TECHNICAL FIELD

The disclosed subject matter relates generally to design of reference signals in a radio access network and, more particularly, in a radio network with connectivity from the sky.

BACKGROUND

There has been a surge of interest in adapting LTE radio access networks to cater for different deployment scenarios. A new innovative trend is to provide “connectivity from the sky.” To provide such connectivity, platforms being considered include drones, balloons, aircrafts, airships, and/or satellites, among others. LTE standards have been designed primarily for a terrestrial network and have not been optimized to provide “connectivity from the sky.” In particular, compared to traditional terrestrial LTE infrastructure that may generally be stationary, these new platforms in the sky are usually non-stationary and may move at high speeds.

In an LTE radio access network, a wireless device or user equipment (UE) typically follows the following access procedures.1) Cell search: Search and acquire synchronization to a cell in the network.2) System information reading: Receive and decode system information necessary for proper operation in the network.3) Random access: Once the system information has been correctly decoded, UE can perform random access to access the network.

In an existing LTE random access design, random access may serve multiple purposes such as initial access when establishing a radio link, transmitting scheduling requests, etc. Among others, one objective of random access may be to achieve uplink synchronization, which may be important for maintaining uplink orthogonality in an LTE network. To preserve orthogonality among different UEs in an OFDMA or SC-FDMA system, the time of arrival (ToA) of each UE's signal may need to be within the cyclic prefix (CP) of the OFDMA or SC-FDMA signal at the base station.

LTE random access can be either contention-based or contention-free. A contention-based random access procedure may generally include four operations. Brief reference is made toFIG. 8, which is a signal flow diagram illustrating a contention-based random-access procedure. Note that the first operation involves physical-layer processing specifically designed for random access, while the random-access response810, the scheduled transmission815and the contention resolution820may follow the same physical-layer processing used in uplink and downlink data transmission. For contention-free random access, the UE uses reserved preambles assigned by the base station. In this case, contention resolution is not needed, and thus only operations805and810may be required.

Still referring toFIG. 8, in terrestrial LTE networks, the eNB estimates a ToA from a received Msg1 transmitted by UE in operation805. Based on the estimate, eNB feeds back the acquired uplink timing in Msg2 to command the UE to perform timing advance in operation810. This timing advance mechanism may help ensure that the subsequent uplink signals from UEs located in different positions in the cell can arrive at the base station within the CP range.

According to LTE standards, random access preambles may be sent in physical random-access channel (PRACH). The PRACH subcarrier spacing may be 1.25 kHz and the preambles may be Zadoff-Chu (ZC) sequences of length 839. A fixed number of preambles may be available in each LTE cell. For example, 64 preambles may be available in each LTE cell. Several preamble formats of different durations of the sequence and cyclic prefix (CP) may be defined to be used for cells of different sizes. The format configured in a cell may be broadcast in the System Information.

ZC sequences have also been used as reference signals for other purposes in LTE networks as well. For example, ZC sequences may be used in the 3 primary synchronization signals (PSS) in LTE networks. There may be 3 length-63 ZC sequences for the PSS and the length-63 ZC sequences may be extended with 5 zeros at the edges and mapped to the center 73 subcarriers in LTE. Together with secondary synchronization signals (SSS), PSS may enable the UE to acquire frequency and symbol synchronization to a cell, frame timing of the cell, and physical-layer cell identity of the cell.

ZC sequences may also be candidates for reference signals in 5G new radio (NR), such as downlink synchronization signals and uplink random access preambles.

It is known that the ambiguity function of ZC sequences does not have the “thumbtack-like” property, which features a single central peak in the Delay-Doppler plane. In particular, there are several peaks in the ambiguity function of ZC sequences in the Delay-Doppler plane, leading to many timing and Doppler ambiguities when ZC sequences are used to acquire timing and frequency synchronization.

Using LTE PRACH as a specific example, LTE PRACH subcarrier spacing is 1.25 kHz and may not handle a Doppler shift (plus residual carrier frequency offsets) larger than 1.25 kHz. Due to the nature of ZC sequences, both delay and frequency shift cause cyclic shift in the observation window of received ZC sequences at the eNB. As a result, two issues may arise. First, it may be difficult if not impossible to separate the two effects (delay and frequency shifts) by observing the composite cyclic shift. Separating the delay and frequency shift may be advantageous, however, to provide estimates thereof. Second, the composite cyclic shift may make sequence A be interpreted as sequence B, leading to misdetection. Although LTE PRACH cyclic shift set has been designed to avoid this situation, the capacity to avoid misdetection may be limited to frequency shifts that are less than 1.25 kHz, which is not sufficient for LTE deployments in high Doppler scenarios. Such scenarios may include a moving base station in the air, such as a satellite or other airborne device.

Approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in the Background section are not prior art to the inventive embodiments disclosed in this application and are not admitted to be prior art by inclusion in the Background section. Therefore, any description contained in the Background section may be moved to the Detailed Description section.

SUMMARY

Example methods, nodes, and systems are operable to resolve time delay and Doppler frequency shift ambiguities when ZC sequences are used to construct reference signals. The reference signals may be any one of different types of radio system reference signals, including, e.g., a downlink synchronization signal or an uplink random access preamble.

Some embodiments are directed to methods of estimating a time delay and/or a frequency shift of a reference signal. Such methods may include receiving a first reference signal that is generated using a first Zadoff-Chu (ZC) sequence, receiving a second reference signal that is generated using a second ZC sequence that is different than the first ZC sequence, and processing the first reference signal and the second reference signal to estimate at least one of the time delay and the frequency shift of the first reference signal and/or the second reference signal.

In some embodiments, processing the received first and second reference signals includes correlating the received first reference signal with a local copy of the first reference signal to generate a first set of correlation results, correlating the received second reference signal with a local copy of the second reference signal to generate a second set of correlation results, detecting a first correlation peak in the first set of correlation results, detecting a second correlation peak in the second set of correlation results, and estimating at least one of the time delay and the frequency shift of at least one of the reference signals using the detected first and second correlation peaks.

Some embodiments provide that processing the received first and second reference signals includes detecting the signals using a set of hypotheses of at least one of the time delay and frequency shift, determining a best hypothesis of the set of hypotheses, and estimating at least one of the time delay and frequency shift according to the best hypothesis.

In some embodiments, the received first and second reference signals are time domain multiplexed. Some embodiments provide that the received first and second reference signals are frequency domain multiplexed. In some embodiments, the received first and second reference signals are time and frequency domain multiplexed.

In some embodiments, the first ZC sequence is generated using a first root and the second ZC sequence is generated using a second root that is different than the first root and the first ZC sequence has a same length as the second ZC sequence.

Some embodiments provide that the second ZC sequence includes a cyclic shift relative to the first ZC sequence. In some embodiments, the second ZC sequence is generated as a conjugate of the first ZC sequence. Some embodiments provide that the first ZC sequence and the second ZC sequence include a same root, a same length and a same cyclic shift.

Some embodiments provide that the first ZC sequence includes a first length and the second ZC sequence includes a second length that is different than the first length.

In some embodiments, the first and second reference signals are downlink synchronization signals in a radio access network. Some embodiments provide that the radio access network is one of an LTE and a 5G network.

In some embodiments, the first and second reference signals are uplink random access preambles in a radio access network. Some embodiments provide that the radio access network is one of an LTE and a 5G network.

Some embodiments are directed to a radio node that is adapted to carry out operations corresponding to any of the methods described herein.

In some embodiments, the radio node is a wireless device. Some embodiments provide that the radio node is a non-stationary radio access node. In some embodiments, the non-stationary radio access node is a satellite. In some embodiments, the non-stationary radio access node is airborne.

Some embodiments disclosed herein are directed to a computer program product comprising a non-transitory computer readable medium storing program code that when executed by a processor of a UE causes the UE to perform operations corresponding to any of the methods described herein.

Some embodiments are directed to a radio communication network that includes a first radio node that includes a transceiver to provide wireless communication over a radio interface, at least one processor coupled to the transceiver, and at least one memory coupled to the at least one processor and storing program code that when executed by the at least one processor causes the at least one processor to perform operations comprising receiving, via the transceiver, a first reference signal that is generated using a first Zadoff-Chu (ZC) sequence, receiving, via the transceiver, a second reference signal that is generated using a second ZC sequence that is different than the first ZC sequence, and processing the first reference signal and the second reference signal to estimate at least one of the time delay and the frequency shift of the first reference signal and/or the second reference signal. The network may include a second radio node that is configured to generate the first and second signals and to transmit the first and second reference signals to the first radio node.

In some embodiments, at least one of the first radio node and the second radio node includes one of a wireless device and a non-stationary radio access node. Some embodiments provide that the non-stationary radio access node includes one of a satellite and an airborne device.

Some embodiments are directed to a wireless communication device (UE) that includes a transceiver to provide wireless communication over a radio interface, at least one processor coupled to the transceiver, and at least one memory coupled to the at least one processor and storing program code that when executed by the at least one processor causes the at least one processor to perform operations including receiving a first reference signal that is generated using a first Zadoff-Chu (ZC) sequence, receiving a second reference signal that is generated using a second ZC sequence that is different than the first ZC sequence, and processing the first reference signal and the second reference signal to estimate at least one of the time delay and the frequency shift of the first reference signal and/or the second reference signal.

In some embodiments, processing the received first and second reference signals includes correlating the received first reference signal with a local copy of the first reference signal to generate a first set of correlation results, correlating the received second reference signal with a local copy of the second reference signal to generate a second set of correlation results, detecting a first correlation peak in the first set of correlation results, detecting a second correlation peak in the second set of correlation results, and estimating at least one of the time delay and the frequency shift of at least one of the reference signals using the detected first and second correlation peaks.

In some embodiments, processing the received first and second reference signals includes generating correlation peaks corresponding to a set of hypothetical values of time delay and/or frequency shift, detecting the first and second reference signals using the set of hypothetical values, determining a best hypothesis of the set of hypothetical values, and estimating at least one of the time delay and frequency shift according to the best hypothesis.

Some embodiments provide that the first ZC sequence is generated using a first root and the second ZC sequence is generated using a second root that is different than the first root and that the first ZC sequence has a same length as the second ZC sequence.

In some embodiments, the second ZC sequence includes a cyclic shift relative to the first ZC sequence. Some embodiments provide that the second ZC sequence is generated as a conjugate of the first ZC sequence. In some embodiments, the first ZC sequence and the second ZC sequence include a same root, a same length and a same cyclic shift. In some embodiments, the first ZC sequence includes a first length and the second ZC sequence includes a second length that is different than the first length. Some embodiments provide that the first and second reference signals are downlink synchronization signals in a radio access network. In some embodiments, the first and second reference signals are uplink random access preambles in a radio access network.

Some embodiments are directed to a computer program product that includes a computer readable storage medium having computer readable code embodied in the computer readable storage medium that when executed by a processor of a wireless communication device causes the wireless communication device to perform operations including receiving a first reference signal that is generated using a first Zadoff-Chu (ZC) sequence, receiving a second reference signal that is generated using a second ZC sequence that is different than the first ZC sequence, and processing the first reference signal and the second reference signal to estimate at least one of the time delay and the frequency shift of the first reference signal and/or the second reference signal.

Some embodiments are directed to methods of estimating a time delay and/or a frequency shift of a reference signal. Operations according to such methods include generating a first reference signal using a first Zadoff-Chu (ZC) sequence, generating a second reference signal using a second ZC sequence that is different than the first ZC sequence, and transmitting the first reference signal and the second reference signal to a radio node that is configured to process the first reference signal and the second reference signal to estimate at least one of the time delay and the frequency shift of the first reference signal and/or the second reference signal.

In some embodiments, methods include multiplexing the first and second reference signals before transmitting the first and second reference signals. Some embodiments provide that the first and second reference signals are time domain multiplexed and/or frequency domain multiplexed.

In some embodiments, the first ZC sequence is generated using a first root and the second ZC sequence is generated using a second root that is different than the first root, and the first ZC sequence has a same length as the second ZC sequence.

In some embodiments, the second ZC sequence includes a cyclic shift relative to the first ZC sequence. Some embodiments provide that the second ZC sequence is generated as a conjugate of the first ZC sequence. In some embodiments, the first ZC sequence and the second ZC sequence include a same root, a same length and a same cyclic shift. In some embodiments, the first ZC sequence includes a first length and the second ZC sequence includes a second length that is different than the first length. Some embodiments provide that the first and second reference signals are downlink synchronization signals in a radio access network. Some embodiments provide that the radio access network is one of an LTE and a 5G network.

In some embodiments, the first and second reference signals are uplink random access preambles in a radio access network. Some embodiments provide that the radio access network is one of an LTE and a 5G network.

Some embodiments are directed to a network node that includes a reference signal generation module that is configured to generate first and second reference signals using first and second ZC sequences and a reference signal transmission module that is configured to transmit the first and second reference signals to a radio node that is configured to estimate the time delay and/or frequency shift using the first and second reference signals.

Some embodiments are directed to a network node that includes a reference signal receiver module that is configured to receive a first reference signal that is generated using a first Zadoff-Chu (ZC) sequence and a second reference signal that is generated using a second ZC sequence and a reference signal processing module that is configured to process the first reference signal and the second reference signal to estimate at least one of the time delay and the frequency shift of the first reference signal and/or the second reference signal.

