Ex-Post Enforcement in Cooperative Spectrum Sharing: A Case Study of the 1695-1710 MHz Band

The limited available spectrum and the inefficiency of spectrum usage has led to the new approach to communication policy of spectrum sharing. The National Telecommunication and Information Administration (NTIA) offered several bands for sharing between government agencies and commercial wireless service providers. Spectrum sharing can be thought of as a reorganization of rights between stakeholders. The principal stakeholders are the Primary User (PU), who holds the spectrum license, and the Secondary User (s) who is using the spectrum temporarily. Any rights management system requires a set of strategies and technologies to enforce the rights and the timing of the enforcement action (ex-ante and ex-post) plays a significant role. The spectrum sharing approaches that have been proposed by the NTIA emphasize ex-ante actions, which are designed to prevent a PU’s signal from harmful interference that could occur by the SU. A comprehensive enforcement framework would include protecting the rights of the SU as well, in addition to having an ex post component that can efficiently and effectively adjudicate claims of interference. In we evaluated the role of ex-post enforcement by modeling how an ex-post only enforcement scheme might work. A model of a geographic region with geographically distributed SUs and a single PU was simulated to determine whether (and when) the ex post approach is superior to an ex-ante approach. Aggregate signal power of the SUs were computed at the PU’s antenna to calculate the interference level. We hypothesized an adjudication system that penalizes the SU for when the interference level reaches certain limit received at the PU’s antenna. The penalty was proportional to the lost value of the lost data by the PU plus the enforcement costs caused by it. By following the approach that had been suggested in the benefits for the PU are: (1) PU will get the value of the lost data and can recover it, (2) PU will gain income from sharing the spectrum within the excluded areas. On the other hand, the disadvantages that the PU may find are: (1) if the lost data cannot be recovered, (2) and if the SUs keep transmitting near the PU for long time that will lead to denial of service (DOS) for the PU. A purely remunerative penalty function may not be the best strategy; it might not be large enough to stop the SU from interfering with the PU. In this paper, we construct a broader framework for evaluating the role of the ex-post enforcement by modeling how an ex-post only enforcement scheme might work, and what the limits are on its effectiveness. This work will extend the work in by exploring the other enforcement sanctions in the cooperative spectrum sharing regime when SUs conduct harmful interference. There are a number of aspects to consider, including what the sanctions are and how they should be applied to enforce the PU’s rights. This paper will study an ex-post-only enforcement to examine the role of ex-post enforcement in a cooperative spectrum sharing regime. A hypothetical scenario of using the recommended protection zones and the involved entities will be used to analyze the current enforcement timing measures and to evaluate the usage of ex-post-only enforcement measures. This hypothetical scenario about the behavior of the SUs is significant / of concern if SU-mobile devices transmit near PU-base station (earth station) or if they are transmitting high power signals within the protection zone. These behaviors will cause harmful interference to the PU signal and data received by the PU will be lost. The suggested ex-ante and ex-post enforcement measures will be explained and analyzed using this hypothetical scenario to see if SU guaranteed or not. Then, ex-post-only enforcement measures will be applied to this scenario and will be analyzed. The analysis of the ex-post-only enforcement measures is to evaluate how these measures might work, and what the limits are on their effectiveness. To evaluate ex-post enforcement, we hypothesize an adjudication system by an enforcer that applies different enforcement sanctions on the Secondary User (SU). The enforcement sanctions will depend on the interference level and its duration caused by the SUs transmission. We will analyze two ex-post-only enforcement mechanisms. First, we will analyze the remunerative penalty approach. Then we will present a graduated response approach. In the graduated response approach, the interference levels will be divided into three different interference event levels received at the PU’s antenna. In the first interference level, the penalty would be a fine proportional to the lost data by the PU plus the expected increase in enforcement costs occasioned by it. In the second interference level, when the SU does not optimize its transmission and decides to continue transmitting closer to the PU’s antenna, more punitive penalties would be applied. The SU then optimizes their transmissions so that the net value of a sequence of transmissions is positive. In the third interference level, if the penalty value exceeds the maximum limit or the SU’s interference duration exceeds a hypothesized maximum duration, other ex-post enforcement sanctions would be suggested like conditionally suspending SU license. We do not consider interference caused by “rogue” or “pirate” radios. There are a number of phenomena that we study in this scenario. First, as the value for SU transmissions increase, SU may find it valuable to risk a higher interference penalty by transmitting closer to the PU’s antenna. The levels of sanctions will encourage the SU to self-manage their transmission so that the value of a sequence of transmissions is positive and not to go through more sanctions forms that may lead them to lose the trust of the PU. While the enforcer role will be discussed in future work, the above approach assumes that adjudication which is applied by the enforcer is immediate and costless. To clarify the restrictions of adjudication costs, it was determined that the hypothetical cost level of a region was equivalent to an exclusion zone in ex-ante enforcement. Having completed that, we articulate the effectiveness of an ex-post enforcement system and its technical requirements.In the current static spectrum allocation, ex-ante-only measures make sense, but with a more dynamic spectrum sharing policy ex-post enforcement must play a role. This study will help researchers develop feasible approaches to adjudication and will help policymakers balance the use of ex-ante and ex-post enforcement techniques in spectrum sharing regimes.