WO2000057568A1

WO2000057568A1 - Doppler spread estimation system

Info

Publication number: WO2000057568A1
Application number: PCT/US2000/005129
Authority: WO
Inventors: Leonid Krasny; Sandeep Chennakeshu; R. David Koilpillai
Original assignee: Ericsson Inc.
Priority date: 1999-03-22
Filing date: 2000-02-29
Publication date: 2000-09-28
Also published as: JP2002540672A; TR200102697T2; CN1351781A; US6563861B1; JP4446606B2; AU3710500A; CN1169310C; EP1166455B1; EP1166455A1; ATE303681T1; DE60022330D1; MY124757A

Abstract

A receiving apparatus Doppler spread estimator includes a receiver receiving signals over a transmission channel that varies over time due to fading and converting the signals to digital samples for processing. A stationarizer operatively associated with the receiver stationarizes the digital samples. An estimator is operatively associated with the stationarizer for estimating spectral density of the stationarized digital samples. The correlator is operatively coupled to the estimator for correlating the spectral density estimates with functions corresponding to hypotheses of plural Doppler spread values to select one of the Doppler spread values as an estimate of actual Doppler spread.

Description

DOPPLER SPREAD ESTIMATION SYSTEM

FIELD OF THE INVENTION This invention relates to wireless communication systems and, more

particularly, to a method and apparatus for Doppler spread estimation in a wireless communication system.

BACKGROUND OF THE INVENTION

In wireless mobile communication systems the transmission channel can vary

with time. This is due to the mobility of the transmitter and/or the receiver. Variations in the

transmission channel are characterized by the Doppler spectrum. The Doppler spectrum is the Fourier transform of the autocorrelation function of the channel's time impulse response. The width of the Doppler spectrum is referred to as the Doppler spread in "Microwave Mobile

Communications," W. C. Jakes, Jr., ed., New York: Wiley, 1974, IEEE Press, 1994, and

"Wireless Communication," Theodore S. Rappaport, Prentice Hall PTR, 1996. The Doppler

spread is commonly used as a measure of the channel variation rate or fading rate.

It is often desirable to estimate the Doppler spread. For example, in order to adaptively optimize a coherent detector in a receiver, the parameters of the adaptation

algorithm are made to be a function of the estimated Doppler spread. Also, in a cellular mobile

telephone system, the handoff process can be enhanced if an estimate of the Doppler spread

is available. This avoids handoff of fast moving users to micro cells.

U.S. Patent Nos. 4,723,303 and 5,016,017 discuss the use of Doppler spread

estimation. Both patents describe estimating the Doppler spread from the received signal itself.

However, these methods may not give a good estimate with a non-stationary received signal. Lars Lindbom, "Adaptive Equalization for Fading Mobile Radio Channels," Techn.Lic.Thesis

No. UPTEC 92124R, November 1992, Department of Technology, Uppsala University,

Uppsala, Sweden, discloses a method of estimating the Doppler spread from a sequence of

channel estimates. This method uses differentials of the channel estimate, which comprise difference of values between two points in time. However, the differential itself is generally

very noisy and needs averaging. As a result, the average gives biased estimates of the Doppler

spread.

Still another proposal improves Doppler estimation by adaptive filters

controlled by the final Doppler spread estimate. However, this is done with a significant

increase of receiver complexity, while the estimate remains biased.

The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a receiver Doppler spread

estimator which utilizes stationarization of a non-stationary received signal.

Broadly, in accordance with one aspect of the invention, there is disclosed a

Doppler spread estimator including receive means for receiving signals over a transmission

channel that varies over time due to fading, and converting the signals to digital samples for

processing. First processing means are operatively associated with the receive means for stationarizing the digital samples. Second processing means are operatively associated with

the first processing means for estimating spectral densities of the stationarized digital samples.

Third processing means are operatively coupled to the second processing means for correlating the spectral density estimates with functions corresponding to hypotheses of plural Doppler

spread values to select one of the Doppler spread values as an estimate of actual Doppler

spread.

It is a feature of the invention that the receive means is adapted to receive code

division multiple access (CDMA) signals.

It is a further feature of the invention that channel coefficients are obtained

directly from the CDMA signals.

It is another feature of the invention that the receive means is adapted to receive signals including pilot symbols spread with a known CDMA spreading code. It is another feature of the invention that the received signal represents a

sequence of transmitted symbols and the first processing means comprises a mixer for mixing

the digital samples with a complex conjugate of the transmitted symbols. The complex

conjugate of the transmitted symbols is derived from a channel estimator.

It is another feature of the invention that the first processing means comprises a low pass filter.

It is another feature of the invention that the second processing means develops

vector data representing spectral components.

It is still another feature of the invention that the third processing means

comprises a multi-channel correlator. The correlator calculates correlations between estimated spectral densities and weighting functions corresponding to the hypotheses of the plural

Doppler spread values. The third processing means includes a selector for selecting the

hypothesis producing a highest correlation value. In accordance with another aspect of the invention, there is disclosed a receiving apparatus Doppler spread estimator including a receiver receiving signals over a transmission channel that varies over time due to fading and converting the signals to digital

samples for processing. A stationarizer operatively associated with the receiver stationarizes

the digital samples. An estimator is operatively associated with the stationarizer for estimating

spectral density of the stationarized digital samples. The correlator is operatively coupled to the estimator for correlating the spectral density estimates with functions corresponding to

hypotheses of plural Doppler spread values to select one of the Doppler spread values as an estimate of actual Doppler spread.

There is disclosed in accordance with a further aspect of the invention a method of estimating Doppler spread of a received signal comprising the steps of receiving the signals

over a transmission channel that varies over time due to fading, and converting the signals to

digital samples for processing; stationarizing the digital samples; estimating spectral density of the stationarized digital samples; and correlating the spectral density estimates with functions

corresponding to hypotheses of plural Doppler spread values to select one of the Doppler

spread values as an estimate of actual Doppler spread.

More particularly, the present invention relates to a method and apparatus for Doppler spread estimation in a wireless communication system. The Doppler spread

information can be used to optimize performance of a communication system. Applications

include both base and mobile station receivers where a coherent detector is the preferred detector type.

Further features and advantages of the invention will be readily apparent from the specification and from the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Figs. 1-3 are block diagrams of receiving apparatus including a Doppler spread

estimator according to the invention; and

Fig. 4 is a block diagram of the Doppler spread estimator according to the

invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Fig. 1, a receiving apparatus 10 according to the invention is

illustrated. The receiving apparatus 10 is adapted for use with known pilot symbols. The

receiving apparatus 10 includes an antenna 12 for receiving radio signals represented at 14. The antenna 12 is coupled to a radio receiver and converter 16. The radio receiver and

converter 16 filters amplified signals and converts them to a suitable form for processing, such

as complex numerical sample values. The signals are in the nature of digital samples for processing. The radio receiver and converter 16 is coupled to a channel estimator 18. The

channel estimator 18 is connected to a Doppler spread estimator 20 and receives known

symbols from a known symbol block 24. The known symbols comprise pilot or reference

symbols also included in the received signal used for determining channel estimates. The

known symbols are stored in a memory or generated locally in a code generator. The channel

estimator 18 correlates the received digital samples with the known symbols to provide channel

estimate to the Doppler spread estimator 20. The Doppler spread estimator 20 estimates the

Doppler spread which is sent to a signal processing block 22. The signal processing block 22

processes the sampled signal to extract information. The signal processing block 22 is also connected to the channel estimator 18 so that after Doppler estimation the channel estimation

can be improved.

Referring to Fig. 2, a block diagram of a receiving apparatus 10' is illustrated. The receiving apparatus 10' is generally similar to the receiving apparatus 10 of Fig. 1. For simplicity, like elements are referenced with like numerals, while modified elements are indicated with primed numerals. Particularly, the receiving apparatus 10' differs in replacing

the known symbols at block 24 with a symbol estimator 24'. This receiving apparatus is used

in an application where symbols are unknown. The channel estimator 18 attempts to estimate

the symbols, which are then stored in the symbol estimator 24'. The estimated symbols are subsequently used by the channel estimator 18 for determining channel estimates, as above.

In the United States code-division multiple access (CDMA) cellular system

known as IS-95, a transmitter transmits a stream of known symbols known as the pilot code.

The pilot code is transmitted on the same channel at the same time as other, information bearing, symbols using different spreading codes. Fig. 3 illustrates a receiving apparatus 10"

used in such a CDMA system. With CDMA systems the channel can be estimated directly to

be used for Doppler spread. In this case, the known symbol block 24 of Fig. 1 is omitted. The

channel estimator 18 correlates the received signal, comprising the pilot code and other codes

in additive superimposition, with the known pilot code, and low pass filters the resultant

complex correlation to obtain channel estimates. The received signal is also correlated with

other codes carrying information it is desired to decode. The results of correlating with

information carrying codes are multiplied by the conjugate of pilot code correlations for the

same delay, and the results added to coherently combine the multi-path signals. In wide band CDMA (WBCDMA) systems, modulation symbol intervals are

much shorter. This allows multiple propagation paths to be resolved with much finer time

resolution.

In mobile communication systems, the received signal in a flat Rayleigh channel

can be described by the following baseband model:

r(t) = s(t) ^■ c(t) + n(t),

where

s(t) = ∑ώpft - n t), n

d„ is an n-th transmitted symbol,/?^ is a transmitted pulse, c(t) is a complex Gaussian process with mean zero and envelope correlation function

σ_c is a variance of the process c(t), J₀(χ) is a Bessel function of the first kind and of order

zero,

ω_d = ω₀v/cg, ω₀ is a carrier frequency, v is a mobile's velocity, c₀ is a light speed, and n(t) is white noise with the spectral density g.

There are two conventional approaches for the basic Doppler spread estimation solution. These approaches are spectral analysis of the received signal or correlation analysis

of the received signal. With spectral analysis of the received signal:

where

r„ c„ and «, are the /-th samples of the processes r(t) , c(t), and n(t) , respectively. For this

algorithm an estimate of the Doppler spread can be expressed in terms of FFT coefficients:

where f_q is a sampling frequency.

With correlation analysis of the received signal:

An estimate of the Doppler spread can be expressed in terms of the correlation function

K(m) , calculated for some value m₀:

fi = K:^l[K (mo)]

2πmo

where K^τ) is a function which is the inverse of the function K_c(τ).

However, a vector r = (r_l5 ..., r_N) is a time-varying, non-stationary random

discrete process. Traditional spectral analysis does not work for non-stationary processes. To

solve this problem, the present invention takes into consideration that d_x - exp(jφ . Then, the algorithm

r, = (d) • / where d _l is an estimate of the symbol d, , transforms the non- stationary vector r to the

stationary vector f = (n,..., / ) .

Indeed, for non-dispersive channels di « dι . Besides, statistical characteristics

of the noises n, andήi = di - Yli are the same, because = 1 . Therefore,

Vι K Cι + lit

It is possible to improve the algorithm (9) by using the moving average:

In this case the errors Adn = d — dn are reduced and Equation (10) becomes more precise.

For CDMA systems, channel coefficients (more exactly, channel coefficients estimates C₍ ) are

directly obtained. In this case, channel coefficient estimates can be processed instead of r_t .

In accordance with the invention the non-stationary vector r is transformed to

reduce the problem to analysis of the stationary vector f . Thereafter, the invention uses the

optimal approach for estimating Doppler spread. This approach uses a criterion of the

maximum of likelihood ratio. The optimal estimation algorithm is obtained by using the following equation: <? Λ (r|/«.)

= o. df_d fi=β

where Λ(r /«. ) is the likelihood ratio which is formed based on the vector r .

Because the vector r has a Gaussian distribution, with the accuracy up to an

inessential constant, the following equation can be written for the log-likelihood ratio:

where G#(/ι. l /d) is a weight function which corresponds to some value of the Doppler

spread ,. The method of estimating Doppler spread in accordance with the invention utilizes stationarization of the received signal, spectral analysis of the stationary vector, and correlation of the spectral estimate. Particularly, for stationarization of the received signal:

I ιM+TVO-1 rt = — ∑ {dnfrn,

Spectral analysis of the stationary vector r = (n,..., f*N) using a fast Fourier

transform (FFT):

Correlation is done in the frequency domain of the spectral estimate S(k) with

L weight functions G/ (/c| c.) which correspond to L values of the Doppler spread ,:

where f_d = (//I), • ••, /_<£L)} is a vector of Doppler spread hypotheses. One of the Doppler spread hypothesis /?/) is selected if:

Fig. 4 illustrates a block diagram of the Doppler spread estimator 20 of Fig. 1.

Stationarization of the received signal is performed using a multiplier 26 and low pass filter 28. The multiplier 26 receives the sampled received signals r„ which are multiplied with the

complex conjugate of the transmitted symbols dn . The radio receiver and converter block

16, see Fig. 1, is the source of the received signal r_n. The source of the complex conjugate of the transmitted symbols can be the channel estimator 18 or the known symbols 24. The output of the multiplier 26 is passed through a low pass filter 28 which reduces statistical errors

Adn = dn — dn of the transmitted symbol estimation and out-of-band noise. The output of the filter 28 is coupled to an input of a second processing block

30. The block 30 estimates the spectral density of the incoming signal using the algorithm

of Equation (15). The output, represented in bold lines, indicates that the output data is in the

form of vector data which are the spectral components S = [<->(--),...,ι-?(./V72)] .

The output of the block 30 is fed to an input of a third processing function 32 in the form of a multi-channel correlator which produces the likelihood ratio metrics using

Equation (16). Each channel of the correlator calculates correlations in frequency domain

between the spectral estimate S(k) and the waiting function G-v(/ .) . The waiting function

m-th channel corresponds to some selective value f/m) of the Doppler

spread. Particularly, these comprise hypotheses of plural Doppler spread values. The likelihood ratio metrics are compared with each other using Equation (16). The outputs are

applied to a max function block 34. On the basis of comparisons between the likelihood ratio

metrics ofE Doppler spread hypotheses, the multi-channel correlator 32 generates the decision

that the Doppler spread is f/m) if the w-th channel has the maximum output value.

The illustrated method for estimating Doppler spread can be used in WCDMA

or IS-95 systems, since in these systems known symbols or pilot symbols are periodically inserted into the data stream at every slot.

As will be appreciated by one of ordinary skill in the art, the present invention

may be embodied as methods or devices. Accordingly, the present invention may take the

form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining hardware and software aspects. The present invention has been described in part with respect to the block diagram illustrations of Figs. 1-4. It will be understood that each

block of the illustrations, and combinations of blocks, can be implemented by computer

program instructions. These program instructions, which represent steps, may be provided to

a processor to produce a machine. Accordingly, blocks of the block diagrams support combinations of means for performing the specified functions in combinations of steps for performing the specified functions. It will be understood that each block of the illustrations, and combinations of

blocks, can be implemented by special purpose hardware-based systems which perform the

specified functions or steps, or combinations of special purpose hardware and computer

instructions.

Thus, in accordance with the invention, a Doppler spread estimator utilizes stationarization of non-stationary received signals and calculates correlation functions which

are compared to hypotheses to determine an accurate and reliable estimate of Doppler spread.

Claims

WE CLAIM:

1. A Doppler spread estimator comprising: receive means for receiving signals over a transmission channel that varies over

time due to fading and converting the signals to digital samples for processing;

first processing means operatively associated with the receive means for

estimating spectral densities of the digital samples; and second processing means operatively coupled to the first processing means for correlating the spectral density estimates with functions corresponding to hypotheses of plural

Doppler spread values to select one of said Doppler spread values as an estimate of actual Doppler spread.

2. The Doppler spread estimator of claim 1 wherein said receive means is

adapted to receive Code-Division Multiple Access (CDMA) signals.

3. The Doppler spread estimator of claim 2 wherein channel coefficients

are directly obtained from the CDMA signals.

4. The Doppler spread estimator of claim 1 wherein said receive means is adapted to receive signals including pilot symbols spread with a known CDMA spreading code.

5. The Doppler spread estimator of claim 1 further comprising third

processing means operatively associated with the receive means for stationarizing the digital samples prior to estimating spectral densities of the digital samples.

6. The Doppler spread estimator of claim 5 wherein said received signal

represents a sequence of transmitted symbols and said third processing means comprises a multiplier for mixing the digital samples with a complex conjugate of the transmitted symbols.

7. The Doppler spread estimator of claim 6 wherein said complex

conjugate of the transmitted symbols is derived from a symbol estimator.

8. The Doppler spread estimator of claim 5 wherein said third processing

means further comprises a low pass filter.

9. The Doppler spread estimator of claim 1 wherein said first processing

means develops vector data representing spectral components.

10. The Doppler spread estimator of claim 1 wherein said second processing means comprises a multichannel correlator.

11. The Doppler spread estimator of claim 10 wherein said correlator

calculates correlations between estimated spectral densities and weighting functions

corresponding to the hypotheses of the plural Doppler spread values.

12. The Doppler spread estimator of claim 11 wherein the second processing means includes a selector for selecting the hypothesis producing a highest

correlation value.

13. A receiving apparatus Doppler spread estimator comprising:

a receiver receiving signals over a transmission channel that varies over time due to fading and converting the signals to digital samples for processing; an estimator operatively associated with the stationarizer for estimating spectral

densities of the stationarized digital samples; and a correlator operatively coupled to the estimator for correlating the spectral

density estimates with functions corresponding to hypotheses of plural Doppler spread values

to select one of said Doppler spread values as an estimate of actual Doppler spread.

14. The receiving apparatus Doppler spread estimator of claim 13 wherein

said receiver is adapted to receive Code-Division Multiple Access (CDMA) signals.

15. The Doppler spread estimator of claim 14 wherein channel coefficients

are directly obtained from the CDMA signals.

16. The receiving apparatus Doppler spread estimator of claim 13 wherein

said receiver is adapted to receive signals including pilot symbols spread with a known CDMA

spreading code.

17. The Doppler spread estimator of claim 13 further comprising a stationarizer operatively associated with the receiver for stationarizing the digital samples prior

to estimating spectral densities of the digital samples.

18. The receiving apparatus Doppler spread estimator of claim 17 wherein

said received signal represents a sequence of transmitted symbols and said stationarizer

comprises a multiplier for mixing the digital samples with a complex conjugate of the

transmitted symbols.

19. The receiving apparatus Doppler spread estimator of claim 18 wherein

said complex conjugate of the transmitted symbols is derived from a symbol estimator.

20. The receiving apparatus Doppler spread estimator of claim 18 wherein

said stationarizer further comprises a low pass filter.

21. The receiving apparatus Doppler spread estimator of claim 13 wherein said estimator develops vector data representing spectral components.

22. The receiving apparatus Doppler spread estimator of claim 13 wherein

said correlator comprises a multichannel correlator.

23. The receiving apparatus Doppler spread estimator of claim 22 wherein

said correlator calculates correlations between estimated spectral densities and weighting

functions corresponding to the hypotheses of the plural Doppler spread values.

24. The receiving apparatus Doppler spread estimator of claim 23 wherein the correlator includes a selector for selecting the hypothesis producing a highest correlation

value.

25. The method of estimating Doppler spread of a received signal,

comprising the steps of: receiving the signals over a transmission channel that varies over time due to

fading and converting the signals to digital samples for processing; estimating spectral densities of the stationarized digital samples; and correlating the spectral density estimates with functions corresponding to

hypotheses of plural Doppler spread values to select one of said Doppler spread values as an

estimate of actual Doppler spread.

26. The method of claim 25 wherein said receiving step receives Code

Division Multiple Access signals.

27. The Doppler spread estimator of claim 26 wherein channel coefficients

are directly obtained from the CDMA signals.

28. The method of claim 25 wherein said receiving step receives signals including pilot symbols spread with a known CDMA spreading code.

29. The Doppler spread estimator of claim 25 further comprising third

30. The method of claim 25 wherein said received signal represents a

sequence of transmitted symbols and said stationarizing step comprises multiplying the digital

samples with a complex conjugate of the transmitted symbols.

31. The method of claim 30 wherein said complex conjugate of the

transmitted symbols is derived from a symbol estimator.

32. The method of claim 30 wherein said stationarizing step comprises low

pass filtering the mixed samples.

33. The method of claim 25 wherein said estimating step develops vector

data representing spectral components.

34. The method of claim 25 wherein said correlating step comprises using a multichannel correlator.

35. The method of claim 34 wherein said correlator calculates correlations

between estimated spectral densities and weighting functions corresponding to the hypotheses

of the plural Doppler spread values.

36. The method of claim 35 wherein the correlating step includes selecting

the hypothesis producing a highest correlation value.