KWACK, Hyun Sun (No.Sindonga Paradium, 318-2Wonchun-dong, Yeongtong-g, Suwon-si Gyeonggi-do 443-380, 103-1103, KR)
| Claims [Claim 1] A cyclic prefix length change method in a wireless communication system including a base station and at least one mobile station, comprising: determining, at the base station, a length of a cyclic prefix based on a channel condition in the middle of data communication session with the mobile station; comparing the determined cyclic prefix length with a previous cyclic prefix length; generating a cyclic prefix length change information based on the comparison result; transmitting a downlink reference signal including the cyclic prefix length change information to the mobile station; and sending the mobile station a data with the cyclic prefix added in the beginning of the data. [Claim 2] The cyclic prefix length change method of claim 1, wherein transmitting a downlink reference signal comprises adding the cyclic prefix length change information to an initial value (Cmit) of a pseudorandom sequence used for generating the downlink reference signal. [Claim 3] The cyclic prefix length change method of claim 1, wherein determining a length of a cyclic prefix comprises receiving an uplink reference signal containing a cyclic prefix length change request transmitted by the mobile station in the middle of data communication session. [Claim 4] The cyclic prefix length change method of claim 3, wherein the cyclic prefix length change request is generated by changing a sequence-shift pattern used for generating the uplink reference signal. [Claim 5] A cyclic prefix length change method in a wireless communication system including a base station and at least one mobile station, comprising: pre-determining, at the mobile station, a cyclic prefix length based on a channel condition in the middle of data communication session with the base station; comparing the pre-determined cyclic prefix length with a previous cyclic prefix length; generating a cyclic prefix length change request based on the comparison result; and transmitting an uplink reference signal including the cyclic prefix length change request to the base station. [Claim 6] The cyclic prefix length change method of claim 5, further comprising: receiving, at the base station, the uplink reference signal; extracting the cyclic prefix length change request from the uplink reference signal; determining the cyclic prefix length for a next transmission period based on the cyclic prefix length change request; transmitting a downlink reference signal including a cyclic prefix length information indicative of the cyclic prefix length for the next transmission period; and sending the mobile station a data with the cyclic prefix added in the beginning of the data. [Claim 7] The cyclic prefix length change method of claim 5, wherein the cyclic prefix length change request is generated by changing a sequence-shift pattern used for generating the uplink reference signal. [Claim 8] A wireless communication system comprising: a base station comprising a cyclic prefix length determiner which determines a cyclic prefix length based on channel condition, a cyclic prefix length change information generator which generates a cyclic prefix length change information by comparing the determined cyclic prefix length and a previous cyclic prefix, a downlink reference signal generator which generates a downlink reference signal including the cyclic prefix length change information, and a modulator which modulates data signal with the cyclic prefix having the determined length to transmission data; and a mobile station comprising a demodulator which demodulates the transmission data obtained by removing the cyclic prefix from the data signal. [Claim 9] The wireless communication system of claim 8, wherein the downlink reference signal generator generates the downlink reference signal by adding the cyclic prefix length change information to an initial value (C imt) of a pseudo-random sequence used for generating the downlink reference signal. [Claim 10] The wireless communication system of claim 8, wherein the mobile station further comprises: a cyclic prefix length determiner which pre-determines a length of the cyclic prefix based on the channel condition; a cyclic prefix length change request generator which generates a cyclic prefix length change request by comparing the pre-determined cyclic prefix length and the previous cyclic prefix; and an uplink reference signal generator which generates an uplink reference signal including the cyclic prefix length change request. [Claim 11] The wireless communication system of claim 10, wherein the uplink reference signal generator generates the cyclic prefix length change request by changing a sequence- shift pattern used for generating the uplink reference signal. [Claim 12] A base station comprising: a cyclic prefix length determiner which determines a cyclic prefix length based on channel condition; a cyclic prefix length change information generator which generates a cyclic prefix length change information by comparing the determined cyclic prefix length and a previous cyclic prefix; a downlink reference signal generator which generates a downlink reference signal including the cyclic prefix length change information; a transmitter which transmits the downlink reference signal to a base station; and a modulator which modulates data signal with the cyclic prefix having the determined length to transmission data. [Claim 13] The base station of claim 12, wherein the downlink reference signal generator generates the downlink reference signal by adding the cyclic prefix length change information to an initial value (Cmit) of a pseudorandom sequence used for generating the downlink reference signal. [Claim 14] The base station of claim 12, further comprising a receiver which receives an uplink reference signal containing a cyclic prefix length change request transmitted by the mobile station. [Claim 15] The base station of claim 14, wherein the mobile station generates the cyclic prefix length change request by changing a sequence-shift pattern used for generating the uplink reference signal. |
Title of Invention: DYNAMIC CYCLIC PREFIX LENGTH CHANGE METHOD AND WIRELESS SYSTEM THEREFOR
Technical Field
[1] The present invention relates to wireless communications and, in particular, to a dynamic cyclic prefix length change method for changing the cyclic prefix dynamically in adaptation to the channel condition during a data communication session and an OFDM-based wireless communication system supporting the dynamic cyclic prefix length change. Background Art
[2] Universal Mobile Telecommunications System (UMTS) is one of the third generation (3G) mobile telecommunication technologies, which is evolved from Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS) and uses Wideband Code Division Multiple Access (WCDMA).
[3] The 3 rd Generation Partnership Project (3GPP), which is responsible for the standardization of UMTS, is working on to significantly extend the performance of UMTS in the work item Long Term Evolution (LTE). LTE is a 3GPP standard that provides for a downlink speed of up to 300 Mbps and is expected to be commercially launched in 2010. In order to fulfill the requirements for the LTE systems, studies have been done in various aspects including minimization of the number of involved nodes in the connections and placing radio protocol as close as to the radio channels.
[4] Particularly, LTE employs Orthogonal Frequency Division Multiplexing (OFDM) with guard interval Cyclic Prefix (hereinafter called CP ), also known as guard interval, to improve robustness against multipath propagation (ghost channel). The CP increase the duration of OFDM symbol so as to mitigate inter-symbol interference caused due to multipath propagation effect and, the orthogonality between subcarriers of the OFDM reduces inter-channel interferences. The receiver can acquire the symbol timing synchronization by using the CP inserted in the guard interval.
[5] The most conventional OFDM-based wireless communication systems use a fixed length CP, which means that the same length of CP is inserted regardless of variation of channel condition. However, the use of fixed length CP may cause considerable performance deterioration when the CP length is less than the channel delay spread. There is therefore a need for a method to adapt the CP length to the variation of channel condition in the OFDM-based wireless communication system. Disclosure of Invention Solution to Problem [6] In order to overcome the problems of the prior art, the present invention provides an
OFDM-based wireless communication system and method that is capable of changing the CP length in adaptation to the channel condition in the middle of a communication session, resulting in improving system performance.
[7] In accordance with an exemplary embodiment of the present invention, a cyclic prefix length change method in a wireless communication system including a base station and at least one mobile station includes determining, at the base station, a length of a cyclic prefix based on a channel condition in the middle of data communication session with the mobile station; comparing the determined cyclic prefix length with a previous cyclic prefix length; generating a cyclic prefix length change information based on the comparison result; transmitting a downlink reference signal including the cyclic prefix length change information to the mobile station; and sending the mobile station a data with the cyclic prefix added in the beginning of the data.
[8] In accordance with another exemplary embodiment of the present invention, a cyclic prefix length change method in a wireless communication system including a base station and at least one mobile station includes pre-determining, at the mobile station, a cyclic prefix length based on a channel condition in the middle of data communication session with the base station; comparing the pre-determined cyclic prefix length with a previous cyclic prefix length; generating a cyclic prefix length change request based on the comparison result; and transmitting an uplink reference signal including the cyclic prefix length change request to the base station.
[9] In accordance with another exemplary embodiment of the present invention, a wireless communication system includes a base station comprising a cyclic prefix length determiner which determines a cyclic prefix length based on channel condition, a cyclic prefix length change information generator which generates a cyclic prefix length change information by comparing the determined cyclic prefix length and a previous cyclic prefix, a downlink reference signal generator which generates a downlink reference signal including the cyclic prefix length change information, and a modulator which modulates data signal with the cyclic prefix having the determined length to transmission data; and a mobile station comprising a demodulator which demodulates the transmission data obtained by removing the cyclic prefix from the data signal.
[10] In accordance with still another exemplary embodiment of the present invention, a base station includes a cyclic prefix length determiner which determines a cyclic prefix length based on channel condition; a cyclic prefix length change information generator which generates a cyclic prefix length change information by comparing the determined cyclic prefix length and a previous cyclic prefix; a downlink reference signal generator which generates a downlink reference signal including the cyclic prefix length change information; a transmitter which transmits the downlink reference signal to a base station; and a modulator which modulates data signal with the cyclic prefix having the determined length to transmission data. Brief Description of Drawings
[11] The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description in conjunction with the accompanying drawings, in which:
[12] FIG. 1 is a schematic diagram illustrating a multipath propagation environment causing inter-symbol interference problem to be solved by an OFDM-based wireless communication system according to an exemplary embodiment of the present invention;
[13] FIG. 2 is a diagram illustrating an LTE user plane protocol stack to be adopted to an
OFDM-based wireless communication system according to an exemplary embodiment of the present invention;
[14] FIG. 3 is a table illustrating types and structures of CPs for use in an LTE system according to an exemplary embodiment of the present invention;
[15] FIG. 4 is a flowchart illustrating a procedure of transmitting downlink reference signal containing CP length change information in a communication method according to an exemplary embodiment of the present invention;
[16] FIG. 5 is a flowchart illustrating a procedure of transmitting uplink reference signal containing CP length change information in a communication method according to an exemplary embodiment of the present invention;
[17] FIG. 6 is a flowchart illustrating a procedure of processing the uplink reference signal carrying CP length change request transmitted by a mobile station in a communication method according to an exemplary embodiment of the present invention;
[18] FIG. 7 is a sequence diagram illustrating operations of a base station and mobile stations in a wireless communication system according to an exemplary embodiment of the present invention when the CP length change is required;
[19] FIG. 8 is a block diagram illustrating a configuration of a wireless communication system including a downlink transmitter and a downlink receiver for transmission of a downlink reference signal according to an exemplary embodiment of the present invention; and
[20] FIG. 9 is a block diagram illustrating a configuration of a wireless communication system including a downlink transmitter and a downlink receiver for transmitting a downlink reference signal according to another exemplary embodiment of the present invention. Mode for the Invention
[21] Exemplary embodiments of the present invention are described with reference to the accompanying drawings in detail. The same reference numbers are used throughout the drawings to refer to the same or like parts. Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention.
[22] The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
[23] In the following description, the guide interval is a signal duration inserted between
OFDM symbols in order to avoid inter-symbol interference. The guard interval is set such that the maximum delay spread is shorter than the guard interval length. The guard interval is also called as Cyclic Prefix (CP) because it consists of a copy of the end of the OFDM symbol.
[24] In the following description, the OFDM symbol duration is equal to the sum of a valid symbol duration carrying data and a guard interval.
[25] In the following description, the downlink reference signals are pilot signals for coherent demodulation of downlink channel and include the cell-specific reference signal shared by all the user equipments within the cell and the terminal- specific reference signal transmitted to a specific user equipment. The downlink reference signals are generated using pseudo-random sequence.
[26] In the following description, the uplink reference signals include the Demodulation
Reference Signal (DMRS) for the coherent demodulation of the uplink channel and the Sounding Reference Signal (SRS) for the frequency domain scheduling of data channels.
[27] In the following description, the terms are borrowed from the 3GPP LTE standards.
[28] FIG. 1 is a schematic diagram illustrating a multipath propagation environment causing inter-symbol interference problem to be solved by an OFDM-based wireless communication system according to an exemplary embodiment of the present invention;
[29] As shown in FIG. 1, the LTE mobile communication is characterized with the
Evolved Radio Access Network (hereinafter called E-RAN) 110 and 112 having only two infrastructure nodes: the Evolved Node B (hereinafter called ENB or Node B) 120, 122, 124, 126, and 128 and the Access Gateway (hereinafter called (AG) 130 and 132. A User Equipment (hereinafter called UE) 101 accesses the Internet Protocol (IP) network via the E-RAN 110 and 112.
[30] The ENB s 120, 122, 124, 126, and 128 correspond to the conventional Node B which provides the UE 101 with radio access service. The ENB s 120, 122, 124, 126, and 128 are responsible for more complex functions than that of the conventional Node B. In the next generation wireless communication system, all user traffics including real time service such as Voice over IP (VoIP) are served through a shared channel. For this reason, there is a need for a device collecting status information of the UE s and scheduling based on the status information. Each of the ENBs 120, 122, 124, 126, and 128 is responsible for scheduling the UE s. In order to achieve the speed of 100 Mbps or faster, the wireless communication system exploits the radio access technology of Orthogonal Frequency Multiplexing (OFDM) on 20 MHz bandwidth. Also, an Adaptive Modulation and Coding (AMC) is supported for determining a modulation scheme and a channel coding rate according to the channel status of the UE 101. In FIG. 1, the radio channel denoted by reference numeral 1 is a direct channel which does not experience unexpected interference or obstacle, the radio channel denoted by reference numeral 2 is a channel reflected a fixed obstacle such as building, and the radio channel denoted by reference numeral 3 is a channel diffracted by moving obstacle such as plane. The guard interval (i.e. CP) is inserted between OFDM symbols to avoid the distortion and delay caused by these delayed channels so as to suppress the inter-symbol interference and inter-channel interference.
[31] Since the fixed CP length is not efficient to dynamically counteract the time-varying channel environment, the ENB and UE measure the channel environment periodically and determine whether to change the CP length. When it is required to change the CP length, the ENB can send the UE the downlink reference signal indicating the change of CP length. The UE updates the information on the CP length with reference to the downlink reference signal indicative of the change of the CP length and acquires the symbol timing synchronization based on the updated CP length. Also, the UE can check the channel environment periodically and, if it is need to change the CP length, sends the uplink control signal indicative of CP length change to the ENB. The CP length change request can be used as a criterion for the ENB to determine the channel condition.
[32] From now on, the term base station is used to denote each of the E-RAN 110 including the ENBs 120, 122, 124, and 130 and the E-RAN 112 including the ENBs 126, 128, and 132; and the term mobile station is used to denote the UE 101.
[33] The LTE user plane protocol stack to be adopted to the OFDM-based wireless communication is described according to the present invention is described hereinafter.
[34] FIG. 2 is a diagram illustrating a user plane protocol stack architecture of an LTE mobile communication system.
[35] As shown in FIG. 2, the UE has a protocol stack including a Packet Data Convergence Protocol (PDPC) layer 205, a Radio Link Control (RLC) layer 210, a Media Access Control (MAC) layer 215, and a Physical (PHY) layer 220. Also, the ENB has a protocol stack including a PDPC layer 240, a RLC layer 235, a MAC layer 230, and a PHY layer 225.
[36] The PDCP layer 205 and 240 is responsible for IP header compression/decompression. The RLC layer 210 and 235 packs the PDCP Packet Data Units (PDUs) into a size appropriate for transmission (hereinafter the data unit delivered from an upper layer entity is called PDU) and performs an Automatic Repeat Request (ARQ) function.
[37] The MAC layer 215 and 230 serves multiple RLC layer entities. The MAC layer 215 and 230 can multiplex the RLC PDUs produced by the RLC layer entities into a single MAC PDU and de-multiplex a MAC PDU into the RLC PDUs.
[38] The physical layer 220 and 225 performs encoding and modulation on the upper layer data to transmit through a radio channel and performs demodulation and decoding on the OFDM symbol received through radio channel to deliver to upper layers. The physical downlink channels include Physical Broadcast Channel (PBCH), Physical Control Format Indicator Channel (PCFICH), Physical Downlink Shared Channel (PDSCH), Physical Hybrid ARQ Indicator Channel (PHICH), Physical Downlink Shared Channel (PDSCH), Physical Multicast Channel (PMCH), Reference Signal (RS), and Synch channel; and the physical uplink channels include Physical Uplink Control Channel (PUCCH), Physical Uplink Shared Channel (PUSCH), Physical Random Access Channel (PRACH), and Sounding Reference Signal (SRS).
[39] In the meantime, the uplink Demodulation Reference Signal (DMRS) is transmitted over the PUCCH and PUSCH, and the DMRS or SRS can carry the information requesting CP length change. The base station can transmit the downlink reference signal (RS) carrying the CP length change request.
[40] FIG. 3 is a table illustrating types and structures of CPs for use in an LTE system according to an exemplary embodiment of the present invention.
[41] Referring to FIG. 3, the extended CP has a length of 512 T s which is over 3 times longer than normal CP, thereby appropriate for the long delay multipath channel environment with high channel interference and delay probability. In case of normal CP, the CP length is 160 T s for the first symbol, i.e. symbol#0, and 144 T s for the second to sixth symbol, i.e. symbol#l to symbol#6, and appropriated for high data rate transmission in a stable channel environment.
[42] In an exemplary embodiment, the base station determines one of the normal CP and extended CP for transmission to a mobile station, depending on the channel en- vironment indicated by the downlink reference signal. That is, the base station checks the delay time and distortion extent of the signal transmitted by the mobile station and transmits, if required, the CP length change request to the mobile station. If the CP length change request is received, the mobile station performs decoding based on the CP indicated by the CP length change request. Also, the mobile station checks the delay and distortion of the signal transmitted by the base station and sends, if required, the CP length change request to the base station. The CP length change request can be carried by the uplink reference signal. The base station and mobile station can send the CP length change request for changing from the normal CP to the extended CP if the channel environment is deteriorated, even in the middle of data communication session.
[43] A base station-triggered CP length change procedure using downlink reference signal is described first. The base station determines the CP length in consideration of the channel condition. Here, the initial value (C imt ) of the pseudo-random sequence for generating the downlink reference signal is set according to equation (1). (1)
[45] n s : Slot number, /: symbol number, NπD ce11 : cell identity
[46] Ncp= 1 for normal CP
[47] N CP = 0 for extended CP
[48] From equation (1), the initial CP length is indicated by the N C p, such that the mobile station performs decoding based on the N C p contained in the downlink reference signal. Even when the mobile and base stations are in the middle of data communication, the base station transmits the downlink reference signal periodically. At this time, the initial value (C mit ) of the pseudo random sequence for generating the downlink reference signal is set according to equation (2).
[49] C mit = 2»>(7(ns+l)+/+l)+2N ID - 11 + N CP (2)
[50] n s : Slot number, /: symbol number, Nn 3 0611 : cell identity
[51] Ncp = 1 if the CP length should be changed in the next period
[52] Ncp = 0 if the CP length should be the same in the next period
[53] If the Ncp is set to 1, this means that the base station transmits the data with a changed
CP length at the next transmission period; otherwise if the N C p is set to 0, this means that the CP length is maintained in the next transmission period. That is, the base station can notify the mobile station of the change of CP length by toggling the N C pθf the downlink reference signal, and the mobile station can acquire the symbol timing synchronization base for the next symbol based on the N C p, thereby facilitating channel condition- adaptive CP length change.
[54] A mobile station-triggered CP length change procedure using uplink reference signal is described hereinafter. While the mobile and base stations are performing data com- munication through a wireless channel, the mobile station transmits the uplink reference signal to the base station periodically. The uplink reference signal classified into the Demodulation Reference Signal (DMRS) and the Sounding Reference Signal (SRS), and both types of reference signal use sequence-shift pattern of base sequence group. According to the LTE specification, the sequence-shift pattern is determined by equation (3).
[55] f ss PUCCH = N ID ceU MOD 30 (PUCCH DMRS or SRS) (3)
[56] f ss PUSCH = (fss PUCCH + Δ ss ) MOD 30 (PUSCH DMRS)
[57] In an exemplary embodiment of the present invention, the CP length information is included in the sequence-shift pattern determined by equation (4).
[58] f ss PUCCH = (N ro ceU + 2 9 N cp ) MOD 30 (PUCCH DMRS or SRS) (4)
[59] Ncp = 1 if the CP length should be change in the next period
[60] Ncp = 0 if the CP length should be same in the next period
[61 ] f ss PUSCH = (fss PUCCH +Δ SS + 2 6 N cp ) MOD 30 (PUCCH DMRS)
[62] N CP = 1 if the CP length should be change in the next period
[63] N CP = 0 if the CP length should be same in the next period
[64] If it is determined that the CP length should be changed based on the channel condition, the mobile station transmits the uplink reference signal containing the CP length change request information toggled as compared to the CP length flag for previous transmission period. The base station can use the CP length change request information contained in the uplink reference signal for determining the channel condition.
[65] FIG. 4 is a flowchart illustrating a procedure of transmitting downlink reference signal containing CP length change information in a communication method according to an exemplary embodiment of the present invention.
[66] Referring to FIG. 4, while performing data communication with a mobile station, the base station checks the channel condition periodically (400). That is, the base station monitors the channel to detect Inter-Symbol Interference (ISI) and/or Inter-Channel Interference (ICI). The base station also performs monitoring of the CP length change request transmitted by the mobile station. The procedure of receiving and processing the CP length change request information is described in more detail later with reference to FIG. 6.
[67] The base station determines whether to change the current CP length, based on the currently measured channel condition (405). For instance, if the ISI or ICI is greater than the corresponding threshold value, this means that the channel condition is bad and thus the expended CP is required. In this case, if the current CP is the normal CP, the base station changes the normal CP to the extended CP and, otherwise, maintains the expended CP. In contrast, if the ISI or ICI is less than the corresponding threshold value, this means that the channel condition is good and thus the normal CP is enough for transmission. In this case, if the current CP is the normal CP, the base station maintains the current CP length and, otherwise, changes the extended CP to the normal CP.
[68] If it is determined to change the Current CP length, the base station sets the CP length change flag N C pto 1 (410). Otherwise, if it is determined to maintain the current CP length, the base station sets the N C p to 0.
[69] After determining whether to change the current CP length, the base station generates a pseudo-random sequence using the initial value (C imt ) reflecting the value of the CP length change flag N C p and generates a downlink reference signal containing the pseudo-random sequence (420)
[70] As a consequence, the base station transmits the downlink reference signal containing the CP length change information such that the mobile station synchronizes symbol timing based on the CP length change information for the next period.
[71] FIG. 5 is a flowchart illustrating a procedure of transmitting uplink reference signal containing CP length change information in a communication method according to an exemplary embodiment of the present invention.
[72] Referring to FIG. 5, while communicating data with a base station, the mobile station measures the channel condition periodically (500). That is, the mobile station monitors to detect the ISI and/or ICI on the channel established with the base station. Whenever the channel condition is measured, the mobile station determines, based on the measured channel condition, whether it is required to change the current CP length (505).
[73] If it is required to change the current CP length, the mobile station sets the CP length change flag N C pto 1 (510). Otherwise, if it is not required to change the current CP length, the mobile station sets the CP length flag N CP to 0 (515).
[74] After setting the CP length change flag N C p, the mobile station generates a sequence- shift pattern with the CP length change flag N C p and generates an uplink reference signal using the sequence-shift pattern. That is, the mobile station generates the uplink reference signal indicative of CP length change request.
[75] As a consequence, the mobile station transmits the uplink reference signal containing the CP length change request to the base station, thereby requesting the base station for changing the current CP length.
[76] FIG. 6 is a flowchart illustrating a procedure of processing the uplink reference signal carrying CP length change request transmitted by a mobile station in a communication method according to an exemplary embodiment of the present invention.
[77] Referring to FIG. 6, the base station receives the uplink reference signal transmitted by a mobile station (600). In the exemplary embodiment of the present invention, it is assumed that the uplink reference signal includes the CP length change information. Here, the uplink reference signal can be one of PUCCH DMRS, PUSCH DMRS, and SRS. Once the uplink reference signal is received, the base station detects the sequence-shift pattern of the DMRS and SRS (605) and compares the currently detected sequence-shift pattern with the previously detected sequence- shift pattern (610).
[78] Next, the base station determines whether the current sequence-shift pattern has changed as compared to the previous sequence-shift pattern (615). If the current sequence-shift pattern has changed as compared to the previous sequence- shift pattern, this means the mobile station requests CP length change and, thus, the base station recognizes the CP length change request and judges the channel condition (620).
[79] FIG. 7 is a sequence diagram illustrating operations of a base station and mobile stations in a wireless communication system according to an exemplary embodiment of the present invention when the CP length change is required.
[80] Referring to FIG. 7, the base station (ENB) is communicating data with the mobile stations (UEl and UE2) are communicating data using the normal CP (700). In the exemplary embodiment of the present invention, it is assumed that the base station establishes a wireless channel with each mobile station in initial configuration with the normal CP. The mobile stations measure the channel conditions and send the reference signals (DMRS/SRS) to the base stations, respectively (705 and 710). In case that the data communication is unstable due to the ISI and/or ICI, each mobile station sends the Demodulation Reference Signal (DMRS) or the Sounding Reference Signal (SRS) indicative of CP length change request to the base station.
[81] If the uplink reference signals, the base station determines whether to change the CP length with reference to the uplink reference signals. If it is determined to change the CP length, the base station generates the initial pseudo-random sequence with N C p set to 1 and generates the downlink reference signal using the pseudo-random sequence. That is, in the exemplary embodiment of FIG. 7, the base station sends the mobile stations the downlink reference signals containing the CP length change information indicating that the data are transmitted with the extended CP at the next transmission period (715 and 720), and then modulates the data with the extended CP and transmits the modulated data to the corresponding mobile stations (725 and 730).
[82] Afterward, the mobile station (UEl) detects that the channel condition becomes better and thus sends the DMRS or SRS containing the CP length change request information to the base station (735). If the reference signal is received, the base station determines to change the CP length based on the reference signal and sends the mobile station (UEl) a downlink reference signal generated using the initial value of the pseudo-random sequence with N C p set to 1 (740). Consequently, the base station sends the mobile station (UEl) the data with the normal CP from the next transmission period (745) while sending the mobile station (UE2) the data with the extended CP.
[83] FIG. 8 is a block diagram illustrating a configuration of a wireless communication system including a downlink transmitter and a downlink receiver for transmission of a downlink reference signal according to an exemplary embodiment of the present invention.
[84] Referring to FIG. 8, a downlink reference signal transmitter includes a channel condition detector 801, a CP length determiner 802, and a reference signal generator 803; and a downlink reference signal receiver includes a CP length change information extractor 804 and a CP length checker 805.
[85] The channel condition detector 801 of the transmitter checks the condition periodically in a data communication session and determines whether the ISI and/or ICI is detected on the channel.
[86] The CP length determiner 802 determines whether a CP length change is required, base on the channel condition detected by the channel condition detector 801. For instance, the CP length determiner 802 quantizes the ISI and/or ICI and determines the CP length base on whether the quantized value is greater than the threshold value. Whether to change the CP length can be determined by comparing the determined CP length and the current CP length.
[87] The CP length determiner 802 outputs the CP length change information to the reference signal generator 803. The reference signal generator 803 generates a pseudorandom sequence with the initial value (C mit ) reflecting the CP length change information Ncp and generates the downlink reference signal using the pseudo-random sequence. That is, the reference signal generator 803 generates the downlink reference signal and transmits the downlink reference signal through RS channel.
[88] The CP length change information extractor 804 of the receiver extracts the CP length change information from the downlink reference signal received through the RS channel. The CP length selector selects the CP length to be used for decoding data transmitted at the next transmission period based on to the currently used CP length and the CP length change information.
[89] FIG. 9 is a block diagram illustrating a configuration of a wireless communication system including a downlink transmitter and a downlink receiver for transmitting a downlink reference signal according to another exemplary embodiment of the present invention.
[90] Referring to FIG. 9, the downlink transmitter 900 includes a modulator 901, a serial/ parallel (S/P) converter 902, an Inverse Fast Fourier Transformer (IFFT) 903, a parallel/serial (P/S) converter 904, a CP inserter 905, and a Digital/Analog (D/ 'A) converter 906. [91] The source data is first input to the modulator 901 of the downlink transmitter 900.
The input source data is modulated by the modulator 901 and the modulated data is output to the S/P converter 902. The modulated data input to the S/P converter 902 in series are converted to N parallel data streams. The N parallel data streams are input to the IFFT 903 and thus inversely Fourier- transformed by the IFFT 903. The inversely Fourier-transformed signals are output to the P/S converter 904 and thus converted to a serial data stream by the P/S converter 904. The serial data stream is input to the CP inserter 905 and a guide interval is inserted to the serial data stream by the CP inserter 905 so as to be output in the form of an OFDM symbol.
[92] In more detail, the CP inserter 905 generates one of an extended CP or a Normal CP based on the serial data stream output from the P/S converter 904 and inserts the CP before each symbol. If the downlink reference signal indicative of CP length change has been transmitted to the receiver at the previous transmission period, the transmitter generates the OFDM symbol by inserting the CP having the changed CP length. The OFDM symbol output from the CP inserter 905 is converted into an analog signal by the D/A converter and then transmitted through a predetermined wireless channel.
[93] The downlink receiver 950 includes an Analog/Digital (AfD) converter 951, a CP remover 952, a Serial/Parallel (S/P) converter 953, an N-POINT Fast Fourier Transformer (FFT) 954, an equalizer 955, a synchronization and channel estimator 956, a Parallel/Serial (P/S) converter 957, and a demodulator 958. The analog signal received through the downlink data channel is input to the A/D converter 951 so as to be converted into digital signal and then output to the CP remover 952. The CP remover 952 removes CP from the digital signal output by the A/D converter 951, thereby outputting CP-removed OFDM signal. The OFDM signal is input to the S/P converter 953 so as to be serial-to-parallel converted by the S/P converter 953 and output to the N-POINT FFT 954 in the form of multiple data streams. The N-POINT FFT 954 performs N-POINT FFT transformation on the multiple signal streams and outputs the FFT transformed signals to the equalizer 955. The FFT-transformed signals are channel-equalized by the equalizer 955. The equalized signals are output to the P/S converter 957 so as to be converted into a serial signal. The signal output by the P/S converter 957 is input to the demodulator 958 so as to be demodulated into transmitted data. The synchronization and channel estimator 956 acquires symbol timing synchronization and estimates channel to configure the taps of the equalizer 955. The length of the CP to be removed is set based on the CP length change information extracted from the downlink reference signal transmitted by the transmitter.
[94] In another exemplary embodiment of the present invention, the base station can transmit the CP length change information by means of a Radio Resource Control (RRC) message other than the physical layer signal. That is, the CP length change in- formation for the downlink and uplink transmission can be transmitted to the mobile station by means of an SIB2 message.
[95] As described above, the dynamic cyclic prefix length change method and system of the present invention is capable of changing the cyclic prefix in adaptation to the channel condition in the middle of a data communication session, thereby improving connection stability. In the dynamic cyclic prefix length change method and system of the present invention, the base station pre-notifies the mobile station of the change of cyclic prefix length, whereby the mobile station can acquire the symbol timing synchronization efficiently. Industrial Applicability
[96] Although exemplary embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.
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