Abstract:
Provided are an orthogonal frequency division multiple assess (OFDMA) system and a method for controlling frequency offsets of subscribers in uplink communication. The OFDMA system solves a problem that the system performance is deteriorated in uplink communications because subscriber stations have different carrier frequency offsets. Each subscriber station compensates for a frequency offset thereof in response to a frequency offset control signal provided by a base station and then transmits an uplink frame to the base station. The base station compensates for an average frequency offset using the uplink frame, which has a small frequency offset shift because each subscriber station compensates for the frequency offset thereof previously, and thus the frequency offset of each subscriber station can be correctly compensated to prevent inter-carrier interference.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims foreign priority under 35 U.S.C. § 119 to Korean Patent Application No. 2004-51527, filed on Jul. 2, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present disclosure relates to digital communications and, more particularly, to controlling frequency offsets of subscribers in uplink communications.  
         [0004]     2. Description of the Related Art  
         [0005]     Orthogonal frequency division multiple assess (OFDMA) has been considered as a new multiple access method in a wireless Metropolitan Area Network (MAN) such as IEEE 802.16a, and as a broadband mobile Internet access network such as flash-type OFDMA. The OFDMA method uses a larger number of subcarriers than a conventional Orthogonal Frequency Division Multiplexing (OFDM) system such as IEEE 802.11a, for example, and allocates a subchannel constructed of a set of a part of the subcarriers to each subscriber. The OFDMA method enables two-dimensional resource allocation in time and frequency domains. In addition, the OFDMA can reduce overhead due to a long guard space required for high-speed transmission because the OFDMA uses a large number of subcarriers. Moreover, the OFDMA can concentrate power on a part of the subcarriers so that efficient control and service area extension can be achieved.  
         [0006]      FIG. 1  illustrates a multiple access communications environment of an OFDMA system, indicated generally by the reference numeral  100 . Referring to  FIG. 1 , a single cell includes a single base station BS and a plurality of subscriber stations SS-1, SS-2 and SS-M corresponding to multiple access users. The subscriber stations transmit/receive communications data via the base station. The direction from the base station to the subscriber stations is called downlink and the direction from the subscriber stations to the base station is called uplink.  
         [0007]      FIG. 2  illustrates a frame structure in the OFDMA environment, indicated generally by the reference numeral  200 . The base station includes data items to be sent to the multiple subscriber stations in a single downlink frame, and transmits the downlink frame to the subscriber stations. Each subscriber station that has received the downlink frame reproduces only data allocated thereto and processes the reproduced data. When the downlink frame is finished, each subscriber station transmits an uplink frame to the base station. Each subscriber station transmits uplink data through a subchannel or subcarrier allocated thereto. The uplink frames transmitted from the subscriber stations are requested to simultaneously arrive at the base station.  
         [0008]     Referring to  FIG. 2 , each of the downlink frames and uplink frames starts with a preamble. The preamble is used for a receiver to detect and reproduce a received signal. A header of the downlink frame includes frame construction information, and a downlink map DL-MAP stores information about which subscriber station receives a data part of the downlink frame. An uplink map UL-MAP of the downlink frame stores information about which subscriber station is allocated how many subchannels through the uplink frame following the downlink frame.  
         [0009]      FIG. 3  illustrates a downlink communications environment, indicated generally by the reference numeral  300 . Referring to  FIG. 3 , a baseband processor  310  of the base station modulates the downlink frame of  FIG. 2  and loads the modulated downlink frame on a carrier frequency fc to transmit it to respective baseband processors  320 ,  330  and  340  of the subscriber stations. Respective receivers of the baseband processors  320 ,  330  and  340  of the subscriber stations receive and demodulate the modulated downlink frame. The baseband processors  320 ,  330  and  340  receive the downlink frame having phases shifted by frequency offsets Δf o,1 , Δf o,2  and Δf o,M  of their receivers, respectively. Accordingly, the respective receivers of the baseband processors  320 ,  330  and  340  estimate their frequency offsets through frequency offset estimators  321 ,  331  and  341  and subtract the estimated frequency offsets from the received downlink frame. Then, a frequency-offset-compensated downlink frame is respectively processed by modems  322 ,  332  and  342  of the subscriber stations.  
         [0010]      FIG. 4  illustrates an uplink communications environment, indicated generally by the reference numeral  400 . Referring to  FIG. 4 , respective transmitters of the baseband processors  320 ,  330  and  340  of the subscriber stations load data items, respectively processed by the baseband processors  320 ,  330  and  340 , on the carrier frequency fc to construct an uplink frame and transmit the uplink frame to the baseband processor  310  of the base station. Here, the respective transmitters of the baseband processors  320 ,  330  and  340  of the subscriber stations transmit data items having phases shifted by frequency offsets Δf o,1 , Δf o,2  and Δf o,M  of the transmitters, respectively. The baseband processor  310  of the base station receives the uplink frame, obtains an average frequency offset Δf o,BS  of the frequency offsets Δf o,1 , Δf 0,2  and Δf o,M  of the transmitters using a frequency offset estimator  311 , and subtracts the average frequency offset Δf o,BS  from the received uplink frame such that the baseband processor  310  processes the average-frequency-offset-compensated uplink frame.  
         [0011]     From constellations of signals transmitted from the baseband processors  320 ,  330  and  340  to the base station  310 , it can be known that the average frequency offset compensation of the base station deteriorates communications performance.  
         [0012]      FIG. 5  shows the results of reproduction of an uplink frame, indicated generally by the reference numeral  500 , carried out by the base station, when the uplink frame is transmitted from four subscriber stations to the base station. Assume that the frequency offsets of first, second and third subscriber stations SS 1 , SS 2  and SS 3  are set to 0, the frequency offset of a fourth subscriber station SS 4  is set to 0.05 times a subcarrier interval, and the uplink frame is transmitted as a binary phase shift keying (BPSK) signal. In addition, a single frame has a 15 OFDM symbol length.  
         [0013]     Referring to  FIG. 5 , the base station estimated frequency offsets using preambles of the uplink frame. However, the base station estimated intermediate values of the frequency offsets so that signals transmitted from all subscriber stations were affected. Particularly, a value received from the fourth subscriber station SS 4  has a large frequency offset so that the phase of the received value is rotated for each symbol, as shown in  FIG. 5 ( d ). That is, the frequency offset of each subscriber station is not correctly compensated to generate inter-carrier-interference. This deteriorates communications performance and decreases a maximum communications transmission rate.  
         [0014]     Therefore, what is desired is an OFDMA system and a frequency offset compensating method capable of correctly compensating for the frequency offset of each subscriber station in uplink communications.  
       SUMMARY OF THE INVENTION  
       [0015]     The present disclosure provides an OFDMA system capable of correctly compensating for the frequency offset of each subscriber station in uplink communication.  
         [0016]     The present disclosure also provides a frequency offset compensating method in the OFDMA system.  
         [0017]     According to an aspect of the present disclosure, there is provided an OFDMA system comprising a plurality of subscriber stations, which compensate for their frequency offsets on data to be transmitted to a base station to construct an uplink frame and transmit the uplink frame to the base station; and the base station compensating for the uplink frame by an intermediate frequency offset obtained by averaging frequency offsets of the uplink frame transmitted from the subscriber stations. The subscriber stations compensate for their frequency offsets on the uplink frame in response to a frequency offset control signal included in a downlink frame transmitted from the base station.  
         [0018]     Preferably, each subscriber station compensates for the downlink frame transmitted from the base station by the frequency offset thereof, and compensates for the uplink frame by the frequency offset compensated on the downlink frame.  
         [0019]     According to another aspect of the present disclosure, there is provided a frequency offset control method of an OFDMA system, comprising selectively compensating for frequency offsets of an uplink frame, respectively generated by a plurality of subscriber stations, in response to a frequency offset control signal provided by a base station; each subscriber station transmitting the uplink frame to the base station; the base station detecting the uplink frame; and estimating an intermediate frequency offset obtained by averaging frequency offsets from preambles of the uplink frame and compensating for the intermediate frequency offset.  
         [0020]     According to another aspect of the present disclosure, there is provided a frequency offset control method of an OFDMA system, comprising a base station generating a downlink frame including a frequency offset control signal and transmitting the downlink frame to each subscriber station; each subscriber station detecting the downlink frame, estimating a frequency offset thereof and compensating for the frequency offset; each subscriber station detecting the frequency offset control signal and generating an uplink frame; each subscriber station compensating a frequency offset of the uplink frame in response to the frequency offset control signal; the base station detecting the uplink frame; and estimating an intermediate frequency offset obtained by averaging frequency offsets from preambles of the uplink frame to compensate for the intermediate frequency offset.  
         [0021]     Accordingly, the OFDMA system of the present disclosure compensates for an average frequency offset using an uplink frame, which has a small frequency offset shift because each subscriber station compensates for the frequency offset thereof previously. Thus, the frequency offset of each subscriber station can be correctly compensated in uplink communications to prevent inter-carrier interference. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The above and other features and advantages of the present disclosure will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which:  
         [0023]      FIG. 1  illustrates a multiple access communications environment of an OFDMA system;  
         [0024]      FIG. 2  illustrates a frame structure in an OFDMA environment;  
         [0025]      FIG. 3  illustrates an OFDMA downlink communications environment;  
         [0026]      FIG. 4  illustrates an OFDMA uplink communications environment;  
         [0027]      FIG. 5  illustrates constellations of signals transmitted from subscriber stations to a base station in the uplink communications environment of  FIG. 4 ;  
         [0028]      FIG. 6  illustrates an OFDMA system having a previous frequency offset compensating function in uplink communications according to an embodiment of the present disclosure;  
         [0029]      FIG. 7  illustrates a frequency offset compensator used for the OFDMA system of  FIG. 6  according to an embodiment of the present disclosure;  
         [0030]      FIG. 8  illustrates a frequency offset compensator used for the OFDMA system of  FIG. 6  according to another embodiment of the present disclosure;  
         [0031]      FIG. 9  illustrates constellations of signals transmitted from subscriber stations to a base station in uplink communications using the OFDMA system of  FIG. 6 ;  
         [0032]      FIGS. 10   a  and  10   b  are graphs showing the relationship between SNR and BER in the conventional OFDMA system of  FIG. 4  and the OFDMA system of  FIG. 6 ; and  
         [0033]      FIG. 11  is a flow chart showing a frequency offset compensating method using the OFDMA system of the present disclosure. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0034]     The present disclosure will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the disclosure are shown. The disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the disclosure to those skilled in the art. Throughout the drawings, like reference numerals refer to like elements.  
         [0035]      FIG. 6  illustrates an OFDMA system  600  according to an embodiment of the present disclosure. Referring to  FIG. 6 , the OFDMA system  600  applies the method of estimating the frequency offset of each subscriber station from a preamble of a downlink frame and compensating for the frequency offset in downlink communication, described above with reference to  FIG. 3 , to uplink communication. Subscriber stations SS 1 , SS 2  and SSM use their frequency offsets, which are respectively estimated by frequency offset estimators  321 ,  331  and  341  when respective receivers of the subscriber stations SS 1 , SS 2  and SSM receive the downlink frame from a base station  310 , for their transmitters  620 ,  630  and  640 .  
         [0036]     When the transmitters  620 ,  630  and  640  of the subscriber stations SS 1 , SS 2  and SSM transmit data to the base station  310 , the transmitters  620 ,  630  and  640  compensate for the frequency offsets e −j2π(Δf     o,1     )t , e −j2π(Δf     o,2     )t  and e −j2π(Δf     o,M     )t , estimated by the frequency offset estimators  321 ,  331  and  341  of the receivers of the subscriber stations, and transmit the compensated frequency offsets, respectively. The base station  310  obtains an average value e j2π(Δf     o,BS     )t  of the frequency offsets using a frequency offset estimator  311  from preambles of an uplink frame at a carrier frequency fc, received from the subscriber stations SS 1 , SS 2  and SSM. The base station  310  compensates for the average frequency offset from the uplink frame, which has a small frequency offset shift because the transmitters  620 ,  630  and  640  of the subscriber stations SS 1 , SS 2  and SSM previously compensate for the frequency offsets of the subscriber stations. Accordingly, the frequency offset of each subscriber station can be correctly compensated in the uplink communication.  
         [0037]     The frequency offset compensation principle is as follows.  
         [0038]     In general, a communications signal is represented by a complex signal s in a Cartesian coordinate form, as shown in Equation 1. The complex signal s includes a real part x and an imaginary part y. 
 
 s=x+jy   [Equation 1]
 
         [0039]     The complex signal s can be also represented in a polar coordinate form as follows. 
 
 s=M·e   jΦ   [Equation 2]
 
         [0040]     The Cartesian coordinate and the polar coordinate have the following relationship between them. 
 
 X=Re[X]+jIm[X]=X   r   +jX   i   =√{square root over (X     r           2     +X     i           2     )}·   e   j tan     −1     (X     i     /X     r     )   =A   X   ·e   jΦ     x    
 
 s=x+jy=√{square root over (x     2     +y     2     )}   e   j tan     −1     (y/x)   =A·e   jΦ   [Equation 3]
 
         [0041]     When the complex signal s has a frequency offset Δf, the following result is obtained. 
 
 r=s·e   j2πΔft   [Equation 4]
 
         [0042]     That is, a reception signal r is received such that the phase of the complex signal s is rotated according to the frequency offset Δf and a time variable t.  
         [0043]     In Equation 4, e j2Δπft  is represented as follows. 
 
 e   j2πΔft =cos(2 πΔft )+ i  sin(2 πΔft )  [Equation 5]
 
         [0044]     A receiver estimates the frequency offset Δf and reversely rotates the phase of the reception signal, which has been rotated by the frequency offset. This is frequency offset compensation. On the basis of this principle, a method of compensating for a frequency offset of a signal in a Cartesian coordinate form reversely rotates the reception signal r by e −j2πΔft  to obtain the communications signal s. 
 
 s=r·e   −j2πΔft   [Equation 6]
 
         [0045]     When Equation 6 is replaced with Equation 5, the following equation is obtained. 
 
 s=r·e   −j2πΔft ( x′+jy ′)[cos(−2 πΔft )+ i  sin(−2 πΔft )  [Equation 7]
 
         [0046]     Here, assume that the reception signal r includes a real part x′ and an imaginary part y′. Equation 7 is arranged as follows. 
 
 s=[x ′ cos(−2 πΔft )− y ′ sin(−2 πΔft )]+ j[y ′ cos(−2 πΔft )+ x ′ sin(−2 πΔft )]  [Equation 8]
 
         [0047]     Therefore, the frequency offset Δf of the reception signal r is compensated to obtain the original complex signal s.  
         [0048]     As shown in  FIG. 7 , a frequency offset compensator for realizing Equation 8 is indicated generally by the reference numeral  700 .  
         [0049]     In the meantime, frequency offset compensation of the signal in a polar coordinate form represented by Equation 2 is as follows. 
 
 s=r·e   −j2πΔft   =M′·e   jΦ   ·e   −j2πΔft   =M′·e   j(Φ′−2πΔft   [Equation 9]
 
         [0050]     Accordingly, the complex signal s is obtained by subtracting only a phase term from the reception signal r.  
         [0051]     Turning to  FIG. 8 , a frequency offset compensator for realizing Equation 9 is indicated generally by the reference numeral  800 .  
         [0052]      FIG. 9  illustrates the results of reproduction of an uplink frame by a base station when the uplink frame is transmitted from four subscriber stations SS 1 , SS 2 , SS 3  and SS 4  to the base station using the OFDMA system of the present disclosure, and indicated generally by the reference numeral  900 . Referring to  FIG. 9 , when frequency offsets of the subscriber stations SS 1 , SS 2 , SS 3  and SS 4  are respectively set to 0, 0, 0 and 0.005 times a subcarrier interval, the subscriber stations SS 1 , SS 2 , SS 3  and SS 4  transmit the uplink frame in a BPSK signal form. Then, the base station reproduces received values with respect to the subscriber stations SS 1 , SS 2 , SS 3  and SS 4  into signal constellation of BPSK.  
         [0053]      FIGS. 10   a  and  10   b  are graphs showing the relationship between SNR and BER of the OFDMA system according to the present disclosure, which are obtained by adding a noise environment to the condition of  FIG. 5  and performing computer simulations.  
         [0054]      FIG. 10   a  shows downlink performance and the result obtained, indicated generally by the reference numeral  1000 , when each subscriber station receives a downlink frame, reproduces data allocated thereto and then detects a bit error. The numeral 1 in brackets shows performance of the conventional OFDMA system of FIG. 3 and the numeral 2 in brackets represents performance of the OFDMA system of the present disclosure, which show similar performances.  
         [0055]      FIG. 10   b  shows uplink performance, indicated generally by the reference numeral  1050 . Referring to  FIG. 10   b , the performance of a subscriber station SS 4  is remarkably deteriorated and the performances of subscriber stations SS 1 , SS 2  and SS 3  are also deteriorated due to the effect of the frequency offset of the subscriber station SS 4  in the conventional OFDMA system. However, the performances of the four subscriber stations are similar in case of the OFDMA system according to the present disclosure.  
         [0056]      FIG. 11  is a flow chart showing a frequency offset compensating method in the OFDMA system of the present disclosure, and indicated generally by the reference numeral  1100 . Referring to  FIG. 11 , in the case of a downlink, a base station generates a downlink frame including a frequency offset control signal in the step  1101 , and transmits the downlink frame to each subscriber station in the step  1102 . Each subscriber station detects the downlink frame in the steps  1111  and  1112 , and estimates the frequency offset thereof from a preamble of the downlink frame to compensate for the frequency offset in the step  1113 . Then, each subscriber station processes data included in the downlink frame, such as a header, DL_MAP, UP_MAP and burst data, in the step  1114 .  
         [0057]     In the case of an uplink, each subscriber station detects the frequency offset control signal and generates an uplink frame in the step  1115 . When the frequency offset control signal is activated in the step  1116 , each subscriber station compensates for a frequency offset of the uplink frame by the frequency offset of the subscriber station in the step  1117  and transmits the uplink frame to the base station in the step  1118 . The base station detects the uplink frame in the steps  1103  and  1104  and estimates an intermediate frequency offset from preambles of the uplink frame to compensate for the intermediate frequency offset in the step  1105 . Then, the base station processes the uplink frame in the step  1106 .  
         [0058]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the pertinent art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.