| Although the development of the next generation (i.e.,5G) of wireless networks is in its initial phase, it is widely agreed that some key technologies will play a vital role in the realization of 5G, such as millimeter waves, massive MIMO, small cells, and new radio air interfaces. Among these, densely deployed small cell networks are believed to be one of the most practical solutions to the 5G promise of truly ubiquitous mobile broadband. However, due to the high interference between small cells it must be equipped with advanced interference cancelation techniques or the interference will offset the benefit brought by the supplication of small cells. Synchronization between densely deployed small cells is the key to the materialization of the benefits a small cell network promises, and is one of the most challenging issues of small cell networks.In this article, we overview the existing network synchronization techniques and address their limitations in small cell network deployment environments. There are two primary synchronization techniques in today’s communication networks. GPS provides the most accurate synchronization accuracy. However, due to environmental and cost limitations, GPS may not be available in some small cell deployment scenarios (e.g., indoors). Another synchronization technique is IEEE 1588, the accuracy of which depends highly on backhaul conditions.We therefore study the feasibility of a network-listening-based solution as a practical alternative to the synchronization problem of a dense small cell network with an application to LTE. Firstly, we provide the mathematical principle behind the synchronization. Then the conceptually basic network synchronization design is proposed. The key ideal is using the time shift to convey synchronization stratum level. CRS (cell-specific reference signal) is used as an exemplary synchronization signal; the property related to the synchronization is elaborately studied. After all the basic discussion, an advanced synchronization design is proprosed with a more legible synchronization signal and a modified synchronization struntum algorithm, thus an improved signal detection performance and the possibility of blind detection is obtained. Some detailed issues, such as the elimination of propogation delay between a macro cell and a small cell, the synchronization period calculation and the initial synchronizarion procedure are also covered in this paper. In conclusion, this paper provides a overall small cell synchronization structure and important details about the synchronization techniques.After the synchronization among small cells and the advanced interference cancelation and cooperation techniques are operated, the high SNR (signal-to-noise) wireless envoroment is established. The question is how to convert the gain in SNR to a practical spectral efficiency; an instinctual solution is the application of a higher order modulation. This part is aimed to study the feasibility of the usage of 256 QAM with all-around perspective. Various simulation performances considering the practical hardware imperfection (i.e., EVM, error vector magnitude) confirm that the EVM parameter is the bottleneck of the gain from 256QAM over 64 QAM (the highest modulation order in LTE release 11). Fignally the impact on the LTE standard from the introducation of 256QAM is also studied with two possible methods. |
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