Third generation partnership project (3GPP) Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology in which transmissions from base stations, e.g., eNodeBs, to wireless devices such as mobile stations (also referred to as user equipment (UE)) are sent using orthogonal frequency division multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in the frequency domain. The basic unit of transmission in LTE is a resource block (RB) which in its most common configuration consists of 12 subcarriers and 7 OFDM symbols (one slot). As shown in the LTE physical resource diagram of FIG. 1, a unit of one subcarrier and 1 OFDM symbol is referred to as a resource element (RE). Thus, an RB consists of 84 REs. An LTE radio subframe is composed of two slots in time and multiple resource blocks in frequency with the number of RBs determining the bandwidth of the system (see 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 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.
The signal transmitted by the eNB in a downlink, which is the link carrying transmissions from the eNB to the UE, 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 (RS) that are transmitted on the downlink. These reference symbols 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 Rel-11 of the LTE standards and in prior releases of the LTE standards, there are multiple types of reference symbols. The common reference symbols are used for channel estimation during demodulation of control and data messages in addition to synchronization. The common reference symbols occur once every subframe. These reference symbols are shown in FIG. 2.
Heterogeneous networks, where the macro cells and the small cells have vastly different transmit powers, 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 macro cell layer are on separate frequencies.
Referring to FIG. 3, the network architecture for LTE allows messages to be sent between eNBs 12 via an X2 interface. An eNB 11 also can communicate with other nodes in the network, e.g., to the Mobility Management Entity (MME) 14 via the S1 interface. In current LTE specifications, methods are specified that allow some self-organizing network (SON) functionality where an eNB 11 can request information regarding another eNB 10 via the MME 14.
As used herein, the term “over the air” is defined as wireless communication. In other words, transmitting “over the air” refers to wireless transmission. Currently, network interface based signaling for over the air synchronization purposes is enabled by means of the S1: eNB Configuration Transfer and S1: MME Configuration Transfer procedures according to the process outlined in FIG. 4. This process is as follows:                1. The eNB1 11a generates an eNB Configuration Transfer message containing a SON Information Transfer IE with a SON Information Request IE set to “Time synchronization Info”;        2. The MME 14 receiving the eNB Configuration Transfer message forwards the SON Information Transfer IE towards a synchronization candidate eNB2 11b indicated in the IE by means of the MME Configuration Transfer message;        3. The receiving eNB2 11b may reply with an eNB Configuration Transfer message towards the eNB1 11a including a SON Information Reply IE with the Timing Synchronization Information IE, which consists of Stratum Level and Synchronization Status of the sending node (additionally the message can include information about availability of the muting function and details of already active muting patterns). These two parameters can be defined as follows:                    a. Stratum Level: indicates the number of hops between the node to which the stratum level belongs to the source of a synchronized reference clock. That is, when the stratum level is M, the eNB is synchronized to an eNB whose stratum level is M−1, which in turn is synchronized to an eNB with stratum level M−2 and so on. The eNB with stratum level 0 is the synchronization source;            b. Synchronization Status: indicates whether the node signalling such parameter is connected (via the number of hops stated in the Stratum Level) to a synchronized reference clock (e.g., a GPS source) or to a non-synchronized reference clock (e.g., a drifting clock);                        4. The MME 14 receiving the eNB Configuration Transfer message from eNB2 11b forwards it to eNB 1 12a by means of the MME Configuration Transfer message;        5. The eNB1 11a selects the best available cell's signal as synchronization source and identifies whether there are neighbour cells interfering with the synchronization source signal. If such interfering cells are identified, e.g., in eNB2's cells, eNB1 sends an eNB Configuration Transfer including information about the cell selected as synchronization source as well as a request to activate muting on certain specific cells. The information on the synchronization source cell may consist of the synchronization RS period, offset, the synchronization node's stratum level;        6. The MME 14 receiving the eNB Configuration Transfer message from eNB1 11a forwards it to eNB2 12b by means of the MME Configuration Transfer message;        7. The eNB2 11b determines whether the muting request from eNB1 11a can be fulfilled and activates muting patterns that are most suitable to such request. eNB responds with an eNB Configuration Transfer message containing muting pattern information such as muting pattern period (period of muted subframes) and muting pattern offset;        8. The MME 14 receiving the eNB Configuration Transfer message from eNB2 11b forwards it to eNB1 12a by means of the MME Configuration Transfer message;        9. If eNB1 11a determines that muting at eNB2's cells is no more needed, eNB1 11a can trigger an eNB Configuration Transfer message containing a muting deactivation request;        10. The MME 14 receiving the eNB Configuration Transfer message from eNB1 11a forwards it to eNB2 11b by means of the MME Configuration Transfer message. The eNB2 11b may then deactivate the muting pattern, i.e., eNB2 11b, may freely transmit on the subframes previously muted.        
The Radio Interface Based Synchronization (RIBS) functions standardized in 3GPP Release 12 and described above have the purpose of enabling a more accurate detection of the synchronization source signal, to improve the synchronization accuracy. Hence, muting patterns activation should enable an enhancement of the synchronization source signal with respect to the case where interference from aggressor cells is not mitigated.
A management system for a wireless network is shown in FIG. 5. The node elements (NE) 11a and 11 b, also referred to as eNodeB, are managed by a domain manager (DM) 16a, also referred to as an operation and support system (OSS). A DM 16a, or DM 16b, referred to herein collectively as domain managers 16, may further be managed by a network manager (NM) 18. Two NEs 11a and 11b are interfaced by X2, whereas the interface between two DMs 16 is referred to as Itf-P2P. The management system may configure the network elements, as well as receive observations associated to features in the network elements. For example, a DM 16 observes and configures NEs 11, while a NM 18 observes and configures a DM 16, as well as an NE 11 via a DM 16.
By means of configuration via the DM 16, the NM 18 and related interfaces, functions over the X2 and S1 interfaces can be carried out in a coordinated way throughout the radio access network (RAN), eventually involving the Core Network, i.e., MME 14 and S-GWs.
FIG. 6 is a block diagram of a general radio base station structure 11 where the digital baseband process 20 contains 3GPP defined layer 1 & 2 functions which generate the baseband in-phase and quadrature (IQ) data signal. The IQ baseband signal is then filtered and up converted in the digital up conversion (DUC) block 22 to a higher sampling rate signal, converted to analog be a digital to analog converter 24 which is later mixed and amplified in an RF front end 26 and sent to the antenna 28 to be to be transmitted down link. For the uplink, the received RF signal is first filtered and mixed in the RF front 26 end and then sampled by an analog to digital converter (ADC) 30 and input to a digital down conversion (DDC) block 32 which channelizes and converts from a higher sampling rate to an IQ baseband sampling rate. The baseband processing 20 will further decode the user information data from the baseband IQ signal.
Current synchronization techniques rely on extraction of timing from Global Navigation Satellite Systems (GNSS) or the Precision Time Protocol (PTP). These techniques have inherent drawbacks. In the case of PTP, transport network packet delay variation and delay asymmetry may require specialized network equipment to mitigate. In the case of GNSS, obstructions to the sky-view and/or jamming from terrestrial interference place constraints on location of the GNSS antenna which may be inconvenient or costly.
The synchronization procedure described above with reference to FIG. 4 does not take into account synchronization source signal propagation delays at the synchronization target node in order to achieve an accurate synchronization to the source signal. Informing the synchronization target node of the geolocation of the transmission points of the synchronization source and the target would make it possible to calculate the minimum propagation delay between synchronization source and synchronization target nodes. An example of such method is described in U.S. Patent Application No. 62/140,736 entitled “Accurate Over the Air Synchronization, filed on Mar. 31, 2015 and herein incorporated by reference in its entirety. However, in a number of cases, such as indoor base station deployments, it might not be possible to know the geolocation of transmission points (TPs), possibly because acquisition of global navigation satellite signal (GNSS) coordinates to geolocate TPs is not possible due to indoor sheltering from GNSS. Also, it may not be possible to compensate for propagation delays arising from non line-of-sight propagation.
Therefore, if an eNB that needs to synchronize detects a number of cells in its neighborhood and if the procedures described with reference to FIG. 4 to acquire Time Synchronization Information were carried out, the eNB would determine the best cell for synchronization and then send muting requests towards nearby aggressors or interfering nodes. However, such procedure may not lead to good synchronization results if the location of the transmission point sending the synchronization source or target signal is indoor or the signal is affected by multipath propagation delays as the signal travels from the synchronization source to the synchronization target.
Indeed, current synchronization requirements for time division duplex (TDD) systems are interpreted as allowing a synchronization margin of +/−1.5 us between cells in a given neighborhood using GNSS as a synchronization reference. Moreover, functions for interference cancellation and interference coordination such as eICIC (enhanced Interference Cancellation and Interference Coordination) benefit from synchronization margins within +/−500 ns between cells in a given neighborhood. Other applications such as Offset Time Difference of Arrival (OTDOA) are used in some jurisdictions to comply with regulatory requirements for location services to meet an accuracy standard of +/−50 m that benefit from even more stringent synchronization accuracy.
Such synchronization accuracy is not achievable by means of the current RIBS function described above due to the lack of knowledge of the propagation delay from the synchronization source transmission point. A synchronization target eNB 500 m away from the synchronization source transmission point would already be subject to a synchronization error equal to the propagation delay from source to target of about 1.66 us. The delay may be even higher if the transmission path from source to target nodes is subject to multipath propagation. Such a mismatch would not meet TDD synchronization requirements and may degrade the performance of a number of functions that require more accurate synchronization.
In addition, for a multiple transmission point scenario, combined signals received by the synchronization target may not be received by the line of sight (LOS) signal of the closest TP. Either because there is no line of sight or because of different TP signal combining, the synchronization signal received at the synchronization target could be affected by a higher propagation delay than the LOS with the closest TP. Even if the geolocation coordinates of the synchronization source TPs are known at the target, the synchronization target eNB would not be able to compensate the exact propagation delays due to multipath combining. Indeed, the synchronization target may assume that the propagation delay is the one from the closest synchronization source's TP, which should provide the strongest signal. However, this may not be the case.