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Femtocells are small, low-cost and low-power cellular base stations optimized for providing wireless voice and broadband services to customers in a relatively small area. Femtocells are considered to be one of the most promising solutions for future wireless communication with all the advantages that they bring, such as better indoor coverage, higher spectrum efficiency and lower energy consumption to name but a few. However time synchronization for femtocells is very challenging. Femtocells are expected to be equipped with cheap and less accurate crystal oscillators, which cannot satisfy the femtocell synchronization requirements. Moreover, femtocells are deployed in a variety of scenarios with different synchronization requirements and so different synchronization solutions are needed. Existing solutions for some of the femtocell synchronization scenarios either are unsuitable for that particular scenario or suffer serious performance degradation. Therefore, this thesis is focused on investigating the time synchronization problem for femtocells and proposing novel synchronization schemes. This thesis mainly investigated two femtocell synchronization methods: synchronization via backhaul and synchronization via neighbouring cells listening. Femtocell synchronization using the IEEE 1588 protocol, which is seen as the major solution for femtocell synchronization via backhaul, suffers from both asymmetric and random delay problems. An ‘improved IEEE 1588’ scheme is firstly proposed, which utilizes additional packets with different sizes in every IEEE 1588 synchronization process to solve both of these problems. Although the simulation results show that the ‘improved IEEE 1588’ scheme successfully overcomes both aforementioned limitations and yields much better synchronization accuracy than the conventional scheme, it also brings an additional undesirable overhead. Therefore, this thesis also presented a ‘variable-length IEEE 1588’ scheme, where the length of transmitted packets varies periodically. Simulation results show that the synchronization accuracy of the ‘variable-length IEEE 1588’ scheme is similar to the ‘improved IEEE 1588’ scheme but with the advantage of a significantly reduced number of transmitted messages. For femtocell synchronization via neighbouring cells listening, listening to other synchronized femtocells is an important approach, especially for femtocells in SOHO (Small Office and Home Office) area where the femtocells overlap with each other and then form a wireless femtocell network. This thesis carefully examines the time synchronization in wireless femtocell networks. First, the receiver-receiver synchronization (RRS), which has proved efficient in WSNs (wireless sensor networks), is applied in wireless femtocell networks. Two new RRS based synchronization schemes are proposed for different scenarios to ensure better availability. Second, a hybrid synchronization scheme based on wireless IEEE 1588 and RRS is proposed for wireless femtocell networks. The synchronization accuracy and robustness of this hybrid scheme are then evaluated through simulations. Finally, the synchronization scenario where the femtocell is connected to more than one synchronization sources is studied. The basic solution for this scenario is to simply select the best synchronization source when it is available. A two-step weighted multiple linear regression (WMLR) based synchronization scheme is proposed for this case, and it is proved via simulations that it provides better synchronization accuracy and better stability than the basic solution, especially when the two synchronization sources are comparable. |
