Patent Document

PRIORITY 
     This application claims priority under 35 U.S.C. §119 to an application entitled “Communication Initialization Method at Medium Access Control Layer in a Cognitive Radio Wireless Communication System” filed in the Korean Intellectual Property Office on Nov. 5, 2005 and assigned Serial No. 2005-105718, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a communication initialization method at a Medium Access Control (MAC) layer in a Cognitive Radio (CR) wireless communication system. 
     2. Description of the Related Art 
     Along with the rapid development of wireless communication systems and a variety of wireless communication services, strict frequency separation is required for coexistence with legacy communication systems. There is also a short supply of spectrum frequencies due to multiple allocations over all of the frequencies which can be commercially used. Deployment of a new wireless platform faces a serious shortage of spectrum resources, particularly in lower frequency bands than a few gigahertzes. To solve this spectrum shortage, the concept of CR has been introduced, which is a frequency-agile technique for sensing allocated but unused spectrum and efficiently sharing the detected spectrum. 
     Many countries have strictly regulated the use of frequency resources subject to their frequency policies, and service providers use frequencies through licensing and allocation from their governments. As opposed to legacy wireless communication systems, the CR technology enables the use of allocated but unused frequency resources in a non-interfering manner with wireless communications from legacy service providers. 
     In order to meet the recent increasing demand for limited frequency resources, there is a pressing need for the CR technology, which has attracted much interest since the U.S. Federal Communications Commission (FCC) commented on the possibility of frequency sharing in a Notice of Proposed Rule Making (NPRM) in December 2003. Such a major example is the standardization of the Institute of Electrical and Electronics Engineers (IEEE) 802.22 Wireless Regional Area Network (WRAN) seeking the development of a CR communication platform. The IEEE 802.22 WRAN system is expected to be deployed in suburban areas of the U.S. or Canada, or in developing countries, aiming to provide wireless communication services using the CR technology in an unused TV frequency band. 
     CR standardization and development is active but in an early stage. There still exists many issues to be handled and most of configurations for the CR technology are yet to be specified. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to communication initialization in a CR wireless communication system. 
     Communication initialization is critical to CR in view of its nature. A CR communication system is required to continually monitor the spectrum of a licensed service provider. Upon sensing the presence of the licensed service provider on the spectrum, the CR communication system has to move to another available extra frequency band. While legacy communication systems operating under the strict regulations of the government have only to deal with problems with regards to given frequency bands, detection over the total available frequency band becomes critical to the CR communication system. Problems generated by different frequency detection results between CR wireless communication systems have to be tackled and above all things, the detection time in the frequency area is highly probable to increase. Because the increased frequency detection time is an obstacle to efficient communications, there exists a need for effectively solving the problem. 
     Accordingly, the present invention is directed to an effective communication initialization method in a MAC layer to solve the problems involved in the CR implementation. 
     An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, the present invention provides a communication initialization method in a MAC layer for minimizing a frequency detection time in a CR communication system. 
     According to an aspect of the present invention, in a communication initialization method in a CR communication system having a service area overlapped with the service area of a licensed communication system which provides communication service in a licensed frequency band, a BS senses total available frequency bands for providing communication service and detects unused frequency bands for licensed communication systems or other CR communication systems. Then, the BS prepares for providing the communication service to CPEs in the unused frequency bands. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a network configuration to be referred to for describing an operation of a CR system according to the present invention; 
         FIG. 2  illustrates transmission of an RTU- 1  signal in a communication initialization method in the CR system according to the present invention; 
         FIG. 3  illustrates a backoff operation for preventing collision between RTU- 1  signals in the communication initialization method in the CR system according to the present invention; 
         FIG. 4  illustrates transmission of channel reports from CPEs in the communication initialization method in the CR system according to the present invention; 
         FIG. 5  illustrates transmission of the RTU- 2  signal after the BS selects a frequency band in the communication initialization method in the CR system according to the present invention; 
         FIG. 6  is a diagram illustrating insertion of the service identification signal in the communication initialization method in the CR system according to the present invention; and 
         FIG. 7  illustrates the communication initialization method in the CR system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
     Before describing a communication initialization method in a CR communication system according to the present invention, the terms used herein will first be defined below. 
     CR System: a wireless communication system using the CR technology; 
     Licensed Primary System: a wireless communication system legally authorized to use a frequency; 
     Base Station (BS): a device located at the center of a cell, for enabling communications of Customer Premise Equipments (CPEs) within its coverage area; and 
     CPE: a user or device that communicates through the BS. 
     In accordance with the present invention, requirements for the CR communication system are defined as follows. 
     1. A CR communication system is limited to a point-to-multipoint cellular communication system. In the cellular system, a BS located at the center of a cell communicates with CPEs within its coverage area. 
     2. Upon sensing the presence of a licensed primary user in the current frequency band, the CR wireless communication system has to immediately move to another unused frequency band, discontinuing the use of the current frequency band. This is mandatory to the CR communication system to avoid interference with licensed primary users. 
     3. The CR wireless communication system has to be equipped to dynamically sense the operation statuses of other wireless communication systems in order not to interfere with existing licensed primary systems. When sensing the presence of any other device at a certain frequency by the frequency sensing function, the CR wireless communication system determines if the device is a licensed primary system or another CR wireless communication system and operates correspondingly. 
     A communication initialization method according to the present invention is an initialization protocol for the MAC layer, independent of the properties of the physical layer. A detailed description of how a frequency band is sensed and how a communication system that has sent a sensed signal is identified in the physical layer will not be provided. The present invention is described on the assumption that requirements for the physical layer for practical implementation of CR can be satisfied. 
     Relative to other wireless communication systems, communication initialization is not so simple in the CR system. Due to the nature of CR system, i.e. detection of an unused frequency band, upon sensing the presence of a licensed primary user in the current frequency band, the frequency band has to be immediately changed. For notational simplicity, five different frequency bands identified by their channel indexes, channel  1  to channel  5  are assumed. 
       FIG. 1  illustrates a network configuration to be referred to for describing an operation of the CR system according to the present invention. 
     Referring to  FIG. 1 , the service area  150  of a BS  100  (CR BS) in the CR system is overlapped with those of BSs  110  and  120  in licensed primary systems and BSs  130  and  140  in neighbor CR systems. The licensed primary systems  110  and  120  use channel  1  and channel  2 , respectively, and the neighbor CR systems  130  and  140  use channel  3  and channel  4 , respectively. A plurality of CPEs  10 - 1  to  10 - 12  are scattered within the service area  150  of the BS  100 . Some of them, i.e. the CPEs  10 - 1 ,  10 - 4 ,  10 - 5 ,  10 - 8 ,  10 - 10 ,  10 - 11  and  10 - 12  are located in overlap areas between the service areas of the BSs  110  to  140  and the service area  150  of the BS  100 . 
     The CR BS  100  and the CPEs  10 - 1  to  10 - 12  can sense frequency bands as well as perform communications. The BS  100  does not have knowledge of the frequency use status of every CPE that it services. Especially when there are a plurality of CR system operators, the BS has more difficulty in determining the frequency use status of neighbor CPEs. If a currently serviced CPE is located in an overlap area between a licensed primary system and the CR BS  100  or between another CR system and the CR BS  100 , these systems affect the CPE, and are affected by signal propagation of the BS  100 . In the illustrated case of  FIG. 1 , although the CR BS  100  may determine through frequency sensing that all of channel  1  to channel  5  are available, in fact, channel  1  and channel  2  are in use for the licensed primary systems  110  and  120 , and the CPEs  10 - 1 ,  10 - 4  and  10 - 5  serviced by the CR BS  100  are within the coverage area of the licensed primary systems  110  and  120 . Also, channel  3  and channel  4  are in use for the other CR systems  130  and  140 . In this case, the point is which channel the CR BS  100  has to select. The CR BS  100  sends frequency sensing request information over the frequency bands sensed as available to the CPEs  10 - 1  to  10 - 12  and selects a frequency band for use based on the result for each frequency band received from the CPEs  10 - 1  to  10 - 12 . Considering the existence of CPEs that cannot interpret particular frequency band information or CPEs that cannot respond due to a power-off state while in the service areas of the licensed primary systems  110  and  120  and the other CR systems  130  and  140 , a new MAC initialization technique is required for the CR BS  100  to select a frequency band. 
     Despite power-off states or during communications, the CR BS  100  has to sense as many frequency bands as possible to thereby determine available frequency bands. After determining the available frequency bands, the CR BS  100  sends a control signal at a preset position in all or part of the frequency bands unused for the licensed primary systems  110  and  120  and the other CR systems  130  and  140 . In the present invention, the control signal is referred to as a Ready to Use (RTU)- 1  signal. The RTU- 1  signal is sent with a very narrow bandwidth, compared to the bandwidth of each frequency band and may have a pattern easily sensible to other CR systems. The RTU- 1  signal contains the Identifier (ID) of a serving BS (BSID), a Training Frequency Set (TFS) that the BS sensed as available, and a known Training Sequence Signal (TSS) by which other BSs or CPEs capable of receiving the RTU- 1  signal can find out the status of the channel (e.g. Signal-to-Interference Noise Ratio (SINR)). The BS periodically sends the RTU- 1  signal over all available frequency bands. 
       FIG. 2  illustrates transmission of the RTU- 1  signal in the communication initialization method in the CR system according to the present invention. 
     Referring to  FIG. 2 , the CR BS periodically sends the RTU- 1  signal for a preset time over all or part of the frequency bands except for those occupied by licensed primary systems or other CR systems detected by the CR BS, that is, over channel  1 , channel  2 , channel  4 , channel  5 , channel  7 , channel  8 , channel  9  and channel  10  in  FIG. 2 . Unused frequency bands are denoted by  203  and  206 . The RTU- 1  signal includes a BSID, a TFS, and a TSS, and is defined as shown in Equation (1):
 
RTU-1={BSID,TFS,TSS}  (1)
 
     The CR BS  100  periodically sends the RTU- 1  signal and receives channel reports from CPEs in response to the RTU- 1  signal. The CR BS  100  excludes a frequency band in which a licensed primary system or another CR system may significantly affect a CPE from the next RTU- 1  transmission based on the channel reports. The transmission periods of the RTU- 1  signal and the channel report are determined by a CR system operator. 
     It may occur that RTU- 1  signals in a commonly available frequency band from a plurality of other CR system operators collide with each other, thereby making it impossible for CPEs or neighbor BSs to reliably interpret the RTU- 1  signals. As a consequence, the available frequency band is rendered unavailable. To solve this problem, each BS sends the RTU- 1  signal in Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) with backoff. Upon sensing the presence of an RTU- 1  signal in the available frequency band before signal transmission, the BS waits for a backoff time. If detecting a licensed primary system during the backoff, the BS immediately stops the RTU- 1  transmission attempt. 
     The backoff operation goes with CSMA/CA. A BS or a CPE monitors if a medium (i.e. a frequency band) is clear before signal transmission. Upon detection of an energy level greater than or equal to a threshold in the medium, the BS or CPE waits, having determined that the medium is in use and activates a backoff timer. A backoff time later, the BS or CPE attempts signal transmission. 
       FIG. 3  illustrates a backoff operation for preventing collision between RTU- 1  signals in the communication initialization method in the CR system according to the present invention. 
     Referring to  FIG. 3 , the CR BS  100  senses an RTU- 1  signal in an available frequency band before signal transmission and attempts the signal transmission after a backoff period  305 . 
     The backoff time can be set and the backoff timer can be operated in many ways, which will not be described herein. CPEs, which have received the RTU- 1  signal, acquire a Training Frequency Set (TFS) and channel status information of each frequency band (for example a signal to noise ratio SINR) from the RTU- 1  signal. The CPEs then feed back channel reports to the CR BS  100  in a plurality of frequency bands in which the licensed primary system and other CR systems have no influence on the CPEs. The number of frequency bands for carrying the channel reports is determined by a service provider. 
       FIG. 4  illustrates transmission of channel reports from CPEs in the communication initialization method in the CR system according to the present invention. 
     Referring to  FIG. 4 , upon receipt from the BS of an RTU- 1  signal in unused frequency bands, a CPE replies with a channel report in CSMA/CA with backoff in a plurality of frequency bands in optimum channel status (e.g. optimum SINR status) and having no influence on a licensed primary system and other CR systems around the CPE, except for frequency bands  405  in use for the licensed primary system and other CR systems as sensed by the CPE. 
     In the illustrated case of  FIG. 4 , the CPE receives the RTU- 1  signal over channel  1 , channel  2 , channel  4 , channel  5 , channel  7 , channel  8 , channel  9  and channel  10 , but sends its channel report to the CR BS over channel  1 , channel  2 , channel  4 , channel  8 , channel  9  and channel  10  except two channels occupied by the licensed primary system and other CR systems, i.e. channel  5  and channel  7 . 
     The channel report contains the ID of the CPE, a Current Band Channel Metric (CBCM), and a Not Available Training Frequency Set (NA-TFS), and is defined by Equation (2):
 
Channel Report={CPE ID,CBCM,NA-TFS}  (2)
 
where CPE ID is an identification number specific to the CPE, which may contain location information of the CPE. CBCM contains the channel status information of each frequency band that has carried the RTU- 1  signal from the BS, and NA-TFS denotes a frequency band set which is available to the BS but not available to the CPE at its location for some reason, e.g. for the reason that the frequency band set is occupied by neighbor systems. NA-TFS functions to notify the CR BS of frequency use status around the CPE, which is not known to the CR BS, i.e. frequency bands used for other CR systems and a frequency band locally used for a licensed primary system, to thereby avoid interference with the licensed primary system and the CR systems. NA-TFS includes information indicating the frequency bands that are not available are occupied by the licensed primary system or other CR systems.
 
     In the case of simultaneous transmission of channel reports from CPEs, they may collide. To overcome this problem, the channel reports are delivered in CDMA/CA with a backoff. 
     As described above, RTU- 1  transmission from the BS and channel report transmission from the CPEs are based on CSMA/CA with backoff. The CR system operator has to set a substantially sufficient time for transmission of the RTU- 1  signal and the channel report signals. When a BS determines that it has received a sufficient number of channel reports, the BS will select an optimum frequency band. Considerations regarding channel report reception and the period of channel report transmission are determined by the service provider. 
     Upon receipt of channel reports from the CPEs, the BS selects the best frequency band according to whether each frequency band is in use or not, and according to channel status information from the CPEs (e.g. mean SINR). The BS selection process of the best frequency band is beyond the scope of the present invention and thus will not described herein. Then the BS notifies the CPEs of the selected frequency band. 
     The BS sends to the CPEs frequency band information associated with the selected frequency band in the frequency bands that delivered the RTU- 1  signal, which is defined by Equation (3):
 
Selected Frequency={BSID,SFS}  (3)
 
     As noted from Equation (3), the selected frequency band information contains the BSID and a Selected Frequency Set (SFS). 
     During communications in the selected frequency band, the BS monitors frequency bands and periodically sends an RTU- 2  signal in all frequency bands sensed as available except for the selected frequency band in current use. As with the case of  FIG. 1 , in the case where a CPE located within the service area of the neighbor CR system  130  has not sent a channel report due to a power-off state, for example, when the BS selects the same frequency band as that of the CR system  130 , the transmission of the RTU- 2  signal enables the CPE to send frequency band information about the selected frequency band in another frequency band. The transmission of the RTU- 2  signal relieves the constraints of sensing excess frequency bands when there are other CPEs to communicate. The RTU- 2  signal contains information about the current frequency band that carries the RTU- 1  signal. 
       FIG. 5  illustrates transmission of the RTU- 2  signal after the BS selects a frequency band in the communication initialization method in the CR system according to the present invention. To avoid collision with RTU- 2  signals from other CR systems, the RTU- 2  signal is periodically sent in CSMA/CA with a backoff. Upon detecting a licensed primary system authorized to use a predetermined frequency band in the frequency band, the BS discontinues transmission of the RTU- 2  signal in the frequency band and updates information about the RTU- 2  signal. 
     Referring to  FIG. 5 , the BS selects optimum frequency bands  501 ,  502  and  504  based on channel reports received from CPEs in response to the RTU- 1  signal which was sent in frequency bands except for frequency bands  503  and  506  in use for the licensed primary system and other CR systems, and then sends the RTU- 2  signal periodically in frequency bands except for these channels  501 ,  502 ,  503 ,  504  and  506 , that is, over channels  5 ,  7 ,  8 ,  9 , and  10 . The RTU- 2  signal contains a BSID, a Current Band List (CBL), a TFS, and a TSS, as defined by Equation (4):
 
RTU-2={BSID,CBL,TFS,TSS}  (4)
 
     After the frequency band selection, the BS inserts a service identification signal identifying the CR system at a preset position of the frequency band so that other CR systems can sense frequency bands with a reduced sensing time. 
       FIG. 6  is a diagram illustrating insertion of the service identification signal in the communication initialization method in the CR system according to the present invention. Referring to  FIG. 6 , a CPE identifies a CR system by a service identification signal  605 . 
       FIG. 7  illustrates the communication initialization method in the CR system according to the present invention. 
     Referring to  FIG. 7 , the BS detects available frequency bands for a frequency searching/sensing period (A). For a BS initialization period (B), the BS selects an optimum frequency band by exchanging an RTU- 1  signal and channel reports with CPEs and completes preparation for communications in the selected frequency band. The BS performs synchronization and process bandwidth requests for CPEs receiving control information from the BS for a CPE initialization period (C), and communications are conducted under the control of the BS for a communication period (D). 
     In accordance with the present invention as described above, the initialization method for the CR communication system enables efficient selection of a frequency band unused by licensed primary users and other CR systems and reduces a frequency sensing time before communications with CPEs, since a BS sends an RTU signal in available frequency bands to the CPEs, the CPEs reply with channel reports, and the BS sends an RTU- 2  signal in extra available frequency bands during communications. 
     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Technology Category: 5