CN109525532B

CN109525532B - Impulse noise suppression method in OFDM communication system

Info

Publication number: CN109525532B
Application number: CN201811625971.XA
Authority: CN
Inventors: 刘鲲; 刘元成; 龙必起; 陈丽恒
Original assignee: Leaguer Microelectronics Co ltd
Current assignee: Leaguer Microelectronics Co ltd
Priority date: 2018-12-28
Filing date: 2018-12-28
Publication date: 2021-07-13
Anticipated expiration: 2038-12-28
Also published as: CN109525532A

Abstract

The invention provides an impulse noise suppression method of an OFDM communication system, which comprises the following steps: s1: carrying out embedded interweaved space-time block coding aiming at the transmitted data symbols to obtain two paths of transmitted symbols; s2: performing IFFT on the two paths of sending symbols respectively, converting the two paths of sending symbols into a time domain, and inserting a protection interval signal; s3: respectively carrying out up-conversion on the two paths of time domain signals obtained in the S2, and transmitting; s4: receiving the signal transmitted in S3 to obtain a received signal, and performing down-conversion on the received signal; s5: carrying out amplitude limiting on the signal subjected to down-conversion; s6: and stripping a guard interval from the signal after amplitude limiting, and performing FFT (fast Fourier transform) conversion to a frequency domain to obtain a frequency domain signal. The symbol interleaving method embedded in space-time block coding (STBC) realizes the dispersion of impulse noise interference with smaller interleaving depth; by combining the impulse noise suppression of time domain amplitude limiting and the diversity reception effect of STBC decoding, the method implements effective suppression aiming at dispersed impulse noise interference and improves the anti-impulse noise performance.

Description

Impulse noise suppression method in OFDM communication system

Technical Field

The invention belongs to the technical field of digital communication, and particularly relates to an impulse noise suppression method in an OFDM communication system.

Background

Impulse noise is an intractable interference in a communication system and is one of the main factors causing communication link failure. The impulse noise mainly comes from factors such as a rectifying circuit, an electrical switch, lightning, an ignition device of an automobile spark plug and the like. The impulse noise has the characteristics of short duration, far higher power than background noise, random occurrence and the like. The wireless communication system is mainly affected by impulse noise in the lower end portions of the VHF band and the UHF band, and the average duration (burst length) of the impulse noise is less than 250us, occurring at intervals greater than several tens of ms. In the power line carrier communication system, the interference of the impulse noise is more serious, and the impulse noise is divided into non-periodic impulse noise and periodic impulse noise. Periodic impulse noise is a main noise in a narrow-band power line system, and is mainly generated by a switching power supply of household appliances and the like, and the generation period of the periodic impulse noise is a half power frequency period (50Hz power frequency, half period is 10 ms). Meanwhile, the burst length is very long (0.833ms to 2.5ms), which completely interferes with one or even several OFDM symbol periods. On the other hand, the power of the impulse noise is 30-50 dB higher than that of the background noise. Therefore, the burst length of the periodic impulse noise is long, the ratio is large, the interference is strong, the suppression difficulty of the receiver is large, and the communication reliability is seriously influenced.

In order to better resist Frequency selective fading and impulse noise interference, OFDM (Orthogonal Frequency Division Multiplexing) technology is widely studied and applied to wireless communication systems such as DAB (digital audio broadcasting), DVB (digital video broadcasting), WLAN (wireless local area network), LTE (long term evolution system) mobile communication, and IEEE P1901 power line carrier communication systems. Compared with a single carrier technology, the multi-carrier OFDM has natural resistance to impulse noise with lower power. This is because, by performing FFT conversion on the received signal, the OFDM receiver spreads impulse noise to the frequency domain subcarriers of the entire OFDM symbol, and reduces the influence of impulse noise interference on demodulation. On the other hand, if the power of impulse noise is high, the impulse noise is spread to the whole OFDM symbol, so that the carrier-to-noise ratio of each subcarrier is seriously deteriorated, and a serious symbol decision error is caused.

At present, the main methods for suppressing impulse noise include time-domain nonlinear suppression, FEC suppression schemes of interleaving and FEC coding, and the like. Time-domain nonlinear suppression, which mitigates the impact of impulse noise on demodulation performance by Clipping (Clipping) or nulling (Blanking) or a combination thereof, on the received signal. The performance improvement resulting from this approach is limited because it introduces additional interference. The FEC restraining method disperses the impulse noise interference by interleaving and corrects error codes by an FEC decoding algorithm to resist the influence caused by the impulse noise. The method has strong dependence on the environment of impulse noise, particularly under the environment of large impulse noise power and long burst length, the interleaving depth and the error correction capability need to be increased, and difficult compromise is carried out between the improvement of performance and the realization complexity of an algorithm.

The above background disclosure is only for the purpose of assisting understanding of the concept and technical solution of the present invention and does not necessarily belong to the prior art of the present patent application, and should not be used for evaluating the novelty and inventive step of the present application in the case that there is no clear evidence that the above content is disclosed at the filing date of the present patent application.

Disclosure of Invention

The invention aims to solve the problems of impulse noise interference, particularly impulse noise interference with large power and long burst length in the prior art, and provides an impulse noise suppression method in an OFDM communication system.

In order to solve the above technical problem, the present invention provides an impulse noise suppression method for an OFDM communication system, including: s1, carrying out embedded interweaved space-time block coding aiming at the sent data symbols to obtain two paths of sent symbols; s2, performing IFFT conversion on the two paths of sending symbols respectively to a time domain, and inserting a protection interval signal; s3, respectively carrying out up-conversion on the two paths of time domain signals obtained in the S2, and transmitting; s4, receiving the signal transmitted in the S3 to obtain a received signal, and performing down-conversion on the received signal; s5, carrying out amplitude limiting on the down-converted signal; s6, stripping a guard interval from the amplitude-limited signal, and performing FFT (fast Fourier transform) to transform the signal to a frequency domain to obtain a frequency domain signal; s7, de-compounding the frequency domain signals; s8, decoding the signal after de-compounding by space-time block coding.

Compared with the prior art, the invention has the beneficial effects that: the symbol interleaving method embedded in space-time block coding (STBC) realizes the dispersion of impulse noise interference with smaller interleaving depth; by combining the impulse noise suppression of time domain amplitude limiting and the diversity reception effect of STBC decoding, the method implements effective suppression aiming at dispersed impulse noise interference and improves the anti-impulse noise performance.

Drawings

FIG. 1 is a schematic diagram of the STBC coder;

FIG. 2 is a schematic diagram of an embedded interleaving STBC coder according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of an exemplary embedded interleaved STBC encoded signal;

FIG. 4 is a system block diagram of an embodiment of the present invention;

FIG. 5 is a de-compounding block diagram of an embodiment of the present invention;

FIG. 6 is a schematic diagram of an embedded interleaved and de-multiplexed signal according to an embodiment of the present invention;

fig. 7 is another schematic diagram of an embedded interleaved and de-multiplexed signal according to an embodiment of the invention.

Detailed Description

The invention is further described with reference to the following figures and embodiments.

Fig. 1 is a schematic diagram illustrating a STBC (Space Time Block Code) encoder. The figure shows a dual transmit antenna application, which is a widely adopted Alamouti scheme with the characteristics of full rate, full diversity and the possibility of using a simple linear decoding algorithm. The input data symbols are split into odd and even branches by a serial-to-parallel conversion 101. The two branches are recombined in time and space domains at the signal recombination 102 to obtain data symbol sequences of the two antennas. The odd/even branches are directly multiplexed to obtain the symbol sequence s of the antenna 1^*1,s2,s ^*3,s4,s ^*5, s 6. The even-numbered sequence branch is subjected to conjugate negation and then is multiplexed with the conjugate of the odd-numbered branch to obtain the symbol sequence-s of the antenna 2^*2,s1,-s ^*4,s3,-s ^*6, s 5. The multiplexing control signal 103 is inverted in the symbol period.

Fig. 2 is a schematic diagram of an embedded interleaving STBC encoder according to an embodiment of the present invention. First, in the impulse noise mode 2, the transmission symbol sequence is repeated one symbol by one symbol in the repeating unit 201, and after the repetition, s1 is regarded as s2, s3 is regarded as s4, and so on. In the impulse noise mode 1, the repetition part 201 is bypassed, that is, no symbol repetition is performed. For the signal from the repetition portion 201, the serial-to-parallel conversion 202 divides the input signal into odd branches and even branches. The odd branches pass through interleaver 203 and the even branches pass through interleaver 204. The outputs of the interleavers 203,204 are connected to a signal combining unit 205, and time domain/space domain signal recombination is performed to obtain symbol sequences 206,207 for two antennas. The multiplexing is controlled by a multiplexing control signal 208.

Fig. 3 is a schematic diagram of an embedded interleaving STBC encoded signal according to an embodiment of the present invention, further illustrating the STBC encoding principle according to an embodiment of the present invention. The interleaver adopts a block interleaving mode of n x 2, namely the interleaver has n rows and 2 columns. In the example of fig. 3, n is 3. The steps of the interleaving algorithm include: A. writing the data symbols into the storage unit column by column; B. reading data symbols line by line after all the data symbols are fully written; C. after all the reading is finished, one interleaving operation is finished, and A is repeated to carry out the next interleaving operation. The multiplexing control 208 is flipped by 2n symbol periods (i.e. the number of symbols interleaved at one time), the odd branch is first selected by antenna 1, and s is output ^*1,s ^*7,s ^*3, the output s2, s8, s4 after the reversal of the multiplexing control signal. The antenna 2 firstly selects an even number of branches and outputs-s^*2,-s^*8,-s^*And 4, outputting the inverted multiplexed control signals s1, s7 and s 3. The parameter n is determined by the impulse noise burst length (see equation (1)):

in which ceil () is a ceiling function, T_IMFor burst length, T is the symbol time.

Fig. 4 is a system block diagram of an embodiment of the invention. The STBC encoder output signal 206 is connected 401,207 to 402. With respect to the

input signals

401 and 402, the IFFT section 403 performs IFFT transformation to transform the signals into the time domain, thereby realizing OFDM modulation. For the signal from IFFT section 403, GI insertion section 405 inserts a guard interval signal, connects to TX section 407, up-converts the signal, sends the signal to a channel, and transmits the signal. In channel section 409, antenna 1 is multiplied by channel impulse response h1, and antenna 2 is multiplied by channel impulse response h 2; in the channel section 411, impulse noise z1 is added to the antenna 1, and impulse noise z2 is added to the antenna 2. Receive combining 413 adds the two antenna signals and adds the background noise w at 414.

The RX unit 417 down-converts the received signal from the channel, and the output is connected to a limiter 418 to perform a limiting process against impulse noise. After GI strip 419 removes the guard interval from the signal from slicer 418, FFT section 420 performs FFT conversion and reconversion back to the frequency domain. The output signal of the FFT section 420 is connected to the demultiplexing section 421 to restore the scrambled symbol sequence. The STBC decoder 422 performs STBC decoding on the signal from the demultiplexed signal 421 to restore the transmission symbol sequence. The channel estimation 423 estimates channel impulse responses for the two antennas and outputs to the STBC decoder 422. With respect to the time domain signal from the RX part 417, the impulse noise estimation part 424 estimates the burst length T of impulse noise_IMAnd impulse noise power I_IMAnd the impulse noise occurrence time. The impulse noise estimation section 424 is connected to the clipping section 418 to control the clipping section; the clipping section clips the signal to suppress impulse noise if and only if the impulse noise occurs. Burst length T from 424_IMAnd impulse noise power I_IMThe parameters are output to the mode control unit 425, and the system mode is determined. First, the burst length T is given according to equation (1)_IMThe interleaving depth n is determined. In addition, the burst length T_IMAnd impulse noise power I_IMAn impulse noise pattern is determined. When T is_IM<T/TH1 or I_IM<TH2, which indicates that the burst length is sufficiently small or the power of the impulse noise is sufficiently small, and determines that the impulse noise mode is 1; otherwise, it is determined as the impulse noise mode 2. As described above, in the impulse noise mode 2, it is determined that the transmission symbol needs to be repeated to enhance the resistance to impulse noise interference. The thresholds TH1 and TH2 may be determined through analysis or simulation of the performance of the communication link.

FIG. 5 is a de-compounding block diagram of an embodiment of the present invention. For an input signal, the serial-to-parallel conversion 501 divides the input signal into odd and even branches. The deinterleaver 502 deinterleaves the odd-numbered tributary signals, and the deinterleaver 503 deinterleaves the even-numbered tributary signals, with a deinterleaver depth n × 2 (example n — 3 in fig. 5). When de-interleaving, firstly writing in column by column, and reading out row by row after full writing. The multiplexer 503 multiplexes the signals for the odd and even branch signals. The multiplexing control 504 is flipped by 2n symbol periods (i.e., the number of symbols interleaved at one time).

Fig. 6 is a schematic diagram of an embedded interleaving and de-multiplexing signal according to an embodiment of the present invention to further explain the effect of the embodiment of the present invention. Since the symbol interleaving is limited between OFDM symbols, for simplicity of illustration, the frequency domain signal example of transmission and reception is shown in fig. 6, but the effect of the embodiment of the present invention is also sufficiently demonstrated. On the other hand, fig. 6 mainly shows the effect of impulse noise dispersion, and therefore, it is assumed that the channel impulse response h1 is h2 is 1. In addition, the impulse noise burst length T shown in fig. 6_IM2nT, i.e. exactly the length of one interleaved block, and n 3. As shown, all symbols of the first interleaved block are contaminated (interfered) by impulse noise, as indicated by the diagonally shaded symbols in the figure. The de-multiplexed symbol sequence yi is obtained according to the de-multiplexing rule shown in fig. 5, and is sent to the STBC decoder 422. According to the Alamouti STBC decoding algorithm, a signal pair consisting of two symbols, such as y1 and y2, is simultaneously sent to STBC decoding, and estimated values of transmitted symbols s1 and s2 are obtained, so that the signal decoding is completed. As shown in fig. 6, only one symbol of the signal pair fed into the STBC decoder is contaminated by impulse noise, and the other symbol is completely free from the impulse noise. The diversity reception effect of STBC decoding is utilized to effectively resist impulse noise interference. On the other hand, when the impulse noise interference is too severe, the system enters impulse noise mode 2. In pattern 2, the sequence of transmitted symbols is repeated, i.e., s1 is s2, s3 is s4, and so on. Therefore, y2 ═ h1 ═ s2+ h2 ═ s1 ═ h1 ═ s1+ h2 ═ s1 ═ h1+ h2 ═ s1, and by using the symbol y2 completely free from impulse noise interference, the transmission symbol can be decoded and recovered, the influence of impulse noise interference is completely avoided, and the impulse noise resistance performance is greatly improved.

Fig. 7 is another schematic diagram of an embedded interleaved and de-multiplexed signal according to an embodiment of the present invention, where n is 1 and the burst length T is T_IMLess than one symbol length. When T is_IM<T/TH1, the system works in impulse noise mode 1, and the STBC decoder decodes according to the normal mode.

The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several equivalent substitutions or obvious modifications can be made without departing from the spirit of the invention, and all the properties or uses are considered to be within the scope of the invention.

Claims

1. A method for suppressing impulse noise in an OFDM communication system is characterized by comprising the following steps:

s1: carrying out embedded interweaved space-time block coding aiming at the transmitted data symbols to obtain two paths of transmitted symbols;

step S1 includes:

s101: repeating or not repeating the transmitted symbol sequence;

s102: carrying out serial-parallel conversion on the sending symbol sequence, and decomposing the sending symbol sequence into odd branches and even branches;

s103: n x 2 block interleaving is carried out on the odd branch symbol sequences;

s104: n x 2 block interleaving is carried out on the even branch symbol sequences;

s105: 2n multiplexing control signals with inverted symbol periods synthesize odd branch symbol sequences and even branch symbol sequences to generate disordered STBC coding symbol sequences;

s2: performing IFFT on the two paths of sending symbols respectively, converting the two paths of sending symbols into a time domain, and inserting a guard interval signal;

s3: respectively carrying out up-conversion on the two paths of time domain signals obtained in the S2, and transmitting;

s4: receiving the signal transmitted in S3 to obtain a received signal, and performing down-conversion on the received signal;

s5: carrying out amplitude limiting on the down-converted signal, and inhibiting impulse noise in a time domain;

s6: stripping a guard interval from the amplitude-limited signal, and performing FFT (fast Fourier transform) conversion to a frequency domain to obtain a frequency domain signal;

s7: de-compounding the frequency domain signals;

s8: and performing space-time block coding decoding on the de-multiplexed signal.

2. The method for suppressing impulse noise in an OFDM communication system according to claim 1, further comprising before the step S8:

and performing pulse noise estimation on the down-converted output signal to obtain a characteristic value of the pulse noise.

3. The method for suppressing impulse noise in an OFDM communication system according to claim 1, further comprising before the step S8:

and determining an impulse noise mode according to the characteristic value of the input impulse noise estimation.

4. The method for suppressing impulse noise in an OFDM communication system according to claim 1, further comprising before the step S8:

and performing channel estimation to obtain a channel estimation value, and providing the channel estimation value for STBC decoding.

5. The method for suppressing impulse noise in an OFDM communication system according to claim 1, wherein step S7 comprises:

s71: carrying out serial-parallel conversion on the symbol sequence, and decomposing the symbol sequence into odd branches and even branches;

s72: n x 2 block de-interleaving is carried out on the odd branch symbol sequences;

s73: n x 2 block de-interleaving is carried out on the even branch symbol sequences;

s74: and synthesizing the odd branch symbol sequence and the even branch symbol sequence by the multiplexing control signal with 2n symbol period turnover, and recovering the original sequential STBC coding symbol sequence.

6. The method of claim 2, wherein the impulse noise estimate estimates impulse noise characteristics including burst length T_IMAnd impulse noise power I_IMAnd the occurrence time of impulse noise。

7. The method of claim 3, wherein the determining the impulse noise pattern is an impulse noise pattern decision, when T is_IM<T/TH1 or I_IM<TH2, determining as impulse noise mode 1; otherwise, it is determined as the impulse noise mode 2.

8. The method of claim 1, wherein step S8 is performed with STBC decoding in the impulse noise mode 1; aiming at the scattered impulse noise interference, the impulse noise interference is suppressed through the diversity action of amplitude limiting on the impulse noise and STBC decoding in the time domain; in impulse noise mode 2, the transmission symbol is recovered using the reception symbol completely free from impulse noise.

9. A computer-readable storage medium storing a computer program for use in conjunction with a computing device, the computer program being executable by a processor to implement the method of any one of claims 1 to 8.