TECHNICAL FIELD

The invention relates to a measuring device and method for receiving communication signals, especially for receiving OFDMA communication signals which have been subjected to multi-path-propagation.

BACKGROUND

In mobile communication systems, the accurate and stable synchronization of timing and frequency is very important for the user equipment - UE . It is a precondition of successful data transmission and reception. The performance of a timing- and frequency-estimation significantly affects the achievable data throughput.

In case of orthogonal frequency-division multiplexing - OFDM based mobile communication system, e.g., long term evolution - LTE, a very high accuracy of the synchronization of timing and frequency is necessary to avoid inter symbol interference - ISI and inter carrier interference - ICI.

Usually, the synchronization has two stages: capturing and tracking. During the capturing stage, a coarse synchronization is performed. Before this stage, the UE doesn't have any synchronization information. This is the case for example just after power on. During the tracking stage, a fine synchronization for data transmission and reception is performed. The tracking stage is performed after the capture stage. In the tracking stage, the accurate and stable synchronization relies on a good estimation of timing and frequency offset, based on which the tracking loop can be adjusted.

In LTE, the existing technologies either estimate the timing and frequency offset separately based on known reference symbols - RS or estimate the timing offset and frequency offset jointly based on a cyclic prefix - CP. In general, RS based estimation has a better accuracy than CP based estimation. RS based estimation is used for fine estimation during the tracking stage. CP based estimation has a very low accuracy. It is mainly used for the coarse estimation during the capture stage.

The above-described methods of estimating frequency offset and timing offset though are not very accurate. This leads to a low reception quality. Moreover, they require a great deal of computational resources.

SUMMARY

Accordingly, an object of the invention is to provide an apparatus and method, which require only a low computational complexity and achieve a high reception quality.

The object is solved by the features of claim 1 for their apparatus and claim 16 for the method. Further it is solved by the features of claim 17 for the associated computer program. The dependent claims contain further developments.

According to a first aspect of the invention, a communication device for determining a timing offset and a frequency offset of a received signal, received through a communication channel, is provided. The communication device comprises a cross correlator, configured to perform a cross correlation of channel estimates of the communication channel, and thereby determine at least two cross correlation metrics. Moreover, the communication device comprises a joint estimator, configured to jointly estimate the timing offset and the frequency offset of the received signal based upon the determined at least two cross correlation metrics. It is thereby possible to achieve a low computational complexity high accuracy determination of the timing offset and the frequency offset.

According to a first implementation form of the first aspect, the communication device further comprises a channel estimator, configured to determine the channel estimates of the communication channel from the received signal. It is thereby possible to determine the timing offset and frequency offset starting from (only) the received signal.

According to a first implementation form of the first implementation form of the first aspect, the channel estimator is configured to determine the channel estimates of the communication channel based upon known pilot symbols within the received signal. It is thereby possible to very accurately and with a low computational complexity, determine the channel estimates .

According to a further implementation form of the first aspect or the previously described implementation forms, each cross correlation metric comprises a correlation value, a time distance value and a frequency distance value. The time distance value is a distance between two OFDM symbols used for the correlation. The frequency distance value is a distance between OFDM sub-carriers or subbands used for the correlation. The cross correlator is then configured to determine the cross correlation value based on the time distance, the frequency distance, and the channel estimates. The cross correlator is moreover configured to determine the at least two cross correlation metrics by determining the correlation value, the time distance value and the frequency distance value from the performed cross correlation. It is thereby possible to determine prerequisites of the joint estimator in a very accurate and low computational complexity manner.

According to a further implementation form of the first aspect or the previously described implementation forms, the cross correlator is configured to determine the cross correlation value according to

wherein

is the correlation value,

is the number of correlations, is a sub-carrier index set of available channel estimates in the I ^th OFDM symbol;

is a frequency distance,

is a time distance,

is an OFDM symbol index,

is a sub-carrier index, is a conjugated complex of the available channel estimate of a sub-carrier k in an OFDM symbol I, and the available channel estimate of a sub-carrier l+b _m in an OFDM symbol k+a _m .

It is thereby possible to determine the cross correlation with a very high accuracy and a very low computational complexity.

According to a further implementation form of the previously described implementation form, if the received signal is received by more than one reception antenna and/or if the received signal was transmitted using more than one transmit antenna, the cross correlator is configured to average the correlation metrics over the reception antennas and/or transmit antennas. A further increase in accuracy can thereby be achieved.

According to a further implementation form of the two previously described implementation forms, wherein if the received signal has undergone beam forming on a transmission side or on a reception side, the cross correlator is configured to correlate channel estimates, which have undergone an identical beam forming pre-coder. A further increase in accuracy can thereby be achieved.

According to a further implementation form of the first aspect or the previously described implementation forms of the first aspect, the joint estimator is configured to determine the timing offset and the frequency offset of the received signal by determining phases of all cross correlation metrics, determined by the cross correlator, formulating a phase vector comprising the phases of all correlation metrics, and jointly determining the timing offset and the frequency offset from the phase vector and at least a part of the correlation metrics. A very high accuracy of the estimation of the timing offset and the frequency offset can thereby be achieved while only incurring a low computational complexity . According to a first implementation form of the previous implementation form, the joint estimator is configured to determine the phases of all cross correlation metrics as follows: wherein

is a frequency distance,

is a time distance, and

is the correlation value.

is the operation of getting the phase of a complex value.

It is thereby possible to determine the phases with a very high accuracy and low computational complexity.

According to a further implementation form of the two previously described implementation forms, the joint estimator is configured to formulate the phase vector as follows:

wherein is the phase vector is the phase of a correlation metric at frequency distance a _m and time distance b _m , is a frequency distance, and

is a time distance. Thereby, it is possible to determine the phase vector with a very high accuracy and a very low computational complexity.

According to a further implementation form of the three previously described implementation forms, the joint estimator is configured to jointly determine the timing offset and the frequency offset as follows:

wherein

is a matrix of all correlation metrics,

is the transposed matrix L,

is a number of cross correlation metrics, is a frequency distance, is a time distance, is a vector of intermediate variables for time

offset and frequency offset vector,

is the intermediate variable of time offset,

is the intermediate variable of frequency offset, is the time offset,

is the frequency offset,

is a phase vector of phases of all correlation metrics. In other words the joint estimator is configured to determine jointly determine the timing offset and the frequency offset based on Least Square Estimation.

It is thereby possible to determine the timing offset and the frequency offset with a high accuracy and a low computational complexity.

According to a further implementation form of the first aspect or any of the previously described implementation forms, the joint estimator is configured to jointly estimate the timing offset and the frequency offset further based on a signal-to-noise-ratio of the communication channel. A further increase in accuracy can thereby be achieved.

According to a further implementation form of the first aspect or the previously described implementation forms, the communication device further comprises a weighting unit configured to determine a weight matrix .

wherein is the weight matrix, is a signal-to-noise-ratio related metric of either

is a frequency distance, is a time distance, is the phase of the correlation metric with

frequency distance a _m and time distance b _m , and is the correlation metric value with frequency distance a _m and time distance b _m . A further increase in accuracy can thereby be achieved.

According to a further implementation form of the previous implementation form, the joint estimator is configured to jointly determine the timing offset and the frequency offset as follows:

wherein

is a matrix of all correlation metrics,

is the transposed matrix L,

is the weight matrix, is a number of cross correlation metrics, is a frequency distance, is a time distance, is a vector of intermediate variables for time

offset and frequency offset,

is the intermediate variable of time offset,

is the intermediate variable of frequency offset, is the time offset,

is the frequency offset,

θ is a phase vector of phases of all correlation metrics .

In other words the joint estimator is configured to determine jointly determine the timing offset and the frequency offset based on Weighted Least Square Estimation. It is thereby possible to further increase the accuracy of the estimation.

According to a further implementation form of the first aspect or the previously described implementation forms, the communication device comprises a synchronizing unit, which is configured to perform a synchronization of the received signal based upon the frequency offset and the time offset determined by the joint estimator. It is thereby possible to very accurately decode the received signal in further processing .

According to a second aspect of the invention, a communication method for determining a timing offset and a frequency offset of a received signal, received through a communication channel, is provided. The communication method comprises performing a cross correlation of channel estimates of the communication channel, and thereby determining at least two cross correlation metrics, and jointly estimating the timing offset and frequency offset of the received signal based upon the determined at least two cross correlation metrics. An especially accurate and low computational complexity estimation can thereby be achieved.

According to a third aspect of the invention, a computer program with program code for performing the previously described method according to the second aspect, when the computer program runs on a computer is provided . Generally, it has to be noted that all arrangements, devices, elements, units and means and so forth described in the present application could be implemented by software or hardware elements or any kind of combination thereof. Furthermore, the devices may be processors or may comprise processors, wherein the functions of the elements, units and means described in the present applications may be implemented in one or more processors. All steps which are performed by the various entities described in the present application as well as the functionality described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if in the following description or specific embodiments, a specific functionality or step to be performed by a general entity is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respect of software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF DRAWINGS

The present invention is in the following explained in detail in relation to embodiments of the invention in reference to the enclosed drawings, in which

Fig. 1 shows an embodiment of the first aspect of the invention in a block diagram;

Fig. 2 shows a further embodiment of the first aspect of the invention in a block diagram;

Fig. 3 shows a time-frequency-diagram of a received signal; Fig. 4 shows results of a frequency offset estimation;

Fig. 5 shows results of a timing offset estimation, and

Fig. 6 shows an embodiment of the second aspect of the invention.

DESCRIPTION OF EMBODIMENTS

First we demonstrate the mathematical background of separate parameter estimation. After this, the construction and function of different embodiments of the first aspect of the invention along Fig. 1 - Fig. 2 are described. Along Fig. 3, further details of the function are described.

With the use of Fig. 4 and Fig. 5, achievable results by use of embodiments of the invention are shown. Finally, along Fig. 6, the function of a method according to the second aspect of the invention is described in detail.

Similar entities and reference numbers in different figures have been partially omitted.

As a background, RS based timing and frequency offset estimation is shown here. It has to be noted that the estimation shown here is a separate estimation of timing offset and frequency offset.

First, timing offset estimation is described:

Where, is the channel estimate of I ^th symbol and k ^th sub-carrier;

a is the number of resource elements - REs between two channel estimates for correlation in frequency direction; is the set of OFDM symbol index available for correlation in frequency direction ; K is the number of channel estimates per available OFDM symbol;

_N {corrF,a) is the number of correlations in frequency direction for averaging;

N _FFT is the FFT size used in OFDM system, and is the estimated timing offset with unit of sample duration in OFDM system.

Secondly, frequency offset estimation is described:

Where, b is the number of OFDM symbols between two channel estimates for correlation in time direction;

T is the duration of each OFDM symbol with unit of second; is the number of correlations in time direction for averaging, and is the estimated frequency offset with unit Hz.

The above-shown RS based solution needs to collect enough correlations to suppress the noise well. However, it is difficult to find enough correlations in case of a narrow-band system, e.g., LTE with 1.4MHz bandwidth - BW, which may lead to a bad synchronization performance. A CP based solution has a very low accuracy and is not suitable for synchronization tracking. When there is no data transmission, the CP of the OFDM symbol is dominated by noise which even further degrades the accuracy .

In Fig. 1, an embodiment of the first aspect of the invention is shown. A communication device 10 comprises a cross correlator 12 and a joint estimator 13. The cross correlator 12 is supplied with channel estimates of a received signal, which was transmitted through a communication channel. The cross correlator 12 performs a cross correlation resulting in a number of correlation metrics. These are handed to the joint estimator 13, which therefrom determines the frequency offset and the timing offset.

In Fig. 2, a more detailed embodiment of the first aspect of the invention is shown. Here, the communication device 10 additionally comprises a channel estimator 11, which is supplied with the received signal. The channel estimator 11 determines channel estimates from the received signal and hands them to the cross correlator 12. Moreover, the communication device 10 comprises a synchronizing unit 14, which is supplied with the frequency offset and the timing offset determined by the joint estimator 13. Additionally, it is supplied with the received signal. The synchronizing unit 14 performs a synchronization of the received signal using the frequency offset and the timing offset. Furthermore, the communication device 10 comprises a weighting unit 15, which is connected to the channel estimator 11 and the joint estimator 13. The weighting unit 15 determines a weight matrix, which is used by the joint estimator 13 to more accurately determine the frequency offset and timing offset. Details of the function of the individual units are given in the following.

For given channel estimates or channel estimates determined by the channel estimator 11 of Fig. 2, the communication device 10 is able to exhaust the useful information. This means that the entire information comprised within the channel estimates is actually used for optimizing the estimation of the timing offset and frequency offset.

The frequency offset and timing offset are estimated jointly in a combined way, which makes good use of all the information to improve the performance .

The cross correlation performed by the cross correlator 12 allows the correlation of two channel estimates with a certain distance in both time and frequency direction. It is possible to determine a correlation of any two channel estimates. The concept of cross correlation is further shown in Fig. 3.

A cross correlation can be identified by which is a pair of

lags, lag a defines the difference between two estimates for cross correlation in terms of sub-carriers for frequency direction and lag b defines the difference between two estimates for cross correlation in terms of OFDM symbols for time direction. Lags here are signed, which give the direction of the cross correlation. When the lags in both time- and frequency-direction are not 0, the phase of the correlation metric includes mixed information of timing- and frequency-offset. Assume there are M cross correlations identified by matrix L with size defines one lagX, i.e., one cross

correlation . the cross correlation is computed as follows:

wherein is the sub-carrier index set of available channel estimates in I ^th OFDM

symbol ;

Note that, can be averaged over different Tx antennas and different

Rx antennas to get a more accurate value. c^ ^am,bm ^ can be averaged or filtered over different SFs to get a more accurate value.

Note that, if beam forming is applied for RS (e.g., demodulation reference symbol - DMRS) , the two channel estimates for cross correlation should undergo the same pre-coder for beam forming.

All the can be vectorized as column vector, that is,

As mentioned before, the phase of each correlation c^ ^am,bm may include the information of both timing offset and frequency offset. That is,

The joint estimator 13 can jointly estimate and by minimizing a

cost function, e.g., based on Least Square Estimation - LSE or Weighted Least Square Estimation - WLSE.

where, w^ ^am,bm) is the weight of item m.

The joint estimation can be done by the following three steps:

1. Compute the phases of cross correlation metrics, that is,

For M cross correlation metrics, the phase vector can be formulated as,

Construct the weight matrix _shall relatively reflect the SNR of either

o

The weight matrix is constructed by the weighting unit 15. Channel fading caused by delay spread and Doppler spread could also be considered to calculate the weight.

The use of the weight matrix W can be skipped for reasons of simplification. Then, LSE (Equation 2-12) is be used in the third step instead of WLSE (Equation 2-9) .

3. Offset estimation using LSE or WLSE

Where, is the transpose operation of matrix.

Note that, if step 2 is skipped for reasons of simplification, Equation 2-9 shall be replaced by,

After the frequency offset and the timing offset have been determined, they are handed on to the synchronizing unit 14, which then synchronizes on the received signal. The timing offset and the frequency offset are used for performing this synchronization. This is a prerequisite for performing further processing on the received signal, for example for performing a decoding of the received signal.

It is important to note that only function blocks and functions of the communication device which are relevant for the present invention have been described. Further function blocks and functions relevant to the remaining reception of signals and also further functions of the communication device have been left out for reasons of conciseness.

A significant performance increase of joint estimation compared to separate estimation of timing offset and frequency offset can be achieved. This is shown in Fig. 4 and Fig. 5 from which it can be derived that for equal SNRs the standard deviation of the timing offset estimates (in T _s (1/(15000 x 2048) seconds), as defined in the LTE standard) and the frequency offset estimates (in Hz) is lower for joint estimation than for separate estimation. Especially, a performance gain in areas of low signal-to-noise-ratio is discernible from Fig. 4 and Fig. 5.

Parameters of the measurement producing in the results shown in Fig. 4 and Fig. 5 are shown in the following:

In Fig. 6, an embodiment of the communication method according to the second aspect of the invention is shown. In a first step 100, a signal is received. In a second step 101, channel estimates of a communication channel through which the signal was received are determined from the received signal. Especially, known reference symbols RS can be used for this determination. The steps 100 and 101 are optional steps.

In a third step 102, a cross correlation of the channel estimates is performed. Based on cross correlation metrics derived in the third step 102, in a fourth step 103, a joint estimation of timing offset and frequency offset is performed.

Regarding the details of the implementation of the individual steps, it is referred to the previous elaborations regarding the function of the communication device.

In the following, some acronyms are explained: