WO2012060751A1

WO2012060751A1 - A radio base station and a method therein for estimating a doppler spread

Info

Publication number: WO2012060751A1
Application number: PCT/SE2010/051203
Authority: WO
Inventors: Oskar Mauritz; Gunnar Peters
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2010-11-04
Filing date: 2010-11-04
Publication date: 2012-05-10
Also published as: CN103181094A; EP2636158A1; EP2636158A4; CN103181094B

Abstract

Embodiments herein relate to a method in a radio base station (12) for estimating Doppler spread of a signal transmitted by a user equipment (10) over a channel in a radio communications network. The radio base station (12) and user equipment (10) are comprised in the radio communications network. The radio base station (12) receives a first signal and a second signal from the user equipment (10), which first signal and second signal are based on the signal transmitted by the user equipment (10). The first and second signals are separated in space, time and/or polarisation. The radio base station (12) estimates a first channel estimate of the received first signal by comparing the received first signal to a known signal, and a second channel estimate of the received second signal by comparing the received second signal to the known signal. The radio base station (12) determines a ratio of the first channel estimate to the second channel estimate, and estimates an autocorrelation function of a function of the determined ratio. The radio base station (12) then estimates the Doppler spread based on the estimated autocorrelation function.

Description

A RADIO BASE STATION AND A METHOD THEREIN FOR ESTIMATING A DOPPLER SPREAD

TECHNICAL FIELD

Embodiments herein relate to a radio base station and a method therein. In particular, embodiments herein relate to estimation of Doppler spread in a radio communications network.

BACKGROUND

In radio communication networks of today, a user equipment communicates information over a radio link to a radio base station, in a so called uplink (UL)

transmission, and the radio base station communicates information to the user equipment in the other direction in a so called downlink (DL) transmission. A Doppler shift is a frequency shift experienced by a receiver of a radio signal when a transmitter and the receiver are moving relative to one another. A Doppler spread is a spread of Doppler shifts for a number of incoming signaling rays to the receiver from the transmitter. The Doppler spread causes variations in a propagation channel between the transmitter and the receiver. Knowledge of the Doppler spread has numerous applications in mobile communications systems. Such applications include receiver algorithms like smoothing filters for channel estimation and radio resource management algorithms such as link adaptation, scheduling, and selection of closed-loop or open-loop spatial multiplexing. The Doppler spread of a radio link is inversely proportional to a channel coherence time. The channel coherence time is the time duration over which a channel impulse response is considered to be correlated, that is, the channel impulse response is correlated to another channel impulse response made during the channel coherence time. A long channel coherence time allows the receiver, for example the radio base station, to follow effects of small scale fading for scheduling and link adaptation, e.g. for using closed-loop spatial multiplexing in downlink and time-dependent scheduling in mobile communications systems such as Evolved Universal Terrestrial Radio Access (E-UTRA) systems. A period for transmitting channel quality information reports from the user equipment to the radio base station should be shorter than the channel coherence time to provide downlink channel state information that is accurate during the report period. Similarly a periodicity of transmitted channel sounding reference signals in the uplink should be shorter than the channel coherence time to enable valid uplink channel quality estimation between the sounding reference signal transmissions.

The Doppler spread may be estimated in a number of ways using Doppler spread estimators. One class of Doppler spread estimators is based on the temporal properties of channel estimates of received signals. In time domain, a propagation channel may be modeled as a tapped delay line with time-dependent channel taps. Assuming that a channel tap C is a Rayleigh fading channel tap, the Doppler spread may be estimated from a measured level-crossing rate of the channel tap C . Several estimators rely on an autocorrelation function R_c of the channel tap C :

where

E is the expected value operator

c denotes complex conjugate,

t is the time, and

τ is the delay.

Theoretically, another autocorrelation function R_c of a Rayleigh fading channel tap is given by

where

C is a constant,

J₀ is the zero-order Bessel function of the first kind, and

f_D is the Doppler spread. From an estimate of the autocorrelation function and the known form of the autocorrelation function the Doppler spread may then be estimated e.g. from the ratio of the second derivative of R_C (T) , R"(t) , to R_C (T) at τ = 0 :

or from a maximum-likelihood estimate,

where f_D denotes the Doppler spread estimate.

The Doppler spread estimate f_D may also be obtained from the auto cova ance functions of functions of the channel taps. An example of such a function of the channel tap is the squared amplitude of the channel tap.

Channel estimates for a certain time instance are typically obtained from a transmission of a pilot or reference signal. Channel estimators are simplified and more efficient if there are continuous or periodic transmissions on fixed frequencies. The shorter the periodicity of the transmissions, the higher Doppler spreads, or shorter channel coherence times, may be estimated leading to a better resolution of the estimates.

An accurate channel estimate requires that the transmitted signal is known at the receiver. In general the transmitted reference or pilot signal is known except for amplitude and phase which are constant during a single transmission. In several mobile

communications systems, e.g. E-UTRA, the mobile may change the transmit power between subsequent transmissions. Such a change alters the amplitude and possibly also the phase of the transmitted signal. The radio base station estimates a composite channel that is composed of the transmitter gain, i.e. amplitude and phase, and a propagation channel. The composite channel is relevant for a compensation in the receiver but not relevant for estimating propagation of the transmission. The autocorrelation function as well as other temporal properties of the composite channel may be very different from those of the propagation channel and therefore Doppler spread estimates based on channel estimates may be erroneous. Generally, Doppler spread estimators encompass the temporal properties relating to time of the received signal on a receive antenna resulting in erroneous Doppler spread estimates leading to a transmission scheme of poor efficiency.

SUMMARY

An object of embodiments herein is to provide a mechanism for estimating a Doppler spread in a reliable manner leading to a better performance of a radio

communications network. According to an aspect of embodiments herein the object is achieved by a method in a radio base station for estimating Doppler spread of a signal transmitted by a user equipment over a channel in a radio communications network. The radio base station and user equipment are comprised in the radio communications network. The radio base station receives a first signal and a second signal from the user equipment, which first signal and second signal are based on the signal transmitted by the user equipment. Also, the first and second signals are separated in space, time and/or polarisation.

The radio base station estimates a first channel estimate of the received first signal by comparing the received first signal to a known signal, and a second channel estimate of the received second signal by comparing the received second signal to the known signal. The radio base station furthermore determines a ratio of the first channel estimate to the second channel estimate, and estimates an autocorrelation function of a function of the determined ratio. The radio base station then estimates the Doppler spread based on the estimated autocorrelation function. In order to perform the method a radio base station is provided for estimating

Doppler spread of a signal transmitted by a user equipment over a channel in a radio communications network. The radio base station is arranged to be comprised in the radio communications network. Furthermore, the radio base station comprises a receiving circuit configured to receive a first signal and a second signal from the user equipment. The first signal and the second signal are based on the signal transmitted by the user equipment, which first signal and second signal are separated in space, time and/or polarisation. In addition, the radio base station comprises a first estimating circuit configured to estimate a first channel estimate of the received first signal by comparing the received first signal to a known signal. Also, the radio base station comprises a second estimating circuit configured to estimate a second channel estimate of the received second signal by comparing the received second signal to the known signal.

The radio base station further comprises a determining circuit configured to determine a ratio of the first channel estimate to the second channel estimate.

Additionally, the radio base station comprises an estimating autocorrelation circuit configured to estimate an autocorrelation function of a function of the determined ratio. The radio base station also comprises an estimating Doppler spread circuit configured to estimate the Doppler spread based on the estimated autocorrelation function.

The estimate of the Doppler spread is based on the ratio of channel estimates of two separated received signals, which signals are based on the same transmitted reference signal. This is performed by using, for example, two receive antennas. Thereby, the changes of the amplitude and phase of the transmitted reference signal do not impact when estimating the Doppler spread. Thus, the Doppler spread estimate is more accurate leading to more efficient transmission schemes, link adaptations, scheduling etc., which result in an improved performance of the radio communications network. Also, higher throughput and data rate may be achieved in the radio communications network with a more accurate Doppler spread estimate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described in more detail in relation to the enclosed drawings, in which:

Fig. 1 is a block diagram depicting a radio communications network,

Fig. 2 is a schematic overview depicting a combined flow chart and signalling scheme in a radio communications network,

Fig. 3 is a block diagram depicting a radio base station,

Fig. 4 is a schematic overview of a graph depicting a curve of an autocorrelation function,

Fig. 5 is a block diagram of a method in a radio base station, and

Fig. 6 is a block diagram of a radio base station. DETAILED DESCRIPTION

Fig. 1 shows a schematic overview of a radio communications network, such as e.g. the Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA) system, Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention some possible options for implementing embodiments disclosed herein.

A User Equipment (UE) 10, served by a radio base station 12, is

communicating over a radio link with the radio base station 12, which communication is a so called uplink (UL) transmission. The radio base station 12 is communicating with the user equipment 10 in a so called downlink (DL) transmission. A Doppler shift is the frequency shift experienced by the radio base station 12 of a radio signal when the user equipment 10 and the radio base station 12 are moving relative to one another with a velocity v. That is, the radio base station 12 may be stationary or moving, and the user equipment 10 may be still or moving, and the difference in velocity between them is v in a direction.

The user equipment 10 transmits an original signal to the radio base station 12 in the UL. As the signal is propagated and reflected along the way to the radio base station 12, different signals, a first and a second signal, originating from the original signal will be received at the radio base station 12. Thus, the radio base station 12 receives the first signal and the second signal from the user equipment 10. The first and second signals are based on the original signal but are separated in, for example, space, time and/or polarisation. As transmission power of the user equipment 0 influences the amplitude and phase, varying transmission power of the user equipment 10 will change channel autocorrelation properties as obtained from a single received signal. Present

embodiments herein provide a mechanism that is not affected by the varying transmission power of the user equipment 10. The radio base station 12 estimates a first channel estimate of the first signal by comparing the received first signal to an initial signal also referred to as a known signal. Furthermore, the radio base station 12 estimates a second channel estimate of the second signal by comparing the received second signal to the initial signal. For example, a reference signal comprised in the received first signal and second signal may be compared to a known reference signal stored in the radio base station 12. The radio base station 12 then determines a ratio of the channel estimates by dividing the first channel estimate with the second channel estimate. The ratio is a dimensionless parameter cancelling out the transmit amplitude and phase dependency of the Doppler spread estimation. The ratio of the channel estimates is then used in a function to estimate an autocorrelation function. This estimated autocorrelation function is then used to estimate the Doppler spread.

The thus more accurate estimated Doppler spread may be used for frequency selective scheduling, link adaptation, modulation and coding for DL transmissions, Multiple Input Multiple Output (MIMO) mode etc.

Fig. 2 shows a schematic combined signaling and flow chart in a radio

communications network. The radio communications network comprises the radio base station 12 and the user equipment 10 moving relative to one another.

Step 201. The user equipment 10 transmits a signal s to the radio base station 12. The signal may comprise reference or pilot symbols known at the radio base station 12. The signal is separated along the way to the radio base station into a first signal s1 and a second signal s2. The first and second signals s1 ,s2 are thus originating from the same transmitted signal, the originating signal, and separated in space, polarisation and/or in time. Signals separated in space are distinguished by receive antennas with some distance between them. Signals separated in polarisation are distinguished by cross-polarised receive antennas. Signals separated in time are distinguished in a channel estimator. Also combinations thereof may be used to distinguish the first and second signal.

The radio base station 12 receives, in the illustrated example, the first signal s1 at a first antenna and the second signal s2 at a second antenna. Step 202. The radio base station 12 estimates the Doppler spread by first estimating a first channel estimate of the first signal and a second channel estimate of the second signal. Then, the radio base station 12 determines a ratio of the first channel estimate to the second channel estimate and estimates an autocorrelation function of a function of the determined ratio. The radio base station 12 then estimates the Doppler spread based on the estimated autocorrelation function. The radio base station 12 has by basing the estimation of the Doppler spread on a ratio of channel estimates of the first and second signals eliminated the transmitter influence. That is, the influence of the amplitude and phase of the transmitted signal on the estimated Doppler spread has been cancelled out.

In Fig. 3 a block diagram depicting a radio base station 12 in a radio

communications network is shown. In the illustrated example the radio base station 12 comprises a first antenna do and a second antenna aj. A baseband equivalent signal, Y_a , is a complex representation of a signal received over an antenna a. The baseband equivalent signal, ΐ°_α , of a received sampled signal for the antenna a due to the transmission of a reference signal S from the user equipment 10 is given by r_a {k) =∑h_a{ t)s(k - l) + n_a (k\ a = 0X ..., A - \ _{t (E}q. 1 )

where

k denotes the sample,

/ denotes a delay index,

t denotes the time,

A is the number of receive antennas,

h_a (l;t) is a composite channel coefficient,

C is the propagation channel, and

ft is noise and interference. is the baseband equivalent signal received over antenna a.Q and ^ (*) is the baseband equivalent signal received over antenna a;. , i.e. in this case a e {ao , aj}

The transmitted signal is a product of the reference signal S and a complex gain factor ¾ ^> with amplitude β and phase φ which is constant during the transmission of the reference signal, used for one channel estimate of the channel, but possibly different between subsequent transmissions.

It should be noted that the composite channel coefficient, h_a , includes the complex gain factor

t) = C_a ( ; i) β^β]ψ . The channel estimates are estimates of the composite channel because β^{β φ} is unknown in the receiver.

For any pair of composite channel coefficients, for example, composite channel coefficient h ( _x , ^' t) for antenna with a delay of l\ and composite channel coefficient h (/₀ ; t) for antenna a₀ and delay /₀, both composite channel coefficients are proportional to the complex gain factor ββ^1ψ of the transmission. An autocorrelation of a ratio of the two separate composite channel coefficients ^~ r will reflect coherence properties of the channel excluding the gain factor variations so that the estimate of the Doppler spread may be unaffected by changes of amplitude and phase of the transmitted reference signal. In particular according to some embodiments herein the radio base station 12 comprises at least two receive antennas, achieving separation in space and/or polarisation, thereby always providing at least two channel coefficients, one for each receive antenna. In the illustrated example, a first channel estimator 303 performs a channel estimation on the baseband equivalent signal Y_a( received over antenna <¾ . Thus, the first channel estimator 303 compares the baseband equivalent signal ^_ao(^) comprising a reference signal with a known reference signal stored in the radio base station 12. This results in a first channel estimate h_a0 (t) . A second channel estimator 305 performs a channel estimation on the baseband equivalent signal ^ (*) received over antenna aj. Thus, the second channel estimator 305 compares the baseband equivalent signal ^r _a\ $) comprising the reference signal with the known reference signal stored in the radio base station 12. This results in a second channel estimate h_al (t) In embodiments herein a normalized ratio g(t) of the composite channel coefficients is considered. The normalized ratio t) may be expressed as

A ratio estimate g(t) is calculated in a channel tap ratio calculator 307, also referred to as a determining circuit. The ratio estimate g(t) is the estimate of g(t) and is obtained from channel estimates h_a0(t) and h_al (t) according to

g(t) = exp{/ arg(¾ (/, ; t) I (l₀;t))} _(Eq. 3)

In some embodiments ₀≠ &_x , and ₀ , are the minimum values of / for the two antennas #₀ ^and ^βι ^{sucn tnat} ( - J 0≠ , = 0,1 .

An autocorrelation function of g{t) is denoted i^ (r) and is given by a normalized autocorrelation function p(x) and the Doppler spread f_D :

R_g (?) = p(f_D ^T) , (Eq- ⁴) where

τ is the delay, i.e. the time difference between two time instances to and t] in the argument of g(t) and hence the dimension of τ is time.

Assuming that h (l t) and _a^ (I₀ ,^' t) of the separated first and second signals are independent and identically distributed random variables, it follows from symmetry that R_g ty) is real. Furthermore, ^_g ( ) equals 1 by definition. In an autocorrelation function estimator 309 an estimate of the autocorrelation function of g(t) , denoted as R_g( ) , is first obtained from channel estimates of the signal. The autocorrelation function is estimated at evenly spaced delays, also known as sampled delays T = k · AT, k = 0, 1, ... , K— 1 from R_g (k - AT) = -^ '∑R_Q{g( AT) - g ((/ + *) · ΔΓ)} , (Eq. 5)

L— K 1=0 where

K is the number of sampled delays,

L is the number of estimated channel samples,

g is the ratio estimate of the normalized ratio g and is obtained from channel estimates according to (Eq. 3),

AT is a delay sampling interval,

/ is a delay index related to a reception time of the first and second signal,

g denotes the complex conjugate of the ratio estimate g , and

k indexes the delay sample.

Once the estimate of the autocorrelation function R_g(j) is obtained, the Doppier spread can be estimated in an analyzer 311. In one embodiment, the Doppier spread may be the maximum-likelihood estimate, i.e. the maximum-likelihood estimate is the Doppier spread that gives the largest probability of the estimate of the autocorrelation function.

A discrepancy function

between the estimated autocorrelation function R_g (k · AT) and a set of pre-calculated ideal autocorrelation functions R (k · AT; f_D ) :

k=0 where.

R(k - Ar,f_D) = p(k - AT - f_D)

The Doppler spread is estimated as the value of that minimizes the discrepancy function i^_{g D} ^■

Embodiments herein allow for robust Doppler spread estimation in systems where the amplitude and phase of the transmitted signal changes between transmissions due to variations in transmission power. By using a ratio of the channel estimates the complex gain factor ββ^]φ from the different channel estimates may be eliminated.

The normalized autocorrelation function p(x) mentioned above may be calculated numerically for uncorrelated Rayleigh fading channel coefficients and is plotted in Fig. 4. The values of the normalized autocorrelation function p(x) are defined along the y-axis and x is defined along the x-axis. Autocorrelation functions for different Doppler spread, scaled versions of p(x), may be compared to the estimated autocorrelation function and from the autocorrelation function that most resembles the estimated autocorrelation function the Doppler spread is estimated. That is, the Doppler spread with the

autocorrelation function that most resembles the estimated autocorrelation function is the estimated Doppler spread.

The method steps in the radio base station 12 for estimating Doppler spread of a signal transmitted by a user equipment 10 over a channel in a radio communications network according to some general embodiments will now be described with reference to a flowchart depicted in Fig. 5. The steps do not have to be taken in the order stated below, but may be taken in any suitable order. The radio base station 12 and user equipment 10 are comprised in the radio communications network.

Step 501. The radio base station 12 receives a first signal and a second signal from the user equipment 10. The first signal and second signal are based on the signal transmitted by the user equipment 0 and the first and second signals are separated in space, time and/or polarisation. In some embodiments, the first signal may be received at a first antenna and the second signal may be received at a second antenna. The first signal may be separated from the second signal by being received at a different point in space compared to the second signal, by being differently polarised than the second signal and/or by being delayed in time compared to the second signal.

Step 502. The radio base station 12 estimates a first channel estimate of the received first signal by comparing the received first signal to a known signal. Step 503. The radio base station 12 estimates a second channel estimate of the received second signal by comparing the received second signal to the known signal.

Step 504. The radio base station 12 determines a ratio of the first channel estimate to the second channel estimate. In some embodiments, the ratio may be a normalised ratio.

Step 505. The radio base station 12 estimates an autocorrelation function of a function of the determined ratio. Step 506. The radio base station 12 estimates the Doppler spread of the transmitted signal based on the estimated autocorrelation function. The Doppler spread may be estimated by comparing the estimated autocorrelation function to at least one pre- calculated autocorrelation function. Furthermore, in some embodiments the Doppler spread may be estimated by minimizing a discrepancy function, which discrepancy function may be calculated as a mean square error between the estimated autocorrelation function and the at least one pre-calculated autocorrelation function.

In order to perform the method a radio base station is provided. Fig. 6 is a block diagram depicting the radio base station 12 for estimating Doppler spread of a signal transmitted by the user equipment 10 over the channel in the radio communications network. The radio base station 12 is arranged to be comprised in the radio

communications network. The radio base station 12 comprises a receiving circuit 601 configured to receive a first signal and a second signal from the user equipment 10. The first signal and second signal are based on the signal transmitted by the user equipment 10 and the first and second signals are separated in space, time and/or polarisation. The receiving circuit 601 may comprise a first antenna <¾ arranged to receive the first signal and a second antenna a arranged to receive the second signal. In some embodiments, the first signal may be separated from the second signal by being received at a different point in space compared to the second signal and may be distinguished by antennas, <¾, and <2y, with a distance between them. The first signal may alternatively or additionally be separated from the second signal by being received differently polarised compared to the second signal and may be distinguished by cross-polarised antennas. Thus, the first antenna <¾, and the second antenna aj may be cross polarised. The first signal may alternatively or additionally be separated in time by being delayed in time compared to the second signal and these signals are then distinguished in a channel estimator. It should be understood that also any combination of separation is possible.

The radio base station 12 further comprises a first estimating circuit 602, corresponding to the first channel estimator 303 in Fig. 3, configured to estimate a first channel estimate of the received first signal by comparing the received first signal to a known signal. The radio base station 12 comprises a second estimating circuit 603, corresponding to the second channel estimator 305 in Fig. 3, configured to estimate a second channel estimate of the received second signal by comparing the received second signal to the known signal. Furthermore, the radio base station 12 comprises a determining circuit 604, corresponding to the channel tap ratio calculator 307 in Fig. 3, configured to determine a ratio of the first channel estimate to the second channel estimate. In some embodiments, the ratio may be a normalised ratio.

The radio base station 12 further comprises an estimating autocorrelation circuit 605, corresponding to the autocorrelation estimator 309 in Fig. 3, configured to estimate an autocorrelation function of a function of the determined ratio. Also, the radio base station 12 comprises an estimating Doppler spread circuit 606, corresponding to the analyser 31 1 in Fig. 3, configured to estimate the Doppler spread based on the estimated autocorrelation function. The estimating Doppler spread circuit 606 may in some embodiments be configured to estimate Doppler spread by comparing the estimated autocorrelation function to at least one pre-calculated autocorrelation function. Furthermore, the estimating Doppler spread circuit 606 may be configured to estimate Doppler spread by minimizing a discrepancy function, which discrepancy function is calculated as a mean square error between the estimated autocorrelation function and the at least one pre-calculated autocorrelation function. The present mechanism for estimating Doppler spread of a signal transmitted by a user equipment may be implemented through one or more processors, such as a processing circuit 607 in the radio base station 12 depicted in Fig. 6, together with computer program code for performing the functions of the embodiments herein. The program code mentioned above may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code for performing the present solution when being loaded into the radio base station 12. One such carrier may be in the form of a CD ROM disc. It is however feasible with other data carriers such as a memory stick. The computer program code may furthermore be provided as pure program code on a server and downloaded to the radio base station 12.

Furthermore, the radio base station 12 may comprise a memory circuit 608 arranged to be used to store estimates, data, applications to perform embodiments herein, or the like.

It should here be mentioned that the radio base station 12 also may be referred to as e.g. a NodeB, an evolved Node B (eNB, eNode B), a base transceiver station, Access Point Base Station, base station router, or any other network unit capable to communicate with a user equipment within a cell served by the radio base station 12, depending e.g. of the radio access technology and terminology used. The user equipment 10 may e.g. be represented by a wireless communication terminal, a mobile cellular phone, a Personal Digital Assistant (PDA), a wireless platform, a laptop, a computer or any other kind of device capable to communicate wirelessly with the radio base station 12. In the drawings and specification, there have been disclosed exemplary embodiments of the invention. However, many variations and modifications can be made to these embodiments without substantially departing from the principles of the present invention. Accordingly, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being defined by the following claims.

Claims

1 . A method in a radio base station (12) for estimating Doppler spread of a signal transmitted by a user equipment (10) over a channel in a radio communications network, which radio base station (12) and user equipment (10) are comprised in the radio communications network, the method comprising

- receiving (501 ) a first signal and a second signal from the user equipment (10), which first signal and second signal are based on the signal transmitted by the user equipment (10), wherein the first and second signals are separated in space, time and/or polarisation,

- estimating (502) a first channel estimate of the received first signal by comparing the received first signal to a known signal,

- estimating (503) a second channel estimate of the received second signal by comparing the received second signal to the known signal,

- determining (504) a ratio of the first channel estimate to the second channel estimate,

- estimating (505) an autocorrelation function of a function of the determined ratio, and

- estimating (506) the Doppler spread based on the estimated autocorrelation function.

2. A method according to claim 1 , wherein the first signal is received at a first

antenna (CIQ), and the second signal is received at a second antenna ( j).

3. A method according to any of claims 1 -2, wherein the first signal is separated from the second signal by being received at a different point in space compared to the second signal, by being differently polarised than the second signal and/or by being delayed in time compared to the second signal.

4. A method according to any of claims 1 -3, wherein the ratio is a normalised ratio.

5. A method according to any of claims 1 -4, wherein the Doppler spread is estimated by comparing the estimated autocorrelation function to at least one pre-calculated autocorrelation function.

6. A method according to claim 5 wherein the Doppler spread is estimated by minimizing a discrepancy function, which discrepancy function is calculated as a mean square error between the estimated autocorrelation function and the at least one pre-calculated autocorrelation function.

7. A radio base station (12) for estimating Doppler spread of a signal transmitted by a user equipment (10) over a channel in a radio communications network, which radio base station (12) is arranged to be comprised in the radio communications network, the radio base station (12) comprises;

a receiving circuit (601 ) configured to receive a first signal and a second signal from the user equipment (10), which first signal and second signal are based on the signal transmitted by the user equipment (10) and wherein the first and second signals are separated in space, time and/or polarisation,

a first estimating circuit (602,303) configured to estimate a first channel estimate of the received first signal by comparing the received first signal to a known signal,

a second estimating circuit (603,305) configured to estimate a second channel estimate of the received second signal by comparing the received second signal to the known signal,

a determining circuit (604,307) configured to determine a ratio of the first channel estimate to the second channel estimate,

an estimating autocorrelation circuit (605,309) configured to estimate an autocorrelation function of a function of the determined ratio, and an estimating Doppler spread circuit (606,31 1 ) configured to estimate the

Doppler spread based on the estimated autocorrelation function.

8. A radio base station (12) according to claim 7, wherein the receiving circuit

comprises a first antenna (<¾>) arranged to receive the first signal and a second antenna (aj) arranged to receive the second signal.

9. A radio base station (12) according to any of claims 7-8, wherein the first signal is separated from the second signal by being received at a different point in space compared to the second signal, by being differently polarised than the second signal and/or by being delayed in time compared to the second signal.

10. A radio base station (12) according to any of claims 7-9, wherein the ratio is a normalised ratio.

1 1. A radio base station (12) according to any of claims 7-10, wherein the estimating Doppler spread circuit (31 1 ,606) is configured to estimate Doppler spread by comparing the estimated autocorrelation function to at least one pre-calculated autocorrelation function.

12. A radio base station (12) according to any of claims 7-1 1 , wherein the estimating Doppler spread circuit (31 1 ,606) is configured to estimate Doppler spread by minimizing a discrepancy function, which discrepancy function is calculated as a mean square error between the estimated autocorrelation function and the at least one pre-calculated autocorrelation function.