Cognitive radio means an intelligent and adaptive radio that knows its environment. Cognitive radio technology is a solution used to enhance utilization of spectrum enabling the users that are not licensed to exploit in an opportunistic way. Unlicensed users are considered as temporary on the spectrum that is licensed, and once a licensed user reclaims it they are expected to vacate. As a result of licensed users reclaiming the spectrum, there are disruptions in both unlicensed and licensed communications that are always difficult to prevent. To address these problems, a spectrum handoff structure is proposed for cognitive technology to help the unlicensed users to leave the spectrum before a licensed user reclaims it to avoid disruptions. In this technology, users that are not licensed coordinate with one another without using a channel that is common. Spectrum mobility is the disparity in the spectrum band.
The existing open spectrum holes are because of the unpredictability of licensed user mobility. Spectrum mobility results to spectrum handoff. This is where a cognitive user’s operation frequency is changed. Depending on licensed users, each hole can send and receive data in cognitive radio. However, due to the appearance of primary users randomly, it becomes hard to have a smooth spectrum resulting in minimum distraction to licensed users. Spectrum handoff falls into two; the reactive approach and the proactive approach (Kaur, Udin & Khosla, 2009). The reactive approach is where secondary users switch the spectrum after realizing a primary user. Although this concept is intuitive, there is reconfiguration hold-up that results to interferences to both the licensed and unlicensed users transmissions. The proactive approach is where the secondary users suspect the future path availability position and switch the spectrum and reconfigure the radio frequency before a licensed user occupies it. This dramatically minimizes the collisions between licensed users and unlicensed users by allowing the unlicensed users to leave the channel before any licensed user reclaims it. The appearance of the primary user on the spectrum band occupied by the unlicensed user triggers the user to leave the band. Therefore, the secondary user tries to reclaim the medium by either remaining in the original channel until the licensed user is through. Then select another channel from the previously sensed to replace his stolen channel or switch to a different channel after realising a spectrum handoff. However, a secondary user may be forced to end its session if it does not succeed in reacquiring the band. Below is an image of a dynamic spectrum which enables cognitive users to share the band.
For a reliable communication, there is a need for spectrum management. The coordination of network and channel related data exchange between secondary users is done through a control channel that is common. To avoid many secondary users selecting a similar channel at the same time, a good channel technique is required to ensure fairness. However, if spectrum handoff is done at the same time by secondary users, it may result in collisions among them. When there are more than one channel for secondary users access, a multichannel spectrum is used for sensing. Such a spectrum is divided into narrowband and wideband schemes. In narrowband sensing method, it's only one channel that’s being sensed at a time resulting in the easier operation and less power is consumed. Secondary users in narrowband method try to find a channel that is free by sensing another channel that is new. For wideband spectrum, several channels are sensed at the same time. This allows a short time in sensing, and it needs some complex technology to implement, and it also consumes much power. However in multiple channels handoff there is a probability of delays. To deal with this challenge, a prediction model can be created to predict the previous channel patterns of the cognitive users. This model estimates when the primary user leaves the spectrum and when it is reclaimed. The figure below shows spectrum handoff.
Some of the challenges facing spectrum mobility management include; a broad range of the available spectrum, heterogeneous events, and dynamic spectrum mobility. Broad range spectrum is where delay occurs when cognitive users reconfigure the frequency operating so as to move on a new band. Heterogeneous events are when cognitive users are expected to give different handoff types. These include; classical inter-cell and spectrum handoff. Classical inter-cell is a handoff for the physical users while spectrum handoff is spectrum mobility. Due to these two handoffs it is necessary to have a comprehensive mobility management scheme. Availability of dynamic spectrum in cognitive radio varies making it hard to give seamless and dependable communications. To hinder inter-cell disruption, a coordination method is adopted where cells use different spectrum. However, the available spectrum bands are spread over a big frequency range.
Moreover, a dynamic admittance control method is created to choose the spectrum bands dependent on the time difference cognitive radio network. Since the spectrum bands that are free varies with time, a vibrant inter-cell spectrum sharing is required. This allows the allocation of resources to be fair. This dynamic spectrum sharing solves the following.Firstly; it negotiates the additional spectrum according to the licensed user. Secondly, it accounts for the maximum cell level and reduces the interference caused by the cells that are near. Below is an inter-cell spectrum sharing structure.
In order to efficiently share spectrum resources, a unified scheme to support cooperation between inter and intra-cell spectrum schemes is required. This framework consists of event monitoring, inter-cell, and intra-cell spectrum sharing. First, event monitoring has two purposes. One is to detect the activities of the primary users. According this event type, the spectrum sharing strategies is determined by the base station (Mavromoustakis, Pallis & Mastorakis, 2014). Cell spectrum sharing enables the base station to evade the disruption to the primary networks by allocating the resources well. If a new user comes in this cell, its acceptance is determined by the base station and the best vacant spectrum is selected. Lastly, inter-cell spectrum sharing maximizes the network levels. It has two functions that include power distribution and spectrum distribution. The power allocation allows the base station to establish the transmission power so as to higher the chances of the cell capacity without disruption to the primary network. In the spectrum allocation, the spectrum bands are determined by bearing in mind the geographical data of primary networks. A block diagram of the cognitive radio network is as shown below.
In conclusion, due to spectrum changes of cognitive radio, a practical scheme for coordinating is considered instead of using a control channel that is common. Two types of channels are incorporated into the spectrum handoff structure to compare the performances. According to the statistics of the channel usage observed, a proactive spectrum is the best handoff criteria for secondary users. Secondary users that can have the capability to predict can foretell the idleness of the spectrum band in future. Thus, the distraction between the licensed users and the unlicensed users will be reduced. This would, therefore, result in the increase of the throughput of secondary users. The apparatus eliminates collisions between the licensed and unlicensed users, a scheme for multiple users’ spectrum should be created for distributing the channel selection. This will, therefore, reduce delays. A spectrum handoff is determined by using two criteria, the length of the channel free period and the probability predicted of a candidate channel is free. This reduces collisions between secondary users and primary users.
Christian, I., Moh, S., Chung, I., & Lee, J. (2012). Spectrum mobility in cognitive radio networks. IEEE Communications Magazine. doi:10.1109/MCOM.2012.6211495
Kaur, P., Udin, M., & Khosla, A. (2009). An efficient spectrum mobility management strategy in cognitive radio networks. doi:10.1109/UKIWCWS.2009.5749388
Mavromoustakis, C. X., Pallis, E., & Mastorakis, G. (2014). Resource Management in Mobile Computing Environments. Dordrecht: Springer