JP5499687B2 - OFDM wireless communication terminal - Google Patents

Description

  The present invention is an OFDM (Orthogonal Frequency-Division Multiplexing) type wireless terminal used in a wireless LAN (local area network) and the like, and in particular, the degradation of throughput due to electromagnetic noise generated from nearby electronic information devices. The present invention relates to a reduced OFDM wireless communication terminal.

  Television broadcasts and wireless communications using OFDM can be easily adapted to poor transmission channel conditions, and are highly resistant to narrowband transmission channel interference, intersymbol interference due to multipath transmission, fading, and timing synchronization errors. It has already been used frequently because it has the advantage of having a frequency utilization efficiency and easily realizing a single frequency network.

  For example, public wireless communication such as WiMAX (Worldwide Interoperability for Microwave Access), wireless LAN, and the like are well known as OFDM wireless communication. As a problem in this case, particularly in a wireless communication terminal for a wireless LAN, when the terminal is built in an electronic information device such as a personal computer (PC), electromagnetic waves generated from the electronic information device in its vicinity or in the vicinity thereof are generated. It is known that bit error rate and throughput are degraded due to noise.

  In this way, when the bit error rate due to reception power, surrounding electromagnetic noise, or the like increases, packet errors increase, the number of packet retransmissions increases, and throughput decreases. Even if there is a 1-bit error in the combined packet, the entire packet must be retransmitted, which has a large effect on the throughput.

  One way to mitigate such performance degradation is to reduce the amount of information transmitted per packet by adaptively changing the modulation method, coding rate, or packet length according to the surrounding electromagnetic environment, thereby reducing packet errors. Means are used to suppress throughput degradation by preventing an increase in rate. The problem with this method is how to evaluate the change in the electromagnetic environment where the communication link is placed, and to select the most appropriate modulation method, encoding and decoding method.

  In the conventional method, the transmission side monitors the ACK signal (ACKnowledgement signal) from the packet transmission destination, and reduces the transmission rate if the ratio of undelivered packets exceeds a certain limit. If the number of undelivered packets is very small, the transmission rate is increased.

On the other hand, on the receiving side, 1) the signal level for each subcarrier is estimated using a pilot signal whose contents transmitted at a fixed frequency and timing in the OFDM signal are known.
2) The noise power for each subcarrier is estimated from the demodulation result of the pilot signal and the received power when there is no signal.
3) The signal / noise power ratio for each subcarrier is estimated using the results of 1) and 2) above, and based on this, the bits input to the soft decision decoder of the error correction code are weighted, that is, the signal / A bit having a small correct reliability and demodulated from a subcarrier having a small noise power ratio is assigned a small weight.

  Problems with the conventional method on the transmission side include the following. For example, even when interference occurs for a specific subcarrier, since it is not known on the transmission side, it is necessary to change the modulation schemes of all subcarriers to be transmitted, which may increase waste. In addition, since it is necessary to repeatedly change the modulation method while monitoring packet non-delivery information, it takes time to converge to the optimum transmission rate, and it does not converge when the time fluctuation of the electromagnetic environment is large. can give.

  On the other hand, the biggest problem of the above countermeasures performed on the receiving side is as follows. This comes from the fact that the influence of noise must be evaluated only by the signal / noise power ratio. When it can be assumed that external noise can be ignored, Gaussian noise (receiver noise) having no frequency dependence can be regarded as dominant, and therefore the influence of noise can be evaluated only by the signal / noise power ratio for each subchannel. However, it usually includes extraneous noise, which is generally non-Gaussian noise with frequency dependence. For example, when the noise is impulse-like, it is known that the bit error rate is often significantly deteriorated compared to the Gaussian noise having the same power. For this reason, if the transmission modulation method and error correction method are set based on only the signal / noise power while ignoring the statistical properties of noise, the influence of noise may be overestimated or underestimated as a result.

  It has been reported that quality degradation (bit error rate: BER) of a digital line caused by an interference wave is estimated from a statistical parameter (amplitude probability distribution: APD) of the interference wave intensity. Here, APD is a time rate at which the amplitude exceeds a certain level L at the measurement time T.

  For example, Non-Patent Document 1 describes a reception quality evaluation method of an OFDM signal using an APD of noise radiated from an electronic device. In particular, in a digital television receiver that receives OFDM radio waves, BER is Gaussian. It has been shown that the encoding effect under noise can be used to approximately evaluate with the APD of interference noise. Non-Patent Document 2 describes a bit error rate estimation method for a digital modulation scheme using a noise APD. For example, the relationship between BER and APD in a digital coherent radio receiver is shown. Non-Patent Document 3 describes a bit error rate estimation method for a multi-level modulation scheme using a noise APD. For example, the relationship between an APD and a BER by a coherent signal is QAM or multiplexing. It is shown that it varies depending on a modulation scheme such as PSK.

  Non-Patent Document 4 describes the effect of weighting input bits to an error correction / Viterbi decoder using noise power for each subcarrier of an OFDM signal. Non-Patent Document 5 describes the effect of weighting of input bits to an error correction / Viterbi decoder using noise frequency information. Non-Patent Document 6 describes a subcarrier weighting technique in a soft decision circuit.

  Patent Document 1 (Japanese Translation of PCT International Publication No. 2009-535911) discloses a system that selects a modulation scheme and an error correction coding scheme based on energy detected in each subchannel of the OFDM system. It is disclosed. Japanese Patent Application Laid-Open No. 2006-501695 discloses a method for determining a data rate for transmission based on a metric for an OFDM system equivalent channel.

  However, it is clear that the OFDM wireless communication terminal of the present invention cannot be realized by simply integrating these disclosures and descriptions.

Special table 2009-535911 gazette Special table 2006-501695 gazette

K. Gotoh et al., "Evaluation of sensitivity of digital TV receiver subjected to intrasystem interference", Proc. Of the 8th Int. Symposium on Electromagnetic Compatibility (EMC Europe 2008), Hamburg, Germany, 727-730, September 8-12 , 2008 K. Wiklundh, "Relation Between the Amplitude Probability Distribution of an Interfering Signal and its Impact on Digital Radio Receivers", IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 48, NO. 3, 537-544, AUGUST 2006 Y. Matsumoto, "On the Relation Between the Amplitude Probability Distribution of Noise and Bit Error Probability", IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 49, NO. 4, 940-941, NOVEMBER 2007 W. Lee and H. Park, "Performance analysis of Viterbi decoder using channel state information in COFDM system", IEEE Trans.Broadcasting, vol. 44, no.4, pp. 488-496, Dec. 1998. Y. Matsumoto, K. Fujii, A. Sugiura, and T. Murakami, "Impact of Electromagnetic Noises from PCs on the Performance of MB-OFDM UWB Devices," 13th IEEE International Symposium on Consumer Electronics, May 2009 Masahiro Morikura, Shuji Kubota, “802.11 High-Speed Wireless LAN Textbook”, 216 pages, 2005

  As described above, in the case of non-Gaussian noise having frequency dependency, for example, impulse-like noise, it is known that the bit error rate is often significantly deteriorated compared to Gaussian noise of the same power. . That is, ignoring the statistical properties of noise and setting the transmission modulation method and error correction method based on only the signal / noise power, the effect of noise is overestimated or underestimated as a result.

  Therefore, in the present invention, the signal modification in the transmission means or the reception means is performed using the relationship between the APD and the APD and the BER estimated from the APD and the BER estimated from the signal / noise power ratio instead of the signal / noise power ratio in the electromagnetic environment evaluation, and the output from the reception means. The bit error rate of the received information.

The present invention relates to an OFDM wireless communication terminal used as the receiving means in a transmission / reception system including a transmitting means for transmitting an OFDM signal and a receiving means for receiving the OFDM signal. The received OFDM signal is received for each subcarrier. Spectrum decomposition means for converting the spectrum into each spectrum; amplitude probability distribution measurement means for generating an amplitude probability distribution for each of the spectra; and control for controlling the amplitude probability distribution to be acquired in a no-signal time zone in the radio wave. Means. In such a configuration, information output from the receiving means using the amplitude probability distribution acquired for each subcarrier in the no-signal time zone for signal modification in the transmitting means or the receiving means. The error correction decoder includes a soft decision decoder, and the reception means uses the amplitude probability distribution acquired in the no-signal time zone. The demodulated bits input to the error correction decoder are weighted . Here, the signal modification in the transmission means relates to the transmission form of the signal, and modifies the signal with respect to the encoding method, the modulation method and the like. The signal modification in the receiving means is a modification of the signal with respect to the signal reception mode, particularly the demodulation method and decoding method.

  In addition, the OFDM wireless communication terminal of the present invention provides the above-described reception in a state in which the relationship between the amplitude probability distribution acquired in advance and the bit error rate is prepared, and the amplitude probability distribution acquired in the no-signal time zone of each subchannel is prepared. The modulation means in the transmission means is controlled from the signal power index acquired in the means so that the bit error rate is not more than a desired limit value.

  Further, the signal power distribution for each subchannel in the transmission means is determined by controlling the bit error rate to be equal to or less than a desired limit value.

  The receiving means predicts the signal power of each subcarrier necessary for obtaining a predetermined bit error rate from the amplitude probability distribution, and each of the signal power of each subcarrier obtained from the spectrum decomposition means Subcarrier signal power comparison means for deriving the ratio is provided, and the demodulation bits are weighted so that the respective ratios of the subcarrier signal power comparison means are set to predetermined values.

  For example, since a wireless LAN terminal is usually used by being incorporated in an information electronic device such as a PC, it is affected by noise from the terminal itself. Speed (throughput) is improved.

It is a block diagram which shows the structural example of the receiving part of an OFDM system packet radio | wireless communication terminal. It is a time-amplitude diagram for demonstrating the definition of APD. It is a figure which shows the example of probability density distribution with respect to signal strength in the case of only receiver noise, and when an interference wave is added to it. It is a table | surface which shows the type | formula which estimates the reception BER of an OFDM signal from APD for every subchannel. It is a figure which shows the relationship between a BER estimation result and subcarrier signal strength.

FIG. 1 shows a configuration example of the receiving unit 50 of the OFDM packet radio communication terminal.
The radio wave received by the antenna is frequency-converted to an intermediate frequency by the frequency conversion unit 1, and the intermediate frequency band signal of a desired frequency band is filtered by the reception filter 2. The filtered intermediate frequency band signal is converted into a digital signal by an AD (analog / digital) converter 3, a guard interval is removed by a guard interval remover 4, and then subjected to Fourier transform by an FFT (fast Fourier) calculator 5. The signal is decomposed into spectrums, adjusted so that the amplitude of the subcarrier signal of each channel becomes constant by the channel equivalent unit 6, converted from a signal point on the phase space to a digital signal by the subcarrier demapping unit 7, and deinterleaved 8, the digital signals divided for each subcarrier are integrated and returned to the original state, and the error correction decoding unit 9 performs error correction of the digital signals and outputs them.

As shown in FIG. 2, the APD is represented by a time rate at which the noise amplitude exceeds a certain amplitude value (threshold value x _k ), and is a function of the threshold value x _k as follows.

ここで、Ｗ_i（ｘ_k）は雑音振幅がある振幅値（スレッショルド値ｘ_k）を超える時間、Ｔ₀は測定時間長、ｐ（ｙ）は確率密度分布である。

Here, W _i (x _k ) is a time when the noise amplitude exceeds a certain amplitude value (threshold value x _k ), T ₀ is a measurement time length, and p (y) is a probability density distribution.

  FIG. 3 shows an example of probability density distribution with respect to signal intensity when only receiver noise is present and when an interference wave is added thereto. From this figure, it can be seen that the probability density distribution when an interference wave is added is clearly different from the case of white noise.

  As a specific part of the present invention, there is a noise APD unit 11 that measures APD with respect to noise in a no-signal time zone with a signal from an FFT (Fast Fourier) arithmetic unit 5. The time zone determined by the unit 10 is used.

  The present invention uses the amplitude probability distribution acquired for each subcarrier in the no-signal time zone for signal modification in the transmission means or the reception means, and uses bits of information output from the reception means. The error rate is suppressed. Here, the signal modification in the transmission means relates to a signal transmission form, and refers to modifying a signal in encoding or modulation with respect to an encoding method or a modulation method. Moreover, using APD for signal modification in transmission means changing the encoding method and the modulation method according to APD. The signal modification in the reception means refers to the signal reception form, in particular, the modification of the signal in the demodulation and decoding with respect to the demodulation method and the decoding method, and the use of APD for signal modification in reception corresponds to the APD. This means changing the demodulation method and decoding method.

  In the configuration of FIG. 1, the configuration for obtaining the APD is specifically as follows. The noise amplitude is obtained by calculating the square root sum of squares of the FFT results (in-phase and quadrature components) in the time zone when no communication signal is received, and a histogram of the time variation of the noise amplitude obtained within a certain time is obtained for each subchannel. To create. APD can be obtained by accumulating this histogram from the one with the larger amplitude.

  In the configuration of FIG. 1, the FFT function of the OFDM receiver is used to perform FFT of noise received in no signal time, and the APD is simultaneously measured for each subcarrier, but is independent of the OFDM receiver. Obviously, a device having an equivalent function may be used.

  Here, it is known that when a noise APD and signal power are given, the demodulation bit error rate of the OFDM signal before error correction can be approximately estimated for various modulation schemes and synchronous detection schemes (non- Patent Documents 1, 2, and 3) are shown in FIG.

  When the output from the noise APD unit 11 is used for signal modification in the transmission means, this output is sent to the BER control unit 12 together with a signal used for channel equivalent in the channel equivalent unit 6. The BER control unit 12 estimates the current BER using information input in advance from the APD of noise and the channel equivalent signal, and controls the modulation scheme and coding rate necessary for controlling this to a target value. Etc. The modulation scheme and coding rate determined in this way are transmitted to the transmitting terminal through the transmission unit 20. The transmitting terminal transmits an OFDM signal according to the transmitted modulation scheme and coding rate.

  Using the above relationship between APD and BER, the modulation method necessary for setting the BER before error correction to a desired value is directly determined from the noise APD and signal power of each subchannel, as shown in function 2 in FIG. Instruct the sender. Since the difference between whether the noise is continuous or impulse appears clearly in the APD, it is more accurate than the index based on the noise power.

  Further, the signal power distribution for each subchannel may be modified instead of changing the modulation method. For example, in order to determine the subcarrier signal strength from the required BER value, the bit error rate estimation by the noise APD and the required signal level estimation may be performed from the relationship between the BER estimation result and the subcarrier signal strength shown in FIG. it can. For this purpose, the required signal levels A and B with respect to the required value of BER are obtained as shown in FIG. 5 according to the results of the APDs of subchannels A and B. At this time, the ratio (actual signal level A / required signal level A) and ratio (actual signal level B / required signal level B) are used as weights of bits demodulated from the channels A and B, respectively.

In any of the above cases, it is assumed that the power of each subcarrier used for BER estimation is obtained, but this is estimated using a pilot signal or the like. In addition, it is desirable that the noise APD is correctly obtained up to a target BER (before error correction: in many cases about 10 ⁻² to 10 ⁻⁴ ) or less.

  When the output from the noise APD unit 11 is used for signal modification in the receiving means, this output is sent to the soft decision setting unit 13 to set the weighting of the decoding bit. In setting the weighting, it is desirable to change the weighting for each channel using the channel equivalent signal from the channel equivalent unit 6. When the output signal of the soft decision setting unit 13 is used for the error correction decoding unit 9, a soft decision decoder is used for the error correction decoding unit 9.

  As shown in function 3 of FIG. 1, the above-mentioned APD measurement result can be used to weight the demodulated bits input to the error correction decoder on the receiving side. As a method of weighting the demodulated bits, for example, a method of predicting the signal power of each subcarrier necessary for obtaining a constant BER from the APD and using a ratio with the actual signal power can be considered. Similarly to the above, for example, from the relationship between the BER estimation result and the subcarrier signal strength shown in FIG. 5, the required signal levels A and B for the required BER values are obtained according to the APD results of the subchannels A and B. Here, the ratio (actual signal level A / required signal level A) and ratio (actual signal level B / required signal level B) are used as weights of bits demodulated from channels A and B, respectively.

  Note that the above-described transmission modulation scheme and transmission power distribution optimization cannot be used for signals that need to be simultaneously received by a plurality of terminals in different noise environments. On the other hand, the countermeasures on the receiving side can be applied to the electromagnetic environment in which each receiver is placed, and can be used properly according to the purpose.

  The present invention can be used for a wireless communication device using an ODFM system, such as a wireless LAN and a wireless PAN, which is mounted on a digital AV device (terrestrial digital tuner, DVD recorder, portable video recorder, car navigation device, etc.).

DESCRIPTION OF SYMBOLS 1 Frequency conversion part 2 Reception filter 3 AD (analog / digital) conversion part 4 Guard interval removal part 5 FFT (fast Fourier) calculation part 6 Channel equivalent part 7 Subcarrier demapping part 8 Deinterleaving part 9 Error correction decoding part 10 Signal Arrival determination unit 11 Noise APD unit 12 BER control unit 13 Soft decision setting unit 20 Transmission unit 50 Reception unit of OFDM packet radio communication terminal

Claims (4)

An OFDM wireless communication terminal used as the receiving means in a transmission / reception system including a transmitting means for transmitting an OFDM signal and a receiving means for receiving the OFDM signal,
Spectral decomposition means for converting the received OFDM signal into a spectrum for each subcarrier;
For each of the spectra, each amplitude probability distribution measuring means for generating an amplitude probability distribution;
Control means for controlling to acquire the amplitude probability distribution in a no-signal time zone in the radio wave,
The amplitude probability distribution acquired for each subcarrier in the no-signal time zone is used for signal modification in the transmission means or the reception means, thereby suppressing the bit error rate of information output from the reception means. Is what
The receiving means comprises a soft decision decoder in an error correction decoder,
An OFDM wireless communication terminal characterized in that weighting of demodulated bits input to an error correction decoder in the receiving means is performed using an amplitude probability distribution acquired in the no-signal time period. From the relationship between the amplitude probability distribution acquired in advance and the bit error rate, the amplitude probability distribution acquired in the no-signal time zone of each subchannel, and the signal power index acquired in the receiving means,
2. The OFDM wireless communication terminal according to claim 1, wherein the modulation scheme in the transmission means is controlled so that the bit error rate is less than or equal to a desired limit value. From the relationship between the amplitude probability distribution acquired in advance and the bit error rate, the amplitude probability distribution acquired in the no-signal time zone of each subchannel, and the signal power index acquired in the receiving means,
2. The OFDM wireless communication terminal according to claim 1, wherein the signal power distribution for each subchannel in the transmission means is controlled to make the bit error rate a desired value. The receiving means predicts the signal power of each subcarrier necessary for obtaining a predetermined bit error rate from the amplitude probability distribution, and calculates the ratio of the signal power of each subcarrier obtained from the spectrum decomposition means. A subcarrier signal power comparing means for deriving,
2. The OFDM radio communication terminal according to claim 1, wherein the demodulation bits are weighted so that the respective ratios of the subcarrier signal power comparison means are set to predetermined values.

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