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The current wireless networks are characterized by a fixed spectrum assignment policy. The license users do not utilize the spectrum all the time. In this way large amount of spectrum is not used efficiently and goes unused. The spectrum is a very limited and the inefficiency in the spectrum usage gives birth to a new communication paradigm i.e. to access the existing wireless spectrum opportunistically. This new paradigm is referred to as Next Generation Networks, Cognitive Radio (CR) Networks as well as Dynamic Spectrum Access (DSA). The novel functionalities and current research challenges of CRN are explained and open research issues are discussed.
The common approach to spectrum allocation is based on statically allocating long-term licenses on portions of the spectrum to providers and their users by government agencies. In , according to the FCC temporal and geographical variation in utilization of assigned spectrum ranges from 15 to 85 percent. This type of spectrum allocation leads to underutilization of spectrum as the number of users to access the spectrum has increased dramatically over the years. To address this problem FCC has approved the usage of unlicensed devices in licensed band. Therefore, the option of reusing assigned spectrum when it is temporarily (and locally) available is frequently referred to as Dynamic Spectrum Access (DSA) which tries to increase the efficiency of spectrum usage. The key enabling technology of DSA networks is cognitive radio (CR) technology, which provides the capability to share the wireless channel with licensed users in an opportunistic manner. A ''Cognitive Radio'' is a radio that can change its transmission parameters based on interaction with the environment in which it operates . The concept of DSA is to identify spectrum holes or white spaces and use them to communicate as shown in Fig 1. White spaces change over time, a cognitive radio is used to "jump" from one chunk of spectrum to another. The novel characteristic of cognitive radio transceiver is a sensing capability of the RF front-end in a large range of frequencies.
There are unique challenges imposed in CRN due to the high fluctuation in the available spectrum and different quality of service (QoS) requirements of applications. In order to address these challenges, each CR user in the CR network must:
â€¢ Determine the portions of spectrum available
â€¢ Select the best available channel
â€¢ Coordinate access to this channel with other users
â€¢ Vacate the channel when a licensed user is detected
These capabilities can be realized through spectrum management functions that address four main challenges: spectrum sensing, spectrum decision, spectrum sharing, and spectrum mobility. The CR network communication components and their interactions are shown in Fig. 2. It is evident from the significant number of interactions that the CR network functionalities necessitate a cross-layer design approach. It is quite evident that spectrum sensing and spectrum sharing cooperate with each other for better spectrum efficiency. Keeping in view the dynamic nature of the underlying spectrum, application, transport, routing, medium access and physical layer functionalities are carried out in a cooperative way in spectrum management and spectrum mobility functions.
This paper presents the definition, architecture and functions of the DSA. In sec 2 we will present the architecture and functions of the DSA Networks. Spectrum Sensing, spectrum management, spectrum mobility and spectrum sharing and their challenges are discussed in section 3. 4, 5 & 6 respectively. Finally, we explain the upper layer issues in Section 7 and conclude the paper in Section 8.
DSA network architecture:
The components of DSA network architecture are shown in the figure 3 and all possible scenarios are considered.
Primary Network: A network which has an exclusive right to a certain spectrum band or licensed network. The components of the primary network are:
Primary user (or licensed user)
Primary base-station (licensed base-station)
Next Generation Network / DSA Network: They do not have license to operate in a desired band. However, spectrum is opportunistically accessed. They can be deployed both as an infrastructure network and an adhoc network as shown. The components of a DSA network are as follows:
DSA Network user (unlicensed user, secondary user)
DSA Network Base Station.
Spectrum Broker is a central network entity that plays a role in sharing the spectrum resources among different DSA networks
Fig 3 
2.1) DSA network Functions:
The DSA Network has the capability to operate in both licensed band as well as in unlicensed band.
Licensed Band: In a licensed band the DSA user will first sense a spectrum hole with the help of cognitive communication techniques in the licensed band. Although the main purpose of DSA network is to determine the best available spectrum, DSA functions in the licensed band are mainly aimed at the detection of the presence of primary users. Different techniques have been proposed which are discussed in detail in the latter section for spectrum sensing. The most important issue is to avoid interference with the primary user in this band. As soon a primary user is detected the CR user should vacate the band and find a new band for communication, this is called spectrum handoff.
Unlicensed Band: Since there are no license holders, all network entities have the same right to access the spectrum bands. Multiple networks can exist in the same area and communicate using the same portion of the spectrum without any handoff.
The main purpose of spectrum sensing is to find spectrum holes to provide more spectrum access opportunities to DSA network users without interfering with the primary network user. The current technology of radio frequency (RF) front-ends is unable to sense and transmit at the same time, which decreases the opportunities to transmit leading to sensing efficiency problem .
One of the most efficient ways of spectrum holes detection is to detect the primary users which are receiving data within the communication range of a DSA user . Practically, however, it is challenging for a cognitive radio to have a direct measurement of a channel between a primary receiver and a transmitter. Since cognitive radio (CR) networks are responsible for detecting the transmission of primary networks and avoiding interference to them, DSA networks should intelligently sense the primary band to avoid missing the transmission of primary users. Thus, sensing accuracy has been considered as the most important factor to determine the performance of CR networks. Hence, recent research has been focused on improving the sensing accuracy for interference avoidance .
For the detection of spectrum holes different techniques have been developed. We will go through these techniques in the following subsection Detection and then in the next section we will discuss the open research issues in this area.