This application claims priority under 35 U.S.C. §119 to a Korean application filed in the Korean Intellectual Property Office on Jan. 5, 2006 and allocated Ser. No. 2006-1473, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to an apparatus and method for communicating data in a broadband wireless communication system, and more particularly, to an apparatus and method for communicating data in a hybrid diversity mode in a broadband wireless communication system, the hybrid diversity mode being a hybrid of a diversity mode and a band Adaptive Modulation and Coding (AMC) mode.
2. Description of the Related Art
Many wireless communication technologies have been proposed for high-speed mobile communications. Among them, Orthogonal Frequency Division Multiplexing (OFDM) is considered as the most powerful next-generation wireless communication technology. OFDM technology is expected to come into commercial use in most wireless communication systems around 2010. The Institute of Electrical and Electronics Engineers (IEEE) 802.16 based Wireless Metropolitan Area Network (WMAN), called “3.5th generation (3.5G) technology”, adopts the OFDM technology as its standard specification.
A broadband wireless communication system using the OFDM technology generally transmits data in a diversity mode or a band AMC mode.
In the diversity mode, data are distributed and transmitted over the entire frequency band and band AMC is possible only in a time domain. The diversity mode can cope well with a wireless channel with frequency-selective characteristics because it uses the entire frequency band by hopping between subbands. In addition, the diversity mode reduces complexity and overhead because it uses a general Medium Access Protocol (MAP) and requires only time-domain scheduling.
In the band AMC mode, only a few best subbands of the entire frequency band are selected and used for data transmission. Therefore, the band AMC mode can increase an average Signal-to-Interference-plus-Noise Ratio (SINR) and thus can provide a high data rate. However, because triggering conditions must be met in order to use the band AMC mode, only a few subscriber stations (SSs) in a cell can use the band AMC mode. In addition, the band AMC mode increases complexity and overhead because it must use a band AMC MAP and requires frequency-domain scheduling as well as time-domain scheduling.
As described above, the use of the band AMC mode is desirable for achieving a high data throughput. However, because the triggering conditions must be met to use the band AMC mode, most SSs in a cell have no choice but to operate in the diversity mode. In the diversity mode, a data rate (or an AMC level) is determined by an average channel value of the entire frequency band. Therefore, the average channel value in the diversity mode is greatly reduced due to subbands with very poor channel conditions.
In use, although data can be transmitted at a high data rate by excluding subbands with very poor channel conditions from data transmission, occasionally data are transmitted at a very low data rate due to the use of the subbands with very poor channel conditions. In this way, there may be an SS that exhibits poor performance in both the diversity mode and the band AMC mode. What is therefore required is a new technology that can enhance the advantages and overcome the disadvantages of the diversity mode and the band AMC mode.
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 enhancing a data throughput in a broadband wireless communication system.
Another object of the present invention is to provide an apparatus and method for excluding, in the case of a diversity mode, frequency subbands with poor channel conditions from data transmission in a broadband wireless communication system.
A further object of the present invention is to provide an apparatus and method for transmitting, using a frequency-selective fading effect, high-rate data to an SS that is moving at a medium or low speed in a broadband wireless communication system.
Still another object of the present invention is to provide an apparatus and method for providing a hybrid diversity mode, which is a hybrid of a diversity mode and a band AMC mode, in a broadband wireless communication system.
According to one aspect of the present invention, there is provided a method for transmitting data to an SS in a broadband wireless communication system. In the method, null subbands are determined using feedback information received from the SS. TX data to be transmitted to the SS are mapped to a diversity zone from which the null subbands has been excluded. The mapped data are OFDM-modulated and transmitted to the SS.
According to another aspect of the present invention, there is provided a method for receiving data in a broadband wireless communication system. In the method, a received signal is OFDM-demodulated to generate frequency-domain data. A channel is estimated using the frequency-domain data. If the broadband wireless communication system operates in a hybrid diversity mode, null subbands with poor channel conditions are determined using the channel estimation value. Information indicating the null subbands is fed back to a transmitter.
According to another aspect of the present invention, there is provided a apparatus for transmitting data in a broadband wireless communication system. The apparatus comprises a controller for determining null subbands using feedback information received from a subscriber station (SS), a subchannel mapper for mapping TX data to be transmitted to the SS to a diversity zone from which the null subbands has been excluded, and an orthogonal frequency division multiplexing (OFDM) modulator for inverse fast Fourier transform processing the mapped data from the subchannel mapper.
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:
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 since they would obscure the invention in unnecessary detail. Also, the terms used herein are defined according to the functions of the present invention. Thus, the terms may vary depending on user's or operator's intent and usage. That is, the terms used herein must be understood based on the descriptions made herein.
The present invention provides a hybrid diversity mode that is a hybrid of a diversity mode and a band AMC mode. In the hybrid diversity mode, a diversity mode is basically used whereby frequency subbands with poor channel conditions are excluded from data transmission.
As illustrated in
Referring to
In addition, controller 100 controls a coding rate of encoder 102 and a modulation level of modulator 104 according to the determined MSC level, and controls a subchannel mapping operation of subchannel mapper 106 based on the determined null subbands.
Encoder 102 encodes an input data bitstream at a predetermined coding rate to output encoded data (i.e., encoded bits or symbols). When the number of the input data bits and the coding rate are k and R, respectively, the number of the output symbols is k/R. Examples of encoder 102 include a convolutional encoder, a turbo encoder, or a Low Density Parity Check (LDPC) encoder.
Modulator 104 maps the output symbols of encoder 102 to signaling points by a predetermined modulation scheme (or modulation level) to output complex symbols. Examples of the predetermined modulation scheme include a Binary Phase Shift Keying (BPSK) modulation scheme, a Quadrature Phase Shift Keying (QPSK) modulation scheme, an 8-ary Phase Shift Keying (8PSK) modulation scheme, a 16-array Quadrature Amplitude Modulation (16QAM) scheme, and a 64-array Quadrature Amplitude Modulation (16QAM) scheme. The BPSK modulation scheme maps one bit (s=1) to one signaling point (complex symbol), the QPSK modulation scheme maps two bits (s=2) to one complex symbol, the 8PSK modulation scheme maps four bits (s=3) to one complex symbol, the 16QAM scheme maps six bits (s=4) to one complex symbol, and the 64QAM scheme maps six bits (s=6) to one complex symbol.
Subchannel mapper 106 maps the output complex symbols of modulator 104 to corresponding zones according to a subchannel allocation scheme provided from the controller, and outputs the mapped complex symbols to corresponding memory addresses of frame buffer 108 corresponding to an actual frame size. Examples of the subchannel allocation scheme include a diversity allocation scheme, a hybrid diversity allocation scheme, and a band AMC allocation scheme, as described above. In the case of the hybrid diversity mode according to the present invention, subchannel mapper 106 maps the complex symbols to corresponding diversity zones. At this point, symbols to be transmitted are mapped to the remaining frequency band except the null subbands that are determined by the feedback information from the SS.
Frame buffer 108 is a buffer for arranging, in accordance with an actual order, complex symbols to be provided to OFDM modulator 110. Based on time synchronization, frame buffer 108 buffers complex symbols and sequentially outputs the buffered complex symbols on an OFDM symbol basis.
OFDM modulator 110 inverse fast Fourier transform (IFFT)-processes and converts the output complex symbols of frame buffer 108 into time-domain sample data, and attaches a copy of a rear portion of the sample data to the front of the sample data to output an OFDM symbol.
DAC 112 converts the sample data of OFDM modulator 110 into an analog signal. RF processor 114 includes a filter and a front-end unit. RF processor 114 RF-processes and converts the output analog signal of DAC 112 into an RF signal, and transmits the RF signal through a TX antenna over a wireless channel. The TX signal of the transmitter undergoes a multipath channel to become a noise-containing signal, and the noise-containing signal is received at an RX antenna of a receiver.
As illustrated in
Referring to
OFDM demodulator 206 removes a Cyclic Prefix (CP) from the output data of ADC 204, and fast Fourier transform (FFT)-processes the resulting data to output frequency-domain data.
Subchannel demapper 208 extracts actual data symbols from the output data of OFDM demodulator 206 to output the actual data symbols to equalizer 210. Also, subchannel demapper 208 extracts symbols (e.g., pilot symbols) of a predetermined location for channel estimation to output the pilot symbols to channel estimator 216.
Channel estimator 216 performs channel estimation using the pilot symbols of subchannel demapper 208, and provides the channel estimation values to controller 200 and equalizer 210. At this point, it is assumed that channel estimator 216 also performs SINR estimation.
Using the channel estimation values from subchannel demapper 208, equalizer 210 channel-compensates the data symbols of subchannel demapper 208 to output channel-compensated symbols. That is, equalizer 210 compensates for various noises generated at the wireless channel.
Demodulator 212 demodulates the output symbols of equalizer 210 according to the modulation scheme used in the transmitter, thereby outputting encoded data. Decoder 214 decodes the encoded data of demodulator 212 to restore original information data.
In the case of the hybrid diversity mode according to the present invention, controller 200 calculates an average channel value of each subband using the channel estimation values from channel estimator 216, and compares the average channel value with a predetermined threshold to determine null subbands. Thereafter, controller 200 generates CQI (e.g., SINR) of the remaining frequency band except the null subbands, and feeds information indicating the null subbands and the CQI back to a BS. For example, the information indicating the null subbands and the CQI are transmitted to the BS over a CQI channel.
Referring to
In step 303, the transmitter analyzes feedback information received from an SS to determine null subbands. In step 305, using the remaining frequency band except the determined null subbands, the transmitter performs scheduling to determine an MCS level and time/frequency resources to which the TX data are to be mapped.
In step 307, the transmitter encodes and modulates the TX data at the determined MCS level. In step 309, the transmitter maps the encoded/modulated data to a diversity zone according to the scheduling results. At this point, the encoded/modulated data are mapped to the remaining frequency band except the null subbands.
In step 311, the transmitter OFDM-modulates the mapped data, converts the OFDM-modulated data into an RF signal, and transmits the RF signal through an antenna.
Referring to
In step 403, the receiver extracts symbols (e.g., pilot symbols) at a predetermined subcarrier location for channel estimation from the frequency-domain data. In step 405, the receiver performs channel estimation using the extracted symbols, thereby acquiring the channel estimation values for the entire frequency band.
In step 407, the receiver determines if a current mode is a hybrid diversity mode. If so, the receiver proceeds to step 409, and if not, the receiver proceeds to step 417. In step 417, the receiver performs other operation such as an operation corresponding to a diversity mode or a band AMC mode.
In step 409, the receiver calculates an average channel value (e.g., SINR) for each subband using the channel estimation values. In step 411, the receiver compares the calculated average channel value with a predetermined threshold to determine null subbands. At this point, the criteria for determining the null subbands and the number of the null subbands may be set by various methods.
In step 413, the receiver generates CQI of the remaining frequency band except the null subbands. The CQI may be an SINR of the remaining frequency band. In step 415, the receiver feeds information indicating the null subbands and the CQI back to a transmitter (such as a BS). The information indicating the null subbands and the CQI may be periodically fed back to the BS over a CQI channel.
Criteria for selecting one of a diversity mode, a band AMC mode, and a hybrid diversity mode can be set by various methods. For example, an optimal transmission mode may be selected according to the moving speed of the SS. The diversity mode is suitable when the moving speed is high (above 30 km/h). The band AMC mode is suitable when the moving speed is low (below 3 km/h). The hybrid diversity mode is suitable when the moving speed is medium (3-30 km/h). Alternatively, the transmission mode may be determined according to the channel condition of the SS (e.g., frequency-selective fading characteristics, average SINR).
The results of a simulation for verifying the performance of the present invention are as follows:
Simulation parameters are shown in Table 1 below.
The graph of
As can be seen from
As the simplest example, the transmission mode is switched according to the moving speed of an SS.
The graph of
As can be seen from
As described above, the present invention excludes the frequency subbands with poor channel conditions from data transmission, thereby making it possible to enhance the FER performance and transmit data at a high MCS level. Also, because the diversity mode is basically used in the SS, the present invention can be applied also to the SS moving at a high speed. Also, because diversity MAP allocation is possible, the hybrid diversity mode according to the present invention is simpler in operation and lower in signaling overhead than the conventional band AMC mode.
Although the downlink transmission has been taken as an example in the above embodiment, the hybrid diversity mode according to the present invention can be similarly applied to uplink transmission.
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 further defined by the appended claims.
Number | Date | Country | Kind |
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2006-0001473 | Jan 2006 | KR | national |