CN1642051A - Method for obtaining optimum guide symbolic power - Google Patents

Abstract

The invention supplies a method to gain the optimal guidance symbol power. It is used the factors that system noise signal ration in the channel estimating, base station and mobile platform relative move speed, coherent testing channel estimating interpolation filter coefficient and it depth, signal modulating module and the functional relation between the optimal guidance symbol power and the data symbol power ratio to gain the method used to gain the optimal power radio. The method of the invention is adopted to gain the optimal bit error ratio to the self-adapting OFDM system at the condition of giving the total emission power.

Description

A kind of method of obtaining the optimal guidance symbol power

Technical field

The invention belongs to wireless communication technology field, particularly optimize OFDM guide symbol power technology.

Background technology

(Orthogonal Frequency Division Multiplexing OFDM) is high speed transmission technology under a kind of wireless environment to OFDM.The main thought of OFDM technology is divided into many orthogonal sub-channels with institute to channel exactly in frequency domain; on each subchannel, use a subcarrier to modulate; and each subcarrier parallel transmission; if it is suitable that protection is chosen at interval; then each subchannel fading rise shows as flat characteristic; and can significantly reduce intersymbol interference (Inter-SymbolInterference, ISI).

Compare with single-carrier system, ofdm system is used a plurality of subcarriers and is transmitted a parallel data flow, be modulated on a plurality of subcarriers through the data flow of encoding and interweave, in having the multidiameter fading channel of frequency selectivity, ofdm system has than the better anti fading performance of single-carrier system, so the OFDM technology has obtained increasingly extensive application in wide-band mobile communication system.

The wireless channel of wide-band mobile communication system has frequency selectivity usually and becomes when being, its channel transfer functions all presents heterogeneity on time domain still is frequency domain.Have various factors to influence the reliability of received signal in the wireless channel of reality, ofdm system must be paid attention to separating timing.In typical frequency selective fading channels such as Rayleigh channel, if the OFDM originating terminal signal is modulated on a plurality of carrier waves with identical power, but because the characteristic of channel heterogeneous, on the different carrier wave of demodulating end, will have different signal to noise ratios (Signal to Noise Ratio, SNR).Therefore when carrying out signal decision, the data that are modulated onto on the high snr carrier have higher decision reliability with respect to the data that transmit on the low signal-to-noise ratio carrier wave.

This priori reliability information that becomes before judgement the time is called channel condition information, and it has dynamically reflected the situation of change of channel.By the confidence level this heterogeneous that channel variation is brought, when Soft decision decoding, must pay attention to.Therefore, at the OFDM demodulating end channel condition information is extracted and to be very important, this also is a kind of extremely important and particular structure that multicarrier system is different from single-carrier system.

And if transmitter adopts non-differential modulation scheme, then receiver just must adopt coherent demodulation.In ofdm system, " being concerned with " means that each subcarrier must be synchronous or phase deviation is known.Therefore in ofdm system,, must estimate dynamically the variation of its channel at its demodulating end if carry out coherent detection.

Channel estimating can be carried out according to those known guide symbols, and these symbols are included in the preceding ofdm signal of transmission.At first, receiver extract the guide symbol place those constantly and the transmission coefficient on the frequency, then, the method interpolation locational transmission coefficient between guide symbol by filtering again.(Pilot Symbol Assisted Modulation PSAM) carries out channel estimating according to the guide symbol that inserts in the data sequence, compares with differential ference spiral, and the snr gain of 2.5～3dB can be arranged to have the auxiliary coherent demodulation of guide symbol of channel estimating.

Each carrier wave of ofdm system can use different modulation according to the condition of channel, such as BPSK, QPSK, 8PSK, 16QAM, 64QAM or the like, is principle with the optimum balance between the availability of frequency spectrum and the error rate.The best modulation system that certain error rate is satisfied in selection can obtain maximum spectral efficiency, the basic principle of Adaptive Modulation that Here it is.The frequency selective fading of multipath channel can cause received signal power to decline to a great extent, and reaches more than the 30dB, and signal to noise ratio also declines to a great extent.Use the modulation system that is complementary with signal to noise ratio can improve the availability of frequency spectrum.As everyone knows, whether good reliability be communication system operation important performance assessment criteria, so system selects BPSK or QPSK modulation usually, can guarantee the signal to noise ratio requirement under the channel worst case like this, but the spectrum efficiency of these two kinds of modulation is too low.If use Adaptive Modulation, terminal just can be used higher modulation when channel is good so, equally in terminal when the base station, modulation can change into 16QAM～64QAM (4～6bit/s/Hz) by BPSK (1bit/s/Hz), the availability of frequency spectrum of whole system is significantly improved, and Adaptive Modulation can make power system capacity be doubled in this case.But anything all has its dual character, and Adaptive Modulation is no exception.It requires signal must comprise certain overhead-bits, and with the modulation system of informing that receiving terminal transmits and adopted, and terminal needs the regular update modulation intelligence, and this will certainly increase more overhead-bits again.For solving this contradiction, OFDM adopts the technology of power control and Adaptive Modulation co-ordination.When channel was good, transmitting power was constant, can enhanced modulation mode (as 64QAM), perhaps reduction transmitting power when low modulation (as QPSK).Power control will average out with Adaptive Modulation, that is to say that it has good channel for a far-end transmitting station, if transmitted power remains unchanged, can use higher modulation scheme such as 64QAM; If power can reduce, the also corresponding reduction of modulation scheme can be used QPSK.

Distortion, frequency deviation also are the factors that must consider when selecting modulation.The non-linear meeting of transmission causes intermodulation distortion, and this moment, signal had higher noise level, and signal to noise ratio generally can be too not high; The frequency shift (FS) that step-out and Doppler frequency shift are caused makes interchannel lose orthogonal property, and only 1% frequency deviation will cause signal to noise ratio decline 30dB.Signal to noise ratio has limited the maximum spectrum utilance can only be near 5～7bit/s/Hz.Adaptive Modulation requires that the performance of channel is had sufficient understanding, if use stronger modulation system on the channel of difference, will produce the very high error rate so, influences the reliability of system.The guide symbol of multi-user OFDM system or can be used for the quality of test channel with reference to code word.Send the code word of a given data, under the situation that satisfies the communication limit, measure the signal to noise ratio of every channel, recently determine optimal modulation system according to this noise.So carrying out channel estimating when receiving terminal adopts adaptive modulation system is very to be necessary.

Under the situation of the total transmit power limited of system, for the auxiliary channel estimating of guide symbol, on the one hand, the power that increases guide symbol can improve the accuracy of channel estimating, but the interference to the data symbol also can increase, along with the increase of guide symbol power, the power of giving data symbol must reduce simultaneously, causes and separates the decline of timing signal to noise ratio; On the other hand, the power that reduces guide symbol then can reduce channel estimated accuracy, particularly when the travelling carriage high-speed mobile.Hence one can see that, and the ratio (pilot-to-data power ratio:PDR) of guide symbol power and data symbol power has significant effects to the performance of system, is an important parameter that urgency is to be optimized.

Summary of the invention

Task of the present invention provides a kind of method of obtaining the optimal guidance symbol power, adopts method of the present invention, for Adaptive OFDM system, under the situation of given total transmitting power, can obtain best bit error rate performance.

A kind of method of obtaining the optimal guidance symbol power of the present invention is characterized in that adopting following step:

The first step is obtained signal to noise ratio γ _b: γ _b=E _bσ ²/ N ₀, wherein, E _bThe transmit energy of every bit of expression, N ₀The one-sided power spectrum density of white Gaussian noise of expression channel, they are all by system and channel decision, ${\mathrm{\σ}}^2=E\left[{\mathrm{\σ}}_i^2\right]$ Represent separable multipath signal average power, wherein σ _i ²The energy of representing i footpath in the separable multipath signal;

In second step, obtain the fading factor ν of steering signal by fading channel:

Wherein, f _p ^lAnd g _q ^kThe interpolation coefficient of expression interpolater, P, Q represent the interpolation filter degree of depth.Determine interpolation filter coefficient, can adopt optimum dimension to receive interpolating method, also can adopt the sinc interpolation method,

The 3rd goes on foot, and obtains the correlation function of the decline estimation of data symbol on the subchannel: ${\mathrm{\Π}}_i^0={\mathrm{\Π}}_i/{\mathrm{\σ}}_i^2$ Be normalized correlation, ${\mathrm{\Π}}_i=\underset{q}{\mathrm{\Σ}}\underset{q^{\′}}{\mathrm{\Σ}}\underset{p}{\mathrm{\Σ}}\underset{p^{\′}}{\mathrm{\Σ}}{g}_q^k{g}_{q^{\′}}^k{f}_p^l{f}_{p^{\′}}^l{R}^i({\mathrm{\τ}}_p,{\mathrm{\ν}}_q)$ Be the correlation function of the decline estimation of the data symbol on the i subchannel, $R^i({\mathrm{\τ}}_p,{\mathrm{\ν}}_q)={\mathrm{\σ}}_i^2{J}_0{\left(2\mathrm{\π}{f}_{d}T{\mathrm{\τ}}_p\right)e}^{-j2\mathrm{\π}{\mathrm{\ν}}_{q}i/N}$ Be the correlation function of i bar mobile wireless subchannel decline, τ _p=(pD _t-l) N _t, ν _q=(qD _f-k), with vector representation be

Wherein, the groups of filter coefficients vector of embarking on journey on the time-frequency domain

With

Represent vectorial transposition, R _Φ=Df ⁱBe i subchannel Φ=D on frequency domain _fThe time R ' (Δ, Φ=D _f) correlation matrix on time domain.In addition, f _dBe maximum doppler frequency, by base station and travelling carriage relative moving speed V decision, J ₀() expression zero Bessel function.

The 4th step, the result above the utilization and formula (2), (3) just can obtain optimum guide symbol power and data symbol power ratio ρ:

1. adopt the optimum PDR of M-PSK modulation to be:

$\mathrm{\ρ}=\sqrt{\frac{({\mathrm{\γ}}_b{\log }_2^{M}K+{\left(f_d{T}_s\right)}^2{\mathrm{\γ}}_b{\log }_2^{M}K+\mathrm{KL})(v{\left(f_d{T}_s\right)}^2{\mathrm{\γ}}_b{\log }_2^{M}K+\mathrm{vLK})}{L{\mathrm{\γ}}_b{\log }_2^{M}K\underset{i=0}{\overset{L-1}{\mathrm{\Σ}}}{\mathrm{\Π}}_i^0+L^{2}v}}---\left(2\right)$

2. adopt the optimum PDR of M-QAM modulation to be:

$\mathrm{\ρ}=\sqrt{\frac{v{\mathrm{LK}}^2+\left(v({\left(f_d{T}_s\right)}^2+v){\mathrm{\γ}}_b{\log }_2^{M}K\right)}{v+{\mathrm{\γ}}_b{\log }_2^{M}K\underset{i=0}{\overset{L-1}{\mathrm{\Σ}}}{\mathrm{\Π}}_i^0}}---\left(3\right)$

Wherein, L represents that Rayleigh fading is distinguishable many through number, is determined by channel situation; K=D _t* D _f-1, T _sBe the cycle of an OFDM symbol, M represents order of modulation, and M-PSK represents the strong control of M rank phase shift, and M-QAM represents M rank quadrature amplitude modulation.

The 5th step, utilize the 4th guide symbol power that obtains of step and the data symbol power value that data symbol power ratio ρ and Adaptive OFDM system have provided, just can obtain the guide symbol performance number of optimum.

Utilize the guide symbol performance number of the optimum that the present invention obtains, design optimum guide symbol, make snr loss's minimum of Adaptive OFDM system, the receptivity optimum.

Need to prove that the channel of considering among the present invention is static irrelevant scattering (WSSUS) rayleigh fading channel, the time of system and a Frequency Synchronization entirely accurate of broad sense.

Operation principle of the present invention: based on the OFDM baseband system model (as shown in Figure 1) of the auxiliary relevant modulation of guide symbol, a binary sequence is mapped as M system symbol, insert after 1 at guide symbol, be multiplexed with { X through anti-phase fast fourier transform 2 (IFFT) _{L, n}, the protection at interval insertion 3 of (Cyclic Prefix) is in order to offset intersymbol interference (ISI) in the ofdm system, transmission signals is through out-of-date change multipath Rayleigh fading channel 6{h then _{L, n}, remove protection interval 9 back received signals and be { r _{L, n}, the demultiplexing multi-carrier signal after the corresponding fast fourier transform 10 (FFT) is { R _{L, k}, obtain demodulating data symbols by channel estimating 11 compensate for channel decline 12.

By top as can be known described, for the auxiliary relevant demodulating system of OFDM guide symbol, under the situation of given total transmitting power, in order to obtain best bit error rate performance, the optimal guidance symbol power that draws and the ratio (PDR) of the power of data symbol have been subjected to the influence of system signal noise ratio, base station and travelling carriage relative moving speed, coherent detection channel estimating interpolation filter coefficient and factors such as the interpolation filter degree of depth and signal order of modulation.Therefore can define optimum ρ:

$\mathrm{\ρ}=f({\mathrm{\γ}}_b,V,f_p^l,g_q^k,M,Q,P)---\left(4\right)$

Wherein, ρ represents optimum guide symbol power and data symbol power ratio, and it is γ _b, V, f _p ^l, g _q ^k, the function of arbitrary variable among M, Q and the P, γ _bThe expression signal to noise ratio, V represents base station and travelling carriage relative moving speed, f _p ^lAnd g _q ^kExpression coherent detection channel estimating interpolation filter coefficient, M represents the signal order of modulation, and Q and P represent the coherent detection channel estimating interpolation filter degree of depth, and f () is expressed as a kind of functional relation, and its function characteristic is as follows:

1. the transmitting power of signal increases, corresponding signal to noise ratio γ _bIncrease, the ratio that is mapped as the power of optimal guidance symbol power and data symbol reduces; The transmitting power of signal reduces, corresponding signal to noise ratio γ _bReduce, the ratio that is mapped as the power of optimal guidance symbol power and data symbol reduces.

2. the variation of base station and travelling carriage relative moving speed, maximum doppler frequency f _dAlso corresponding changing, then optimum PDR value also changes.That is, relative moving speed increases, and corresponding optimum PDR increases; Relative moving speed reduces, and corresponding optimum PDR reduces.

3. the time domain interpolation filter coefficient f of channel estimating in the coherent detection _p ^lThe optimum PDR value of variable effect.

4. the frequency domain interpolation filter coefficient g of channel estimating in the coherent detection _q ^kThe optimum PDR value of variable effect.

5. interpolation filter degree of depth Q and P also influence optimum PDR value, if the interpolation degree of depth is no more than the coherence bandwidth of channel, then along with the increase of Q and P, optimum PDR is corresponding to be reduced; Interpolation filter degree of depth Q and P reduce, and optimum PDR value increases; But the interpolation degree of depth surpasses the channel coherence bandwidth, along with the increase of Q and P, and the corresponding increase of optimum PDR; Interpolation filter degree of depth Q and P reduce, and optimum PDR value reduces.

6. the exponent number M of system modulation determines the size of PDR, and the PDR of high order modulation is greater than the PDR of low-order-modulated.But when adopting optimum PDR, channel estimating increases accurately, so transmitting power is constant, and can enhanced modulation mode (as 64QAM, M is bigger), perhaps reduction transmitting power when low modulation (as QPSK, M is less).

According to top function characteristic, the present invention has provided concrete function f () expression formula, and f () can also have other expression certainly.The present invention has provided the method for optimal guidance symbol power value in the ofdm system that adopts M-QAM modulation and M-PSK modulation.

For Adaptive OFDM system, adopt the present invention to provide a kind of method of obtaining the optimal guidance symbol power, under the situation of given total transmitting power, can obtain best bit error rate performance; This method can be used for determining that PDR branch optimum in the Rayleigh fading environment is equipped with the signal to noise ratio degradation that the imperfect channel estimating of compensation causes; And optimum choose that PDR can improve the whole system performance effectively and the transmitting power that do not increase system.

Description of drawings:

Fig. 1: self adaptation OFDM baseband system model

Wherein, the 1st, guide symbol inserts, and the 2nd, inverse fast fourier transform, the 3rd, add Cyclic Prefix, the 4th, pulse shaping filter, the 5th, multiplier, the 6th, Rayleigh fading, the 7th, adder, the 8th, noise, the 9th, remove Cyclic Prefix, the 10th, fast fourier transform, the 11st, channel estimating, the 12nd, signal compensation, the 13rd, remove guide symbol.

Fig. 2 is a kind of method flow schematic diagram that obtains the optimal guidance symbol power of the present invention

Fig. 3 is the performance comparison diagram that the present invention draws with COSSAP emulation.

Embodiment

The first step in the ofdm system that adopts the 16QAM modulation (being M=16), is chosen channel and is ChannelA M.1225Vehicular, and the amplitude in each footpath is a rayleigh distributed, selects three footpath channels (being L=3) for use, and the power spectrum in each footpath is that Classic composes.The setting carrier frequency is 2GHz, and signal bandwidth is 20MHz, sub-carrier number N=4096, and the then symbol cycle is 200 μ s, and 50 μ s are got at interval in protection, and the interval of guide symbol on frequency domain and time domain all is 4 (to be D _t=D _f=4).In addition, the signal to noise ratio of setting channel is: E _b/ N ₀=12dB.

Second step, set the interpolation filter degree of depth and be 3 (that is, and Q=P=3), by the previous step front provide system parameters and following shown in the sinc function:

$f_p^l=\sin c(\frac{l}{D_t}-p)---\left(3\right)$

$g_q^k=\sin c(\frac{k}{D_f}-q)$

Can determine the interpolation coefficient f that adopts the sinc interpolation method _p ^lAnd g _q ^kWherein, l express time index, k represents Frequency Index.

In the 3rd step,, just can obtain the optimal power ratio in native system: ρ=1.5 according to the result who draws previously and the given characteristic of channel.

The 4th step, other situation of ρ=1.5 and ρ ≠ 1.5 is carried out emulation, drawn the performance comparison diagram (as shown in Figure 3) under the different ρ conditions.As can be seen from the figure, under the certain condition of total emission power, have only when ρ=1.5 bit error rate of system (BER) minimum, the i.e. best performance of system.

As seen, the optimal guidance symbol that use the present invention provides and the power ratio of data symbol design guide symbol, can optimize the performance of Adaptive OFDM system.

Claims (2)

1, a kind of method of obtaining the optimal guidance symbol power is characterized in that adopting following step:

The first step is obtained signal to noise ratio γ _b: γ _b=E _bσ ²/ N ₀, wherein, E _bThe transmit energy of every bit of expression, N ₀The one-sided power spectrum density of Gauss's self noise of expression channel, ${\mathrm{\σ}}^2=E\left[{\mathrm{\σ}}_i^2\right]$ Represent separable multipath signal average power, wherein σ _i ²The energy of representing i footpath in the separable multipath signal;

In second step, obtain the fading factor of steering signal by fading channel

Wherein, f _p ^lAnd g _q ^kThe interpolation coefficient of expression interpolater, P, Q represent the interpolation filter degree of depth; Determine interpolation filter coefficient, can adopt optimum dimension to receive interpolating method, also can adopt the sinc interpolation method,

The 3rd goes on foot, and obtains the correlation function of the decline estimation of data symbol on the subchannel: ${\mathrm{\Π}}_i^0={\mathrm{\Π}}_i/{\mathrm{\σ}}_i^2$ Be normalized correlation, ${\mathrm{\Π}}_i=\underset{q}{\mathrm{\Σ}}\underset{q^{\′}}{\mathrm{\Σ}}\underset{p}{\mathrm{\Σ}}\underset{p^{\′}}{\mathrm{\Σ}}{g}_q^k{g}_{q^{\′}}^k{f}_p^l{f}_{p^{\′}}^l{R}^i({\mathrm{\τ}}_p,v_q)$ Be the correlation function of the decline estimation of the data symbol on the i subchannel, $R^i({\mathrm{\τ}}_p,v_q)={\mathrm{\σ}}_i^2{J}_o\left(2\mathrm{\π}{f}_{d}T{\mathrm{\τ}}_p\right)e^{-j2\mathrm{\π}{v}_{q}i/N}$ Be the correlation function of i bar mobile wireless subchannel decline, τ _p=(pD _t-l) N _t, ν _q=(qD _f-k), with vector representation be

Wherein, the groups of filter coefficients vector of embarking on journey on the time-frequency domain With

Represent vectorial transposition, $R_{\mathrm{\Φ}=D_f}^i$ Be i subchannel Ф=D on frequency domain _fThe time R ⁱ(Δ, Ф=D _f) correlation matrix on time domain; In addition, f _dBe maximum doppler frequency, by base station and travelling carriage relative moving speed V decision, J ₀() expression zero Bessel function;

The 4th step, the result above the utilization and formula (2), (3) just can obtain optimum guide symbol power and data symbol power ratio ρ:

At first, adopt the optimum PDR of M-PSK modulation to be:

$\mathrm{\ρ}=\sqrt{\frac{({\mathrm{\γ}}_b{\log }_2^{M}K+{\left(f_d{T}_s\right)}^2{\mathrm{\γ}}_b{\log }_2^{M}K+\mathrm{KL})(v{\left(f_d{T}_s\right)}^2{\mathrm{\γ}}_b{\log }_2^{M}K+\mathrm{vLK})}{L{\mathrm{\γ}}_b{\log }_2^{M}K\underset{i=0}{\overset{L-1}{\mathrm{\Σ}}}{\mathrm{\Π}}_i^0+L^{2}v}}---\left(2\right)$

Then, adopt the optimum PDR of M-QAM modulation to be:

$\mathrm{\ρ}=\sqrt{\frac{\mathrm{vL}{K}^2+(v{\left(f_d{T}_s\right)}^2+v){\mathrm{\γ}}_b{\log }_2^{M}K}{v+{\mathrm{\γ}}_b{\log }_2^{M}K\underset{i=0}{\overset{L-1}{\mathrm{\Σ}}}{\mathrm{\Π}}_i^0}}---\left(3\right)$

Wherein, L represents that Rayleigh fading is distinguishable many through number, is determined by channel situation; K=D _t* D _f-1, T _sBe the cycle of an OFDM symbol, M represents order of modulation, and M-PSK represents the strong control of M rank phase shift, and M-QAM represents M rank quadrature amplitude modulation;

The 5th step, utilize the 4th guide symbol power that obtains of step and the data symbol power value that data symbol power ratio ρ and Adaptive OFDM system have provided, just can obtain the guide symbol performance number of optimum.

2, a kind of method of obtaining the optimal guidance symbol power according to claim 1 is characterized in that described channel is static irrelevant scattering (WSSUS) rayleigh fading channel, the time of system and a Frequency Synchronization entirely accurate of broad sense.

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