CN101018083A - Dopla frequency shift compensation method in the MPSK mobile communication system - Google Patents

Abstract

The disclosed Doppler shift compensation method in MPSK mobile communication system comprises: fixing the receiver local carrier frequency, coherent demodulating and recovering clock to receive the features of both sides by data package head; unbiased estimating the code phase difference, then using simple LMS algorithm to compensate other offset and initial phase. This invention combines phase estimation and LMS algorithm, and uses less reference bit, and compensates fast.

Description

The compensation method of Doppler frequency shift in the MPSK mobile communication system

Technical field

The invention belongs to moving communicating field, be specifically related to the compensation method of MPSK mobile communication system Doppler frequency shift.

Background technology

In mobile communication system, because the relative motion of communicating pair, the frequency shift (FS) that the recipient is taken place is called Doppler frequency shift, and Doppler frequency shift is directly proportional with both sides' speed of related movement.This shows, for the very fast LEO satellite communication system of mobile communication, the particularly speed of service, Doppler frequency shift is bigger usually, for MPSK (MPSK multiple phase shift keying) mobile communication system of using coherent demodulation, if Doppler frequency shift is not compensated, then local carrier can't reach with signal carrier synchronously, and the generation frequency departure, and then cause the deviation of demodulation phase, and make the judgement of data and regeneration mistake occur, can not obtain correct data.To the compensation of frequency shift, all be earlier carier restoration to be come out at present, as making a start with inserting pilot tone system, receiving terminal comes out carrier extract with simple narrow band filter, and this method is owing to need seldom adopt for a large amount of power of allocation of pilots; Or carry out Nonlinear Processing to received signal, and carrier component is extracted, as Costas ring etc., this method needs complicated carrier recovery circuit, and required precision is higher.

Summary of the invention

The objective of the invention is to provide the compensation method of Doppler frequency shift in a kind of MPSK mobile communication system, need the shortcoming of complicated carrier recovery circuit to overcome prior art for the MPSK mobile communication system.

The present invention realizes by following step:

A, at the signal receiving end of mobile communication system, the characteristic that the reference symbols sn information of utilizing every bag datagram header is known altogether for mobile communication system signal receiving end and transmitting terminal both sides, between the adjacent code element of the baseband signal after the clock recovery because the phase difference that Doppler frequency shift causes does not have inclined to one side estimation;

The phase estimation value that b, utilization obtain is carried out the phase place correction to each baseband signal code element;

C, select the desired signal of reference symbols sn Given information, the revised reference symbols sn baseband signal of phase place is made the LMS algorithmic statement as input signal as algorithm;

D, with the revised data symbols baseband signal of phase place as input signal, the less phase deviation that stays after utilizing the LMS algorithm restrained to the phase place correction and the initial phase deviation or the clock recovery phase deviation of modulation compensate, at this moment, with the hard decision result of input signal desired signal as algorithm;

E, to the hard decision result of the LMS algorithm output signal final result of method by way of compensation, come the entrained information of decision signal.

The compensation method of Doppler frequency shift of the present invention does not need to received signal that carrier wave recovers, and directly uses former modulated carrier frequencies to carry out coherent demodulation, thereby has saved carrier recovery circuit, has simplified communication system, makes the operation that it can be more stable.

The present invention starts with from the estimation to phase deviation, on the basis of fixed receiver local carrier frequency, at first the phase deviation between the code element is not had inclined to one side estimation, used a kind of very simple and utilize seldom bit data packet head code element (reference symbols sn) then as the LMS algorithm of desired signal.Phase estimation and LMS algorithm combine, and the demodulation phase deviation that can cause the Doppler frequency shift (in the receiving filter filter range) of any size compensates.

Description of drawings

Fig. 1 is the schematic diagram of mobile communication system that the present invention is applied to; Fig. 2 is the flow chart that makes the LMS algorithmic statement among the step c; Fig. 3 is the LMS algorithm signal flow graph in the steps d; Fig. 4 is the emulation ber curve of different reference symbols sn numbers, and abscissa is signal to noise ratio 0≤SNR≤10 (dB), and ordinate is the error rate, and theory represents the coherent demodulation theoretical curve, and R is the ber curve that obtains under the different reference symbols sn; When Fig. 5 had provided R=10, the ber curve under the different Doppler frequency shifts, theory were represented the coherent demodulation theoretical curve, Δ θ _DopPhase deviation between the code element that Doppler frequency shift causes is different Δ θ among the figure _DopCorresponding ber curve.

Embodiment

Further describe below in conjunction with 1 pair of application of the present invention of accompanying drawing.Transmitting terminal in mobile communication is modulated up-conversion f to transmission symbol through QPSK _c, transmitting to receiving terminal by channel, the intermediate frequency unit received signal of receiving terminal is passed through coherent demodulation and clock recovery again, uses the compensation that method of the present invention is carried out Doppler frequency shift then, and each step before the clock recovery does not belong to content of the present invention.

Of the present inventionly mainly comprise two major parts: phase estimation and LMS adaptive algorithm; Wherein, the LMS adaptive algorithm is core of the present invention.Because message transmission rate is very high, can think that every packet Doppler frequency shift is constant; Phase place is the integration of frequency, and for constant frequency shift (FS), there is the doppler phase deviation that equates in the phase deviation linearity between the promptly adjacent code element.

At first need to set up the model of whole communication system:

In conjunction with Fig. 1, to establish channel and be multiple additive white Gaussian noise channel (complex AWGN), its expression-form is suc as formula (1):

n(t)＝n _c(t)+jn _s(t) (1)

Wherein, n _c(t) and n _s(t) be separate Gaussian process.

Transmitting terminal, every packet is made up of R reference symbols sn R (k) and D data code element D (k), and data are S (k) through the MPSK modulating baseband signal after packing, pass through suitable rf modulations method again, and modulation signal is sent in the channel.Can be expressed as through the signal after the carrier modulation:

Wherein,  (k) is the MPSK phase modulation;

Be modulation initial phase, f _cBe carrier frequency.

Receiving terminal, for MPSK, log ₂M is the bit number of each code element representative, and Δ f is a Doppler frequency deviation, and B is the bit rate (bit/s) of digital signal, and the signal that has Doppler frequency shift and noise can be expressed as:

Warp is with f _cAfter the coherent demodulation of local carrier, clock recovery, its each symbol recovery is:

Wherein:

${\mathrm{\&theta;}}_{\mathrm{dop}}\left(k\right)=\frac{2\mathrm{\&pi;\&Delta;f}\&CenterDot;{\log }_{2}M}{B}\left(\mathrm{<figure-callout\; id="2"\; label="mod"\; filenames="A20071007178900101.png"\; state="\{\{state\}\}">mod</figure-callout>}2\mathrm{\&pi;}\right)\&CenterDot;k={\mathrm{\&Delta;\&theta;}}_{\mathrm{dop}}\&CenterDot;k---\left(5\right)$

Because the existence of noise, the phase deviation of establishing between the adjacent code element is Δ θ (k), k=1, and 2 ..., R+D, then in multiple additive white Gaussian noise channel, if signal power is 1, noise power is σ _n ², the phase deviation Δ θ (k) between the code element～N (Δ θ then _Dop, 2/ σ _n ²).

After setting up model of communication system, the present invention will carry out according to the following steps:

The characteristic that a, the reference symbols sn information of utilizing every bag datagram header are known altogether for mobile communication system signal receiving end and transmitting terminal both sides is to the phase difference θ (k) between the adjacent code element of the baseband signal y after the clock recovery (k)

There is not inclined to one side estimation;

Theoretically, when signal to noise ratio is very big, ignore noise item, have:

${\mathrm{\&Delta;\&theta;}}_{\mathrm{dop}}={\mathrm{\&Delta;\&theta;}}_{\mathrm{dop}}\&CenterDot;(k+1)-{\mathrm{\&Delta;\&theta;}}_{\mathrm{dop}}\&CenterDot;k$

Wherein,

Be phase angle.

But in actual conditions because the existence of noise, can only utilize sample (k=1,2 ..., R) average

The population mean Δ θ that does not have inclined to one side estimation Δ θ (k) _DopDo not estimate that partially revised phase error is very little and have, utilize the LMS algorithm to compensate.Order:

${\mathrm{\&Delta;\&theta;}}_{\mathrm{dop}}=\stackrel{\&OverBar;}{\mathrm{\&theta;}}+\mathrm{\&Delta;}---\left(7\right)$

Phase deviation Δ θ between the code element that causes of the Doppler frequency shift that will estimate exactly _Dop, the phase deviation of Δ for estimating that the back is remaining,

To Δ θ _DopThe order of accuarcy of estimation will determine the compensation situation of algorithm to the Doppler frequency shift phase error.That is:

$=\frac{1}{R}\underset{k=1}{\overset{R}{\mathrm{\&Sigma;}}}E\left[\mathrm{\&Delta;\&theta;}\left(k\right)\right]=\frac{1}{R}\underset{k=1}{\overset{R}{\mathrm{\&Sigma;}}}{\mathrm{\&Delta;\&theta;}}_{\mathrm{dop}}={\mathrm{\&Delta;\&theta;}}_{\mathrm{dop}}$

The phase place that b, utilization obtain does not have inclined to one side estimation

, each baseband signal code element y (k) is carried out the phase place correction, obtain z (k), be about to the as far as possible little of linear phase deviation adjustment; Process

The revised baseband signal z of phase place (k) can be expressed as:

C, utilize reference symbols sn e ^{J  (k)}(k=1,2 ... R) as desired signal d (k), with the revised reference symbols sn baseband signal of these phase places z (k) (k=1,2 ..., R) as input signal, make the LMS algorithmic statement;

D, beyond the reference symbols sn in every bag data, the revised signal element z of phase place (K) (k=R+1 ..., R+D), with its hard decision e as a result ^{J  ` (k)}(k=R+1 ... R+D) as desired signal, utilize the LMS algorithm restrained to continue phase deviation Δ remaining after the phase estimation is compensated, the initial phase deviation to modulation compensates simultaneously; Vector LMS algorithm among step c and the d can be described as three following formula:

X (k)=w ^*(k) z (k) (algorithm output) (11)

E (k)=d (k)-x (k) (error signal) (12)

W (k+1)=w (k)+2ue ^*(k) z (k) (weight iteration) (13)

Fig. 2 and Fig. 3 have provided among step c and the d corresponding to the signal flow of formula 11 to formula 13, and z (k) is as the input signal of algorithm; D (k) is a desired signal, and the difference of step c and d is the different of desired signal and input signal; Error signal e (k) by desired signal d (k) and output signal x (k) is constantly adjusted weights signal w (k), makes weight adjusted value each time can catch up with the variation of input signal, and makes error signal always very little.Weight initial value in the steps d need use the result among the step c.U is an adaptive step, chooses u=0.15 among the present invention.

E, hard decision be e as a result ^{J  ` (k)}(k=R+1 ... R+D) the output result of method by way of compensation, come the entrained information of decision signal.

According to top description this method is carried out emulation.If 100 code elements of every bag, reference symbols sn R={10,12,20}, data symbols D=100-R; Choose the QPSK modulation system, the phase place of modulation signal is  (k)={ ± π/4, ± 3 π/4}; Make initial weight w (0)=0.θ ₀=0, at random }; Average power signal is 1.The Monte-Carlo emulation mode is used in the calculating of the error rate, promptly utilizes the method for statistical average.

Because the LMS algorithm is a kind of adaptive equalization to regular variation error, and the random phase error of bringing for white Gaussian noise, can't compensate, therefore the error rate behind the LMS algorithm compensation can only be unlimited levels off to the theoretical ber curve of coherent demodulation, and can't surpass this curve.

Fig. 4 has provided the ber curve under the different condition, and as can be seen from the figure, when reference symbols sn was 10, compensation performance was the poorest, and 12 characters are weaker than 20 characters slightly.Therefore, for general condition, can select R=20.

When Fig. 5 has provided R=10, different Doppler frequency shift Δ θ _DopUnder ber curve, as can be seen from the figure, for different Doppler frequency shifts, the algorithm compensation performance does not have difference.

Claims (2)

1.MPSK the compensation method of Doppler frequency shift in the mobile communication system is characterized in that this method realizes by following step:

A, at the signal receiving end of mobile communication system, the reference symbols sn of utilizing every bag datagram header does not have inclined to one side estimation for the characteristic that mobile communication system signal receiving end and transmitting terminal both sides know altogether to the phase difference between the adjacent code element of the baseband signal after the clock recovery;

The phase estimation value that b, utilization obtain is carried out the phase place correction to each baseband signal code element;

C, with the reference symbols sn Given information as desired signal, the revised reference symbols sn baseband signal of phase place makes the LMS algorithmic statement as input signal;

D, the revised data symbols baseband signal of phase place as input signal, the less phase deviation that stays after utilizing the LMS algorithm restrained to the phase place correction compensates, initial phase deviation to modulation compensates simultaneously, at this moment, with the hard decision result of input signal as desired signal;

E, to the hard decision result of the LMS algorithm output signal final result of method by way of compensation, come the entrained information of decision signal.

2. the compensation method of MPSK mobile communication system Doppler frequency shift according to claim 1, when it is characterized in that among the step a phase difference estimated, utilize sample (k=1,2 ..., R) average (

) there is not an inclined to one side estimation population mean (Δ θ _Dop).

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