1. Field of the Invention
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) communication system, and more particularly to an apparatus and a method for generating a preamble sequence for minimizing the interference between neighbor cells in an OFDM communication system.
2. Description of the Related Art
Generally, a wireless communication system includes a Node B and user equipment (UE) in order to provide users with wireless communication services. The Node B and the UE support the wireless communication services by using transmission frames. Accordingly, the Node B and the UE must obtain mutual synchronization for transmitting and receiving the transmission frames. To this end, the Node B transmits a synchronizing signal to the UE to allow the UE to recognize a start point of the frame transmission. Upon receiving the synchronizing signal from the Node B, the UE checks the frame timing of the Node B and demodulates the transmission frames received therein according to the frame timing. In general, a preamble sequence, which is preset between the Node B and the UE, is used as a synchronizing signal.
In addition, a preamble sequence used in an OFDM communication system must have a low peak to average power ratio (PAPR). A long preamble sequence is transmitted to the UE from the Node B. The long preamble sequence includes a preamble sequence required for coarse synchronization and a short preamble sequence required for micro frequency synchronization. In addition, the short preamble sequence is transmitted to the Node B from the UE so as to obtain the micro frequency synchronization. The following examples described why the preamble sequence of the OFDM communication system must have a low PAPR. First, the OFDM communication system is a multi carrier communication system employing a plurality of carriers, that is, a plurality of sub-carriers, so orthogonality of the sub-carriers becomes an important factor. Thus, each sub-carrier has its own phase such that the sub-carriers are orthogonal to each other. If the phase is changed during the sub-carrier transmission/reception procedure, the sub-carriers may overlap with each other. In this case, the overlapped sub-carriers may deviate from a linear interval of an amplifier provided in the OFDM communication system, so a normal signal transceiving operation is impossible. For this reason, the OFDM communication system uses the preamble sequence having a minimized PAPR.
The OFDM communication system transmits data to various UEs by time-multiplexing one frame. In addition, the OFDM communication system transmits a frame preamble sequence for a predetermined interval from a start point of the frame in order to indicate the beginning of the frame transmission. Since data to be transmitted to each UE may be irregularly included in one frame, a burst preamble sequence indicating a start of the data may be transmitted prior to the transmission of each data. Accordingly, the UE must receive a data preamble sequence in order to recognize the start point of the data transmission. That is, the UE must synchronize with the start point of data transmission in order to receive the data, so the UE needs to be synchronized with the preamble sequence commonly used in the OFDM communication system before receiving a signal.
In the meantime, a source coding scheme, a channel coding scheme and a modulation scheme of the OFDM communication system are identical to those of communication systems which do not employ the OFDM scheme. A Code Division Multiple Access (CDMA) communication system transmits data by spreading the data. However, the OFDM communication system transmits data after performing Inverse Fast Fourier Transform (IFFT) with regard to the data and inserting a guard interval signal into the data. Thus, different from the CDMA communication system, the OFDM communication system can transmit broadband signals with a relatively simple hardware structure. The OFDM communication system combines a plurality of bit/symbol arrays after modulating data and inputs parallel bit/symbol arrays into an IFFT unit corresponding to a frequency domain, thereby outputting IFFT time domain signals. The IFFT time domain signal is obtained by multiplexing a broadband signal into a plurality of narrow band sub-carrier signals. In addition, a plurality of modulation symbols are transmitted through the IFFT procedure during one OFDM symbol interval.
If the above IFFT OFDM symbols are transmitted in the OFDM communication system, interference may occur between previously-transmitted OFDM symbols and currently-transmitted OFDM symbols. In order to avoid the interference between the OFDM symbols, the guard interval signal is inserted into the data. The guard interval signal can be created through a “cyclic prefix” scheme, in which predetermined number end samples of the OFDM symbols in the time domain are copied and inserted into effective OFDM symbols, or a “cyclic postfix” scheme, in which predetermined number of beginning samples of the OFDM symbols in the time domain are copied and inserted into effective OFDM symbols. Sizes of the beginning and end samples used in the cyclic prefix scheme and the cyclic postfix scheme are predetermined in the OFDM communication system. A receiver can time-frequency synchronize the OFDM symbols by using the characteristics of the guard interval signals, which are created by inserting the copied beginning or end parts of the OFDM symbols into the effective OFDM symbols.
A transmission signal from a transmitter becomes distorted while passing through a wireless channel environment, and the receiver may receive the distorted transmission signal. Upon receiving the distorted transmission signal from the transmitter, the receiver obtains time-frequency synchronization by using a preamble sequence, which is preset between the transmitter and the receiver, and performs channel estimation. After that, the signals are demodulated into frequency domain symbols through fast Fourier transform (FFT). Then, the receiver performs a channel decoding process and a source decoding process, which correspond to the channel coding in the transmitter, with regard to the demodulated symbols, thereby decoding the symbols into information data.
The OFDM communication system employs the preamble sequence when performing the frame timing synchronization, frequency synchronization and channel estimation. It is also possible for the OFDM communication system to perform the frame timing synchronization, frequency synchronization and channel estimation by using the guard interval and pilot sub-carriers instead of the preamble sequence. When the preamble sequence is employed, known symbols are transmitted with every frame or at a start point of a data burst. At this time, estimated time-frequency-channel information is used for updating transmission data by using information related to guard interval signals and sub-carriers.
Hereinafter, a preamble sequence structure employed in a conventional OFDM communication system will be described with reference to FIGS. 1 and 2.
FIG. 1 is a diagram showing a structure of a long preamble sequence used in the conventional OFDM communication system. The current OFDM communication system uses the same preamble sequence in a down link (DL) and an up link (UL). Referring to FIG. 1, the long preamble sequence includes four 64-length sequences and two 128-length sequences. According to the characteristic of the OFDM communication system, the cyclic prefix is added in front of the four 64-length sequences and in front of the two 128-length sequences. In addition, the signals before the IFFT are frequency domain signals and the signals after the IFFT are time domain signals. The long preamble sequence shown in FIG. 1 is a time domain long preamble sequence obtained after the IFFT has been carried out.
In addition, the long preamble sequence in a frequency domain before the IFFT is represented as follows:
                    S        ⁡                  (                                    -              100                        :            100                    )                    =                          ⁢                        {                                                    +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            -              j                        ,            0            ,            0            ,            0            ,                                          -                                                                  ⁢                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                                                                  ⁢                1                            -              j                        ,            0            ,            0            ,            0            ,                                          -                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                                                                  ⁢                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            -              j                        ,            0            ,            0            ,            0            ,                                          -                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            -              j                        ,            0            ,            0            ,            0            ,                                          +                                                                  ⁢                1                            -              j                        ,            0            ,            0            ,            0            ,                                          +                1                            -              j                        ,            0            ,            0            ,            0            ,                                          -                1                            -              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          -                                                                  ⁢                1                            +              j                        ,            0            ,            0            ,            0            ,                                          -                1                            +              j                        ,            0            ,            0            ,            0            ,                                          -                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          -                                                                  ⁢                1                            -              j                        ,            0            ,            0            ,            0            ,                                                  ⁢            0            ,            0            ,            0            ,            0            ,                                          -                                                                  ⁢                1                            -              j                        ,            0            ,            0            ,            0            ,                                          +                1                            -              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          -                1                            -              j                        ,            0            ,            0            ,            0            ,                                          -                                                                  ⁢                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            -              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          -                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                                                                  ⁢                1                            -              j                        ,            0            ,            0            ,            0            ,                                          -                1                            -              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          -                1                            +              j                        ,            0            ,            0            ,            0            ,                                          -                                                                  ⁢                1                            -              j                        ,            0            ,            0            ,            0            ,                                          +                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                1                            -              j                        ,            0            ,            0            ,            0            ,                                          -                1                            -              j                        ,            0            ,            0            ,            0            ,                                          +                                                                  ⁢                1                            -              j                        ,            0            ,            0            ,            0            ,                                          +                1                            -              +                        ,            0            ,            0            ,            0            ,                                          -                1                            -              j                        ,            0            ,            0            ,            0            ,                                          -                1                            +              j                        ,            0            ,            0            ,            0            ,                                          -                                                                  ⁢                1                            +              j                        ,            0            ,            0            ,            0            ,                                          -                1                            -              j                        ,            0            ,            0            ,            0            ,                                          +                1                            -              j                        ,            0            ,            0            ,            0            ,                                          -                1                            +              j                        ,            0            ,            0            ,            0            ,                                          +                                                                  ⁢                1                            +              j                                }                *                  sqrt          ⁡                      (            2            )                          *                  sqrt          ⁡                      (            2            )                                ⁢                                    P        ⁡                  (                                    -              100                        :            100                    )                    =                        {                                    -              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,                          0              -              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,                          0              +              1                        ,            0            ,                          -                                                          ⁢                                                          ⁢              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,                          0              -              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          -                                                          ⁢                                                          ⁢              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,                          0              -              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,                          0              -              1                        ,            0            ,                          -                                                          ⁢                                                          ⁢              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,                          0              +              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,                          0              +              1                        ,            0            ,                          -                                                          ⁢                                                          ⁢              1                        ,            0            ,                          -              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,                          0              +              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,                          0              +              1                        ,            0            ,                                                  ⁢                                                  ⁢            0            ,                          0              -                                                          ⁢              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,                          0              +              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          +                                                          ⁢              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,            0            ,                          -              1                        ,            0            ,                          -              1                        ,            0            ,                          -              1                        ,                          0              -              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,            0            ,                          -                                                          ⁢              1                        ,            0            ,                          -              1                        ,            0            ,                          -              1                        ,            0            ,                          -              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,                          0              +              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          -                                                          ⁢              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          +              1                        ,                          0              +              1                        ,            0            ,                          -              1                        ,            0            ,                          -              1                        ,            0            ,                          -              1                        ,            0            ,                          -                                                          ⁢              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,                          0              -              1                        ,            0            ,                          -              1                        ,            0            ,                          +              1                        ,            0            ,                          -              1                                }                *                  sqrt          ⁡                      (            2            )                          *                  sqrt          ⁡                      (            2            )                                ⁢                
In the above long preamble sequence of the frequency domain, the numbers shown in S(−100:100) and P(−100:100) represent positions of sub-carriers employed when performing the IFFT, which will be described later with reference to FIG. 3. S(−100:100) represents a frequency domain sequence including four 64-length sequences and P(−100:100) represents a frequency domain sequence including two 128-length sequences. In S(−100:100) and P(−100:100), sqrt(2) signifies root 2, and sqrt(2)*sqrt(2) signifies two-step amplification for increasing the transmit power of S(−100:100) and P(−100:100).
A structure of a short preamble sequence will be described with reference to FIG. 2.
FIG. 2 is a diagram showing a structure of a short preamble sequence of a conventional OFDM communication system. Referring to FIG. 2, the short preamble sequence includes two 128-length sequences. The cyclic prefix is added in front of the two 128-length sequences. In addition, the short preamble sequence shown in FIG. 2 is a time domain short preamble sequence after the IFFT. P(−100:100) is a frequency domain short preamble sequence.
The long preamble sequence must be created by taking the following factors into consideration.
(1) Low PAPR
In order to maximize the transmission rate of a power amplifier provided at a transmission terminal of a transmitter of an OFDM communication system, the OFDM symbol must have a low PAPR. As mentioned above, after the IFFT has been performed with regard to the signal, the signal is input into a power transmitter. At this time, since the power transmitter has a non-linear characteristic, the low PAPR is required. That is, it is necessary for the time domain OFDM symbol corresponding to an IFFT output terminal to have a low PAPR, and the OFDM symbols must have uniform distribution so as to minimize the PAPR. In other words, if symbols having a low cross correlation are provided in an IFFT input terminal of a transmitter, that is, in the frequency domain, the PAPR can be reduced.
(2) Parameter estimation required for communication initialization
Parameter estimation includes channel estimation, frequency offset estimation, and time offset estimation.
(3) Low complexity and overhead
(4) Coarse frequency offset estimation
The long preamble sequence created by taking the above factors into consideration has the following functions.
(1) The long preamble sequence including the four 64-length sequences is used for time offset estimation and coarse frequency offset estimation.
(2) The long preamble sequence including the two 128-length sequences is used for micro frequency offset and channel estimation.
The long preamble sequence is used in the OFDM communication system for the purpose of following objects.
(1) A first preamble sequence of a forward PDU (protocol data unit)
(2) Initial ranging
(3) Bandwidth request ranging
In addition, the short preamble sequence is used in the OFDM communication system for the purpose of following objects.
(1) Backward data preamble sequence
(2) Periodic ranging
Since the OFDM communication system can obtain precise synchronization by performing initial ranging and periodic ranging, the backward data preamble sequence is mainly used for the purpose of channel estimation. The PAPR, performance and complexity must be considered when performing the channel estimation. In a case of a short preamble sequence, the PAPR is 3.5805dB, and various channel estimation algorithms including minimum mean square error (MMSE) and least square (LS) can be used.
A mapping relationship between the sub-carriers and the preamble sequence when performing the IFFT in the conventional OFDM communication system will be described with reference to FIG. 3.
FIG. 3 is a diagram illustrating a mapping relationship between sub-carriers and the preamble sequence when performing the IFFT in the conventional OFDM communication system.
Referring to FIG. 3, the OFDM communication system employs 256 sub-carriers including #−128 to #127 sub-carriers, and 200 sub-carriers including #−100 to #100 sub-carriers are actually used. In FIG. 3, numbers shown at a front of an IFFT unit represent frequency components, that is, the numbers of the sub-carriers. Herein, from among the 256 sub-carriers, 200 sub-carriers except for a #0 sub-carrier, #−128 to #−101 sub-carriers, and #101 to #127 sub-carriers are actually used. When the #0 sub-carrier, #−128 to #−101 sub-carriers, and #101 to #127 sub-carriers are transmitted, null data (that is, zero data) is inserted into the sub-carriers due to following reasons. First, the null data are inserted into the 0th sub-carrier because the #0 sub-carrier represents the DC component at a reference point of the preamble sequence, that is, in a time domain after the IFFT has been carried out. In addition, the null data is inserted into the 28 sub-carriers including #−128 to #−101 sub-carriers and 27 sub-carriers including #101 to #127 sub-carriers in order to insert a guard interval in a frequency domain because the above sub-carriers correspond to a high frequency band in the frequency domain.
Thus, if a frequency domain preamble sequence S(−100:100) or P(−100:100) is input into the IFFT unit, the IFFT unit maps the frequency domain preamble sequence S(−100:100) or P(−100:100) into corresponding sub-carriers and performs IFFT, thereby outputting a time domain preamble sequence.
A structure of a transmitter provided in a conventional OFDM communication system will be described with reference to FIG. 4.
FIG. 4 is a block diagram illustrating a structure of a transmitter provided in a conventional OFDM communication system. Referring to FIG. 4, if information bits to be transmitted are generated, the information bits are input into a symbol mapper 411. Upon receiving the information bits, the symbol mapper 411 modulates the information bits into symbols through a predetermined modulation scheme and sends the symbols to a serial to parallel converter 413. The predetermined modulation scheme includes a quadrature phase shift keying (QPSK) scheme and a 16 quadrature amplitude modulation (16QAM) scheme. Upon receiving the symbols from the symbol mapper 411, the serial to parallel converter 413 converts the symbols such that the symbols match with an A-point, which is an input number of an inverse fast Fourier transformer (hereinafter, referred to as IFFT unit) 419 and sends the symbols to a selector 417. In addition, a preamble sequence generator 415 generates preamble sequences under the control of a controller (not shown) and sends the preamble sequences to the selector 417. The selector 417 selects a signal output from the serial-to-parallel converter 413 or a signal output from the preamble sequence generator 415 according to a predetermined scheduling scheme and sends the signal to the IFFT unit 419.
Upon receiving the signal from the selector 417, the IFFT unit 419 performs A-point IFFT with regard to the signal and sends the signal to a parallel-to-serial converter 421. In addition, the parallel-to-serial converter 421 receives a cyclic prefix signal having a predetermined length in addition to the signal output from the IFFT unit 419.
The “cyclic prefix” signal is a guard interval signal. The guard interval signal can be inserted into the data through a “cyclic prefix scheme”, in which predetermined number of end samples of the OFDM symbols in the time domain are copied and inserted into effective OFDM symbols, or a “cyclic postfix” scheme, in which predetermined number of beginning samples of the OFDM symbols in the time domain are copied and inserted into effective OFDM symbols. Sizes of the beginning and end samples used in the cyclic prefix scheme and the cyclic postfix scheme are predetermined in the OFDM communication system. In FIG. 4, the guard interval signals are inserted into the data through the cyclic prefix scheme. The parallel-to-serial converter 421 converts the signal output from the IFFT unit 419 and the guard interval signal into serial signals and sends the serial signals to a digital-to-analog converter 423. Upon receiving the serial signals from the parallel-to-serial converter 421, the digital to analog converter 423 converts the serial signals into analog signals and sends the analog signals to an RF (radio frequency) processor 425. The RF processor 425 includes a filter and a front end unit (not shown). The RF processor 425 processes the analog signals output from the digital-to-analog converter 423 such that the signals can be transmitted through air and sends the signals through an antenna.
A preamble sequence and a method for creating the preamble sequence in a conventional OFDM communication system employing N transmit antennas will be described.
In order to estimate the channels of the data transmitted through N transmit antennas in a receiver of the OFDM communication system employing N transmit antennas, it is necessary to transmit preamble sequences through N transmit antennas. However, if at least two antennas are provided in the OFDM communication system, a preamble sequence transmission scheme for estimating the channels of the data transmitted through the transmit antennas has not been yet suggested.
If the preamble sequence is allocated by assigning different sub-carriers to each transmit antenna, the receiver can perform channel estimation only for the sub-carriers assigned for the preamble sequence and cannot perform channel estimation for remaining sub-carriers. Therefore, if at least two transmit antennas are provided, a preamble sequence transmission rule is required for channel estimation with regard to all of the sub-carriers transmitted through the transmit antennas.
In addition, as mentioned above, the OFDM communication system must use a preamble sequence having a low PAPR. Studies and research have been extensively carried out for providing a preamble sequence having a low PAPR in a current OFDM communication system employing one transmit antenna. Similar to the OFDM communication system employing one transmit antenna, the OFDM communication system employing a plurality of transmit antennas must use a preamble sequence having a low PAPR. However, a method for providing the preamble sequence having the low PAPR in the OFDM communication system employing a plurality of transmit antennas has not yet been suggested. Accordingly, it is necessary to provide a method for generating the preamble sequence having low PAPR in the OFDM communication system.
Hereinafter, a preamble sequence and a method for generating the preamble sequence in a cellular OFDM communication system will be described.
First, since the cellular OFDM communication system has a multi-cell structure, all of the Node Bs of the cellular OFDM communication system must transmit the preamble sequence with the same time interval in order to minimize the interference between the neighbor Node Bs. If the Node Bs transmit the preamble sequence with different time intervals, interference between Node Bs may increase, thereby degrading performance of the preamble sequence and deteriorating synchronization and channel estimation performance.
In short, if all of the Node Bs transmit the preamble sequence with the same time interval, interference between the Node Bs can be minimized. Therefore, it is necessary to provide a method for generating the preamble sequence capable of obtaining synchronization and maximizing channel estimation performance by minimizing interference between neighbor Node Bs.