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Friday, June 17, 2011

The Concept of cognitive radio Base Stations


There are two trends in cognitive radio; one trend is the so-called “Multiple Systems,” which switches wireless communication systems according to the radio conditions. The other trend is the so-called “Dynamic Spectrum Access,” which recognizes spare frequencies of a primary system and allocates them to be used for communication of a secondary system to such an extent that the primary system would not be affected. From a terminal point of view, SDR (Software Defined Radio) terminals that support multiple wireless systems have been proposed. Mobile terminals can measure radio information and report that to the base station, and the base station decides whether to switch to other systems according to this report from the mobile terminal.


However, that is not sufficient for maximizing system capacity and satisfying requirements for user communication quality because system load and information that can be acquired from the network (e.g., the number of terminals that connect to an access point) are not taken in account. The concept of cognitive radio based on “Multiple Systems” approach is shown in the illustration below. In this figure, the concept is expressed using both the frequency domain and time domain.

For example, we assume we can communicate with three systems: A, B and C.
In this illustration, when the current time is t2, cognitive terminal (CT) D communicates using the frequency of System A, and when the current time becomes t3, terminal D changes the wireless communication system from System A to System B to communicate using the frequency of System B. As shown in the illustration, the number of wireless communication systems used simultaneously is not limited to one, and the cognitive system can transmit and receive data with multiple wireless communication systems simultaneously. Furthermore, terminals of the cognitive radio system (cognitive terminal) switch the wireless communication system frequently according to the radio conditions as stated.
Therefore, in cognitive radio, the corresponding node need not know which wireless system is being used. Based on this concept, two requirements to achieve cognitive radio are as below:

1. System architecture for fast system handover, which can reflect radio environments that change dynamically, and system load and information that can be acquired from the network.
2. Assignment of one local IP address to the terminal regardless of the number of wireless communication systems that the terminal communicates with.

Provided that EV-DO (cdma2000 1x Evolution Data Optimized) is system A, WiMAX (Worldwide Interoperability for Microwave Access) is system B and wireless LAN is system C in the illustration above, terminal D can communicate with EV-DO, wireless LAN, and WiMAX adaptively according to the radio conditions. However, terminal D can use different radio systems, as shown in the illustration, only when terminal D is located in the area where EV-DO, WiMAX, and wireless LAN are in service. The service areas of each system differ from each other due to the difference in frequency performance and difference in service (carrier, bit rate, and charge, for example), its on this basis that the architecture of the cognitive base station has to be considered.

When we set up a cognitive BS (Base Station) that supports multiple wireless systems like that shown below, only the center area of the base station, which is covered by all kinds of wireless communication systems, can satisfy the conditions shown in the illustration above. This architecture is simple and easy to construct; however, an area where cognitive radio can be adopted is narrow and limited. Actually, WiMAX access points are not always located in the same place where EV-DO access points are located; therefore, this architecture is not suitable and it would seem that the realization probability of the architecture is low.
To expand the area that satisfies the conditions shown in first illustration, the cognitive base station can be described as below:

1. The area of the cognitive base station is equivalent to the area in which access points of a cellular system are covered, which is the widest area among the access points of other wireless systems.

2. A cognitive base station has the function of access points of a cellular system, the function of access points of WiMAX and wireless LAN in the cognitive base station area, and a control node to integrate these functions. The concept of a cognitive BS (base station) based on this definition is shown below.
From this definition, the area that satisfies the conditions shown in first illustration can be expanded. Actually, WiMAX access points are not always located in the same place where the EV-DO access point is located; therefore, this architecture is more realistic. Moreover, placing a control node inside the cognitive base station is one characteristic. The control node controls these systems below the IP layer. Thus, converging multiple access points of multiple systems inside the cognitive base station enables us to treat the radio resources spread throughout the cognitive base station area as an “internal” radio resource of the cognitive base station. Consequently, that is expected to achieve fast system handover. Based on the concept described above a system architecture of a cognitive radio base station is as illustrated below;


From this definition, the area that satisfies the conditions shown in first illustration can be expanded. Actually, WiMAX access points are not always located in the same place where the EV-DO access point is located; therefore, this architecture is more realistic. Moreover, placing a control node inside the cognitive base station is one characteristic. The control node controls these systems below the IP layer. Thus, converging multiple access points of multiple systems inside the cognitive base station enables us to treat the radio resources spread throughout the cognitive base station area as an “internal” radio resource of the cognitive base station. Consequently, that is expected to achieve fast system handover. Based on the concept described above a system architecture of a cognitive radio base station is as illustrated below;


The Cognitive base station consists of PDSN (Packet Data Serving Node) to integrate an EV-DO access point to the cognitive base station, ASNGW (Access Service Network Gateway) to integrate multiple WiMAX access points, PDIF (Packet Data Interworking Function) to integrate multiple wireless LAN access points, control node, monitoring node in addition to multiple access points of multiple radio systems. The monitoring node collects radio information and system load from each access point and information that can be acquired from network side and recognizes radio condition. Based on the information from the monitoring node, the control node switches the communication system in a packet-by-packet basis. Moreover, location of control node is not above each access point, but above PDSN, ASN-GW and PDIF. PDSN terminates PPP (Point-to-Point Protocol) session to the EV-DO terminal function, ASN-GW controls multiple WiMAX access points, and IPSec (Security Architecture for Internet Protocol) tunnel is established between PDIF and the terminal, and PDIF controls multiple access points of wireless LAN. Therefore, it is reasonable for future system migration to locate control node above PDSN/ASN-GW/PDIF.

To place control node to converge wireless systems inside the cognitive base station and its location above PDSN, ASN-GW and PDIF are major characteristics of our system and these are main difference from MIRAI architecture. Cognitive terminal consists of EV-DO terminal module, WiMAX CPE (Customer Premises Equipment) module, wireless LAN access terminal module and control node (application) to integrate the data received from these modules.





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