WO2016134763A1

WO2016134763A1 - Pilot pattern for wifi ofdma

Info

Publication number: WO2016134763A1
Application number: PCT/EP2015/053946
Authority: WO
Inventors: Genadiy Tsodik; Shimi Shilo; Doron Ezri; Oded Redlich; Le LIU; Jiayin Zhang
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2015-02-25
Filing date: 2015-02-25
Publication date: 2016-09-01
Also published as: CN107210878A; CN107210878B

Abstract

A method for generating a pilot pattern within a data frame (100) for a data transmission arrangement employing orthogonal frequency division multiple access, OFDMA, is provided. One data frame comprises a multitude of OFDM symbols (120) to be consecutively transmitted in time. The method comprising the steps of: transmitting a first pilot (132) at a first frequency of one of the OFDM symbols and transmitting a second pilot (134) at a second frequency of one of the OFDM symbols, wherein the second frequency is different from the first frequency, wherein the first frequency and the second frequency are assigned to a first communication device (20A).

Description

Pilot pattern for WiFi OFDMA

TECHNICAL FIELD The present invention relates to the technical field of data transmission in communication networks. Particularly, the invention relates to a method for generating a pilot pattern within a data frame for a data transmission arrangement employing orthogonal frequency division multiple access, OFDMA, to a data transmission arrangement configured to carry out this method and to a data transmission system.

BACKGROUND

In communication networks, modulation schemes are usually used to modulate or encode data before transmitting the data via a communication channel from a transmitter to a receiver or a multitude of receivers. A communication channel can be a wire-bound or a wireless transmission path between the transmitter and the receiver. The transmission path may be configured for one-way communication (simplex), two-way alternate communication (half duplex) or two-way simultaneous communication (duplex) between two communicating entities.

Several modulation and encoding schemes are known and can be used, for example, depending on the characteristics of the communication channel, according to the desired data transmission parameters, and according to the needs of the participating

communication entities.

One of these encoding schemes is orthogonal frequency-division multiplexing, OFDM.

OFDM uses multiple orthogonal carriers for encoding data to be transmitted such that several parallel data streams are channels are generated. Subcarrier signals are used to carry data on these several parallel data streams and each subcarrier is modulated with a modulation scheme.

Orthogonal frequency-division multiple access, OFDMA, is a further development of OFDM and is configured for multi user access by assigning one or more subcarriers to individual receiving devices or users, respectively.

OFDMA may for example be used for data transmission in WiFi systems. A WiFi frame usually consists of two main parts: preamble and data. Each of these includes special signals that are used for carrier frequency offset, CFO, estimation. In the first stage the initial CFO estimation is carried out based on the preamble contents (specifically, the Legacy Short Training Field, L-STF, and the Legacy Long Training Field, L-LTF, signals). The second and the last stage is the CFO tracking during the data portion of the WiFi frame. The WiFi preamble consists of two preambles, L-STF and L-LTF which are based on known training sequences transmitted in repetitive manner. The WiFi receiver uses this repetition within the signals to estimate the initial CFO. The data portion includes reserved tones, named "pilots", which are also known to the receiver and thus allow continuous CFO estimation and correction along the frame. These pilots are spread over the entire bandwidth (and throughout the entire frame duration) to provide diversity and allow correct CFO estimation in various channel conditions.

In current WiFi receivers, CFO estimation and correction mechanism may be crucial to achieving good system performance in various environments. CFO pilots are located at each OFDM symbol starting from L-LTF. During the data portion the receiver usually compares between pilots in two consecutive OFDM symbol and updates the CFO value.

SUMMARY

An object of the invention is to improve effectiveness and efficiency of an OFDMA communication network, in particular of link impairment estimation.

This object is achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures. The invention is based on the following findings:

A part of the signal processing at a receiver is the estimation and the correction of impairments created by the analog part and the wireless environment of the communication network. An issue is the estimation and the correction of the carrier frequency offset (CFO) caused by clock mismatch between transmitter and receiver and possible Doppler effects caused by a wireless transmission link. Existing technologies introduced several solutions to this problem, some of which are incorporated into the frame structure. However all the solutions assume that only one client can transmit at any particular time and that the transmitted signal occupies the entire bandwidth. These solutions may provide good performance in an OFDM system and allow receivers to accommodate very high CFOs. These techniques become less effective if OFDM is extended to OFDMA where each client occupies only part of the available bandwidth (which may be very narrow) and a number of clients can transmit simultaneously, each on its own respective part of the spectrum. The main difference between OFDM and OFDMA is the multiple access capability of OFDMA. In OFDMA, multiple clients occupy the entire bandwidth and transmit or receive simultaneously. The frequency granularity of OFDMA may differ between the standards. Some of them allow very narrow bandwidth to be allocated to a particular client and others may allow a broader bandwidth to be allocated to the particular client.

According to a first aspect of the invention, a method for generating a pilot pattern within a data frame for a data transmission arrangement employing orthogonal frequency division multiple access, OFDMA, is provided. One data frame comprises a multitude of OFDM symbols to be consecutively transmitted in time. The method comprises the following steps: transmitting a first pilot at a first frequency of one of the OFDM symbols and transmitting a second pilot at a second frequency of one of the OFDM symbols, wherein the second frequency is different from the first frequency, wherein the first frequency and the second frequency are assigned to a first communication device.

This method may particularly be implemented in a wireless data communication network according to one of the IEEE 802.1 1 standards, particularly in OFDMA based WiFi technology adopted by the IEEE 802.1 1 ax standard in order to allow CFO estimation. In such a communication network, multiple subscribers are provided which are configured to receive data from and transmit data to a data transmission arrangement, which could be referred to as access point. Thus, the method as described above and hereinafter may advantageously be implemented in a scenario where multiple subscribers share a predetermined bandwidth according to the principles of OFDMA to receive and transmit data. Especially in such a scenario, every one of the multiple subscribers is assigned the first and second frequency each of which comprises a pilot. Thus, every one of the multiple subscribers is able to estimate link impairment since every subscriber is assigned frequencies comprising a pilot independent from the exact bandwidth assignment to the clients.

In other words, the pilots may not be transmitted at fixed frequencies regardless of the assignment of subscribers to the available frequencies but such that the pilots can be transmitted at different frequencies depending on the assignment of subscribers to frequencies.

In the method as described above and hereinafter, pilots are spread over the frequency and the subcarriers assigned to one and the same communication device, i.e. subscriber.

Multiple pilots are located in the bandwidth assigned to one communication device. Thus, the structure of the frame allows each communication device to estimate and correct the carrier frequency offset independently from the presence and the operation of other communication devices. The method as described above and hereinafter and the resulting frame structure are designed in consideration of challenges that particularly arise when OFDMA is implemented in WiFi communication networks. The pilot pattern within a data frame as described herein may lead to a reduced packet error rate, PER, and may improve mobility conditions in wireless data networks. At least some of the embodiments of the method described herein may generate data frames that can be easily reused to channel tracking. OFDMA may be described as a data transmission protocol in a wireless data transmission arrangement between a multitude of or at least two communication devices and an access point, wherein each one of the multitude of communication devices is configured to send and receive data packets to and from the access point, respectively. Each one of the multitude of communication devices is configured to use a predetermined bandwidth and transmit and/or receive data to and from the access point simultaneously using the predetermined bandwidth.

According to an embodiment of the invention, each OFDM symbol is divided such that parts of the OFDM symbol are transmitted in a multitude of subcarriers at different frequencies and the first pilot is provided in a first subcarrier at the first frequency and the second pilot is provided in a second subcarrier at the second frequency, wherein the first subcarrier and the second subcarrier are assigned to the first communication device.

The method as described above and hereinafter and the data frame generated with this method may specifically enhance and improve link impairment estimation, for example channel estimation, particularly carrier frequency offset, CFO, estimation. This particularly applies to the scenario where the available bandwidth is divided between multiple communication devices (clients, subscribers) since the method and the generated data frame ensure that every communication device receives at least one pilot such that link estimation can be performed by any one of the communication devices.

The first pilot and the second pilot can be contained in the same or in different OFDM symbols which are provided at different frequencies, i.e. at different subcarriers in order to achieve frequency diversity.

A data frame may be described as a matrix with two dimensions frequency and time, wherein an OFDM symbol is transmitted with predetermined time duration at a predetermined bandwidth (totally available bandwidth). The totally available bandwidth is divided into multiple subcarriers such that an OFDM symbol is transmitted in multiple subcarriers.

In other words, one OFDM symbol comprises a multitude of subcarriers. In OFDMA, a first plurality of subcarriers (first bandwidth) is assigned to the first communication device and a second plurality of other subcarriers (second bandwidth) is assigned to a second

communication device. The method according to this embodiment comprises the step of generating pilots such that in each of the first bandwidth and the second bandwidth, at least two pilots are arranged at different subcarriers.

A pilot is a known symbol, signal sample or signal sequence provided in an OFDM symbol and is used for estimating channel impairment. A pilot does not transmit any user data but control data, wherein any participating communication device knows the signal pattern and compares the received pilot with the known pattern. The result of this comparison facilitates estimation of channel impairment.

In this embodiment, the pilots are assigned to the subcarriers such that diversity in frequency is provided and robustness to frequency selective channels is allowed. According to a further embodiment of the invention, the method as described above and hereinafter further comprises the step of generating a first OFDM symbol and a second OFDM symbol, wherein the first OFDM symbol comprises the first pilot and the second OFDM symbol comprises the second pilot. In addition to the diversity of pilots in frequency described above, in this embodiment the pilots are provided in different OFDM symbols to spread them over time in order to achieve time diversity.

According to a further embodiment of the invention, the first pilot and the second pilot are provided in non-consecutive OFDM symbols.

Non-consecutive OFDM symbols are symbols which are not transmitted immediately one after the other, i.e. a first OFDM symbol and a second OFDM symbol which are transmitted such that further OFDM symbols transmitted in between them. Thus, the pilots are spread over time.

According to a further embodiment of the invention, multiple instances of the first pilot are provided at the first frequency. One instance of the first pilot is a repetitive transmission of the first pilot. Multiple instances of the first pilot may be referred to as a subset of pilots. This embodiment spreads pilots over time in one and the same subcarrier, i.e. at the same frequency. According to a further embodiment of the invention, a time lag between two successive instances of the multiple instances of the first pilot varies.

A time lag may be defined as the time duration between the transmission of two successive or consecutive instances of the multiple instances of the first pilot. Thus, the time duration between transmitting the first instance of the first pilot and the second instance of the first pilot may be different from the time duration between transmitting the second instance of the first pilot and the third instance of the first pilot. For example, the time duration between successive instances of pilots may increase or decrease. A processing gain of the method described herein can be increased when the time lag is shorter and therefore, the processing gain can be adapted according to current

requirements.

According to a further embodiment of the invention, the time lag between two successive instances of the multiple instances of the first pilot increases in time.

The time gap between successive instances of pilots in the first subcarrier is growing in time. In other words, at the beginning of data transmission, an initial processing gain is maximized, wherein with increasing duration of data transmission, the time lag between successive pilots is increased in order to reduce signalling overhead.

According to a further embodiment of the invention, the method further comprises the steps of: generating a third OFDM symbol comprising a third pilot, wherein the third pilot is provided at the first frequency.

The method according to this embodiment can provide pilots for a multitude of consecutive OFDM symbols such that time diversity in OFDMA can be achieved.

According to a further embodiment of the invention, the first pilot and the third pilot are provided in consecutive OFDM symbols.

Thus, the first pilot and the third pilot provide accumulation of pilots in time as they are transmitted without or virtually without time delay between each other. Multiple pilots transmitted continuously in time, i.e. the first and the third pilot transmitted in consecutive OFDM symbols, are used to improve estimation of link impairment, thus the processing gain can be increased. According to a further embodiment of the invention, the first pilot and the third pilot represent a group of pilots provided in consecutive OFDM symbols, wherein multiple groups of pilots are provided in one data frame in different subcarriers.

Thus, groups of pilots are provided in order to increase processing gain, wherein the groups of pilots are spread over time and frequency to achieve time diversity and frequency diversity.

According to a further embodiment of the invention, the first group of pilots and the second group of pilots are arranged in different OFDM symbols in the data frame.

Thus, the groups of pilots do not overlap in time and the total duration of continuously transmitted pilot patterns is increased by providing consecutive pilots in time.

According to a further embodiment of the invention, the method as described above and hereinafter further comprises the steps of: providing a fourth pilot in the first OFDM symbol, wherein the fourth pilot and the first pilot are provided at different frequencies.

Thus, the first pilot and the fourth pilot are located at different subcarriers to achieve frequency diversity.

According to a further embodiment of the invention, the fourth pilot and the second pilot are provided at different frequencies.

Thus, the total number of subcarriers in which a pilot is transmitted is increased. For example, the subcarriers with the first pilot and the second pilot may be assigned to a first communication device and the subcarrier with the fourth pilot may be assigned to a second communication device such that any communication device is assigned a subcarrier which comprises a pilot in order to allow any communication device to carry out link impairment estimation.

The subcarriers at which the first, second, third and fourth pilots are transmitted may vary during the operation time of an access point. Particularly, some or all of the pilots may be transmitted at other frequencies if the frequency assignment or subcarrier allocation per communication device is changed.

The method as described above and hereinafter may be summarized, alternatively described and additionally characterized as follows:

Link impairment estimation, particularly CFO estimation, in the IEEE 802.1 1 standards is based on pilots which are spread over the entire available bandwidth. This data frame generated with the method described herein provides diversity in frequency and allows robustness to frequency selective channels. However, in OFDMA based WiFi, where a client may be allocated a very narrow allocation, a CFO pilot may occupy the same subcarrier for the entire frame duration. It is proposed to spread the CFO pilots over the bandwidth within one resource unit, RU, in order to achieve similar diversity in OFDMA based WiFi. The described method can significantly improve CFO estimation performance, reduce the error probability respective to the CFO estimation even if the channel suffers from very poor signal to noise ratio, SNR, inside a single RU and allow reuse of the CFO pilots for channel tracking along the packet.

It is one central idea of the method as described above and hereinafter to provide a new pilot structure, where the pilots are spread over the entire bandwidth, even in very narrow band allocations. The number of available CFO pilots in existing technologies may be preserved and rearranged. Following existing WiFi pilot patterns, the pilots are located at the same subcarrier in every OFDM symbol. Thus, the following is assumed: a number of OFDM symbols per RU is denoted as N; the maximum number of the CFO pilots per RU is equal to the number of OFDM symbols; the total number of the subcarriers in an RU is denoted as K^*N, where K depends on the predetermined bandwidth of the RU defined by the used data transmission standard; N pilots can be located at any N out of K^*N subcarriers to achieve high CFO estimation performance. The proposed method and the resulting pilot pattern are based on at least some of the following design principles. The pilot pattern is designed to allow best coverage of the following aspects: correct CFO estimation in frequency selective channel conditions, accurate CFO estimation for very small CFO values, and maximum processing gain. The criteria mentioned above may not be achieved using a single pilot pattern design. For example, maximizing the processing gain limits the performance in frequency selective channels. Thus, different pilot pattern designs as described with reference to different embodiments above are proposed where one criterion is maximized along with reasonable degradation in terms of other aspects. When performing CFO estimation, a receiver typically aggregates less OFDM symbols to reduce the latency of data processing. Small CFO values may require larger gaps between the pilots to improve the CFO estimation accuracy and pilots that are spread in frequency yield a higher diversity gain. Assuming almost constant CFO value along the packet, CFO estimation converges after a small number of the OFDM symbols.

According to a further aspect of the invention, a data transmission arrangement is provided, comprising an interface configured to wirelessly transmit data to a first communication device and to a second communication device and a data frame generator configured to generate orthogonal frequency division multiple access, OFDMA, frames, wherein the data frame generator is configured to carry out the method for generating a pilot pattern within a data frame for a data transmission arrangement employing orthogonal frequency division multiple access, OFDMA, as described above and hereinafter. The data transmission arrangement may be an access point according to one of the WiFi IEEE 802.1 1 standards, particularly according to IEEE 802.1 1 ax.

The details provided above with reference to the method for generating a pilot pattern within a data frame for a data transmission arrangement employing orthogonal frequency division multiple access, OFDMA, apply similarly to the data transmission arrangement. Particularly, the data transmission arrangement may be configured such that the data frame generator or any other structural component carries out the method steps described above. However, these details are not repeated herein. The data transmission arrangement may implement the method as described above and hereinafter in hardware and/or in software.

According to a further aspect of the invention, a data transmission system is provided, comprising a data transmission arrangement as described above and hereinafter. The data transmission system further comprises a first communication device and a second

communication device, wherein the first communication device and the second

communication device are configured to estimate link impairment of a data transmission link between the data transmission arrangement and the first communication device and the second communication device, respectively, based on a received data frame, particularly based on the pilots contained in the received data frame. BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the invention will be described with respect to the following figures, in which: Fig. 1 shows a diagram of bandwidth allocation in OFDM and OFDMA;

Fig. 2 shows an example of coherence bandwidth of three communication devices; Fig. 3 schematically shows an OFDMA data frame generated according to the rules of a method according to an embodiment;

Fig. 4 schematically shows an OFDMA data frame generated according to the rules of a method according to another embodiment;

Fig. 5 schematically shows an OFDMA data frame generated according to the rules of a method according to another embodiment;

Fig. 6 shows a data transmission system according to an embodiment;

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows bandwidth allocation to consumer device(s) in OFDM (left hand side) and OFDMA (right hand side).

In OFDM the totally available bandwidth of a data transmission channel of a data

transmission arrangement, for example of an IEEE 802.1 1 access point, is assigned to one user, for example User₀ indicated in Fig. 1 . Contrary to this approach, in OFDMA, the totally available bandwidth is divided and assigned to a multitude of users, such that every one of the multitude of users has been assigned its own frequency domain or range, which is referred to as subcarrier in the context of OFDMA. In the specific example of Fig. 1 , the bandwidth is assigned to four users, namely User₀, User_! , User₂, and User₃.

In OFDMA technology, the available bandwidth is divided between several clients where the minimal frequency and time resource is known as Resource Unit (RU). Each station transmits and/or receives within its respective specified RUs while the number of RUs can vary from a single RU to all RUs (entire bandwidth). All the impairments, including CFO, should be estimated per client inside the allocated RUs. The bandwidth of one RU may vary between the standards of wireless data transmission. Several alternatives for RU size may be possible, such as for example 24+2 tones (~2MHz). The channel models adopted by IEEE 802.1 1 standards have a maximal delay spread of above Ο.δμεβα Such a delay spread results in coherence bandwidth of around 1 MHz. If existing CFO pilot structures are re-used for OFDMA when a single RU is allocated per client, there is only one CFO pilot per OFDM symbol available for the respective client.

Moreover all the pilots in all the OFDM symbols are located at the same subcarrier. As a result, a single pilot may suffer from very poor channel conditions. Therefore, using only one pilot per client may ruin the CFO estimation. Hence, the method for generating a pilot pattern within a data frame for a data transmission arrangement employing orthogonal frequency division multiple access, OFDMA, may overcome this drawback.

Fig. 2 exemplarily shows the channel response over the frequency of three communication signals assigned to three users User_! , User₂, and User₃.

Spreading multiple pilots in frequency yields diversity and allows correct CFO estimation even if a single pilot suffers from very low signal to noise ratio (SNR), as can be seen at about 3 MHz of the User₃ communication signal and at about 8 MHz of the User₂

communication signal. The move to OFDMA leads to potentially narrower bandwidth allocations per client (e.g. a single RU). In such a case, only a single subcarrier may be used as a CFO pilot, which can lead to significant degradation of the CFO estimation accuracy due to very low SNR experienced on this one pilot subcarrier, particularly if the available bandwidth is divided such that one communication device is assigned a frequency with bad SNR.

The method for generating a pilot pattern described herein overcomes this drawback as the pilots are generated in an OFDMA scenario at different frequencies such that any

communication device receives at least two pilots at different frequencies.

A new pilot pattern should be designed to accommodate channel selectivity and allow CFO estimation per RU.

Figs. 3 to 5 describe exemplary data frames 100 generated according to an embodiment of the method described herein.

Fig. 3 shows a data frame 100 comprising multiple OFDM symbols 120 presented in a matrix with two dimensions time 102 and frequency 104. A column of this matrix corresponds to one OFDM symbol 120 and a row corresponds to one subcarrier 1 10 at a specified frequency. A first group of pilots 140 comprising four pilots (two of which are denoted as first pilot 132 and third pilot 136) in subsequent OFDM symbols 121 , 122, 123, 124 and 125 is arranged in a first subcarrier 1 1 1 . The subcarriers 1 12, 1 13, 1 14 do not comprise any pilots. A second group of pilots 142 comprising four pilots (one of which is denoted as second pilot 134) is arranged in the subcarrier 1 15 such that the second group of pilots 142 does not overlap the first group of pilots 140 in time.

Two additional groups of pilots 144, 146 are shown in Fig .1 . All groups of pilots are transmitted consecutively in time (not overlap on time-axis and at most one pilot per OFDM symbol) and at different subcarriers in order to achieve frequency diversity.

In this embodiment, particularly maximum processing gain may be achieved as pilots are located at consecutive OFDM symbols. In this case multiple pilot pairs can be used to estimate the CFO, thus the processing gain is increased. The main properties of this design are: N pilots are divided into M groups; Each group consists of N/M pilots; The pilots of each group are located at the same subcarrier along N/M consecutive OFDM symbols; Each group is located at a different subcarrier to achieve frequency diversity; Each group is located at different OFDM symbols to allow CFO tracking along the entire packet.

The subcarriers which contain the groups of pilots are chosen such that the subcarriers cover the entire bandwidth of the data frame. For example, the i^th group of pilots is allocated at the subcarrier (i-1 )[K/M] and per allocated subcarrier the pilots are placed at N/M consecutive OFDM symbols starting from symbol (i-1 )[N/M].

This embodiment may have the following advantages and effects: enable symbol by symbol CFO estimation inside each group; enable CFO interpolation on the transition between the groups; enable averaging of group results to achieve diversity in frequency. Fig. 4 illustrates a data frame 100 the structure of which basically corresponds to the structure of the data frame 100 shown in Fig. 3 and which is therefore not repeated herein.

In a subcarrier 1 1 1 , pilots are arranged equidistant in time (one pilot is provided in every fourth OFDM symbol, wherein the first pilot 132 is provided in the first OFDM symbol 121 ).

In a subcarrier 1 15, pilots are arranged similarly to those shown in the subcarrier 1 1 1 but with a time offset of two OFDM-symbols. The second pilot 134 is provided in the subcarrier 1 15 in the OFDM-symbol 123. The time lag 150 between subsequent pilots in one subcarrier is constant in time and the same in all subcarriers 1 1 1 , 1 15.

The subcarriers 1 1 1 , 1 15 containing pilots are spread over the frequency of the data frame. Particularly, these subcarriers are not adjacent subcarriers but further subcarriers are arranged between them in frequency. The pilot pattern of subcarrier 1 19 corresponds to the pilot pattern of subcarrier 1 1 1 .

It should be noted that the explanations provided with reference to Fig. 4 could also apply to a data frame containing any number of subcarriers of which any subcarriers could contain pilots. Therefore, the principle described with reference to two subcarriers is applicable to any number of subcarriers. This also applies to the remaining embodiments shown in Figs. 3 and 5.

In order to achieve better accuracy of the CFO estimation, especially for small CFO values, a long gap duration in time should be used between two CFO pilots. In this case, the CFO is accumulated and the estimation performance is improved. The design properties are as following: N pilots are divided into M groups; Each group consists of N/M pilots; The pilots of each group are located at the same subcarrier at N/M non-consecutive OFDM symbols; Each group is located at a different subcarrier to achieve diversity. In this case the processing gain is less compared to the pilot pattern shown in Fig. 3 because only close pilots can be used for CFO estimation.

For example, the i^th group of pilots is allocated at the subcarrier (i-1 )[K/M] and pilots of each group are placed at N/M OFDM symbols while the j^th pilot is placed at symbol (j- 1 )[N/M]+offset. The offset may be zero for odd groups and N/(2M) for even symbols.

This embodiment may have the following advantages and effects: all the pilots allocated in current OFDM symbol may be averaged; pilots may be interpolated in frequency to allow CFO estimation with shifted pilots; N/M symbol aggregation for CFO estimation between two consecutive pilots of each group.

Fig. 5 illustrates a data frame 100 the structure of which basically corresponds to the structure of the data frame 100 shown in Figs. 3 and 4 and which is therefore not repeated herein.

In a subcarrier 1 1 1 , pilots are arranged such that the time lag between subsequent pilots in the same subcarrier increases in time (a first instance 132A is provided in the first OFDM- symbol, the second instance 132B is provided in the fourth OFDM-symbol, the third instance 132C is provided in the ninth OFDM-symbol and the fourth instance 132D is provided in the sixteenth OFDM-symbol). In a subcarrier 1 15, pilots are arranged in the same OFDM-symbols than in the subcarrier 1 1 1 . The time lag between subsequent pilots in one subcarrier increases in time and is the same in all subcarriers 1 1 1 , 1 15. The number of pilot groups can be chosen in such a way that the number of pilots in each OFDM symbol is enough to allow reasonable channel estimation.

Taking into account that CFO changes slowly in time, the processing gain may be more important at the beginning. Thus, a hybrid pilot pattern is described in Fig. 5 where time gap between the pilots is growing with time. This idea allows maximizing the initial processing gain and in addition allowing accurate CFO estimation within the packet. The main principles of this design are: N pilots are divided into M groups; Each group consists of N/M pilots; The pilots of each group are located at the same subcarrier at N/M OFDM symbols with a growing gap between the symbols; Each group is located at a different subcarrier to achieve frequency diversity.

This embodiment may have the following advantages and effects: all the pilots allocated in current OFDM symbol may be averaged; small number of symbols to be aggregated at the start of a data frame; CFO interpolation for later OFDM symbols.

Fig. 6 describes a data transmission system with a data transmission arrangement 10 and two communication devices 20A, 20B which receive data from and transmit data to the data transmission arrangement 10 via a wireless data link 30. The data transmission arrangement 10 comprises a data frame generator 14 and a data transmission interface 12, for example an air interface.

The data frame generator is configured to carry out the method as described with reference to any one of the embodiments herein.

Although the invention is described with reference to specific features, implementation forms, and embodiments, it is evident that various modifications and combinations can be made thereto without departing from the spirit and scope of the invention. The description and the figures are, accordingly, to be regarded simply as an illustration of the invention as defined by the appended claims, and are contemplated to cover any and all modifications, variations, combinations, or equivalents that fall within the scope of the invention. List of reference signs

10 data transmission arrangement

12 data transmission interface

14 data frame generator

20A first communication device

20B second communication device

30 data transmission link

100 data frame

102 time

104 frequency

1 10 subcarrier

1 1 1 -1 15 subcarriers

120 OFDM symbol

121 -125 OFDM symbols

132 first pilot

132A-132D instances of the first pilot

134 second pilot

136 third pilot

138 fourth pilot

140-144 groups of pilots

150 time lag between consecutive pilots in one subcarrier

K number of subcarriers

N number of OFDM symbols

Claims

1 . Method for generating a pilot pattern within a data frame (100) for a data transmission arrangement employing orthogonal frequency division multiple access, OFDMA;

wherein one data frame comprises a multitude of OFDM symbols (120) to be consecutively transmitted in time;

the method comprising the steps of:

transmitting a first pilot (132) at a first frequency of one of the OFDM symbols;

transmitting a second pilot (134) at a second frequency of one of the OFDM symbols, wherein the second frequency is different from the first frequency;

wherein the first frequency and the second frequency are assigned to a first communication device (20A).

2. Method according to claim 1 further comprising

wherein each OFDM symbol is divided such that parts of the OFDM symbol are transmitted in a multitude of subcarriers (1 10) at different frequencies;

providing the first pilot (132) in a first subcarrier (1 1 1 ) at the first frequency and the second pilot (134) in a second subcarrier (1 15) at the second frequency;

assigning the first subcarrier (1 1 1 ) and the second subcarrier (1 15) to the first communication device (20A).

3. Method according to claim 1 or 2,

further comprising the step of

generating a first OFDM symbol (121 ) and a second OFDM symbol (125);

wherein the first OFDM symbol (121 ) comprises the first pilot (132) and the second

OFDM symbol (125) comprises the second pilot (134).

4. Method according to any one of the preceding claims,

wherein the first pilot (132) and the second pilot (134) are provided in non- consecutive OFDM symbols (120).

5. Method according to any one of the preceding claims,

wherein multiple instances (132A, 132B, 132C, 132D) of the first pilot are provided at the first frequency.

6. Method according to claim 5,

wherein a time lag (150) between two successive instances of the multiple instances of the first pilot varies.

7. Method according to claim 6,

wherein the time lag (150) between two successive instances of the multiple instances of the first pilot increases in time.

8. Method according to any one of the preceding claims,

further comprising the steps of

generating a third OFDM symbol (122) comprising a third pilot (136);

wherein the third pilot (136) is provided at the first frequency.

9. Method according to claim 8,

wherein the first pilot (132) and the third pilot (136) are provided in consecutive OFDM symbols (121 , 122).

10. Method according to claim 8 or 9,

wherein the first pilot (132) and the third pilot (136) represent a group of pilots (140) provided in consecutive OFDM symbols;

wherein multiple groups of pilots (140, 142, 144) are provided in one data frame in different subcarriers.

1 1 . Method according to claim 10,

wherein a first group (140) of pilots and a second group (142) of pilots are arranged i different OFDM symbols in the data frame.

12. Method according to any one of the preceding claims,

further comprising the steps of

providing a fourth pilot (138) in the first OFDM symbol (121 );

wherein the fourth pilot and the first pilot are provided at different frequencies.

13. Method according to claim 12,

wherein the fourth pilot (138) and the second pilot (134) are provided at different frequencies.

14. Data transmission arrangement (10), comprising

an interface (12) configured to wirelessly transmit data to a first communication device (20A) and a second communication device (20B);

a data frame generator (14) configured to generate orthogonal frequency division multiple access, OFDMA, frames; wherein the data frame generator (14) is configured to carry out the method according to any one of the preceding claims.

15. Data transmission system, comprising

a data transmission arrangement (10) according to claim 14;

a first communication device (20A) and a second communication device (20B);

wherein the first communication device (20A) and the second communication device (20B) are configured to estimate link impairment of a data transmission link (30) between the data transmission arrangement (10) and the first communication device (20A) and the second communication device (20B), respectively, based on a received data frame.