WO2019137808A1

WO2019137808A1 - Time and frequency offset estimation using repeated data symbols

Info

Publication number: WO2019137808A1
Application number: PCT/EP2018/086666
Authority: WO
Inventors: Jukka Tapaninen
Original assignee: Nordic Semiconductor Asa
Priority date: 2018-01-11
Filing date: 2018-12-21
Publication date: 2019-07-18
Also published as: GB201800491D0

Abstract

A method of estimating timing and frequency offsets in a radio frequency signal comprising at least one frame including a plurality of sub-frames (4b,d,g,i). Each sub-frame (4b,d,g,i) comprises a plurality of time positions and a plurality of sub- carrier frequencies. A first instance of a data symbol (70a) is received at a first time position (12.2) on a first sub-carrier frequency (14.2). A second instance of the data symbol (70b) is received at the first time position (12.2) on a second sub-carrier frequency (14.8). A third instance of a data symbol is received at a second time position (12.9) on the first sub-carrier frequency (14.2). The first and second instances of the data symbol (70a, 70b) are correlated to obtain a timing offset estimate. The first and third instances of the data symbol (70a, 70c) are correlated to obtain a frequency offset estimate.

Description

TIME AND FREQUENCY OFFSET ESTIMATION USING REPEATED DATA SYMBOLS

Technical Field

The present invention relates to improving estimations of the time and frequency offsets associated with radio communication systems, particularly cellular-based systems such as Long Term Evolution (LTE) systems suited for Internet of Things applications.

Background

Throughout the course of the past few decades, the extent and technical capabilities of cellular-based radio communication systems have expanded dramatically. A number of different cellular-based networks have been developed over the years, including the Global System for Mobile Communications (GSM), General Packet Radio Services (GPRS), Enhanced Data rates for GSM Evolution (EDGE), and Universal Mobile Telecommunications System (UMTS), where GSM, GPRS, and EDGE are often referred to as second generation (or "2G") networks and UMTS is referred to as a third generation (or "3G") network.

More recently, the Long Term Evolution (LTE) network, a fourth generation (or "4G") network standard specified by the 3^rd Generation Partnership Project (3GPP), has gained popularity due to its relatively high uplink and downlink speeds and larger network capacity compared to earlier 2G and 3G networks. More accurately, LTE is the access part of the Evolved Packet System (EPS), a purely Internet Protocol (IP) based communication technology in which both real-time services (e.g. voice) and data services are carried by the IP protocol. However, while "classic" LTE connections are becoming increasingly prevalent in the telecommunications industry, further developments to the communication standard are being made in order to facilitate the so-called "Internet of Things" (loT), a common name for the inter-networking of physical devices, sometimes called "smart devices", providing physical objects that may not have been connected to any network in the past with the ability to communicate with other physical and/or virtual objects. Such smart devices include: vehicles; buildings; household appliances, lighting, and heating (e.g. for home automation); and medical devices. These smart devices are typically real-world objects with embedded electronics, software, sensors, actuators, and network connectivity, thus allowing them to collect, share, and act upon data. These devices may

communicate with user devices (e.g. interfacing with a user's smartphone) and/or with other smart devices, thus providing "machine-to-machine" (or "machine type") communication. However, the development of the LTE standards makes it more practical for them to connect directly to the cellular network.

3GPP have specified two versions of LTE for such purposes in Release 13 of the LTE standard. The first of these is called "NarrowBand loT" (NB-loT), sometimes referred to as "LTE Cat NB1", and the second is called "enhanced Machine Type Communication" (eMTC), sometimes referred to as "LTE Cat M1". It is envisaged that the number of devices that utilise at least one of these standards for loT purposes will grow dramatically in the near future.

From a communications perspective, LTE standards (including NB-loT and eMTC) use orthogonal frequency division multiple access (OFDMA) as the basis for allocating network resources. This allows the available bandwidth to be shared between user equipment (UE) that accesses the network in a given cell, provided by a base station, referred to in LTE as an "enhanced node B", "eNodeB", or simply "eNB". OFDMA is a multi-user variant of orthogonal frequency division multiplexing (OFDM), a multiplexing scheme in which the total bandwidth is divided into a number of non-overlapping sub-bands, each having its own sub-carrier frequency. In OFDM, unlike other frequency division multiplexing (FDM) schemes, each of these sub-carriers are orthogonal to one another such that cross-talk between sub- bands is ideally eliminated, removing the need for inter-carrier guard bands.

In order to achieve this orthogonality, the spacing Af between the sub-carriers is k

set such that D/ =— , where T_u is the "useful symbol duration" (the receiver-side

^Tu

window size) and k is a positive integer (and is usually set to 1 ). Therefore with N sub-carriers, the total bandwidth B can be expressed as B = NAf. These sub- carriers are then shared between multiple users, thus providing multiple access. At the physical layer, in the downlink of an LTE connection, each data frame is 10 ms long and is constructed from ten sub-frames, each of 1 ms duration. Each sub-frame contains two slots of equal length, i.e. two 0.5 ms slots. Each slot (and by extension, each sub-frame and each frame) will typically contain a certain number of "resource blocks" (where each sub-frame has twice as many resource- blocks as a slot and each frame has ten times as many resource blocks as a sub- frame). A resource block is 0.5 ms long in the time domain and is twelve sub- carriers wide in the frequency domain. Generally speaking, there are seven OFDM symbols per slot and thus fourteen OFDM symbols per sub-frame. These resource blocks can be visualised as a grid of "resource elements", where each resource element is 1/14 ms long and one sub-carrier wide, such that there are eighty-four resource elements per resource block (i.e. seven multiplied by twelve) and one hundred and sixty-eight resource elements per sub-frame.

It will be appreciated that, generally speaking, the physical layer in LTE supports two different cyclic prefix lengths, normal and extended. If using the normal cyclic prefix mode, there are seven OFDM symbols per slot as outlined above. If using the extended cyclic prefix mode, there are only six OFDM symbols per slot - however, extended cyclic prefix mode is generally not employed in practical network deployments and in NB-loT only normal cyclic prefix is supported.

The exact number of resource blocks that exist in each slot (and by extension, each sub-frame and each frame) depends on the bandwidth configuration of the radio communication system. For example, in LTE eMTC Release 13, the LTE radio channel may have a bandwidth of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, or 20 MHz, where the channel is divided up into narrowbands of 1.4 MHz width.

As LTE eMTC and NB-loT are intended to operate in extremely low signal-to-noise ratio (SNR) conditions, the OFDM data symbols are typically repeated in a number of sub-frames. For example, in eMTC, each sub-frame may be repeated identically up to 2048 times. Thus, in conventional LTE systems known in the art per se, a given data symbol may be repeated numerous times across different sub-frames. NB-loT, for example, provides for repetitions of complex (IQ) symbols to be combined prior to detection. This is used in narrowband physical downlink control channel (NPDCCH) and narrowband physical downlink shared channel (NPDSCH) transmission.

In practical LTE systems, a local oscillator is used for down-conversion of incoming radio frequency (RF) signals to a baseband frequency or to an intermediate frequency. However, if this local oscillator is not properly synchronised with the carrier signal, a carrier frequency offset can occur. This often arises due to a mismatch between the frequencies in use by the transmitter and the receiver, but can also arise due to the Doppler effect when the transmitter and/or the receiver is moving. It is important to be able to estimate and compensate for this carrier frequency offset in order to carry out LTE communications effectively.

Another important parameter to estimate in LTE is the timing offset. The timing at which incoming symbols are expected to be received is critical for being able to demodulate the received signal properly. Knowing the timing offset is also important to ensure that the timings for particular transmissions and/or receptions are synchronised between the transmitter and receiver so as to prevent

transmissions occurring at incorrect times. A timing offset can arise, for example, due to the distance between the transmitter and the receiver. In LTE, a UE that is farther away from the eNodeB will experience a greater timing offset than a UE that is nearer to the eNodeB.

Summary of the Invention

When viewed from a first aspect, the present invention provides a method of estimating timing and frequency offsets in a radio frequency signal comprising at least one frame including a plurality of sub-frames, each sub-frame comprising a plurality of time positions and a plurality of sub-carrier frequencies, wherein the method comprises:

receiving a first instance of a data symbol at a first time position of the sub- frame on a first sub-carrier frequency of the sub-frame;

receiving a second instance of the data symbol at the first time position of the sub-frame on a second sub-carrier frequency of the sub-frame;

receiving a third instance of a data symbol at a second time position of the sub-frame on the first sub-carrier frequency of the sub-frame; correlating the first instance of the data symbol with the second instance of the data symbol to obtain a timing offset estimate; and

correlating the first instance of the data symbol with the third instance of the data symbol to obtain a frequency offset estimate.

This first aspect extends to a radio receiver device arranged to estimate timing and frequency offsets in a radio frequency signal comprising at least one frame including a plurality of sub-frames, each sub-frame comprising a plurality of time positions and a plurality of sub-carrier frequencies, wherein the radio receiver device is arranged to:

receive a first instance of a data symbol at a first time position of the sub- frame on a first sub-carrier frequency of the sub-frame;

receive a second instance of the data symbol at the first time position of the sub-frame on a second sub-carrier frequency of the sub-frame;

receive a third instance of a data symbol at a second time position of the sub-frame on the first sub-carrier frequency of the sub-frame;

correlate the first instance of the data symbol with the second instance of the data symbol to obtain a timing offset estimate; and

correlate the first instance of the data symbol with the third instance of the data symbol to obtain a frequency offset estimate.

When viewed from a second, complimentary aspect, the present invention provides a method of transmitting a radio frequency signal comprising at least one frame including a plurality of sub-frames, each sub-frame comprising a plurality of time positions and a plurality of sub-carrier frequencies, wherein the method comprises: transmitting a first instance of a data symbol at a first time position of the sub-frame on a first sub-carrier frequency of the sub-frame;

transmitting a second instance of the data symbol at the first time position of the sub-frame on a second sub-carrier frequency of the sub-frame; and

transmitting a third instance of a data symbol at a second time position of the sub-frame on the first sub-carrier frequency of the sub-frame.

This second aspect extends to a radio transmitter device arranged to transmit a radio frequency signal comprising at least one frame including a plurality of sub- frames, each sub-frame comprising a plurality of time positions and a plurality of sub-carrier frequencies, wherein the radio transmitter device is arranged to:

transmit a first instance of a data symbol at a first time position of the sub- frame on a first sub-carrier frequency of the sub-frame;

transmit a second instance of the data symbol at the first time position of the sub-frame on a second sub-carrier frequency of the sub-frame; and

transmit a third instance of a data symbol at a second time position of the sub-frame on the first sub-carrier frequency of the sub-frame.

When viewed from a third, complimentary aspect, the present invention provides a radio communication system comprising a radio transmitter device and a radio receiver device, wherein the radio transmitter device is arranged to:

transmit a third instance of a data symbol at a second time position of the sub-frame on the first sub-carrier frequency of the sub-frame;

and wherein the radio receiver device is arranged to:

receive the first, second, and third instances of the data symbol;

Those skilled in the art will appreciate that embodiments of the present invention provide an improved method of transmitting and receiving data symbols that can be used to estimate both the frequency offset and the timing offset, where the symbols used for the timing offset are all contained within the same sub-frame. Thus, in accordance with such embodiments, it is possible to obtain the timing and frequency offset estimates with a first pair of instances of the data symbol separated in frequency (the first and second instances of the data symbol) and a second pair of instances of the data symbol separated in time (the first and third instances of the data symbol), where these pairs are all located within the same sub-frame. By correlating repeated instances of the same symbol, symbol polarity (i.e. the 'sign' of the symbol) is removed because correlating the symbol with a repetition of itself involves multiplying it by its own complex conjugate, i.e. the phase information is removed. The Applicant has appreciated that embodiments of the present invention advantageously make it possible to utilise data symbols for time and frequency offset estimation instead of relying on the use of known pilot symbols (i.e. where the receiver relies on using a predetermined symbol transmitted at a particular time and on a particular frequency to make estimates).

Any timing offset will cause a linear phase shift as a function of the sub-carrier index. By correlating repeated data symbols that are separated in frequency, this linear phase shift, and therefore the timing offset, can be determined. Conversely, a frequency offset is determined by analysing the phase shift between repeated data symbols that are separated in time on the same sub-carrier frequency.

The Applicant has appreciated that this approach is particularly advantageous as it does not necessarily rely on data symbols being repeated across consecutive sub- frames which can be problematic because certain sub-frames typically have particular protocol-specific roles (e.g. providing control commands) and so it is rare in most circumstances to have consecutive sub-frames that are able to support retransmissions as the repetitions would typically take place at a later sub-frame rather than the adjacent sub-frame.

Having a spacing of one or more sub-frames between the instances of the data symbols that are combined to obtain the frequency offset estimates reduces the frequency offset estimation range - for example, the range may be limited to a Nyquist rate of 500 Hz with one sub-frame separation between repetitions.

However, by having the repetitions within the sub-frame, in accordance with embodiments of the present invention, the frequency offset estimate range is improved - for example, the range may be extended to a Nyquist rate of 1000 Hz with one slot (i.e. half a sub-frame) separation between the repetitions.

The Applicant has appreciated that having the repetitions of the data symbol within the sub-frame may also result in the required estimates of timing and frequency offsets being obtained more quickly than with conventional approaches, because these estimates may be obtained in a single sub-frame. This may be beneficial for having the receiver able to decode transmissions more quickly than with conventional approaches.

The approach outlined by the Applicant aids in maintaining synchronisation when repetition across multiple sub-frames is not suitable, e.g. after a long transmission. This approach also reduces the risk of residual timing and/or frequency offsets that may degrade data reception performance.

While it is possible to obtain the timing and frequency offset estimates with the first pair of instances of the data symbol separated in frequency (the first and second instances of the data symbol) and the second pair of instances of the data symbol separated in time (the first and third instances of the data symbol), in some embodiments the sub-frame further comprises a fourth instance of the data symbol at the second time position of the sub-frame and on the second sub-carrier frequency of the sub-frame. This fourth instance can then be combined with the other three instances of the data symbol to yield further timing and/or frequency offset estimates or to enhance the timing and frequency offset estimates made using the first, second, and third instances of the data symbol. For example, in some embodiments the receiver is arranged to correlate the second instance of the data symbol with the fourth instance of the data symbol to obtain a further frequency offset estimate. In some potentially overlapping embodiments, the receiver is arranged to correlate the third instance of the data symbol with the fourth instance of the data symbol to obtain a further timing offset estimate.

Those skilled in the art will appreciate that embodiments of the present invention may be readily applied to a variety of radio communication protocols known in the art per se. However, the Applicant has appreciated that embodiments of the present invention are particularly advantageous for LTE applications, including "classic" LTE as well as LTE loT protocols such as NB-loT and eMTC. Thus, in some embodiments, the frame comprises an LTE frame and each sub-frame is an LTE sub-frame, wherein each data symbol comprises an OFDM symbol.

There are many suitable mappings between the repeated instances of the data symbol that may be used in order to allow the frequency and timing offset estimates to be made in accordance with embodiments of the present invention. For example, the frequency separation between the first and second instances of the data symbol (i.e. the separation between the first and second sub-carrier frequencies) could be any suitable separation. In other words, so long as the first and second instances are on different sub-carriers within the sub-frame, it will be possible to acquire the timing offset estimate.

Similarly, the timing separation between the first and third instances of the data symbol (i.e. the separation between the first and second timing positions) may take any suitable value; so long as the first and third instances take place at different time positions within the sub-frame, it will be possible to acquire the frequency offset estimate.

However, in some embodiments a frequency separation between the first and second sub-carrier frequencies is equal to half of the number of the plurality of sub- carrier frequencies of the sub-frame. In some potentially overlapping embodiments, a timing separation between the first and second timing positions is equal to half of the number of the plurality of time positions of the sub-frame. This may be visualised as dividing the sub-frame resource grid into four equal quadrants, where each instance of the repeated data symbol appears in a different quadrant, but at the same relative position within its respective quadrant as the other instances of the repeated data symbol. If N is the number of time positions and M is the number of sub-carrier frequencies, then a data symbol at the x^th time position at the y^th sub- carrier frequency, i.e. at co-ordinates (x, y), will have repeats at (x+N/2, y) and (x, y+M/2). A repeat may also be located at (x+N/2, y+M/2) in embodiments where a fourth instance of the data symbol is provided.

For example, in LTE each sub-frame comprises a grid of twelve sub-carriers by fourteen time positions (i.e. two consecutive LTE resource blocks) and so each quadrant would be a grid of six sub-carriers by seven time positions, i.e. N = 14. In the set of embodiments where each instance of the repeated data symbol appears in a different quadrant, but at the same relative position within its respective quadrant, this would result in a particular data symbol at the second time position on the third sub-carrier frequency, i.e. having co-ordinates (2, 3), having repeats at (8, 3) and (2, 10), and optionally at (8, 10). As explained above, each resource element may be used to carry OFDM symbols over the Evolved UMTS Terrestrial Radio Access (E-UTRA) air interface. Typically this is to carry data, however this is not the only purpose served by these resource elements. In addition to conveying data, these resource elements may be used to carry "common reference symbols" (CRS), where these CRS are used to aid demodulation. In noisy conditions it may be necessary to average or filter a number of these CRS in order to obtain a suitable "channel estimate", i.e. an estimate of the characteristics of the channel such as attenuation, phase shifts, and noise.

These CRS, or averaged CRS, are then used to aid the demodulation of the OFDM data symbols. In general, the averaging of the CRS should be carried out using a symmetrical sliding window in both time and frequency, i.e. CRS from the past and the future relative to a given data symbol should be used to demodulate the data symbol (e.g. an average of the CRS received in the previous five and following five resource elements relative to a resource element containing the data to be demodulated). The positions of the CRS may limit which positions within the resource grid are available for mapping repetitions, particularly if the quadrants are equal in size. However, in accordance with some embodiments of the invention, the mapping of the repeated instances of the data symbol may be chosen so as to avoid disrupting the CRS and to make use of the remaining resource elements that are available for repetitions.

Brief Description of Drawings

Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is a timing diagram of a typical LTE NB-loT frame;

Fig. 2 is a diagram showing the resource block structure of a typical standalone mode NB-loT sub-frame;

Fig. 3 is a diagram showing the resource block structure of a typical in-band mode NB-loT sub-frame;

Fig. 4 is a block diagram of an LTE receiver in accordance with an embodiment of the present invention;

Fig. 5 is a diagram showing the resource block structure of a standalone mode NB-loT sub-frame received by the receiver of Fig. 4; and Fig. 6 is a diagram showing the resource block structure of an in-band mode NB-loT sub-frame received by the receiver of Fig. 4.

Detailed Description

Fig. 1 is a timing diagram of a typical LTE NB-loT frame 2. The data frame 2 is 10 ms long and is constructed from ten sub-frames 4a-j, each of 1 ms duration.

The data frame 2 is followed by another data frame 6 which is also constructed from ten sub-frames (not shown). Those skilled in the art will appreciate that these data frames 4, 6 are just two examples and that in practical radio communication systems, there will be many more data frames, one after another.

Each sub-frame 4a-j within the NB-loT data frame 2 has a specific role associated with it, known in the art per se, that are outlined briefly below.

The first sub-frame 4a is a narrowband physical broadcast channel (NPBCH) sub- frame that carries the master information block (MIB). The MIB contains thirty-four bits and is transmitted over a time period of 640 ms, i.e. 64 radio frames. The MIB provides: the system frame number; hyper frame number; the scheduling and size of the system information block (discussed in further detail below); the system information value tag; a flag indicating whether access class barring is applied; and flags indicating the operation mode with any mode specific values.

The second sub-frame 4b, fourth sub-frame 4d, seventh sub-frame 4g, and ninth sub-frame 4i are narrowband physical downlink shared channel (NPDSCH) sub- frames. An NPDSCH sub-frame is arranged to carry data as well as paging messages, system information, and/or a random access request (RAR) message.

The third sub-frame 4c and eighth sub-frame 4h are invalid sub-frames and cannot be used.

The fifth sub-frame 4e is a narrowband system information block (NSIB) sub-frame that carries the system information block (SIB). There are several types of SIB messages including SIB1 , SIB2, and SIB3. The SIB1 message is transmitted with a fixed periodicity of 20ms on the fifth sub-frame 4e. Because the period and sub- frame location are known to the UE a priori, the UE will look for the physical downlink control channel (PDCCH) and system information radio network temporary identifier (SI-RNTI) needed for gaining access to the network on that particular radio frame and sub-frame combination. After the UE decodes SIB1 , it can determine scheduling information for other SIB messages which is included within the SIB1 message. SIB2 messages are used to provide the UE with information about individual physical layer channel capabilities, configuration etc.

The sixth sub-frame 4f is a narrowband primary synchronisation signal (NPSS) sub-frame which is used by the UE to perform cell searching operations, including time synchronisation, frequency synchronisation, and cell identity detection.

Those skilled in the art will appreciate that the roles of some of these sub-frames 4a-j may vary in practical systems, depending on the exact configuration in use.

In order to determine estimates of any frequency offset, a conventional receiver may make use of repeated data symbols present within multiple NPDSCH sub- frames, e.g. in at least two of the second sub-frame 4b, fourth sub-frame 4d, seventh sub-frame 4g, and ninth sub-frame 4i. However, as it can be seen, in this configuration, there are no two NPDSCH sub-frames 4b,d,g,i that are consecutive, and thus there is always at least one sub-frame between repeats of a given data symbol. While it may be possible in some configurations to have a small number of consecutive NPDSCH sub-frames, in general this is severely limited due to certain sub-frames having fixed roles, e.g. the NPBCH and NSIB sub-frames.

Fig. 2 is a diagram showing the resource block structure of a typical standalone mode NB-loT NPDSCH sub-frame 4b,d,g,i. Those skilled in the art will appreciate that the term "standalone mode" used in relation to NB-loT means that the NB-loT band is outside of the bandwidth of any particular LTE carrier.

In general, each sub-frame 4a-j (including those sub-frames 4a,c,e,f,g,h,j that are not NPDSCH sub-frames 4b,d,g,i) contains two slots 10a, b of equal length, i.e. two 0.5 ms slots 10a, b. Each slot 10a, b (and by extension, each sub-frame 4a-j and each frame 2) will typically contain a certain number of "resource blocks" 8a, b (where each sub-frame 4a-j has twice as many resource-blocks as a slot 10a, b and each frame 2 has ten times as many resource blocks as a sub-frame 4a-j).

A resource block 8a, b is 0.5 ms long in the time domain, divided into seven time positions 12 of duration 1/14 ms, and is twelve sub-carriers 14 wide in the frequency domain. Generally speaking, there are seven OFDM symbols per slot (i.e. one per time position 12) and thus fourteen OFDM symbols per sub-frame 4a-j. These resource blocks 8a, b can be visualised as a grid of "resource elements" 16, where each resource element 16 is one time position 12 (1/14 ms) long and one sub-carrier 14 wide, such that there are eighty-four resource elements 16 per resource block 8a, b (i.e. seven multiplied by twelve) and one hundred and sixty- eight resource elements 16 per sub-frame 4a-j.

In standalone mode, the NPDSCH sub-frames 4b,d,g,i are arranged to carry data symbols 18 and narrowband reference signals (NRS) 20, 22. There are two types of NRS 20, 22: "antenna port 0" NRS 20; and "antenna port 1" NRS 22. The LTE standard set out by 3GPPP defines what are known as "antenna ports". Briefly, these antenna ports do not necessarily correspond to physical antennas, but are logical entities that are distinguished by their respective reference signal sequences. Multiple antenna port signals can be transmitted via a single physical antenna. Conversely, a single antenna port can be spread across multiple physical antennas. There are a number of these NRS 20, 22 per sub-frame 4b,d,g,i but their locations within the sub-frame 4b,d,g,i (i.e. the specific resource elements 16 that they are located at) may vary in practice depending on the configuration in use.

In standalone mode, any resource elements 16 that are not used for NRS 20, 22 are arranged to carry data symbols 18. In general, the actual content of a particular data symbol 18 in an NPDSCH sub-frame 4b,d,g,i may be repeated in a

subsequent NPDSCH sub-frame 4b,d,g,i, either later within the same sub-frame 2 or in a subsequent sub-frame 6.

Fig. 3 is a diagram showing the resource block structure of a typical in-band mode NB-loT sub-frame, where like reference numerals indicate like elements to those described above with reference to Figs. 1 and 2. Those skilled in the art will appreciate that the term "in-band mode" used in relation to NB-loT means that the NB-loT band is within the bandwidth of an LTE carrier.

As before, each sub-frame 4a'-j' contains two slots 10a', b' of equal length, i.e. two 0.5 ms slots 10a', b'. Similarly, each resource block 8a', b' is 0.5 ms long in the time domain, divided into seven time positions 12' of duration 1/14 ms, and is twelve sub-carriers 14' wide in the frequency domain.

Similarly to the standalone mode, in the in-band mode the NPDSCH sub-frames 4b',d',g',i' are arranged to carry data symbols 18' and narrowband reference signals (NRS) 20', 22'. However, in addition to these, the NPDSCH sub-frames 4b',d',g',i' also carry control symbols 24' and CRS 26' when operating in the NB-loT in-band mode.

The actual content of any particular data symbol 18' in an NPDSCH sub-frame 4b',d',g',i' may be repeated in a subsequent NPDSCH sub-frame 4b',d',g',i', either later within the same sub-frame 2' or in a subsequent sub-frame 6'.

Fig. 4 is a block diagram of an LTE receiver 28 in accordance with an exemplary embodiment of the present invention. The LTE receiver 28 comprises: an analogue/digital front-end 30; a fast Fourier transform module 32; a resource element de-mapping module 34; an equaliser 36; a demodulation module 38; a descrambling module 40; a rate de-matching and hybrid automatic repeat request (HARQ) module 42; a decoding module 44; a cyclic redundancy check (CRC) module 46; a synchronisation module 48; a time tracking module 50; a channel estimation module 52; an automatic frequency control (AFC) module 54; an automatic gain control (AGC) module 56; and memory 58.

The analogue/digital front-end 30 is arranged to receive incoming signals 60 that are received via an upstream antenna 62. The incoming signals 60 received via the upstream antenna 62 are input to the fast Fourier transform module 32 and the synchronisation module 48. The synchronisation module 48 extracts coarse timing information, AFC information, and AGC information from the incoming signals 60 and outputs this information as a further signal 64 which may, for example, be fed back to the front-end module 30 to control gain, timing, and frequency offsets during synchronisation.

The FFT 30 converts the incoming signals 60 from the time domain to the frequency domain in a manner well known in the art per se. The FFT 30 also takes as an input a signal produced by the time tracking module 50 as will be explained in further detail below.

The frequency domain signal produced by the FFT 30 is input to the de-mapping module 34 which de-maps the data symbols from the time-frequency grid to a single stream suitable for further processing. The structure of the time-frequency grid is described in further detail below with reference to Figs. 5 and 6. The symbol stream produced by the de-mapping module 34 is input to the equaliser 36 which balances the amplitudes and phases of the different frequency components of the symbol stream.

The de-mapping module 34 also produces a feedback output signal that is input to the time tracking module 50, the channel estimator 52, the AFC module 54, and the AGC module 56.

Once the equaliser 36 has balanced the frequency components of the symbol stream, it produces an output that is fed to the demodulation module 38. Using a known modulation/demodulation scheme, the demodulation module 38

demodulates the symbol steam received from the equaliser 38 to produce a baseband signal suitable for input to the de-scrambling module 40. The

de-scrambling module 40 restores the signal produced by the demodulation module 38 into an intelligible form suitable for decoding. However, before inputting the de-scrambled signal to the decoding module 44, it is passed through the rate de-matching and HARQ module 42.

The rate de-matching and HARQ module 42 carries out a de-puncturing algorithm that reverses the puncturing steps carried out by the transmitter. Those skilled in the art will appreciate that puncturing is a process carried out by the transmitter wherein only certain coded bits are selected for transmission while the others are discarded. The receiver 28 has dummy bits stored in the memory 58 that it can insert at the points of the de-scrambled signal where information was discarded by the transmitter. The decoding module 44 then decodes the de-punctured signal and inputs the decoded data signal to the CRC module 46 which performs error correction in order to produce the output signal 66.

The AFC module 54 and AGC module 56 provide feedback signals to the analogue/digital front-end 30 that the analogue/digital front-end 30 uses to control the reference frequency and gain respectively.

Fig. 5 is a diagram showing the resource block structure of a standalone mode NB- loT sub-frame that is received by the receiver 28 of Fig. 4, in accordance with an embodiment of the present invention.

In standalone mode, the NPDSCH sub-frames 4b,d,g,i each comprise two 0.5 ms slots 10a, b as discussed previously. As described with reference to Fig. 2, any resource elements 16 that are not used for NRS 20, 22 are arranged to carry data symbols 18.

However, unlike the structure of a conventional standalone mode NPDSCH sub- frame, the NPDSCH sub-frames 4b,d,g,i are divided into four quadrants 68a-d. A data symbol 70a, located in the first quadrant 68a, is repeated three times 70b-d in the other three quadrants 68b-d. In this example, the mapping between the repeated data symbols 70a-d is such that each repeat of the data symbol 70a-d is in the same relative position within its respective quadrant 68a-d. In other words, the data symbol 70a-d is located at (2, 2) within the local co-ordinate system of each respective quadrant 68a-d.

The quadrants 68a-d are equally sized and therefore the frequency separation between the data symbol 70a in the first quadrant 68a and the repeated data symbol 70b in the second quadrant 68b is six sub-carriers (half of the total number of sub-carriers 14, i.e. half of twelve) so that they are located on carriers 14.2 and 14.8 respectively. The first data symbol 70a and the second data symbol 70b are at the same time position 12.2. In other words, the first data symbol 70a is located at (2, 2) within the co-ordinate system of the whole sub-frame 4b,d,g,i and the second data symbol 70b is located at (2, 2 + 6) = (2, 8) within the co-ordinate system of the whole sub-frame 4b,d,g,i

Similarly, the time separation between the data symbol 70a in the first quadrant 68a and the repeated data symbol 70c in the third quadrant 68c is seven time positions (half of the total number of time positions 12, i.e. half of fourteen) so that it is located at time position 12.9. The first data symbol 70a and the third data symbol 70c are on the same sub-carrier 14. Thus the third data symbol 70d is located at (2 + 7, 2) = (9, 2) within the co-ordinate system of the whole sub-frame 4b,d,g,i

The repeated data symbol 70d in the fourth quadrant 68d is separated: from the second data symbol 70b in frequency by six sub-carriers 14; from the third data symbol 70c in time by seven time positions 12; and from the first data symbol 70a in frequency by six sub-carriers 14 and in time by seven time positions 12. The fourth data symbol 70d is therefore on the same sub-carrier 14.8 as the second data symbol 70b and at the same time position 12.9 as the third data symbol 70c. In other words, the fourth data symbol 70d is located at (2 + 7, 2 + 7) = (9, 9) within the co-ordinate system of the whole sub-frame 4b,d,g,i.

The method by which these repeated symbols can be used to recover estimates of time and frequency offset will now be described.

A Fourier transform is expressed as per Equation 1 below: x(t) X(f)

Equation 1: Fourier transform where x(t) is a time-domain signal and X(f) is a frequency domain signal.

The Fourier transform of a time-shifted signal is expressed as per Equation 2 below: x(t + At) <® X(f)e^]2nM

Equation 2: Fourier transform of a timing offset where At is the timing offset and j is the imaginary unit.

Similarly, the Fourier transform of a frequency-shifted signal is expressed as per Equation 3 below: x(t)e^]2nAt <® X(f + Af)

Equation 3: Fourier transform of a frequency offset

In order to determine the current timing offset, the receiver 28 performs cross- correlation between two data symbols that are at the same time position 12 that are separated in frequency, e.g. the first data symbol 70a and the second data symbol 70b (or, equivalently, between the third data symbol 70c and the fourth data symbol 70d).

Cross-correlation of time domain elements separated in frequency by Af yields an estimate of the timing offset as per Equation 4 below:

Equation 4: Cross correlation of time domain signals separated in frequency

Conversely, in order to determine the current frequency offset, the receiver 28 performs cross-correlation between two data symbols that are on the same sub- carrier 14 that are separated in time, e.g. the first data symbol 70a and the third data symbol 70c (or, equivalently, between the second data symbol 70b and the fourth data symbol 70d). Cross-correlation of frequency domain elements separated in time by At yields an estimate of the frequency offset as per Equation 5 below:

Equation 5: Cross correlation of frequency domain signals separated in time Fig. 6 is a diagram showing the resource block structure of an in-band mode

NB-loT sub-frame that is received by the receiver 28 of Fig. 4 in accordance with an embodiment of the present invention, wherein like reference numerals indicate like components to those described above with reference to Fig. 5.

Like in the standalone mode, in the in-band mode the NPDSCH sub-frames

4b',d',g',i' each comprise two 0.5 ms slots 10a', b'. However, as described with reference to Fig. 3, the NPDSCH sub-frames 4b',d',g',i' also carry control symbols 24' and CRS 26'. While the CRS 26' are in the same relative positions within each quadrant 68a'-d', the control symbols 24' are only located in the first quadrant 68a' and second quadrant 68b'.

The existence of these control symbols 24' (which are required by the standard) means that the corresponding positions 72' within the third quadrant 68c' and fourth quadrant 68d' cannot be used for repeated data symbols when the mapping scheme being used is to position repeats in the same relative positions within each quadrant 68a'-d'. However, the remaining positions 18' that do not carry control symbols 24' or CRS 26' can be used to carry repeated data symbols 70a'-d' in the same relative positions within each quadrant 68a'-d'.

The first and second data symbols 70a', b' thus have the same time position 12.4'; and the third and fourth data symbols 70c', d' also have the same time position 12.1 T. Similarly, the first and third data symbols 70a', c' are on the same

sub-carrier 14.4'; and the second and fourth data symbols 70b', d' are on the same sub-carrier 14.10'.

The positions 72' within the third quadrant 68c' and fourth quadrant 68d' that cannot be used for repeated data symbols may instead carry non-repeated data. However, the resource elements at these positions 72' may carry data symbols that are repeated at the same relative positions within only the third quadrant 68c' and fourth quadrant 68d', i.e. at the same time position 12' and on different sub-carriers 14'. This will at least allow estimations of the timing offset to be made by cross correlating the respective data symbols, even if no estimate of the frequency offset can be made from such a pairing.

While the mapping scheme in the embodiments described in detail has the repeated data symbols located at the same relative positions 70a-d, 70a'-d' within each respective quadrant 68a-d, 68a'-d', other mapping schemes where this is not the case could be used instead. However, separating pairs of repeated data symbols by a consistent amount makes combining them easier, as no further normalisation steps are required in order to determine which symbols are repetitions of one another and should be correlated to determine the timing and/or frequency offsets.

Thus it will be appreciated by those skilled in the art that embodiments of the present invention provide an improved method of transmitting and receiving data symbols that can be used to estimate both the frequency offset and the timing offset, where the symbols used for the timing offset are all contained within the same sub-frame. Those skilled in the art will appreciate that the specific embodiments described herein are merely exemplary and that many variants within the scope of the invention are envisaged.

Claims

1. A method of estimating timing and frequency offsets in a radio frequency signal comprising at least one frame including a plurality of sub-frames, each sub- frame comprising a plurality of time positions and a plurality of sub-carrier frequencies, wherein the method comprises:

receiving a third instance of a data symbol at a second time position of the sub-frame on the first sub-carrier frequency of the sub-frame;

correlating the first instance of the data symbol with the second instance of the data symbol to obtain a timing offset estimate; and

2. The method as claimed in claim 1 , wherein the sub-frame further comprises a fourth instance of the data symbol at the second time position of the sub-frame and on the second sub-carrier frequency of the sub-frame.

3. The method as claimed in claim 2, further comprising correlating the second instance of the data symbol with the fourth instance of the data symbol to obtain a further frequency offset estimate.

4. The method as claimed in claim 2 or 3, further comprising correlating the third instance of the data symbol with the fourth instance of the data symbol to obtain a further timing offset estimate.

5. The method as claimed in any preceding claim, wherein the frame comprises an LTE frame and each sub-frame is an LTE sub-frame, wherein each data symbol comprises an OFDM symbol.

6. The method as claimed in any preceding claim, wherein a frequency separation between the first and second sub-carrier frequencies is equal to half the plurality of sub-carrier frequencies of the sub-frame.

7. The method as claimed in any preceding claim, wherein a timing separation between the first and second timing positions is equal to half the plurality of time positions of the sub-frame.

8. A radio receiver device arranged to estimate timing and frequency offsets in a radio frequency signal comprising at least one frame including a plurality of sub- frames, each sub-frame comprising a plurality of time positions and a plurality of sub-carrier frequencies, wherein the radio receiver device is arranged to:

9. A method of transmitting a radio frequency signal comprising at least one frame including a plurality of sub-frames, each sub-frame comprising a plurality of time positions and a plurality of sub-carrier frequencies, wherein the method comprises:

transmitting a first instance of a data symbol at a first time position of the sub-frame on a first sub-carrier frequency of the sub-frame;

transmitting a third instance of a data symbol at a second time position of the sub- frame on the first sub-carrier frequency of the sub-frame.

10. The method as claimed in claim 9, further comprising transmitting a fourth instance of the data symbol at the second time position of the sub-frame on the second sub-carrier frequency of the sub-frame.

11. The method as claimed in claim 9 or 10, wherein the frame comprises an LTE frame and each sub-frame is an LTE sub-frame, wherein each data symbol comprises an OFDM symbol.

12. The method as claimed in any of claims 9 to 11 , wherein a frequency separation between the first and second sub-carrier frequencies is equal to half the plurality of sub-carrier frequencies of the sub-frame.

13. The method as claimed in any of claims 9 to 12, wherein a timing separation between the first and second timing positions is equal to half the plurality of time positions of the sub-frame.

14. A radio transmitter device arranged to transmit a radio frequency signal comprising at least one frame including a plurality of sub-frames, each sub-frame comprising a plurality of time positions and a plurality of sub-carrier frequencies, wherein the radio transmitter device is arranged to:

15. A radio communication system comprising a radio transmitter device and a radio receiver device, wherein the radio transmitter device is arranged to:

transmit a third instance of a data symbol at a second time position of the sub-frame on the first sub-carrier frequency of the sub-frame; and wherein the radio receiver device is arranged to:

receive the first, second, and third instances of the data symbol;