Cognitive Radio and its networking technology

With the rapid development of wireless communication technology, spectrum resources, especially those under 3 GHz, become increasingly tense. Due to the emergence of Wireless LAN technology and wireless network technology, more and more people are using these technologies to connect to the Internet and telecommunication networks wirelessly. Most of these technologies use non-authorized frequency bands to make these frequencies saturated. However, some other authorized frequency bands (such as radio and television business frequency bands) need to be protected by the telecom network, to avoid interference. As a result, the usage efficiency of these special frequency bands is very low, and the authorized frequency bands account for a majority of the spectrum resources, which virtually results in a huge waste of spectrum resources. Figure 1 is the result of the Berkeley University radio Research Center (BWRC ~ Spectrum Utilization in the 6 GHz band [1]. As shown in figure 1, some frequency bands are not used by users for most of the time, and some other frequency bands are only occupied in part of the time, A small portion of the remaining band is used by many different users for competition. In fact, the shortage of spectrum resources is not due to the unreasonable allocation of spectrum resources. Increasing spectrum utilization and making full use of limited spectrum resources in various regions has become the focus of communication experts.

Figure 1 spectrum resource usage within 6 GHz in the Urban Area

To solve the problem of "Scarcity" of spectrum resources, the basic idea is to increase the utilization rate of spectrum resources as much as possible. To this end, Dr. Joseph Mitola of the Royal Swedish Institute of Technology proposed the concept of Cognitive Radio [2]. In this sense, Cognitive Radio's initial mourning aims to improve spectrum utilization, enables cognitive wireless communication devices to discover and use spectral holes for communication [3] [4].

1. Introduction to key technologies of Cognitive Radio

In a relatively complete sense, cognitive radio systems should have functions such as detection, analysis, learning, reasoning, and adjustment. The realization of these functions requires a series of technologies to support, it mainly includes definition and measurement of interference temperature, dynamic spectrum allocation, transmission power control, and original user detection.

(1) Interference Temperature

The interference temperature was proposed by the FCC to characterize the interference that unauthorized users have on authorized user receivers in the shared band. The system sets an interference temperature threshold to ensure normal operation of authorized users. This threshold is determined by the worst signal-to-noise ratio that authorized users can work normally. An unauthorized user is disturbed by an authorized user. Once the accumulated interference, including unauthorized user signals, exceeds the interference temperature threshold, the authorized user system cannot work normally. Otherwise, it can ensure that authorized users and non-authorized users work normally at the same time.

(2) dynamic spectrum allocation

The bandwidth requirements, number of available channels, and locations of CR networks change at any time. The traditional dynamic spectrum allocation methods for voice and wireless networks are not fully applicable. In addition, to achieve full dynamic spectrum allocation is subject to many policies, standards and access protocols. Therefore, the current research on CR-based DSA is mainly based on the spectrum sharing pool strategy. The basic idea of the spectrum sharing pool is to combine a portion of the spectrum allocated to different businesses into a public spectrum pool and divide the entire spectrum pool into several subchannels, therefore, the channel is the basic unit of spectrum allocation [5]. Mitola proposed the initial framework of the standard wireless etiquette protocol to regulate the negotiation mechanism of spectrum selection among users. Game theory is an effective tool for analyzing real-time user interaction processes.

(3) transmission power control

The premise of using CR technology to achieve spectrum sharing is to ensure no interference to authorized users, and the power distribution of cognitive users in each distributed operation is the main cause of interference, therefore, we need to explore a distributed power control method suitable for CR technology.

(4) original User Detection

In Cognitive Radio, detection of original users is an important aspect. In practice, this situation often occurs when the authorized users are working at a low power level or are blocked and cannot be promptly and effectively detected by the Cognitive Radio System, the Cognitive Radio system considers the frequency band used by authorized users to be one or more "spectrum holes" and tries to establish its own communication in this frequency band, in this case, Cognitive Radio users may forcibly occupy the authorized spectrum, which is absolutely not allowed during the work of Cognitive Radio.

2. Networking Technology in Cognitive Radio

Cognitive Radio Network is an intelligent multi-user wireless communication network. Its basic features are embodied in the following aspects:

(1) grasp the radio environment through real-time monitoring of the surrounding environment by the CR user receiver;

(2) Adjust the RF front-end parameters of the radio transceiver in real time through learning the communication environment;

(3) smooth communication through self-organizing cooperation among multiple users;

(4) control the communication of competing users through reasonable resource allocation, so that every communication process can proceed smoothly.

2.1 Cognitive Radio Networks

How the transmitter accurately locates the receiver in a dynamic environment is a major research aspect of Cognitive Radio, which depends on the networking scheme of Cognitive Radio. Generally, there are several Cognitive Radio networking solutions [6]:

(1) Central Control Structure

Its main feature is that the control design of Transmit signals is relatively simple, but it is subject to the establishment of the base station, as shown in Figure 2:

Figure 2 Central Control Structure

(2) Distributed Control Structure

The main disadvantage is that the control design of transmitting signals is relatively difficult, and the network construction technology is not mature, as shown in 3:

Figure 3 distributed control structure

(3) mesh Control Structure

Its main feature is that local communication uses Ad-hoc routing, and non-local communication uses access nodes for communication, as shown in Figure 4:

Figure 4 mesh Control Structure

Because the 3rd types of structures combine the advantages of the first two types to facilitate the communication between users, the following are all discussed based on this structure.

When a Cognitive Radio user enters the local communication network, it should have the following five basic functions to ensure seamless communication. This is mainly:

(1) able to discover nearby users;

(2) access nodes can be found;

(3) Ability to constantly update information of nearby users;

(4) Establish a communication path with the receiving node in the wireless self-Networking Mode in the render network;

(5) communication with other receiver nodes through the access node can be established.

Figure 5 shows a refined Cognitive Radio Network with multiple nodes. The Access Node is used to establish a communication path between different networks, which is equivalent to the connection link between different networks in Figure 4. Among them, nodes 15, 23, and 65 are newly connected nodes, all of which communicate through Channel A, but the information transmission distance is limited; nodes 42, 60, and 96 are existing nodes, but they do not receive the "Hello" message sent by the newly added nodes. Other nodes also exist, the difference is that they all receive the "Hello" message sent by the newly added node.

Figure 5 Cognitive Radio Network Structure

A common public control channel is required for a node to discover its surrounding nodes and Exchange Control Information conveniently between nodes in the ingress network. This also has two options: one is adaptive public control channel. At this time, the transmission of each command is carried out in the cognitive channel equivalent to the node, as a result, the complexity of the system is greatly increased. In addition, a pre-allocated control channel can be used, which is easy to implement, however, a considerable amount of spectrum resources need to be allocated to control the channel, which deviates from the spirit of Cognitive Radio.

2.2 Communication Between Cognitive Radio users

When A Cognitive Radio user enters a catv network, it needs to adjust itself to channel A Based on the location information obtained from GPS. In this case, Channel A sends A "Hello" packet to A nearby user to indicate its identity. Each information package should contain the node's ID number, location information, the most suitable spectrum holes, and a series of adjacent users with a single hop. In addition, by limiting the broadcast information of each node on Control Channel A, more nodes can discover their neighboring users through this channel.

As shown in figure 6, nodes 15, 23, and 65 broadcast their own information through control channel A. The difference is that they can discover different neighboring nodes, but they are all concentrated around themselves, that is, the Broadcast Transmission range is limited. In addition, the Access Node can interact with other domestic networks.

Figure 6 search for near users

The Access Node is mainly used to transmit information packets between different regions. It should know the location information of all nodes in the region, this allows users in other networks to Process Communication requests between local nodes in a timely manner. In addition, the Access Node can send a "Hello" broadcast packet with a high power level through the public channel to all nodes in the domestic network; mobile Cognitive Radio nodes have higher power limits than access nodes. Since the Access Node sends messages at a high level, all Cognitive Radio users should be able to receive the "Hello" packet from the Access Node, this includes information related to the GPS of the Access Node, so that all nodes in the network can know the location information of the Access Node. With this location information, every Cognitive Radio user can know the distance between himself and the Access Node, so as to adjust the transmission power properly, enable access nodes and Cognitive Radio users to communicate at an appropriate power level.

Figure 7 Access Node Broadcast

Figure 7 Basic Station nodes send information to all nodes in the primary network through channel B + at a high level of power, because the transmission power level is high, therefore, the local node should be able to receive this message normally.

When the local node receives information from the base station node, it will adjust the transmission power level based on the distance between itself and the base station node, and then feedback information to the base station node through channel B, 8. Node 25 has a low transmission power level because it is close to the base station. node 66 has a high transmission power level because it is far away from the base station; the distance from node 21 is located between the above two nodes, and the transmission power level is between them.

Figure 8 CR node feedback

A node can locate a node other than a few hops in the same network. The information package can be passed down one by one node. In this way, each node can obtain information about the node at the distance of its own two hops. If the receiving node is not within the two hops of the source node, search for the nodes within the three hops, and so on.

3. Conclusion

The core idea of Cognitive Radio is to enable devices to discover "spectrum holes" and make rational use of the "holes" discovered, however, this is based on the spectrum management structure and authorized users to enable the authorized frequency band. This paper introduces the generation and characteristics of Cognitive Radio, and provides a network model for Cognitive Radio and the communication model between users in the network. It can be seen that mobile Ad hoc technology plays an important role in the networking and communication of Cognitive Radio. Since CR technology has its own huge advantages and remarkable characteristics, it has developed rapidly in recent years. However, CR technology is not yet mature, and some of the key technologies need further research.