CN103001916B - Time domain reshaping method of orthogonal frequency division multiplexing (OFDM) communication system - Google Patents

Abstract

The invention discloses a time-domain shaping method of an OFDM communication system, which is used for reducing the loss of information energy caused by cross-border in system communication, and belongs to the technical field of communication. The method of the present invention performs time-domain shaping on the data part of the OFDM frame structure, concentrates the signal energy in the middle part of the OFDM symbol, and has less energy on both sides, thereby effectively reducing the loss of information energy caused by the signal crossing the boundary and improving the system efficiency. performance. The method of the invention is simple to implement and can be used in various existing OFDM communication systems.

Description

A Time Domain Shaping Method for OFDM Communication System

technical field

The invention relates to the field of communication technology, in particular to a time domain shaping method of an OFDM communication system, which is used to reduce the loss of information energy caused by cross-border in system communication.

Background technique

Orthogonal Frequency Division Multiplexing (OFDM, Orthogonal Frequency Division Multiplexing) is a multi-carrier broadband digital modulation technology. OFDM technology has the characteristics of good anti-interference ability, high spectrum utilization rate and large transmission capacity. The orthogonal modulation and demodulation in each sub-channel is achieved by inverse fast Fourier transform (IFFT) or fast Fourier transform ( FFT) to achieve. In the traditional OFDM system, after the serial-to-parallel conversion of the input bit sequence is completed, the corresponding modulation mapping is completed according to the modulation method adopted to form a modulation information sequence pair, and the IFFT is performed on the information pair to calculate the time domain of the OFDM modulated signal The sampling sequence is added with the cyclic prefix CP, and then D/A conversion is performed to obtain the time domain waveform of the OFDM modulated signal. The receiver first performs A/D conversion on the received time-domain signal, removes the cyclic prefix CP, and obtains the sampling sequence of the OFDM modulated signal, and performs FFT on the sampling sequence to obtain the original modulation information sequence.

The OFDM system forms a guard interval GI by introducing a cyclic prefix to effectively counteract Inter-Symbol Interference (ISI for short) and Inter-Carrier Interference (ICI for short) caused by multipath delay. However, a too wide guard interval period will make the system efficiency worse, so a low guard interval OFDM scheme has emerged. Regardless, in some cases the signal still crosses the guard interval, resulting in loss of signal energy, as well as ISI and ICI.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the signal energy loss problem in the existing OFDM communication system due to the signal crossing the guard interval, and to provide a time-domain shaping method for the OFDM communication system, thereby reducing the system communication due to cross-border The loss of information energy caused by it improves the system performance.

The present invention specifically adopts the following technical solutions to solve the above technical problems:

A time-domain shaping method for an OFDM communication system,

At the sending end of the OFDM communication system, the frequency domain signal of the transmitted data obtained after digital modulation is processed as follows: the odd numbered term of the frequency domain signal is multiplied by -1, and the even numbered term remains unchanged to obtain a new frequency domain signal; perform inverse fast Fourier transform on the new frequency domain signal; multiply the odd term of the signal obtained by the inverse fast Fourier transform by -1, and keep the even term unchanged to obtain the time domain of the final transmitted data shaping signal;

At the receiving end of the OFDM communication system, the following processing is performed on the time-domain shaping signal of the received transmission data: the odd numbered term of the time-domain shaping signal is multiplied by -1, and the even numbered term remains unchanged to obtain a new time-domain signal ; performing fast Fourier transform on the new time domain signal; multiplying the odd items of the signal obtained by the fast Fourier transform by -1, and keeping the even items unchanged, to obtain the frequency domain signal of the transmitted data.

The invention performs time-domain shaping on the data part of the OFDM frame structure, concentrates the signal energy in the middle part of the OFDM symbol, and has less energy on both sides, thereby effectively reducing the loss of information energy caused by the signal crossing the boundary and improving system performance .

Description of drawings

Fig. 1 is the frame structure of OFDM communication system;

Fig. 2 a is the flowchart of the transmitting end of the basic OFDM time domain shaping scheme;

Fig. 2b is the flowchart of the receiving end of the basic OFDM time domain shaping scheme;

Figure 3a is a flow chart of the sending end of the DCO-OFDM time domain shaping scheme;

Figure 3b is a flow chart of the receiving end of the DCO-OFDM time domain shaping scheme;

Figure 4a is a frequency domain diagram obtained by the traditional wireless OFDM method after a pseudo-random sequence with a period of 127 is modulated by BPSK, and Figure 4b is a time domain diagram obtained;

Fig. 5 a is the frequency domain diagram obtained by using the method of the present invention to carry out time domain shaping after the pseudo-random sequence with a period of 127 is modulated by BPSK, and Fig. 5 b is the obtained time domain diagram;

Figure 6a is the frequency domain diagram obtained by the traditional DCO-OFDM method after the pseudo-random sequence with a period of 63 is modulated by BPSK, and Figure 6b is the obtained time domain diagram;

Fig. 7a is a frequency domain diagram obtained by using the method of the present invention to perform time domain shaping after the pseudo-random sequence with a period of 63 is modulated by BPSK, and Fig. 7b is the obtained time domain diagram.

Detailed ways

The technical scheme of the present invention is described in detail below in conjunction with accompanying drawing:

The idea of the present invention is to improve the existing OFDM communication system, perform time-domain shaping on the data part of the OFDM frame structure, concentrate the signal energy in the middle part of the OFDM symbol, and have less energy on both sides, thereby reducing the signal caused by signal crossing. loss of information energy. The structure of the OFDM signal frame is shown in Figure 1, and the present invention only reshapes the data part in it, and keeps the other parts unchanged. The present invention specifically adopts the following methods:

At the sending end of the OFDM communication system, the frequency domain signal of the transmitted data obtained after digital modulation is processed as follows: the odd numbered term of the frequency domain signal is multiplied by -1, and the even numbered term remains unchanged to obtain a new frequency domain signal; perform inverse fast Fourier transform on the new frequency domain signal; multiply the odd term of the signal obtained by the inverse fast Fourier transform by -1, and keep the even term unchanged to obtain the time domain of the final transmitted data shaping signal;

At the receiving end of the OFDM communication system, the following processing is performed on the time-domain shaping signal of the received transmission data: the odd numbered term of the time-domain shaping signal is multiplied by -1, and the even numbered term remains unchanged to obtain a new time-domain signal ; performing fast Fourier transform on the new time domain signal; multiplying the odd items of the signal obtained by the fast Fourier transform by -1, and keeping the even items unchanged, to obtain the frequency domain signal of the transmitted data.

The time-domain waveform obtained by using the above scheme is actually the shaping of the time-domain waveform obtained by the traditional OFDM method, that is, the 0th to 1st points to the right point, the Go to N-1 points and move to the left in turn points, so that the signal energy is concentrated in the middle part of the OFDM symbol.

The method of the invention can be used in a traditional wireless OFDM communication system, and can also be used in a non-coherent system such as OFDM in an optical communication system. In order to facilitate the public's understanding, the technical solution of the present invention will be further described below by taking a wireless OFDM communication system and a DCO-OFDM (DC-offset optical OFDM) communication system as examples.

For a wireless OFDM communication system, the sending end of the system first digitally modulates the communication data by means of QAM or QPSK, and converts the transmitted digital signal into the mapping of subcarrier amplitude and phase to obtain the frequency domain signal of the modulated transmitted data. Sequence, denoted as: X(k)(k=0,1,…,N-1); Divide the frequency domain signal X(k)(k=0,1,…,N-1) into even items X( 2m)(m=0,1,…,N/2-1) and odd items X(2m+1)(m=0,1,…,N/2-1); keep even items X(2m)( m=0,1,…,N/2-1) remains unchanged, multiply the odd term X(2m+1)(m=0,1,…,N/2-1) by -1 to get a new frequency domain signal; perform N-point IFFT operation on the new frequency domain signal to obtain x'(n)(n=0,1,...,N-1); then divide the sequence x'(n) into even-numbered items x' (2r)(r=0,1,...,N/2-1), odd item x'(2r+1)(r=0,1,...,N/2-1); the sequence x'(n )’s even-numbered items x’(2r)(r=0,1,…,N/2-1) remain unchanged, and odd-numbered items x’(2r+1)(r=0,1,…,N/2-1 ) multiplied by -1 to obtain the final shaped time-domain signal x(n)(n=0,1,...,N-1). The above shaping process is shown in Figure 2a. The transmitter adds the shaped time domain signal to the guard interval (or cyclic prefix) to form OFDM symbols, and then adds synchronization sequences, channel estimation sequences, etc. during framing to obtain and output baseband signals.

For the received baseband signal, the receiving end of the system performs time synchronization, fractional multiplier estimation and correction first, and then obtains the shaped time domain signal x″(n)(n=0,1,…,N-1), Multiply the odd term x"(2m+1)(m=0,1,...,N/2-1) of the signal sequence x"(n)(n=0,1,...,N-1) by -1 , the even term x″(2m)(m=0,1,…,N/2-1) remains unchanged, and a new time-domain signal is obtained; X″ is obtained after performing N-point FFT operations on the new time-domain signal (k)(k=0,1,...,N-1); the odd item X"(2r+1)( r=0,1,...,N/2-1) multiplied by -1, keeping the even term X″(2r)(r=0,1,...,N/2-1) unchanged, to get the final frequency domain The signal X'(k) (k=0,1,...,N-1) is the digitally modulated frequency domain signal of the digital sequence to be transmitted by the sending end. The above process is shown in Figure 2b. Corresponding digital demodulation is performed on the frequency domain signal X′(k) (k=0, 1, . . . , N−1) to obtain the transmitted data bit stream.

In the traditional optical transmission system, the receiving end adopts direct detection, and the intensity of the generated optical signal is related to the input electrical signal, and the information is carried on the optical intensity, which requires the input electrical signal to be non-negative, while OFDM is a bipolar signal. For applications in optical transmission systems, OFDM symbols are required to be positive real numbers. One of these methods is the DCO-OFDM (DC Offset Optical OFDM) method. For the DCO-OFDM communication system, the digitally modulated frequency domain signal x(n)(n=1,…,N/2-1) of the information bit stream transmitted by the sending end needs to be mapped accordingly to realize the real number of OFDM symbols , the frequency domain signal after realization The mapping expression for is as follows:

${<span\; class="notranslate">\; \{</span>{\mathrm{<span\; class="notranslate">\; the\; s</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; \}</span>}_{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\left[\begin{array}{cccc}\mathrm{<span\; class="notranslate">\; 0</span>}& {<span\; class="notranslate">\; \{</span>{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; \}</span>}_{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}^{\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}& \mathrm{<span\; class="notranslate">\; 0</span>}& {<span\; class="notranslate">\; \{</span>{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; \}</span>}_{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}^{\mathrm{<span\; class="notranslate">\; 1</span>}}\end{array}\right]$

Similarly, the signal sequence The odd-numbered items s(2m+1)(m=0,1,…,N/2-1) are multiplied by -1, and the even-numbered items s(2m)(m=0,1,…,N/2-1) Keeping it unchanged, a new frequency domain signal is obtained; the time domain signal sequence y'(n)(n=0,1,...,N-1) is obtained after performing N-point IFFT operations on the new frequency domain signal; The odd term y'(2r+1)(r=0,1,...,N/2-1) of the domain signal sequence y'(n) is multiplied by -1, and the even term y'(2r)(r=0, 1,...,N/2-1) remains unchanged, and the final shaped time-domain signal y(n) (n=0,1,...,N-1) is obtained. The above process is shown in Figure 3a. The transmitter adds the shaped time domain signal y(n) to the guard interval (or cyclic prefix) to form OFDM symbols, and then adds synchronization sequences, channel estimation sequences, etc. during framing to obtain baseband signals and output them.

For the received baseband signal, the receiving end of the system performs time synchronization, fractional multiplier estimation and correction first, and then obtains the shaped time domain signal y″(n) (n=0,1,…,N-1); Multiply the odd term y″(2m+1)(m=0,1,…,N/2-1) of the signal sequence y″(n)(n=0,1,…,N-1) by -1 , the even term y″(2m) (m=0,1,…,N/2-1) remains unchanged, and a new time-domain signal is obtained; after performing N-point FFT operation on the new time-domain signal, s″( n)(n=0,1,…,N-1); the odd term s″(2r+1)(r=0,1,…,N/2-1) of the frequency domain signal sequence s″(n) ) multiplied by -1, the even term s″(2r) (r=0,1,…,N/2-1) remains unchanged, and the signal sequence s′(n)(n=0,1,…,N -1); take the first signal sequence s'(n) item, get That is, the digitally modulated frequency domain signal of the data transmitted by the sending end. The above process is shown in Figure 3b. right Corresponding digital demodulation is performed to obtain the original data bit stream transmitted by the sending end.

In order to verify the effect of the method of the present invention, the traditional wireless OFDM communication system and DCO-OFDM communication system are compared with the wireless OFDM communication system and DCO-OFDM communication system adopting the method of the present invention. Figure 4a and Figure 4b are the frequency domain diagram and time domain diagram obtained by the traditional wireless OFDM method after the pseudo-random sequence with a period of 127 is modulated by BPSK; Figure 5a and Figure 5b are the pseudo-random sequence with a period of 127 respectively After BPSK modulation, the frequency domain diagram and time domain diagram obtained by using the method of the present invention to carry out time domain shaping; Fig. 6a and Fig. 6b are respectively obtained by the traditional DCO-OFDM method after the pseudo-random sequence with a period of 63 is modulated by BPSK Frequency domain diagram and time domain diagram; Fig. 7a and Fig. 7b are the frequency domain diagram and time domain diagram obtained by using the method of the present invention to perform time domain shaping after the pseudo-random sequence with a period of 63 is modulated by BPSK respectively. It can be seen from Figure 4b and Figure 5b that Figure 5b is actually a translation of Figure 4b, which concentrates the energy of the signal in the middle part of the OFDM symbol, and the energy on both sides is less, so that when the signal crosses the boundary, the loss of signal energy is relatively small. Less, the impact on the system is also reduced to a minimum. Similarly, it can be seen from Fig. 6b and Fig. 7b that in the DCO-OFDM communication system, the time-domain shaping scheme of the present invention also makes the energy of the OFDM symbol concentrate in the middle part, so that when the signal crosses the boundary, it can also reduce the impact on the system. The influence of the system guarantees the performance of the system.

Claims (3)

1. A time-domain shaping method of an OFDM communication system, characterized in that, At the sending end of the OFDM communication system, the frequency domain signal of the transmitted data obtained after digital modulation is processed as follows: the odd numbered term of the frequency domain signal is multiplied by -1, and the even numbered term remains unchanged to obtain a new frequency domain signal; perform inverse fast Fourier transform on the new frequency domain signal; multiply the odd term of the signal obtained by the inverse fast Fourier transform by -1, and keep the even term unchanged to obtain the time domain of the final transmitted data shaping signal; At the receiving end of the OFDM communication system, the following processing is performed on the time-domain shaping signal of the received transmission data: the odd numbered term of the time-domain shaping signal is multiplied by -1, and the even numbered term remains unchanged to obtain a new time-domain signal ; performing fast Fourier transform on the new time domain signal; multiplying the odd items of the signal obtained by the fast Fourier transform by -1, and keeping the even items unchanged, to obtain the frequency domain signal of the transmitted data. 2. the time domain shaping method of OFDM communication system as claimed in claim 1, is characterized in that, described OFDM communication system is wireless OFDM communication system, and described time domain shaping method is specifically as follows: At the sending end of the wireless OFDM communication system, the frequency domain signal X(k) of the transmitted data obtained after digital modulation, k=0,1,...,N-1, is divided into even-numbered items X(2m), m= 0,1,…,N/2-1, and odd items X(2m+1), m=0,1,…,N/2-1; keep even items X(2m), m=0,1, ..., N/2-1, unchanged, multiply odd items X(2m+1), m=0, 1,..., N/2-1 by -1 to get a new frequency domain signal; the new After performing N-point IFFT operation on the frequency domain signal, x'(n) is obtained, n=0,1,...,N-1, and then the sequence x'(n) is divided into even-numbered items x'(2r), r= 0,1,...,N/2-1, odd items x'(2r+1), r=0,1,...,N/2-1; the even items x'(2r) of the sequence x'(n) ), r=0,1,...,N/2-1, unchanged, odd item x'(2r+1), r=0,1,...,N/2-1, multiplied by -1 to get the final The shaped time-domain signal x(n), n=0,1,...N-1; At the receiving end of the wireless OFDM communication system, the odd-numbered items x"(2m+1), m=0,1, …, N/2-1, multiplied by -1, even number item x″(2m), m=0, 1,…, N/2-1, keep unchanged, get a new time-domain signal; for the new The time-domain signal is subjected to N-point FFT operation to obtain X″(k), k=0,1,…,N-1; the signal sequence X″(k), k=0,1,…,N-1, in Odd entries of X″(2r+1), r=0,1,…,N/2-1, multiplied by -1, keeping even entries X″(2r),r=0,1,…,N/2 -1, unchanged, the final frequency-domain signal X′(k), k=0, 1, . . . , N-1 is obtained. 3. the time domain shaping method of OFDM communication system as claimed in claim 1, it is characterized in that, described OFDM communication system is DC bias optical OFDM communication system, and described time domain shaping method is specifically as follows: At the transmitting end of the DC biased optical OFDM communication system, the frequency domain signal of the transmitted data after digital modulation and real digitization Odd items s(2m+1), m=0,1,...,N/2-1, multiplied by -1, even items s(2m), m=0,1,...,N/2-1, Keeping it unchanged, a new frequency-domain signal is obtained; the time-domain signal sequence y'(n) is obtained after performing N-point IFFT operations on the new frequency-domain signal, n=0,1,...,N-1; the time-domain The odd term y'(2r+1) of the signal sequence y'(n), r=0,1,...,N/2-1, multiplied by -1, the even term y'(2r), r=0,1 ,...,N/2-1, remain unchanged, and obtain the final shaped time domain signal y(n), n=0,1,...,N-1,; At the receiving end of the DC biased optical OFDM communication system, the odd term y"(2m+1) of the received shaped time domain signal y"(n), n=0,1,...,N-1, , m=0,1,…,N/2-1, multiplied by -1, even item y″(2m), m=0,1,…,N/2-1, unchanged, to get a new time domain Signal; get s″(n) after N-point FFT operation is carried out to this new time-domain signal, n=0,1,…,N-1,; The odd number item s″ of frequency-domain signal sequence s″(n) (2r+1), r=0,1,…,N/2-1, multiplied by -1, even term s″(2r), r=0,1,…,N/2-1, remains unchanged , get the signal sequence s'(n), n=0,1,...,N-1; take the first signal sequence s'(n) item, get x′(n), That is, the digitally modulated frequency domain signal of the data transmitted by the sending end.