GB2573532A

GB2573532A - Method for extracting synchronisation data

Info

Publication number: GB2573532A
Application number: GB1807505.1A
Authority: GB
Inventors: Ornstein Mecklenburg Kasper; Johnsson Henningsson Gustav
Original assignee: ARM Ltd; Advanced Risc Machines Ltd
Current assignee: ARM Ltd
Priority date: 2018-05-08
Filing date: 2018-05-08
Publication date: 2019-11-13
Anticipated expiration: 2038-05-08
Also published as: WO2019215229A1; GB201807505D0; GB2573532B

Abstract

There is provided a method of extracting synchronisation data from a plurality of subframes in a received signal, the subframes comprising symbol components S602 having associated synchronisation data components and associated cyclic shift coefficients, where the associated synchronisation data components are repeated between the subframes. The method includes modifying the symbol components to compensate for the associated cyclic shift coefficients S603, accumulating the modified symbol components S604, decoding the modified symbol components S605 and determining the synchronisation data from the accumulated modified components S606. In an NB-IoT signal, a Narrowband Secondary Synchronisation Signal (NSSS) comprising subframes is transmitted every two system frames (20ms) and an identical NSSSs every eight frames (80ms). The cyclic shift coefficients within a subframe convey timing information determined by the system frame number. In order to artificially reduce SNR a UE may accumulate identical subframes once they are received. By modifying the cyclic shift component of the received symbol components, the UE is able to accumulate frames more quickly, instead of waiting to receive symbol components with identical cyclic shift components. The symbol components may be modified by changing signs and/or swapping real or imaginary parts.

Description

METHOD FOR EXTRACTING SYNCHRONISATION DATA

Technical Field

The present disclosure relates to a method for extracting synchronisation data from a received signal. In particular, but not exclusively, the present disclosure relates to a method of extracting synchronisation data from a received Narrowband Internet of Things (NB-IoT) signal.

Background

NB-IoT is the fifth-generation of mobile communication technologies standard developed within the 3rd Generation Partnership Project, 3GPP. One of the purposes is to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as improved efficiency and lowered costs. In a typical UMT system, wireless devices or terminals also known as mobile stations and/or user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks.

When a UE, such as a wireless communication device, is powered on it will attempt to connect to a network. The process of attempting to connect to a network includes a frequency band scan, time and frequency synchronisation, and a cell search. There are predetermined frequency bands in which the UE will perform the frequency band scan and while performing the frequency band scan the UE will search for a Narrowband Primary Synchronization Signal (NPSS), which has a known pattern. The NPSS carries primary synchronisation data, which the UE uses to perform timing synchronisation and to estimate a carrier frequency offset (CFO).

Having performed the frequency band scan and detected a downlink NB-IoT signal, the UE identifies a Narrowband Secondary Synchronisation Signal (NSSS) in the downlink NB-IoT signal. The NSSS carries secondary synchronisation data, which the UE uses to perform a cell search. The NB-IoT specification defines 504 unique Narrowband Physical Cell identifiers (NCelllDs), one of which is assigned to each cell in an NB-IoT network. The UE determines the NCelHD from the NSSS. The NSSS conveys additional timing information dependent on the System Frame Number (SFN) of the frame in which the NSSS is transmitted.

An NSSS is transmitted once every two system frames, or once every 20ms. Due to the additional timing information, identical NSSSs are transmitted once every eight system frames, or once every 80ms. In order to artificially reduce the Signal to Noise Ratio (SNR) of a received downlink signal, a UE may accumulate identical NSSSs over multiple system frames.

Summary

According to a first aspect, there is provided a method of extracting synchronisation data from subframes in a received signal, where the subframes contain symbol components having associated synchronisation data components and associated cyclic shift coefficients. The method includes modifying the symbol components to compensate for the associated cyclic shift coefficients, accumulating the modified symbol components, decoding the modified symbol components, and determining the synchronisation from the accumulated decoded modified symbol components.

By modifying symbol components to compensate for the associated cyclic shift coefficients, a UE is able to accumulate data more quickly than would be permitted by methods in which data is only accumulated between subframes having identical symbol components.

According to an example, modifying the symbol components comprises changing signs of symbol components and/or swapping real and imaginary parts of symbol components. Modifying symbol components by changing signs and/or swapping real and imaginary parts allows the method to be performed at a relatively low computational cost, reducing the time taken, and the energy consumption, for a UE performing the modification of symbol components.

According to a second aspect, there is provided a device operable to extract synchronisation data from a plurality of subframes in a received signal, where the subframes contain symbol components having associated synchronisation data components and associated cyclic shift coefficients. The device includes processing circuitry and a memory. The memory is configured to store modified symbol component values. The processing circuitry configured to modify the symbol components to compensate for the associated cyclic shift coefficients, accumulate the modified symbol components in the memory, decode the modified symbol components;

and determine the synchronisation data from the accumulated decoded modified symbol components.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a block diagram illustrating time multiplexing in frames of a downlink NB-IoT signal.

Figure 2 shows a resource map illustrating an example of resource element allocation in a subframe of an NB-IoT signal configured for standalone deployment.

Figure 3 shows a resource map illustrating an example of resource element allocation in a subframe of an NB-IoT signal configured for in-band deployment.

Figure 4 shows Argand diagram representations of cyclic shift coefficients in anNB-IoT signal.

Figure 5 is a block diagram showing a UE operable to extract synchronisation data from a received downlink signal.

Figure 6 is a flow diagram showing an exemplary routine for extracting synchronisation data from a received downlink signal.

Figure 7 is a flow diagram showing an exemplary routine for receiving and processing data within a downlink signal.

Detailed Description

Figure 1 illustrates time multiplexing between physical channels and signals in a downlink NB-IoT signal. In the illustrated example, the downlink NB-IoT signal is deployed on an anchor carrier, and accordingly includes synchronisation data and broadcast information. The downlink signal is arranged in system frames (hereafter referred to as frames), each frame having a System Frame Number (SFN) and a duration of 10ms. Each frame is further arranged in 10 subframes numbered from 0 to 9, each subframe having a duration of 1ms. As shown, subframe 0 of an even numbered frame contains the Narrowband Physical Broadcast Channel (NPBCH). Subframe 5 of the even numbered frame contains a Narrowband Primary Synchronisation Signal (NPSS).

Subframe 9 of the even numbered frame contains a Narrowband Secondary Synchronisation Signal (NSSS). The remaining subframes of the even numbered frame contain Narrowband Physical Downlink Shared Channel (NPDSCH) data or Narrowband Physical Downlink Control Channel (NPDCCH) data. An odd numbered frame is arranged similarly to the even numbered frame, but subframe 9 contains NPDSCH data or NPDCCH data, as opposed to an NSSS.

In the present example, an NSSS is transmitted every two frames or every 20ms, but identical NSSSs are only transmitted once every eight frames or every 80ms, as will be described in more detail hereafter. In order to artificially reduce SNR, a UE may accumulate NSSS data over multiple subframes separated by intervals of 80ms.

Figure 2 shows a resource map illustrating an example of resource element allocation in an NB-IoT signal, for a subframe containing NSSS data. The subframe contains 14 symbols labelled I = 0,..., 13, and 12 subcarriers labelled k = 0,..., 11. The subframe therefore contains a total of 168 resource elements. In this example, the NB-IoT signal is configured for standalone deployment, and the subframe is dedicated entirely to the NB-IoT signal. In this example, 132 resource elements, corresponding to the last 11 symbols in the subframe, are occupied by NSSS data.

Figure 3 shows a resource map illustrating another example of resource element allocation in an NB-IoT signal, again for a subframe containing NSSS data. In this example, the NB-IoT signal is configured for in-band deployment, and shares the subframe with a Long Term Evolution (LTE) carrier. In this example, 24 resource elements are occupied by the LTE Cell-specific Reference Signal (CRS) and 25 resource elements of the first three symbols are occupied by LTE Physical Downlink Control Channel (PDCCH) data. In this example, 116 resource elements of the last 11 symbols are occupied by NSSS data.

In examples in accordance with the present invention, a base station transmits a signal arranged in subframes. Some of the subframes in the transmitted signal convey synchronisation data, for example secondary synchronisation data indicative of a unique cell identifier assigned to a cell in which the base station in located. The subframes conveying synchronisation data contain symbol components, each of which is mapped to a resource element in a frequency domain representation of the transmitted signal.

In some examples, synchronisation data components are repeated between subframes. In an example in which subframes convey secondary synchronisation data indicative of a unique cell identifier, a UE may repeatedly perform a cell search using the secondary synchronisation data, such that the cell identifier is correctly identified in cases where the UE moves between cells. Furthermore, a UE may be configured to accumulate data received from multiple subframes in order to artificially reduce the Signal to Noise Ratio (SNR) and accurately determine the synchronisation data conveyed by the subframes. In some applications, UEs are deployed in locations having a very low SNR (e.g. lower than OdB, lower than -5dB, or lower than -lOdB). Such applications include NB-IoT UEs deployed as smart electricity meters or smart gas meters located in basements or other enclosed areas of buildings.

In some examples, symbol components have associated cyclic shift coefficients. A symbol component in the frequency domain is given by the product of a synchronisation data component and a cyclic shift coefficient. The cyclic shift coefficients within a subframe convey additional information to that conveyed by the synchronisation data components. In some examples, a received signal is arranged in frames, each frame having an SFN and being arranged in subframes. The cyclic shift coefficients within a subframe convey timing information determined by the SFN of the frame in which the subframe is transmitted. It is envisaged that in other examples, cyclic shift coefficients may convey information other than timing information.

An example of an NSSS sequence in a downlink NB-IoT signal, according to the 3GPP specification, has symbol components given by Equation (1) below:

_ ,min'(n'+l) (J) dy(n) = Q(n)bg(m)e ⁷ i3i where j = V—Ϊ, and for n = 0,1,..., NSSSi_ength — 1, the complex number dy(n) is a symbol component mapped to the n^th resource element containing NSSS data in the f^th NSSS subframe. In this example, NSSS_length = 132, corresponding to symbol components of the last 11 symbols in the f^th NSSS subframe.

In the above sequence, b_q(m) is a binary mask dependent on the m = n mod 128 and q = [7V_I^^ceII/126j, where /V|Q^Cel1 is one of 504 unique cell identifiers.

The synchronisation data component of the symbol component d(n) is given by Equation (2):

_ ,n-un'(n'+l) (2) b_q(rri)e ¹ i3i where n' = n mod 131 and u = mod 126 + 3. the sequence of synchronisation data components can be decoded to deduce the cell identifier

The cyclic shift coefficient Q-(n) of the symbol component dy(n) is given by Equation (3):

Q(n) = e~}^2nefⁿ, (3) where the cyclic shift is given by Equation (4):

/n_f\

Of = -rrr -jr mod 4, ^f 132'2' with being the SFN of the frame in which the f^th subframe is transmitted. As described above with reference to Figure 1, an NSSS is transmitted only in frames having even SFNs such that = 0,2,4,6,.... Table 1 sets out the cyclic shifts for NSSS subframes in the first eight frames of the transmitted signal, along with the first four terms of the respective sequences followed by cyclic shift coefficients in each subframe.

ⁿf	Of	Q(0)	Q(i)		Q(3)
0	0	1	1	1	1
2	1/4	1	-j	-1	j
4	1/2	1	-1	1	-1
6	3/4	1	L	-1	zZ

Table 1

Figure 4 illustrates the respective sequences followed by the cyclic shift coefficients for the NSSS subframes in the first eight frames of a received downlink NB-IoT signal. Each arrow is an Argand diagram representation of a cyclic shift coefficient. As shown, cyclic shift coefficients from each of the four NSSS subframes have a different respective sequence. The respective sequences in this example comprise p^th roots of unity for integer values of p. For example, the respective sequence for the NSSS subframe in frame 2 comprises fourth roots of unity. The respective sequence for the NSSS subframe in frame 4 comprises square roots of unity. In further examples, the respective sequences followed by cyclic shift coefficients in a signal may comprise other roots of unity or may comprise complex numbers which are not roots of unity. In the present example, the cyclic shift coefficients affect the phase, but not the magnitudes, of the symbol components. Furthermore, the respective sequences are cyclic, repeating after p terms.

In the present example, the cyclic shifts are cyclic, repeating after four NSSS subframes, such that the respective sequences followed by the cyclic shift coefficients repeat after four NSSS subframes. Specifically, the cyclic shifts in this example satisfy Equation (5):

( (5)

0y₊l = i + — } mod 1. ⁷

In other examples, cyclic shifts repeat after different numbers of subframes. For examples in which cyclic shift coefficients convey timing information and in which the respective sequences repeat after a predetermined number of subframes, the conveyed timing information is repeated accordingly. Other data subframes in the signal may be arranged periodically with a greater period than that of the subframes conveying synchronisation data. In such examples, and in other examples, it may be necessary for a UE to determine the phase of the received signal such that the UE processes the data in the subframes correctly, as will be described in more detail hereafter.

In the present example, symbol components in a subframe have synchronisation data components that are repeated between subframes of a transmitted signal, but different subframes have different cyclic shift coefficients. The different cyclic shift coefficients prevent a UE receiving the signal from accumulating the symbol components from the subframes. In some cases, cyclic shift coefficients are repeated within subsets of the subframes, allowing a UE to accumulate corresponding symbol components between the subsets. As described above, in an example of a downlink NB-IoT signal, an NSSS is transmitted once every two frames or once every 20ms, but due different cyclic shift coefficients, an identical NSSS is transmitted once every eight frames or once every 80ms. A UE may therefore accumulate NSSS data from identical subframes once every 80ms. In situations where it is necessary for the UE to accumulate data over a large number of subframes, for example when the UE is deployed in a location with a very low SNR, the accumulation, and hence the cell search, may take a long time, for example of the order of seconds or more.

As mentioned above, the present invention provides a method of extracting synchronisation data from subframes in a received signal, where the subframes contain symbol components having associated synchronisation data components and associated cyclic shift coefficients. The terms “associated synchronisation data components” and “associated cyclic shift coefficients” refer to the corresponding synchronisation data components and cyclic shift coefficients in the transmitted signal. The method includes modifying the symbol components to compensate for the associated cyclic shift coefficients, accumulating the modified symbol components, decoding the modified symbol components, and determining the synchronisation data from the accumulated decoded modified symbol components. In some examples, the modified symbol components are accumulated first, and then decoded. In other examples, the modified symbol components are decoded first, and then accumulated.

In some examples, a UE modifies symbol components without first needing to determine the cyclic shift coefficients of the symbol components in each subframe. In cases where it is challenging or impossible for a UE to determine the cyclic shift coefficients without first accumulating the symbol components, it is advantageous for the UE to modify the symbol components without first needing to determine the associated cyclic shift coefficients. Such an example is described in detail hereafter.

In some examples, cyclic shift coefficients within each of a set of subframes follow a respective sequence, the respective sequences differing between the subframes. A UE may determine the information conveyed by the cyclic shift coefficients by determining the respective sequence of cyclic shift coefficients in a subframe. In such examples, a UE modifies symbol components within the subframes such that the modified symbol components have modified cyclic shift coefficients following a common sequence shared between the plurality of subframes. In some examples, the modified cyclic shift coefficients follow a common constant sequence. In other examples, the modified cyclic shift coefficients follow a common varying sequence. In some examples, the modified cyclic shift coefficients follow a common sequence corresponding to one of the respective sequences followed by associated cyclic shift coefficients in a subframe, and it is therefore unnecessary for the UE to modify the symbol components in that subframe.

In an example in which a UE receives a downlink NB-IoT signal containing a series of NSSS subframes as described with reference to Equation (1), in which the cyclic shift coefficients are given by Equation (3), modifying symbol components is equivalent to multiplying the n^th symbol components of the f^th subframe by C/_+a(n), where a is an integer that is independent of f and represents a phase difference, in terms of numbers of NSSS subframes, between a predetermined series of NSSS subframes and the received series ofNSSS subframes. The modified cyclic shift coefficients C(n) are given by Equation (6):

C(ri) = Cf(ri) Cf_+a(ri) = βΙ^2πθαη. (6)

Table 2 sets out the common sequence followed by modified cyclic shift coefficients C(n) for four different values of the phase shift a.

a	C(0)	C(l)	C(2)	C(3)
0	1	1	1	1
1	1	/	-1	-/
2	1	-1	1	-1
3	1	zZ	1	L

Table 2

As shown in Table 2, the common sequence followed by the modified symbol components is different for different values of the phase shift a. In this example, each of the common sequences comprise q^th roots of unity for integer values of q. Each of the common sequences is cyclic, repeating after q terms.

In the present example, the associated cyclic shift coefficients of the symbol components in each of the NSSS subframes convey timing information dependent on the SFN of the frame in which the NSSS is transmitted. As shown in Table 2, the common sequences followed by the modified symbol components are different for different values of the phase shift a, and hence the integer a is indicative of the timing information conveyed by the cyclic shift coefficients. In this example, the UE modifies the symbol components without first determining the value of the integer a. In one example, modifying the symbol components is equivalent to multiplying the symbol components in by Cp(n) for β = 0,1,2, 3. The UE then determines, from the resulting common sequence followed by the modified cyclic shift coefficients, the phase shift a between the predetermined series of NSSS subframes and the received series of NSSS subframes, and hence determines the timing information conveyed by the associated cyclic shift coefficients of the symbol components in the received signal.

In the present example, the respective sequences followed by the cyclic shift coefficients repeat after four NSSS subframes. Accordingly, modifying the symbol components to compensate for the cyclic shift coefficient coefficients comprises performing a same set of processing operations on symbol components of subframes separated by four NSSS subframes. For the purpose of modifying symbol components to compensate for associated cyclic shift coefficients, the UE therefore only needs to store a relatively small amount of data and/or code, corresponding to the processing operations performed over one cycle of the respective sequences. In other examples, sequences followed by cyclic shift coefficients may repeat after other numbers of subframes.

In examples according to the present invention, a UE accumulates modified symbol components, thereby determining accumulated modified symbol components. In the example described above in which a UE receives a downlink NB-IoT signal, the UE accumulates modified symbol components over /V_acc NSSS subframes and determines accumulated modified symbol components d_acc(ⁿ)> given by Equation (7).

^acc 1

i=0 where d^^rec\n) is the n^th symbol component from the i^th received NSSS subframe. The accumulated modified symbol components d_acc(ri) have modified cyclic shift coefficients given by C(n). The UE may determine, from the accumulated modified symbol components d_acc(ri), the timing information conveyed by the associated cyclic shift coefficients in the received signal

V di^rec\n)Co(m).

Due to the respective sequences followed by the cyclic shift coefficients repeating after four NSSS subframes, Q*(n) = C/m), where m = (4 -/1-0,) mod 4, and therefore Equation (7) can advantageously be expressed as shown in Equation (8):

Nicc^_l d-acci/l) i=0

Due to the simplification of Equation (8), a UE is only required to perform four different processing operations in order to modify the symbol components, corresponding to the four values of Cq (m) for m = 0,1, 2, 3.

In some examples, modifying symbol components to compensate for associated cyclic shift coefficients includes changing signs of symbol components and/or swapping real and imaginary parts of symbol components. In some such examples, modifying the symbol components is performed without performing complex multiplications. Changing the sign and/or swapping the real and imaginary parts of a complex number is less computationally expensive than performing a complex multiplication, and therefore modifying symbol components by changing signs and/or swapping real and imaginary parts of the symbol components may reduce the time taken, and the energy consumption, for a UE to perform the modification of symbol components.

In the example described above in which a UE receives a downlink NB-IoT signal, modifying the symbol components is equivalent to multiplying each symbol component by one of the elements of the set {l,j, — 1, —j}. Therefore, each processing operation corresponding to a modification of a symbol component is equivalent to changing the sign and/or swapping the real and imaginary parts of the symbol components. In the example above in which a UE accumulates synchronisation data over N_acc subframes, the total computational cost of modifying the symbol components is given by 1321V_acc complex additions, 331V_acc real sign changes, and 641V_acc complex sign changes and/or swaps of real and imaginary parts.

Figure 5 shows an example of a UE 100 operable to extract synchronisation data from a plurality of subframes in a received signal in accordance with the present invention. The UE 100 includes a receiver 110, a processing unit 120, and memory 130, connected by a system bus 140.

Figure 6 shows an exemplary routine performed by the processing unit 120 of the UE 100 in order to extract synchronisation data from a plurality of subframes in a received signal.

The processing unit 120 initialises, at S601, an accumulation value in the memory 130 to zero. The processing unit 120 receives, at S602, a symbol component from a subframe containing synchronisation data. The processing unit 120 modifies, at S603, the received symbol component to compensate for an associated cyclic shift coefficient of the symbol component. The processing unit 120 adds, at S604, the modified symbol component to the accumulation value stored in the memory 130.

The processing unit repeats steps S602 to S604 for corresponding symbol components in each of a set of subframes in which synchronisation data components are repeated, thereby determining an accumulated modified symbol component.

The processing unit 120 repeats steps S601 to S604 for each symbol component in the subframes. The processing unit decodes, at S605, the modified symbol components, and determines, at S606, the synchronisation data conveyed by the synchronisation data components within the subframes. In a further example, the processing unit 120 decodes each modified symbol component individually, and then accumulates the decoded modified symbol components. In this further example, step S605 is performed between steps S603 and S604.

Figure 7 shows an exemplary routine in which a UE 100 receives and processes data from a downlink NB-loT signal. The UE 100 performs, at S701, a frequency band scan and detects, at S702, the downlink NB-loT signal. The UE 100 performs, at S703, primary synchronisation by detecting an NPSS and using the detected NPSS to perform timing synchronisation and to determine a CFO estimate.

Having performed primary synchronisation, the UE 100 extracts, at S704, time domain symbols from the received signal, and removes cyclic prefixes from the time domain symbols, the UE 100 performs, at S705, frequency compensation, which includes adjusting the tuning of the receiver 110 to reduce the CFO between the base station and the UE. The UE 100 performs, at S706, a Fast Fourier Transform (FFT) operation on the extracted symbols, and thereby determines symbol components in the frequency domain.

The UE 100 extracts, at S707, synchronisation data from multiple NSSS subframes in the received signal, in accordance with the present invention. In this example, the UE 100 executes the routine of Figure 6 in order to extract the synchronisation data. The UE 100 determines, at S708, timing information conveyed by the associated cyclic shift coefficients of symbol components in the received signal. In this example, the UE determines uses the accumulated modified symbol components to determine the timing information.

The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. For example, the described methods may be performed by software or by an application-specific integrated circuit (ASIC) of a UE. Furthermore, the described methods may be used to extract other types of data from a received signal, provided that symbol components of the received signal have associated data components that are repeated between subframes of the signal, and further have associated cyclic shift coefficients. For example, the described methods may be used to extract primary synchronisation data from a received signal, or data other than synchronisation data.

It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

1. A method of extracting synchronisation data from a plurality of subframes in a received signal, the subframes comprising symbol components having associated synchronisation data components and associated cyclic shift coefficients, the associated synchronisation data components being repeated between the subframes, the method comprising:

modifying the symbol components, thereby to compensate for the associated cyclic shift coefficients;

accumulating the modified symbol components;

decoding the modified symbol components; and determining, from the accumulated decoded modified symbol components, the synchronisation data.

2. The method of claim 1, wherein the cyclic shift coefficients of symbol components in each of the plurality of subframes follow a respective sequence, the respective sequences being different for at least two of the plurality of subframes, and wherein the modified symbol components have modified cyclic shift coefficients following a common sequence shared between the plurality of subframes.

3. The method of claim 2, wherein a respective sequence followed by the associated cyclic shift coefficients in a subframe conveys timing information, and wherein the method comprises determining, from the common sequence shared between the plurality of subframes, the timing information.

4. The method of claim any previous claim, wherein modifying the symbol components comprises performing a same set of processing operations on symbol components of subframes separated by a predetermined number of subframes.

5. The method of any of claims 2 to 4, wherein the respective sequence for a subframe of the plurality of subframes comprises pth roots of unity, wherein p is a first integer, and wherein the common sequence followed by the modified symbol components comprises qth roots of unity, wherein q is a second integer.

6.

The method of claim 4 or 5, wherein the first integer p is equal to four.

7.

The method of any previous claim, wherein the associated cyclic shift coefficients are given by:

wherein:

Q(n) is the cyclic shift coefficient of the nth symbol component of an /th subframe of the plurality of subframes;

6? is a cyclic shift associated with the /th subframe, wherein modifying the samples is equivalent to multiplying the nth symbol component of the /th subframe by Cf+a(n), a being an integer that is independent of

8.

9.

The method of claim 7, wherein the cyclic shifts satisfy:

mod 1.

The method of any previous claim, wherein modifying the symbol components comprises changing signs of symbol components and/or swapping real and imaginary parts of symbol components.

10. The method of any previous claim, wherein the synchronisation data is indicative of a unique cell identifier corresponding to a base station from which the signal was transmitted.

11. The method of any previous claim, wherein the received signal is a Narrowband Internet of Things (NB-IoT) signal, and each of the plurality of subframes comprises a Narrowband Secondary Synchronisation Signal (NSSS).

12. A device operable to extract synchronisation data from a plurality of subframes in a received signal, the subframes comprising symbol components having associated synchronisation data components and associated cyclic shift coefficients, the associated synchronisation data components being repeated between the subframes, the device comprising:

a memory configured to store modified symbol component values; and processing circuitry configured to:

modify the symbol components, thereby to compensate for the associated cyclic shift coefficients;

accumulate the modified symbol components in the memory;

decode the modified symbol components; and determine, from the accumulated decoded modified symbol components, the synchronisation data.

13. The device of claim 12, wherein the cyclic shift coefficients of symbol components in each of the plurality of subframes follow a respective sequence, the respective sequences being different for at least two of the plurality of subframes, and wherein the modified symbol components have modified cyclic shift coefficients following a common sequence shared between the plurality of subframes.

14. The device of claim 13, wherein a respective sequence followed by the associated cyclic shift coefficients in a subframe conveys timing information, and wherein the processing circuitry' is configured to determine, from the common sequence shared between the plurality of subframes, the timing information.

15. The device of any of claim 12 to 14, wherein modifying the symbol components comprises performing a same set of processing operations on symbol components of subframes separated by a predetermined number of subframes.

16. The device of any of claims 13 to 15, wherein the respective sequence for a subframe of the plurality of subframes comprises pth roots of unity, wherein p is a first integer, and wherein the common sequence followed by the modified symbol components comprises qth roots of unity, wherein q is a second integer.

17.

18.

The device of claim 16, wherein the first integer p is equal to four.

The device of any of claims 12 to 17, wherein the associated cyclic shift coefficients are given by:

wherein:

Q(n) is the cyclic shift coefficient of an nth symbol component of an /th subframe of the plurality of subframes;

6f is a cyclic shift associated with the /th subframe, wherein modifying the samples is equivalent to multiplying the nth symbol component of the fth subframe by C/+a(n), a being an integer that is independent of

19.

20.

The device of claim 18, wherein the cyclic shifts satisfy:

mod 1.

The device of any of claims 12 to 19, wherein modifying the symbol components comprises changing signs of samples and/or swapping real and imaginary parts of symbol components.

21. The device of any of claims 12 to 20, wherein the synchronisation data is indicative of a unique cell identifier corresponding to a base station from which the signal was transmitted.

22. The device of any of claims 12 to 21, wherein the received signal is a Narrowband Internet of Things (NB-loT) signal, and each of the plurality of subframes comprises a Narrowband Secondary Synchronisation Signal (NSSS).

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