Reference signal for wireless communication systems

Abstract

The present invention relates to the transmission and / or reception of reference symbols. A first reference symbol includes a symbol and a cyclically shifted portion of the symbol, wherein the cyclically shifted portion has a cyclic shift length. A second reference symbol includes a cyclically shifted version of the first reference symbol relative to the first reference symbol by the cyclic shift length. These first and second reference symbols are transmitted continuously from at least one antenna.

Description

Technical Field

[0001] The embodiments of the present invention relate to reference signals used in wireless communication systems. Prior Technology

[0002] In a wireless communication system, it may be desirable to have an accurate estimate of the communication channel between devices communicating wirelessly. When reciprocal channel estimation is used in a time-division duplex (TDD) system, an uplink and downlink channel difference calibration is typically performed. Specific communication standards include a reference signal that can be used to estimate a communication channel. In certain communication systems, the downlink frame and uplink frame are not aligned. This can pose technical challenges to channel estimation and / or calibration. Aligning the uplink and downlink frames can be even more difficult because different parts of the communication system process the reference signal. Summary of the Invention

[0003] The innovations described in the technical solutions each have several forms, and none of them individually bears responsibility for their intended attributes. Without limiting the scope of the technical solutions, some significant features of the present invention will now be briefly described.

[0004] One aspect of the present invention is a method for transmitting a reference symbol. The method includes generating a first reference symbol comprising a symbol and a cyclically shifted portion of the symbol. The cyclically shifted portion has a cyclic shift length. The method includes generating a second reference symbol, wherein the second reference symbol comprises a cyclically shifted version of the first reference symbol by a cyclic shift relative to the first reference symbol up to the cyclic shift length. The method also includes continuously transmitting the first and second reference symbols via at least one antenna.

[0005] The cyclic shift portion may be a cyclic prefix, wherein in the first reference symbol, the symbol follows the cyclic prefix.

[0006] The frame offset between the uplink symbol and the downlink symbol can be greater than the cyclic shift length.

[0007] Generating the first reference symbol may involve adding the cyclic shift in a time domain. Generating the first reference symbol may involve applying weighted overlap and adding roll-off. In some applications, the first reference symbol may be generated based on at least one of a Gold sequence or a Zadoff-Chu sequence. Generating the second reference symbol may involve applying a frequency domain phase ramp to at least the symbol of the first reference symbol.

[0008] The method may include applying phase ramping to the first and second reference symbols to pre-compensate for at least one frame offset between the uplink and downlink symbols. Applying phase ramping may also pre-compensate for an additional offset. Applying phase ramping may also pre-compensate for a phase rotation specified in a new radio standard.

[0009] The transmission may include a time-domain duplex (TDD) transmission. The first and second reference symbols may be orthogonal frequency division multiplexing symbols. The first and second reference symbols may include at least one of discrete Fourier transform extended orthogonal frequency division multiple access symbols, interleaved frequency division multiple access symbols, or other time-domain waveform symbols.

[0010] Another aspect of the present invention is a system for transmitting reference symbols. The system includes: at least one antenna; a baseband unit; and a radio frequency (RF) processing unit communicating with the baseband unit. The baseband unit and the RF processing unit are configured together to: generate a first reference symbol; generate a second reference symbol; and cause the first and second reference symbols to be continuously transmitted from the at least one antenna. The first reference symbol includes a symbol and a cyclically shifted portion of the symbol. The cyclically shifted portion has a cyclic shift length. The second reference symbol includes a cyclically shifted version of the first reference symbol relative to the first reference symbol by the cyclic shift length.

[0011] These first and second reference symbols can be transmitted as part of a time-domain duplex (TDD) transmission. The frame offset between the uplink and downlink symbols can be greater than the cyclic shift length.

[0012] The radio frequency processing unit may include a remote radio unit. The radio frequency processing unit may include a fronthaul circuitry system.

[0013] The RF processing unit can be configured to perform an inverse fast Fourier transform on one of the frequency domain versions of the symbol provided by the baseband unit. The RF processing unit can also be configured to add the cyclically shifted portion to the first reference symbol.

[0014] The baseband unit may include a phase ramp block configured to apply phase ramping to the symbol. The second reference symbol may be generated based on an output signal from one of the phase ramp blocks.

[0015] Another aspect of the present invention includes a non-transitory, computer-readable storage device containing computer-executable instructions. When executed by a baseband unit, these computer-executable instructions cause execution of a method. The method includes: generating a first reference symbol comprising a symbol and a cyclically shifted portion of the symbol, the cyclically shifted portion having a cyclic shift length; generating a second reference symbol, wherein the second reference symbol includes a cyclically shifted version of the first reference symbol relative to the first reference symbol by the cyclic shift length; and continuously transmitting the first and second reference symbols via at least one antenna.

[0016] Another aspect of the present invention is a method for processing a reference symbol. The method includes receiving a reference symbol from at least one antenna and processing the reference symbol. The reference symbol includes a portion of a first transmitted reference symbol and a portion of a second transmitted reference symbol. The first transmitted reference symbol includes a symbol and a portion of the symbol that has been cyclically shifted by a cyclic shift length. The second transmitted reference symbol includes a version of the first transmitted reference symbol that has been cyclically shifted relative to the first transmitted reference symbol by the cyclic shift length.

[0017] Processing this reference symbol may include taking into account a frame offset between the uplink and downlink symbols. Processing this reference symbol may also include taking into account another timing offset between downlink transmission and uplink reception.

[0018] The reference symbol can be pre-compensated from a transmitter side.

[0019] This reference symbol can be generated based on a downlink channel status information reference signal sequence. The same channel status information reference signal sequence can be used to generate this reference symbol and for downlink channel status information processing.

[0020] The method may include generating at least one channel estimate based on the processing. The method may also include generating antenna calibration coefficients based on the processing.

[0021] The process may include applying a phase ramp in the frequency domain. The process may include using prior information to account for frame offset and / or any other timing offset between uplink reception and downlink transmission. The process may include removing the time-domain sample corresponding to the cyclic shift length of the reference symbol. The process may include cyclically shifting the sample in the time domain.

[0022] At least one remote radio unit may perform at least a portion of the processing. At least a portion of the processing may be performed by the fronthaul circuitry.

[0023] The frame offset can be greater than the cyclic shift length.

[0024] The reception may include time-division duplex (TDD) reception. The first and second transmitted reference symbols are orthogonal frequency division multiplexing symbols.

[0025] Another aspect of the present invention is a system for processing reference symbols. The system includes: at least one antenna; a baseband unit; and a radio frequency (RF) processing unit communicating with the baseband unit. The baseband unit and the RF processing unit are configured together to receive a reference symbol from the at least one antenna and process the reference symbol to take into account (i) a frame offset between an uplink symbol and a downlink symbol and (ii) another timing offset between downlink transmission and uplink reception. The reference symbol includes a portion of a first transmitted reference symbol and a portion of a second transmitted reference symbol. The first transmitted reference symbol includes a symbol and a cyclically shifted portion of the symbol having a cyclic shift length. The second transmitted reference symbol includes a cyclically shifted version of the first reference symbol relative to the first transmitted reference symbol by the cyclic shift length.

[0026] The radio frequency processing unit can be configured to remove time-domain samples corresponding to the cyclic shift length from the reference symbol.

[0027] The baseband unit can be configured to generate at least one channel estimate based on the processing of the reference symbol.

[0028] Another aspect of the present invention includes a non-transitory, computer-readable storage device containing computer-executable instructions. When executed by a baseband unit, these computer-executable instructions cause execution of a method. The method includes: receiving a reference symbol from at least one antenna, the reference symbol including a portion of a first transmitted reference symbol and a portion of a second transmitted reference symbol, wherein the first transmitted reference symbol includes a symbol and a cyclically shifted portion of the symbol having a cyclic shift length, and wherein the second transmitted reference symbol includes a cyclically shifted version of the first transmitted reference symbol relative to the first transmitted reference symbol by the cyclic shift length; and processing the reference symbol.

[0029] For the purpose of summarizing the invention, specific aspects, advantages, and novel features of the invention have been described herein. It should be understood that not all of these advantages may be achieved according to any particular embodiment. Therefore, the invention may be embodied or practiced in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or implied herein. Simple Explanation of the Diagram

[0030] Embodiments of the invention will now be described with reference to the accompanying drawings and non-limiting examples.

[0031] Figure 1A is a timing diagram for one example of a new radio (NR) frame, showing the frame structure and calibration reference signal timing. Figure 1B is a timing diagram for another example of an NR frame, showing the frame structure and calibration reference signal timing.

[0032] Figure 2A is an example timing diagram illustrating a timing offset between an uplink (UL) frame and a downlink (DL) frame. Figure 2B is another example timing diagram illustrating a timing offset between a UL frame and a DL frame.

[0033] Figure 3 is an example timing diagram of one of the calibration reference symbols according to an embodiment.

[0034] Figure 4A is a schematic block diagram of a system for transmitting a calibration reference signal according to an embodiment.

[0035] Figure 4B is a schematic block diagram of a system for transmitting a calibration reference signal according to another embodiment.

[0036] Figure 5 is a schematic block diagram of a system for processing a received calibration reference signal according to an embodiment.

[0037] Figure 6 is a schematic block diagram of a system for processing a received calibration reference signal according to another embodiment.

[0038] Figure 7 illustrates a timing diagram of a calibration reference signal using weighted overlap and added roll-off according to one embodiment.

[0039] Figure 8 is a timing diagram illustrating a timing sequence for transmitting and receiving calibration reference signals according to an embodiment.

[0040] Figure 9 is a block diagram illustrating an example network system including a baseband unit and a remote radio unit according to an embodiment.

[0041] Figure 10 is a flowchart of an exemplary method for transmitting a calibration reference signal according to an embodiment.

[0042] Figure 11 is a flowchart of an exemplary method for processing a received calibration reference signal according to an embodiment.

[0043] Figure 12 is a diagram illustrating one example of a multiple-input multiple-output (MIMO) network environment in which reference signals can be wirelessly transmitted and received. Implementation

[0044] [Cross-reference to priority claims] [] This application claims priority to U.S. Provisional Patent Application No. 63 / 093,449, filed October 19, 2020, entitled “REFERENCE SIGNAL FOR WIRELESS COMMUNICATION SYSTEMS,” the disclosure of which is incorporated herein by reference in its entirety and for all purposes.

[0045] The following description of certain embodiments presents various descriptions of specific embodiments. However, the innovations described herein may be embodied in many different ways, for example, as defined and covered by the claims of this invention. In this description, references to the same element symbols may indicate the same or functionally similar elements in the drawings. It will be understood that the elements drawn in the drawings are not necessarily drawn to scale. Furthermore, it will be understood that some embodiments may include more than one set of elements drawn in a drawing and / or include a subset of the elements drawn in a drawing. In addition, some embodiments may incorporate any suitable combination of features from two or more drawings.

[0046] In a Time Division Duplex (TDD) communication system, a reference signal can be transmitted from one antenna of a first node to one antenna of a second node. In a particular TDD system, an uplink channel estimate can be used to estimate downlink channel quality based on channel reciprocity. To estimate the downlink channel quality based on reciprocity, a calibration can be performed for one of the uplink and downlink channel differences caused by the transmission and reception circuitry. Channel reciprocity can be used for downlink transmission beamforming. Any suitable channel estimate and / or calibration data (e.g., a calibration coefficient) can be generated based on a received reference signal. It is expected that multiple different remote radio units (RRUs) and / or other network nodes will perform over-the-air calibrations over time (e.g., periodically).

[0047] In a TDD frame architecture, downlink and uplink frames are often misaligned in various applications. In these applications, a frame offset exists between the uplink and downlink frames. This frame offset can pose challenges for calibration when the symbol boundaries of a downlink (DL) transmitted waveform are misaligned with the symbol boundaries of an uplink (UL) received signal.

[0048] This invention relates to transmitting a reference signal including a reference symbol and / or processing a received reference signal that can be used to calibrate UL / DL channels across all remote radio units (RRUs). A first reference symbol may comprise a symbol and a cyclically shifted portion of that symbol. A second reference symbol may be a cyclically shifted version of the first reference symbol relative to the first reference symbol by a length equal to the length of the cyclically shifted portion of that symbol. The first and second reference symbols may be continuously transmitted wirelessly via at least one antenna. A received reference symbol may comprise a portion of the transmitted first reference symbol and a portion of the transmitted second reference symbol. This is attributable to a receiver window landing on a symbol between two transmitted symbols due to UL / DL frame offset. The received reference symbol can be processed to take into account a frame offset between the uplink and downlink frames and the length of the cyclically shifted portion of the symbol. Therefore, regardless of frame offset, the symbol can be recovered from the received reference symbol. Symbols can be used to generate channel estimates and / or other calibration data.

[0049] Figure 1A is a timing diagram for an example of a new radio (NR) frame, illustrating the frame structure and timing of the calibration reference signal. The frame structure can be used in an Open Radio Access Network (ORAN) architecture with TDD Coordinated Multipoint (COMP) wireless communication. For example, the ORAN architecture can be based on ORAN Option 7.x and / or Option 8.

[0050] As shown in Figure 1A, the NR frame structure can include 7 downlink (DL) slots, one special slot (SSF), and 2 uplink (UL) slots. The duration of a 3GPP NR slot is equivalent to a 3GPP LTE subframe in terms of data scheduling. The special slot (SSF) lies between the DL and UL slots. Each group of 7 DL slots, one special slot, and 2 UL slots can be 5 milliseconds (ms) long. Other durations are possible depending on the TDD frame structure. Groups of 7 DL slots, one special slot, and 2 UL slots can be repeated.

[0051] A Special Time Slot (SSF) can contain a DL symbol, a UL symbol, and a flexible symbol that can be configured as DL, UL, or used as a protection cycle between DL and UL. In Figure 1A, the SSF consists of a DL portion and a flexible portion. Calibration can occur within one flexible cycle of the SSF. In the illustrated example, the flexible cycle in the SSF is 7 symbols long.

[0052] Figure 1B is a timing diagram of another example of an NR frame structure, illustrating the frame structure and calibration reference signal timing. In the NR frame structure of Figure 1B, special time slot S contains 2 DL symbols, 6 flexible symbols, and 6 UL symbols. The two DL symbols may contain physical downlink control channel (PDCCH) communication. Calibration can occur within the 4 symbols preceding the flexible portion of special time slot S. A first group of network antennas can transmit calibration reference symbols in the 2 symbols preceding the flexible symbols, and a second group of network antennas can transmit calibration reference symbols in the 2 symbols following the flexible symbols. Each of the two groups of network antennas can receive one calibration reference symbol, which includes portions of two transmitted calibration reference symbols from the other group. The 6 UL symbols of the special time slot can be used for receive probe reference signal (SRS) communication from the user equipment. If calibration is not performed in special time slot S, 4 symbols are available for physical downlink common channel (PDSCH) communication. In some cases, the UL portion of a special time slot can be reduced to support a longer PDSCH.

[0053] In LTE or NR, the UL and DL symbols may not be perfectly aligned. Therefore, the symbols used for calibration reference signal transmissions in multiple groups (e.g., antenna groups G0 and G1) may be misaligned. Antenna groups G0 and G1 may each be contained in one or more remote radio units. Group G0 may transmit a reference signal (RS) measured by group G1 as channel estimate 0. Group G1 may then transmit an RS measured by group G0 as channel estimate 1. An example of the timing of transmission and reception by groups G0 and G1 during a specific time slot is illustrated in Figures 1A and 1B. By comparing the difference between channel estimate 0 and channel estimate 1, a gNode B (gNB) can calculate a calibration coefficient for each antenna. In one example, the group G0 transmission in DL mode and the group G1 reception in UL mode are not symbol aligned, and vice versa. In the ORAN architecture, time-domain samples can be processed in fronthaul (FH) circuitry systems with a Common Public Radio Interface (CPRI) and / or in remote radio units (RRUs) with enhanced CPRI (eCPRI). The migration of time-domain sample processing in FH circuitry systems and / or one or more RRUs can make aligning transmit and receive time-domain symbol boundaries more difficult. For example, understanding a timing offset and / or alignment carried over to symbol boundaries on an FH and / or RRU side can be more challenging.

[0054] The present invention relates to a calibration reference signal waveform that makes the calibration procedure transparent to different RRH and / or FH implementations used for processing time-domain samples. The reference signal disclosed herein can be processed in the time domain without knowing the frame offset between the uplink and downlink frames. Regardless of the frame offset, the calibration signal can be recovered by frequency-domain processing.

[0055] Figure 2A is an example timing diagram illustrating a timing offset between UL and DL frames. In the example shown in Figure 2A, the UL and DL frames have a frame offset TA offset of one time-domain sample. This frame offset can be a transmit-to-receive switching time or a receive-to-transmit switching time. The frame offset is labeled FO in Figure 2A. There is a gap T delta of one time-domain sample between the end of a transmit-to-receive switch and the boundary of the next uplink symbol. This gap is labeled Delta in Figure 2A. The gap T delta can be equal to a time slot length minus twice the frame offset. As an example, for a 100 MHz system bandwidth, the frame offset TA offset can be 1600 time-domain samples and the gap T delta can be 1184 time-domain samples. As another example, for a 50 MHz system bandwidth, the frame offset TA offset can be 800 time-domain samples and the gap T delta can be 592 time-domain samples. Each time slot or frame has a length corresponding to a cyclic shift length CP plus a calibration reference symbol length N. The length can be measured in time-domain samples. At a symbol boundary, the received UL frame and the DL transmission frame are misaligned. The frame offset is longer than the cyclic shift portion of a calibration reference symbol. Therefore, a receiver should receive data between two adjacent OFDM symbols from a transmitter.

[0056] Figure 2B illustrates a second exemplary timing diagram of a timing offset between UL and DL frames for an NR system operating with a system bandwidth of 100 MHz. In the illustrated example, the UL and DL frames have a frame offset of TA offset = 1600 time-domain samples. In Figure 2B, each slot or frame has a length of 288 + 4096 time-domain samples. This is an example of a cyclic shift length CP from Figure 2A plus a calibration reference symbol length N. Figure 2B is an example in New Radio (NR). The symbol may be an Orthogonal Frequency Domain Multiplexing (OFDM) symbol. In some other embodiments, the reference symbol according to any suitable principles and advantages disclosed herein may be other types of reference symbols, such as Discrete Fourier Transform-Spread-FDMA (DFT-Spread-FDMA) symbols, Interleaved FDMA (IFDMA) symbols, or other time-domain waveform symbols.

[0057] The exemplary timing diagrams in Figures 2A and 2B pertain to the transmission of a reference signal between antenna groups. One of the two consecutively transmitted calibration reference symbols is transmitted from each antenna group G0 and G1. These consecutively transmitted reference symbols are transmitted uninterruptedly back-to-back. Further details regarding the calibration reference symbols will be provided below, for example, with reference to Figure 3.

[0058] Each antenna group G0 and G1 receives a calibration reference symbol comprising a portion of two consecutively transmitted calibration reference symbols. An antenna group G0 or G1 and its corresponding receiver processing circuitry can be configured to receive between two consecutively transmitted calibration reference symbols from a transmitter. This can be a result of the frame offset between the uplink and downlink symbols being greater than the length of a cyclically shifted portion of a calibration reference signal. For example, antenna G0 of the first group receives a calibration reference symbol 22. Due to misalignment between the uplink and downlink symbols, calibration reference symbol 22 comprises two different portions of the transmitted reference signal. Similarly, antenna G1 of the second group receives a calibration reference symbol 24, which, due to misalignment between the uplink and downlink symbols, comprises two different portions of the transmitted reference signal. Due to the structure of the transmitted calibration reference symbols, the received calibration reference symbol is received at the receiver side with a cyclic shift. The receiver side can then process a received calibration reference symbol to take this cyclic shift into account.

[0059] Figure 3 is an exemplary timing diagram of a calibration reference symbol according to one embodiment. A calibration reference signal may include two consecutive calibration reference OFDM symbols, which include a first calibration reference symbol 31 and a second calibration reference symbol 32.

[0060] The first calibration reference symbol 31 includes a symbol 33 and a cyclically shifted portion 34 of the symbol 33. The first calibration reference symbol 31 may consist of a symbol 33 and a cyclically shifted portion 34. The cyclically shifted portion 34 is a cyclic prefix in Figure 3. In the first calibration reference symbol 31, the symbol 33 immediately follows the cyclically shifted portion 34. In the example shown in Figure 3, the cyclically shifted portion 34 has a length of 288 time-domain samples. This length of the cyclically shifted portion 34 may be set by a standard or other specification. The length of the cyclically shifted portion 34 is a cyclic prefix length in Figure 3. In the example shown in Figure 3, the symbol 33 contains 4096 time-domain samples. The length of the OFDM symbol 33 may be set by a standard or other specification. The cyclically shifted portion 34 contains the last 288 time-domain samples of the symbol 33.

[0061] The second calibration reference symbol 32 is a cyclically shifted version of the first calibration reference symbol 31. Specifically, the second calibration reference symbol 32 is the first calibration reference symbol 31 cyclically shifted to the left by the length of the cyclic shift portion 34. The second calibration reference symbol 32 includes a symbol 35 and a cyclically shifted portion 36. In the second calibration reference symbol 32, symbol 35 contains the same time-domain samples as symbol 33 of the first calibration reference symbol 31, except that the time-domain samples are arranged in a different order by cyclically shifting to the length of the cyclic shift portion 36. The time-domain samples of symbol 33 of the first calibration reference symbol 31 are the first 4096 time-domain samples of the second calibration reference symbol 32. Therefore, in the first and second calibration reference symbols 31 and 32, the time-domain samples of symbol 33 are repeated back-to-back. The cyclically shifted portion 36 of the second calibration reference symbol 32 has the same length as the cyclically shifted portion 34 of the first calibration reference symbol. If the lengths of the cyclically shifted portions 34 and 36 are different, the second calibration reference symbol 32 can be shifted to the left up to the length of the cyclically shifted portion 36.

[0062] In some embodiments, to account for a receive-side frame offset, the first and second calibration reference symbols 31 and 32 may be further cyclically shifted to reach that frame offset. In the example illustrated in Figure 2B, the frame offset is 1600 time-domain samples. The time-domain cyclic shift may correspond to a frequency-domain phase ramp.

[0063] On the receiving side, a received calibration reference symbol 24 may include a portion of the first calibration reference symbol 31 and a portion of the second calibration reference symbol 32. The structure and continuous transmission of calibration reference symbols 31 and 32 can result in a continuous phase in the received calibration reference symbol 24 at the receiving side. Figure 3 illustrates that the calibration reference signal 24 may be received at a time corresponding to the middle of the first and second transmitted calibration reference symbols 31 and 32. This corresponds to the timing diagram in Figure 2B. The received calibration reference symbol 24 includes some time-domain samples of symbol 33, time-domain samples from the cyclically shifted portion 36, and some time-domain samples from symbol 35. The received calibration reference symbol 24 includes all time-domain samples present in symbol 33, but does not actually receive the entire symbol 33. Similarly, the received calibration reference symbol 24 includes all time-domain samples present in symbol 35, but does not actually receive the entire symbol 35. In addition to all time-domain samples present in symbols 33 or 35, the received calibration reference symbol 24 contains additional time-domain samples corresponding to the length of the cyclically shifted portion 36.

[0064] Symbol 33 can be determined from the received calibration reference symbol 24 by removing a portion corresponding to a cyclic shift and subtracting any other offsets in the processing chain from the cyclic shift delivery frame offset. Another instance of an offset occurs in the middle of a cyclic prefix rather than at the end of that cyclic prefix, using an RRU of a Fast Fourier Transmission Window. Time-domain samples corresponding to the cyclic shift length can be removed by time-domain processing. The cyclic shift of the delivery frame offset minus other offsets can be performed by phase ramping in frequency-domain processing. This phase ramping in frequency-domain processing can be avoided if the offset has been pre-compensated from the transmitter side.

[0065] A baseband unit (BBU) that processes a received calibration reference symbol may not be able to access the forward circuitry and / or one or more RRUs that performed the previous processing. When transmitting a calibration reference signal including one of the first and second calibration reference symbols 31 and 32, the BBU may receive a continuous phase calibration reference signal via the forward circuitry and / or one or more RRUs.

[0066] Although the embodiments disclosed herein may be discussed with reference to a cyclic prefix, any suitable principles and advantages disclosed herein may be applied to continuous calibration reference symbols with cyclic suffixes.

[0067] Figure 4A is a schematic block diagram of a system 40 for transmitting a calibration reference signal according to an embodiment. The system 40 includes a baseband unit 41, a radio frequency processing unit 42, and at least one antenna 43. The radio frequency processing unit 42 may include a remote radio unit and / or a fronthaul processing circuit system.

[0068] The baseband unit 41 can receive calibration reference information and generate symbols based on the received calibration reference information. The calibration reference information may include a reference signal index, scrambling information, similar information, or any suitable combination thereof. The illustrated baseband unit 41 includes a reference symbol generation block 44, a phase ramping block 45, and a frequency domain sampling block 46. The baseband unit 41 can perform frequency domain processing. The baseband unit 41 processes the baseband signal. The baseband unit 41 may include a digital signal processor.

[0069] Reference symbol generation block 44 can generate a first symbol for a calibration reference symbol. This first symbol can be a frequency domain symbol. The first frequency domain symbol can be represented by {A0, A1, … A4095}. The second frequency domain symbol can be represented by {B0, B1, … B4095}. The second frequency domain symbol Bk can be represented by the following equation, where… The number of time-domain samples where N represents the cyclic shift length and is the sign: against , in , .

[0070] Phase ramping block 45 applies phase ramping to the first symbol to generate a second symbol cyclically shifted relative to the first symbol. Frequency domain sampling block 46 samples the first and second symbols. Sampling may involve in-phase (I) samples and quadrature (Q) samples. Frequency domain sampling block 46 buffers the samples and sequences them for transmission to RF processing unit 42. Frequency domain sampling block 46 provides a one-symbol delay of the second symbol relative to the first symbol, such that the second symbol is the next symbol in the sequence after the first symbol. Both the first and second symbols may undergo an additional frequency domain (FD) phase ramp to pre-compensate for frame offset, eliminating the need for additional phase ramp compensation on the receiver side. Two FD phase ramps may also be combined into a single phase ramp for the second symbol.

[0071] Figure 4B is a schematic block diagram of a system 40' for transmitting a calibration reference signal according to one embodiment. System 40' can apply phase ramps to a first and second symbol to pre-compensate for frame offset, wherein two FD frame ramps are combined for the second symbol. System 40' can generate and transmit two consecutive, time-cyclic calibration reference symbols that can be processed at the receiver without phase correction. Given an initial calibration sequence, the two calibration reference symbols can be mapped to a comb spectrum, and different phase adjustments can be applied to each symbol. This phase adjustment can be applied in the time domain. The phase adjustment can be a phase ramp taking into account either the frame offset TA offset or the gap T delta. The second calibration reference symbol can be phase-shifted so that after frequency domain processing (e.g., inverse fast Fourier transform and cyclic prefix addition), the second calibration reference symbol is cyclically consecutive to the first calibration reference symbol. The two frequency domain calibration reference symbols can be provided to an RF processing unit (such as a fronthaul) where an inverse fast Fourier transform (IFFT) is applied and a cyclic prefix is ​​added.

[0072] Unlike LTE, in NR, a transmitter and a receiver may have different carrier frequencies, which can cause phase rotation problems without compensation. Different carrier frequencies can introduce different phase offsets between symbols. Phase compensation can be applied in NR to avoid phase rotation across ODFM symbols. A transmitter and a receiver can each apply phase compensation based on their own center frequency. In this way, a UE can operate without knowing the center frequency at the gNB, where a Synchronization Signal / Physical Broadcast Channel (SS / PBCH) block, Residual Minimum System Information (RMSI), or a Bandwidth Part (BWP) is transmitted relative to that gNB. At a transmitter, phase compensation can be applied to a symbol before upconversion. At a receiver, phase compensation can be applied after downconversion. The phase term used for phase compensation can be known at both the gNB and the UE. In some embodiments, gNB-side phase compensation can be applied at the fronthaul.

[0073] In the calibration reference symbol generation disclosed herein, the calibration reference symbol can be phase-compensated at either the receiving or transmitting side. This phase compensation can supplement the phase compensation value specified by the NR applied at the fronthaul. The pre-compensation can be carrier frequency specific.

[0074] Referring to Figure 4B, system 40' includes a baseband unit 41', which has a reference symbol generation block 44, phase ramping blocks 45' and 49, and a frequency domain sampling block 46. In baseband unit 41', the first phase ramping block 45' applies phase ramping to a first symbol to generate a second symbol cyclically shifted relative to the first symbol and also includes pre-compensation for frame offset. Alternatively or additionally, the first phase ramping block 45' may pre-compensate for one or more other offsets (such as a gap between the end of a transmission-to-receive handover and the boundary of the next uplink symbol). The second phase ramping block 49 applies phase ramping to the first symbol to generate a pre-compensated first symbol cyclically shifted relative to the first symbol to pre-compensate for frame offset. Alternatively or additionally, the second phase ramping block 49 may pre-compensate for one or more other offsets (such as a gap between the end of a transmission-to-receive handover and the boundary of the next uplink symbol). For each calibration symbol, a phase correction signal can be stored in the frequency domain sampling block 46. Alternatively, a common phase adjustment term can be stored and cyclic shifts for each antenna can be generated during transmission.

[0075] In some applications, an additional phase ramp compensation can be applied to pre-compensate for a phase rotation specific to one OFDM symbol as specified by a standard. For example, in an NR standard, a phase rotation is applied to each OFDM symbol for a duration of 1 millisecond. One embodiment of pre-compensation shown in Figure 4B may apply a further additional phase offset to the first symbol to pre-compensate for the phase rotation applied to both the first and second calibration symbols. In a second example, a phase ramp may be applied to the second calibration symbol to pre-compensate for the phase rotation applied to both the first and second calibration symbols.

[0076] The baseband unit 41, baseband unit 41', and other baseband units disclosed herein may include any suitable physical hardware for performing the described functions. This physical hardware may include at least one processor and memory. The illustrated blocks may be implemented using a dedicated circuit system for a specific block and / or using shared hardware programmed and / or configured to perform the functions of two or more blocks. Baseband unit 41 may be connected to radio frequency processing unit 42 via an optical fiber.

[0077] Referring to Figures 4A and 4B, the RF processing unit 42 can be an RRU or a fronthaul unit. In some examples, the RF processing unit 42 can be implemented as a combination of an RRU and a fronthaul unit. The RF processing unit 42 can process RF signals. The illustrated RF processing unit 42 includes an IFFT block 47 and a cyclic prefix block 48. The IFFT block 47 can transform the symbols received from the BBU 41 from the frequency domain to the time domain. A first time domain symbol generated by an IFFT using a first frequency domain symbol can be represented by {a0, a1, …, a4095} = IFFT{A0, A1, … A4095}. A second time domain symbol generated by an IFFT using a second frequency domain symbol can be represented by {b0, b1, …, b4095} = IFFT{B0, B1, … B4095}. Due to the frequency domain phase ramp of phase ramp block 45 or 45', the effective time domain second symbol becomes one of the first symbols cyclically shifted (to the left): {b0, b1, …, b4095} = {a288, a289, …, a4095, a0, a1, …, a287}.

[0078] Cyclic prefix block 48 can add a cyclic prefix to the time domain symbol. Cyclic shift portions 34 and 36 are examples of cyclic prefixes. The calibration reference symbol output by RF processing unit 42 can incorporate any suitable principles and advantages of calibration reference symbols 31 and 32 of FIG3. The time domain symbol can include in-phase and quadrature symbols. The RF processing unit 42 and other RF processing units disclosed herein can include any suitable physical hardware for performing the described functions. The illustrated blocks can be implemented using a dedicated circuit system for a specific block and / or shared hardware configured to perform the functionality of two or more blocks.

[0079] Antenna 43 can wirelessly transmit a calibration reference symbol, including one of the continuous calibration reference symbols output by radio frequency processing unit 42. In some examples, multiple antennas can transmit calibration reference signals.

[0080] As shown in Figure 3, a received calibration reference symbol 24 can be received between two consecutively transmitted calibration reference symbols 31 and 32. Depending on the TDD frame offset, the receiver can observe a calibration reference symbol undergoing a different cyclic shift. In the example shown in Figure 2B, an uplink symbol is received at TA offset = 1600 time-domain samples before a downlink symbol. Therefore, the received calibration reference symbol can be represented by {a2495, a2496, …, a2783, a2784, a2785, …, a4095, a0, …, a2782, a2783}, where the cyclic prefix sensed by the receiver is {a2495, a2496, …, a2783} and the OFDM symbol sensed by the receiver is {a2784, a2785, …, a4095, a0, …, a2782, a2783}. The received calibration reference symbol 24 is cyclically shifted to the left by the difference between the UL / DL frame offset and the cyclic shift length, which is then transmitted as the first calibration reference symbol 31. In this example, the frame offset is 1600 and the cyclic shift length is 288. Therefore, in this example, the cyclic shift of the received calibration reference symbol 24 is 1600 – 288 = 1312.

[0081] Figure 5 is a schematic block diagram of a system 50 for processing a received calibration reference signal according to one embodiment. System 50 includes at least one antenna 51, a radio frequency (RF) processing unit 52, and a baseband unit 53. The RF processing unit 52 may include a remote radio unit and / or a fronthaul processing circuitry. In some applications, the baseband unit 53 may include any suitable combination of the features of the baseband unit 41. In some applications, the RF processing unit 52 may include any suitable combination of the features of the RF processing unit 42.

[0082] Antenna 51 can receive a transmitted calibration reference symbol. In some examples, multiple antennas can receive the calibration reference symbol. As discussed above, a received calibration reference symbol can be a cyclically shifted version of a calibration reference signal symbol, representing an amount of cyclic shift frame offset. In the examples of Figures 2 and 3, the cyclic shift compensation or pre-compensation is to the left. In some other applications, the cyclic shift can be in the opposite direction.

[0083] System 50 can apply phase ramp compensation to compensate for time-domain cyclic shifts on the receiver side. In certain examples, time-domain cyclic shifts at the transmission wavelengths on each transmit antenna can be pre-compensated, and receiver-side compensation may not be required.

[0084] The RF processing unit 52 can be an RRU or a fronthaul unit. In some examples, the RF processing unit 52 can be implemented as a combination of an RRU and a fronthaul unit. The RF processing unit 52 can process time-domain samples in the RF domain. The illustrated RF processing unit 52 includes a cyclic prefix removal block 54 and a Fast Fourier Transform (FFT) block 55. The cyclic prefix removal block 54 removes the initial time-domain samples of the received calibration reference symbols corresponding to the cyclic shift length. The FFT block 55 transforms the symbols output by the cyclic prefix removal block from the time domain to the frequency domain.

[0085] The baseband unit 53 may include a phase ramp block 56 and a channel estimation block 57. The phase ramp block 56 may take into account frame offset and any other DL / UL shift values. In embodiments using pre-compensation, this phase ramp can be avoided. For example, in the examples illustrated in Figures 2A, 2B, and 3, the phase ramp block 56 may compensate for cyclic shift in the received calibration reference signal by applying a phase ramp corresponding to a frame offset minus the cyclic prefix length. As another example, in an application where downlink symbols are received before uplink symbols by a frame offset, the phase ramp block 56 may compensate for cyclic shift in the received calibration reference signal by applying a phase ramp corresponding to a frame offset and any other DL / UL shift values. The channel estimation block 57 may process the output of the phase ramp block 56 and generate a channel estimate and / or other calibration data. The baseband unit 53 may calculate calibration coefficients from one or more antennas including antenna 51 based on the processed received reference symbols. The calibration coefficients, once calculated, can be based on the channel estimates generated by channel estimation block 57.

[0086] Figure 6 is a schematic block diagram of a system 60 for processing a received calibration reference signal according to one embodiment. In practice, relatively long offsets can pose technical challenges to channel estimation. For example, with a frame offset of 1600 time-domain samples, the cyclic shift attributable to this frame offset can be greater than 1 / 4 of the length of an OFDM symbol. Since the SRS time-domain window may be adversely affected, this level of frame offset may not be entirely transparent to UE processing. Therefore, prior information about the offset may be desired. In some applications, instead of phase ramp compensation, prior information about an expected cyclic shift at the receiver side can be used in a channel estimation stage to adjust the position of the time-domain channel estimation window for each antenna to extract an accurate channel estimate for each antenna. A baseband unit 63 includes a channel estimation block 67. The channel estimation block 67 can adjust a time-domain position based on prior information about the expected cyclic shift at the receiver side to extract a channel estimate for each antenna. The baseband unit 63 can calculate calibration coefficients from one or more of the included antennas 51 based on the processed received reference symbols. The calculated calibration coefficients can be based on the channel estimate generated by the channel estimation block 67.

[0087] In applications using Weighted Overlap and Addition (WOLA) transmission, calibration reference signals can be implemented. Figure 7 illustrates a timing diagram of a calibration reference signal with WOLA roll-off applied to both sides of an OFDM symbol according to one embodiment. The WOLA roll-off reduces the Adjacent Channel Leakage Ratio (ACLR). Figure 7 illustrates calibration reference symbols 71 and 72. These symbols are the same as symbols 31 and 32, except that WOLA roll-off is applied at the beginning and end of each of calibration reference symbols 71 and 72. The calibration reference symbols disclosed herein are robustly transmitted with WOLA values ​​of appropriate normalized weights. The overlapping portion of time-domain samples of consecutive calibration reference symbols can have weighting coefficients that sum to 1.

[0088] The timing diagram in Figure 2B indicates that a calibration reference procedure can occur within 5 symbols of time-domain samples. Even with full control of an RRU and fronthaul timing case, at least four symbols can be used for a calibration reference procedure. A minimum amount of time for calibration corresponds to 1 frame offset (FO) + 1 transmit-to-receive transition (TR) + 1 symbol + 1 TR + 1 Symb + 1 TR. This is equivalent to FO + 3 TR + 2 symbols. In the example in Figure 2B, 1 symbol = 4096 + 288 time-domain samples, 1 TR > 1200 or 1600 time-domain samples (3 TRs = 3600~4800), and 1 FO = 1600 time-domain samples. In this example, 1 FO + 3 TR totals approximately 5200~6400 time-domain samples. At least 4 symbols are involved in a calibration reference procedure. Therefore, even assuming complete control over the transmission and reception timing on the RRU side, only one additional symbol can be saved. In the five symbols in Figure 2B, 1 can be considered as an additional cost to account for the differences in transmission and reception timing on the RRU side.

[0089] Figure 8 is a timing diagram illustrating a timing sequence for transmitting and receiving calibration reference signals according to one embodiment. In this embodiment, a single switch between transmission and reception may exist in a time slot used for calibration. Antennas of a first group G0 and antennas of a second group G1 may use the same or substantially the same transmission / reception switching waveform. In a special time slot 81 or 82, only one switch from DL to UL may exist. The first group G0 and the second group G1 may alternate a DL / UL pattern to transmit and receive calibration reference signals in the special time slot. In special time slot 81, the first group G0 transmits calibration reference signals and the second group G1 receives calibration reference signals. Then, in special time slot 82, the first group G0 receives calibration reference signals and the second group G1 transmits calibration reference signals. In this embodiment, two additional symbols after the last downlink time slot before a special time slot are sufficient for calibration. By transmitting and receiving calibration reference signals in special time slots with the timing shown in Figure 8, there is no additional transmission-to-reception or receive-to-transmission transition specifically for calibration. For each group, there is only one handover transition. This handover transition will be implemented regardless of whether a downlink transitions from before a specific time slot to an uplink transitions after that specific time slot.

[0090] A calibration reference signal can also be transmitted at the end of a DL portion of a time slot. This calibration reference signal can be reused in conjunction with a Channel Status Information Reference Signal (CSI-RS). Group G0 can transmit CSI-RS on a specific time slot (e.g., specific time slot 81). Group G1 can transmit CSI-RS on alternating specific time slots (e.g., specific time slot 82). RRUs associated with G0 / G1 can use CSI-RS for calibration. One or more User Equipment (UEs) can use CSI-RS for CSI reporting.

[0091] The calibration reference signal disclosed herein can be any suitable reference signal. For example, the calibration reference signal disclosed herein can be implemented based on a sounding reference signal (SRS), a CSI-RS, a universal linear frequency modulated pulse (chirp) sequence, the like, or any suitable combination thereof. A calibration reference symbol can be generated based on a Gold sequence, a Zadoff-Chu sequence, a UL SRS sequence, a DL CSI-RS sequence, or a DL demodulation reference signal (DMRS) sequence. One resource of a calibration reference signal can be communicated and / or configured to the UE so that they can use the same CSI RS sequence for DL ​​CSI processing.

[0092] Although some embodiments are described with reference to the use of full OFDM symbols for calibration, any suitable principles and advantages disclosed herein can be applied to applications in which a portion of OFDM symbols (e.g., half symbols) are used for calibration. For example, half symbols can be used to transmit a calibration reference symbol. At a receiver side, a BBU side can receive frequency domain samples, perform an IFFT back to the time domain, extract a corresponding calibration reference signal, and perform an FFT back to the frequency domain to obtain a channel estimate for a calibration reference signal.

[0093] A network system can be configured to generate, transmit, receive, and / or process calibration reference signals based on any suitable principles and advantages disclosed herein. The network system can use channel estimation determined based on the calibration reference signals to calibrate RRU antennas for UL / DL reciprocal channel estimation used in wireless communication. The network system can exchange TDD MIMO information with the UE. Figure 9 illustrates an example network system. This network system can operate in any suitable network environment (such as network environment 230 in Figure 12 and / or any suitable network environment). The network system may include any suitable RRU, fronthaul circuitry, and / or BBU disclosed herein.

[0094] Figure 9 is a block diagram illustrating an exemplary network system 900 comprising a baseband unit 902 and a remote radio unit 920 according to an embodiment. The network system 900 of Figure 9 can generate, transmit, receive, and process reference signals according to any suitable principles and advantages disclosed herein. The baseband unit 902 may be coupled to at least one remote radio unit 920. The baseband unit 902 is an example of a processing unit that can determine, generate, and / or process calibration reference signals according to any suitable principles and advantages disclosed herein. As illustrated, the baseband unit 902 may be coupled to a plurality of remote radio units 920. These remote radio units 920 may be distributed. The remote radio units 920 and / or fronthaul circuitry can perform the radio frequency processing disclosed herein.

[0095] A remote radio unit 920 may include one or more antennas (such as at least one first antenna 932 and one second antenna 934) for wireless communication. For example, the wireless communication may be MIMO wireless communication. A remote radio unit may include any suitable number of antennas and / or antenna arrays. Antennas 932 and 934 of RRU 920 are coupled to a transceiver 924. Transceiver 924 may perform any suitable features described with reference to the radio frequency processing unit disclosed herein. Transceiver 924 includes a receiver and a transmitter. The receiver may process signals received via antennas 932 and / or 934. The receiver may include a block of radio frequency processing unit 52 of Figures 5 and 6. Transceiver 924 may provide the processed signal to one RRU interface 918 included in BBU 902. Transceiver 924 may include any suitable number of receive paths. The transmitter may process signals received from BBU 902 for transmission via antennas 932 and / or 934. The transmitter of transceiver 924 can provide signals to antennas 932 and / or 934 for transmission. The transmitter may include blocks of the radio frequency processing unit 42 of Figures 4A and / or 4B. Transceiver 924 may include any suitable number of transmission paths. Transceiver 924 may include different transmission and reception paths for each of antennas 932 and 934.

[0096] As illustrated, BBU 902 includes a processor 904, a calibration block 906, a channel estimator 908, a data storage 914, a beamformer 916, an RRU interface 918, and a bus 919. Bus 919 couples several components of BBU 902. Data can be transferred between components of BBU 902 via bus 919.

[0097] Processor 904 may include any suitable physical hardware configured to perform the functionality described herein. Processor 904 may manage communication between network system 900 and UE and / or network nodes. For example, processor 904 may schedule traffic and cause control information to be sent to the UE. Processor 904 may include a processor configured with specific executable instructions, a microprocessor, a microcontroller, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a programmable logic device (such as a field-programmable gate array (FPGA)), the like, or any combination thereof designed to perform the functions described herein. In some applications, processor 904 may be implemented by any suitable combination of computing devices and / or discrete processing circuitry.

[0098] Calibration block 906 can generate reference signals and process received reference signals to calculate calibration coefficients. For example, calibration block 906 can apply phase ramping to generate a second reference symbol based on a first reference symbol. As another example, calibration block 906 can process a received reference symbol to take into account frame offset and cycle shift length. Calibration block 906 can coordinate the transmission and / or reception of reference signals for calibration. These reference signals can be used to estimate the UL and DL channels between RRUs (transmitted from antenna group G0 to antenna group G1 and received by antenna group G1, and vice versa) and to derive calibration coefficients to facilitate reciprocal channel estimation and beamforming. Therefore, calibration block 906 can generate a channel estimate of calibration reference signals transmitted from antenna group G0 to group G1 and also from antenna group G1 to antenna group G0. Then, calibration block 906 can use these channel estimates to derive calibration coefficients for all RRU antennas for reciprocal beamforming purposes. Calibration block 906 can determine the timing of the timing slots used for the transmission and reception of the reference signal based on any suitable principles and advantages of the timing diagrams disclosed herein. Calibration block 906 can be implemented by a dedicated circuit system and / or by the circuit system of processor 904.

[0099] Channel estimator 908 can generate channel estimates based on a processed calibration reference signal. Channel estimator 908 can generate channel estimates for various links in a wireless communication environment. Channel estimator 908 can be implemented using dedicated circuitry and / or the circuitry of processor 904. In some examples, channel estimator 908 may include circuitry for channel estimation of SRS and / or CSI-RS. Channel estimator 908 can generate any suitable calibration data from the processed calibration reference signal.

[0100] As illustrated, processor 904 communicates with data storage 914. Data storage 914 may store instructions that can be executed by one or more of processor 904, calibration block 906, or channel estimator 908 to implement any suitable combination of the features described herein. Data storage 914 may hold information associated with one or more of the antenna, network conditions, network traffic information, channel estimation, or the like, to which calibration is desired. Data storage 904 may store any other suitable data for baseband unit 902.

[0101] Beamformer 916 generates parameters for nodes serving the UE. These parameters may include one or more of the following: transmission mode, time, frequency, power, beamforming matrix, frequency modulation allocation, or channel ranking. Beamformer 916 determines optimal parameters for network-wide optimization to facilitate downlink data transmission for the RRU 920 coupled to the BBU 902. Similar functionality for receiving uplink data transmission can be implemented. Beamformer 916 is an example of an advanced precoding block that enhances wireless communication in a TDD MIMO network. Beamformer 916 can apply calibration coefficients generated from any suitable calibration data produced by a received calibration reference signal disclosed herein.

[0102] The illustrated processor 904 communicates with the RRU interface 918. The RRU interface 918 can be any suitable interface for providing signals to and receiving signals from an RRU 920. As an example, the RRU interface 918 can be a common public radio interface.

[0103] Figure 10 is a flowchart of an exemplary method 100 for transmitting a calibration reference signal according to an embodiment. Method 100 can be performed by any suitable TDD MIMO system disclosed herein. The calibration reference signal may include OFDM symbols. Any suitable principles and advantages associated with generating the calibration reference signal disclosed herein may be implemented in method 100. Method 100 can be performed by any suitable hardware (such as the systems of Figures 4 and / or 9). The operation of any of the methods disclosed herein can be performed appropriately in any suitable order.

[0104] In block 102, a first reference symbol is generated, comprising a symbol and a cyclically shifted portion of that symbol. The cyclically shifted portion has a cyclic shift length. The cyclically shifted portion may be a cyclic prefix. Generating the first reference symbol involves adding a cyclically shifted portion in a time domain. Examples of the first reference symbol include the first calibration reference symbol 31 of FIG3 and the first calibration reference symbol 71 of FIG7. WOLA may be applied when generating the first reference symbol.

[0105] In block 104, a second reference symbol is generated. The second reference symbol is a cyclically shifted version of the first calibration symbol relative to the first reference symbol by a cyclic shift length. Generating the second reference symbol may include applying a phase ramp to at least the symbol of the first reference symbol. Examples of the second reference symbol include the second calibration reference symbol 32 of FIG3 and the second calibration reference symbol 72 of FIG7.

[0106] In certain examples, phase ramping may be applied to the first and second reference symbols to pre-compensate for at least one frame offset and / or any additional DL / UL offset between the uplink and downlink symbols.

[0107] In block 106, the first and second reference symbols are transmitted continuously. This transmission may occur during a specific time slot. The first and second reference symbols are transmitted wirelessly via at least one antenna.

[0108] Figure 11 is a flowchart of an exemplary method 110 for processing a received calibration reference signal according to an embodiment. Method 110 can be performed by any suitable TDD MIMO system disclosed herein. The calibration reference signal can be transmitted by method 100. The calibration reference symbol may include an OFDM symbol. Any suitable principles and advantages associated with processing the calibration reference signal disclosed herein can be implemented in method 110. Method 110 can be performed by any suitable hardware (such as any of the systems of Figures 5, 6, or 9). The operation of any of the methods disclosed herein can be performed appropriately in any suitable order.

[0109] In block 112, a reference symbol is received via at least one antenna. The reference symbol includes a portion of a first transmitted reference symbol and a portion of a second transmitted reference symbol. The first transmitted reference symbol includes a symbol and a portion of that symbol that has been cyclically shifted by a cyclic shift length. The second transmitted reference symbol is a cyclically shifted version of the first transmitted reference symbol relative to the first transmitted reference symbol by the cyclic shift length.

[0110] In block 114, a cyclically shifted portion of the received reference signal can be removed. This may involve removing a time-domain sample of the reference signal corresponding to the cyclic shift length. Time-domain processing can be performed in an RRU and / or fronthaul circuitry system.

[0111] In block 116, reference symbols are processed in a frequency domain to take into account a frame offset and cyclic shift length between uplink and downlink symbols. Frequency domain processing may include applying a phase ramp. Frequency domain processing may include using prior information to take into account frame offset and / or any other DL / UL shift values. Following the processing in block 116, at least one channel estimate and / or other calibration data (e.g., coefficients) may be generated.

[0112] Figure 12 is a diagram illustrating one example of a multiple-input multiple-output (MIMO) network environment 230 in which a reference signal can be wirelessly transmitted. Various UEs can wirelessly communicate with one of the network systems in the MIMO network environment 230. Such wireless communication can achieve high throughput. The antennas of the MIMO network environment 230 used for wireless communication with the UEs can be distributed. Channel estimation for channels between different nodes can be performed in the MIMO network environment 230 based on a reference signal according to any suitable principles and advantages disclosed herein.

[0113] Various standards and / or protocols can be implemented in the MIMO network environment 230 to wirelessly transmit data between a base station and a wireless communication device. Some wireless devices may use an Orthogonal Frequency Division Multiplexing (OFDM) digital modulation scheme to communicate via a physical layer. Typical standards and protocols for wireless communication in environment 230 may include: 3GPP Long Term Evolution (LTE), Advanced Long Term Evolution (Advanced LTE), 3GPP New Radio (NR), also known as 5G, Global System for Mobile Communications (GSM), GSM Evolution Enhanced Data Rate (EDGE), WiMAX, and the IEEE 802.11 standard (which may be referred to as Wi-Fi). In some systems, a Radio Access Network (RAN) may include one or more base stations associated with one or more Evolved Node Bs (also typically referred to as Enhanced Node Bs, eNodeBs, or eNBs), gNBs, or any other suitable Node B (xNBs). In some other embodiments, a Radio Network Controller (RNC) may be provided as a base station. A base station provides a bridge between a wireless network and a core network (such as the Internet). A base station may be included to facilitate data exchange between wireless communication devices used in the wireless network. A base station may perform reference signal channel estimation based on any suitable principles and advantages disclosed herein.

[0114] A wireless communication device may be referred to as a User Equipment (UE). The UE may be a device used by a user, such as a smartphone, a laptop, a tablet, a cellular phone, a wearable computing device (such as smart glasses, a smartwatch, or a headset), one or more network-connected devices (e.g., consumer network-connected devices or industrial plant equipment), an industrial robot with connectivity, or a vehicle. In some embodiments, the UE may include a sensor or other network-connected devices configured to collect data and wirelessly provide the data to a device (e.g., a server) connected to a core network (such as the Internet). Such devices may be referred to as Internet of Things (IoT) devices. A downlink (DL) transmission typically refers to communication from a base transceiver station (BTS) or eNodeB to a UE. An uplink (UL) transmission typically refers to communication from the UE to a BTS.

[0115] Figure 12 illustrates a collaborative or cloud-based radio access network (C-RAN) environment 230. In environment 230, eNodeB functionality is subdivided between a baseband unit (BBU) 240 and multiple remote radio units (RRUs) (e.g., RRU 255, RRU 265, and RRU 275). The network system of Figure 12 includes BBU 240 and RRUs 255, 265, and 275. An RRU may include multiple antennas, and one or more of these antennas may be used as a transmit-receive point (TRP). An RRU and / or a TRP may be referred to as a servo node. BBU 240 may be physically connected to an RRU, such as via a fiber optic connection. BBU 240 can provide operational information to an RRU to control the transmission and reception of signals from the RRU, along with control data and payload data for transmission. The RRU can provide data received from UEs within a service area associated with the RRU to the network. As shown in Figure 12, RRU 255 provides services to devices within a service area 250. RRU 265 provides services to devices within a service area 260. RRU 275 provides services to devices within a service area 270. For example, wireless downlink transmission services can be provided to service area 270 to transmit data to one or more devices within service area 270.

[0116] In environment 230, a network system can wirelessly communicate with a UE via distributed MIMO. For example, UE 283 can wirelessly transmit MIMO data using the network system's antennas (including at least one antenna of RRU 255, at least one antenna of RRU 265, and at least one antenna of RRU 275). As another example, UE 282 can wirelessly transmit MIMO data using a distributed antenna including at least one antenna of RRU 255 and at least one antenna of RRU 265. As yet another example, UE 288 can wirelessly transmit MIMO data using a distributed antenna including at least one antenna of RRU 255 and at least one antenna of RRU 275. For example, any suitable principles and advantages of the reference signal channel estimation disclosed herein can be implemented in such distributed MIMO applications.

[0117] The illustrated RRUs 255, 265, and 275 include multiple antennas and can provide MIMO communication. For example, an RRU may be equipped with various numbers of transmit antennas (e.g., 2, 4, 8, or more) that can be used simultaneously for transmission to one or more receivers (such as a UE). A receiving device may include more than one receive antenna (e.g., 2, 4, etc.). A receive antenna array can be configured to simultaneously receive transmissions from the RRU. Each antenna included in an RRU can be individually configured to transmit and / or receive according to a specific time, frequency, power, and direction. Similarly, each antenna included in a UE can be individually configured to transmit and / or receive according to a specific time, frequency, power, and direction. Configuration can be provided by BBU 240.

[0118] The service area shown in Figure 12 can provide communication services to user devices of heterogeneous groups. For example, service area 250 may include a cluster of UEs 290, such as a group of devices associated with users participating in a large event. Service area 250 may also include an additional UE 292 located away from UE cluster 290. A mobile user device 294 may move from service area 260 to service area 270. Another example of a mobile user device is a vehicle 286, which may include a transceiver for wireless communications for real-time navigation, in-vehicle data services (e.g., streaming video or audio), or other data applications. Environment 230 may include semi-mobile or stationary UEs configured for wireless communications, such as a robotic device 288 (e.g., a robotic arm, an autonomous driving unit, or other industrial or commercial robot) or a television set 284.

[0119] A user device 282 may be located within one of the areas having overlapping services (e.g., service area 250 and service area 260). Devices in environment 230 may have different performance requirements, and in some instances, these performance requirements may conflict with the requirements of other devices.

[0120] Channel estimation (such as channel estimation between a UE and an RRU) performed in network environment 230 using a reference signal based on any suitable principles and advantages disclosed herein can implement any suitable principles and advantages of the calibration reference signal disclosed herein. An accurate estimation of a wireless communication channel based on a calibration reference signal can be used for calibration and / or for precoding.

[0121] Depending on the implementation, specific actions, events, or functions of any of the programs or algorithms described herein may be performed in a different sequence, and may be added, combined, or omitted entirely (e.g., not all described operations or events are necessary for the practice of the program or algorithm). Furthermore, in some embodiments, operations or events may be performed simultaneously, for example, through multithreading, interrupt handling, or multiple processors or processor cores, or in other parallel architectures (rather than sequentially).

[0122] Unless otherwise specifically stated or understood in the context of its application, the conditional language used herein (such as in particular "can," "could," "might," "may," "for example," "like," and the like) is generally intended to convey that certain embodiments include, while other embodiments do not include, specific features, elements, and / or operations. Therefore, this conditional language is not generally intended to imply that one or more embodiments require features, elements, and / or operations in any way, or that one or more embodiments necessarily include logic for decision-making (with or without additional input or prompts), regardless of whether such features, elements, and / or steps are included in or will be performed in any particular embodiment. The terms "comprising," "including," and the like are synonymous and used inclusively in an open-ended manner, and do not exclude additional elements, features, actions, operations, etc. Furthermore, the terms "in this document," "above," "below," and similar terms, when used in this application, should refer to the entire application and not any specific part thereof. Where the background allows, the use of singular or plural terms in the detailed description of certain embodiments above may also include either the plural or the singular, respectively. Also, the term "or" is used in its inclusive sense (and not in its exclusive sense) such that, when used, (for example) to connect a list of elements, the term "or" means one, some, or all of the elements in that list.

[0123] Unless otherwise specifically stated, transitional language such as the phrase "at least one of X, Y, Z" should be understood in the context of how an item, term, etc., can be X, Y, or Z or any combination thereof (e.g., X, Y, and / or Z). Therefore, this transitional language is generally not intended and should not imply that certain embodiments require the presence of at least one of X, at least one of Y, or at least one of Z.

[0124] Unless otherwise explicitly stated or understood from the context, articles such as "a" or "an" should generally be interpreted as including one or more of the described items. Therefore, phrases such as "configured to..." are intended to include one or more of the referred devices. These one or more referred devices may also be configured together to carry out the stated description. For example, "configured to carry out descriptions A, B, and C" could include a first processor configured to carry out description A working in conjunction with a second processor configured to carry out descriptions B and C.

[0125] As used generally herein, the term "coupled" refers to two or more elements that can be directly coupled to each other or coupled via one or more intermediate elements. Similarly, as used generally herein, the term "connected" refers to two or more elements that can be directly connected or connected via one or more intermediate elements. A connection may be made via an empty interlayer and / or via a wire and / or via an optical fiber and / or via any other suitable connection.

[0126] As used herein, the term "determine" encompasses a wide variety of actions. For example, "determining" can include calculating, operating, processing, deriving, generating, obtaining, searching (e.g., searching a table, a database, or another data structure), determining, and the like via a hardware component without user intervention. Furthermore, "determining" can include receiving (e.g., receiving information), accessing (e.g., accessing data in memory), and the like via a hardware component without user intervention. Moreover, "determining" can include parsing, selecting, choosing, constructing, and the like via a hardware component without user intervention.

[0127] While the above detailed description has shown, described, and pointed out novel features applicable to various embodiments, it is understood that various omissions, substitutions, and changes may be made to the form and details of the illustrated apparatus or algorithm without departing from the spirit of the invention. For example, circuit blocks and / or method blocks described herein may be deleted, moved, added, subdivided, combined, arranged in a different order, and / or modified. These blocks may be implemented in various different ways. Any part of any of the methods disclosed herein may be performed in connection with specific instructions stored on a non-transitory computer-readable storage medium and executed by one or more processors. It will be appreciated that some embodiments described herein may be embodied in a form that does not provide all the features and benefits set forth herein, as some features may be used or practiced separately from other features. The scope of certain embodiments disclosed herein is indicated by the appended claims rather than by the foregoing description. All changes derived from the equivalent meanings and scope of the claims will be covered within their scope.

[0128] 22: Calibration Reference Symbol 24: Calibration reference symbol / Received calibration reference symbol / Calibration reference signal 31: First calibration reference symbol / calibration reference symbol 32: Second Calibration Reference Symbol / Calibration Reference Symbol 33: Symbols / Orthogonal Frequency Division Multiplexing (OFDM) Symbols 34: Part of Circular Shift 35: Symbol 36: Part of Circular Shift 40: System 40': System 41: Baseband Unit (BBU) 41': Baseband Unit (BBU) 42: Radio Frequency Processing Unit 43: Antenna 44: Reference symbol generation block 45: Phase Rise Block 45': Phase ramp block / First phase ramp block 46: Frequency Domain Sampling Block 47: Inverse Fast Fourier Transform (IFFT) Block 48: Cyclic prefix block 49: Phase ramp block / Second phase ramp block 50: System 51: Antenna 52: Radio Frequency Processing Unit 53: Baseband Unit (BBU) 54: Remove blocks with cyclic prefixes 55: Fast Fourier Transform (FFT) Block 56: Phase Rise Block 57: Channel Estimated Block 60: System 63: Baseband Unit (BBU) 67: Channel Estimated Block 71: Calibration Reference Symbol / First Calibration Reference Symbol 72: Calibration Reference Symbol / Second Calibration Reference Symbol 81: Special Time Slot 82: Special Time Slot 100: Method 102: Square 104: Square 106: Square 110: Method 112: Square 114: Square 116: Square 230: Network environment / Multiple-input multiple-output (MIMO) network environment / Environment / Collaborative or cloud-based radio access network (C-RAN) environment 240: Baseband Unit (BBU) 250: Service Area 255: Remote Radio Unit (RRU) 260: Service Area 265: Remote Radio Unit (RRU) 270: Service Area 275: Remote Radio Unit (RRU) 282: User Equipment (UE) 283: User Equipment (UE) 284: Television 288: User Equipment (UE) / Robotic Device 290: User Equipment (UE) Cluster 292: User Equipment (UE) 294: Mobile User Equipment 900: Network System 902: Baseband Unit (BBU) 904: Processor 906: Calibration Block 908: Channel Estimator 914: Data Storage 916: Beamformer 918: Remote Radio Unit (RRU) Interface 919: Busbar 920: Remote Radio Unit (RRU) 932: First Antenna / Antenna 934: Second antenna / antenna

Claims

1. A method for transmitting reference symbols, the method comprising: Generate a first reference symbol comprising a symbol and a cyclically shifted portion of the symbol, the cyclically shifted portion having a cyclic shift length; generate a second reference symbol comprising a cyclically shifted version of the first reference symbol relative to the first reference symbol by the cyclic shift length; and continuously transmit the first and second reference symbols via at least one antenna, wherein a frame offset between an uplink symbol and a downlink symbol is greater than the cyclic shift length.

2. The method of request item 1, wherein the cyclic shift portion is a cyclic prefix, and in the first reference symbol, the symbol follows the cyclic prefix.

3. The method of claim 1, further comprising receiving and processing by a node a reference symbol comprising a portion of the first reference symbol and a portion of the second reference symbol, wherein the reference symbol is the only portion of the first reference symbol and the second reference symbol received and processed by the node.

4. The method of claim 1, wherein generating the second reference symbol includes applying a frequency domain (FD) phase ramp to the symbol of the first reference symbol at least once.

5. The method of request 1, wherein generating the first reference symbol includes adding the cyclic shift in a time domain.

6. The method of request 1, wherein generating the first reference symbol includes applying weighted overlap and adding roll-off.

7. The method of claim 1 further includes applying phase ramping to the first and second reference symbols to pre-compensate for at least one frame offset between the uplink symbol and the downlink symbol.

8. As in request item 7, wherein the applied phase ramp is also pre-compensated for an additional offset.

9. The method of claim 7, wherein the applied phase ramp is also pre-compensated for a phase rotation specified in a new radio standard.

10. The method of request item 1, wherein the transmission includes a time-domain duplex (TDD) transmission.

11. The method of claim 1, wherein the first and second reference symbols are orthogonal frequency division multiplexing (OFDM) symbols.

12. The method of claim 1, wherein the first and second reference symbols comprise at least one of a discrete Fourier transform extended orthogonal frequency division multiple access symbol, an interleaved frequency division multiple access symbol, or other time-domain waveform symbols.

13. The method of claim 1, wherein the first reference symbol is generated based on at least one of the Gold sequence or the Zadoff-Chu sequence.

14. A system for transmitting reference symbols, the system comprising: At least one antenna; One baseband unit; and an RF processing unit communicating with the baseband unit; wherein the baseband unit and the RF processing unit are configured together to: generate a first reference symbol comprising a symbol and a cyclically shifted portion of the symbol, the cyclically shifted portion having a cyclic shift length; generate a second reference symbol comprising a cyclically shifted version of the first reference symbol relative to the first reference symbol by the cyclic shift length; and cause the first and second reference symbols to be continuously transmitted from the at least one antenna, wherein a frame offset between an uplink symbol and a downlink symbol is greater than the cyclic shift length.

15. The system of claim 14, wherein the radio frequency processing unit includes a remote radio unit.

16. The system of claim 14, wherein the radio frequency processing unit includes a fronthaul circuit system.

17. The system of claim 14 further includes receiving and processing, by a node, a reference symbol comprising a portion of the first reference symbol and a portion of the second reference symbol, wherein the reference symbol is the only portion of the first reference symbol and the second reference symbol received and processed by the node.

18. The system of claim 14, wherein the first and second reference symbols are transmitted as part of a time-domain duplex (TDD) transmission.

19. The system of claim 14, wherein the baseband unit includes a phase ramp block configured to apply phase ramp to the symbol, and wherein the second reference symbol is generated based on an output signal from the phase ramp block.

20. The system of claim 14, wherein the radio frequency processing unit is configured to perform an inverse fast Fourier transform on a frequency domain version of the symbol provided by the baseband unit and to add the cyclically shifted portion to the first reference symbol.

21. A method for processing reference symbols, the method comprising: Receiving a reference symbol from at least one antenna, the reference symbol comprising a portion of a first transmitted reference symbol and a portion of a second transmitted reference symbol, wherein the first transmitted reference symbol comprises a symbol and a cyclically shifted portion of the symbol having a cyclic shift length, and wherein the second transmitted reference symbol comprises a cyclically shifted version of the first transmitted reference symbol relative to the first transmitted reference symbol by the cyclic shift length; and processing the reference symbol, wherein processing the reference symbol includes taking into account a frame offset between an uplink symbol and a downlink symbol, and wherein the frame offset is greater than the cyclic shift length.

22. The method of request 21, wherein processing the reference symbol includes taking into account another timing offset between downlink transmission and uplink reception.

23. The method of claim 21, wherein the reference symbol is pre-compensated from a transmitter side.

24. The method of request item 21, wherein the reference symbol is generated based on a downlink channel status information reference signal sequence.

25. The method of request item 24, wherein the same channel status information reference signal sequence is used to generate the reference symbol and for downlink channel status information processing.

26. The method of claim 21 further includes generating at least one channel estimate based on the processing.

27. The method of claim 21 further includes generating antenna calibration coefficients based on the processing.

28. The method of claim 21, wherein the processing includes applying a phase ramp in a frequency domain.

29. The method of claim 21, wherein the processing includes using prior information to take into account a frame offset and / or any other timing offset between uplink reception and downlink transmission.

30. The method of claim 21, wherein the process includes removing the time-domain sample of the reference symbol corresponding to the cyclic shift length.

31. The method of request item 21, wherein the processing includes cyclically shifting samples in a time domain.

32. The method of claim 21, wherein at least one remote radio unit performs at least a portion of the processing.

33. The method of claim 21, wherein the fronthaul circuit system performs at least a portion of the processing.

34. The method of claim 21, wherein the reference symbol is a unique portion of the first transmitted reference symbol and the second transmitted reference symbol received by the at least one antenna and processed in the process.

35. The method of claim 21, wherein the reception includes a time-domain duplex (TDD) reception.

36. The method of claim 21, wherein the first and second transmitted reference symbols are orthogonal frequency division multiplexing (OFDM) symbols.

37. A system for processing reference symbols, the system comprising: At least one antenna; One baseband unit; and an RF processing unit communicating with the baseband unit; wherein the baseband unit and the RF processing unit are configured together to: receive a reference symbol from the at least one antenna, the reference symbol including a portion of a first transmitted reference symbol and a portion of a second transmitted reference symbol, wherein the first transmitted reference symbol includes a symbol and a cyclically shifted portion of the symbol having a cyclic shift length, and wherein the second transmitted reference symbol includes a cyclically shifted version of the first reference symbol relative to the first transmitted reference symbol by the cyclic shift length; and process the reference symbol to take into account (i) a frame offset between an uplink symbol and a downlink symbol and (ii) another timing offset between downlink transmission and uplink reception, wherein processing the reference symbol includes taking into account a frame offset between an uplink symbol and a downlink symbol, and wherein the frame offset is greater than the cyclic shift length.

38. The system of claim 37, wherein the radio frequency processing unit is configured to remove time-domain samples corresponding to the cyclic shift length from the reference symbol.

39. The system of claim 37, wherein the baseband unit is configured to generate at least one channel estimate based on the processing of the reference symbol.

40. The system of claim 37, wherein the reference symbol is a unique portion of the first transmitted reference symbol and the second transmitted reference symbol received by the at least one antenna and processed in the process.

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