources that can be used by sensing frequency resources that are not used in the primary network periodically or aperiodically, and can be used. Data is transmitted / received using various frequency resources.

  In cognitive radio, a cognitive system (secondary network) that performs communication based on this information by monitoring the priority network (primary network) that uses the original communication band and the frequency band and time that the primary network does not use. ) Exists. Basically, the primary network can always use a preferentially assigned communication band, such as a television system. The secondary network performs communication by devices belonging to its own system without interfering with the primary network. The secondary network is premised on using a frequency allocated to the above-described television system as the primary network. Among these, especially in this secondary network, there is a device that performs communication using the frequency determined by itself. For example, it is possible to acquire the position information of itself and information on the frequency of the primary network that can be used at the position. There is a device that can. In addition, in this secondary network, when such position information and frequency information of the primary network cannot be acquired, there are some which perform communication under the control of another device.

  In cognitive radio communication, communication is performed according to the following procedure. First, the secondary network detects a time when the primary network is not performing wireless communication and a frequency band that is not used. Next, the secondary network confirms whether or not each device in the primary network performs interference by communication by a device belonging to its own system. Wireless communication is performed according to bandwidth or time.

K.B. Letaief and W. Zhang, "Cooperative communications for cognitive radio networks," Proceeding of the IEEE, Vol. 97, No. 5, May 2009 E. Peh and Y.C. Liang, "Optimization for cooperative sensing in cognitive radio networks," IEEE WCNC 2007, pp. 27-32, Hong Kong, Mar 2007 W. Zhang, R.K. Mallik, and K.B. Letaief, "Optimization of cooperative spectrum sensing with energy detection in cognitive radio networks," IEEE Trans. Wireless Commun., Vol. 8, No. 12, pp. 5761-5766, Dec 2009

  By the way, the frequency detection by the secondary network is performed only to acquire an available frequency channel for the secondary network to communicate by itself. However, the possibility that an empty frequency channel can be acquired in the presence of the primary network decreases particularly with the increase of the secondary network. For this reason, the channel acquisition possibility by this second network can be significantly improved by cooperating with each other between secondary users (see, for example, Non-Patent Document 1).

  In the case where there are a large number of secondary networks, if each secondary network notifies the detection result of the frequency channel in each time slot, the time of frequency detection operation performed with emphasis on each other will inevitably become long. . As a result of the concentration of notifications of detection results in the control station, the operation of the control station becomes complicated, and if a frequency channel is assigned to each secondary network, a huge bandwidth is required. This also leads to wasted traffic, energy and bandwidth used throughout the system.

  On the other hand, it has been reported that the accuracy and efficiency of frequency detection are not affected so much even if there is no notification of information from more secondary networks (see, for example, Non-Patent Document 2). . On the contrary, the disclosed technique of Non-Patent Document 2 shows that the detection capability can be improved without notification of information from many secondary users. In addition, it has been reported that, when notifying the detection result to the control station, the consumption of energy and bandwidth can be suppressed by sharing the work part of the secondary system.

  Furthermore, according to the disclosure technique of Non-Patent Document 3, a method for searching for a secondary user that is emphasized in frequency detection is shown. However, a method for directly selecting an optimal number of secondary users that is emphasized in frequency detection is not particularly disclosed.

  In the disclosed technology of Non-Patent Document 2, it is required to notify the control station of the SN ratio of the signal received from the primary user for all secondary networks, but the frequency band when the number of secondary systems is large The problems related to the notification of detection are as described above.

  Therefore, the present invention has been devised in view of the above-described problems, and the object of the present invention is to save the communication amount to the control station and the bandwidth to be used even when the number of secondary systems increases. It is an object of the present invention to provide a cognitive radio communication system and method capable of performing the above.

  In order to solve the above-described problems, the present invention provides a primary control system for performing its own wireless communication from a plurality of secondary systems so as not to interfere with the wireless communication in the primary system to a control station that controls the entire system. When notifying the detection result of the presence / absence of the presence of the primary system, it is necessary to identify the secondary system that cannot be detected in cooperation with each other in detecting the presence of the primary system, and not accept the notification of the detection result from there. Characterize

  According to the present invention having the above-described configuration, it is possible to save the communication amount to the control station and the band to be used even when the number of secondary systems increases.

It is a figure which shows the structural example of the cognitive radio | wireless communications system to which this invention is applied. It is a figure which shows the result of having analyzed the relationship between the bandwidth and energy saved, and the number of secondary systems. It is a figure which shows the result of having analyzed the relationship between the threshold value of SN, and the number of secondary systems.

  Hereinafter, embodiments of the present invention will be described in detail.

  FIG. 1 shows a configuration of a cognitive radio communication system 1 to which the present invention is applied. The cognitive radio communication system 1 monitors a frequency band and a time that are not used by the primary system 10 that preferentially uses a communication band, and performs communication based on this information. A plurality of secondary systems 20 and a control station 30 that controls the entire cognitive radio communication system 1 are provided. That is, the secondary system 20 performs communication by selectively using a frequency band that is not used by the primary system 10 to the last.

  Basically, the primary system 10 can always use a preferentially assigned communication band, such as a television system. The secondary system 20 is premised on performing communication by devices belonging to the own system without interfering with the primary system 10.

  The primary system 10 includes a base station 12 and a device 11 that can access the base station 12.

  The device 11 includes, for example, a notebook personal computer (notebook PC), various portable information terminals such as a mobile phone. The device 11 can perform wireless communication with at least the base station 12 and further can perform wireless packet communication with another device 11 via the base station 12.

  The base station 12 is responsible for acquiring beacons transmitted from the device 11 and synchronizing them with each other in order to establish a link with the device 11. The base station 12 transmits radio waves using a frequency channel assigned to a television receiver (not shown).

  Similarly, the plurality of secondary systems 20 include a coordinator that serves as a base station in its own network and a device that performs communication.

  If attention is paid to such a secondary system 20, a channel in a frequency band not used by the primary system 10 is used, or wireless communication is performed in a time zone when the primary system 10 is not performing wireless communication.

  In the present invention, the presence / absence of the primary system 10 is detected in order to perform wireless communication by itself so as not to interfere with wireless communication in the primary system 10 from the plurality of secondary systems 20. At this time, in detecting the presence of the primary system 10, the secondary system 20 reports the detection result to the control station 30 in cooperation with each other. However, when there is a secondary system 20 that cannot report a detection result in cooperation with each other among the other secondary systems 20, the secondary system 20 is identified as needed. Then, the control station 30 performs control so as not to receive the notification of the detection result from the secondary system 20 that cannot report the detection result in cooperation with each other.

  Specifically, in this secondary system 20, when the SN ratio when the presence of the primary system 10 is detected is higher than a preset SN ratio threshold, a detection report composed of a 1-bit signal is controlled. The detection report composed of a 1-bit signal is not transmitted to the control station 30 when the SN ratio when transmitting to the station 30 and detecting the presence of the primary system 10 is equal to or less than a preset SN ratio threshold. And When the control station 30 does not receive a detection report from the secondary system 20, it identifies that the secondary system 20 has not detected the presence of the primary system 10.

  In the cognitive radio communication system 1 to which the present invention is applied, when frequency detection is performed with emphasis between the secondary systems 20, if the number of secondary systems 20 is large, the detection result is sent to the control station 30 for each secondary system. If the reports are sequentially reported from 20, the operation of the control station becomes complicated as a result of concentration of the notification of detection results, and if a frequency channel is allocated to each secondary system 20, a huge bandwidth is required, and the detection notification And the total time will be very long.

  Therefore, in the present invention, under the OR rule, while allowing the decrease in the frequency channel detection capability within the allowable range, the amount of communication to the control station 30 and the band to be used are limited even when the secondary system 20 increases. Savings are possible. That is, the number of secondary systems 20 that notify the frequency detection result is suppressed.

  At this time, the suppression of the number of secondary systems 20 that notify the frequency detection result is performed by deleting the secondary systems 20 that cannot perform frequency detection by emphasizing each other when performing frequency channel detection. At this time, the SN ratio threshold is set for the detection signal of the primary system 10 by the following method.

  First, in order to emphasize each other and perform frequency detection, an upper limit of the number of secondary systems 20 that notify the control station 30 of the frequency detection result is determined. This also corresponds to the maximum allowable range of error possibility when performing frequency detection to which the present invention is applied. The error possibility here is defined as the sum of frequency detection opportunity loss and loss related to transmission of the frequency detection result to the control station 30 with emphasis on each other (hereinafter referred to as transmission loss).

  Here, in the cognitive radio communication system 1 to which the present invention is applied, when the above-described OR rule and frequency energy detection are executed, the N secondary systems 20 that perform detection notification in cooperation with each other are the primary system 10. The presence / absence of presence / absence detection is transmitted to the control station 30 as a 1-bit signal. If such a signal is transmitted at each timing after frequency detection, the error possibility described above is given by the following equation (1).

・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (1)

Here, i = 1, 2,... N is an index number of the secondary system 20, pm _{, i} is a detection opportunity loss, and _{pf, i} is a transmission loss. Here, when the SN ratio of the primary system 10 is given by the density function f (γ _i ), these are given by the following equations (2) and (3).

... (2)

(3)

Here, λ _i is an energy threshold, and γ _i is the SN ratio of the primary system 10 detected by the i-th secondary system 20. u is a factor for detecting the time-bandwidth, and is a parameter for simplification. Gamma and (·), Γ (m, x) = ∫ x ∞t m-1 e -t dt has a fully gamma function, incomplete gamma function.

The following equation (4) is Marcum's Q function. Here, I _m-1 (•) is a modification of the Bessel function and is in the order of m-1.

···········(Four)

  In the OR rule, when one secondary system 20 or many secondary systems 20 detect the primary system 10 and notify the control station 30 of “1” with a 1-bit signal, the control station 30 H1 ″ (the detection that the primary system 10 has been detected) is determined. In other cases, the control station 30 determines a hypothesis of “H0” (that the primary system 10 has not been detected).

In the present invention, the SN ratio threshold (γ _T ) of the primary system 10 is set. In the OR rule and the frequency detection face, frequency detection is performed only when the SN ratio of the primary system 10 detected in the secondary system 20 is higher than γ _T , and in the subsequent period, the secondary system 20 makes 1 to the control station 30. Notification of detection results consisting of bits is performed. In other cases, the detection result is not reported to the control station 30. When the notification of the detection result composed of 1 bit is not performed to the control station 30, the control station 30 indicates that the primary system 10 has not been detected in the secondary system 20 during the detection period. It is determined that the determination is “0”. Since the change in error probability is below the upper limit that the system can tolerate, the SNR threshold γ _T will be selected.

In the present invention, the hypothesis of H0 is based on the assumption that the primary system 10 itself is not communicating because signal detection by the primary system 10 is not performed. For this reason, the transmission loss is not related to the SN ratio. The possibility of transmission loss is given by equation (3) in the i-th secondary system 20. However, in the case of H1, it is assumed that the primary system 10 is communicating. In the i-th secondary system 20, H1 is determined only when γ _i > γ _T and E _i > λ _i . Here, E _i is the energy collected at the frequency detected by the i-th secondary system 20. The possibility of reception failure proposed in the present invention is given by the following equation (5).

···········(Five)

As the detection reports from the secondary system 20 to the control station 30 decrease, the possibility of error in frequency detection at the control station 30 deteriorates. The possibility of error is due to the increase in transmission loss described above. This is given by the following equation (6).
... (6)

  Here, when it is assumed that all the secondary systems 20 have the same SN ratio of the primary system 10, the equation (7) is modified as follows.

・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (7)

  N: number of secondary systems, λ: energy threshold, f (γ): probability density function of SN ratio.

That is, in this equation (7), when the presence of the primary system 10 is detected and the secondary systems 20 that cannot be detected in cooperation with each other are excluded, the upper limit of the number of secondary systems 20 that can be detected in cooperation with each other is set. in terms of determining, and shall be based on the maximum allowable range of error possibilities epsilon _d in performing the frequency detection threshold gamma _T of the SN ratio is to be determined based on the epsilon _d .

In the present invention, the probability density function f (γ) of the S / N ratio in the primary system 10, the allowable reduction range ε (for example, the upper limit of the error possibility) in the control station 30, the number N of the secondary systems 20, the energy It can be used to search for the SN ratio threshold γ _T under circumstances where the threshold λ is known.

The energy threshold λ is searched by assuming additional detection under the assumption that each secondary system 20 has the least error potential.
In the present invention, the metrics used to evaluate the proposed scheme are normalized and saved energy and wavelength as a detection result and are given by the following equation (8).

・ ・ ・ ・ ・ ・ ・ ・ ・ (8)

  That is, it is needless to say that the metrics for evaluating the communication described above may be provided based on p obtained by the equation (8) of the probability density function f (γ) of the SN ratio.

  Hereinafter, embodiments of the present invention will be described by taking numerical analysis as an example. Assume the channel given by Rayleigh and assume that the energy threshold λ is a value that allows ideal frequency detection (a value that achieves the minimum error probability). The probability density of the SNR of the primary system 10 in the channel given by Rayleigh is given by

・ ・ ・ ・ ・ ・ ・ ・ ・ (9)

FIG. 2 shows the bandwidth and energy saved in the case where the frequency detection is emphasized by the OR rule when the present invention is applied. The average SN ratio in the primary system 10 is set at 5 to 10 dB. The upper limit for the change in error probability is assumed. It can be seen that applying the present invention saves bandwidth and energy. If there are 10 secondary systems 20 and the average SNR (= r _avg ) is 10 dB, energy and bandwidth of 60% or more are saved. Moreover, when 50 secondary systems 20 exist, 80 to 90% of energy and bandwidth are saved.

  FIG. 3 shows a result of investigating the dependency of the quantity of the secondary system 20 on the SN ratio threshold in the primary system 10. In FIG. 3, the average of the S / N ratio of the primary system 10 and the upper limit of the change in error probability are used as parameters. These SN ratio thresholds have the tendency described in the above description.

1 Cognitive Radio Communication System 10 Primary System 11 Device 12 Base Station 20 Secondary System 30 Control Station

Claims (4)

A control station that controls the entire system;
A primary system for preferential wireless communication;
A plurality of secondary systems for detecting the presence or absence of the primary system and notifying the control station of the presence or absence of the primary system in order to perform wireless communication by itself so as not to interfere with wireless communication in the primary system,
The cognitive radio communication system characterized in that the control station specifies a secondary system that cannot be detected in cooperation with each other when detecting the presence of the primary system, and does not accept notification of the detection result therefrom. The secondary system transmits a detection report composed of a 1-bit signal to the control station when the SN ratio when the presence of the primary system is detected is higher than a preset SN ratio threshold, When the SN ratio when the presence of the primary system is detected is not more than a preset SN ratio threshold, a detection report composed of a 1-bit signal is not transmitted to the control station,
2. The cognitive system according to claim 1, wherein if the detection report is not received from the secondary system, the control station identifies that the secondary system did not detect the presence of the primary system. Wireless communication system. When detecting the presence of a primary system and eliminating secondary systems that cannot be detected in cooperation with each other, determining the upper limit of the number of secondary systems that can be detected in cooperation with each other, Based on the maximum allowable range of error probability ε _d at
Threshold gamma _T of the SN ratio, it is determined based on the following epsilon _d
The cognitive radio communication system according to claim 2, wherein N: number of secondary systems, λ: energy threshold, and f (γ): probability ratio function of SN ratio. Evaluating the communication described above based on p obtained by the formula of probability density function f (γ) of the following SN ratio
The cognitive radio communication system according to claim 3.