Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) technology is a mobile broadband wireless communications technology in which transmissions from base stations (referred to as enhanced, or evolved, Node Bs (eNBs)) to mobile stations (referred to as User Equipments (UEs)) are sent using Orthogonal Frequency Division Multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in frequency. The basic unit of transmission in LTE is a Resource Block (RB), which in its most common configuration consists of 12 subcarriers and seven OFDM symbols (one slot). A unit of one subcarrier and one OFDM symbol is referred to as a Resource Element (RE), as illustrated in FIG. 1. Thus, an RB consists of 84 REs. An LTE radio subframe is composed of two slots in time and multiple RBs in frequency with the number of RBs determining the bandwidth of the system, as illustrated in FIG. 2. Furthermore, the two RBs in a subframe that are adjacent in time are denoted as an RB pair. Currently, LTE supports standard bandwidth sizes of 6, 15, 25, 50, 75, and 100 RB pairs. In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.
The signal transmitted by the eNB in a downlink (the link carrying transmissions from the eNB to the UE) subframe may be transmitted from multiple antennas and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. In order to demodulate any transmissions on the downlink, a UE relies on Reference Symbols (RSs) that are transmitted on the downlink. These RSs and their position in the time-frequency grid are known to the UE and hence can be used to determine channel estimates by measuring the effect of the radio channel on these symbols. In LTE Release 11 and prior releases of LTE, there are multiple types of RSs. Common Reference Symbols (CRSs) are used for channel estimation during demodulation of control and data messages in addition to synchronization. The CRSs occur once every subframe.
Heterogeneous, or multi-layer, deployments of LTE networks (referred to as heterogeneous networks) can be used to, e.g., increase capacity and achievable data rates in areas where needed. A heterogeneous network includes a macro cell layer including a number of macro cells served by corresponding eNBs (sometimes referred to as macro eNBs or macro cell eNBs) and a small cell layer including a number of small cells served by corresponding small or small cell eNBs (e.g., Homes eNBs (HeNBs), pico eNBs, micro eNBs, femto eNBs, etc.). The macro eNBs serving the macro cells and the small eNBs serving the small cells have vastly different transmit powers and may be deployed in two main ways. In the first deployment type, the small cell layer and the macro cell layer share the same carrier frequencies, which creates interference between the two layers. In the second deployment type, the small cell layer and the macro cell layer are on separate frequencies.
Many features of LTE technology benefit from the eNBs in the system being synchronized with each other with respect to transmit timing and frequency. Synchronization of eNBs is typically done using a Global Navigation Satellite System (GNSS), such as Global Positioning System (GPS), or by using network based methods such as IEEE 1588v2. However, such methods may not be available to all eNBs, particularly small cell eNBs in a heterogeneous network. As such, there is a need for systems and methods for transmit timing and frequency synchronization for small cell eNBs in a heterogeneous network.