The hierarchical spectrum sharing is a technique which categorizes wireless users in the hierarchical spectrum sharing system into primary users and secondary users in terms of spectrum-using privilege. In such a hierarchical spectrum sharing system, a primary user and a secondary user can share the same frequency band but the primary user has a privilege of using the frequency band and is oblivious to the co-exiting secondary user, and thus the primary user can use the frequency band at any moment. To the contrary, the secondary user can only use the frequency band when there is a suitable opportunity, such as in a condition that the primary user is transmitting signals at a low signal quality, a low signal quality is acceptable to the primary user, and etc. The hierarchical spectrum sharing technique can be applied to a variety of scenarios, for example the well-known cognitive radio, the heterogeneous network (such as, a macro/femto network, a macro/pico network, a macro/micro network and so on), and etc. Moreover, it has many advantages such as flexibility and ability of improving spectrum efficiency and thus it is critical for the next generation networks.
However, for the hierarchical spectrum sharing system, the main problem lies in the cross-tier interference, especially the secondary user's interference with the primary users in its vicinity because the primary users are oblivious to the co-exiting secondary users and thus have no knowledge about interference from the secondary user, which brings a great difficulty to cope with such interference. Therefore, it usually requires the interference from the secondary user to be limited to a certain level so as to have a tolerable effect on the primary users.
The macro/femto heterogeneous network is one of typical hierarchical spectrum sharing systems and has received a considerable attention recently. The FAP (Femtocell Access Point) is possible to offload the burden from the macrocell and improve the coverage of the cellular network. Moreover, the femtocell can help to plug the indoor coverage holes, especially when the signal from the macro cell can not penetrate homes due to difficult radio propagation conditions.
As is known, the FAP has a limited transmission range, usually in a home or office area, but it can offer immense capacity improvement for the network due to the ability to reuse the frequency band more often. However, unauthorized macro user equipments (MUEs) can only connect to its macro eNodeB (MeNB) and are not allowed to connect to the FAP even if they are located in the Femtocell Transmission Range (FTR). Therefore, the MUEs may suffer heavy cross-tier interference which is a problem to be tackled urgently.
The article “On exploiting cognitive radio to mitigate interference in macro/femto heterogeneous networks” by S.-M Cheng, et al. (IEEE Wireless Commun., vol. 18, no. 3, pp. 40-47, June, 2011) has proposed a possible solution to tackle the problem about the cross-tier interference. In this article, the MUE will adopt a HARQ (Hybrid Automatic Repeat Request) scheme wherein one packet can be repeatedly transmitted within the next several timeslots when the previous transmission attempt fails. For the purpose of illustration, reference will be made to FIG. 1 and FIG. 2 to describe the solution disclosed in that article.
Referring to FIG. 1, it schematically illustrates a diagram of a simplified architecture for a solution for femtocell interference mitigation in a macro/femto heterogeneous network as disclosed in the above-mentioned article. As illustrated in the figure, the system comprises a MeNB, a MUE, a FAP and a FUE (Femto User Equipment). The message sent to the MUE will be received by both the MUE and the FUE, and, similarly, the information fed back from the MUE to the MeNB will be received by both the MeNB and the FAP; and the message directed to FUE will also be received by the MUE and interfere with MUE. According to the flow chart as shown in FIG. 2, the FAP will keep silence during the MeNB's first transmission attempt (Step S201) and obverse the ARQ feedback from the MUE (Step S202), that is to say, the ARQ feedback from MUE will be received by both the MeNB and the FAP (as shown by the long dash arrow in FIG. 1) instead of only by the MeNB. On one hand, if the ARQ feedback is an ACK, which means the packet being successfully received at the MUE, the FAP will do nothing but keep observing the subsequent ARQ feedbacks from the MUE; on the other hand, if the ARQ feedback is a NACK, which means that the MUE fails to decode the packet successfully and a retransmission is required, the FAP will send its own packet to the FUE during the MeNB's retransmission (Step S203). The reason that the FAP could transmit its packet during the retransmission period lies in that the SINR requirement at MUE for decoding the packet from MeNB during this period is not as high as the first transmission attempt due to HARQ scheme and thus the MUE can tolerate a moderate interference from FAP.
In the aforesaid solution, the cross-tier interference has mitigated and the spectrum efficiency has been improved. However, with the explosive increase in demand for higher data rates and lower power consumptions, there is still a need in the art to further improve the spectrum efficiency.