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In the previous sections, the theoretical findings and solutions for spectrum sharing in xG networks are investigated. Although there already exists a vast amount of research in spectrum sharing, there are still many open research issues for the realization of efficient and seamless open spectrum operation. In the following, we detail the challenges for spectrum sharing in xG networks along with some possible solutions.

Common control channel (CCC)

Many spectrum sharing solutions, either centralized or distributed, assume a CCC for spectrum sharing . It is clear that a CCC facilitates many spectrum sharing functionalities such as transmitter receiver handshake , communication with a central entity , or sensing information exchange. However, due to the fact that xG network users are regarded as visitors to the spectrum they allocate, when a primary user chooses a channel, this channel has to be vacated without interfering. This is also true for the CCC. As a result, implementation of a fixed CCC is infeasible in xG networks. Moreover, in a network with primary users, a channel common for all users is shown to be highly dependent on the topology, hence, varies over time. Consequently, for protocols requiring a CCC, either a CCC mitigation technique needs to be devised or local CCCs need to be exploited for clusters of nodes . On the other hand when CCC is not used, the transmitter receiver handshake becomes a challenge. For this challenge, receiver driven techniques proposed in may be exploited.

Dynamic radio range

Radio range changes with operating frequency due to attenuation variation. In many solutions, a fixed range is assumed to be independent of the operating spectrum . However, in xG networks, where a large portion of the wireless spectrum is considered, the neighbors of a node may change as the operating frequency changes. This effects the interference profile as well as routing decisions. Moreover, due to this property, the choice of a control channel needs to be carefully decided. It would be much efficient to select control channels in the lower portions of the spectrum where the transmission range will be higher and to select data channels in the higher portions of the spectrum where a localized operation can be utilized with minimized interference. So far, there exists no work addressing this important challenge in xG networks and we advocate operation frequency aware spectrum sharing techniques due the direct interdependency between interference and radio range.

Spectrum unit

Almost all spectrum sharing techniques discussed in the previous sections consider a channel as the basic spectrum unit for operation. Many algorithms and methods have been proposed to select the suitable channel for efficient operation in xG networks. However, in some work, the channel is vaguely defined as ‘‘orthogonal non-interfering’’ ,‘‘TDMA, FDMA, CDMA, or a combination of them’’ , or ‘‘a physical channel as in IEEE 802.11, or a logical channel associated with a spectrum region or a radio technology’’ . In other work, the channel is simply defined in the frequency dimension as frequency bands . It is clear that the definition of a channel as a spectrum unit for spectrum sharing is crucial in further developing algorithms. Since a huge portion of the spectrum is of interest, it is clear that properties of a channel may not be constant due to the effects of operating frequency. On the contrary, a channel is usually assumed to provide the same bandwidth as other channels . Furthermore, the existence of primary users and the heterogeneity of the networks that are available introduce additional challenges to the choice of a spectrum unit/channel. Hence, different resource allocation units such as CSMA random access, TDMA time slots, CDMA codes, as well as hybrid types can be allocated to the primary users. In order to provide seamless operation, these properties need to be considered in the choice of a spectrum unit. In , the necessity of a spectrum space for a spectrum unit is also advocated. The possible dimensions of the spectrum space are classified as power, frequency, time, space, and signal. Although not orthogonal, these dimensions can be used to distinguish signals . For this purpose, we describe a three dimensional space model for modeling network resources that has been proposed in . Although this work focuses on heterogeneity in next generation/fourth generation (NG/4G) networks, as discussed in , it can be easily incorporated into xG networks. Based on this three dimensional resource-space, a novel Virtual Cube concept has been proposed in order to evaluate the performance of each network. The Virtual Cube concept defines a unit structure based on the resource allocation techniques used in existing networks. The resource is modeled in a three dimensional resource-space with time, rate, and power/code dimensions as shown in Fig. 14. The time dimension models the time required to transfer information. The rate dimension models the data rate of the network. Thus, the capacity of different networks with the same connection durations but different data rates are captured in the rate dimension. Furthermore, in the case of CDMA networks, the bandwidth increase due to the multi-code transmissions is also captured in this dimension. The power/code dimension models the energy consumed for transmitting information through the network. Note that, the resource in terms of available bandwidth can be modeled using the time and rate dimensions. However, the cost of accessing different networksvary in terms of the power consumed by the wireless terminal. Hence, a third dimension is required. Each network type requires different power levels for transmission of the MAC frames because of various modulation schemes, error coding and channel coding techniques. As a result, the resource differences in these aspects are captured in the power dimension. Using this model, heterogeneous access types in existing networks as well as xG network spectrum can be modeled based on a generic spectrum unit. We advocate that determining a common spectrum unit is crucial for efficient utilization of the wireless spectrum and seamless operability with existing primary networks.