Method of controlling TDD Tx/Rx switching timing in cloud radio access network

Abstract

Provided is a method for controlling a time division duplexing (TDD) Tx/Rx switching timing in a cloud radio access network (CRAN) that can finely control a switching timing between transmission and reception of TDD signals with an additional component added to a digital unit (DU) and a radio unit (RU) in the CRAN.

Claims

1. A method of controlling a time division duplexing (TDD) Transmit/Receive (Tx/Rx) switching timing in a cloud radio access network (CRAN) in which at least one radio unit (RU) including a first RU frame timer for generating a first type of frame at the RU and a second RU frame timer for generating a second type of frame at the RU is connected with a digital unit (DU) having a first DU frame timer for generating the first type of frame at the DU and a second DU frame timer for generating the second type of frame at the DU, the method comprising: determining, by the at least one RU, a boundary of a wireless frame signal transmitted from the DU to synchronize the first RU frame timer with the first DU frame timer, the DU being connected with the RU; receiving, by the DU, from the at least one RU, a report comprising a frame offset, wherein the frame offset is received from the at least one RU through an upper layer message; calculating, in advance by the DU, link delay information for the at least one RU based on their respective frame offset, wherein the link delay information is calculated based on T.sub.12=T.sub.34=(T.sub.14T.sub.offset)/2, and wherein: (i) T.sub.12 is a delay of a downlink signal from an output point (R1) of the DU to an input point (R2) of the at least one RU; (ii) T.sub.14 is a frame timing difference between the output point (R1) and an input point (R4) of the DU; (iii) T.sub.offset is the frame offset between the input point (R2) and an output point (R3) of the at least one RU; and (iv) T.sub.34 is a delay of an uplink signal from the output point (R3) of the at least one RU to the input point (R4) of the DU; transmitting, by the DU, the link delay information to at least one RU; updating, by the at least one RU, the first RU frame timer based on the link delay information received from the DU; transmitting, by the at least one RU, a first acknowledgement message in response to the updating of the first RU frame timer; updating, by the at least one RU, frame timing information for the second type of frame based on information about a difference between a timing for the first type of frame and a frame timing for the second type of frame, delivered from the DU transmitting, by the at least one RU, a second acknowledgement message in response to the updating of the frame timing information for the second type of frame; and controlling a TDD Tx/Rx switching timing by replacing Tx/Rx switching timing information for each antenna carrier (AxC) delivered from the DU to the first RU frame timer and the second RU frame timer; wherein the first type of frame is a Common Public Radio Interface (CPRI) frame, and wherein the second type of frame is an Evolved Universal Terrestrial Radio Access (E-UTRA) frame.

2. The method of claim 1, wherein the determining of the boundary comprises checking the boundary of the wireless frame signal using a sync&timing subchannel for each hyper frame of the wireless frame signal transmitted from the DU.

3. The method of claim 1, wherein each of the first RU frame timer and second RU frame timer is configured using a timer with an error of less than 10 ns.

4. The method of claim 2, wherein each of the first RU frame timer and second RU frame timer is configured using a timer with an error of less than 10 ns.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features, and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which:

(2) FIGS. 1a to 1d show several connection forms of a cloud radio access network (CRAN) that is supported by a common public radio interface (CPRI) standard;

(3) FIG. 2 is a structural diagram showing a CPRI frame and a sub channel;

(4) FIG. 3 is a structural diagram showing a basic frame for a CPRI line bit rate of 614.4 Mbps;

(5) FIG. 4 is an exemplary diagram of the CRAN in which a method of controlling a time division duplexing (TDD) Tx/Rx switching timing according to an embodiment of the present invention;

(6) FIG. 5 is a timing chart illustrating a method of controlling a TDD Tx/Rx switching timing in the CRAN according to an embodiment of the present invention; and

(7) FIG. 6 is a diagram illustrating a reference point for correcting a link delay in a CRAN having a single hop configuration.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(8) Preferred embodiments of a method of controlling a time division duplexing (TDD) Tx/Rx switching timing in a cloud radio access network (CRAN) will be described in detail below with reference to the accompanying drawings.

(9) As described above, the terms DU and RU are widely used in the CRAN while the terms REC and RE are used corresponding to the DU and RU in a common public radio interface (CPRI) which is one of the interfaces used in the CRAN. Hereinafter, the present invention will be described using these terms interchangeably.

(10) As well known, the CPRI standard may be applied to a universal mobile telecommunication system (UMTS), a WiMAX forum mobile system profile according to the IEEE Std 802.16-2009, an evolved UMTS terrestrial radio access (E-UTRA), and other wireless standards. The present invention will be described by exemplifying E-UTRA which is a wireless interface of 3rd generation partnership project (3GPP) long term evolution (LTE).

(11) FIG. 2 is a structural diagram showing a CPRI frame and subchannels. As described in FIG. 2, each CPRI wireless frame (node B frame) is 10 ms in length, and each hyper frame is 66.7 s in length because one CPRI wireless frame consists of 150 hyper frames. One hyper frame consists of a total of 256 basic frames, such that each basic frame is 260.42 ns (=1/3.84 MHz) in length.

(12) FIG. 3 is a structural diagram showing a basic frame for a CPRI line bit rate of 614.4 Mbps. As shown in FIG. 3, user data is contained as a digital baseband IQ stream in an IQ data block within a CPRI basic frame and then delivered, and the RU (RE) receives the user data, converts the received data into an analog signal, amplifies the analog signal, and then propagates the amplified analog signal (to terminals) over the air. CPRI control & management (C&M) data and synchronization information are delivered through a CPRI subchannel (specifically, the control word within the CPRI basic frame) and used only by the DU (REC) and RU (RE), irrespective of the LTE layer.

(13) Each basic frame consists of one control word and 15 payloads, and a total of 256 control words in one hyper frame are gathered to form 64 subchannels. The length of one word (T) of one basic frame varies depending on the line bit rate of the CPRI link, and according to CPRI ver.4.2 (released in Sep. 29, 2010), the length ranges from 8 bits (1 byte) to 128 bits (16 bytes).

(14) FIG. 4 is an exemplary diagram of the CRAN in which a method for controlling a TDD Tx/Rx switching timing according to an embodiment of the present invention. As shown in FIG. 4, the CRAN, to which the method of controlling the TDD Tx/Rx switching timing according to an embodiment of the present invention may be applied, may be configured by connecting a plurality of RUs (REs) to one DU (REC), and FIG. 4 shows a configuration in which a plurality of RUs (REs) are connected to one DU (REC) by star and chain topologies.

(15) The DU (REC) includes a DU_CPRI_frame_timer and a DU_E-UTRA_frame_timer, which are used to generate a CPRI frame and an E-UTRA frame, respectively. In addition, all the RUs (REs) include an RU_CPRI_frame_timer and an RU_E-UTRA_frame_timer. It is desirable that each DU_CPRI_frame_timer, DU_E-UTRA_frame_timer, RU_CPRI_frame_timer, and RU_E-UTRA_frame_timer are implemented as a timer having an error of less than 10 ns in order to finely control the TDD Tx/Rx switching timing.

(16) Table 1 shows the content of 64 subchannels configured with one hyper frame within the CPRI wireless frame.

(17) TABLE-US-00001 TABLE 1 subchannel purpose of number Ns subchannel Xs = 0 Xs = 1 Xs = 2 Xs = 3 0 sync&timing sync byte HFN BFN-low BFN-high 1 slow C&M slow C&M slow C&M slow C&M slow C&M 2 L1 inband prot version startup L1-reset-LOS pointer p 3 reserved reserved reserved reserved reserved 4 Ctrl_AxC low Byte Ctrl_AxC Ctrl_AxC Ctrl_AxC Ctrl_AxC 5 Ctrl_AxC low Byte Ctrl_AxC Ctrl_AxC Ctrl_AxC Ctrl_AxC 6 Ctrl_AxC high Byte Ctrl_AxC Ctrl_AxC Ctrl_AxC Ctrl_AxC 7 Ctrl_AxC high Byte Ctrl_AxC Ctrl_AxC Ctrl_AxC Ctrl_AxC 8 reserved reserved reserved reserved reserved . . . . . . . . . . . . . . . . . . 15 reserved reserved reserved reserved reserved 16 vendor specific vendor specific vendor specific vendor specific vendor specific . . . . . . . . . . . . . . . . . . p 1 vendor specific vendor specific vendor specific vendor specific vendor specific pointer p fast C&M fast C&M fast C&M fast C&M fast C&M . . . . . . . . . . . . . . . . . . 63 fast C&M fast C&M fast C&M fast C&M fast C&M

(18) When the DU (REC) and the respective RUs (REs) are connected by the CPRI link, each RU (RE) checks a signal boundary of the CPRI frame that is transmitted from the DU (REC) by using a synchronization signal of the CPRI frame received from the DU (REC), for example, a sync byte and hyper frame number (HFN) byte constituting subchannel #0 that is a sync&timing subchannel for each hyper frame, as shown in Table 1, and then determines a start timing of the RU_CPRI_frame_timer of the RU (RE) to perform an initial synchronization with DU_CPRI_frame_timer.

(19) FIG. 5 is a timing chart illustrating a method for controlling a TDD Tx/Rx switching timing in the CRAN according to an embodiment of the present invention. As show in FIG. 5, according to the method of controlling the TDD Tx/Rx switching timing in the CRAN of the present invention, first, the DU (REC) detects in advance link delays between the DU (REC) and the respective RUs (REs) in order to match frame boundaries of a wireless frame signal transmitted from the RU (RE) and then transmits the link delays to the respective RUs (REs) (S10). In this case, the link delay may vary according to a type or length of a cable connecting the DU (REC) and RU (RE).

(20) FIG. 6 is a diagram illustrating a reference point for correcting a link delay in the CRAN having a single hop configuration. As shown in FIG. 6, a reference point for correcting a cable (link) delay defined in the CPRI specification may be an input or output point of an apparatus, for example, a connector between the REC and RE.

(21) In FIG. 6, the reference points R1 and R4 respectively indicate an output point and an input point of REC (DU), and the reference points R2 and R3 respectively indicate an output point and an input point of RE (RU). A reference number Ra indicates an antenna. T.sub.12 indicates a delay of a downlink signal from the output point R1 of the REC (DU) to the input point R2 of the RE (RU), and T.sub.34 indicates a delay of an uplink signal from the output point R3 of the RE (RU) to the input point R4 of the REC (DU).

(22) T.sub.offset is a frame offset between an input signal of the reference R2 and an output signal of the reference point R3. Lastly, T.sub.14 is a frame timing difference between the output signal of the reference R1 and the input signal of the reference R4. The REC (DU) may be aware in advance of the frame offset T.sub.offset of each RE (RU) by a report through an upper layer message from each RE (RU).

(23) As such, the DU (REC) receives the frame offset T.sub.offset through an upper layer message from the RU (RE) and then calculates a link delay (T.sub.12=T.sub.34) using the delivered frame offset T.sub.offset. The link delay (T.sub.12=T.sub.34) may be calculated by Equation 1 below:
(Equation 1)
T.sub.12=T.sub.34=(T.sub.14T.sub.offset)/2(1)

(24) In an embodiment as shown in FIG. 4, a method of calculating a link delay in the CRAN having a multi-hop structure is also described in the CPRI standard, and since the present invention does not relate to a method of calculating a CPRI link delay, such detailed descriptions thereof will be omitted.

(25) Referring back to FIG. 5, when receiving the link delay information from the DU (REC), the RU (RE) updates a CPRI frame timer in consideration of the link delay information (S20). For example, if a link delay between the DU (REC) and any RU (RE) is 50 s, the RU (RE) corrects the CPRI_frame_timer by 50 s in consideration of the link delay and then transmits an acknowledgement message Ack. to the DU (REC) (S30). Thus, the RU_CPRI_frame_timer is operated in synchronization with the DU_CPRI_frame_timer, with an error of less than 10 ns, and the DU (REC) repeats such operations with respect to all RUs (REs).

(26) Although the length of one wireless frame (node B frame) is 10 ms, which is the same as that of one wireless frame in the E-UTRA, that is, the LTE standard, the wireless frame boundaries therebetween may not be matched. Considering this, the DU (REC) transmits information about a difference between the CPRI frame timing and the E-UTRA frame timing to all RUs (REs) (S40), and each RU (RE) updates E-UTRA frame timing information (S50) using the frame timing difference information, and then transmits an acknowledgement message Ack. to DU (REC) (S60).

(27) Thus, the RU_E-UTRA_frame_timer is operated in synchronization with the DU_E-UTRA_frame_timer, with an error of less than 10 ns, and the DU (REC) repeats the operations on all RUs (REs). After completion of the operations, all of the RUs (REs) connected to the DU (REC) are operated using RU_CPRI_frame_timers and RU_E-UTRA_frame_timers in synchronization with the DU_CPRI frame-timer and DU_E-UTRA_frame_timer of the DU (REC).

(28) Lastly, the DU (REC) transmits Tx/Rx switching timing information to each RU (RE) for each an antenna carrier AxC (S70), and each RU (RE) performs precise TDD Tx/Rx switching timing control with an error of less than 10 ns using the RU_CPRI_frame_timer and RU_E-UTRA_frame_timer. Here, one antenna carrier AxC refers to an amount of digital baseband U-plane data required by one independent antenna device to receive or transmit one carrier.

(29) According to the method of controlling a TDD Tx/Rx switching timing in a CRAN of the present invention, it is possible to finely control a switching timing between transmission and reception of a TDD signal without an additional component added to the DU and RU.

(30) The preferred embodiment of the method of controlling a TDD Tx/Rx switching timing in a CRAN environment of the present invention has been described in detail with reference to the accompanying drawings. However, the preferred embodiment is merely exemplary, and various modifications and changes are possible within the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

(31) That is, the embodiment has been described by exemplifying the CPRI timer and the E-UTRA timer, and may be applied to an OBSAI timer and a WiMAX timer. Thus in the appended claims, a CPRI_OBSAI_frame_timer indicates the CPRI_frame_timer or OBSAI_frame_timer, and also a WiMAX_E-UTRA_frame_timer indicates the WiMAX_frame_timer or E-UTRA_frame_timer.