CN102055708A - Timing synchronization scheme of multi-band orthogonal frequency division multiplexing (OFDM) ultra wide-band system - Google Patents

Abstract

Aiming at the multi-band OFDM ultra-wideband system adopted by the IEEE 802.15.3a proposal and the ECMA-368 standard, the invention designs a complete timing synchronization method suitable for the system. The frame detection and coarse timing are performed by the maximum autocorrelation method based on the information of the first frequency band of the preamble sequence, and the fine timing is performed by the minimum energy ratio method based on the information of all three frequency bands. The timing position is corrected twice to ensure the performance of the algorithm. The timing synchronization method can control the residual timing deviation in a small range, so that it can be absorbed by frequency domain channel estimation and equalization, and the scheme has low complexity.

Description

Timing Synchronization Scheme for Multiband OFDM UWB System

technical field

The invention relates to a timing synchronization scheme of a multi-band OFDM ultra-wideband system. Aiming at the IEEE 802.15.3a proposal and the multi-band OFDM ultra-wideband system adopted by the ECMA-368 standard, frame detection, coarse timing and fine-tuning suitable for the system are designed. A complete scheme of timing to find the symbol start bit for correct demodulation.

Background technique

Ultra-wideband (UWB) wireless communication technology has the characteristics of low power spectral density, high transmission rate, and strong anti-multipath interference ability. It will be applied in short-distance high-speed wireless communication, penetrating imaging and measurement, etc. There are many systems to realize UWB, among which multi-band orthogonal frequency division multiplexing (MB-OFDM) technology is suggested as the physical layer standard of IEEE 802.15.3a for indoor personal communication, and adopted by European ECMA-368 standard. Orthogonal Frequency Division Multiplexing (OFDM) is an efficient data transmission technology. It transmits data in parallel through mutually orthogonal subcarriers. It has high frequency band utilization and strong anti-multipath interference ability. OFDM technology has been widely used in communications and other fields, such as European standards DAB, DVB, ADSL, IEEE802.11a and HIPERLAN II. In the traditional narrowband OFDM system, if the timing deviation is ahead but does not exceed the guard interval, it will only bring inter-subcarrier interference (ICI), which needs to be compensated in the frequency domain by channel estimation. If it lags behind, it will also bring inter-symbol interference (ISI), resulting in the loss of signal-to-noise ratio, and the system performance is severely degraded.

The MB-OFDM ultra-wideband system is different from the traditional single-band OFDM system, and has the following characteristics: different frequency bands have different channel responses and carrier frequency offsets; for power-limited UWB systems, zero suffix (ZP) is used instead of cyclic prefix (CP) ) to avoid power loss; a frequency hopping mechanism controlled by a time-frequency code is adopted, so that a wrong timing position will cause wrong de-hopping, and the system performance will be greatly deteriorated. For the ZP-OFDM system of UWB, small timing deviation can be compensated by overlapping and adding (OLA) method while performing frequency domain channel equalization. Assuming that the guard interval length is N _g and the number of multipaths is L, the OLA method can be described as adding the last N _g numbers and the first N _g numbers of the OFDM symbols after timing and placing them at the beginning, so that the ZP-OFDM symbols becomes an equivalent CP-OFDM symbol. When the residual timing offset d belongs to [-(N _g -L), 0], the overlap-add operation is equivalent to the cyclic shift of the OFDM symbol in the time domain, so it can be compensated by the pilot in the frequency domain, otherwise it will introduce ISI, which degrades system performance.

The ECMA-368 standard specifies an ultra-wideband MB-OFDM system with a transmission rate of up to 480Mbps. The frequency band uses the undivided 3.1G-10.6GHz and divides it into 14 sub-bands, each with a bandwidth of 528MHz. , the three sub-bands form a group, and the frequency hopping mechanism controlled by the time-frequency code realizes the conversion of the center frequency of the carrier of the radio frequency part. The FFT length is 128, which includes 100 information subcarriers, 12 pilot subcarriers, 10 guard subcarriers, one DC subcarrier and 5 empty subcarriers. After the IFFT at the sending end, a ZP with a length of 37 is added as a guard interval to form an OFDM symbol with a length of 165. Each frame begins with a preamble sequence, which consists of 24 synchronization symbols and 6 channel estimation symbols. The structure of the synchronization sequence in the time-frequency domain is shown in Figure 1, and its time-frequency codes are 1, 2, 3, 1, 2, 3. A brief block diagram of the system is shown in Figure 2.

The UWB channel impulse response recommended by the IEEE 802.15.3a working group can be expressed as:

$\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; x</span>}\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\underset{\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; nk</span>}}\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&tau;</span>}}_{\mathrm{<span\; class="notranslate">\; nk</span>}}<span\; class="notranslate">\; )</span>$

Where X is a lognormal random variable, representing the amplitude gain of the channel; N is the number of observed clusters, K(n) is the number of multipaths received in the nth cluster, α=p _nk β _nk , p _nk is a discrete random variable with equal probability +1 and -1, and β _nk is the channel coefficient of the kth path in the nth cluster that obeys the lognormal distribution. T _n is the arrival time of the nth cluster, and T _nk is the time delay of the kth path in the nth cluster, both of which obey the Poisson distribution. In addition, the multipath average power into a double exponential decay model

$\mathrm{<span\; class="notranslate">\; E.</span>}<span\; class="notranslate">\; [</span>{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&Omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}}{\mathrm{<span\; class="notranslate">\; \&Gamma;</span>}}}{\mathrm{<span\; class="notranslate">\; e</span>}}^{<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; \&tau;</span>}}_{\mathrm{<span\; class="notranslate">\; nk</span>}}}{\mathrm{<span\; class="notranslate">\; \&gamma;</span>}}}$

为簇到达率，γ为径到达率。IEEE802.15.3a工作组确定了四种标准UWB信道模型CM1～CM4，四种模型的上述变量的分布参数不同，其中CM4信道最为恶劣。Where E[.] represents the statistical average, Ω ₀ is the average power of the first path,

is the cluster arrival rate, and γ is the path arrival rate. The IEEE802.15.3a working group has determined four standard UWB channel models CM1-CM4, and the distribution parameters of the above-mentioned variables of the four models are different, among which the CM4 channel is the worst.

Contents of the invention

The purpose of the present invention is to design a complete scheme suitable for the timing synchronization of the multi-band OFDM ultra-wideband system adopted by the IEEE 802.15.3a proposal and the ECMA-368 standard, aiming to control the residual timing deviation at a certain level through an appropriate timing algorithm. In the range, it can be absorbed by frequency domain channel estimation and equalization.

Technical scheme of the present invention:

The frame detection and coarse timing are performed by the maximum autocorrelation method based on the information of the first frequency band of the preamble sequence, and the fine timing is performed by the minimum energy ratio method based on the information of all three frequency bands. The timing position is corrected twice, and the residual timing deviation is controlled in the range of [-(N _g -L), 0], which ensures the performance of the algorithm, and at the same time, the scheme has low complexity.

Beneficial effects of the present invention:

The present invention designs a timing synchronization scheme of a multi-band OFDM ultra-wideband system. The scheme takes into account both timing synchronization performance and system implementation complexity, and uses an algorithm with low complexity to effectively correct the timing deviation, that is, to ensure reception The demodulation performance of the machine can restore the original data correctly, and reduce the implementation cost, which has practical guiding significance for the hardware realization of the ultra-wideband communication receiver.

Description of drawings

Figure 1 is a structural diagram of the preamble synchronization sequence

Figure 2 is a simplified block diagram of the MB-OFDM-UWB system

Figure 3(a) is the autocorrelation peak with autocorrelation window length 128

Figure 3(b) is the autocorrelation peak with autocorrelation window length 132

Figure 4 False alarm and missing alarm probability under different thresholds

Fig.5 Coarse timing deviation probability under different channel models

Fig.6 Fine timing root mean square error under different energy window lengths

Figure 7 Fine timing root mean square error under different number of symbols

Fig.8 Fine timing deviation probability lines under different channel models

Detailed ways

Below in conjunction with accompanying drawing and by embodiment the specific embodiment of the present invention will be further described:

The present invention has designed a kind of timing synchronization scheme of multi-band OFDM ultra-wideband system, and it is characterized in that: this scheme comprises the following steps:

a. Before the receiver performs timing synchronization, the down-converted local oscillator is initially set at the center frequency of the first frequency band;

b. Frame detection uses the first two training symbols of the first sub-band to find the modulus C _i of its autocorrelation value, and use this as the judgment quantity to set a threshold value G, when C _i , C _i+1 , When C _i+2 is greater than G, it is confirmed that the frame is detected, so that the corresponding i=μ at this time, where i is the sample number, and μ is the position of the detected frame;

c. Carry out rough timing, find the maximum value of C _i within the range of i greater than or equal to μ and less than or equal to μ+M, and set the sample number i _ct corresponding to this maximum value as the coarse timing synchronization position, and M is an OFDM symbol length;

d. Correct the synchronous position of the coarse timing, the corrected synchronous position i _ct ' of the coarse timing is equal to the value before the correction minus δ _ct , where δ _ct is the maximum lag length of the coarse timing;

e. Start de-frequency hopping according to the corrected coarse timing position, divide the energy sum of the previous window of the three frequency band information by the energy sum of the adjacent next window, and record the ratio as D _i , and i belongs to a certain range Find the minimum value of D _i , then the sample sequence number i _ft corresponding to the minimum value is determined as the fine timing synchronization position;

f. Correct the fine timing synchronization position, the fine timing synchronization position i _ft ' after correction is equal to the value before correction minus δ _ft , where δ _ft is the maximum delay length of fine timing.

When performing the autocorrelation operation in step b, the length of the autocorrelation window is N, where N is the number of subcarriers in each frequency band.

The maximum lag length of the coarse timing mentioned in step d refers to the maximum value among the four largest coarse timing lag deviations under the four ultra-wideband channel models.

The window length for calculating the energy ratio in step e is less than or equal to N _g -L, where N _g is the length of the guard interval, and L is the maximum number of multipaths selected based on experience.

i described in step e belongs to a certain range means that i is greater than or equal to i _ct ' and less than or equal to i _ct '+δ _ct -γ, where i _ct ' is the corrected rough timing described in step d in claim 1 Synchronization position, δ _ct is the maximum lag length of the coarse timing described in step d in claim 1, and γ is the maximum value among the four maximum coarse timing advance deviations under the four ultra-wideband channel models.

The information on the three frequency bands mentioned in step e may be the third symbol of each frequency band in the preamble sequence, or may be multiple symbols of each frequency band.

The maximum delay length of the fine timing mentioned in step f refers to the maximum value among the four maximum fine timing lag deviations under the four ultra-wideband channel models.

Example

The present invention is applied in the multi-band OFDM ultra-wideband system of the ECMA-368 standard, and the UWB system is simulated. As shown in Figure 3, it is the autocorrelation value C _i in the frame detection and coarse synchronization, and it can be seen that the correlation window length is The correlation peak corresponding to 128 is very sharp, and 132 will cause the correlation peak to appear in a flat area, which will affect the coarse timing performance. Figure 4 shows the false alarm and missing alarm probabilities corresponding to different thresholds G when the signal-to-noise ratio is 0dB, CM4 channel, and the correlation window length is 128. It can be seen that the performance is the best when G is 0.35-0.5.

Fig. 5 is a rough timing deviation probability diagram when the signal-to-noise ratio is 0dB, the correlation window length is 128, and G is 0.425 under AWGN and CM1-CM4 channels. It can be seen that the timing performance is the worst and the deviation is the largest under the CM4 channel. The maximum lead deviation is 2, and the maximum lag deviation is 32, so the correction value δ _ct =32. It is easy to see that under several channel models, it cannot be satisfied that all d _i ' belong to [-(N _g -L _i ), 0]. For example, under CM4 channel, d ₄ ∈ [9, 32], after correction, d ₄ ∈ [-23, 0]. L ₄ =26, [-(N _g -L ₄ ),0]=[-11,0]. [-23, 0] does not belong to [-11, 0], so the OLA method cannot be corrected; in the CM1 channel, d ₁ ∈ [-1, 8], after correction, d ₁ '∈ [-33, -24] , L ₁ =8, [-(N _g -L ₁ ), 0]=[-29, 0]. [-33, -24] does not belong to [-29, 0], and the OLA method cannot be corrected either. Therefore, fine timing is required, and the comparison range of fine timing is [i _ct ', i _ct '+34]. In order to keep the complexity low, the channel multipath number is not estimated, but a larger multipath number 26 is selected according to experience, and the energy window length is less than or equal to 11. Fig. 6 is the root mean square error (MSRE) curve of the fine timing deviation when the energy window length H is 5, 6, 8, 10 and 11 under the CM4 channel. It can be seen that at 12dB, the MSREs of different window lengths are all less than 2, and it is not that the larger the window, the better the effect. performance. Figure 7 shows the MSRE curves of the energy ratio calculated by using a single symbol and multiple symbols under the CM4 channel and the window length is 5, which are respectively the third symbol, the third to the sixth symbol and the third to the sixth symbol of the preamble sequence Eighth symbol. It can be seen that the greater the number of symbols, the better the timing performance, because the use of multiple symbols can smooth the noise.

Through the above simulations, considering the complexity of implementation, we finally determine the length of the energy window to be 5, and use the third to eighth synchronization symbols of the preamble sequence for fine timing. Fig. 8 is a fine timing deviation probability diagram when the signal-to-noise ratio is 0 dB under AWGN, CM1-CM4 channels. It can be seen that the maximum lag deviation of the fine timing is 3, then the correction δ _ft =3, it is easy to see that this can ensure that under all channel models CM1~CM4, the fine timing deviation after the second correction falls within [-(N _g -L _i ), within 0]. So far we have achieved the desired goal, and the residual timing deviation can be absorbed by frequency domain channel equalization.

Claims (7)

1. a timing synchronization scheme of a multi-band OFDM ultra-wideband system, characterized in that: the scheme may further comprise the steps: a. Before the receiver performs timing synchronization, the down-converted local oscillator is initially set at the center frequency of the first frequency band; b. Frame detection uses the first two training symbols of the first sub-band to find the modulus C _i of its autocorrelation value, and use this as the judgment quantity to set a threshold value G, when C _i , C _i+1 , When C _i+2 is greater than G, it is confirmed that the frame is detected, so that the corresponding i=μ at this time, where i is the sample number, and μ is the position of the detected frame; c. Carry out rough timing, find the maximum value of C _i within the range of i greater than or equal to μ and less than or equal to μ+M, and set the sample number i _ct corresponding to this maximum value as the coarse timing synchronization position, and M is an OFDM symbol length; d. Correct the synchronous position of the coarse timing, the corrected synchronous position i _ct ' of the coarse timing is equal to the value before the correction minus δ _ct , where δ _ct is the maximum lag length of the coarse timing; e. Start de-frequency hopping according to the corrected coarse timing position, divide the energy sum of the previous window of the three frequency band information by the energy sum of the adjacent next window, and record the ratio as D _i , and i belongs to a certain range Find the minimum value of D _i , then the sample sequence number i _ft corresponding to the minimum value is determined as the fine timing synchronization position; f. Correct the fine timing synchronization position, the fine timing synchronization position i _ft ' after correction is equal to the value before correction minus δ _ft , where δ _ft is the maximum delay length of fine timing. 2. the timing synchronization scheme of a kind of multi-band OFDM ultra-wideband system according to claim 1, is characterized in that: when doing autocorrelation calculation in the step b, autocorrelation window length gets N, and wherein N is the number of each frequency band The number of subcarriers. 3. the efficient realization method of a kind of digital cross-correlator according to claim 1, it is characterized in that: the maximum lag length of the coarse timing described in the step d refers to four kinds of ultra-wideband channel models under four kinds The maximum value in the maximum coarse timing lag deviation. 4. the timing synchronization scheme of a kind of multi-band OFDM ultra-wideband system according to claim 1, is characterized in that: the window length that seeks energy ratio among the step e is less than or equal to Ng _- L, wherein _Ng is guard interval length, L is the maximum number of multipaths selected based on experience. 5. the efficient implementation method of a kind of digital cross-correlator according to claim 1, is characterized in that: the maximum lag length of fine timing described in the step f refers to four kinds of ultra-wideband channel models under four kinds The maximum value in the maximum fine timing lag deviation. 6. the timing synchronization scheme of a kind of multi-band OFDM ultra-wideband system according to claim 1, it is characterized in that: i described in the step e belongs to a certain range and refers to that i is greater than or equal to i _ct ' and less than or equal to i _ct '+δ _ct -γ, where i _ct ' is the corrected coarse timing synchronization position described in step d in claim 1, δ _ct is the maximum lag length of the coarse timing described in step d in claim 1, and γ is The maximum of the four largest coarse timing advance deviations under the four UWB channel models. 7. the efficient realization method of a kind of digital cross-correlator according to claim 1, it is characterized in that: three frequency band information described in the step e can be the 3rd symbol of each frequency band in the preamble sequence, also can for multiple symbols for each frequency band.

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