Wireless communication technologies are very extensively used in daily life, and various types of wireless communication technologies have rapidly developed. Data other than voice information can currently be transferred using third generation radio communication technology, such as International Mobile Telecommunications (IMT) 2000, which followed Code Division Multiple Access (CDMA) communication technology, designated second generation technology. Recently, technology for providing data at a higher rate and at lower cost through next-generation wireless communication systems, such as Wireless Broadband (Wibro) technology, has been developed.
A newly developed wireless communication system is allocated and uses a frequency band which is not used with existing technology due to the problem of coexistence with existing technology. Due to the current development of various types of wireless communication systems, most frequencies in a band of several gigahertz (GH) have already been allocated, and thus few available frequencies remain.
In order to solve the problem, J. Mitola proposed Cognitive Radio (CR) technology, which is radio technology for sensing surrounding environments and determining radio transmission parameters, such as frequency, a modulation method, and power, and which automatically finds an available frequency according to region and time and thus enables desired communication without harming other licensed radio stations in the area.
Such cognitive radio technology has been popularized as technology for increasing the efficiency of utilization of frequency resources in the present situation, in which frequency resources are exhausted due to the increase in demand for wireless communication.
Cognitive radio-based next-generation wireless communication protocols include a physical layer (PHY), which is a first layer, and a data link layer, which is a second layer, the data link layer being classified into a Medium Access Control (MAC)/Radio Link Control (RLC) layer and a Radio Resource Management (RRM) layer.
The physical layer, the first layer, is a layer for transmitting/receiving signals between a base station and a mobile station or between mobile stations through communication channels. The signal transmission/reception methods of the physical layer include an overlay transmission method, an underlay transmission method, and a cooperative transmission method.
The overlay transmission method is a method of searching the entire frequency spectrum for an available spectrum hole and transmitting data at the frequency of the found spectrum hole, as shown in FIG. 1. The underlay transmission method is a method of transmitting signals in a wide frequency band so that the power level of the signals is not greater than a noise level, as shown in FIG. 2. The cooperative transmission method is a method of transmitting/receiving data in cooperation with neighboring terminals as well as a given terminal when the given terminal communicates with a base station.
The MAC/RLC layer is a layer for designing the format of a MAC frame. Generally, a MAC frame format in a wireless communication system is configured on the basis of a single spectrum. The location of data transmitted in a single spectrum within a MAC frame is detected using control information contained in the MAC frame. A user can check the control information and can determine which portion of a physical layer (PHY) frame is being used to transmit the data thereof. Further, in order to transmit/receive data using two or more different types of spectra, all system modules using respective spectra must be provided. When a single system module is provided, only information about a single MAC and a single PHY can be detected, and thus data can be transmitted or received using only a single spectrum.
The RRM layer allocates available spectrum holes to users in the sequence of the users' requests while detecting spectrum holes.
The operations of respective layers constituting the above-described cognitive radio-based next-generation wireless communication system have the following problems.
In the case of a physical layer (PHY), signals are transmitted or received using either an overlay transmission method or an underlay transmission method, but not both. Each of these transmission methods has its own unique problems. That is, the overlay transmission method is problematic in that, when a paying user appears during communication using a spectrum hole, the overlay transmission method must interrupt the communication using the spectrum hole, so that communication is frequently interrupted. The underlay transmission method is problematic in that, since the distance over which communication is performed must be as short as possible, communication is impossible if a terminal moves far away from a base station. Further, the cooperative transmission method is problematic in that, since a base station must manage the cooperative transmission time and transmission power of each terminal, the signaling overhead and complexity of the base station are increased.
In the case of the MAC/RLC layer, the format of a MAC frame is closely related to the frequency band and frame length of a physical layer. In an existing wireless communication system, a single MAC layer corresponds to a single physical layer. In contrast, in the cognitive radio-based next-generation wireless communication system, a single integrated MAC must manage a plurality of physical layers so as to maximize transmission efficiency. Further, since the usable frequency band of the cognitive radio-based system and the duration of a spectrum hole are variable, MAC must collectively manage the operation of a physical layer that varies with time. However, since the conventional MAC frame format is fixed, there is a problem in that it cannot support a plurality of physical layers and, in addition, a MAC frame that supports both a variable frequency band and a variable spectrum hole size cannot be configured.
FIG. 3 is a diagram showing a conventional MAC frame format. In a convention system, since a usable frequency band and the length of a single frame are preset, data symbols correspond to specific times at specific frequencies in a one-to-one manner. Further, since the conventional system uses only a single physical layer, there is no need to select a physical layer for processing a MAC Protocol Data Unit (PDU) processed by a MAC layer. However, in the cognitive radio-based wireless communication system, since the size of a usable frequency bandwidth and time are not preset, a conventional frame format having a fixed frame length and a fixed frequency band cannot be used, and there is a need to select a physical layer for processing a MAC PDU, so that the conventional frame format must be revised.
In the RRM layer, since spectrum holes are assigned according to the sequence of requests regardless of the type of service requested by users, real-time service, sensitive to delay, is frequently moved to different spectrum holes, and thus interruption may occur. Further, the RRM layer is problematic in that a spectrum hole having a long maintenance time and a large bandwidth is assigned to a user experiencing low Quality of Service (QoS), so that the spectrum may be wasted.