Abstract:
An apparatus and method for determining a pilot pattern in a Broadband Wireless Access (BWA) communication system are provided. In the pilot pattern determining method, an Orthogonal Frequency Division Multiplexing (OFDM) demodulator generates frequency-domain data by fast-Fourier-transform (FFT)-processing a received signal. A pilot pattern decider calculates a coherence bandwidth and a coherence time using subcarrier values received from the OFDM demodulator, and selects one of a plurality of pilot patterns according to the ratio of the coherence bandwidth to the coherence time.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119 to an application filed in the Korean Intellectual Property Office on Dec. 29, 2005 and assigned Serial No. 2005-132859, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and in particular, to an apparatus and method for adaptively changing a pilot pattern according to a link in an Orthogonal Frequency Division Multiplexing (OFDM) communication system.  
         [0004]     2. Description of the Related Art  
         [0005]     Although there are many wireless communication technologies proposed as candidates for high-speed mobile communications, OFDM is considered the most prominent future-generation wireless communication technology. It is expected that OFDM will be adopted for most wireless communication applications by the year 2010. OFDM has been adopted as a standard for an Institute of Electrical and Electronics Engineers (IEEE) 802.16 Wireless Metropolitan Area Network (WMAN) categorized into the 3.5 th  Generation (3.5G) technology.  
         [0006]     In a conventional OFDM communication system, a transmitter simultaneously sends pilot subcarrier signals to a receiver with transmission of data subcarrier signals. The receiver performs synchronization acquisition, channel estimation, and Base Station (BS) identification using the pilot subcarrier signals. A transmission rule for sending the pilot subcarrier signals is known as “pilot pattern”.  
         [0007]     The pilot pattern is determined in consideration of coherence bandwidth and coherence time. The coherence bandwidth is the maximum bandwidth over which a channel is relatively constant or non-distorting in the frequency domain, and the coherence time is the maximum time for which the channel is relatively constant in the time domain. Since the channel can be assumed to be constant over the coherence bandwidth for the coherence time, one pilot signal suffices for synchronization acquisition, channel estimation and BS identification.  
         [0008]     Conventionally, a fixed pilot pattern is used and existing adaptive pilot pattern techniques focus on optimization of pilot power and throughput. A technique for adaptively changing a pilot pattern according to a link status (e.g. coherence bandwidth and coherence time) is yet to be developed.  
         [0009]     Radio channels are said to be a wide range of random channels and it is difficult to always ensure optimum performance over these random channels with the conventional fixed pilot pattern. Assuming that a pilot pattern is created using the same number of pilots, a layout of pilot subcarriers affects performance according to the link status, which in turn directly influences channel estimation performance. That is, the channel estimation performance may increase according to the pilot pattern. Since the channel estimation performance has an influence on Bit Error Rate (BER), it is important to select a pilot pattern that minimizes channel estimation errors.  
       SUMMARY OF THE INVENTION  
       [0010]     An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide an apparatus and method for adaptively changing a pilot pattern according to a link in a BWA communication system.  
         [0011]     An object of the present invention is to provide an apparatus and method for selecting a pilot pattern that minimizes channel estimation errors in a BWA communication system.  
         [0012]     A further object of the present invention is to provide an apparatus and method for adaptively changing an uplink pilot pattern according to an uplink in a BWA communication system.  
         [0013]     Another object of the present invention is to provide an apparatus and method for selecting a pilot pattern based on the Mean Square Error (MSE) of frequency-domain data in a BWA communication system.  
         [0014]     According to the present invention, in an apparatus for determining a pilot pattern in a BWA communication system, an OFDM demodulator generates frequency-domain data by fast-Fourier-transform (FFT)-processing a received signal. A pilot pattern decider calculates a coherence bandwidth and a coherence time using subcarrier values received from the OFDM demodulator, and selects one of a plurality of pilot patterns according to the ratio of the coherence bandwidth to the coherence time.  
         [0015]     According to the present invention, in a method of determining a pilot pattern in a BWA communication system, subcarrier values are generated by FFT-processing a received signal. A coherence bandwidth and a coherence time are calculated using the subcarrier values. One of a plurality of pilot patterns is selected according to the ratio of the coherence bandwidth to the coherence time. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0017]      FIG. 1  is a block diagram of a transmitter in a BWA communication system according to the present invention;  
         [0018]      FIG. 2  is a block diagram of a receiver in the BWA communication system according to the present invention;  
         [0019]      FIG. 3  is a detailed block diagram of a pilot pattern decider illustrated in  FIG. 2 ;  
         [0020]      FIG. 4  is a flowchart illustrating an operation for feeding back a pilot pattern in the receiver in the BWA communication system according to the present invention;  
         [0021]      FIG. 5  is a graph illustrating a first embodiment of a pilot pattern selection method according to the present invention;  
         [0022]      FIG. 6  is a graph illustrating a second embodiment of a pilot pattern selection method according to the present invention; and  
         [0023]      FIG. 7  is a graph comparing the present invention using an adaptive pilot pattern with a conventional technology using a fixed pilot pattern in terms of performance. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail for the sake of clarity and conciseness.  
         [0025]      FIG. 1  is a block diagram of a transmitter in a BWA communication system according to present invention. The transmitter is a relative concept, in that it is a Mobile Station (MS) on the uplink and a BS on the downlink.  
         [0026]     Referring to  FIG. 1 , the transmitter includes a pilot pattern generator  100 , an encoder  102 , a modulator  104 , a data mapper  106 , a frame buffer  108 , an OFDM modulator  110 , a Digital-to-Analog Converter (DAC)  112  and a Radio Frequency (RF) processor  114 .  
         [0027]     In operation, the encoder  102  encodes an input information bit stream at a coding rate and outputs the resulting coded bits or code symbols. With the number of the information bits being denoted by k and the coding rate being denoted by R, the number of the code symbols is k/R. The encoder  102  may be a convolutional encoder, a turbo encoder or a Low Density Parity Check (LDPC) encoder.  
         [0028]     The modulator  104  generates complex symbols by mapping the coded symbols to signal points according to a modulation scheme or modulation level. For example, the modulation scheme is one of Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 8-ary Phase Shift Keying (8PSK), 16-ary Quadrature Amplitude Modulation (16QAM) and 64QAM. One bit (s=1) is mapped to one signal point (i.e. complex symbol) in BPSK, two bits (s=2) are mapped to one signal point in QPSK, three bits (s=3) are mapped to one signal point in 8PSK, four bits (s=4) are mapped to one signal point in 16QAM and six bits (s=6) are mapped to one signal point in 64QAM.  
         [0029]     The data mapper  106  maps the complex symbols to subcarriers according to a control signal received from a higher layer and provides the mapped complex symbols to memory addresses in the frame buffer  108  of an actual frame size.  
         [0030]     The pilot pattern generator  100  selects a pilot pattern according to feedback information (i.e. pilot pattern selection information) received from a receiver and outputs pilot symbols to corresponding addresses of the frame buffer  108  according to the selected pilot pattern. For example, if a frame size is 64×64 and one frame delivers 64 pilot symbols, a 16×4 first pilot pattern and a 4×16 pilot pattern are available. According to the first pilot pattern, the pilot symbols are mapped equidistantly at an interval of 16 on the frequency axis and at an interval of 4 on the time axis. The second pilot pattern indicates that the pilot symbols are mapped equidistantly at an interval of 4 on the frequency axis and at an interval of 16 on the time axis. The first pilot pattern is viable when a coherence bandwidth is greater than a coherence time, while the second pilot pattern is used when the coherence time is greater than the coherence bandwidth.  
         [0031]     The frame buffer  108  is a buffer for ordering the complex symbols in an actual transmission order, prior to input to the OFDM modulator  110 . The frame buffer  108  sequentially outputs the buffered complex symbols based on timing synchronization on an OFDM symbol basis.  
         [0032]     The OFDM modulator  110  converts the complex symbols received from the frame buffer  107  to time-domain sample data by Inverse-Fast-Fourier-Transform (IFFT)-processing the complex symbols, and adds a copy of a last part of the sample data to the sample data, thereby generating an OFDM symbol.  
         [0033]     The DAC  112  converts the sample data to an analog signal. The RF processor  114 , including a filter and a front-end unit, processes the analog signal to an RF signal and ends the RF signal on a radio channel through a transmit antenna. The transmitted signal experiences a multi-path channel, is added with noise, and then arrives at a receive antenna of the receiver.  
         [0034]      FIG. 2  is a block diagram of a receiver in the BWA communication system according to the present invention. The receiver is a relative concept, in that the BS is the receiver on the uplink and the MS is the receiver on the downlink.  
         [0035]     Referring to  FIG. 2 , the receiver includes a pilot pattern decider  200 , an RF processor  202 , an Analog-to-Digital Converter (ADC)  204 , an OFDM demodulator  206 , a subcarrier demapper  208 , an equalizer  210 , a demodulator  212 , a decoder  214  and a channel estimator  216 .  
         [0036]     In operation, the RF processor  202 , including a front-end unit and a filter, downconverts an RF signal received on a radio channel to a baseband signal. The ADC  204  converts the analog baseband signal received form the RF processor  202  to a digital signal.  
         [0037]     The OFDM demodulator  206  removes a Cyclic Prefix (CP) from the digital data and generates frequency-domain data by FFT-processing the CP-removed data.  
         [0038]     The subcarrier demapper  208  extracts data symbols from the OFDM-demodulated data received from the OFDM demodulator  206  and outputs the data symbols to the equalizer  210 . It also extracts pilot symbols at pilot subcarrier positions and provides the extracted pilot symbols to the channel estimator  216 .  
         [0039]     The channel estimator  216  performs channel estimation using the pilot symbols. The equalizer  210  channel-compensates the received data symbols based on channel estimate values received from the channel estimator  216 , that is, compensates for noise created on the radio channel.  
         [0040]     The demodulator  212  demodulates the symbols received form the equalizer in accordance with the modulation scheme used in the transmitter and outputs the resulting coded data. The decoder  214  decodes the coded data corresponding to the coding method used in the transmitter, thereby recovering the original information data.  
         [0041]     The pilot pattern decider  200  calculates the mean square error (MSE) of each of the channel-compensated subcarrier values received from the equalizer  210  and calculates a coherence bandwidth and a coherence time by accumulating the MSEs on the frequency and time axes. The pilot pattern decider  200  selects a pilot pattern by comparing the ratio of the coherence bandwidth to the coherence time with a threshold value and generates feedback information indicating the selected pilot pattern. The feedback information is sent to the transmitter on a feedback channel.  
         [0042]     If the coherence bandwidth is greater than the coherence time, a pilot pattern is selected which maps pilot symbols densely along the time axis. If the coherence time is greater than the coherence bandwidth, a pilot pattern is selected which maps pilot symbols densely along the frequency axis.  
         [0043]      FIG. 3  is a detailed block diagram of the pilot pattern decider  200  illustrated in  FIG. 2 .  
         [0044]     Referring to  FIG. 3 , the pilot pattern decider  200  includes a frame buffer  300 , a mean square error (MSE) calculator  302 , a frequency-axis accumulator  304 , a time-axis accumulator  306 , a first arranger  308 , a second arranger  310 , a first selector  312 , a second selector  314 , a first adder  316 , a second adder  318 , a ratio calculator  320  and a pilot pattern selector  322 .  
         [0045]     In operation, the frame buffer  300  buffers the frequency-domain frame data received form the equalizer  210 . A 64×64 frame size (40964 subcarrier values) is assumed. The MSE calculator  302  calculates the MSEs of the subcarrier values (received complex symbols) received from the frame buffer  300 . The MSEs can be calculated in any one of known methods and a description of the MSE calculation will not be provided herein.  
         [0046]     The frequency-axis accumulator  304  accumulates the calculated 64×64 MSE values along the frequency axis or column by column, thereby creating  64  accumulation values. The time-axis accumulator  306  accumulates the calculated 64×64 MSE values along the time axis or row by row, thereby creating  64  accumulation values.  
         [0047]     The first arranger  308  orders the 64 accumulation values received from the frequency-axis accumulator  304  in a descending order, and the second arranger  310  orders the 64 accumulation values received from the time-axis accumulator  306  in a descending order.  
         [0048]     The first selector  312  selects a number of (e.g. 32) accumulation values among the ordered accumulation values from the first arranger, starting from the highest accumulation value, and the second selector  314  selects a number of (e.g. 32) accumulation values among the ordered accumulation values from the second arranger  310 , starting from the highest accumulation value.  
         [0049]     The first adder  316  generates a value C b  indicating the coherence bandwidth by summing the accumulation values received the first selector  312 , and the second adder  318  generates a value C t  indicating the coherence time by summing the accumulation values received from the second selector  314 .  
         [0050]     The ratio calculator  320  calculates the ratio R of the coherence bandwidth to the coherence time by dividing C b  by C t  and then calculating the Log  10  of the division result.  
         [0051]     The pilot pattern selector  322  compares the ratio R with a threshold value, selects a pilot pattern based on the comparison adaptively according to the link, and generates feedback information indicating the selected pilot pattern.  
         [0052]     The ratio R may be inaccurate when too a small number of MSEs are in an entire frame. In this case, the sum of C b  and C t  is compared with a threshold value. If the sum is less than the threshold value, a pilot pattern is selected based on the average of previous C b  to C t  ratio.  
         [0053]      FIG. 4  is a flowchart illustrating an operation for feeding back a pilot pattern in the receiver in the BWA communication system according to the present invention.  
         [0054]     Referring to  FIG. 4 , the receiver downconverts an RF signal received through an antenna to a baseband signal and OFDM-demodulates the baseband signal, thus acquiring frequency-domain data in step  401 .  
         [0055]     In step  403 , the receiver extracts symbols at subcarrier positions (i.e. pilot symbols) from the frequency-domain data and performs channel estimation using the extracted symbols.  
         [0056]     The receiver channel-compensates the frequency-domain data using channel estimate values in step  405  and calculates the MSEs of the channel-compensated subcarrier values (complex symbols) in step  407 .  
         [0057]     In step  409 , the receiver accumulates the MSE values along the frequency axis, selects a number of accumulation values in a descending order, and sums them. The sum is the coherence bandwidth C b . In step  411 , the receiver accumulates the MSE values along the time axis, selects a number of accumulation values in a descending order, and sums them. The sum is the coherence time C t .  
         [0058]     The receiver calculates the ratio R of the coherence bandwidth to the coherence time by dividing C b  by C t  and calculating the Log  10  of the division result in step  413 .  
         [0059]     In step  415 , the receiver compares the ratio R with 1. If R is less than 1, which implies that the coherence bandwidth is greater than the coherence time, the receiver selects a first pilot pattern in which pilot symbols are mapped more densely along the time axis in step  417 .  
         [0060]     If R is equal to or greater than 1, which implies that the coherence time is greater than the coherence bandwidth, the receiver selects a second pilot pattern in which pilot symbols are mapped more densely along the frequency axis in step  419 .  
         [0061]     After selecting the pilot pattern, the receiver generates feedback information indicating the selected pilot pattern and sends the feedback information on the feedback channel to the transmitter in step  421 .  
         [0062]      FIG. 5  is a graph illustrating a first embodiment of a pilot pattern selection method according to the present invention. A 64×64 frame size and mapping of 64 pilot symbols per frame are assumed.  
         [0063]     Referring to  FIG. 5 , a pilot pattern is selected in which the ratio R (C b /C t ) converges to 1. If the coherence bandwidth is greater than the coherence time, the 16×4 first pilot pattern is selected in which pilot symbols are mapped at an interval of 16 along the frequency axis and at an interval of 4 along the time axis. If the coherence time is greater than the coherence bandwidth, the 4×16 second pilot pattern is selected in which pilot symbols are mapped at an interval of 4 along the frequency axis and at an interval of 16 along the time axis. In this manner, although the same number of pilot symbols are allocated to each frame, the layout of the pilot symbols is adapted to the link status.  
         [0064]      FIG. 6  is a graph illustrating a second embodiment of a pilot pattern selection method according to the present invention.  
         [0065]     Referring to  FIG. 6 , two thresholds are set for pilot pattern switching. If the ratio R (C b /C t ) is greater than a first threshold, the 16×4 first pilot pattern is selected. If the ratio R is between the first and second thresholds, an 8×8 third pilot pattern is selected in which pilot symbols are mapped at an interval of 8 along the frequency axis and at an interval of 8 along the time axis. If the ratio R is less than the second threshold, the 4×16 second pilot pattern is selected. This pilot pattern selection method leads to robust operation against factors such as sudden channel changes or frame delay by use of a smooth transitional pattern. Preferably, the first and second thresholds are empirically obtained.  
         [0066]     A simulation was performed to verify the performance of the present invention, under the following parameters listed in Table 1.  
                           TABLE 1                                       Carrier frequency   5 GHz           FFT size   1024 (16 users)           Bandwidth   40 MHz           Symbol length   25.6 μs           Guard interval   3.2 μs (T/8)           Max delay spread   5 ns, 10 ns           Mobile velocity   5 km/h, 160 km/h           Modulation   QPSK           Channel estimator   Bilinear interpolation           Pilot patterns used   3 (4 × 16, 8 × 8, 16 × 4)           Pilot density   &lt;2%           User frame size   64 × 64           Channels   AWGN, 3-ray indoor                      
 
         [0067]      FIG. 7  is a graph comparing the present invention using an adaptive pilot pattern with a conventional technology using a fixed pilot pattern in terms of performance.  
         [0068]     Referring to  FIG. 7 , the horizontal axis represents received signal strength (E b /N o ) and the vertical axis represents Bit Error Rate (BER). As noted from the graph, the adaptive pilot pattern method of the present invention outperforms the conventional fixed pilot pattern method.  
         [0069]     As described above, the present invention advantageously reduces channel estimation errors by adaptively mapping the same number of pilot symbols in a different layout according to the link status. The minimization of channel estimation errors leads to the increase of data BER. Also, the present invention reduces signaling overhead by switching a pilot pattern on a frame-by-frame basis.  
         [0070]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.