Report itu-r m. 2038 Technology trends
Flexible spectrum sharing 4.1 A method for flexible inter-operator spectrum sharing
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- 4.1.1.3 Description of the proposed method
- 4.1.1.3.1 Load control on the shared and proprietary frequency
- 4.1.1.3.1.1 Proprietary frequency carriers are not fully loaded
- 4.1.1.3.1.2 Proprietary frequency carriers are fully loaded
- 4.1.1.3.2 Increased flexibility: dynamic adjustment of the default and target sets
- 4.1.1.3.2.1 Information exchanged
- 4.1.1.3.2.2 Shared spectrum management with the CC
- 4.1.2 Conclusion and perspectives
4 Flexible spectrum sharing4.1 A method for flexible inter-operator spectrum sharingThe radio-frequency spectrum resource is scarce and expensive. Spectrum efficiency of the cellular systems must, therefore, be optimized. This is critical especially in the presence of several competing operators in the same frequency band. Spectrum resources available to each operator should be dynamically adjusted to its needs. Sharing of frequency carriers between different operators is a method to optimize the use of these resources. The spectrum allocation process is nowadays not very flexible. Indeed, operators are allocated a fixed amount of spectrum and they are committed to a priori some objectives in terms of amount of traffic and coverage. If they do not meet these commitments, part of their spectrum can be reallocated to other operators in need. However, this method may take a long time and is not flexible enough. Besides, it is not suitable if the under-utilization of spectrum occurs in peaks. A dynamic way of reallocating spectrum according to traffic needs is necessary. This Report proposes a method to enable sharing of frequency carriers between different operators by dynamically adjusting a set of thresholds without any need of load information exchange between the operators, thereby prioritizing spectrum efficiency.
4.1.1 Description4.1.1.1 PreambleFrequency carriers can be shared between several operators to improve spectrum utilization. For example, in the following, frequency spectrum is used by two operators A and B, FA and FB are “proprietary frequencies”, only network A and B respectively can use them. Fshared are two frequency carriers shared among networks A and B. That implies that mobile users of both networks may be assigned to these shared frequencies, depending on the load conditions. In this case, each network can potentially use up to four frequency carriers. Radio resource can therefore be adapted to the traffic needs, resulting in improved spectrum utilization. 
The objective of this technology trend is to propose a method for the management of these shared frequency carriers, bearing in mind the four principles mentioned above, by defining: – admission rules of mobile users, either by direct access or hand-over; – load control; – priority management between operators for the use of shared carriers.
4.1.1.2 TerminologyFour thresholds are used in order to manage the load or the number of users on the frequency carriers: – Tadmission: new call admission threshold; – THO accept: hand-over requests admission threshold; – THO depart: hand-over departure threshold; – Tdrop: drop threshold, to start disconnecting some mobile users to ensure that certain quality of service is maintained (drop calls anyway since outage conditions). Difference between Tdrop and THO_depart should be defined by outage risks to warrant the best trade-off between outage probability and overall resources utilization. 4.1.1.3 Description of the proposed methodIn case of the shared frequencies, two sets of these four thresholds are defined: – Default threshold set refers to values of thresholds that are used to prevent the use of the shared frequency carriers. – Target threshold set refers to values of thresholds when shared frequency carriers are used. Each operator has the freedom to vary the four thresholds used on the shared frequency, provided that they remain within the default and the target threshold sets. The target and default threshold sets can be pre-defined or changed dynamically during the operations. Basically, each operator uses a target set of thresholds that is different from the other operators. Priority rules are generated automatically by the relative difference of target thresholds between the operators. For example, if operator A uses a Tdrop of 14 dB while operator B uses a Tdrop of 20 dB, operator A begins to drop its mobile users before operator B starts to do so. Dynamic setting of the priority rules is enabled in this way. Moreover, the cost of using these frequency carriers can be easily derived by comparing the respective threshold values used.
There is a set of thresholds for each operator on the shared frequencies, that are dynamically adjusted, depending on the loading conditions, as explained in the two cases below. 4.1.1.3.1.1 Proprietary frequency carriers are not fully loadedThe mobile users on the shared frequency carriers are to be handed-over to the proprietary frequencies. Admission of mobile users on the shared frequency carriers is stopped and the hand-over of the users from the shared to the proprietary carriers has to be favoured. In this case, Tadmission and THO accept decreases, and THO depart as well to make the users leave the shared frequency. The Tdrop can remain at the same level in order to control the outage on the shared frequency. The adjustment of the thresholds should not be blind in order not: – to overload the proprietary frequency carriers; – to overload the system by performing too many inter-frequency handovers. The final result is that all users will leave the shared frequency, either by hand-over and if not, at the end of their call. 4.1.1.3.1.2 Proprietary frequency carriers are fully loadedIn this case the objective is to increase the load on the shared frequencies. New call requests are placed on the shared frequency carriers. Here, Tadmission, as well as THO depart and THO_accept are increased. Tadmission_proprietary and THoaccept_proprietary are the thresholds used on the proprietary frequency.
After analysing the access request, based on the current utilization of the shared spectrum and other concurrent access requests, the CC will send only an ACCESS INCREASE message if some bandwidth can be offered to the operator B on the shared spectrum. Note that the message will contain the set of target thresholds to be applied by the operator. This is enough to inform the operator B it can access the shared spectrum. 4.1.1.3.2.2 Shared spectrum management with the CCFigure 11 displays the call admission procedure for this method. The example is given for HO calls but is equally applied to new calls. 
Shared spectrum is accessed only when the proprietary spectrum is saturated. Before sending a SHARED SPECTRUM ACCESS REQUEST message to the CC, the overflow situation is first determined by the local controller. This is done by counting the number of unsatisfied requests over the observation time. During this observation time the access requests may be queued if such facilities exist and services might tolerate some waiting delay. Otherwise they are simply dropped. After receiving the SHARED SPECTRUM ACCESS REQUEST, calls which have been buffered waiting for resources are accepted on the shared spectrum if enough bandwidth available. Otherwise, calls which have been eventually queued have to be dropped. This can be done either by sending a specific message to the local controller or simpler by timeout operations.
Priority rules and distribution of the cost between the operators are automatically derived from the specific values given to the default and target sets. More flexibility in the adjustment of resource available to each operator can be provided by the notification of the availability of a frequency for sharing and the dynamic adjustment of the thresholds sets. As mentioned earlier, four principles have to be respected to enable sharing of frequency between operators: – This method consists of an algorithm that enables dynamic adjustment of resources available to each operator. – By using the algorithm, these shared frequency carriers are to be used only when all proprietary frequency carriers are fully loaded. Regulatory controls and agreements can further enforce this. – Priority between operators sharing these frequency carriers is determined dynamically by the values of the thresholds, enabling optimal load management policies and control of interference. The cost of the use of the shared frequency can be shared between the operators, the proportion for each operator being derived from the values of target threshold set. – Exchange of load or confidential or sensitive information concerning each operator is not needed. In short, the proposed approach makes the sharing of frequency carriers feasible by dynamically controlling a set of adjustable thresholds for call admission and termination, resulting in better utilization of the scarce spectrum.
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