WO2019029790A1

WO2019029790A1 - Method and apparatus for spectrum shaping

Info

Publication number: WO2019029790A1
Application number: PCT/EP2017/069971
Authority: WO
Inventors: Matthew Baker; Karri Markus Ranta-Aho; Jorma Johannes Kaikkonen
Original assignee: Nokia Technologies Oy
Priority date: 2017-08-07
Filing date: 2017-08-07
Publication date: 2019-02-14

Abstract

A method comprises receiving a signal at a transmitter which comprises a first reference signal to which a first filter has been applied and a second reference signal to which said first filter has not been applied. These first and second reference signals are used to determine information about the first filter. This information may be filter coefficients.

Description

METHOD AND APPARATUS FOR SPECTRUM SHAPING

FIELD

This disclosure relates to a method and apparatus, and in particular but not exclusively to a method and apparatus used where a filter is applied to data before it is transmitted.

BACKGROUND

A communication system can be seen as a facility that enables communication between two or more devices such as user terminals, machine- like terminals, base stations and/or other nodes by providing carriers between the communication devices. A communication system can be provided for example by means of a communication network and one or more compatible communication devices. The communication may comprise, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia and/or content data and so on. Non-limiting examples of services provided include two-way or multi-way calls, data communication or multimedia services and access to a data network system, such as the Internet.

In a wireless system at least a part of communications between at least two stations occurs over wireless interfaces. Examples of wireless systems include public land mobile networks (PLMN), satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). A local area wireless networking technology allowing devices to connect to a data network is known by the tradename Wi-Fi. Wi-Fi is often used synonymously with WLAN.

The wireless systems can be divided into cells, and are therefore often referred to as cellular systems. A user can access a communication system by means of an appropriate communication device or terminal. A communication device of a user is often referred to as user equipment (UE). A communication device is provided with an appropriate signal receiving and transmitting apparatus for enabling communications, for example enabling access to a communication network or communications directly with other users. The communication device may access a carrier provided by a station, for example a base station of a cell, and transmit and/or receive communications on the carrier.

A communication system and associated devices typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. Communication protocols and/or parameters which shall be used for the connection are also typically defined. Examples of standardised radio access technologies include GSM (Global System for Mobile), EDGE (Enhanced Data for GSM Evolution) Radio Access Networks (GERAN), Universal Terrestrial Radio Access Networks (UTRAN) and evolved UTRAN (E-UTRAN). An example of standardized communication system architectures is the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. The LTE is being standardized by the 3rd Generation Partnership Project (3GPP). The LTE employs the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access. Further developments of LTE are sometimes referred to as LTE Advanced (LTE-A).

SUMMARY

According to a first aspect, there is provided a method comprising: receiving a signal comprising a first reference signal to which a first filter has been applied and a second reference signal to which said first filter has not been applied; and using said first and second reference signals to determine information about said first filter.

The information may comprise filter coefficients.

The method may comprise receiving first data to which said first filter has been applied.

The method may comprise using said information about said first filter to reverse the effects of said first filter on first data.

According to a second aspect, there is provided a method comprising: causing a signal to be transmitted, said signal comprising a first reference signal to which a first filter has been applied and a second reference signal to which said first filter has not been applied. The method may comprise transmitting first data to which said first filter has been applied.

One or more of the following features may be used in conjunction with one or both of the first and second aspects. The first filter may comprise a spectrum shaping filter.

The first filter may reduce a peak to average power ratio of transmitted data and/or suppress spectral leakage of the transmitted data.

One of said first and second reference signals may comprise a BPSK signal.

The BPSK signal may be a π/2 BPSK signal.

One of said first and second reference signals may comprise a sequence of phase shifts.

The sequence of phase shifts may comprise a Zadoff-Chu sequence. The first data may comprise BPSK data.

The first reference signal, said second reference signal and said first data may be time multiplexed.

The first reference signal, said second reference signal and said first data may be provided by different symbols of a DFT-S-OFDM wave form.

According to a third aspect, there is provided an apparatus, the apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to: receive a signal comprising a first reference signal to which a first filter has been applied and a second reference signal to which said first filter has not been applied; and use said first and second reference signals to determine information about said first filter.

The information may comprise filter coefficients.

The at least one memory and the computer code may be configured with the at least one processor to cause the apparatus to receive first data to which said first filter has been applied. The at least one memory and the computer code may be configured with the at least one processor to cause the apparatus to use said information about said first filter to reverse the effects of said first filter on first data.,

According to a fourth aspect, there is provided an apparatus, the apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to: cause a signal to be transmitted, said signal comprising a first reference signal to which a first filter has been applied and a second reference signal to which said first filter has not been applied.

The at least one memory and the computer code may be configured with the at least one processor to cause the apparatus to transmit first data to which said first filter has been applied.

One or more of the following feature may be used in conjunction with one or both of the third and fourth aspects.

The first filter may comprise a spectrum shaping filter.

One of said first and second reference signals may comprise a BPSK signal.

The BPSK signal may be a π/2 BPSK signal.

The information may comprise filter coefficients.

The method may comprise receiving first data to which said first filter has been applied. The method may comprise using said information about said first filter to reverse the effects of said first filter on first data.

According to a fifth aspect, there is provided an apparatus comprising: means for receiving a signal comprising a first reference signal to which a first filter has been applied and a second reference signal to which said first filter has not been applied; and means for using said first and second reference signals to determine information about said first filter.

The information may comprise filter coefficients.

The receiving means may receive first data to which said first filter has been applied.

The apparatus may comprise means for using said information about said first filter to reverse the effects of said first filter on first data.,

According to a sixth aspect, there is provided a method comprising: means for causing a signal to be transmitted, said signal comprising a first reference signal to which a first filter has been applied and a second reference signal to which said first filter has not been applied.

The transmitting means may be for transmitting first data to which said first filter has been applied.

One or more of the following feature may be used in conjunction with one or both of the fifth and sixth aspects.

The first filter may comprise a spectrum shaping filter.

One of said first and second reference signals may comprise a BPSK signal.

The BPSK signal may be a π/2 BPSK signal.

The first reference signal, said second reference signal and said first data may be time multiplexed. The first reference signal, said second reference signal and said first data may be provided by different symbols of a DFT-S-OFDM wave form.

According to an aspect, there is provided a method comprising: receiving first data to which a first filter has been applied, a reference signal and information relating to said first filter; and using information about said first filter to process said first data.

The information about said first filter may comprise one or more filter coefficients.

The first data and said reference signal are modulated differently.

According to an aspect, there is provided a method comprising: causing transmitting of first data to which a first filter has been applied, a reference signal and information relating to said first filter.

It should be appreciated that one or more of the features described in relation the first and/or second aspects may be used with this aspect and/or the previous aspect.

According to another aspect, there is provided an apparatus in a communication device, the apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to: receive first data to which a first filter has been applied, a reference signal and information relating to said first filter; and use information about said first filter to process said first data.

According to another aspect, there is provided an apparatus in a communication device, the apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to cause transmitting of first data to which a first filter has been applied, a reference signal and information relating to said first filter. A computer program comprising program code means adapted to perform the herein described methods may also be provided. In accordance with further embodiments apparatus and/or computer program product that can be embodied on a computer readable medium for providing at least one of the above methods is provided.

Various other aspects and further embodiments are also described in the following detailed description of examples embodying the invention and in the attached claims.

BRIEF DESCRIPTION OF FIGURES

Some embodiments will now be described in further detail, by way of example only, with reference to the following examples and accompanying drawings, in which:

Figure 1 shows a schematic example of a system where some embodiments may be implemented;

Figure 2 shows an example of a communication device;

Figure 3 shows a control apparatus;

Figure 4 show an example of a transmitter chain;

Figure 5 shows an example of a receiver chain;

Figure 6 shows a method of one embodiment; and

Figure 7 shows a method of another embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following certain exemplifying embodiments are explained with reference to a wireless communication system serving devices adapted for wireless communication. Therefore, before explaining in detail the exemplifying embodiments, certain general principles of a wireless system, components thereof, and devices for wireless communication are briefly explained with reference to system 10 of Figure 1 , device 20 of Figure 2 and apparatus of Figure 3, to assist in understanding the described examples.

A communication device can be used for accessing various services and/or applications provided via a communication system. In wireless communication systems the access is provided via a wireless access interface between wireless communication devices and an appropriate access system. A device may access wirelessly a communication system via a base station. A base station site can provide one or more cells of a cellular system. In the Figure 1 example, a base station 12 can provide e.g. three cells on different carriers. In addition to the base station 12, at least one serving cell can also be provided by means of another station or stations. For example, at least one of the carriers may be provided by a station that is not co-located at base station 12. This possibility is denoted by station 1 1 in Figure 1 . Interaction between the different stations and/or controllers thereof can be arranged in various manners. Each mobile device 20 and base station may have one or more radio channels open at the same time and may receive signals from more than one source.

A base station node can be connected to a data network 18 via an appropriate gateway 15. A gateway function between the access system and another network such as a packet data network may be provided by means of any appropriate gateway node, for example a packet data gateway and/or an access gateway. A communication system may thus be provided by one or more interconnect networks and the elements thereof, and one or more gateway nodes may be provided for interconnecting various networks.

Figure 2 shows a schematic, partially sectioned view of a communication device 20 that a user can use for communications. Such a communication device is often referred to as user equipment (UE) or terminal. An appropriate communication device may be provided by any device capable of sending and receiving radio signals. Non-limiting examples include a mobile station (MS) such as a mobile phone or what is known as a 'smart phone', a portable computer provided with a wireless interface card or other wireless interface facility, personal data assistant (PDA) provided with wireless communication capabilities, or any combinations of these or the like. A mobile communication device may provide, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia, positioning data, other data, and so on. Users may thus be offered and provided numerous services via their communication devices. Non-limiting examples of these services include two-way or multi-way calls, data communication or multimedia services or simply an access to a data communications network system, such as the Internet. A mobile device is typically provided with at least one data processing entity 23, at least one memory 24 and other possible components 29 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications via base stations and/or other user terminals. The data processing, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets. This apparatus is denoted by reference 26.

A user may control the operation of the device 20 by means of a suitable user interface such as key pad, voice commands, touch sensitive screen or pad, combinations thereof or the like. A display 25, a speaker and a microphone are also typically provided. Furthermore, a mobile communication device may comprise appropriate connectors (either wired or wireless) to other devices and/or for connecting external accessories, for example hands-free equipment, thereto.

The device 20 may receive and transmit signals 28 via appropriate apparatus for receiving and transmitting signals. In Figure 2 transceiver apparatus is designated schematically by block 27. The transceiver apparatus may be provided with cognitive radio capability. The transceiver may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the mobile device.

The communication device can access a communication system based on various access techniques, for example those based on the third Generation Partnership Project (3GPP) specifications or any other suitable specifications. Some embodiments may be provided in the context of so-called 5G or New Radio standards. Of course, other embodiments may be provided in the context of other standards.

Figure 3 shows an example of an apparatus 300 provided in or associated with a base station, as shown in Figure 1 .The apparatus comprises at least one memory 301 , at least one data processing unit or at least one data processor 302, 303 and an input/output interface 304. Via the interface the apparatus can be coupled to a receiver and a transmitter of the base station. Typically radio communication receivers require a phase reference in order to demodulate received radio signals. The phase reference is typically provided by a pilot or reference signal (RS) transmitted together with a data signal to be demodulated.

Currently a RS is used which the receiver knows to have been transmitted with the same phase as the data signal. The receiver can thus determine the phase of the data signal directly.

Alternatively, a RS is transmitted with a different phase from that of the data signal, together with explicit signalling of an indication of the phase difference between the RS and the data signal. The receiver can derive the phase of the data signal.

These systems may be satisfactory if the receiver does not need any additional information about processing steps applied to the transmitted signal. However, in some cases it may be desirable for the transmitter to perform filtering on the data signal. This may degrade the signal reception at the receiver unless the associated filter parameters or coefficients are known to the receiver.

For example, it has been proposed that applying a spectral shaping filter to a π/2-rotated BPSK (binary phase shifted keying) modulated signal can reduce the PAPR (peak to average power ratio) of the transmitted signal. This may enable a higher transmit power to be used and increase coverage. However, such filters also increase the time-domain spread of the signal such that an equaliser in the receiver needs to process a larger number of taps in order to achieve the same output SINR (signal to interference plus noise ratio).

One way to handle the use of such filters is for the RS to be transmitted without passing through the spectral shaping filter. This may help to achieve a better channel estimate as the filter does not distort the RS. This spectral shaping filter is specified such that it is known to the receiver and the received data signal can be equalised accordingly. However, only one filter can be used, as the filter would need to be specified for the transmitter and the receiver. This may prevent the filter from being designed differently in different UEs. This means that advantage may not be taken of applied power amplifier design, or technology improvements. This may prevent dynamic changes to the filter depending on, for example, available transmit power at the transmitter and/or channel conditions.

Another way to handle the use of such filters is for the transmitter to apply the same spectral shaping filter to the RS as to the data signal so that the filter coefficients do not need to be known at the receiver. However, it this may degrade channel estimation performance.

Some embodiments address one or more of the previously discussed issues. In some embodiments, a filter may be varied on the transmit side without degrading channel estimation performance significantly.

Some embodiments may provide a method by which the properties of a transmitter filter may be provided or indicated to a receiver.

In one embodiment, the transmitted filtered data signal is multiplexed with two different RSs. The filter at the transmitter side is applied to the data signal and to the first RS, but not to the second RS. By performing channel estimation independently on the two RSs, the receiver can deduce the filter applied to the data signal and can adapt its equaliser accordingly.

In one embodiment, one of the RSs is transmitted less often than the other of the RSs. For example, the filtered RS may be transmitted less frequently, as the applied filter would not change frequently. For example, it may be that a given transmitter may consistently apply just one filter.

In some embodiments, a training phase at a beginning of a data burst or at a beginning of the connection may be provided. In some embodiments, both the filtered RS and the non-filtered RS are transmitted in the training phase so that the receiver can reliably deduce the filter. Later, only the non-filtered RS is used, with the receiver using the earlier-obtained and stored filter parameterization. To minimise any difference in the channel impulse response between the received first reference signal and the second reference signal, the first and second reference signals are transmitted adjacent in time or at least close together. The reference signals may be provided by adjoining symbols in the time domain.

In some embodiments, the one or more reference signals may be uplink reference signals. For example, in some embodiments, filtering may be used on one type of RS (e.g. SRS (sounding reference signal) for sounding the link for scheduling purposes), while not use filtering for another type of RS (such as DMRS (demodulation reference signal) used for demodulation). It should be appreciated that one or more alternative reference signals may alternatively or additionally be used.

In the cases were the filter is used to improve PAPR, this may provide advantages in the context of uplink signals.

In some embodiments, the one or more reference signals may be downlink reference signals. For example, there may be one or more of a DMRS (demodulation reference signal) associated with user specific control, DMRS associated with user specific data, user specific CSI-RS (channel state information reference signal) to estimate the channel and a DMRS associated with a broadcast PBCH (physical broadcast channel). In one embodiment, the PBCH DMRS is filtered and one or more other RS are not filtered, or vice versa. It should be appreciated that one or more alternative reference signals may alternatively or additionally be used.

In some embodiments, the data signal and first RS are BPSK signals (for example, π/2-rotated BPSK) and the second RS comprises a sequence of phase shifts (for example, a Zadoff-Chu sequence). In this example the first RS would be a pre-determined sequence of BPSK constellation points, i.e. the RS format is BPSK, whereas the second RS is ZC-sequence defined constellation points, i.e. the RS format is ZC. One of the two reference signals would then go through the spectrum shaping filter while the other is unfiltered. The filtered reference signal can be either of these example signals, with the other signal not being filtered.

It should be appreciated that the reference signals will be dependent on the standard in which embodiments are being implemented. Any other suitable reference signal may alternatively or additionally be used.

In some embodiments, the first and second RSs are inserted into the data signal in a time-multiplexed manner, for example by mapping them to different symbols of a DFT-S-OFDM (discrete Fourier transformation spread orthogonal frequency division multiplexing) waveform. In some embodiments, frequency domain pulse shaping may be used.

In some embodiments, the waveform may take any other suitable form, for example CP-OFDM (cyclic prefix OFDM).

In another embodiment, the transmitted filtered data signal is multiplexed with just one RS. The filter is applied to the data signal but not to the RS, and a set of coefficients describing the filter are signalled separately between the transmitter and the receiver. In this embodiment, the data signal and RS may be modulated differently. For example the data signal is BPSK-modulated (for example π/2 BPSK) and the RS comprises a sequence of phase shifts (for example, a Zadoff-Chu sequence). The reference signal(s) in this embodiment can be any of the one or more reference signals discussed previously.

The signalling of the filter coefficients may take place via a physical layer control channel and/or by a higher layer configuration message. In some cases, the filter coefficients may be chosen by the transmitter and signalled to the receiver. The receiver may or may not acknowledge or confirm that the transmitter may use the signalled filter. In other cases, the filter coefficients to be used by the transmitter may be chosen by the receiver and signalled to the transmitter.

A transmitter of an embodiment will now be described with reference to Figure 4. This may be for π/2 BPSK with time domain pulse shaping. Data 412 is provided as an input to a spectrum shaping block 416. The spectrum shaping block has a π/2 constellation block 410, and a filter 408. The filter is a spectrum shaping filter to shape the spectrum. This is to reduce the signal PAPR by shaping the spectrum. The first filter may reduce a peak to average power ratio of transmitted data and/or suppress spectral leakage of the transmitted data.

The spectrum shaping block provides an output to an M-point DFT (discrete Fourier transform) block 406. This block provides an output to a subcarrier mapping block 404. The subcarrier block 404 maps the symbols to sub-carriers.

The subcarrier mapping block 404 provides an output to an IFFT (inverse fast Fourier transform) block 400. The IFFT block converts the signal from the time domain to a representation in the frequency domain. The IFFT block 400 provides an output to a CP (cyclic prefix) block 402 which applies a cyclic prefix. This provides an output to an RF part, not shown, for transmitting to a receiver.

Alternatively or additionally, the filter may be between the M-point DFT block 406 and the subcarrier mapping block 404.

Alternatively or additionally, the filter may be after the subcarrier mapping block 404.

Alternatively or additionally, the filter may be after the IFFT block 400. In the case, where there are two reference signals, the data 412 comprises the data to be transmitted and the first RS which will be processed by the blocks shown in Figure 4. The second reference signal will bypass the spectrum shaping block and is input to the M-point DFT block 406.

In the case where there is only one reference signal which is not filtered, that will bypass the spectrum shaping block and is input to the M-point DFT block 406.

Reference is made to Figure 5 which shows a receiver for receiving signals from the transmitter of Figure 4. Data received from the transmitter is received by an antenna and down converted from the radio frequency. This is not shown.

The received data is then processed to remove the cyclic prefix by the cyclic prefix removal block 502 and output to the FFT block 500. The FFT block converts the signal from the frequency domain to the time domain.

The processed signal output from the FFT block 500 is input to an equaliser 504. This equalizer is a frequency domain channel equalizer that reduces the signal distortion caused by the radio channel.

The output of the equaliser 504 is provide to a M point inverse DFT function

506.

The output of the M point IDFT block 506 is input to a reverse filter block 508. The reverse of the transmit filtering is done here so that the effect of the transmitter side spectrum shaping filter is reversed so that the receiver is able to reproduce the signal that was existing at the input of the transmit side spectrum shaping function. The filter information can be determined by: a) The standard defines the spectrum shaping filter to be used in a particular scenario;

b) Properties of the filter are negotiated between the transmitter and the receiver. (This is as previously discussed)

c) The transmitter sends at least one RS without spectrum shaping and at least one RS with spectrum shaping. The receiver knows which RS is spectrum shaped and which is not, compares the two and derives the filter parameters from that. These filter parameters are used by the block 508.

The output of the reverse filter is input to a constellation phase detection block 510, the output of which provides the recovered data.

Reference is made to Figure 6 which shows a method of one embodiment. In step S1 , first data and a first reference signal are filtered but not a second reference signal. This may be carried out in an apparatus in a transmitting entity. That entity may be a UE or access node such as a base station or the like.

In step S2, the first data, first reference signal and second reference signal are transmitted by the transmitting entity.

In step S3, the first data, first reference signal and second reference signal are received by a receiving entity. That may be the other of a UE or access node such as a base station or the like.

In step S4, an apparatus of the receiving entity compares the first reference signal and second reference signals to determine filter parameters. The step may assume that a filtered signal = unfiltered signal multiplied by a filter. Thus if the known RS without filtering (unfiltered RS) with the known RS with filtering (filtered RS), the filter characteristics can be determined. Similar techniques to those used in channel equalization may be used where the channel is determined from the known RS and then the received signal is multiplied with the inverse of the channel to eliminate the channel's impact.

In step S5, the filter parameters are used to process the received first data. Reference is made to Figure 7 which shows another method of one embodiment. In step T1 , the first data is filtered and not reference signal. In some embodiments, the reference signal is filtered. This may be carried out in an apparatus in a transmitting entity. That entity may be a UE or access node such as a base station or the like.

In step T2, the first data, reference signal and filter coefficients are transmitted by the transmitting entity;

In step T3 the first data, the reference signal and filter coefficients are received by a receiving entity. That may be the other of a UE or access node such as a base station or the like.

In step T4 the filter parameters are used to process the received first data.

The required data processing apparatus and functions may be provided by means of one or more data processors. The described functions may be provided by separate processors or by an integrated processor. The data processors may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASIC), gate level circuits and processors based on multi core processor architecture, as non-limiting examples. The data processing may be distributed across several data processing modules. A data processor may be provided by means of, for example, at least one chip. Appropriate memory capacity can be provided in the relevant devices. The memory or memories may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. One or more of the steps discussed in relation to Figures 6 and 7 may be performed by one or more processors in conjunction with one or more memories.

An appropriately adapted computer program code product or products may be used for implementing the embodiments, when loaded or otherwise provided on an appropriate data processing apparatus. The program code product for providing the operation may be stored on, provided and embodied by means of an appropriate carrier medium. An appropriate computer program can be embodied on a computer readable record medium. A possibility is to download the program code product via a data network. In general, the various embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Embodiments of the inventions may thus be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

It is noted that whilst embodiments have been described in relation to certain architectures, similar principles can be applied to other systems. Therefore, although certain embodiments were described above by way of example with reference to certain exemplifying architectures for wireless networks, technologies and standards, embodiments may be applied to any other suitable forms of communication systems than those illustrated and described herein. It is also noted that different combinations of different embodiments are possible. It is also noted herein that while the above describes exemplifying embodiments of the invention, there are several variations and modifications which may be made to the disclosed solution without departing from the spirit and scope of the present invention.

Claims

1 . A method comprising:

receiving a signal comprising a first reference signal to which a filter has been applied and a second reference signal to which said filter has not been applied; and

using said first and second reference signals to determine information about said filter.

2. A method as claimed in claim 1 , wherein said information comprises filter coefficients.

3. A method as claimed in claim 1 or 2 comprising receiving data to which said filter has been applied.

4. A method as claimed in claim 3, comprising using said information about said filter to reverse the effects of said filter on data.,

5. A method comprising:

causing a signal to be transmitted, said signal comprising a first reference signal to which a filter has been applied and a second reference signal to which said filter has not been applied.

6. A method as claimed in claim 5 comprising transmitting data to which said filter has been applied.

7. A method as claimed in any preceding claim, wherein filter comprises a spectrum shaping filter.

8. A method as claimed in any preceding claim, wherein at least one of said first and second reference signals comprises a BPSK signal.

9. A method as claimed in any preceding claim, wherein at least one of said first and second reference signals comprises a sequence of phase shifts.

10. A method as claimed in claim 9, wherein said sequence of phase shifts comprises a Zadoff-Chu sequence.

1 1 . A method as claimed in claim 3 or 6, or any claim appended thereto, wherein data comprises BPSK data.

12. A method as claimed in claim 3 or 6, or any claim appended thereto, wherein said first reference signal, said second reference signal and said data are time multiplexed.

13. A method as claimed in claim 12, wherein said first reference signal, said second reference signal and said data are provided by different symbols of a

DFT-S-OFDM wave form.

14. An apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to:

receive a signal comprising a first reference signal to which a filter has been applied and a second reference signal to which said filter has not been applied; and

use said first and second reference signals to determine information about said filter.

15. An apparatus as claimed in claim 14, wherein the information comprises filter coefficients.

16. An apparatus as claimed in claim 14 or claim 15, wherein the at least one memory and the computer code are be configured with the at least one processor to cause the apparatus to receive data to which said filter has been applied.

17. An apparatus as claimed in any of claims 14 to 16, wherein the at least one memory and the computer code may be configured with the at least one processor to cause the apparatus to use said information about said filter to reverse the effects of said filter on data.

18. An apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to:

cause a signal to be transmitted, said signal comprising a first reference signal to which a filter has been applied and a second reference signal to which said filter has not been applied.

19. An apparatus as claimed in claim 18, wherein the at least one memory and the computer code are be configured with the at least one processor to cause the apparatus to transmit data to which said filter has been applied.

20. An apparatus as claimed in any of claims 14 to 19, wherein the filter comprises a spectrum shaping filter.

21 . An apparatus as claimed in any of claims 14 to 20, wherein the filter reduces a peak to average power ratio of transmitted data and/or suppress spectral leakage of the transmitted data.

22. An apparatus as claimed in any of claims 14 to 21 , wherein one of said first and second reference signals comprises a BPSK signal.

23. An apparatus as claimed in claim 22, wherein the BPSK signal is a π/2 BPSK signal.

24. An apparatus as claimed in any of claims 14 to 23, wherein one of said first and second reference signals comprises a sequence of phase shifts.

25. An apparatus as claimed in claim 24, wherein the sequence of phase shifts comprises a Zadoff-Chu sequence.

26. An apparatus as claimed in any of claims 16, 17 and 19, wherein the data comprises BPSK data.

27. An apparatus as claimed in any of claims 16, 17 and 19 or any claim appended thereto, wherein the first reference signal, said second reference signal and said data are time multiplexed.

28. An apparatus as claimed in any of claims 16, 17 and 19 or any claim appended thereto, wherein the first reference signal, said second reference signal and said data are provided by different symbols of a DFT-S-OFDM wave form.

29. An apparatus as claimed in any of claims 14 to 28, wherein the information comprises filter coefficients.