CN107210878B

CN107210878B - Pilot patterns for WIFI OFDMA

Info

Publication number: CN107210878B
Application number: CN201580074493.6A
Authority: CN
Inventors: 根纳季·特所迪克; 希米·西隆; 多伦·埃兹里; 奥德·里德里; 刘乐; 张佳胤
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2015-02-25
Filing date: 2015-02-25
Publication date: 2020-04-28
Anticipated expiration: 2035-02-25
Also published as: CN107210878A; WO2016134763A1

Abstract

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

Description

Pilot patterns for WIFI OFDMA

Technical Field

The present invention relates to the technical field of data transmission in communication networks. In particular, the present invention relates to a method for generating a pilot pattern within a data frame for a data transmission apparatus employing orthogonal frequency division multiple access, OFDMA (OFDMA), to a data transmission apparatus configured to perform the method and to a data transmission system.

Background

In a communication network, data is typically modulated or encoded using a modulation scheme before being transmitted from a transmitter to one or more receivers via a communication channel. The communication channel may be a wired or wireless transmission path between the transmitter and the receiver. The transmission path may be configured for unidirectional communication (simplex), bidirectional alternate communication (half duplex) or bidirectional simultaneous communication (duplex) between two communicating entities.

Several modulation and coding schemes are known and may be used, for example, according to the characteristics of the communication channel, according to desired data transmission parameters, and according to the needs of the entities involved in the communication.

One of these coding schemes is orthogonal frequency division multiplexing, OFDM (OFDM). OFDM encodes data to be transmitted using a plurality of orthogonal carriers, so that several parallel data streams are generated as channels. A subcarrier signal is 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 (orthogonal frequency-division multiple access), is a further development of OFDM and is configured for multi-user access by allocating one or more sub-carriers to respective receiving devices or users, respectively.

OFDMA may be used, for example, for data transmission in WiFi systems. WiFi frames typically include two main parts: a preamble and data. Each of these includes a dedicated signal for carrier frequency offset, CFO, estimation. In the first phase, initial CFO estimation is performed based on preamble content, in particular, Legacy Short Training Field L-STF (L-STF) and Legacy Long Training Field L-LTF (L-LTF), signals. The second, and final, phase is CFO tracking during the data portion of the WiFi frame. The WiFi preamble includes two preambles, L-STF and L-LTF based on known training sequences sent in a repeated fashion. The WiFi receiver uses the repetitions within the signal to estimate the initial CFO. The data portion includes reserved carriers (reserved tones), called "pilots," which are also known to the receiver, thus enabling 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 enable correct CFO estimation under various channel conditions.

In current WiFi receivers, CFO estimation and correction mechanisms may be critical to achieve good system performance under various environments. The CFO pilots are located at every OFDM symbol starting with the L-LTF. During the data portion, the receiver typically compares the pilots in two consecutive OFDM symbols and updates the CFO values.

Disclosure of Invention

The invention aims to improve the effectiveness and efficiency of an OFDMA communication network, in particular to link impairment (link impairment) estimation.

This object is achieved by the features of the independent claims. Further implementations are apparent from the dependent claims, the description and the drawings.

The present invention is based on the following findings:

part of the signal processing at the receiver is the estimation and correction of impairments created by the radio environment and analog parts of the communication network. The problem is the estimation and correction of the Carrier Frequency Offset (CFO) caused by the clock mismatch between the transmitter and the receiver and possibly the doppler effect caused by the wireless transmission link. The prior art introduces several solutions to this problem, some of which are incorporated into the frame structure. However, all 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 OFDM systems and enable the receiver to tolerate very high CFO. These techniques become less effective in extending OFDM to OFDMA, where each client occupies only a portion of the available bandwidth (which may be very narrow) and many clients are able to transmit simultaneously, each client transmitting on its respective corresponding portion 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 standards. Some of them allow very narrow bandwidths to be allocated to particular clients, while others may allow wider bandwidths to be allocated to particular clients.

According to a first aspect of the present invention, there is provided a method for generating a pilot pattern within a data frame for a data transmission apparatus employing Orthogonal Frequency Division Multiple Access (OFDMA). One data frame includes a plurality of OFDM symbols to be transmitted consecutively in time. The method comprises the following steps: transmitting a first pilot of one of the OFDM symbols at a first frequency; and transmitting a second pilot of one of the OFDM symbols at a second frequency, wherein the second frequency is different from the first frequency, wherein the first frequency and the second frequency are allocated to the first communication device.

In particular, the method may be implemented in a wireless data communication network according to one of the IEEE 802.11 standards, in particular a wireless data communication network employing an OFDMA-based WiFi technology adopted by the IEEE 802.11ax standard, to enable CFO estimation. In such a communication network, a plurality of subscribers are provided which are configured to receive data from and transmit data to a data transmission device, which may be referred to as an access point. Thus, the method described above and below may advantageously be implemented in the following scenarios: a plurality of subscribers share a predetermined bandwidth to receive and transmit data according to the principle of OFDMA. In particular, in such a scenario, each of the plurality of subscribers is assigned a first frequency and a second frequency, each of the first frequency and the second frequency including a pilot. Thus, each of the plurality of subscribers is able to estimate the link impairment, since each subscriber is assigned a frequency comprising a pilot, independently of the exact bandwidth allocation to the client.

In other words, the pilot may not be transmitted at a fixed frequency, independent of the subscriber's allocation to available frequencies, but rather, may be enabled to be transmitted at different frequencies depending on the subscriber's allocation to frequencies.

In the methods described above and below, the pilot is spread over the frequencies and subcarriers allocated to the same communication device, i.e., subscriber. A plurality of pilots are located in the bandwidth allocated to one communication device. Thus, the structure of the frame enables each communication device to estimate and correct the carrier frequency offset independently of the presence and operation of the other communication devices. The method described in the context and the resulting frame structure are designed taking into account the challenges that arise in implementing OFDMA, particularly in WiFi communication networks.

The pilot patterns within the data frames described herein may result in a reduced Packet Error Rate (PER) and may improve mobility conditions of the wireless data network. At least some of the embodiments of the methods described herein may generate data frames that can be easily reused for channel tracking.

OFDMA may be described as a data transmission protocol between a plurality or at least two communication devices and an access point in a wireless data transmission arrangement, wherein each of the plurality of communication devices is configured to transmit and receive data packets to and from the access point, respectively. Each of the plurality of communication devices is configured to simultaneously transmit data to and/or receive data from the access point using a predetermined bandwidth and using the predetermined bandwidth.

According to an embodiment of the present invention, each OFDM symbol is divided such that portions of the OFDM symbol are transmitted in a plurality of subcarriers at different frequencies, and first pilots are provided in first subcarriers at a first frequency and second pilots are provided in second subcarriers at a second frequency, wherein the first and second subcarriers are allocated to a first communication device.

The method described above and below and the data frames generated using the method may in particular improve and improve link impairment estimation, such as channel estimation, in particular carrier frequency offset, CFO (CFO), estimation. This is particularly applicable in scenarios where the available bandwidth is divided between multiple communication devices (clients, subscribers), as the method and the generated data frames ensure that each communication device receives at least one pilot so that any one of the communication devices can perform link estimation.

The first pilot and the second pilot may be included in the same or different OFDM symbols and disposed at different frequencies, i.e., different subcarriers, to achieve frequency diversity.

A data frame may be described as a matrix having two dimensions, frequency and time, where OFDM symbols are transmitted with a predetermined bandwidth (total available bandwidth) using a predetermined duration. The total available bandwidth is divided into a plurality of subcarriers such that OFDM symbols are transmitted in the plurality of subcarriers.

In other words, one OFDM symbol includes a plurality of subcarriers. In OFDMA, a first plurality of subcarriers (first bandwidth) is allocated to a first communication device and a second plurality of other subcarriers (second bandwidth) is allocated to a second communication device. The method according to this embodiment comprises the steps of: the pilots are generated such that at least two pilots are arranged at different subcarriers in each of the first and second bandwidths.

The pilots are known symbols, signal samples, or signal sequences disposed in the OFDM symbols and are used to estimate channel impairments. The pilots do not transmit any user data but control data, wherein any participating communication devices know the signal pattern and compare the received pilots with the known pattern. The result of this comparison facilitates estimation of channel impairments.

In this embodiment, the allocation of pilots to subcarriers provides frequency diversity and gives robustness to frequency selective channels.

According to another embodiment of the invention, the method as described above and below further comprises the steps of: a first OFDM symbol and a second OFDM symbol are generated, wherein the first OFDM symbol includes first pilots and the second OFDM symbol includes second pilots.

In addition to the diversity of the pilots in frequency as described above, the pilots are also placed in different OFDM symbols in this embodiment to spread the pilots in time in order to achieve time diversity.

According to another embodiment of the present invention, the first pilots and the second pilots are arranged in non-contiguous OFDM symbols.

A discontinuous OFDM symbol is a first OFDM symbol and a second OFDM symbol that are not transmitted next to one another, i.e., transmitted such that further OFDM symbols are transmitted between them. Thus, the pilot is spread in time.

According to another embodiment of the present invention, multiple instances of the first pilot are disposed at the first frequency.

One example of a first pilot is a repeated transmission of the first pilot. The multiple instances of the first pilot may be referred to as a pilot subset. This embodiment spreads the pilots in time in the same sub-carrier, i.e. at the same frequency.

According to another embodiment of the invention, the time delay between two successive instances of the multiple instances of the first pilot is different.

The time delay may be defined as the duration between the transmission of two successive instances or consecutive instances of the multiple instances of the first pilot. Thus, a duration between transmitting a first instance of the first pilot and transmitting a second instance of the first pilot may be different than a duration between transmitting the second instance of the first pilot and transmitting a third instance of the first pilot. For example, the duration between successive instances of pilot may be increased or decreased.

The processing gain of the method described herein can be increased when the time delay is short, and thus the processing gain can be adaptively adjusted according to current requirements.

According to another embodiment of the invention, the time delay between two successive instances of the plurality of instances of the first pilot increases with time.

The time interval between successive instances of the pilot in the first subcarrier gradually increases over time. In other words, the initial processing gain is greatest at the beginning of the data transmission, where as the duration of the data transmission increases, the time delay between successive pilots increases to reduce the signaling overhead.

According to another embodiment of the invention, the method further comprises the steps of: a third OFDM symbol is generated that includes third pilots, wherein the third pilots are disposed at the first frequency.

The method according to this embodiment can provide pilots for a plurality of consecutive OFDM symbols, enabling time diversity in OFDMA.

According to another embodiment of the present invention, the first pilots and the third pilots are arranged in consecutive OFDM symbols.

Thus, the first pilot and the third pilot provide an accumulation of pilots in time when they are transmitted with no or almost no time delay between each other. The estimation of the link impairment is improved using a plurality of pilots transmitted consecutively in time, i.e., the first pilot and the third pilot transmitted in consecutive OFDM symbols, so that the processing gain can be increased.

According to another embodiment of the present invention, the first pilots and the third pilots represent pilot groups disposed in consecutive OFDM symbols, wherein a plurality of pilot groups are disposed in different subcarriers in one data frame.

Thus, the pilot set is set to increase processing gain, where the pilot set is spread in time and frequency to achieve time diversity and frequency diversity.

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

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

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

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

According to another embodiment of the invention, the fourth pilot and the second pilot are arranged at different frequencies.

Therefore, the total number of subcarriers transmitting pilots increases. For example, subcarriers with first and second pilots may be allocated to a first communication device and subcarriers with fourth pilots to a second communication device such that either communication device is allocated subcarriers including pilots to enable either communication device to perform link impairment estimation.

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

The methods described in the context may be summarized, or described and otherwise characterized as follows:

link impairment estimation, and in particular CFO estimation, in the IEEE 802.11 standard is based on pilots spread over the entire available bandwidth. The data frame generated using the methods described herein provides diversity in frequency and confers robustness to the frequency selective channel. However, in OFDMA-based WiFi, where clients may be allocated a very narrow allocation, the CFO pilots may occupy the same subcarriers for the entire frame duration. It is proposed to spread the CFO pilot over the bandwidth within one resource unit, RU, to achieve similar diversity in OFDMA based WiFi. The method can significantly improve CFO estimation performance, reduce the probability of error corresponding to CFO estimation, and allow the CFO pilot to be reused for channel tracking along a packet, even if the channel suffers from very poor signal to noise ratio, SNR, in a single RU.

One core idea of the method described in the context is to provide a new pilot structure, wherein the pilots are spread over the whole bandwidth, even in case of very narrow band allocations. The number of available CFO pilots in the prior art may be reserved and rearranged. Following the existing WiFi pilot pattern, the pilots may be located at the same subcarriers in each OFDM symbol. Therefore, the following assumptions are made: representing the number of OFDM symbols per RU as N; the maximum number of CFO pilots per RU is equal to the number of OFDM symbols; representing a total number of subcarriers in the RU as K × N, wherein K depends on a predetermined bandwidth of the RU defined by a data transmission standard used; the N pilots can be located at any N subcarriers of the K × N subcarriers to achieve high CFO estimation performance.

The proposed method and the generated pilot pattern are based on at least some of the following design principles. The pilot pattern is designed to enable optimal coverage of the following: correct CFO estimation under frequency selective channel conditions; accurate CFO estimation for very small CFO values; and a maximum processing gain. The above criteria cannot be implemented using a single pilot pattern design. For example, maximizing processing gain limits the performance of frequency selective channels. Thus, different pilot pattern designs are proposed as described with reference to the different embodiments above, wherein one criterion is maximized with reasonable degradation in other respects.

When performing CFO estimation, the receiver typically aggregates fewer OFDM symbols to reduce the latency of data processing. A small CFO value may require a larger inter-pilot spacing to improve CFO estimation accuracy and a pilot spread in frequency yields higher diversity gain. Assuming an almost constant CFO value along the packet, the CFO estimate converges after a small number of OFDM symbols.

According to another aspect of the present invention, there is provided a data transmission apparatus including: an interface configured to wirelessly transmit data to a first communication device and a second communication device; and a data frame generator configured to generate an Orthogonal Frequency Division Multiple Access (OFDMA) frame, wherein the data frame generator is configured to perform the method for generating a pilot pattern within the data frame for a data transmission device employing Orthogonal Frequency Division Multiple Access (OFDMA) described above and below.

The data transmission means may be an access point according to one of the WiFi IEEE 802.11 standards, in particular according to IEEE 802.11 ax.

The details provided above with reference to the method for generating a pilot pattern within a data frame for a data transmission apparatus employing orthogonal frequency division multiple access, OFDMA, are equally applicable to the data transmission apparatus. In particular, the data transmission device may be configured such that the data frame generator or any other structural component performs the above-described method steps. However, these details are not repeated here. The data transmission means may implement the method described above and below in hardware and/or software.

According to another aspect of the present invention there is provided a data transfer system comprising a data transfer device as described above and below. 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 a 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 the received data frame, in particular based on pilots comprised in the received data frame.

Drawings

Embodiments of the invention will be described with respect to the following drawings, in which:

fig. 1 shows a diagram of bandwidth allocation in OFDM and OFDMA;

FIG. 2 shows an example of coherence bandwidths of three communication devices;

figure 3 schematically illustrates an OFDMA data frame generated according to the rules of the method of an embodiment;

figure 4 schematically illustrates an OFDMA data frame generated according to a rule of a method of another embodiment;

figure 5 schematically illustrates an OFDMA data frame generated according to a rule of a method of another embodiment;

fig. 6 shows a data transmission system according to an embodiment.

Detailed Description

Fig. 1 shows bandwidth allocation to one or more consumer devices in OFDM (left-hand side) and OFDMA (right-hand side).

In OFDM, the total available bandwidth of a data transmission channel of a data transmission device, e.g. an IEEE 802.11 access point, is allocated to one user, e.g. the user indicated in fig. 1₀. In contrast to this approach, in OFDMA, the total available bandwidth is divided and partitionedA plurality of users are allocated such that each of them is allocated its own frequency domain or range, which in the context of OFDMA is called a subcarrier. In the specific example of fig. 1, bandwidth is allocated to four users, i.e., users₀User, user₁User, user₂And the user₃。

In OFDMA technology, the available bandwidth is divided among several clients, where the smallest frequency and time Resource is called a Resource Unit (RU). Each station transmits and/or receives within its respective designated RU, and the number of RUs can vary from a single RU to all RUs (the entire bandwidth). All impairments including CFO should be evaluated per client within the allocated RU.

The bandwidth of one RU may vary between wireless data transmission standards. Several alternatives for RU size may be feasible, such as 24+2 subcarriers (-2 MHz). The channel model adopted by the IEEE 802.11 standard has a maximum delay spread of 0.8 microseconds or more. Such delay spread results in a coherence bandwidth of about 1 MHz. If the existing CFO pilot structure is reused for OFDMA when each client is allocated a single RU, there is only one CFO pilot available for the respective client per OFDM symbol. Also, all pilots in all OFDM symbols are located at the same subcarrier. Thus, a single pilot may suffer from very poor channel conditions. Thus, using only one pilot per client may destroy the CFO estimate. Accordingly, the method for generating a pilot pattern within a data frame for a data transmission apparatus employing Orthogonal Frequency Division Multiple Access (OFDMA) may overcome this disadvantage.

FIG. 2 exemplarily shows allocation to three users-users₁User, user₂And the user₃The channel response of the three communication signals over frequency.

Spreading multiple pilots across frequency creates diversity and enables correct CFO estimation even if a single pilot suffers from very low signal-to-noise ratio (SNR), such as at a user₃At about 3MHz of the communication signal and at the user₂Visible at about 8MHz of the communication signal. The evolution to OFDMA results in 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 may result in a significant reduction in CFO estimation accuracy due to the very low SNR experienced on this pilot subcarrier, especially when the available bandwidth is divided such that one communication device is allocated frequencies with poor SNR.

The method for generating pilot patterns described herein overcomes this drawback because pilots are generated at different frequencies in an OFDMA scenario such that at least two pilots at different frequencies are received by any communication device. The new pilot pattern should be designed to accommodate channel selectivity and enable CFO estimation per RU.

Fig. 3-5 depict an exemplary data frame 100 generated according to an embodiment of the method described herein.

Fig. 3 shows a data frame 100 comprising a plurality of OFDM symbols 120, the plurality of OFDM symbols 120 being present in a matrix having two dimensions of time 102 and frequency 104. The columns of the matrix correspond to one OFDM symbol 120 and the rows correspond to one subcarrier 110 at a specified frequency.

A first pilot group 140 is arranged in the first sub-carrier 111, the first pilot group 140 comprising four pilots (two of which are denoted as first pilot 132 and third pilot 136) in

successive OFDM symbols

121, 122, 123, 124 and 125. The

subcarriers

112, 113, 114 do not include any pilots. Second pilot group 142 is arranged in subcarriers 115 such that second pilot group 142 does not overlap in time with first pilot group 140, second pilot group 142 including four pilots (one of which is denoted as second pilot 134).

Two

further pilot groups

144, 146 are shown in figure 1. All pilot groups are transmitted continuously in time (non-overlapping in time axis and at most one pilot per OFDM symbol) on different sub-carriers to achieve frequency diversity.

In this embodiment, in particular, maximum processing gain can be achieved when the pilots are located at consecutive OFDM symbols. In this case, the CFO can be estimated using a plurality of pilot pairs, thereby increasing the processing gain. The main properties of the design are: dividing the N pilots into M groups; each group includes N/M pilots; the pilots of each group are located at the same subcarriers along N/M consecutive OFDM symbols; each group is located at a different subcarrier to achieve frequency diversity; each group is located at a different OFDM symbol to enable CFO tracking along the entire packet.

The subcarriers containing the pilot groups are selected such that the subcarriers cover the entire bandwidth of the data frame. For example, the ith pilot group is allocated at subcarrier (i-1) [ K/M ] and pilots are placed at N/M consecutive OFDM symbols starting from symbol (i-1) [ N/M ] for each allocated subcarrier.

This embodiment may have the following advantages and effects: enabling symbol-by-symbol CFO estimation within each group; enabling CFO insertion for transfers between groups; so that the group results can be evenly distributed to achieve frequency diversity.

Fig. 4 shows a data frame 100, the structure of said data frame 100 substantially corresponding to the structure of the data frame 100 shown in fig. 3 and is therefore not repeated here.

In the subcarriers 111, the pilots are arranged equidistantly in time (one pilot is provided every four OFDM symbols, wherein the first pilot 132 is provided in the first OFDM symbol 121).

In subcarriers 115, pilots are arranged similar to those shown in subcarriers 111 but with a time offset of two OFDM symbols. Second pilots 134 are provided in subcarriers 115 in OFDM symbol 123. The time delay 150 between successive pilots in one subcarrier is constant in time and the same in all

subcarriers

111, 115.

The

subcarriers

111, 115 containing the pilot are spread over the frequency of the data frame. In particular, these subcarriers are not adjacent subcarriers, but further subcarriers are arranged between their frequencies.

The pilot pattern for subcarrier 119 corresponds to the pilot pattern for subcarrier 111.

It should be noted that the description provided with reference to fig. 4 is equally applicable to data frames containing any number of subcarriers, any of which may contain pilots. Thus, the principles described with reference to two subcarriers are applicable to any number of subcarriers. The same applies to the remaining embodiments shown in fig. 3 and 5.

To achieve better accuracy of CFO estimation, especially for small CFO values, a longer interval duration should be used between the two CFO pilots. In this case, CFOs are accumulated and estimation performance is improved. The design properties are as follows: dividing the N pilots into M groups; each group includes N/M pilots; the pilots of each group are located at the same subcarriers at the N/M discontinuous OFDM symbols; each group is located at a different subcarrier to achieve diversity. In this case, the processing gain is small compared to the pilot pattern shown in fig. 3, because only close pilots can be used for CFO estimation.

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

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

Fig. 5 shows a data frame 100, the structure of which data frame 100 substantially corresponds to the structure of data frame 100 shown in fig. 3 and 4, and is therefore not repeated here.

In sub-carriers 111, the pilots are arranged such that the time delay between successive pilots in the same sub-carrier increases with time (first instance 132A is set in the first OFDM symbol, second instance 132B is set in the fourth OFDM symbol, third instance 132C is set in the ninth OFDM symbol and fourth instance 132D is set in the sixteenth OFDM symbol).

In subcarrier 115, pilots are arranged in the same OFDM symbol as in subcarrier 111. The time delay between successive pilots in one subcarrier increases with time and is the same in all

subcarriers

111, 115.

The number of pilot groups may be chosen such that the number of pilots in each OFDM symbol is sufficient to allow reasonable channel estimation.

Considering that CFO varies slowly over time, the processing gain may be more important at the beginning. Thus, a hybrid pilot pattern is depicted in fig. 5, where the time intervals between pilots gradually increase over time. This idea maximizes the initial processing gain and additionally enables accurate CFO estimation within a packet. The main principle of the design is as follows: dividing the N pilots into M groups; each group includes N/M pilots; the pilots of each group are located at the same subcarriers at N/M OFDM symbols with gradually increasing intervals 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 pilots allocated in the current OFDM symbol may be evenly distributed; a small number of symbols to be aggregated at the beginning of a data frame; CFO insertion for later OFDM symbols.

Fig. 6 depicts a data transmission system with a data transmission apparatus 10 and two

communication devices

20A, 20B, the two

communication devices

20A, 20B receiving data from the data transmission apparatus 10 and transmitting data to the data transmission apparatus 10 via a wireless data link 30.

The data transmission apparatus 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 perform the method described with reference to any of the embodiments herein.

While the invention has been described with reference to particular features, implementations and embodiments, it will be apparent that various modifications and combinations can be made thereto without departing from the spirit and scope of the invention. Accordingly, the specification and figures are to be regarded in an illustrative manner only of the invention defined by the appended claims, and are intended to cover any and all modifications, variations, combinations, or equivalents that fall within the scope of the invention.

List of reference numerals:

10 data transmission device

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 (F)

110 sub-carriers

111-115 subcarriers

120 OFDM symbol

121-125 OFDM symbols

132 first pilot

Examples of 132A-132D first pilots

134 second pilot

136 third pilot

138 fourth Pilot

140-144 pilot groups

150 time delay between successive pilots in one subcarrier

Number of K subcarriers

Number of N OFDM symbols

Claims

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

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

the method comprises the following steps:

transmitting a first set (140) of pilots in the OFDM symbol (120) at a first frequency, wherein the first set comprises a plurality of first pilots (132) that are each transmitted at the first frequency;

transmitting a second set (142) of pilots in the OFDM symbol (120) at a second frequency, wherein the second set of pilots comprises a plurality of second pilots (134) that are each transmitted at the second frequency, and wherein the second frequency is different from the first frequency;

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

2. The method of claim 1, further comprising:

wherein each OFDM symbol is divided such that portions of the OFDM symbol are transmitted in a plurality of subcarriers (110) at different frequencies;

arranging the first pilot (132) in a first subcarrier (111) at the first frequency and the second pilot (134) in a second subcarrier (115) at the second frequency;

allocating the first subcarrier (111) and the second subcarrier (115) to the first communication device (20A).

3. The method according to claim 1 or 2, further comprising the steps 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. The method according to claim 1 or 2,

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

5. The method according to claim 1 or 2,

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

6. The method of claim 5, wherein the first and second light sources are selected from the group consisting of,

wherein a time delay (150) between two successive instances of the plurality of instances of the first pilot is different.

7. The method of claim 6, wherein the first and second light sources are selected from the group consisting of,

wherein the time delay (150) between two successive instances of the plurality of instances of the first pilot increases over time.

8. The method of claim 3, further comprising the steps of:

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

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

9. The method of claim 8, wherein the first and second light sources are selected from the group consisting of,

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

10. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

wherein the first pilot group (140) and the second pilot group (142) are arranged in different OFDM symbols in the data frame.

11. The method of claim 3, further comprising the steps of:

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

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

12. The method of claim 11, wherein the first and second light sources are selected from the group consisting of,

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

13. A data transmission apparatus (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 an orthogonal frequency division multiple access, OFDMA, frame;

wherein the data frame generator (14) is configured to perform the method according to any of the preceding claims.

14. A data transmission system comprising:

the data transmission device (10) according to claim 13;

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 a link impairment of a data transmission link (30) between the data transmission apparatus (10) and the first communication device (20A) and the second communication device (20B), respectively, based on the received data frames.