WO2010054688A1

WO2010054688A1 - Methods, apparatuses, computer programs for time synchronization

Info

Publication number: WO2010054688A1
Application number: PCT/EP2008/065411
Authority: WO
Inventors: Natividade Albert Lobo
Original assignee: Nokia Corporation
Priority date: 2008-11-12
Filing date: 2008-11-12
Publication date: 2010-05-20

Abstract

A method comprising: determining a mathematical moment of time-varying signal power for a received signal; and using the mathematical moment to synchronize with the received signal. A method comprising: selecting a first putative code sequence; selecting a first portion of a received signal in dependence upon the first putative code sequence; performing a correlation on the selected first portion of the received signal using the first putative code sequence; selecting a second putative code sequence; selecting a second portion of the received signal in dependence upon the second putative code sequence, wherein the second portion is different to the first portion as a consequence of the first putative code being different to the second putative code; performing a correlation on the selected second portion of the received signal using the first putative code sequence.

Description

TITLE

Methods, apparatuses, computer programs for time synchronization.

FIELD OF THE INVENTION

Embodiments of the present invention relate to time synchronization. In particular, they relate to methods, apparatus and computer programs for enabling time synchronization

BACKGROUND TO THE INVENTION

The Ecma Standards ECMA-368 and ECMA-369 specify a MultiBand Orthogonal Frequency Division Modulation (MB-OFDM) scheme to transmit information. The MB- OFDM scheme satisfies the requirements of ultra wideband (UWB) physical layer (PHY) for a wireless personal area network (PAN),

Ultra Wideband (UWB) provides for short range, high speed data communications such as, for example, file transfer, high-resolution video streaming etc between apparatuses (e.g. desktop peripherals, personal devices such as, for example, mobile cellular telephones)

MB-OFDM uses the unlicensed 3 100 - 10 600 MHz frequency band dividing it into 14 bands, each with a bandwidth of 528 MHz. Time-frequency codes (TFCs) support up to ten channels in each band. A total of 122 sub-carriers (including 100 data carriers) are used per band to transmit the information.

A Physical Layer Convergence Protocol Protocol Data Unit (PPDU) is illustrated in Figure 6 of ECMA-368. The PPDU or frame is composed of three components (in order of transmission): a preamble, a header, and a data unit.

The preamble comprises a packet/frame synchronization sequence and a channel estimation sequence. The preamble provides for synchronization.

A unique preamble sequence is assigned to each time-frequency code (TFC). There are either 24 or 12 symbols in the packet/frame synchronisation sequence. The symbol has a length, over which an inverse fast Fourier transform is performed in a transmitter apparatus and over which a fast Fourier transform is performed in the receiver apparatus, of 242.42 ns (128 samples). A zero-padded suffix has a length of 70.08ns (37 samples). Consequently, a slot has a length of 312.5 ns (165 samples).

The header conveys necessary information to aid decoding of the data unit at a receiver. The header has a field identifying the TFC of the transmitted data.

Synchronization is the process whereby a receiver establishes a time frame for the timing of received data. The receiver is then able to identify components in the received data and where they begin and/or end.

As described in "A Low-Complexity Synchronization Design for MB-OFDM Ultra- Wideband Systems' by Zhenzhen et al, ICC 08 IEEE International Conference, p3807- 3813, the start of a symbol may be identified using a significant correlation.

However, there are circumstances when there is no line of sight or no single strong path between transmitter apparatus and receiver apparatus. The transmitted energy may be spread out over a large number of multipath rays that are received at the receiver apparatus at different times.

BRIEF DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

According to a first set of embodiments of the invention there is provided a method comprising: determining a mathematical moment of time-varying signal power for a received signal; and using the mathematical moment to synchronize with the received signal.

According to the first set of embodiments of the invention there is provided a computer program which when executed by a processor enables the processor to: determine a mathematical moment in a reference time frame of signal power for a received signal; and use the mathematical moment to shift the reference time frame.

According to a first set of embodiments of the invention there is provided an apparatus comprising: a processor configured to determine a mathematical moment in a reference time frame of signal power for a received signal and to use the mathematical moment to synchronize with the received signal.

According to a first set of embodiments of the invention there is provided an apparatus comprising: means for determining a mathematical moment in a reference time frame of signal power for a received signal; and means for using the mathematical moment to shift the reference time frame.

According to an example in a second set of embodiments of the invention there is provided a method: comprising: determining a characteristic for multiple data samples; calculating a weighted sum of the multiple determined characteristics, where a weight applied to the determined characteristic of a sample is a value of a defined time- dependent function for the timing of the sample; processing the weighted sum to obtain a time value; and using the time value to synchronize with the data samples.

According to this example in the second set of embodiments of the invention there may be provided one or more of the following features in any appropriate combination: wherein the characteristic is a measureable quantitative parameter inherent in each data sample and the distribution in time of the parameter value among the data samples is code dependent. wherein a weight is applied to a parameter for a sample, the weight being a function of the time of the sample in a reference time frame. wherein a weight is applied to a parameter for a sample, the weight the time of the sample in a reference time frame. wherein the processing of the weighted sum comprises re-normalising using the unweighted sum of the multiple determined characteristics. wherein using the time value to synchronize with the data samples comprises shifting a reference time frame by an offset calculated as a difference between the time value and a reference value. wherein using the reference value is an ideal time value wherein the reference value is code dependent and the offset is code dependent, further comprising, after synchronizing, selecting a portion of the data samples wherein a timing of an initial sample in the portion of data samples is code dependent, further comprising folding the portion of the data samples. further comprising correlating each of a plurality of codes against the portion of the data samples to identify the code associated with the data samples, wherein the reference value is code independent and the offset is code independent, further comprising, after synchronizing, selecting a portion of the data samples wherein a timing of an initial sample in the portion of data samples is code independent. further comprising converting the selected portion of the data samples from the time domain to the frequency domain.

A computer program which when executed by a processor enables the processor to perform any one of these methods

According to an example in the second set of embodiments of the invention there is provided a computer program which when executed by a processor enables the processor to: determine a weight for each one of a plurality of data samples; determine a parameter for each on of the plurality of data samples; calculate a weighted sum comprising the summation, over the plurality of data samples, of the product of the determined weight for a sample and the determined parameter for that sample; and process the weighted sum to obtain a time value for synchronization.

According to this example in the second set of embodiments of the invention there may be provided one or more of the following features in any appropriate combination: wherein a distribution in time of the parameter among the data samples is dependent upon which one of multiple codes was used in the creation of the data samples, wherein the weight for a sample is determined as a function of the timing of the sample in a reference time frame. wherein the weight for a sample is the timing of the sample in a reference time frame, wherein the processing of the weighted sum comprises re-normalising using an unweighted sum of the multiple determined parameters, the computer program when executed by a processor, further enables synchronization by shifting a reference time frame by an offset calculated as a difference between the time value and a reference value.

According to an example in the second set of embodiments of the invention there is provided an apparatus comprising: a processor configured to determine a weight for each one of a plurality of data samples; configured to determine a parameter for each on of the plurality of data samples; configured to calculate a weighted sum comprising the summation, over the plurality of data samples, of the product of the determined weight for a sample and the determined parameter for that sample; and configured to process the weighted sum to obtain a time value for synchronization.

According to this example in the second set of embodiments of the invention there may be provided one or more of the following features in any appropriate combination: wherein a distribution in time of the parameter among the data samples is dependent upon which one of multiple codes was used in the creation of the data samples. wherein the weight for a sample is determined as a function of the timing of the sample in a reference time frame, wherein the weight for a sample is the timing of the sample in a reference time frame.

According to an example in the second set of embodiments of the invention there is provided an apparatus comprising: means for determining a characteristic for multiple data samples; means for calculating a weighted sum of the multiple determined characteristics, where a weight applied to the determined characteristic of a sample is a value of a defined time-dependent function for the timing of the sample; means for processing the weighted sum to obtain a time value; and means for using the time value to synchronize with the data samples.

According to an example in a third set of embodiments of the invention there is provided a method comprising: measuring, at different times in a reference time frame over a time period, parameter values for a received signal; calculating, using the measured parameter values, a statistical value that varies when a distribution of parameter values varies over the time period; and shifting the reference time frame by a difference between the calculated value and a reference value to synchronize with the received signal.

According to this example in the third set of embodiments of the invention there may be provided one or more of the following features in any appropriate combination: wherein the reference value is an expected statistical value; wherein the reference value is code dependent. According to an example in a third set of embodiments of the invention there is provided a method comprising: selecting a first putative code sequence; selecting a first portion of a received signal in dependence upon the first putative code sequence; performing a correlation on the selected first portion of the received signal using the first putative code sequence; selecting a second putative code sequence; selecting a second portion of the received signal in dependence upon the second putative code sequence, wherein the second portion is different to the first portion as a consequence of the first putative code being different to the second putative code; and performing a correlation on the selected second portion of the received signal using the first putative code sequence.

According to this example in the third set of embodiments of the invention there may be provided one or more of the following features in any appropriate combination: further comprising: identifying one of the putative code sequences as the most probable code sequence based upon the results of the correlations, wherein selecting a portion of a received signal comprises: determining a characteristic for multiple data samples of the received signal; calculating a weighted sum of the multiple determined characteristics, where a weight applied to the determined characteristic of a sample is a value of a defined time- dependent function for the timing of the sample in a reference time frame; processing the weighted sum to obtain a time value; using a difference between a code-dependent reference value and the obtained time value to identify within the reference time frame a portion of the received signal for selection. wherein selecting a portion of a received signal comprises: determining a first mathematical moment in a reference time frame of signal power for the received signal; and using a difference between a the first mathematical moment and a code-dependent reference value to identify within the reference time frame a portion of the received signal for selection.

A computer program which when executed by a processor enables the processor to perform these methods.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of various examples of embodiments of the present invention reference will now be made by way of example only to the accompanying drawings in which: Fig. 1 schematically illustrates a system comprising a transmitter apparatus and a receiver apparatus;

Fig. 2 schematically illustrates a method that is performed by the receiver apparatus to shift a reference time frame;

Fig 3A schematically illustrates in detail a method for calculating a time value used to shift the reference time frame;

Fig 3B schematically illustrates the shifting of the reference time frame;

Fig 4 schematically illustrates a method for identifying the code used by the transmitter apparatus for the received data samples processed in method of Fig 2;

Fig 5A schematically illustrates a method for time to frequency domain conversion of the received data samples processed in the method of Fig 2;

Fig 5B schematically illustrates a method for folding a received signal; and

Figures 6A and 6B illustrate different embodiments of processors

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

Fig 1 schematically illustrates a system 2 comprising a transmitter apparatus 4 and a receiver apparatus 6. The transmitter apparatus 4 transmits wirelessly a signal 8 which is received by the receiver apparatus 6.

The signal 8 received by the receiver apparatus 6 may comprise a plurality of individual multipath rays 8A, 8B that have taken different routes to the receiver apparatus 6. For example, in the illustrated example there is a ray 8A that is received directly from the transmitter apparatus 4 and there is a ray 8B that is reflected by surface 12 before reception by the receiver apparatus 4. It will be appreciated that as the ray 8B has a longer path to the receiver apparatus 6 it will appear to be delayed relative to the ray 8A.

The transmitter apparatus 4 may apply one of a plurality of different codes to the signal 8 before transmission. Each different code may impart a different quality of a code- dependent characteristic to the signal 8. For example, the system 2 may use a Multiband Orthogonal Frequency Division Modulation (MB-OFDM) scheme, such as that specified by the Ecma Standards ECMA- 368 and ECMA-369, to transmit information via the signal 8 between the transmitter apparatus 4 and the receiver apparatus 6.

The signal 8 may, for example, correspond to a coded synchronization sequence such as the packet/frame synchronization sequence of the Physical Layer Convergence Protocol Data Unit (PPDU) of WCMA-368 A unique synchronization sequence is assigned to each time-frequency code (TFC). As described in s 10.2 of ECMA-368, the contents of which are hereby incorporated by reference, the TFC is used to select the appropriate time-domain sequence (Tables 4 through 10 of ECMA-368) and appropriate cover sequence (Table 21). Each symbol of the synchronization sequence is created by multiply the appropriate value of the cover sequence with each of the symbols of the extended time-domain sequence. The time-domain sequence, which is TFC dependent, therefore imparts a characteristic to the synchronization sequence that is TFC dependent. The synchronization sequence therefore has a code-dependent characteristic that is measurable as a quantitative parameter that varies sequence to sequence as different codes are used.

Fig. 2 schematically illustrates a method 20 that is performed by the receiver apparatus 6 to shift, as illustrated in Fig 3B, a reference time frame 31.

At block 22, the receiver apparatus 6 selects a reference time frame 31 (Fig 3B). This may occur, for example, by selecting an arbitrary reference point between the end of one slot and the start of the next slot in a synchronization sequence. As an example, the end of a slot may be detected by a sudden drop in energy which corresponds to the zero padding sequence between symbols.

At block 22, the receiver apparatus 6 calculates a time offset value 34 and then at block 26 the time value is used to shift or offset the reference time frame 31 to achieve synchronization as a synchronized time frame 35.

The time offset value 34 may be a code-dependent value that is separately determined for each possible code or may be a constant value for each code. When the time offset value 34 is code-dependent, each code will have a different synchronized time frame 35.

Fig 3 schematically illustrates a method 40 for calculating a time value 32. The method is suitable for use in block 24 of Fig 2.

At block 42, the receiver apparatus 6 determines a characteristic value for each of multiple received data samples.

The characteristic value relates to a measureable quantitative parameter inherent in each data sample. The characteristic is preferably a parameter that is code-dependent and therefore has a different value depending upon the code used to generate the data. The distribution of the parameter value among the received data samples may be code dependent and a mathematical moment (in the time domain) of the parameter value may be used to discriminate between the different code-dependent distributions of the parameter.

For example, in ECMA-368 the time-domain sequence, which is TFC dependent, imparts a characteristic to the synchronization sequence that is TFC dependent. As an example, the distribution of signal power in a synchronization sequence is TFC dependent. The signal power is a measureable quantitative parameter for each sample of a symbol in the synchronization sequence and a mathematical moment (in the time domain) of the signal power may be used to discriminate between the different distributions of signal power for the different TFCs.

At block 44, the receiver apparatus 6 calculates a weighted sum of the multiple determined characteristics.

The weighted sum is the summation of the parameter values for each sample, over the samples for the symbol. A weighting is applied to each parameter value in the sum. The weighting applied to a parameter value of a particular sample is a value of a defined time-dependent function at the timing of the particular sample. In the example of ECMA-368, the defined time-dependent function may be a linear function in time such that the weighting applied to a parameter value of a particular sample is proportional to the timing of the particular sample.

At block 46, the receiver apparatus 6 processes the weighted summation to obtain a measured time result 32 with respect to the reference time frame 31.

In the example where the defined time-dependent function is a linear function in time, the result of the summation is re-normalized. The result of the weighted summation is divided by a re-normalising value. This obtains a measured time result by converting the dimensionality of the weighted sum from [parameterpme] to [time].

In one embodiment, the re-normalising value is calculated using the parameter values determined at block 42. The re-normalizing value is the un-weighted sum of the parameter values for each sample, over the samples for the symbol.

At block 48, the receiver apparatus 6 uses the measured time result 32 to calculate an offset time 34 for adjusting the reference time frame 31 to create the synchronized time frame 35. The offset 34 is calculated as the difference between the measured time result 32 and a reference time result 33.

The reference time result 33 is calculated by performing block 42 with respect to expected data samples rather than received data samples and then proceeding to block 44 and 45 of Fig 3A. The time value produced by block 46 is the reference time result 33. As the parameter is code-dependent, the results of blocks 42, 44 and 46 will vary with code. A different reference time result 33 may therefore be calculated for each code. A set of reference time results 33 may therefore be produced where there is a reference time result for each code. For the calculation of the reference time result 33, it may be that although the distribution of a parameter across a symbol is code-dependent that the total value of the parameter for the symbol is code independent. In this circumstance, the re-normalizing value would be a constant for all codes. The reference time result 33 or set of reference time results 33 are pre-determined as they relate to expected ideal data rather than real data. It is therefore possible to pre-calculate these values and store them in memory, so that they are accessed by performing a look-up at block 48 of Fig 3A. In one embodiment, it is assumed that the set of reference time results 33 has a reasonably narrow distribution and a representative reference time results 34 is then used by block 48 for all codes. This produces a representative offset 34. The representative reference time 33 may be a reference time 33 for a particular code or an average (mean or median) of the reference times 33 for all codes.

In another embodiment, at block 48, a set of offsets 34 may be produced where there is an offset 34 for each code. The set of offsets 34 is calculated as the difference between the measured time result 32 and the set of reference time results 34. In this scenario, there may be a different synchronized time frame for each of the codes. Therefore different down-stream processes such as correlation (Fig 4), and/or time to frequency domain conversion (Fig 5A) and/or data processing (Fig 5B) may occur at different times for different codes and therefore occur on different data portions.

Fig 4 schematically illustrates a method 50 for identifying the code used by the transmitter apparatus 4 for the received data samples processed in method 20 of Fig 2.

At block 51 , a code is selected.

At block 52, a portion of the received data samples is selected for correlation. As an example, the set of offsets 34 may be accessed and the offset for the selected code determined. This offset gives the timing in the reference time frame 31 of the expected start of the synchronisation sequence for the selected code. The portion of the data samples selected for correlation in MB-OFDM has a length of 128 samples (NFFT) and a start dependent upon the code dependent offset 34. A reduced number of correlations may be required because time synchronisation has already been achieved.

At block 53, the selected code is correlated against the selected portion of the data samples and a correlation result is produced and stored.

At block 54, it is checked whether there are additional codes that have not yet been correlated. If this is the case, the method restarts at block 51 with the selection of a new code. If, however, correlations for all the codes have been calculated, then the method moves to block 55, where the code with the highest correlation score is identified. This identifies the code used by the transmitter apparatus 4 for the received data samples processed in method 20 of Fig 2. It may also allow for some fine scale adjustment to the synchronized time frame 35.

Fig 5A schematically illustrates a method 56 for time to frequency domain conversion of the received data samples processed in method 20 of Fig 2. A transform, in this case a fast Fourier transform (FFT), is performed on the received data samples that occur in the transform period after the start of the newly created synchronized time frame 35. In the example of ECMA-368, the transform period is 242.42 ns (165 samples).

If a representative offset 34 is used to create the synchronized time frame 35, then the time to frequency domain conversion can occur immediately. That is there is no requirement to identify the code in the time-domain (e.g. using correlation) before the time to frequency domain conversion. If necessary, the code identification may occur later by reading fields in subsequently received data.

It is of course also possible for the time to frequency domain conversion to occur after correlation.

Fig 5B schematically illustrates a method 58 for folding a received signal that should include zero padding. It may be desirable to fold a received signal to replicate the effect of a cyclic prefix in the receiver apparatus 6. The start of the newly created synchronized time frame 35 is point A and the point a transform period after the start of the newly created synchronized time is point B. The n data samples that occur before A are added to the n data samples that occur before B. The n data samples that occur after B are added to the n data samples that occur after A. This process may occur before correlation to improve efficiency.

Figures 6A and 6B illustrate processors that implement the logic and routines that enables the receiver apparatus 6 to perform the methods illustrated in the Figs.

The logic and routines may be implemented using a programmable processor and stored instructions such as software or firmware as illustrated in Fig 6A. Alternatively , the logic and routines may be implemented using a dedicated processor comprising dedicated circuits as illustrated in Fig 6. The processor may be provided as a module or parts of the processor may be provided as a module, where 'module' refers to a unit or apparatus that excludes certain parts/components that would be added by an end manufacturer or a user.

Referring to Fig 6A, a processor 64 is configured to read from and write to a memory 62. The processor 64 and the memory 62 are operationally coupled and any number or combination of intervening elements can exist (including no intervening elements)

The processor 64 may also comprise an output interface via which data and/or commands are output by the processor and an input interface via which data and/or commands are input to the processor.

The memory 62 stores a computer program 61 comprising computer program instructions that control the operation of the processor 64 when loaded into and executed by the processor 64. The computer program instructions provide the logic and routines that enables the processor 64 to perform the methods illustrated in the Figs. The processor 64 by reading the memory 62 is able to load and execute the computer program 61.

Although the memory 62 is illustrated as a single component it may be implemented as one or more separate components some or all of which may be integrated/removable and/or may provide permanent/semi-permanent/ dynamic/cached storage.

Referring to Fig 6B, a processor 70 is configured as a collection of discrete components 72. The components are operationally coupled and any number or combination of intervening elements can exist (including no intervening elements)

Typically the components will include one or more components for performing arithmetic and/or logical functions and one or more components for storing data.

References to 'computer-readable storage medium', 'computer program product', 'tangibly embodied computer program' etc. or 'computer', 'processor' etc. should be understood to encompass not only computers having different architectures such as single /multi- processor architectures and sequential (e.g. Von Neumann)/parallel architectures but also specialized circuits such as field-programmable gate arrays (FPGA), application specific circuits (ASIC), signal processing devices and other devices. References to computer program, instructions, code etc. should be understood to encompass software for a programmable processor or ffirmware such as, for example, the programmable content of a hardware device whether instructions for a processor, or configuration settings for a fixed-function device, gate array or programmable logic device etc.

The computer program 61 may arrive at the apparatus 6 via any suitable delivery mechanism 66. The delivery mechanism 66 may be, for example, a computer-readable storage medium, a computer program product, a memory device, a record medium such as a CD-ROM or DVD, an article of manufacture that tangibly embodies the computer program 61. The delivery mechanism may be a signal configured to reliably transfer the computer program 61.

The apparatus 6 may propagate or transmit the computer program 61 as a computer data signal.

Particular examples of the blocks 42, 44, 46 and 48 of Fig 3A will now be described Although these examples relate to MB-OFDM as defined by ECMA-368 it should be realised from the foregoing that some embodiments of the invention have broader application that this technology. That is these implementation details described particular embodiments of the invention and not all embodiments of the invention necessarily have these features.

First Embodiment

The first embodiment calculates the first moment of the signal power.

Calculating the measured time value 32

Definitions:

V_k ≡ V(t_k) Equation 1

t_k ≡ k.Δt Equation 2 V[t] be the baseband received symbol

t_k is the timing of each sample as measured in the reference time frame 31.

k is the sample number k=0, 1 , 2... N

N= Ω *NSYM , where Ω is the oversampling rate and NSYM is the number of samples per symbol, 165 for MB-OFDM

Δt is the sample period and is equal to 1/f ₈ where f _s is the sampling frequency (528 MHz for MB-OFDM)

At blocks 44 and 46, calculate the first mathematical moment

where P is the symbol power

represents the complex conjugate

Calculating the reference time value 33

Definitions:

Vj[t'] is the first baseband symbol for the packet/frame synchronization sequence based upon TFC j and t' is measured from the beginning of the symbol. For MB-OFDM V_j[t'] has a value for the FFT period (128 samples) and is zero valued for the remaining zero padding period (37 samples)

k is the sample number k=0, 1 , 2... N where N= NSYM

NSYM is the number of samples per symbol, 165 for MB-OFDM

Δt is the sample period and is equal to 1/f _s where f _s is the sampling frequency (528 MHz for MB-OFDM)

Calculate the expected first mathematical moment

where P_j is the symbol power assuming TFC number j

where * represents the complex conjugate

Calculating the offset 34 (first example) a) use

to obtain η

b) Shift the arbitrary reference point by ηto obtain synchronisation

Calculating the offset 34 (alternative example) a) use

to obtain <η >

where

(M is 10 for MB-OFDM)

b) Shift the arbitrary reference point by <η_>to obtain synchronisation

In the preceding implementation it is assumed that *^■ j j The attached annex verifies this assumption.

Second Embodiment

The second embodiment is a generalization of the first embodiment. The integral or summation over time of the product of a characteristic x_k and a function of time f(t) is calculated. When x_k is signal power and f(t) is time, this embodiment is the same as the first embodiment.

The characteristic is a measureable quantitative parameter x_k inherent in each data sample.

The integral represents a statistical value that represents how parameter values x_k are distributed over the time period of the integral. As the distribution of parameter values x_k varies over the time period, the statistical value varies. The distribution may be code dependent and hence the statistical value may be code-dependent.

The function f(t) may, for example, be any suitable function of t. It may, for example be a polynomial in t, or an exponentiation having time as a base raised to a power n where n is a non-zero number (complex, real, rational or natural) or another function that varies in a desired manner with time. It may be desirable, for example, for f(t) to be a function that decreases in time so that a greater weighting is applied to early portions of received signals. It will be recognised by those skilled in the art that when f(t) is an exponentiation having time as a base raised to a power n where n is a natural number greater than zero i.e. f(t)= t^Λn., that the calculated integral or sum represents the nth mathematical moment.

The characteristic x_k may, for example, may be chosen so that it is larger for samples occurring earlier in time compared to samples occurring later in time and/or chosen so that the effect of narrowband interferers is reduced. The characteristic x_k may, for example, be any suitable function of signal power P. It may, for example be a polynomial in P, or an exponentiation having power P as a base raised to a power m where m is a non-zero number (complex, real, rational or natural) or another function that varies in a desired manner with power.

When the characteristic x_k is an exponentiation having power as a base raised to a power m i.e. P^Λm, then a greater contribution may be obtained from stronger signals by having m>1 and a lesser contribution from stronger signal may be achieved by having m<1. A greater contribution from early signals may be obtained by defining m as a function of time that decreases with increasing time. The effect of narrowband interferers may be reduced by defining m as a function of power P that decreases with P.

Thus components f(t) and/or x_k may be chosen in dependence upon a channel model. For example, for short range signals it may be expected in some circumstances that total signal power may be spread across many multipath rays. In this scenario, x_k may, for example, be defined as P^Λm where m<1 so that there is a lesser contribution from stronger signals. For example, for long range signals it may be expected in some circumstances that total signal power will be in no more than a few high power rays. In this scenario, x_k may, for example, be defined as P^Λm where m>1 so that the effect of noise is minimised.

Calculating the measured time value 32

V[t] is the baseband received signal t is measured from an arbitrary reference point

x_k is a determined characteristic of a sample k of the baseband received signal

Calculate a weighted sum S, over the samples of the signal, of the multiple determined characteristics x_k, where a weight w_k applied to the determined characteristic x_k of a sample k is a value of a defined time-dependent function f(t) for the timing of the k^th sample t_k i.e. w_k = f(t_k) Equation 15

k is the sample number k=0, 1 , 2... Ν where N= Ω *NSYM Ω is the oversampling rate NSYM is the number of samples per symbol, 165 for MB-OFDM

Processing the weighted sum to obtain a time value:

where F[f(t)] =t

X is the summation of x_k over the samples of the symbol

In the first embodiment described above, the statistical value S/X is the first mathematical moment: VftJ is the baseband received symbol

f(t) = t= k.Δt

F(t)= t

In another embodiment, the statistical value S/X could be the second mathematical moment: x_k = V(t_k). V(t_k)^* V[t] is the baseband received symbol f(t) = t² = (k.Δt) ² F(t)= t^1/2 (square root)

Calculating the reference time value 33

V_j[t'] is an expected signal or one of a plurality of expected signals j

Xιg is a characteristic of a sample k of the expected signal j

Calculate a weighted sum S, over the samples of the expected signal, of the multiple determined characteristics X_k, where a weight w_k applied to the determined characteristic x_k of a sample k is a value of a defined time-dependent function f(t) for the timing of the k^th sample t_k i.e. w_k = f(t_k)

k is the sample number k=0, 1. 2...N where N= NFFT

NFFT is the number of samples per FFT period, 128 for MB-OFDM

Processing the weighted sum to obtain a time value

where F[f(t)] =t

X_j is the summation of x_k over the samples of an expected signal

In the first embodiment described above, the statistical value S/X is the first mathematical moment:

Vj [t] is the first baseband symbol for the packet/frame synchronization sequence based upon TFC j and t' is measured from the beginning of the symbol. f(t) = t= k.Δt F(t)= t

Calculated time offset 34

As described in relation to the first embodiment.

Dealing with Noise

If the values of S (Equation 15) and X (Equation 17) were calculated in the absence of an information signal, then they represent respective noise values S_nOj_Se and X_noise .

If the values of S (Equation 15) and X (Equation 17) were then calculated in the presence of an information signal (and noise), then they represent respective 'dirty information'

Value

Assuming that the noise is statistically invariant, then the values of Sj_nfo and Xj_nf0. for a 'pure' information signal without noise can be determined, where S_inf₀ = S_nOise + info -

The values of S_infOand X_inf0 may then be used to calculate T (Equation 16)

The blocks illustrated in the Figs may represent steps in a method and/or sections of code in the computer program 61. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some steps to be omitted.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

Features described in the preceding description may be used in combinations other than the combinations explicitly described.

Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not.

Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

Annex

In the derivation below we show that the process of calculating the center of gravity of the received signal from the arbitary reference point yields terms that are approximately the weighted mean delays of each path plus the center of gravity of the synchronisation signal. In the table below we present some definitions of variables used in the discussion. The variable names are similar to the parameters defined in the MB-OFDM USB specification.

Variable Description Value

MB - OFDM UWB fs Sampling Frequency 528MHz

Ts Sampling period — 528 s

NFFT Total Number of subcarriers (FFT size) 128

NZPS Number of Samples in zero - padded suffix 37

NSYM Total Number of Samples per Symbol 165

TSYM Symbol interval 312.5 ns

TFFT IFFT and FFT period 242.42 ns

TZPS Zero- Padded Duration in time 70.08 ns Δf Subcarrier frequency spacing 4.125MHz

Let ⁽LJ [t] represent the unit step function, equal to 0 for t < 0 and 1 for t > 0. Consider the synchronisation pulse Us_ync[t]. The baseband signal is

The receiver function, Vs_ync[t], is the sum of several delayed rays of gain oc\ . Let the i^th ray have a gain of α\ and a delay of ^i. Then

Substituting for Sync[t]

Fsyn_c3B[fl is sampled at intervals of -^- where o_sanp represents the oversamp ling rate.

^Jsamp

LeUT =-^

Os amp

Simplifying we get Define

Define Signal Power by

where the * represents the complex conjugate. We calculate Center of Gravity of signal by

Then

Substituting equation 1 in equation 5

Using the distributive law we get Using

We can find integer Jc₁ such that

Let

We assume that from the ith path the contribution only begins at the k, instant and ends after (NFFT O_samp- 1 + k, ) We therefore make the following transformation for k

Θ[i] = Min[ o_samp NSYM-I, 0_Samp NFFT - 1 + ki] k → ki + k and sum between 0 and Θ [i]

< k_Dur δT > Σ =

Simplifying

[( i j j) ])) )

Conditions satisfied by Ik, when the expression in the summation is non zero

There are at o_samp values of Ik for each value of r that make a contribition to the summation. Therefore we do the summation for k first Simplying Let

We get

We define two cases where n = O and where n != O

Then we have

Considering the case < kour ^T >_n÷o first, because of the correlation properties of

This is zero because there does not exist a single value of k where the supports of the Pulse

overlap.

Next considering the term < ^our δT >_n=o

There are precisely o_saiφ values of k where the supports of the Pulse functions

overlap. This happens for each value of r, and for s = r

Summing over the non - zero values of k, we get

This is T, measured time value 32 and its component parts are given by the equations below.

is T_j , reference time 33, the distance of the center of gravity of the synchronising signal from the start of the signal, and can be approximated by rcog, the distance 33 of the center of gravity of the synchronising signal from the start of the synchronisation TFC signal. τcog can be precalculated and stored for each TFC.

Thus, the second term simplifies to

represents a kind of weighted mean of all the delays in the paths and is denoted by 34 in Fig 3B.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

I/we claim:

Claims

1. A method comprising: determining a mathematical moment of time-varying signal power for a received signal; and using the mathematical moment to synchronize with the received signal.

2. A method as claimed in claim 1 , wherein the mathematical moment is a first mathematical moment.

3. A method as claimed in claim 2, wherein determining the first mathematical moment comprises calculating a weighted sum of signal power for samples of the received signal, where a weight applied to the signal power of a sample is the timing of that sample in a reference time frame.

4. A method as claimed in any one of claims 1 to 3, wherein determining the mathematical moment further comprises re-normalising the calculated weighted sum.

5. A method as claimed in claim 4, wherein renormalizing comprises dividing the weighted sum by an unweighted sum of signal power for samples of the received signal.

6. A method as claimed in any one of claims 1 to 5, wherein using the mathematical moment to synchronize with the received signal comprises shifting a reference time frame by an offset calculated as a difference between the first mathematical moment and a reference value.

7. A method as claimed in claim 6, wherein the reference value is an ideal mathematical moment.

8. A method as claimed in claim 6 or 7, wherein the reference value is code dependent and the offset is code dependent.

9. A method as claimed in any one of claims 6 to 9, further comprising selecting a portion of the received signal wherein a timing of a start to the portion is code dependent and positioned the offset value from an origin of the reference time frame.

10. A method as claimed in claim 9, further comprising folding the portion of the received signal.

11. A method as claimed in claim 9 or 10, further comprising correlating each of a plurality of codes against the portion of the received signal to identify a code used in generating the received data signal.

12. A method as claimed in claim 6 or 7, wherein the reference value is code independent and the offset is code independent.

13. A method as claimed in any one of claims 6 to 9, further comprising, after synchronizing, selecting a portion of the received signal wherein a timing of a start to the portion of the received signal is code independent

14. A method as claimed in claim 13, further comprising converting the selected portion of the received signal from the time domain to the frequency domain.

15. A computer program which when executed by a processor enables the processor to perform any one of methods 1 to 14.

16. A computer program which when executed by a processor enables the processor to: determine a mathematical moment in a reference time frame of signal power for a received signal; and use the mathematical moment to shift the reference time frame.

17, A computer program as claimed in claim 16, wherein the mathematical moment is a first mathematical moment.

18. A computer program as claimed in claim 16 or 17, wherein determining the first mathematical moment comprises calculating a weighted sum of signal power for samples of the received signal, where a weight applied to the signal power of a sample is the timing of that sample in the reference time frame.

19. A computer program as claimed in claim 16, 17 or 18, wherein determining the mathematical moment further comprises re-normalising the calculated weighted sum.

20. A computer program as claimed in any one of claims 16 to 19, wherein using the mathematical moment to shift the reference time frame comprises shifting the reference time frame by an offset calculated as a difference between the first mathematical moment and a reference value.

21. A computer program as claimed in any one of claims 16 to 20 wherein the reference value is code dependent and the offset is code dependent.

22. A computer program as claimed in any one of claims 16 to 21, enabling the processor to select a portion of the received signal wherein a timing of a start to the portion is code dependent and positioned the offset value from an origin of the reference time frame.

23. A computer readable medium tangibly embodying a computer program as claimed in any one of claims 16 to 22.

24. An apparatus comprising: a processor configured to determine a mathematical moment in a reference time frame of signal power for a received signal and to use the mathematical moment to synchronize with the received signal.

25. An apparatus as claimed in claim 24, wherein the mathematical moment is a first mathematical moment.

26. An apparatus as claimed in claim 24 or 25, wherein determining the first mathematical moment comprises calculating a weighted sum of signal power for samples of the received signal, where a weight applied to the signal power of a sample is the timing of that sample in the reference time frame.

27. An apparatus as claimed in claim 24, 26 or 26, wherein determining the mathematical moment further comprises re-normalising the calculated weighted sum.

28. An apparatus as claimed in any one of claims 24 to 27, wherein using the mathematical moment to shift the reference time frame, comprises shifting the reference time frame by an offset calculated as a difference between the first mathematical moment and a reference value.

29. An apparatus as claimed in any one of claim 28 wherein the reference value is code dependent and the offset is code dependent.

30. An apparatus as claimed in any one of claims 28 or 29, enabling the processor to select a portion of the received signal wherein a timing of a start to the portion is code dependent and positioned the offset value from an origin of the reference time frame.

31. An apparatus comprising: means for determining a mathematical moment in a reference time frame of signal power for a received signal; and means for using the mathematical moment to shift the reference time frame.

32. A method comprising: determining a characteristic for multiple data samples; calculating a weighted sum of the multiple determined characteristics, where a weight applied to the determined characteristic of a sample is a value of a defined time- dependent function for the timing of the sample; processing the weighted sum to obtain a time value; and using the time value to synchronize with the data samples.

33. A method as claimed in claim 32, wherein the characteristic is a measureable quantitative parameter inherent in each data sample and the distribution in time of the parameter value among the data samples is code dependent.

34. A method as claimed in claim 33, wherein a weight is applied to a parameter for a sample, the weight being a function of the time of the sample in a reference time frame.

35. A method as claimed in claim 33, wherein a weight is applied to a parameter for a sample, the weight the time of the sample in a reference time frame.

36. A method as claimed in any one of claims 32 to 35, wherein the processing of the weighted sum comprises re-normalising using the unweighted sum of the multiple determined characteristics.

37. A method as claimed in any one of claims 32 to 36, wherein using the time value to synchronize with the data samples comprises shifting a reference time frame by an offset calculated as a difference between the time value and a reference value.

38. A method as claimed in claim 37, wherein using the reference value is an ideal time value

39. A method as claimed in claim 37 or 38, wherein the reference value is code dependent and the offset is code dependent.

40. A method as claimed in any one of claims 37 to 39, further comprising, after synchronizing, selecting a portion of the data samples wherein a timing of an initial sample in the portion of data samples is code dependent.

41. A method as claimed in claim 40, further comprising folding the portion of the data samples.

42. A method as claimed in claim 40 or 41, further comprising correlating each of a plurality of codes against the portion of the data samples to identify the code associated with the data samples.

43. A method as claimed in claim 37 or 38, wherein the reference value is code independent and the offset is code independent.

44. A method as claimed in any one of claims 37 to 40, further comprising, after synchronizing, selecting a portion of the data samples wherein a timing of an initial sample in the portion of data samples is code independent.

45. A method as claimed in claim 44, further comprising converting the selected portion of the data samples from the time domain to the frequency domain.

46. A computer program which when executed by a processor enables the processor to perform any one of methods 32 to 35.

47. A computer program which when executed by a processor enables the processor to: determine a weight for each one of a plurality of data samples determine a parameter for each on of the plurality of data samples calculate a weighted sum comprising the summation, over the plurality of data samples, of the product of the determined weight for a sample and the determined parameter for that sample; process the weighted sum to obtain a time value for synchronization.

48. A computer program as claimed in claim 47, wherein a distribution in time of the parameter among the data samples is dependent upon which one of multiple codes was used in the creation of the data samples.

49. A computer program as claimed in claim 47 or 48, wherein the weight for a sample is determined as a function of the timing of the sample in a reference time frame.

50. A computer program as claimed in claim 47, 48 or 49, wherein the weight for a sample is the timing of the sample in a reference time frame.

51. A computer program as claimed in any one of claims 47 to 50, wherein the processing of the weighted sum comprises re-normalising using an unweighted sum of the multiple determined parameters.

52. A computer program as claimed in any one of claims 47 to 51, which when executed by a processor, further enables synchronization by shifting a reference time frame by an offset calculated as a difference between the time value and a reference value.

53. An apparatus comprising: a processor configured to determine a weight for each one of a plurality of data samples; configured to determine a parameter for each on of the plurality of data samples; configured to calculate a weighted sum comprising the summation, over the plurality of data samples, of the product of the determined weight for a sample and the determined parameter for that sample; and configured to process the weighted sum to obtain a time value for synchronization.

54. An apparatus as claimed in claim 53, wherein a distribution in time of the parameter among the data samples is dependent upon which one of multiple codes was used in the creation of the data samples.

55. An apparatus as claimed in claim 53 or 54, wherein the weight for a sample is determined as a function of the timing of the sample in a reference time frame.

56. An apparatus as claimed in claim 53, 54 or 55, wherein the weight for a sample is the timing of the sample in a reference time frame.

57. An apparatus as claimed in any one of claims 53 to 56, wherein the processor is configured to process the weighted sum by re-normalisation using an unweighted sum of the determined parameters.

58. An apparatus as claimed in any one of claims 53 to 57, wherein the processor is configured to enable synchronization by shifting a reference time frame by an offset calculated as a difference between the time value and a reference value.

59. An apparatus comprising: means for determining a characteristic for multiple data samples; means for calculating a weighted sum of the multiple determined characteristics, where a weight applied to the determined characteristic of a sample is a value of a defined time- dependent function for the timing of the sample; means for processing the weighted sum to obtain a time value; and means for using the time value to synchronize with the data samples.

60. A method comprising: measuring, at different times in a reference time frame over a time period, parameter values for a received signal calculating, using the measured parameter values, a statistical value that varies when a distribution of parameter values varies over the time period; and shifting the reference time frame by a difference between the calculated value and a reference value to synchronize with the received signal.

61. A method as claimed in claim 60, wherein the reference value is an expected statistical value

62. A method as claimed in claim 60 or 61 , wherein the reference value is code dependent.

63. A method comprising: selecting a first putative code sequence; selecting a first portion of a received signal in dependence upon the first putative code sequence; performing a correlation on the selected first portion of the received signal using the first putative code sequence; selecting a second putative code sequence; selecting a second portion of the received signal in dependence upon the second putative code sequence, wherein the second portion is different to the first portion as a consequence of the first putative code being different to the second putative code; performing a correlation on the selected second portion of the received signal using the first putative code sequence.

64. A method as claimed in claim 63, further comprising: identifying one of the putative code sequences as the most probable code sequence based upon the results of the correlations

65. A method as claimed in claim 63 or 64, wherein selecting a portion of a received signal comprises: determining a characteristic for multiple data samples of the received signal; calculating a weighted sum of the multiple determined characteristics, where a weight applied to the determined characteristic of a sample is a value of a defined time- dependent function for the timing of the sample in a reference time frame; processing the weighted sum to obtain a time value; using a difference between a code-dependent reference value and the obtained time value to identify within the reference time frame a portion of the received signal for selection.

66. A method as claimed in claim 63 or 64, wherein selecting a portion of a received signal comprises: determining a first mathematical moment in a reference time frame of signal power for the received signal; and using a difference between a the first mathematical moment and a code-dependent reference value to identify within the reference time frame a portion of the received signal for selection.

67. A computer program which when executed by a processor enables the processor to perform any one of methods 63 to 66.