Cognitive Radio

1.COGNITIVE RADIO Cognitive radio will lead to a revolution in wireless communication with significant impacts on technology as well as regulation of spectrum usage to overcome existing barriers. Cognitive radio, including SDR as enabling technology, is suggested for the first time in to realize a flexible and efficient usage of spectrum. Cognitive radio is an enhancement of SR which again emerged from SDR. Thus, cognitive radio is the consequent step from a flexible physical layer to a flexible system as a whole similar to reconfigurable radio. The term cognitive radio is derived from “cognition”. According to Wikipedia cognition is referred to as 

Mental processes of an individual, with particular relation

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Mental states such as beliefs, desires and intentions

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Information processing involving learning and knowledge

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Description of the emergent development of knowledge and concepts within a group

Resulting from this definition, the cognitive radio is a self-aware communication system that efficiently uses spectrum in an intelligent way. It autonomously coordinates the usage of spectrum in identifying unused radio spectrum on the basis of observing spectrum usage. The classification of spectrum as being unused and the way it is used involves regulation, as this spectrum might be originally assigned to a licensed communication system. To enable transparency to the consumer, cognitive radios provide besides cognition in radio resource management also cognition in services and applications. The mental processes of a cognitive radio based on the cognition circle fare depicted Figure 1 Cognition is illustrated at the example of flexible radio spectrum usage and the consideration of user preferences. In observing the environment, the cognitive radio decides about its action. An initial switching on may lead to an immediate action, while usual operation implies a decision making based on learning from observation history and the consideration of the actual state of the environment. The Federal Communications Commission (FCC) has identified in the following (less revolutionary) features that cognitive radios can incorporate to enable a more efficient and flexible usage of spectrum: o

Frequency Agility – The radio is able to change its operating frequency to optimize its use in adapting to the environment.

o

Dynamic Frequency Selection (DFS) – The radio senses signals from nearby transmitters to choose an optimal operation environment.

o

Adaptive Modulation – The transmission characteristics and waveforms can be reconfigured to exploit all opportunities for the usage of spectrum

o

Transmit Power Control (TPC) – The transmission power is adapted to full power limits when necessary on the one hand and to lower levels on the other hand to allow greater sharing of spectrum.

o

Location Awareness – The radio is able to determine its location and the location of other devices operating in the same spectrum to optimize transmission parameters for increasing spectrum reuse.

o

Negotiated Use – The cognitive radio may have algorithms enabling the sharing of spectrum in terms of prearranged agreements between a licensee and a third party or on an ad-hoc/real-time basis.

decide observe - measurements - preferences

current state

learn

history

reasoning

environment - radio spectrum - user

act - spectrum allocation - message to user

Figure 1: Mental processes of a cognitive radio based on the cognitioncycle

Strictly following this definition modern Wireless Local Area Networks (WLANs) can already be regarded as cognitive radios: IEEE 802.11 devices operate with a listen-before-talk spectrum access, dynamically change the operation frequencies and control their transmission power. In late research, cognitive radios are also referred to as “spectrum agile radios” to indicate an emphasis on dynamic spectrum usage. Mangold et al.,focus thereby on IEEE 802.11k for radio resource measurements as an approach to facilitate the development of spectrum agile radios, while Mangold et al. introduce spectrum agile radios as a society of value oriented machines. Basic concepts are taken there from social science to classify the social action of independent decision-makers. This understanding of cognitive radios is summarized in the following definition of cognitive radio from Haykin: 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 radio frequency 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: (i.) highly reliable communication whenever and wherever needed and (ii.) efficient utilization of the radio spectrum.

1. Cognitive tasks The major tasks of the cognitive radio can be characterized with the cognitive cycle which is discussed next.

1.1 Cognitive cycle In general the cognitive cycle is a continuous process comprising of the following steps s) Sensing, b) Understanding, c) Deciding and d) Adapting, as described in Figure 2-1. Cognitive Radio exploits specifically this cycle in a manner that spectrum is the main figure to be sensed, and all the subsequent process focuses also on how to manage the spectrum based on the observations.

Sense Sense

Adapt Adapt

Decide Decide

Understand Understand

Figure 1-1. Generic cognitive cycle. The major tasks of the cognitive radio include [Haykin 2005]:

1. radio-scene analysis, 2. channel identification, and 3. dynamic spectrum management and transmit-power control. Radio-scene analysis performed in the receiver comprises the estimation of interference temperature of the surrounding radio environment of the receiver, detection of spectrum holes, and predictive modelling of the environment. Channel identification performed in the receiver is needed for coherent detection of the message signal as well as for improving the spectrum utilization. Finally, dynamic spectrum management and transmit-power control performed in the transmitter make decision on the transmission parameters based on the information provided by radio-scene analysis and channel identification. The cognitive cycle comprising the cognitive tasks is presented in Figure. The feedback channel between the receiver and the transmitter is the facilitator for intelligence in the cognitive radio. The feedback channel is needed to transmit the following information [Haykin 2007a]:

• • •

centre frequencies and bandwidths of the spectrum holes, combined variance of interference plus thermal noise in each spectrum hole, estimate of SNR for adaptive transmission.

Figure 2 presents a cognitive radio link where the transmitter and receiver are located in different cognitive radio devices. The cognitive radio devices are transceivers and therefore also the transmitter side includes a unit for radio scene analysis to sense the spectrum in the vicinity of the transmitter. However, this sensing unit belongs to a different link and therefore is not depicted in the figure 2. With cooperative detection, the spectrum sensing information from the receiver and the transmitter can be combined to provide more reliable information about the availability of spectrum holes.

Userm-1 User 2

TransmitTransmitter power control

User 1

Userm Radio environment

Dynamic spectrum management

Estimate of message Channel state signal of user 1 Receiver estimation Radio-scene analysis

Spectrum holes, interference situation,

Feedback

Figure2. Cognitive cycle for cognitive radio link.

1.2 Radio-scene analysis Radio-scene analysis encompasses spectrum awareness that can be classified into passive and active awareness as presented in Figure 3. The classification and the passive awareness presentation are taken from our article about spectrum awareness presented in book about cognitive and cooperative networks [Höyhtyä 2007]. In the passive awareness, the knowledge about electromagnetic environment, i.e. the spectrum use pattern, is received outside own secondary communication system. The knowledge about electromagnetic environment can be received from existing communication system, i.e., primary and secondary users negotiate for spectrum usage [Tonmukayakul 2004]. For example, the base station of the existing (primary) communication system like television broadcasts frequency environment to the CR terminals (SUs). The spectrum use pattern can be obtained also from a server [Raman 2005] or database [Brown 2005]. In addition, in a policy based approach, the primary system use is defined a priori [Mangold 2004]. This could lead to a rather static secondary usage without optimally exploiting spectrum holes (i.e., temporarily unused frequency band of primary user). Another form of awareness is that secondary users actively sense the surrounding radio environment and adapt their transmissions based on the measurements. In non-cooperative situation nodes make their decisions independently based on the observations about the spectrum environment. In cooperative situation local measurements will be combined and signalled to all SU stations without using the frequencies occupied by the primary users (PUs) [Weiss 2004] before decisions about spectrum use are made. When the active method, also referred to as opportunistic spectrum use [DARPA XG] is used, the primary users do not need to know anything about SUs. CR systems may employ either or both forms of awareness, thus the discussed approaches should not be viewed as mutually exclusive. The reliable sensing of spectrum environment is perhaps the most important attribute of CR. It should ensure adaptive transmission in wide bandwidths without causing interference to primary users [Čabrić 2005]. The key challenge of spectrum sensing is the detection of weak signals in a noisy environment with a very small probability of miss detection. The same problem is present in radar systems [Skolnik 2002].

Figure 3. Spectrum sensing classification.

2. Challenges in cognitive radios and networks The cognitive radio has no sense of sight which severely limits the ability to detect the environment. This can lead to the hidden terminal problem where the sensing secondary user is unaware of the presence of a primary user because it cannot reliably detect its presence. A PU terminal and a SU terminal can be separated by some physical obstacle opaque to radio signals. They can also be out-ofrange of each other so that the reliable sensing of primary transmission becomes impossible. Two such terminals are said to be hidden from each other [Tobagi 1975]. One example of hidden terminal problem is a digital TV which lies at the cell edge where the power of received signal can be barely above the sensitivity of the receiver [Krenik 2005]. If the CR is not capable of detecting TV signal, it can start to use the spectrum and interfere with the signal the digital TV is trying to decode. This problem can be avoided if the sensitivity of CR outperforms primary user receiver by a large margin [Čabrić 2004], [Krenik 2005]. The hidden terminal problem is also present in WLAN systems which operate on open bands. In WLANs based on the IEEE 802.11 standard, the problem is tackled by using carrier sense multiple access with collision avoidance (CSMA/CA) scheme as the multiple access method. In CSMA/CA a station wishing to transmit first listens to the channel and only transmits if the channel is sensed “idle”. If the channel is sensed busy before transmission, the transmission is deferred for a random interval, which reduces the probability of collisions on the channel. In a noncooperative game, the hidden terminal problem can cause unpredictable moves and thus lead to a bad situation. Cooperation and distributed methods help to avoid hidden terminal problem and thus reduce interference to the primary system. Access point (AP) is needed in an ad hoc wireless network to realize control-theoretic cooperation between secondary users. Spectrum sensing information of the nodes will be handled and combined in the access point [Weiss 2004]. Based on that information the occupancy vector is defined and distributed to the nodes in the network. Occupancy vector can be a simple binary vector in which 1 refers to the channel in use and 0 for a free channel. In a four-channel system where only the second channel is free, the occupancy vector is 1011. It is not adequate to determine whether a band is free. The cognitive radio must also estimate the amount of interference and noise that would exist in the free subband to make sure that the transmission power of the cognitive radio does not violate the interference limit of the system.

The complexity of the cognitive radio is an important aspect. The benefits from the use of cognitive capability must exceed the cost of introducing the cognitiveness which inherently adds to the complexity of the system. The cognitive radio must be capable of operating over wide bandwidths because the spectrum holes can be spread over large bandwidths. The cognitive radio must be able to sense wide bandwidths as well as transmit on wide range of bandwidth, which places challenges on the antenna design. In particular, the transmission may be spread to several narrow sub-bands and the emission to adjacent bands which are used by the primary users must be avoided. In a cognitive network information is exchanged between the nodes. The amount of control information is an important issue since the transmission of the control information can become the bottleneck if the amount of control information is large. In an ad hoc network of cognitive radios, all control information is sent over the wireless links resulting in significant traffic amounts if not properly planned. The emergent behaviour apparent in the cognitive network due to the adaptations in the time varying operating environment is a key issue. When a set of adaptive equipment are connected, the uncontrolled adaptations can lead to fundamental problems. Emergent behaviour in cognitive radios can be classified into:

1. positive emergent behaviour, which is characterized by order, and 2. negative emergent behaviour, which is characterized by disorder (e.g. traffic jams and chaotic behaviour). It is important to be able to detect negative emergent behaviour which is difficult. In positive emergent behaviour predictability is easier.