CN114553645A - Method for reducing inter-carrier interference of orthogonal frequency division multiplexing system - Google Patents
Method for reducing inter-carrier interference of orthogonal frequency division multiplexing system Download PDFInfo
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- CN114553645A CN114553645A CN202210258333.9A CN202210258333A CN114553645A CN 114553645 A CN114553645 A CN 114553645A CN 202210258333 A CN202210258333 A CN 202210258333A CN 114553645 A CN114553645 A CN 114553645A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
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- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
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- H—ELECTRICITY
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- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
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- H—ELECTRICITY
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- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
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Abstract
The invention belongs to the technical field of wireless communication, and particularly relates to a method for reducing interference between carriers of an orthogonal frequency division multiplexing system. In the method provided by the invention, based on the idea that the interference of adjacent subcarriers with the same amplitude and the same sign to other subcarriers is the lowest, a scrambler and a symbol interleaver are utilized to change the sequence of modulation symbols, so that more opposite adjacent subcarriers appear in the sequence, and the inter-carrier interference in an orthogonal frequency division multiplexing system is reduced. The redundancy rate of the conventional self-elimination method is at least 1/2, and the system cannot be applied when the transmission efficiency is limited. The scheme provided by the invention can reduce the interference among the sub-carriers by using simple operation while ensuring the frequency spectrum efficiency of the system to be unchanged, and improves the performance of the orthogonal frequency division multiplexing system.
Description
Technical Field
The invention belongs to the technical field of wireless communication, and particularly relates to a method for reducing Inter-Carrier Interference (ICI) in an Orthogonal Frequency Division Multiplexing (OFDM) multi-Carrier system.
Background
In the OFDM system, due to carrier frequency deviation caused by synchronization error, unstable oscillator operating frequency, doppler shift spreading, and other factors, the received signal spectrum is shifted, orthogonality between subcarriers is destroyed, and ICI is generated. OFDM is based on strict orthogonality, and if ICI is not suppressed by effective measures, the system error performance will be deteriorated, and a severe floor effect will be brought. Aiming at the problem that ICI in a system seriously damages the performance of the system, an ICI Self-Cancellation (Self-Cancellation) method is proposed, and compared with other ICI Cancellation methods, the Self-Cancellation method is simple to realize, can effectively resist interference and is widely applied.
The main idea of the self-elimination method is as follows: at the transmitting end, before data is subjected to IFFT, the modulated signals are precoded, and data signals with equal amplitude and opposite signs are transmitted on adjacent odd-even carriers, for example: x0＝-X1，X2＝-X3，X4＝-X5…, when receiving, it also takes corresponding receiving measures, specifically, combines and receives the adjacent data, that is, after FFT, it receives by subtracting the adjacent data. The self-elimination method can effectively reduce the interference coefficient and improve the performance of the system.
However, since the self-cancellation method is equivalent to repetition coding, redundant data is added, the transmission efficiency is only 1/2 at the maximum, the spectrum utilization rate is greatly reduced, and the application is limited when the system limits the redundancy rate. The invention thus exploits this theoretical basis for self-elimination: the ICI coefficients of adjacent sub-carriers are close, and a method for reducing ICI and improving the error code performance of an OFDM system is provided on the basis of not reducing the transmission efficiency.
Disclosure of Invention
The invention provides a simple ICI elimination method for an OFDM system, which can reduce the influence caused by ICI and improve the transmission performance on the premise of not reducing the frequency spectrum efficiency.
For ease of understanding, the method employed in the present invention is illustrated as follows:
carrier frequency offset which destroys orthogonality among sub-carriers inevitably affects a system in a communication process, and carrier signal waveform distortion causes ICI. The process of introducing frequency offset in an OFDM system is shown in fig. 1.
The time domain expression of the data carrier symbols at the receiving end is as follows:
here ε represents the normalized value of the carrier frequency offset, represented by Δ fNTsDenoted by Δ f is a fixed value of the carrier frequency offset, TsThe duration of the data subcarrier symbol is indicated. N is the number of subcarriers in the OFDM system and w (N) represents the additive white gaussian noise in the channel.
If y (k) is used to represent the corresponding frequency domain expression, i.e. the data of k-th sub-carrier, then y (k) is expressed as follows:
wherein W (k) is the time-frequency transform result of Gaussian white noise, X (m) is the signal modulated on the mth data subcarrier, and the expression
After transformation, the following can be obtained:
the formula (3) is introduced into the formula (2) to obtain:
s (m-k) is an ICI coefficient, and represents the interference generated between the mth and kth sub-carriers, which is specifically expressed as follows:
the first term in equation (4) represents the current carrier signal, and the frequency offset of 0 may cause amplitude distortion and phase distortion of the current carrier. The second term is a carrier signal of an interference part, and when the frequency offset factor is 0, the limit of equation (5) is 0, so that when the frequency offset exists, ICI may be generated when a receiver receives a signal, so that each sub-carrier may be affected by other carriers, and when receiving, the receiving performance of the system may be reduced, so that the performance of the system becomes very poor. Fig. 2 shows a variation rule of the ICI coefficient, where the total number of subcarriers in the OFDM system is 64, the normalized frequency offsets are respectively set to 0.1, 0.2, 0.3, and 0.5, and m is 0, it can be seen that, under different normalized frequency offsets, the mutual influence degrees between subcarriers are different, and when the normalized frequency offset is very large, serious interference is generated between adjacent data subcarriers.
The technical scheme of the invention is as follows:
in order to solve the problem that ICI seriously deteriorates the system performance, it is necessary to think of reducing ICI caused by frequency offset. Interference of the l-th and l + 1-th sub-carriers to the k-th (k ≠ l, l +1) -th sub-carrier is as follows:
the adjacent inverse self-cancellation method suggests that if adjacent subcarriers are opposite, the interference of the two subcarriers to the kth carrier is reduced to:
fig. 3 shows the interference contrast for other sub-carrier positions when adjacent sub-carriers are identical and opposite. If the adjacent subcarriers are opposite, the adjacent subcarriers are marked as an opposite subcarrier pair. It is clear that the larger the number of opposite subcarrier pairs in an OFDM symbol, the smaller the sum of the interference experienced. Therefore, on the premise of not reducing transmission efficiency, multiple scrambling codes are carried out during BPSK modulation, and multiple symbol interleaving is carried out during other modulation, so that the number of opposite subcarrier pairs is increased. In addition, as can be seen from fig. 2 and 3, the current sub-carrier mainly affects the adjacent sub-carriers, so if the number of the opposite sub-carrier pairs is the same, the ICI coefficients of the two sub-carriers in each opposite sub-carrier pair can be made to be closer by avoiding the adjacent opposite sub-carrier pairs, so as to cancel more ICI.
Aiming at different modulation modes, the invention provides corresponding different methods, which specifically comprise the following steps:
1. after a binary signal source is generated by adopting a BPSK modulated OFDM system, an ICI (inter-carrier interference) elimination algorithm of a scrambler is utilized:
s1, defining adjacent symbol pairs with bits of 0 and 1 as opposite subcarrier pairs, counting the number of the opposite subcarrier pairs in the binary source, and determining whether the opposite subcarrier pairs are adjacent, specifically: defining the length of a binary information source to be N, judging whether an nth bit signal is different from an N +1 th bit signal or not, if so, enabling the nth bit signal and the N +1 th bit signal to be opposite subcarrier pairs, adding 1 to the number parameter num _ s count of the opposite subcarrier pairs, and skipping the judgment of a next bit signal, otherwise, directly judging the next bit signal; if the opposite subcarrier pair exists, simultaneously judging whether the opposite subcarrier pair is adjacent to the last opposite subcarrier pair or not, if so, not performing any processing, and if not, adding 1 to the count of the parameter num _ n; until all N bit signals are judged;
s2, changing the initial state of the scrambler, generating sequences after different scrambling codes: the structure of the scrambling code register is shown in FIG. 4, and the generator polynomial used is x7+x4+1, scrambling by xoring the last bit in the shift register with the sign bit;
s3, respectively counting the number of opposite subcarrier pairs and the number of nonadjacent subcarrier pairs in the bit sequence after scrambling in the same way as S1;
s4, selecting the sequence with the most number of opposite subcarrier pairs in S1 and S3, if the sequence has the same number of opposite subcarrier pairs, taking the sequence with the most number of the opposite subcarrier pairs, recording the initial state of a scrambler corresponding to the sequence, sending the initial state to a receiver, carrying out BPSK modulation and sending signals;
and S5, the receiver receives the signal, and after demodulation, the descrambling operation is carried out according to the initial state of the scrambler.
2. After an OFDM system modulated by M-PSK and M-QAM generates a binary signal source and carries out corresponding modulation to obtain a modulation symbol, an ICI elimination algorithm of a symbol interleaver is utilized:
s1, defining adjacent modulation symbols with equal amplitude and opposite signs as opposite subcarrier pairs, counting the number of the opposite subcarrier pairs in the modulation symbols, and determining whether the opposite subcarrier pairs are adjacent, specifically: defining the length of a modulation symbol as N, judging whether the nth modulation symbol is opposite to the (N +1) th modulation symbol or not for the nth modulation symbol, if so, judging that the nth and the (N +1) th modulation symbols are opposite subcarrier pairs, counting the number parameter num _ s of the opposite subcarrier pairs and adding 1, and skipping the judgment of the next modulation symbol, otherwise, directly judging the next modulation symbol; if the opposite subcarrier pair exists, simultaneously judging whether the opposite subcarrier pair is adjacent to the last opposite subcarrier pair or not, if so, not performing any processing, and if not, adding 1 to the count of the parameter num _ n; until all N modulation symbols are judged;
s2, changing the seed of the random interleaver, and interleaving the modulated sequence in symbol level;
s3, respectively counting the number of opposite subcarrier pairs and the number of nonadjacent subcarrier pairs in the symbol sequence after random interleaving in the same way as S1;
s4, selecting the sequence with the most number of opposite subcarrier pairs in S1 and S3, if the sequence has the same number of opposite subcarrier pairs, selecting the sequence with the most number of the opposite subcarrier pairs, recording the interleaver seed corresponding to the sequence and sending the interleaver seed to the receiver;
s5, the receiver receives the signal, and demodulates after de-random interleaving according to the interleaving seed.
The invention has the advantages that based on the idea that the interference of adjacent subcarriers with the same amplitude and opposite signs to other subcarriers is the lowest, the invention utilizes the scrambler and the symbol interleaver to change the sequence of the modulation symbol sequence, so that more opposite adjacent subcarriers appear in the sequence, thereby reducing the inter-carrier interference in the orthogonal frequency division multiplexing system. The redundancy rate of the conventional self-elimination method is at least 1/2, and the system cannot be applied when the transmission efficiency is limited. The scheme provided by the invention can reduce the interference among the sub-carriers by using simple operation while ensuring the frequency spectrum efficiency of the system to be unchanged, and improves the performance of the orthogonal frequency division multiplexing system.
Drawings
FIG. 1 is a process for introducing frequency offset in an OFDM system;
fig. 2 shows the ICI coefficient variation law under different normalized frequency offsets in the OFDM system;
FIG. 3 shows the interference contrast for other sub-carrier positions when the adjacent sub-carriers are the same and the adjacent sub-carriers are opposite;
FIG. 4 is a structure of a scrambling code register;
fig. 5 shows the effect of ICI cancellation algorithm using scrambler on improving the error performance of BPSK modulated OFDM system;
fig. 6 shows the effect of ICI cancellation algorithm using symbol interleaver on improving the error performance of QAM modulated OFDM systems.
Detailed Description
The technical method of the invention is described in detail below with reference to the accompanying drawings and examples:
in the simulation of the error rate performance, the channel is AWGN with constant normalized frequency offset, and the number of subcarriers of the OFDM system is N-64.
Fig. 5 shows an error performance curve for improving ICI by using a scrambler under BPSK modulation, where after generating bit sequences, the transmitter scrambles the bit sequences 10 times, 20 times, and 40 times, and then selects a sequence with the most number of adjacent bits (equivalent to an opposite subcarrier pair) from the sequences (including an original sequence), where the number of the opposite subcarrier pair is the same, and then takes the sequence with the most number of the opposite subcarrier pair, stores the corresponding scrambler initial state, and sends the scrambler initial state and data information together to a receiver, thereby performing correct descrambling.
Fig. 6 shows an error performance curve for improving ICI by using a symbol interleaver under QAM modulation, where a transmitter generates bit information and modulates, then randomly interleaves the modulation symbol sequences for 10 times, 20 times, and 40 times with different interleaving seeds, and then selects a sequence with the largest number of opposite subcarrier pairs from the sequences (including an original sequence), where the number of opposite subcarrier pairs is the same, and takes the sequence with the largest number of non-adjacent opposite subcarrier pairs, stores the corresponding interleaving seeds, and sends the interleaving seeds and data information together to a receiver, thereby performing correct de-symbol random interleaving.
In addition, simulation proves that the scrambling code adopted in BPSK modulation has better performance than symbol interleaving because the scrambler can change the number of 0 and 1 in the bit sequence.
Claims (2)
1. A method for reducing the interference between the carriers of the orthogonal frequency division multiplexing system, adopt the orthogonal frequency division multiplexing system of BPSK modulation to produce the binary signal source bit, characterized by that, comprising the following steps:
s1, defining adjacent symbol pairs with bits of 0 and 1 as opposite subcarrier pairs, counting the number of the opposite subcarrier pairs in the binary source, and determining whether the opposite subcarrier pairs are adjacent, specifically: defining the length of a binary information source to be N, judging whether an nth bit signal is different from an N +1 th bit signal or not, if so, judging that the nth bit signal and the N +1 th bit signal are opposite subcarrier pairs, adding 1 to the number parameter num _ s count of the opposite subcarrier pairs, and skipping the judgment of a next bit signal, otherwise, directly judging the next bit signal; if the opposite subcarrier pair exists, simultaneously judging whether the opposite subcarrier pair is adjacent to the last opposite subcarrier pair or not, if so, not performing any processing, and if not, adding 1 to the count of the parameter num _ n; until all N bit signals are judged;
s2, changing the initial state of the scrambler to generate sequences with different scrambles;
s3, respectively counting the number of opposite subcarrier pairs and the number of nonadjacent subcarrier pairs in the bit sequence after scrambling in the same way as S1;
s4, selecting the sequence with the most number of opposite subcarrier pairs in S1 and S3, if the sequence has the same number of opposite subcarrier pairs, taking the sequence with the most number of the opposite subcarrier pairs, recording the initial state of a scrambler corresponding to the sequence, sending the initial state to a receiver, carrying out BPSK modulation and sending signals;
and S5, the receiver receives the signal and carries out descrambling operation according to the initial state of the scrambler after demodulation.
2. A method for reducing the interference between the carriers of the orthogonal frequency division multiplexing system, adopt M-PSK and M-QAM orthogonal frequency division multiplexing system of modulation to produce the binary signal source and carry on the corresponding modulation and get the modulation symbol, characterized by that, comprising the following steps:
s1, defining adjacent modulation symbols with equal amplitude and opposite signs as opposite subcarrier pairs, counting the number of the opposite subcarrier pairs in the modulation symbols, and determining whether the opposite subcarrier pairs are adjacent, specifically: defining the length of a modulation symbol as N, judging whether the nth modulation symbol is opposite to the (N +1) th modulation symbol or not for the nth modulation symbol, if so, judging that the nth and the (N +1) th modulation symbols are opposite subcarrier pairs, counting the number parameter num _ s of the opposite subcarrier pairs and adding 1, and skipping the judgment of the next modulation symbol, otherwise, directly judging the next modulation symbol; if the opposite subcarrier pair exists, simultaneously judging whether the opposite subcarrier pair is adjacent to the last opposite subcarrier pair or not, if so, not performing any processing, and if not, adding 1 to the count of the parameter num _ n; until all N modulation symbols are judged;
s2, changing the seed of the random interleaver, and interleaving the modulated sequence in symbol level;
s3, respectively counting the number of opposite subcarrier pairs and the number of nonadjacent subcarrier pairs in the symbol sequence after random interleaving in the same way as S1;
s4, selecting the sequence with the most number of opposite subcarrier pairs in S1 and S3, if the sequence has the same number of opposite subcarrier pairs, selecting the sequence with the most number of the opposite subcarrier pairs, recording the interleaver seed corresponding to the sequence and sending the interleaver seed to the receiver;
s5, the receiver receives the signal, and demodulates after de-random interleaving according to the interleaving seed.
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| CN115134202A (en) * | 2022-06-24 | 2022-09-30 | 常熟理工学院 | ICI (inter-carrier interference) suppression method based on OFDM (orthogonal frequency division multiplexing) system inter-subcarrier |
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