Spectrum Scarcity Issue Cognitive Radio Computer Science Essay

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Radio spectrum is a valuable resources shared by different wireless service providers. It can be repeatedly use by frequency reuse techniques in global system of mobile communication etc. Radio Spectrum management and utilization are the key research paradigms on which the most researcher are working on. The figure 2.1 has been used over 100 years for spectrum management.

The technical and policy aspects of spectrum management are extensively studied. The technical aspect of spectrum management is concern with technology and physical world phenomena that affects the spectrum utilization and policy aspect takes in to account of the economical and political factors. The International Telecommunication Union radio communication Sector has to manage radio frequency allocation for different emerging technologies and devise new standards for it. The spectrum is divided into several segments for specific wireless services. The licensed user has full right to divide the allocated spectrum into fixed number of frequency channels within a specific geographical area.

Figure 2.1: Schematic representation of division of the radio spectrum and radio ranges.

FCC is an independent organization of United State of America established by communications Act of 1934. Its main task is to regulate interstate and international communications by radio, television, wired, satellite and cable [2]. In 2002 FCC published spectrum policy report that aims to adopt new technologies and create new technological resources. Radio spectrum is a limited resource by nature. The report concluded that Cognitive radio can enable better spectrum usage and improved network efficiency [28]. Thus cognitive radio can increase secondary market and effective spectrum utilization. FCC then published a report called Notice of proposed rule making (NPRM) using cognitive radio Technology [4]. Similar model is proposed by Ofcom in United Kingdom which defined the "command and control strategy" for spectrum management.

According to aforementioned report the "command and control" model will be reduced from 95.7% to 21.6%, and secondary market model will be extended form 5% to 71%, and finally spectrum commons model will increase from 4.9% to 6.9%. [7].

The aim of "command and control" model is to make license inflexible, promote spectrum liberalization, to remove spectrum restriction to introduce new services and methods. Graphical representations of these models are shown in figure 2.2 , 2.3 and 2.4.

Command and control model, see figure 2.2.

Secondary Market model, see figure 2.3.

Spectrum commons model, see figure 2.4.

Figure 2.2: Command and control model indicates gradual decrease in spectrum limitation. Spectrum linearization is the key issue of this model.

Figure 2.3: Gradual increase of secondary user spectrum utilization.

Figure 2.4: Improvement in spectrum commons model.

Figure 2.5: Radio spectrum measurements [3].

Figure 2.5 shows, that most of the licensed spectrum is unused and free. This reflects that license owner is not using the spectrum all the time and there are white holes/ white spaces/spectrum opportunity. The given spectrum is severely underutilized in the middle of radio spectrum. To solve the spectrum scarcity, to increase spectrum utilization and to fulfill the white spaces cognitive radio technology is used to achieve this goal. Similar study is conducted at Berkley wireless research center (BWRC) of Berkley University [29].

Figure 2.6: Power spectral density (dB) vs Frequency Measurement from 0 to 6 GHz spectrum utilization at Berkley wireless research center (BWRC) [29].

The measurement consist of a power spectral density of a radio signal between 0 and 6 GHz and samples were collected at 20 GHz for the duration of 50 seconds as shown in Figure 2.6. The radio spectrum is highly underutilized form 3 GHz to 6GHz resulting in wastage of the radio spectrum. Such problem creates spectrum scarcity for modern wireless communication system. The experimental measurement conducted in the north sub urban of Brno (Czech Republic) in, July 2008 reveal spectrum under utilization in different period of time[3]. The upper plot presents the maximum power from frequency 0 to 3 GHz. The average power of the upper plot is given in the lower plot of figure 2.7.

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Figure 2.7: Maximum overall spectrum usage plot over 6 days in the city of suburban of Brno, Czech Republic [4].

The Dynamic Spectrum Access is emerging technology to cope with spectrum scarcity. Cognitive radio employing dynamic spectrum access technology has to operate with minimum interference among primary and secondary users.

2.2 The Cognition Cycle

The cognitive radio cycle first proposed by J.Mitola[51] and described by different authors in various paradigm covering multi features of cognitive radio. The CR is combination of intelligent signal processing and RF flexibility and as a result the cognitive radio cycle was implanted in radio equipments.

According to Simon Haykin [1] cognitive radio can be defined as

" Cognitive radio is an intelligent wireless communication system that is aware of its surrounding environment (i.e., outside world), and uses the methodology of understanding-by-building to learn from the environment and adapt its internal states to statistical variations in the incoming RF stimuli by making corresponding changes in certain operating parameters (e.g,transmit-power, carrier-frequency, and modulation strategy) in real-time, with two primary objectives in mind:

• Highly reliable communications whenever and wherever needed;

• Efficient utilization of the radio spectrum."

The different actions taken by CR in cognition cycle are given in figure 2.8 [1]. To define it, a state machine embedded in cognitive radio to learn, adjusts and reacts to surrounding radio environment changes.

Figure 2.8: Cognitive Radio Cycle

The radio will collect information about present operating environment as illustrated by outside world through direct observation. The importance of collected information is determined at Orient. According to that the cognitive radio will use either Plan or Decide alternative for operation. Wireless communication channels are considered to be unstable therefore cognitive user will adjust its resources through Act alternative. This process is repeated to improve its operations Learn and in creating new model states, seeking new alternatives.

2.3 Network Architecture for Cognitive Radio Network

According to [16] the architecture of a cognitive radio and its functional framework can be presented.

Network components: The basic cognitive radio network model is shown in Figure 2.9. The cognitive radio can be classified into two types; the primary network (licensed network) is the commercial network in the operation. There are two kinds of users, licensed (Primary User) and Unlicensed (Secondary user) in cognitive radio network.

1. Licensed user (Primary User)

Licensed user is the owner of a radio spectrum. A customer pay monthly/yearly basis to government authorities. Licensed user is also called Primary user has the right and priority to use the spectrum anytime, anywhere in the world. The primary base station controls various operating parameters for PU. The operation of PU has not to be affected by unlicensed users.

2. Unlicensed user (Secondary User)

Unlicensed user or Secondary user has no authority to use any radio spectrum without permission. Cognitive radio provides an opportunity for Secondary user to use the spectrum, whenever Primary user is not using the radio spectrum.

.

Figure 2.9: Cognitive Radio Network Model

Licensed Users: Primary Users

Unlicensed Users: Secondary Users

Cognitive radio using advance wideband access technology can access both the licensed and unlicensed bands of communication channels. The Primary network uses licensed spectrum for operation. The SU has to monitor the presence of PU in the network. In absence of PU the spectrum band can be used by SU. Hence a complicated frame work of spectrum management is developed for SU to utilize the vacant band of the PU.

Cognitive radio designed to provide better QoS for secondary user applications. Spectrum management policies are devised to meet the QoS requirements of Secondary Users. Spectrum management techniques are given below

Spectrum Sensing (Monitoring the presence of PU)

Spectrum decision (Allocation of vacant channel)

Spectrum Sharing (Avoiding Multiple User Colliding)

Spectrum Handoff/Mobility (To move SU due to presence of PU to other vacant channel)

2.4 Standard and Regulation

Cognitive radio standards are under careful investigation. The IEEE standards coordinating committee (SCCI 41) on dynamic spectrum accessing work is related to cognitive radio network [5]. The responsibilities of IEEE standard coordinating committee 41 (Dynamic Spectrum Access Networks) consist of maintaining the standards developed by the committee in accordance with IEEE-SA standard board operating Manual. Further, to cooperate with other standard developing organizations.

For next generation radio network and spectrum management IEEE initiated 1900 Standards committee. The IEEE P.1900 committee was formed in 2005 to formulate standards for new emerging radio networks.

2.4.1. IEEE standardization on cognitive radio

IEEE standardization related to cognitive radio include IEEE 802.22 Working group for the development of physical (PHY) and Medium Access Control (MAC) for Wireless Regional Area Network (WRAN) operating in TV broadcast band .WRAN's operate in unused channels in the VHF/UHF TV bands between 54 and 862 MHz[7].

The motivation of developing IEEE 802.22 standard was to utilize the unused Digital T.V spectrum to provide broadband services in rural areas. The accessibility to broadband services is not that critical in suburban areas as compared to rural area where establishment of DSL network is expensive for few customers. The operating TV bands chosen to provide broadband service have high-quality propagation characteristics covering larger area. In U.S.A many T.V channels highly unoccupied [8]. The 802.22 network will provide high quality voice and data services with suitable QoS.

The cognitive radio networks for the first time deployed in U.S.A for commercial operation. The IEEE 802.22 standard is devising rules and regulation which can accommodate system specification consisting of frequency range of operation from 54 -862 MHz and suggestions are circulated to extend to 900 MHz.

IEEE 802.22 System: consist of a fixed point-to-multipoint wireless access system with base station managing the coverage for area called cell as shown in Figure 2.13. The base station (complex electronic equipment) controls the traffic on upstream and downstream channels. Furthermore, a unique feature is embedded in base station called distributed sensing. In it combine effort is made for to detect vacant T.V band [11].

Figure 2.10: Cognitive Radio Network Model [11].

The IEEE system Specification

Coverage: 32 Km to 100 Km (considering power issue) with

Spectral efficiency: 0.5 bit/ (sec/Hz) to 5 bit/ (sec/Hz).

TV channel: 6 MHz

Data rate will be 18 Mbps for a secondary user.

The 802.22 system operate in dynamic spectrum sharing environment taking account of other licensed users usage of the spectrum. Multi-carrier modulation is employed in IEEE system 802.22 to support flexibility in the traffic for a user on upstream and downstream.

2.4.2. ITU-R standardization on cognitive radios

ITU-R is conducting standardization activities related to cognitive radio networks. The standardization activities are carried out in the Study Group 1 (SG 1) which is responsible for spectrum management [6]. The IEEE is important sector member and contributes to ITU-R standardization activities.

The ITU Radio communication Assembly in 2010 sent Question ITU-R 241-1/5 on cognitive radio system in the mobile service. ITU-R 241/8 lists the following issues to be studied in ITU-R WP 5A [6].

What is the ITU definition of cognitive radio systems?

What are the closely related radio technologies (e.g. smart radio, reconfigurable radio, policy-defined adaptive radio and their associated control mechanisms) and their functionalities that may be a part of cognitive radio systems?

What key technical characteristics, requirements, performance and benefits are associated with the implementation of cognitive radio systems?

What are the potential applications of cognitive radio systems and their impact on spectrum management?

What are the operational implications (including privacy and authentication) of cognitive radio systems?

What are the cognitive capabilities that could facilitate coexistence with existing systems in the mobile service and in other radio communication services, such as broadcast, mobile satellite or fixed?

What spectrum-sharing techniques can be used to implement cognitive radio systems to ensure coexistence with other users?

How can cognitive radio systems promote the efficient use of radio resources?

2.5 Industrial Applications

There is rapid transition from analog to digital technology in TV broadcasting. Many countries around globe currently broadcast both analog TV and digital TV (DTV) simultaneously. Most of analog services will be switched-off at different times in countries.

There will be vacant channel or white spaces in the TV bands. The FCC has proposed in U.S to allow unlicensed working in the white spaces in the TV spectrum: 76-88 MHz, 174-216 MHz, and 470-608 MHz's. The FCC has discussed three important methods for operation of TV bands: control signals, position determination and cognitive radio with dynamic frequency selection.

The main issue in the implementation of cognitive radios on TV bands is to reduce interference between licensed and unlicensed user. In [8] provides explanation for application of cognitive radios on vacant TV bands and through simulation it is verified that an occupied DTV signal can be identified with certainty in fading environment. The commercial operation of a cognitive radio network is yet to be seen. Some cognitive radio product is available which are discussed below.

2.5.1 Adapt4's XG1

Adapt4 developed first commercial CR product available in the market with XG1 label in 2004 [8]. XG1 is an OFDM-based system operating in licensed frequency band in 217-220 MHz as secondary user. The throughput rate of Adapt's XG1 is 192 Kbps. The CR network deployed with XG1 supports point-to-point and point-to-multipoint with multi-hopping architectures. The network identifies vacant channels and uses them for transmission and avoid channel when other activity is detected.

Adapt4's XG1 consist of following capabilities [Adapt4]:

Frequency hopping,

Dynamic Power Management,

Automatic Configuration

Dynamic Frequency Selection

Fig 2.11: XG1â„¢ Cognitive Radio

The XG1 cognitive radio network constantly observes the activity of the license user in the band and identifies unused channel. In CR OFDM systems a set of sub-carriers of 45 is created, each of width 6.25 KHz and rapidly hops among them using each channel for 10ms. When another licensed user is sensed, the network stops using the carries until they become vacant again.

The XG1 equipments detect the licensed users and forward this information to a central station. The central station processes this information and create usage map covering entire XG1 network with coverage up to 80 km. The Interference avoidance is achieved with help of two features consisting of frequency-hopping and dynamic power control for a device. The Frequency-hopping minimize the amount of time that any single frequency is used. The radio uses minimum amount of transmission power to maintain reliable communication link with central station.

2.5.2 Cognichip

France Telecom developed simple low cost radio device called Cognichip. It has to be operated in the ultra high frequency (UHF) spectrum of 470-870 MHz described in [9]. The Cognichip can detect spectrum holes in the TV spectrum and can support centralized architecture. The network consists of base station and User devices. The user device with transceiver chip measures of radio signal strength indicator (RSSI) for spectrum occupancy. The sensing time for one channel is 24 ms. The user device cannot receive data during sensing the spectrum.

The Cognichip support a centralized architecture such that base station collects the RSSI information from the user device and updates the channel occupation information in real time and makes decisions on the spectrum usage. The dynamic frequency selection in reception mode takes 23.2 ms and transmission mode 23.6ms.

2.5.3. Rockwell chip

The Rockwell Collins has developed a broadband spectrum sensor for frequency band 30 MHz -2.5 GHz [10]. It is power efficient with power consumption is below 2.5 W and scan the spectrum at 18 GHz.

2.6 xMax Commercial Cognitive radio Network

The xG Technology developed first industrial commercial xMax cognitive radio cellular network and presently it is deploying in Florida for U.S Army training center. xMax will provide mobile communication service with larger coverage area. The network deployed will consist of base station of BSN-250, a central switching center xMSC and special handset of TX70 handsets. A reference model of xMax cognitive radio network supporting data and voice service is given in figure 2.9 [21].

Figure 2.9: Network Architecture of xMax cognitive radio network.

Features of xMax Network

Base station (BSN) is 18 channel mobile VOIP transiver device .It channelize the spectrum from 902-928 MHz into 18 discrete channels, and is only in used when there is traffic for a mobile equipment registered with a particular channel.

Access Network Gateway (ANG), called the xMSC is the central switching center that provides service of call processing, IP packet delivery services, mobility and other signaling related functions.

The xMax system provide soft handoffs features with capability of (make-before-break) time slots are acquired before breaking a connection resulting in reliable roaming and a seamless user experience.

Ethernet switch aggregate BSN links in use.

Firewall provides NAT (network address translation) services.

SIP proxy server provides SIP call control, xG's SIP message compression technology.

Network Monitor handle end to end network management and monitoring services.

Proxy DHCP server is used for IP addresses services.

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