COMMUNICATION SYSTEM

TECHNICAL FIELD

Embodiments herein relate to a transmitting device, a receiving device and methods performed therein. Furthermore, a computer program and a computer readable storage medium are also provided herein. In particular, embodiments herein relate to enable communication between the transmitting device and the receiving device, in a wireless communication network.

BACKGROUND

In a typical wireless communication network, wireless devices, also known as wireless communication devices, mobile stations, stations (STA) and/or user equipments (UE), communicate via a Radio Access Network (RAN) to one or more core networks (CN). The RAN covers a geographical area and provide radio coverage over service areas or cells, which may also be referred to as a beam or a beam group, with each service area or beam being served or controlled by a radio network node such as a radio access node e.g., a Wi-Fi access point or a radio base station (RBS), which in some networks may also be denoted, for example, a "NodeB" or "eNodeB". The radio network node communicates over an air interface operating on radio frequencies with the wireless device within range of the radio network node.

A Universal Mobile Telecommunications network (UMTS) is a third generation (3G) telecommunications network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several radio network nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural radio network nodes connected thereto. This type of connection is sometimes referred to as a backhaul connection. The RNCs and BSCs are typically connected to one or more core networks.

Specifications for the Evolved Packet System (EPS), also called a Fourth

Generation (4G) network, have been completed within the 3 ^rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases, for example to specify a Fifth Generation (5G) network also referred to as New Radio (NR). The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E- UTRAN/LTE is a variant of a 3GPP radio access network wherein the radio network nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of an RNC are distributed between the radio network nodes, e.g. eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially "flat" architecture comprising radio network nodes connected directly to one or more core networks, i.e. they are not connected to RNCs. To compensate for that, the E- UTRAN specification defines a direct interface between the radio network nodes, this interface being denoted the X2 interface.

Industrial communications have stringent latency and/or reliability requirements, in particular ultra-reliable low latency communications (URLLC), require determinism, short packet sizes, low latency, and high reliability, see M. Luvisotto et al. Ultra High

Performance Wireless Control for Critical Applications: Challenges and Directions. IEEE Trans on Industrial Informatics, 2016, issue 99. Determinism means that updated sensing or control data must reach a receiving device, such as a wireless device, at precise time instants. One way to achieve determinism is by introducing a synchronized time division multiple access (TDMA) structure, where time is divided into time-slots and the receiving devices know approximately when to expect a packet. In URLLC the messages in industrial applications are often very short, with a payload in the order of 100 bits or smaller. Wireless devices such as sensors and actuators typically produce/consume data at rates varying from 0.1 Hz to 10 ⁵ Hz so the URLLC needs low latency of the

communications and as URLLC often involve the transmission of safety critical messages whose loss could cause serious harm to people or equipment, the packets should be delivered with low delay and high reliability.

Hence, the design of signals with low overhead is important and beneficial in URLLC since that will shorten the packets and in several applications the packets and/or time-slots may need to be as short as a few με. Fine time synchronization is often necessary in time-synchronized Orthogonal Frequency Division Multiplexing (OFDM), because variations in the propagation delay and multi-path propagation introduce timing inaccuracies that must be compensated at the receiver in order to obtain good

performance.

One method to achieve a low overhead in time-synchronized OFDM signals is to add a cyclic prefix (CP) that is correlated with a tail of an OFDM symbol in the time domain. The CP can serve two purposes. First, it can be used to combat the effect of inter-symbol interference. Second, it can be used for fine time synchronization. Since the CP is generated by inserting a copy of the tail as a prefix to the OFDM symbol, correlating the tail and the CP of the received signal yields a statistic that can be used to time synchronize the received signal. Fig. 1 A shows real parts of the received signal over the CP and over the tail of one OFDM symbol. The Signal to Noise Ratio (SNR) is 40 dB. The length of the CP is 0.8 με and the samples are taken at a rate of 20 MHz. Since the channel is additive white Gaussian noise (AWGN) with no time dispersion, the two portions, i.e. the CP and the tail, of the signal overlap almost perfectly and the correlation would give an accurate estimate of the start of the OFDM symbol. The advantage of this method is that it avoids the need to add special symbols for time synchronization, and results in lower overhead than traditional designs such as those used in 802.11 a/g/n/ac/ax physical layers (PHY), which include short and long training fields. However, in fading channels the method does not perform very well, especially when both the packet and the CP are short, as e.g. required in URLLC applications. Fig. 1 B shows the real part of the received signal, corresponding to the tail and CP of one OFDM symbol. The SNR is 40 dB. The length of the CP is 0.8 us and the samples are taken at a rate of 20 MHz. The propagation channel is an indoor propagation channel with a root mean square delay spread less than 100 ns. Unlike Fig. 1A, the real parts of the tail and the CP do not agree very well, due to the fading channel and to the effect of Transmission (TX) and RX filters, wherein energy spills from tail of one OFDM symbol to the following CP, and the performance of the method suffers and thus the synchronization suffers as well. This problem can be alleviated by enlarging the CP or by averaging over several OFDM symbols. None of those solutions are ideally suited for e.g. URLLC applications, since in URLLC it is desired to use very short CP's and the packets consist of few OFDM symbols, possibly as little as one single OFDM symbol.

Commonly used designs of OFDM signals often introduce Demodulation

Reference Symbols (DMRS) which are interleaved together with data symbols. DMRS are frequency domain constellation symbols which are known at the receiving device receiving the DMRS. For example, LTE and 802.1 1a/g/n/ac/ax make use of the DMRSs. In LTE downlink DMRSs are used by the receiving device in order to perform channel estimation, and in 802.1 1 DMRSs are called pilots and are used for phase tracking, i.e. to estimate how the phase changes. The overhead introduced by the DMRS may be lowered by making the DMRS more sparse and increasing the density of data symbols.

Fig. 2Error! Reference source not found, shows an example of a frequency response of an indoor fading channel over a 20 MHz band. The maximum Doppler shift is 3 Hz. In order to estimate the channel and perform coherent demodulation, the

subcarriers of the DMRS should be placed closer in the frequency domain than the coherence bandwidth of the channel. The subcarrier spacing must also be taken into account when determining the DMRS subcarriers. For example, as a rough approximation, in the example of Fig. 2, the DMRS should be separated by not more than 500 kHz. In a system using the 802.1 1 ax numerology using a subcarrier spacing of 78125 Hz, this would mean that every 6-th subcarrier should be a DMRS. That is, 40 out of 242 useful subcarriers should be DMRS subcarriers, although to obtain good performance as many as one half of the sub-carriers should carry DMRS. In other words, somewhere between 20% and 50% of the useful subcarriers should carry DMRS. Although this overhead may be acceptable in some situations, even lower overhead is desirable for applications carrying a short payload such as URLLC applications.

SUMMARY

An object herein is to provide a mechanism that improves performance of a wireless communications network in an efficient manner.

According to an aspect the object is achieved by providing a method performed by a transmitting device for handling communication of data towards a receiving device in a wireless communications network. The transmitting device encodes first data bits mapped to constellation symbols to generate differentially encoded M:ary - Phase Shift Keying (M- PSK) symbols in a frequency domain, which data bits modulate one or more first

Orthogonal Frequency Division Multiplexing (OFDM) symbols in a packet. The

transmitting device further repeats at least a part of the differentially encoded M-PSK symbols. The transmitting device then transmits to the receiving device, a packet comprising the differentially encoded M-PSK symbols and the repeated at least part of the differentially encoded M-PSK symbols.

According to another aspect the object is achieved by providing a method performed by a receiving device for handling communication of data from a transmitting device in a wireless communications network. The receiving device receives, from the transmitting device, first data bits modulating one or more first OFDM symbols in a packet, which packet comprises differentially encoded M-PSK symbols and a repeated at least part of the differentially encoded M-PSK symbols. The receiving device estimates a phase shift based on the repeated at least part of the differentially encoded M-PSK symbols and de-rotates the differentially encoded M-PSK symbols by the estimated phase shift. The receiving device further combines the differentially encoded M-PSK symbols

corresponding to pairs of the repeated at least part of the differentially encoded M-PSK symbols and corresponding at least part of the differentially encoded M-PSK symbols, and demodulates the combined M-PSK symbols in a differential demodulator. The receiving device further, if the first data bits are correctly decoded, passes the first data bits to a media access control (MAC) layer.

It is furthermore provided herein a computer program comprising instructions, which, when executed on at least one processor, cause the at least one processor to carry out any of the methods above, as performed by the receiving device or the transmitting device. It is additionally provided herein a computer-readable storage medium, having stored thereon a computer program comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to any of the methods above, as performed by the receiving device or the transmitting device.

According to yet another aspect a transmitting device for handling communication of data towards a receiving device in a wireless communications network is herein provided. The transmitting device is configured to encode first data bits mapped to constellation symbols to generate differentially encoded M-PSK symbols in a frequency domain, which data bits modulate one or more first OFDM symbols in a packet. The transmitting device is configured to repeat at least a part of the differentially encoded M- PSK symbols; and to transmit, to the receiving device, a packet comprising the

differentially encoded M-PSK symbols and the repeated at least part of the differentially encoded M-PSK symbols.

According to still another aspect a receiving device for handling communication of data from a transmitting device in a wireless communications network is herein provided. The receiving device is configured to receive, from the transmitting device, first data bits modulating one or more first OFDM symbols in a packet, which packet comprises differentially encoded M-PSK symbols and a repeated at least part of the differentially encoded M-PSK symbols. The receiving device is further configured to estimate a phase shift based on the repeated at least part of the differentially encoded M-PSK symbols and to de-rotate the differentially encoded M-PSK symbols by the estimated phase shift. The receiving device is further configured to combine, the differentially encoded M-PSK symbols corresponding to pairs of the repeated at least part of the differentially encoded M-PSK symbols and corresponding at least part of the differentially encoded M-PSK symbols, to demodulate the combined M-PSK symbols in a differential demodulator, and, if the first data bits are correctly decoded, to pass the first data bits to a MAC layer.

The differential encoding enables the receiving device to determine how the M- PSK symbols have been rotated relative one another and thus the receiving device may determine which symbol has been transmitted. An advantage of embodiments herein is that by repeating the differentially encoded M-PSK symbol, residual phase shifts may be corrected at the receiving device, which residual phase shifts may be due to small timing inaccuracies, or allowing fine time synchronization in the frequency domain. Thus, embodiments allow the transmission of very short packets consisting only of data symbols and are well suited for e.g. URLLC and industrial applications, resulting in an improved performance of the wireless communications network in an efficient manner.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described in more detail in relation to the enclosed drawings, in which:

Fig. 1Ais a graph depicting real parts of the received signal over the CP and the tail of one OFDM symbol;

Fig. 1 Bis a graph depicting real parts of the received signal over the CP and the tail of one OFDM symbol for a fading channel;

Fig. 2 is a graph depicting frequency response for an indoor fading channel;

Fig. 3 is a schematic diagram depicting a wireless communications network according to embodiments herein;

Fig. 4 shows a combined signaling scheme and flowchart according to embodiments herein;

Fig. 5 shows a method performed by a transmitting device according to embodiments herein;

Fig. 6 shows a method performed by a receiving device according to embodiments

herein;

Fig. 7 is an illustration of residual timing inaccuracies at the receiver; Fig. 8Ais a flowchart of the method performed by the transmitting device according to an embodiment herein;

Fig. 8Bis a flowchart of the method performed by the receiving device according to an embodiment herein;

Fig. 9A illustrates repeated symbols according to embodiments herein;

Fig. 9B illustrates repeated symbols according to embodiments herein;

Fig. 9Cillustrates repeated symbols according to embodiments herein;

Fig. 9Dshows a comparison using the repetition according to embodiments herein and not using it;

Fig. 10 is a block diagram depicting a transmitting device according to embodiments

herein; and

Fig. 1 1 is a block diagram depicting a receiving device according to embodiments herein.

DETAILED DESCRIPTION

Embodiments herein relate to wireless communications networks in general. Fig. 3 is a schematic overview depicting a wireless communications network 1. The wireless communication network 1 comprises one or more RANs and one or more CNs. The wireless communication network 1 may use one or a number of different technologies, such as New Radio (NR), Wi-Fi, Long Term Evolution (LTE), LTE-Advanced, 5G,

Wideband Code Division Multiple Access (WCDMA), Global System for Mobile

communications/enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide

Interoperability for Microwave Access (WMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. Embodiments herein relate to recent technology trends that are of particular interest in a 5G context, however, embodiments are also applicable in further development of the existing wireless communication networks such as e.g. WCDMA and LTE.

In the wireless communication network 1 , a receiving device 10, such as a wireless device, a mobile station, a non-access point (non-AP) STA, a STA, a user equipment and/or a wireless terminals, may communicate via one or more Access

Networks (AN), e.g. a RAN, to one or more core networks (CN). It should be understood by the skilled in the art that "wireless device" is a non-limiting term which means any terminal, wireless communications terminal, user equipment, Machine Type

Communication (MTC) device, Device to Device (D2D) terminal, or node e.g. smart phone, laptop, mobile phone, sensor, relay, mobile tablets or even a small base station communicating within a service area. It should further be noted that the receiving device 10 may be a radio network node receiving transmissions from a wireless device.

The wireless communication network 1 comprises a transmitting device 12. The transmitting device 12 may be any wireless device or a radio network node providing radio coverage over a geographical area referred to as service area 11 or cell, which may be provided by one or more beams or a beam group where the group of beams is covering the service area of a first radio access technology (RAT), such as NR, 5G, LTE, Wi-Fi or similar. A radio network node may also serve multiple cells, and may be a transmission and reception point e.g. a radio-access network node such as a Wireless Local Area Network (WLAN) access point or Access Point Station (AP STA), an access controller, a base station e.g. a radio base station such as a NodeB, an evolved Node B (eNB, eNode B), a base transceiver station, a radio remote unit, an Access Point Base Station, a base station router, a transmission arrangement of a radio base station, a stand-alone access point or any other network unit capable of communicating with a wireless device within the service area served by the radio network node depending e.g. on the radio access technology and terminology used. The transmitting device 12 communicates with the receiving device 10 within the wireless communication network.

It should here be noted that the transmitting device 12 is illustrated as a radio network node and the receiving device 10 is illustrated as a wireless device. However, the transmitting device may be the wireless device and the receiving device may be the radio network node.

Embodiments disclosed herein allow the transmission of very short packets consisting only of data symbols and is well suited for e.g. URLLC and industrial applications. Embodiments herein enable shortened packets by having the transmitting device to encode first data bits mapped to constellation symbols to generate differentially encoded M-PSK symbols in the frequency domain, which data bits modulate one or more first OFDM symbols in the packet. Hence, differentially encoded M-PSK symbols are data bits encoded and mapped to constellation symbols such as any M-PSK symbols. The data bits transmitted thus depend not only on current signal symbol but also on the previous symbol. The transmitting device further repeats at least a part of the differentially encoded M-PSK symbols, such as one of the M-PSK symbols. The transmitting device then transmits to the receiving device, the packet comprising the differentially encoded M- PSK symbols and the repeated at least part of the differentially encoded M-PSK symbols.

The differential encoding enables shortening of the packet, since a training field in the packet is not needed to determine M-PSK symbol at the receiving device for a channel estimation over a fading channel and by repeating a differentially encoded M- PSK symbol residual phase shifts may be corrected. The repetition enables the receiving device to determine how the M-PSK symbols have been rotated relative one another and thus the receiving device may correct phase rotations due to time synchronization inaccuracies.

Fig. 4 is a combined flowchart and signaling scheme according to some

embodiments herein.

Action 401. The transmitting device 12 encodes the first data bits mapped to constellation symbols to generate differentially encoded M-PSK symbols in the frequency domain, which data bits modulate one or more first OFDM symbols, e.g. a 1 ^st OFDM symbol, in the packet.

Action 402. The transmitting device 12 repeats at least the part of the differentially encoded M-PSK symbols wherein at least a part means one or more M-PSK symbols, e.g. repeat a second M-PSK symbol.

Action 403. The transmitting device 12 may further rotate the repeated part, e.g. rotate the repeated second M-PSK symbol.

Action 404. The transmitting device 12 transmits to the receiving device 10, the packet comprising the differentially encoded M-PSK symbols and the repeated part of the differentially encoded M-PSK symbols. Hence, the packet comprises e.g. the second M- PSK symbol and the additional repeated second M-PSK symbol (rotated).

Action 405. The receiving device 10 receives the packet and estimates a phase shift based on the repeated at least part of the differentially encoded M-PSK symbols.

Action 406. The receiving device 10 de-rotates the differentially encoded M-PSK symbols by the estimated phase shift.

Action 407. The receiving device 10 combines the M-PSK symbols corresponding to pairs of the repeated at least part of the differentially encoded M-PSK symbols and corresponding at least part of the differentially encoded M-PSK symbols. For example, the second M-PSK symbol is combined with the repeated second M-PSK symbol.

Action 408. The receiving device 10 demodulates the combined M-PSK symbols in a differential demodulator.

Action 409. The receiving device 10 then when the first data bits are correctly decoded, passes the first data bits to the MAC layer.

Embodiments herein address the design of low overhead OFDM signals. It is provided methods to design very short packets, as short as one single OFDM symbol, and with a very low overhead. Embodiments may be adapted to TDMA systems where the receiving device 10 knows approximately when a packet is expected. In the proposed design, a packet can be as short as one OFDM symbol. The useful sub-carriers in the first OFDM symbol in the packet (and possibly the only symbol in the packet) convey only data symbols, also referred to as frequency domain constellation symbols or simply

constellation symbols. The constellation symbols are drawn from an M-PSK constellation, e.g. Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 8-PSK or 16-PSK, which are differentially encoded in the frequency domain, e.g. Differential Binary Phase Shift Keying (DBPSK), Differential Quadrature Phase Shift Keying (DQPSK) etc. A small subset of the constellation symbols are repeated and then a CP may be inserted. In practice the CP is as short as possible in order to cope with the propagation delay, multi- path channel and the timing uncertainties due to clock drift. The repeated constellation symbols may be subject to a rotation. The time domain signal may be generated by applying an Inverse Fast Fourier Transform (IFFT).

Any further OFDM symbols, if present, may be formatted according to prior art, e.g. in the same way as the OFDM symbols carrying payload in 802.11 a/g/n/ac/ax or LTE, and may use any modulation and coding scheme (MCS).

The method actions performed by the transmitting device 12, exemplified herein as a radio base station, for handling communication of data towards the receiving device 10, exemplified herein as a wireless device, in the wireless communications network according to some embodiments will now be described with reference to a flowchart depicted in Fig. 5. The actions do not have to be taken in the order stated below, but may be taken in any suitable order. Actions performed in some embodiments are marked with dashed boxes.

Action 501. The transmitting device 12 may add a first frame check sequence (FCS) to the first data bits corresponding to the one or more first OFDM symbols in the packet.

Action 502. The transmitting device 12 may encode the FCS and the first data bits. Action 503. The transmitting device 12 may further interleave the encoded FCS and first data bits.

Action 504. The transmitting device 12 may map the interleaved FCS and first data bits to the constellation symbols drawn from a M-PSK constellation.

Action 505. The transmitting device 12 encodes the first data bits mapped to constellation symbols to generate differentially encoded M-PSK symbols in the frequency domain, which first data bits corresponds to the one or more first OFDM symbols in the packet.

Action 506. The transmitting device 12 repeats at least the part of the differentially encoded M-PSK symbols, e.g. one or more M-PSK symbols. The part of the differentially encoded M-PSK symbols and the repeated part of the differentially encoded M-PSK symbols may be adjacent M-PSK symbols in the frequency domain.

Action 507. The transmitting device 12 may rotate the repeated at least part of the differentially encoded M-PSK symbols. This is performed for better signal dynamics differentiating the phases of the part and the repeated part.

Action 508. The transmitting device 12 may transform to the time domain, the rotated at least part of the differentially encoded M-PSK symbols. For example, the transmitting device 12 may transform the rotated at least part of the differentially encoded M-PSK symbols in an IFFT process.

Action 509. The transmitting device 12 may further insert the CP (in the time domain).

Action 510. The data may comprise data bits that are segmented into two parts, a first part comprising first data bits and a second part comprising second data bits. The first data bits modulate the one or more first OFDM symbols in the packet, and the second data bits modulate one or more second OFDM symbols. The second data bits may modulate the one or more second OFDM symbols according to a wireless

communications standard having a physical layer based on OFDM such as an 802.11 standard or some other OFDM-based standard, i.e. the transmitting device 12 may omit differential encoding and may also omit the repeating and rotating of the constellation symbols modulating the one or more second OFDM symbols. The transmitting device 12 may then assemble the packet by joining the one or more first OFDM symbols with the one or more second OFDM symbols. For example the one or more first OFDM symbols may comprise a 1 ^st OFDM symbol in the packet, while the one or more second OFDM symbols may comprise a 2 ^nd , a 3 ^rd and further OFDM symbols in the packet. It should also be understood that the one or more first OFDM symbol may comprise the 2 ^nd or the 3 ^rd OFDM symbol and then the one or more second OFDM symbols may comprise a fourth OFDM symbol or the 1 ^st OFDM symbol.

Action 511. The transmitting device 12 transmits, to the receiving device 10, the packet comprising the differentially encoded M-PSK symbols and the repeated at least part of the differentially encoded M-PSK symbols. The packet may further comprise the rotated at least part of the differentially encoded M-PSK symbols and/or the CP. The transmitting device may in some embodiments transform the assembled packet to the analog domain and then transmit the transformed packet.

The method actions performed by the receiving device 10, exemplified herein as a wireless device, for handling communication of data from the transmitting device 12, exemplified herein as a radio network node, in the wireless communications network according to some embodiments will now be described with reference to a flowchart depicted in Fig. 6. The actions do not have to be taken in the order stated below, but may be taken in any suitable order. Actions performed in some embodiments are marked with dashed boxes.

Action 601. The receiving device 10 receives, from the transmitting device, first data bits modulating the one or more first OFDM symbols in the packet, which symbols comprise the differentially encoded M-PSK symbols and the repeated at least part of the differentially encoded M-PSK symbols. The repeated part of the differentially encoded M- PSK symbols and the part of the differentially encoded M-PSK symbols may be adjacent in the frequency domain. The receiving device 10 may also receive, from the transmitting device, second data bits modulating to the one or more second OFDM symbols in the packet, also referred to as one or more remaining OFDM symbols.

Action 602. The receiving device 10 may, when the receiving device 10 receives the first data bits and also second data bits in a signal, segment the received packet into two groups of OFDM symbols, wherein the first data bits modulate the one or more first OFDM symbols, and the second data bits modulate the one or more second OFDM symbols in the packet.

Action 603. The receiving device12 may, when the packet received further comprises the CP, remove the CP from the packet (or from the received time domain signal).

Action 604. The receiving device 12 may convert the received signal to a frequency domain e.g. by means of a Fast Fourier Transform (FFT).

Action 605. The receiving device 12 may de-rotate the at least part of the M-PSK symbols whose rotation is known a-priori at the receiving device 12 for being repeated and rotated at the transmitting device.

Action 606. The receiving device 12 estimates the phase shift, e.g. introduced by a timing offset, based on the repeated at least part of the differentially encoded M-PSK symbols. The receiving device knows which at least part of the differentially encoded M- PSK symbols is being repeated. Action 607. The receiving device 12 then de-rotates the differentially encoded M- PSK symbols by the estimated phase shift e.g. in order to compensate for the timing offset.

Action 608. The receiving device 12 combines the differentially encoded M-PSK symbols corresponding to pairs of the repeated at least part of the differentially encoded M-PSK symbols and corresponding at least part of the differentially encoded M-PSK symbols. The receiving device 12 may combine the pairs of repeated differentially encoded M-PSK symbols in order to improve the receiver performance.

Action 609. The receiving device 10 then demodulates the combined M-PSK symbols in a differential demodulator.

Action 610. The receiving device 10 may decode, de-interleave and perform a frame check on the first data bits.

Action 611. The receiving device 10, if the first data bits are correctly decoded, passes the first data bits to the MAC layer.

Action 612. The receiving device 10 may re-encode, if the first data bits are correctly decoded, the first data bits and re-generate a frequency domain signal that was transmitted.

Action 613. The receiving device 10 may further use the re-generated frequency domain signal for channel estimation.

Action 614. The receiving device 10 may then perform coherent decoding of the second data bits.

The following example will help illustrate the benefits of embodiments herein. 802.1 1 ax numerology is used in the example but is not limiting to the solution. Note that even though protocol refers to payloads of the order of 100 bits, there is always protocol overhead associated with e.g. the Medium Access Control (MAC) header, etc. Hence, suppose that a MAC layer, at the transmitting device 12, delivers a Protocol Data Unit (PDU) consisting of 196 bits. First, a 32 bit Frame Check Sequence (FCS), such as Cyclic Redundancy Check (CRC), may be computed and added to the data bits, i.e. the 196 bits. Then 6 tail bits are appended and a rate ½ convolutional code is applied. The encoded payload consists of 2*(196 + 32 + 6) = 468 bits. Assume also that the bandwidth is 20 MHz and that the CP length is 0.8 μβ. Finally, a Quadrature Phase Shift Keying (QPSK) constellation is used.

Consider first a URLLC system based on 802.1 1ax by straightforward

modifications. If TDMA is used and transmissions occur at rates from 10's of Hz to several kHz, then automatic gain control (AGC), direct current (DC) compensation, coarse time synch and other tasks may be performed by an outer loop, as opposed to the usual 802.11 per packet operation. Moreover, in an industrial URLLC setting, there is no need to take backward compatibility into account. In addition, it is reasonable to assume that a MAC header is enough and there is no need for a PHY header. Thus, this assumed system has a very lean packet format where the packet consists of one Long Training Field (LTF) OFDM symbol followed by data symbols. 802.11 ax supports several LTF lengths and a short LTF of 6.4 με + CP is assumed to be used. There are 234 useful subcarriers in one OFDM symbol in 802.1 1ax, and 8 pilot subcarriers. Hence, using QPSK, a payload of 234*2 = 468 bits can be carried in one OFDM symbol.

The packet of length would be (0.8+6.4+0.8+12.8)με = 20.8 μβ.

According to embodiments herein, the packet length would be (0.8+12.8)μ3 = 13.6

Thus, we see from this realistic example that the packet duration is reduced from 20.8 με to 13.6 με, a quite significant reduction. Assuming a scheduled TDMA system with a slot duration of 13.6 με, embodiments herein allow an increase of approximately 50% in the system capacity, as well as reduced latency, with respect to an analogous system based on slots of 20.8 με.

These gains come at the cost of a degradation of coverage due to non-coherent differential demodulation. Roughly, the loss of link performance is 3 dB compared to coherent (non-differential) detection. This loss is likely to be acceptable in e.g. URLLC applications where the required coverage is in the order of 10m to 100m. In the proposed design, all useful sub-carriers in the OFDM system (i.e. excluding unused guard and DC sub-carriers) can carry user data.

The packet design disclosed herein addresses the design of very short packets for OFDM systems targeting communication networks focusing on reliability such as URLLC and industrial applications.

We assume that a TDMA structure is in place, and that transmissions are scheduled, i.e. radio resources, such as slots, symbols or similar, are assigned for transmissions. Moreover, the receiving device 10 and the transmitting device 12 are synchronized. If transmissions occur at rates from 10's of Hz to several kHz, then AGC, DC compensation, coarse time synch, carrier frequency offset correction and other tasks can be performed by an outer loop (as opposed to the usual 802.1 1 per packet operation). Moreover, in a deterministic, scheduled industrial URLLC setting, there is no need to take backward compatibility into account so the legacy preambles commonly used in 802.11 are not necessary. In addition, a MAC header may be enough for the packet recipient to determine the destination of the packet, while the duration is given by the time-slot length. Therefore there is no need for a PHY header.

However, one important problem must still be solved at the receiver side, namely fine time synchronization, which must be performed for each incoming packet. The receiver knows with some certainty the time of arrival of the packet, as it knows its assigned time slot. However, the propagation delays are unknown, there is multi-path propagation, and there is always some residual clock drift. Nonetheless, it is assumed that the coarse timing synchronization is enough to guarantee that the residual timing error is smaller than the duration of the CP. Using 802.1 1 ax numerology as an example, with a short 0.8 με CP, this means that there is a residual timing inaccuracy of up to 0.8 με, and the receiver must be able to cope with this timing offset. The situation is illustrated in Fig. 7. Note that while the samples collected are enough to recover any data carried by the OFDM symbol, the timing offset introduces phase shifts at the output of the Fast Fourier Transform (FFT). Failure to compensate these phase shifts may result in non-negligible performance degradations.

Fig. 8A shows a flowchart of a method performed by the transmitting device 12 according to an embodiment herein. The packet is formatted and transmitted according to the following steps.

Action 801. The data bits may be segmented into two parts. The first part modulates the one or more first OFDM symbols in the packet, while the second part modulates the one or more second OFDM symbols.

Action 802. The transmitting device 12 may add a first FCS to the first data bits corresponding to the one or more first OFDM symbols in the packet.

Action 803. The first data bits are then encoded, interelaved and mapped to constellation symbols drawn from an M-PSK constellation, such as BPSK, QPSK, 8-PSK, 16-PSK, etc.

Action 804. The constellation symbols are differentially encoded to generate differentially encoded M-PSK symbols, e.g. DBPSK, DQPSK, etc, or any variant of a differentially encoded phase shift keying such as π/4-DQPSK.

Action 805. A selected number of differentially encoded M-PSK symbols are repeated. In practice it is enough to repeat around 5% or fewer of said symbols. The locations of the selected M-PSK symbols are known at the receiving device 10. Action 806. A rotation may be applied to the repeated symbols. The purpose is to avoid co-phasing of nearby symbols and to add randomness to the signal. This rotation is known at the receiver.

Action 807. From the frequency domain, differentially encoded, repeated and rotated M-PSK symbols are transformed to the time domain via e.g. an IFFT.

Action 808. The transmitting device 12 inserts a CP.

Action 809. The input data symbols in the second part modulate OFDM symbols according to prior art, e.g. following the 802.1 1a/g/n/ac/ax standard. Thus, the transmitting device 12 may add a second FCS to the second data bits corresponding to the one or more second OFDM symbols in the packet.

Action 810. The second data bits are then encoded, interleaved and mapped to constellation symbols drawn from any quadrature amplitude modulation (QAM) constellation, such as BPSK, QPSK, 16QAM, 64QAM, 256QAM, etc.

Action 811. The transmitting device may then insert a pilot sequence.

Action 812. The constellation symbols are transformed to the time domain via e.g. an IFFT.

Action 813. The transmitting device 12 inserts a CP.

Action 814. The packet is assembled by joining the one or more first OFDM symbol and the one or more second OFDM symbols.

Action 815. The packet is then transformed to the analog domain in a digital-to- analog converter (DAC).

Action 816. The transmitting device then transmits the packet. Observe that the one or more first OFDM symbols in the packet carries only user data. Fig. 8B shows a flowchart of a method performed by the receiving device 10 according to an embodiment herein.

The packet is received according to the following steps.

The receiving device may open the RX window approximately at the beginning of the time slot.

Action 821. Digital baseband RX samples are segmented into two groups or parts of OFDM symbols. The first group corresponds to the one or more first OFDM symbols and the second group to the one or more second OFDM symbols.

Action 822. The CP is removed from the samples corresponding to the one or more first OFDM symbols.

Action 823. The samples are converted to the frequency domain via an FFT. Action 824. The samples corresponding to symbols repeated and rotated are de- rotated. The rotation angles are known a-priori at the receiving device 10.

Action 825. The samples that correspond to pairs of repeated transmitted symbols are used to estimate the phase shift introduced e.g. by a timing offset.

Action 826. The received frequency domain samples are de-rotated by the angle estimated in the previous action, in order e.g. to compensate for a timing offset.

Action 827. The samples corresponding to pairs of repeated symbols are coherently combined.

Action 828. The frequency domain samples are fed to a differential demodulator. Action 829. This is followed by decoding, and de-interleaving (these two operations are combined in the decoder block in Fig. 8B).

Action 830. A frame check is performed, and if the data has been correctly decoded, it is passed to the MAC layer. Otherwise the frame is lost, action 831.

Action 832. If e.g. the packet consists of more than one OFDM symbol, and the FCS in the one or more first OFDM symbols is correct, then the data is re-encoded, following the steps in the transmitter chain in fig. 8A, up to and including symbol rotation. These OFDM symbols can then be used as DMRS to perform channel estimation, fine timing and frequency correction, etc. A coherent demodulator can be used to decode the samples in the second part, according to prior art methods, action 833.

Action 834. If the second data bits are correctly decoded these are passed to the

MAC layer.

Considering a simplified scenario where the FFT size is 16 and all subcarriers are used, and where all the data can be transmitted in one OFDM symbol. Referring to Fig. 5, suppose that the encoded payload consists of 28 bits.

After the interleaver, the bits are processed by the symbol mapper which outputs 14 QPSK constellation symbols. The differential encoder applies differential encoding to transform the 14 QPSK input symbols into 14 DQPSK symbols. These symbols are labeled DQ1 to DQ14 in Fig. 9A.

The repetition block selects two of the 14 DQPSK symbols, say symbols number 4 and 1 1 , and repeats them. This is illustrated in Fig. 9AError! Reference source not found..

The rotation block may apply a rotation only to the repeated symbols. This is illustrated in Fig. 9B. In this example a 180 degree rotation is applied to the repetition of DQ4, and a 90 degree rotation is applied to the repetition of DQ1 1. The rotated repeated symbols are labeled RDQ4 and RDQ1 1 in Fig. 9B. Note that RDQ4 = (-1)*RDQ4, while RDQ11 = j*DQ11. The resulting sequence of symbols shown in Fig. 9B are sent to the IFFT block wherein the 16 symbols are converted to the time domain via an IFFT, followed by CP insertion, DAC, upmixing, amplification and transmission.

The receiver opens a reception (RX) window approximately at the beginning of the time slot. After down-mixing and Analogue to Digital converter (ADC), the receiver discards the CP. Note that fine timing is not performed, so that there is a residual timing offset, as illustrated in Fig. 7.

The samples corresponding to the first (and only) OFDM symbol are fed to the

FFT block, which outputs frequency domain samples. Because of the residual timing offset, these samples may be phase shifted. The output of the FFT block is illustrated in Fig. 9C. The frequency domain samples are denoted R1 to R16.

The frequency domain samples corresponding to repeated, rotated DQPSK samples are de-rotated. Recalling that the rotations were 180 and 90 degrees, the de- rotated samples are DR5 = (-1)*R5 and DR13 = (-j)*R13.

The frequency domain samples are fed to the block that estimates the phase shift in Fig. 6. The phase shift Θ between adjacent frequency domain samples, due to timing offsets, can be estimated as Θ =arg{conj(R4)* DR5 + conj(R12)* DR13}. Here conj(.) denotes the complex conjugate of a complex number and arg{.} the argument of a complex number.

Next, the de-rotation block in Fig. 6 de-rotates all frequency domain received samples, R1 to R16, by an angle Θ. Let's label these samples by dR1 ,dR2, ... ,dR16.

The repeated samples may be coherently combined: dR4combined =

(dR4+dR5)/2 and dR12combined = (dR12+dR13)/2.

Finally, the 14 samples dR1 , dR2, dR3, dR4combined, dR6, dR7, dR8, dR9, dR10, dR11 , dR12combined, dR14, dR15, dR16 are fed to the differential demodulator.

The demodulated bits are decoded and the frame check (e.g. a CRC check) is performed. If the bits have been correctly decoded they are passed onto the MAC, otherwise the frame is lost.

Consider a scenario where the 802.11 ax numerology is also employed. Suppose that the CP is 0.8 us long and that the time offset shown in Fig. 7 is 0.5 us.

First, suppose that the repetition and rotation blocks in Fig. 5Error! Reference source not found, are omitted. Differential modulation is able to cope with some timing offsets, but at the expense of receiver performance. The effect of the time offset on the received signal is shown in Fig. 9D. The circles correspond to noisy received DQPSK symbols. In contrast, the stars in Fig. 9D show the same received DQPSK symbols when the repetition and rotation blocks in the transmission of Fig. 5 is applied, and the receiver processing is performed as indicated in Fig. 6. The character of the 90 degree phase shifts in the DQPSK constellation are clearly visible in the stars but not in the circles. This translates into improved receiver performance.

Embodiments herein provide quite significant performance gains with respect to a similar packet format based only on differential encoding, and omitting the repetition and rotation blocks in the transmitting device of Fig. 10.

Fig. 10 is a schematic block diagram depicting, in two embodiments, the transmitting device 12 for handling communication of data towards the receiving device in the wireless communications network.

The transmitting device 12 may comprise a processing circuitry 1001 , e.g. one or more processors or similar, being configured to perform the method herein.

The transmitting device 12 may comprise an encoding module 1002. The transmitting device 12, the processing circuitry 1001 , and/or the encoding module is configured to encode first data bits mapped to constellation symbols to generate differentially encoded M-PSK symbols in the frequency domain, which data bits corresponds to the one or more first OFDM symbols in the packet.

The transmitting device 12 may comprise a repeating module 1003. The transmitting device 12, the processing circuitry 1001 , and/or the repeating module 1003 is configured repeat at least the part of the differentially encoded M-PSK symbols. The part of the differentially encoded M-PSK symbols and the repeated part of the differentially encoded M-PSK symbols may be adjacent in the frequency domain.

The transmitting device 12 may comprise a transmitting module 1004, e.g. a transmitter or a transceiver. The transmitting device 12, the processing circuitry 1001 , and/or the transmitting module 1004 is configured to transmit, to the receiving device 10, the packet comprising the differentially encoded M-PSK symbols and the repeated at least part of the differentially encoded M-PSK symbols.

The transmitting device 12 may comprise a rotating module 1005. The transmitting device 12, the processing circuitry 1001 , and/or the rotating module 1005 may be configured to rotate the repeated at least part of the differentially encoded M-PSK symbols; and the packet comprises the rotated at least part of the differentially encoded M-PSK symbols. The transmitting device 12 may comprise a transforming module 1006. The transmitting device 12, the processing circuitry 1001 , and/or the transforming module 1006 may be configured to transform, to the time domain, the rotated at least part of the differentially encoded M-PSK symbols.

The transmitting device 12 may comprise an inserting module 1007. The transmitting device 12, the processing circuitry 1001 , and/or the inserting module 1007 may be configured to insert the cyclic prefix; and the packet further comprises the cyclic prefix.

The data may comprise data bits that are segmented into two parts; the first data bits modulate the one or more first OFDM symbols in the packet, and second data bits modulate the one or more second OFDM symbols. The second data bits may modulate the one or more second OFDM symbols according to a wireless communications standard having a physical layer based on OFDM.

The transmitting device 12 may comprise an assembling module 1008. The transmitting device 12, the processing circuitry 1001 , and/or the assembling module 1008 may be configured to assemble the packet by joining the one or more first OFDM symbols and the one or more second OFDM symbols. The transmitting device 12, the processing circuitry 1001 , and/or the transmitting module 1004 may be configured to transform the packet to the analog domain and then to transmit the transformed packet.

The transmitting device 12 may comprise an initiating module 1009. The transmitting device 12, the processing circuitry 1001 , and/or the initiating module 1009 may be configured to:

add a first frame check sequence, FCS, to the first data bits modulating or corresponding to the one or more first OFDM symbols in the packet;

encode the FCS and the first data bits;

interleave the encoded FCS and first data bits; and to

map the interleaved FCS and first data bits to the constellation symbols drawn from a M-PSK constellation.

The transmitting device 12 further comprises a memory 1010 comprising one or more memory units. The memory 1010 comprises instructions executable by the processing circuitry 1001 to perform the methods herein when being executed in the transmitting device 12. The memory 1010 is arranged to be used to store e.g. information, data such as encoding schemes, packet information, repetition information, rotation information, etc. The methods according to the embodiments described herein for the transmitting device 12 are respectively implemented by means of e.g. a computer program 1011 or a computer program product, comprising instructions, i.e., software code portions, which, when executed on at least one processor, cause the at least one processor to carry out the actions described herein, as performed by the transmitting device 12. The computer program 1011 may be stored on a computer-readable storage medium 1012, e.g. a disc, a USB, or similar. The computer-readable storage medium 1012, having stored thereon the computer program, may comprise the instructions which, when executed on at least one processor, cause the at least one processor to carry out the actions described herein, as performed by the transmitting device 12. In some embodiments, the computer- readable storage medium may be a non-transitory computer-readable storage medium. Thus, the transmitting device 12 may comprise the processing circuitry and the memory, said memory comprising instructions executable by said processing circuitry whereby said transmitting device is operative to perform the methods herein.

Fig. 11 is a schematic block diagram depicting, in two embodiments, the receiving device 10 for handling communication of data from the transmitting device 12 in the wireless communications network.

The receiving device 10 may comprise a processing circuitry 1101 , e.g. one or more processors or similar, being configured to perform the method herein.

The receiving device 10 may comprise a receiving module 1102, e.g. a receiver or transceiver. The receiving device 10, the processing circuitry 1101 , and/or the receiving module 1 102 is configured to receive, from the transmitting device, first data bits modulating the one or more first OFDM symbols in the packet, which packet comprises differentially encoded M-PSK symbols and the repeated at least part of the differentially encoded M-PSK symbols.

The receiving device 10 may comprise an estimating module 1103. The receiving device 10, the processing circuitry 1101 , and/or the estimating module 1 103 is configured to estimate the phase shift based on the repeated at least part of the differentially encoded M-PSK symbols. The repeated part of the differentially encoded M- PSK symbols and the part of the differentially encoded M-PSK symbols may be adjacent in the frequency domain.

The receiving device 10 may comprise a de-rotating module 1104. The receiving device 10, the processing circuitry 1101 , and/or the de-rotating module 1104 is configured to de-rotate the differentially encoded M-PSK symbols by the estimated phase shift. The receiving device 10 may comprise a combining module 1105. The receiving device 10, the processing circuitry 1101 , and/or the combining module 1 105 is configured to combine, the differentially encoded M-PSK symbols corresponding to pairs of the repeated at least part of the differentially encoded M-PSK symbols and corresponding at least part of the differentially encoded M-PSK symbols.

The receiving device 10 may comprise a demodulating module 1106. The receiving device 10, the processing circuitry 1 101 , and/or the demodulating module 1106 is configured to demodulate the combined M-PSK symbols in a differential demodulator.

The receiving device 10 may comprise a passing module 1107. The receiving device 10, the processing circuitry 1 101 , and/or the passing module 1107 is configured to, if the first data bits are correctly decoded, pass the first data bits to a media access control, MAC, layer.

The receiving device 10, the processing circuitry 1 101 , and/or the de-rotating module 1 104 may further be configured to de-rotate the at least part of the differentially encoded M-PSK symbols known a-priori at the receiving device for being repeated and rotated at the transmitting device.

The receiving device 10 may comprise a removing module 1108. The receiving device 10, the processing circuitry 1101 , and/or the removing module 1108 may be configured to, wherein the packet received further comprises the cyclic prefix, remove the cyclic prefix from the packet (or from the received OFDM symbols).

The receiving device 10 may comprise a segmenting module 1109. The receiving device 10, the processing circuitry 1 101 , and/or the segmenting module 1109 may be configured to, when receiving second data bits, segment the received packet into two groups of OFDM symbols, wherein the first data bits modulate the one or more first OFDM symbols, and the second data bits modulate the one or more second OFDM symbols.

The receiving device 10 may comprise a remaining module 1110. The receiving device 10, the processing circuitry 1101 , and/or the remaining module 11 10 may be configured to: re-encode, if the first data bits are correctly decoded, the first data bits and re-generate a frequency domain signal that was transmitted; use the re-generated frequency domain signal for channel estimation; and to perform coherent decoding of the second data bits.

The receiving device 10 may comprise a converting module 1111. The receiving device 10, the processing circuitry 1101 , and/or the converting module 11 11 may be configured to convert the received signal to the frequency domain. The receiving device 10 may comprise a decoding module 1112. The receiving device 10, the processing circuitry 1 101 , and/or the decoding module 11 12 may be configured to decode, de-interleave and perform a frame check on the first data bits.

The receiving device 10 further comprises a memory 1113 comprising one or more memory units. The memory 11 13 comprises instructions executable by the processing circuitry 1 101 to perform the methods herein when being executed in the receiving device 10. The memory 1 113 is arranged to be used to store e.g. information, data such as decoding schemes, packet information, repetition information, rotation information etc.

The methods according to the embodiments described herein for the receiving device 10 are respectively implemented by means of e.g. a computer program 1114 or a computer program product, comprising instructions, i.e., software code portions, which, when executed on at least one processor, cause the at least one processor to carry out the actions described herein, as performed by the receiving device 10. The computer program 11 14 may be stored on a computer-readable storage medium 1115, e.g. a disc, a USB, or similar. The computer-readable storage medium 1 115, having stored thereon the computer program, may comprise the instructions which, when executed on at least one processor, cause the at least one processor to carry out the actions described herein, as performed by the receiving device 10. In some embodiments, the computer- readable storage medium may be a non-transitory computer-readable storage medium. Thus, the receiving device 10 may comprise the processing circuitry and the memory, said memory comprising instructions executable by said processing circuitry whereby said receiving device is operative to perform the methods herein. As will be readily understood by those familiar with communications design, means or modules may be implemented using digital logic and/or one or more

microcontrollers, microprocessors, or other digital hardware. In some embodiments, several or all of the various functions may be implemented together, such as in a single application-specific integrated circuit (ASIC), or in two or more separate devices with appropriate hardware and/or software interfaces between them. Several of the functions may be implemented on a processor shared with other functional components of a wireless terminal or network node, for example.

Alternatively, several of the functional elements of the processing means discussed may be provided through the use of dedicated hardware, while others are provided with hardware for executing software, in association with the appropriate software or firmware. Thus, the term "processor" or "controller" as used herein does not exclusively refer to hardware capable of executing software and may implicitly include, without limitation, digital signal processor (DSP) hardware, read-only memory (ROM) for storing software, random-access memory for storing software and/or program or application data, and non-volatile memory. Other hardware, conventional and/or custom, may also be included. Designers of communications receivers will appreciate the cost, performance, and maintenance tradeoffs inherent in these design choices.

It will be appreciated that the foregoing description and the accompanying drawings represent non-limiting examples of the methods and apparatus taught herein. As such, the inventive apparatus and techniques taught herein are not limited by the foregoing description and accompanying drawings. Instead, the embodiments herein are limited only by the following claims and their legal equivalents.