METHOD AND APPARATUS FOR PERFORMING DIGITAL COMMUNICATIONS
A wired and wireless communication method, and more particularly a method and apparatus for reducing a PAPR (Peak to Average Power Ratio) in an OFDM (Orthogonal Frequency Division Multiplexing) system. A transmitter of the OFDM system designates, as a coset leader, a vector capable of minimizing a PAPR in a general standard array of (n, k) linear block codes and transmits a sequence having a minimum PAPR by adding each coset leader to an n-bit codeword corresponding to k-bit information and producing U (≦2n-k) number of vectors. Then, a receiver of the OFDM system can easily recover an original transmission signal using a syndrome of a received vector if the receiver identifies information of the syndrome and the coset leader. Subsequently, the OFDM system can be designed irrespective of the number of carriers, i.e., N, and enhance the system's performance by increasing a value of U. Moreover, the OFDM system different from the conventional SLM (Selective Mapping) system does not have to transmit, to the receiver, information indicating which signal has been selected.
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This application claims priority to an application entitled “METHOD AND APPARATUS FOR DIGITAL COMMUNICATIONS”, filed in the Korean Industrial Property Office on Jan. 23, 2002 and assigned Serial No. 2002-3901, the contents of which are hereby incorporated by reference herein. This application is a divisional of and claims priority under 35 U.S.C. § 121 to U.S. application Ser. No. 10/349,414 filed in the U.S. Patent and Trademark Office on Jan. 22, 2003, the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates to a wired and wireless communication method, and more particularly a method and apparatus for reducing a PAPR (Peak to Average Power Ratio) in an OFDM (Orthogonal Frequency Division Multiplexing) system.
2. Description of the Related Art
In a typical parallel data processing system, an entire signal frequency band is divided into N number of frequency sub-channels, which do not overlap one another. Respective symbols are modulated through the sub-channels, and frequency division multiplexing is applied to the N sub-channels.
Hereinafter, the operation of the conventional OFDM system will be described with reference to
The analog signal transmitted through the air interface 17 is inputted into an LPT A/D 18. The LPT A/D 18 converts the analog signal received from the channel 17 into a digital signal and then outputs the digital signal to a guard interval remover 19. The guard interval remover 19 removes the guard interval inserted into the digital signal and then outputs, to a serial to parallel converter 20, the digital signal from which the guard interval is removed. The serial to parallel converter 20 converts the digital signal from which the guard interval is removed, i.e., the serial OFDM signals, into parallel OFDM signals, and then outputs the parallel OFDM signals in a unit of d0, d1, . . . , dn-1 bits. The parallel OFDM signals are inputted into a DFT (Discrete Fourier Transform) circuit 21. The DFT circuit 21 carries out DFT for the parallel OFDM signals and then outputs Fourier Transformed symbols. The symbols are inputted into a signal demapper 22. The signal demapper 22 demodulates the symbols and then outputs the parallel data, X bits. The parallel data are inputted into a parallel to serial converter 23. The parallel to serial converter 23 converts the parallel data into one serial data stream and then outputs the serial data stream.
In an OFDM communication system using N number of carriers, it is assumed that a kth OFDM signal is represented as a modulated signal Ai,k (i=0, 1, . . . , N−1) allocated to an ith carrier in a given symbol duration T. Each modulated signal Ai,k is one of the symbols in a constellation plot based on modulation. Using the modulated signal Ai,k, an envelope having a complex value of an OFDM baseband signal is as follows.
In the above Equation 1, g(t) denotes a rectangular pulse having width T, and T denotes an OFDM symbol duration. To maintain the orthogonality between OFDM carriers, an ith carrier frequency fi can be represented as the following Equation 2 in terms of a center frequency fc.
fi=fc+iΔf [Equation 2]
In the above Equation 2, Δf means a bandwidth of one carrier, and is an integral multiple of an OFDM symbol rate 1/T.
Looking into several prominent characteristics of the OFDM system, when the OFDM system is compared with a single carrier system identical with the OFDM system in a transmission bandwidth and data transmission rate, a duration of one symbol to be transmitted in the OFDM system is approximately a multiple N of the duration of one symbol to be transmitted in the single carrier system, in the case where data to be transmitted is distributed on N carriers. As a result, the duration of one symbol in the OFDM system is longer than that of one symbol in the single carrier system. In addition, if a guard interval is added in a time domain, the degradation of transmission characteristics due to delay becomes less even though the number of multiple paths is increased.
Further, since data distributed on the entire transmission band is transmitted, an interference signal affects only a portion of the data in the case where the interference signal exists in a specific frequency band, and the OFDM system can efficiently improve its performance using an interleaver and an error correcting code.
Conventionally, in a multi-carrier transmission method, the peak envelope power of a multi-carrier signal increases in proportion to the number of carriers. If N signals in the OFDM system overlap in the same phase, the peak envelope power increases by a multiple of N of the average power. A PAPR is referred to as a peak to average power ratio of a multi-carrier signal. If the PAPR increases, A/D (Analog/Digital) or D/A (Digital/Analog) conversion is complicated and the efficiency of an RF (Radio Frequency) power amplifier is reduced.
Thus, research to reduce the PAPR is actively conducted and reducing the PAPR is one of problems to be necessarily addressed in order to efficiently implement the OFDM system having superior performance in RF and optical communications.
Where a symbol sequence of {A0, A1, . . . AN-1} having complex values is transmitted through N number of carriers, an OFDM baseband signal s(t) is represented as the following Equation 3 with respect to time tε[0, T].
A PAPR of the OFDM baseband signal s(t) is defined by the following Equation 4.
PAPR(s)=Maximum instantaneous power of s(t)/Average power of s(t) [Equation 4]
Referring to the above Equation 4, where Ai is an MPSK (Multiple Phase-Shift Keying) modulated symbol and the average power has a value of N, the maximum instantaneous power can have a value of N2 and the PAPR has a value of N.
Thus, if an OFDM signal is generated using a symbol sequence, it has very high maximum instantaneous power and a high PAPR. Since IDFT (Inverse Discrete Fourier Transform) and DFT are used for modulation and demodulation in the OFDM system, baseband OFDM symbols in an arbitrary symbol duration are represented as N number of sample values and hence the baseband OFDM symbols can be represented as follows.
As defined in the above Equation 5, L*N-point IDFT is considered to produce the PAPR. L is an over sampling factor. If a sequence of N modulated inputs is A={A0, A1, . . . , AN-1}, a sequence A′={A0′, A1′, . . . , ALN-1′}={A0, A1, . . . , AN-1, 0, 0, . . . , 0} including (L−1)N number of 0's is considered to take the LN-point IDFT. After taking the L*N-point IDFT of the sequence A in one symbol duration using the sequence A′, an nth sample is represented as follows.
Because the calculation of the PAPR with respect to continuous signals is complicated, the PAPR is calculated by considering only LN samples of OFDM signals associated with the sequence A. That is, the PAPR considering LN-point IDFT samples of the sequence A is defined as follows.
As defined in the above Equation 6, s[n] is a sample by the LN-point IDFT, and L denotes an oversampling factor. Further, E is an operator taking the mean of values of OFDM signal s[n] for all n. The case of L=1 is referred to as Nyquist sampling. It is well known that the PAPR can be sufficiently obtained if the oversampling factor L is 4, in order to obtain the PAPR shown in the above Equation 5 being a function of actually continuous time.
Several methods for reducing the PAPR in the OFDM communication system have been suggested. In a simplest method for reducing the PAPR, signal clipping is considered to limit a maximum size of a signal to a predetermined size or less.
The conventional clipping is the simplest method for reducing the PAPR, but has several problems. At first, the clipping causes amplitude of an OFDM signal to be distorted, and hence self-interference is generated to increase a BER (Bit Error Rate). Further, because the distortion of the OFDM signal is non-linear, it causes out-of-band frequency characteristics to be degraded.
On the other hand, in another method for reducing the PAPR in the OFDM system, a Golay sequence becomes an important factor in reducing the PAPR in the OFDM system. Where only the Golay sequence is used, there is an advantage in that a value of the PAPR is limited to 2 (3 dB). However, there is a disadvantage in that a code rate is rapidly reduced as the number of carriers increases.
A conventional error-correcting code technique can be used to reduce the PAPR in the OFDM system. In the conventional technique, only a codeword having peak envelope power of a small value is selected such that an OFDM signal can be generated to reduce the entire PAPR. However, there is a problem in that a code rate is greatly reduced as the number of carrier signals increases.
There is SLM (Selective Mapping) as another conventional technique. A basic concept of the SLM is to generate a plurality of OFDM signals indicating the same information. That is, the SLM generates U number of sequences indicating the same information and transmits a sequence having a smallest PAPR among the U sequences.
Hereinafter, the operation of a transmitter of the conventional SLM-based OFDM system will be described with reference to
The U number of sequences a(u) for u=1, 2, . . . , U in the time domain produced by the IDFT of each sequence A(u) is given by Eq. 7:
a(u)=IDFT{A(u)},u=1,2, . . . , U [Equation 7]
All sequences a(1) to a(u) including the same information A, and a signal to be actually transmitted among the sequences a(1) to a(u) has the smallest PAPR value.
Theoretically, as the number of independent sequences, i.e., a value of U, increases, the characteristic of the PAPR becomes better. However, if the value of U increases, the complexity of the system increases.
Also, there is another significant disadvantage. Side information must be transmitted from a transmitter to a receiver because the receiver must identify which sequence is actually used at the transmitter among U number of sequence to recover original information. Moreover, if the side information have an error during transmission, a burst error can be caused, thereby greatly degrading the system's performance. Thus, the side information should be highly protected during transmission.
There is a PTS (Partial Transmit Sequence) method as another conventional technique. The conventional PTS method divides an input sequence into M number of independent and partial blocks and then shifts a phase of each partial block, thereby generating a plurality of sequences and reducing the PAPR. Side information associated with the phase shift should also be transmitted such that a receiver can recover original information.
SUMMARY OF THE INVENTIONTherefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an OFDM (Orthogonal Frequency Division Multiplexing) communication method and apparatus capable of reducing a PAPR (Peak to Average Power Ratio) without affecting the efficiency of information transmission.
It is another object of the present invention to provide an OFDM (Orthogonal Frequency Division Multiplexing) communication method and apparatus capable of reducing a PAPR (Peak to Average Power Ratio) irrespective of the number of carriers without transmitting additional side information.
In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of an OFDM (Orthogonal Frequency Division Multiplexing) transmitter, comprising: an (n, k) linear block encoder for outputting an n-bits codeword c=(c0, c1, . . . , cn-1) when a k-bits information block d=(d0, d1, . . . , dk-1) is inputted; U number of adders for generating signals a1, a2, . . . , aU (U≦2n-k) by performing bit-wise modulo-2 addition of a set of coset leaders e1, e2, . . . , eU and the codeword c, the coset leaders associated with different syndromes being selected in relation to a parity-check matrix H for an (n, k) code; U number of m-ary signal mappers each mapping an output of a respective adder to a signal symbol in a unit of m bits and then generating a discrete signal xi consisting of N number of symbols; U number of LN-point IDFT (Inverse Discrete Fourier transform) circuits each transforming the discrete signal xi from a respective m-ary signal mapper into a discrete signal yi consisting of LN number of samples in a time domain, L being an oversampling factor; and a peak detector for searching for a signal yj having a smallest maximum value among the LN number of samples to detect a signal having minimum peak power among U number of discrete signals y1, y2, yU from the LN-point IDFT circuits.
BRIEF DESCRIPTION OF THE DRAWINGSThe above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
Now, a technique for reducing a PAPR (Peak to Average Power Ratio) in accordance with the present invention will be described in detail with reference to
If noise on a channel is indicated as e when a codeword c of (n, k) linear block codes is transmitted, a received vector is defined as r=c+e or ri=ci+ei at i=1, 2, . . . , n. If H is an (n−k)×n parity-check matrix, then Hct=0, where t is a transposed matrix. The syndrome s of the received vector r is defined as follows.
The syndrome is affected by an error caused on a channel rather than by the transmitted codeword. If e+C={x|x=e+c, cεC} denotes a coset of a code C with a vector e, the syndrome of a vector x belonging to the coset e+C is as follows.
Hxt“=H(e+c)t=He” [Equation 9]
Thus, all vectors within the coset e+C have syndromes related to the vector e.
The simplest method for decoding a block code is to use the standard array. Referring to
According to a method for generating a general standard array, the standard array of (n, k) block codes is generated as shown in
Because a vector within the coset can be selected as a corresponding coset leader, a vector for minimizing the PAPR within the coset is designated as a coset leader. On the basis of the coset leader, the PAPR can be reduced using the generated standard array.
In a preferred embodiment of the present invention, where a codeword corresponding to information to be transmitted is ci, U number of vectors of the coset of ci+eu at u=1, 2, . . . , U are generated using previously designated coset leaders eu at u=1, 2, . . . , U, and a vector having a smallest PAPR among the vectors is transmitted. Thus, if a receiver identifies information related to a syndrome and a coset leader, an original transmission signal ci can be simply recovered using only a syndrome of a received vector.
Referring to
Then, coset leaders 105, i.e., e1, e2, . . . , eU have different syndromes in relation to a parity-check matrix H for an (n, k) code, and the coset leaders are stored in a receiver and transmitter to be used later on or in syndrome computation. Signals a1, a2, . . . , aU are generated by bit-wise modulo-2 addition of the coset leaders and the codeword c, wherein U≦2n-k. The (n, k) code has error correcting capability by maintaining a value of U smaller than a value of 2n-k and restrictedly selecting coset leaders e1, e2, . . . , eU.
In the preferred embodiment of the present invention, ECC (error-correcting code) encoders 110 can be optionally used to appropriately correct an error on a channel. In this case, if each n-bit signal ai is inputted into a respective (mN, n) ECC encoder 110, an mN-bit output bi is generated. Where the ECC is not used, relationships of bi=ai and n=mN are satisfied.
On the other hand, the mN-bit output bi is inputted into an m-ary signal mapper 120 and then a discrete signal xi consisting of N number of symbols is produced, wherein each symbol corresponds to m bits. Then, the discrete signal xi consisting of the N symbols is converted to a discrete signal yi consisting of LN number of samples in a time domain by an LN-point IDFT circuit 130, wherein L is an oversampling factor.
Peak detector 140 detects a signal having minimum peak power among U number of discrete signals y1, y2, . . . , yU by finding a signal yi having a smallest maximum value among LN number of samples. The discrete signal xj corresponding to a signal yj is applied to N-point IDFT, the discrete signal xj applied to the N-point IDFT is filtered by a low frequency band-pass filter, and then an OFDM signal for the information block d=(d0, d1, . . . , dk-1) is produced.
Hereinafter, an operation of an OFDM receiver in accordance with the present invention will be described with reference to
Finally, an n-bit codeword c is produced by correcting the coset leader ei associated with the vector r by the equation c=r−ei. The k-bit information block d is obtained from the codeword c. In other words, the receiver can produce a syndrome of a received signal to determine a coset leader used upon transmitting, and then produce an original transmission vector by adding the coset leader to the received vector.
Side information associated with a transmitted signal must be sent to the receiver in the conventional SLM, while additional side information does not have to be transmitted to a receiver if the receiver identifies only information of a coset leader using a standard array in accordance with the present invention. While the efficiency of a frequency is slightly degraded in the above-described two methods, where the standard array in accordance with the present invention is used, the reduction of the frequency efficiency can be compensated to some degree by using a block linear code with a high code rate.
Features and advantages of the present invention have been broadly disclosed such that the following claims of the present invention can be better understood. Other features and advantages configuring the following claims of the present invention will be described in detail. Those skilled in the art will appreciate that the idea and specific embodiments of the present invention described above can be implemented by a design or modification of another structure for accomplishing an object similar to the present invention's object.
Further, those skilled in the art will appreciate that the idea and specific embodiments of the present invention described above can be implemented by a design or modification of another structure in order to accomplish an object identical to the present invention's object. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope of the invention. Accordingly, the present invention is not limited to the above-described embodiments, but the present invention is defined by the claims which follow, along with their full scope of equivalents.
As apparent from the above-description, a transmitter designates, as a coset leader, a vector capable of minimizing a PAPR (Peak to Average Power Ratio) in a general standard array and transmits a sequence having a minimum PAPR on the basis of the coset leader in accordance with the present invention. In accordance with the present invention, a receiver can simply recover an original transmission signal using a syndrome of a received vector if the receiver identifies information of the syndrome and the coset leader.
In accordance with the present invention, an OFDM (Orthogonal Frequency Division Multiplexing) system can be designed irrespective of the number of carriers, i.e., N, and enhance the system's performance by increasing a value of U (≦2n-k). Moreover, the OFDM system in accordance with the present invention is superior to the conventional SLM (Selective Mapping) technique because it does not have to transmit information indicating which signal has been selected.
Claims
1. An OFDM (Orthogonal Frequency Division Multiplexing) receiver, comprising:
- an N-point DFT (Discrete Fourier Transform) circuit for transforming a signal produced by sampling a received baseband OFDM signal into a signal x=(x0, x1,..., xN-1) consisting of N number of samples through N-point DFT;
- an m-ary signal demapper for producing an n-bit binary vector r=(r0, r1,..., rn-1) in response to the signal x outputted from the N-point DFT circuit; and
- an adder for receiving a syndrome of the binary vector r=(r0, r1,..., rn-1) produced using a parity-check matrix H for an (n, k) code and thereby producing a coset leader ei, performing a modulo-2 addition of the binary vector r and the coset leader ei, and producing an n-bit codeword c.
2. The OFDM receiver as set forth in claim 1, further comprising:
- an ECC (Error-Correcting code) decoder connected to an output terminal of the m-ary signal demapper.
3. A method for decoding an OFDM (Orthogonal Frequency Division Multiplexing) signal in an OFDM system for dividing an entire signal frequency band into N number of sub-channels which do not overlap and performing frequency division multiplexing, comprising the steps of:
- (a) transforming a signal produced by sampling a received baseband OFDM signal into a signal x=(x0, x1,..., xN-1) consisting of N number of samples in a frequency domain through N-point DFT (Discrete Fourier Transform);
- (b) allowing an m-ary signal demapper to produce an n-bit binary vector r=(r0, r1,..., rn-1) in response to the signal x;
- (c) producing a syndrome of the binary vector r using a parity-check matrix H for an (n, k) code and producing a coset leader ei; and
- (d) producing an n-bit codeword c by performing correction through a modulo-2 addition of the binary vector r and the coset leader ei, and producing a k-bit information block from the codeword c.
4. The method as set forth in claim 3, wherein the step (b) comprises the step of:
- producing the n-bit binary vector r through an ECC (Error-Correcting code) decoder connected to an output terminal of the m-ary signal demapper.
Type: Application
Filed: Sep 21, 2007
Publication Date: Jan 10, 2008
Applicant: SAMSUNG ELECTRONICS CO., LTD. (Suwon-si)
Inventor: Jung-Min SON (Kyongsangbuk-do)
Application Number: 11/859,643
International Classification: H04J 11/00 (20060101);