WO2014139554A1

WO2014139554A1 - Measuring device and measuring method for high resolution time synchronization in ofdm systems

Info

Publication number: WO2014139554A1
Application number: PCT/EP2013/054872
Authority: WO
Inventors: Stefan Schmidt
Original assignee: Rohde and Schwarz GmbH and Co KG
Current assignee: Rohde and Schwarz GmbH and Co KG
Priority date: 2013-03-11
Filing date: 2013-03-11
Publication date: 2014-09-18
Anticipated expiration: 2015-09-11
Also published as: CN105144648B; CN105144648A

Abstract

A measuring device for measuring a symbol start time in a first signal of a device under test comprises reception means for receiving the first signal and processing means for processing the first signal. The processing means comprise synchronization calculation means (32) for calculating the symbol start time comprising channel estimation means (40) for estimating channel response parameters, phase means (41) for calculating phase values from the channel response parameters, differentiating means (42) for performing a differentiation of the phase values, and calculation means (44) for calculating the symbol start time from the differentiated phase values.

Description

Measuring device and measuring method for high resolution time synchronization in OFDM systems

The invention relates to a measuring device and a

measuring method for accurately measuring a symbol start time in a signal of a device under test.

In regular OFDM receivers, time synchronization is usually performed by means of correlation of known pilot symbols and received pilot symbols. This method though requires a great deal of computing power and is limited by principle on an accuracy of no more than the distance between two sample points. For example, the international patent application WO

2008/116534 Al shows a method and system for determining a channel impulse response in an OFDM transmission system including determining the timing of the OFDM symbols with the system and method shown there only a relative low synchronization accuracy can be reached.

Accordingly, the object of the present invention is to create a measuring device and measuring method for

accurately measuring a symbol start time in a signal of a device under test.

The object is solved by the features of claim 1 for the measuring device and by the features of claim 9 for the measuring method. The dependent claims contain further developments.

An inventive measuring device for measuring a symbol start time in a first signal of a device under test comprises reception means for receiving the first signal, and processing means for processing the first signal. The processing means further comprise synchronization

calculation means for calculating the symbol start time comprising channel estimation means for estimating channel response parameters, phase means for calculating phase values from the channel response parameters,

differentiating means for performing a differentiation of the phase values, and calculation means for calculating the symbol start time from the differentiated phase values. It is therefore possible to accurately calculate the actual start time.

The processing means advantageously comprise

synchronization estimation means for determining an estimated symbol start time. The calculation means are then set up for calculating the symbol start time by adding a value derived from the differentiated phase values to the estimated symbol start time. By use of this two-step approach, a reduced computational complexity can be reached.

Advantageously, the phase means comprise phase calculation means for calculating phase values from the channel response parameters and phase unwrapping means for

performing a phase unwrapping of the phase values. This allows a simple and accurate calculation of the phase values .

The processing means advantageously further comprise averaging means for calculating a weighted average of the differentiated phase values. The calculation means are then set up for calculating the symbol start time by adding the weighted average to the estimated symbol start time. This allows an increase in accuracy. Advantageously, the averaging means comprise accumulation means for performing an accumulation of at least two differentiated phase values and normalization means for determining the weighted average by performing a

normalization of the accumulated differentiated phase values. A simple averaging is therefore reached.

The normalization means are advantageously set up for normalizing the accumulated differentiated phase values with a preset parameter. This allows an easy calculation of the start time.

Advantageously, the channel estimation means estimate the channel response parameters by using received pilot symbols within the first signal and known pilot symbols defined by an employed communication standard. An easy channel estimation is thereby reached. An inventive measuring method serves the purpose of measuring a symbol start time in a first signal of a device under test. The method comprises receiving the first signal and processing it. Moreover the method comprises calculating the symbol start time by estimating channel response parameters, calculating phase values from the channel response parameters, performing a

differentiation of the phase values, and calculating the symbol start time from the differentiated phase values. It is therefore possible to accurately calculate the actual start time.

An exemplary embodiment of the invention is now further explained with respect to the drawings as a non-limiting example only, in which Fig. 1 shows several OFDM symbols in a first time diagram;

Fig. 2 shows several OFDM symbols in a second time diagram;

Fig. 3 shows an OFDM symbol and a number of relevant parameters in a time diagram;

Fig. 4 shows an embodiment of the inventive measuring device in a block diagram;

Fig. 5 shows a first detail of the embodiment of the inventive measuring device in a block diagram;

Fig. 6 shows a second detail of the embodiment of the inventive measuring device in a block diagram, and

Fig. 7 shows an embodiment of the inventive measuring method .

First, we demonstrate along Fig. 1-3 the underlying problem and the general principle of the present

invention. Along Fig. 4-6, the construction and function of an embodiment of the inventive measuring device are shown. Finally, along Fig. 7, the function of an

embodiment of the inventive measuring method is shown.

Similar entities and reference numbers in different figures have been partially omitted. The present invention is set up for working with

communication standards using known pilot symbols, as for example OFDM. From an output of an OFDM modulator at a transmitter to the input of an OFDM demodulator at a receiver, we are dealing with I/Q samples in the time- domain while we are dealing with I/Q samples in the frequency-domain at other processing stages.

The relationship between I/Q samples in time-domain x (n) and I/Q samples in frequency-domain X(k) is given through the Discrete Fourier Transform (DFT) : Q-x$(rj*2* *k*nlN_DFT)

with symb

N DFT N s.ymb

where T_symb is the OFDM symbol length [sec], N_symb is the OFDM symbol number [number of samples] and T_s=l/f_s, where f_s is the sampling frequency [Hz] .

An exemplary synchronization approach in OFDM systems is the continuous evaluation of an appropriate correlation function m(t) . Here one exploits that the end of each OFDM symbol is repeated at its beginning as Cyclic Prefix (CP) before transmission. A possible correlation function could be

where x (n) , N_symb and N_w denote the sampled OFDM signal, the OFDM symbol length [number of samples] and correlation window length [number of samples] respectively.

In Fig. 1, this correlation function 16a is plotted versus an ideal OFDM signal for N_w = N_Cp , where N_Cp is the cyclic prefix length [number of samples] . Fig. 1 shows two OFDM symbols 11a, lib, each including an end portion 12a, 12b which is used as the cyclic prefix 10a, 10b at the

beginning of the respective OFDM symbol 11a, lib. One can observe that there are distinct correlation peaks 17a, 18a, 19a of a correlation function 16a at the symbol boundaries that allow for accurate synchronization.

However, there are cases where the transmitter weights the OFDM signal after cyclic prefix (CP) insertion with a window function before transmission. If the receiver now chooses the correlation window length N_w = N_CP this leads to a severe degradation of the correlation peak. Thus, a robust thresholding for correlation peak detection is not possible. In order to circumvent this problem, the

correlation window length has to be reduced N_w< N_CP. Thus, in turn one looses the distinct correlation peak and obtains a correlation plateau instead. This can easily be seen from Fig. 2, in which the changed correlation

function 16b and the correlation plateaus 17b, 18b and 19b are shown. Moreover the windowing function 9 is shown in Fig. 2.. The timing estimation accuracy is limited by the plateau length N_CP-N_W and we have I T_syncerror | < N_CP-N_W, where Tsyncerror is the synchronization error. Even if we assume that there are no channel effects and no Additional White Gaussian Noise (AWGN) the inequality | T_syncerror I <N_Cp-N_w cannot be tightened.

In order to increase synchronization accuracy, a timing estimation technique is proposed, which is efficient, i.e. I Tgyncerror I→0 for SNR → ∞ if we imply certain restrictions on the channel.

In case of a channel with a non-frequency selective phase, the digital baseband model of the channel is given as

N_DFT -l

H(k)= h(n)*exp(-j*2* π* k* n/ N_DFT) = a(k)*exp(j* <p) for

=0

k {0,...,N_DPT-\} (1) where a(k) and φ are real numbers. This is possible, if the transmission channel between a transmitter and a receiver produces no phase drift over frequency. This is for example achieved in laboratory setups with a short over-the-air channel or a cable channel.

In Fig. 3 an OFDM symbol 11 is shown. The OFDM symbol 11 comprises an end part 12, which is used as cyclic prefix 10 before the OFDM symbol. The OFDM symbol 11 has the length T_symb. The cyclic prefix 10 and the end part 12 of the OFDM symbol 11 each have the length T_Cp.

Moreover, in Fig. 3 an ideal receiver window 13 is shown. This indicates the ideal time interval, in which a

receiver attempts to receive the OFDM symbol 11. In this case, the receiver starts receiving the OFDM symbol 11 earlier by a duration T_readoffset than the actual start time tref of the OFDM symbol 11 to account for an inaccurate synchronization. T_{re a}dof f set is usually picked as one half of the duration T_Cp of the cyclic prefix 10. In reality though, a synchronization error T_syncerror occurs. This error changes the start time t_sync used by the receiver for the actual receiver window 14.

Fig. 4 shows an embodiment of the inventive measuring device 1. The inventive measuring device 1 comprises reception means 20, which comprise for example an antenna and a preamplifier. Moreover, the measuring device 1 comprises analog processing means 21, connected to an analog-digital-converter 22. Connected to the analog- digital-converter 22, the measuring device 1 comprises a digital processing means 23, which in turn is connected to displaying means 24. The analog processing means 21, the digital processing means 23 and the display means 24 are connected to furthermore comprised control means 25. The measuring device 1 is connected to a device under test 2, in this example a mobile telephone. The device under test 2 is connected to the reception means 20. In this example, the connection is performed using a cable.

Alternatively, an over-the-air connection is possible. The reception means 20 receives a signal, for example an OFDM signal from the device under test 2 and processes it. For example, a pre-amplification is performed. The resulting signal is passed on to the analog processing means 21, which perform further analog processing. For example, a filtering and a frequency reduction to the baseband or to an intermediate frequency are performed. A resulting analog signal is passed on to the analog-digital-converter 22, which converts it to a digital signal. This digital signal is passed on to the digital processing means 23, which performs further digital processing. For example, a synchronization, a demodulation and a decoding is

performed. As part of the digital processing means 23, the measuring of the symbol start time is performed.

Regarding the detailed function of the digital processing means 23, it is referred to Fig. 5 and Fig. 6. Results of the digital processing means 23 are passed on to the display means 24 and displayed. The function of the analog processing means 21, the digital processing means 23 and the display means 24 are controlled by the control means 25.

In Fig. 5, a detail of the embodiment of Fig. 4 is shown. Here only the digital processing means 23 are depicted.

The signal of the analog-digital-converter 22 of Fig. 4 is passed on to synchronization estimation means 30, which estimates t_ref, for example by the use of the before mentioned correlation. The estimate can be written as:

^ ref ^ ref ^syncerror where T_syncerror is the synchronization error. The signal and the synchronization estimation are passed on to processing means 31, which perform further digital processing, for example a demodulation and a decoding. The function of the processing means 31 though is of no concern to the present invention. Moreover, the digital processing means comprise synchronization calculation means 32, which are also supplied with the digital signal of the analog-digital- converter 22 of Fig. 4. Moreover, the synchronization calculation means 32 are provided with the synchronization estimation of the synchronization estimation means 30. The synchronization calculation means 32 calculate a precise symbol start time in the signal of the device under test 2 of Fig. 4 and pass on the exact synchronization

information to the processing means 31. Regarding the function of the synchronization calculation means 32, it is referred to Fig. 6.

Fig. 6 shows a further detail of the embodiment of the inventive measuring device 1 of Fig. 4. Here the

synchronization calculation means 32 of Fig. 5, which are comprised by the digital processing means 23 of Fig. 4, are shown in detail. The synchronization calculation means 32 comprise fourier transformation means 39, channel estimation means 40, which are connected to phase means 41. The phase means 41 comprise phase calculation means 41a, which are connected to the channel estimation means 40 and phase unwrapping means 41b which are connected to the phase calculation means 41a. Moreover, the

synchronization calculation means 32 comprise

differentiation means 42, which are connected to the phase unwrapping means 41b of the phase means 41. Moreover, the differentiation means 42 are connected to averaging means 43. The averaging means 43 comprise accumulation means 43a, which are connected to the differentiation means 42, and normalization means 43b, which are connected to the accumulation means 43a. Moreover, the synchronization calculation means 32 comprise calculation means 44, which are connected to the normalization means 43b of the averaging means 43. The time offset

^offset ^— T_reaci₀ffset T_Sy_ncerr0r is calculated as shown in the following. The fourier transformation means 39 are provided with the digitized signal of the analog-digital-converter 22 of Fig. 4. The fourier transformation means 39 perform a fourier

transformation, advantageously a fast fourier

transformation, in order to transform the signal into the frequency domain. The channel estimation means are

provided with the signal in the frequency domain and with the synchronization estimation of the synchronization estimation means 30 of Fig. 5. The channel estimation means 40 perform a channel estimation based on known pilot symbols and based on received pilot symbols within the digital signal. The pilot symbols are known from the employed communication standard. The channel estimation means 40 produces channel response parameters as a result. As described earlier, the channel response parameters comprise information, which can be used to calculate an exact synchronization. The channel estimation means 40 estimate the channel

H(k),ks {k₀,k₀ + \,..., k₀ + L - \) for L pilot subcarriers . For the sake of brevity, we assume here that the L pilots are distributed continuously over the subcarriers k. However, our proposed approach can easily be adapted to other distributions.

In an alternative embodiment, the synchronization

calculation means 32 do not comprise the fourier

transformation means 39 and the channel estimation means 40. In this case, the fourier transformation means 39 and the channel estimation means 40 are located within the processing means 31. In this case, the synchronization calculation means 32 receives an input signal for the phase means 41 from the processing means 31. This

embodiment is advantageous, since for decoding an OFDM signal, a channel estimation and a fourier transformation are needed anyway. In this alternative embodiment

calculation complexity can therefore be reduced.

By making use of the circular shift property of the DFT

X (n-m) <-> exp ^■ j * 2* π *^■ m X[k]

N where ( . ) N_DFT denotes the modulo N_DFT operator and we assume that x[n]<→X[k], and according to the equation (1) the output (k) of the channel estimator can be modeled as

H(k) = a(k)*Qxp + n(k) for

ks _? H^~~ 1_? ..._? ^~~h L where N_offset = Nsyncerror+ N_readoffset and Nsyncerror and N_readoffset are the synchronization error and the readoffset in multiples of T_s [number of samples] respectively, and n(k) is White Gaussian Noise (WGN) .

The channel response parameters are transmitted to the phase means 41, more precisely to the phase calculation means 41a. The phase calculation means 41a determine phase values .

y(k) -2*π*- N₀f_fiet + <P + n'(k)

N DFT These phase values contain the synchronization error as described earlier. These phase values are passed on to the phase unwrapping means 41b, which perform a phase

unwrapping. This means that a continuous phase is

calculated from the phase values, which can comprise phase jumps. The resulting unwrapped phase values are passed on to the differentiation means 42, which calculate

differentiated unwrapped phase values.

N _ff

yXk) = -2* *^^ + n"{k)

N_DFT for ks {k₀ + \,..., k₀ + L - \

In an alternative embodiment, the phase means 41 do not comprise phase unwrapping means 41b. In this case, the phase calculation means 41a are directly connected to the differentiation means 42. The differentiation means 42 are then connected to unwrapping means set up for removing the discontinuities that are removed by the phase unwrapping means 41b in the above described setup. These unwrapping means are then connected to the averaging means 43.

The differentiated unwrapped phase values are then passed on to the averaging means 43, which perform a weighted average of the differentiated unwrapped phase values. This is done by handing the differentiated unwrapped phase values to the accumulation means 43a, which accumulate at least two successive differentiated unwrapped phase values. A large number of differentiated unwrapped phase values can be accumulated by the accumulation means 43a.

^ DFT k=k₀ +l After finishing the accumulation 43a, the resulting sum the differentiated unwrapped phase values is passed on ^■ the normalization means 43b, which performs a

normalization using a predefined parameter, thus from y ' accumulated we obtain an estimate v

N J accumulated Γ DFT

i offset

2*7i* (L - l) 0 ^υ T ¹ offset = N ⁱ offset

The preset parameter used for normalization is

configurable and therefore can be changed during operation of the measuring method and measuring device.

The resulting value T_offset now corresponds to T_syncmor + T_readoffset .

It is passed on to the calculation means 44, which

calculates the symbol start t_ref time adding the weighted average _offset to the estimated symbol start time t_sync provided by the synchronization estimation means 30 of Fig. 5.

Finally we have: 5 t_ref — t_sync + T_offset

Alternatively, the averaging means 43 can be omitted. In this case, the differentiated unwrapped phase values resulting from the differentiation means 42 are directly0 passed on to the calculation means 44. In this case

though, the calculation means 44 has to perform a

normalization of the resulting value of the differentiation means 42 before calculating the symbol start time.

In Fig. 7, an exemplary embodiment of the inventive measuring method is shown. In a first step 100, a

synchronization estimation is performed. A correlation of known pilot symbols with received pilot symbols can be used. The result is an estimated symbol start time t_sync. Either as part of performing the synchronization

estimation or independent of it, moreover this step includes performing a fourier transformation of the signal into the frequency domain. In a second step 101, a channel estimation is performed. For example, known pilot symbols and the received pilot symbols are used. Resulting channel response parameters are used in a third step 102 to calculate phase values. These are subjected to a phase unwrapping in a fourth step 103. The unwrapped phase values are differentiated in a fifth step 104. The

resulting differentiated unwrapped phase values are accumulated in a sixth step 105. In this step, a

predefined number of unwrapped phase values is added. In a seventh step 106, a normalization of the accumulated differentiated unwrapped phase values is performed. This normalization is performed using a predefined

normalization parameter. In an eights step 107, the resulting value from the seventh step 106 is added to the synchronization estimation value resulting in the symbol start time. The invention is not limited to the examples and

especially not to any OFDM communication standard. Many different communication standards can be used. The

characteristics of the exemplary embodiments can be used in any advantageous combination of the features claimed in the claims and/or described in the description and/or drawn in the drawings .

Claims

1. Measuring device for measuring a symbol start time (t_ref) in a first signal of a device under test,

comprising:

- reception means (20) for receiving the first signal, and

- processing means (21, 22, 23, 25) for processing the first signal,

characterized in that

the processing means (21, 22, 23, 25) comprise

synchronization calculation means (32) for calculating the symbol start time (t_ref) comprising:

- channel estimation means (40) for estimating channel response parameters,

- phase means (41) for calculating phase values from the channel response parameters,

- differentiating means (42) for performing a

differentiation of the phase values, and

- calculation means (44) for calculating the symbol start time (t_ref) from the differentiated phase values.

2. Measuring device according to claim 1,

characterized in that

the processing means (21, 22, 23, 25) comprise

synchronization estimation means (30) for determining an estimated symbol start time (t_sync) , and

the calculation means (44) are set up for calculating the symbol start time (t_ref) by adding a value derived from the differentiated phase values to the estimated symbol start time (t_sync) ·

3. Measuring device according to claim 1 or 2,

characterized in that

that the phase means (41) comprise: - phase calculation means (41a) for calculating phase values from the channel response parameters, and

- phase unwrapping means (41b) for performing a phase unwrapping of the phase values.

4. Measuring device according to any of the claims 1 to 3, characterized in that

the processing means (21, 22, 23, 25) further comprise averaging means (43) for calculating a weighted average of the differentiated phase values, and

the calculation means (44) are set up for calculating the symbol start time (t_ref) by adding the weighted average to the estimated symbol start time (t_sync) .

5. Measuring device according to claim 4,

characterized in that

the averaging means (43) comprise:

- accumulation means (43a) for performing an accumulation of at least two differentiated phase values, and

- normalization means (43b) for determining the weighted average by performing a normalization of the accumulated differentiated phase values.

6. Measuring device according to claim 5,

characterized in that

the normalization means (43b) are set up for normalizing the accumulated differentiated phase values with a

configurable preset parameter.

7. Measuring device according to any of the claims 1 to 6, characterized in that

the channel estimation means (40) are set up for

estimating the channel response parameters by using received pilot symbols within the first signal and known pilot symbols, defined by an employed communication standard .

8. Measuring device according to any of the claims 1 to 7, characterized in that

the first signal is an OFDM signal using a cyclic prefix for each symbol.

9. Measuring method for measuring a symbol start time (t_ref) in a first signal of a device under test, comprising the following steps:

- receiving the first signal, and

- processing the first signal,

characterized in that

the method comprises calculating the symbol start time (t_ref) with the following steps:

- estimating channel response parameters,

- calculating phase values from the channel response parameters ,

- performing a differentiation of the phase values, and

- calculating the symbol start time (t_ref) from the

differentiated phase values.

10. Measuring method according to claim 9,

characterized in that

the method further comprises:

- determining an estimated symbol start time (t_sync) , and

- calculating the symbol start time (t_ref) by adding a value derived from the differentiated phase values to the estimated symbol start time (t_sync) .

11. Measuring method according to claim 9 or 10,

characterized in that

the method further comprises: - calculating phase values from the channel response parameters, and

- performing a phase unwrapping of the phase values.

12. Measuring method according to claim 11,

characterized in that

the method further comprises:

- calculating a weighted average of the differentiated unwrapped phase values, and

- calculating the symbol start time (t_ref) by adding the weighted average to the estimated symbol start time (t sync / ·

13. Measuring method according to claim 12,

characterized in that

the step of calculating the weighted average comprises:

- performing an accumulation of at least two

differentiated phase values, and

- determining the weighted average by performing a normalization of the accumulated differentiated phase values .

14. Measuring method according to claim 13,

characterized in that

the step of performing a normalization comprises normalizing the accumulated differentiated phase values with a configurable preset parameter.

15. Measuring method according to any of the claims 9 to 14,

characterized in that

the step of channel estimation comprises estimating channel response parameters by using received pilot symbols within the first signal and known pilot symbols, defined by an employed communication standard.

16. Measuring method according to any of the claims 9 to 15,

characterized in that

the first signal is an OFDM signal using a cyclic prefix for each symbol.