WO2005041514A1

WO2005041514A1 - Method for synchronisation on transmission of ofdm signals

Info

Publication number: WO2005041514A1
Application number: PCT/DE2004/002309
Authority: WO
Inventors: Gerhard Fettweis
Original assignee: Technische Universität Dresden
Priority date: 2003-10-18
Filing date: 2004-10-18
Publication date: 2005-05-06
Also published as: DE112004001837D2

Abstract

The invention relates to a method for synchronisation on transmission of OFDM signals, whereby an OFDM data symbol is transmitted over a transmission channel with several individual carriers and a synchronisation of the received signals and a channel estimation is carried out in a receiver. A known first synchronisation symbol is transmitted parallel to the OFDM data symbol with the same symbol frequency and in the same frequency band and a synchronisation and a channel estimation are carried out with the known synchronisation symbol. The aim of the invention is to minimise signal superimposition for symbols, or the complexity of the synchronisation and the channel estimation. Said aim is achieved, whereby a second known synchronisation symbol is transmitted, parallel to the transmission of the OFDM data symbols, the first and the second synchronisation symbol are not fully correlated with each other and the synchronisation and the channel estimation are carried out with both synchronisation symbols.

Description

Synchronization method for the transmission of OFDM signals

The invention relates to a method for synchronization in the transmission of OFDM signals, in which an OFDM data symbol is transmitted via a transmission channel with a plurality of individual carriers, and a synchronization from the received signals and a channel estimation is carried out at a receiver, in parallel with the OFDM data symbol transmit a known first synchronization symbol with the same symbol clock and in the same frequency band and with the aid of the known synchronization symbol a synchronization and a channel estimation is carried out.

Orthogonal Frequency Division Multiplexing (OFDM) is an increasingly popular modulation method for new wired and wireless standards (e.g. ADSL, DAB, DVB-T, iFi wireless LAM). This modulation method is also discussed for new future standards (e.g. 4G cellular and wireless USB 2.0).

The basic idea of OFDM is to combine the transmission of information over a set of many parallel carriers that are modulated in the same symbol period. By arranging the carriers next to one another and orthogonally to one another, a combined modulated symbol can be generated by an inverse Fast Fourier transformation. For this purpose, the modulation process of a set of N data symbols a _N on N individual carriers can be expressed by generating an output signal y _k with k = [0, ..., Ν-1]

For a receiver, the demodulation process can be carried out simply by a Fast Fourier transform followed by a symbol detector. In particular, no complicated channel equalization is necessary.

This simple consideration applies to AWGN channels (AWGN = additive white Gaussian noise) without a delay spread. In typical radio channels, however, the receiver reaches echoes of the transmitted signals and generates a delay spread, which leads to inter-symbol interference (ISI = inter-symbol interference), which necessitates complex channel equalization. In order to still be able to use a simple receiver, a so-called guard interval (GI = guard interval) is inserted between two successive OFDM symbols. Its length is chosen such that the inter-symbol interference is limited to this protection interval and the sampling values of the OFDM symbols are therefore free of inter-symbol interference. The protection interval can be designed in various ways, for example the implementation as a cyclic prefix (CP = cyclic prefix) is customary, for example the last L of the N samples of a sequence of this sequence being prefixed. In the following, an OFDM symbol is to be understood as a unit consisting of the samples of the actual symbol sequence and the protection interval.

Even with a guard interval, the samples of the OFDM symbols on the receiver are distorted by the frequency selectivity of the channel, which is another consequence of the delay spread. In the usual design of the system, the frequency selectivity on each subcarrier acts as a multiplicative link between the data symbol a _t and a channel coefficient. A multiplication of the subcarriers by the respective inverse channel coefficient is then sufficient for the equalization. The channel coefficients have to be estimated.

When the OFDM signal is transmitted from the low-frequency so-called baseband to the assigned one high-frequency band converted and downmixed back to baseband at the receiver, for example, there is a shift in the spectrum of the OFDM signal, ie the subcarrier, relative to their expected frequency position. The reason for this is that the local oscillators used for mixing in the transmitter and receiver do not work at exactly the same frequency. Interference between the signals of the individual subcarriers then occurs at the receiver, as a result of which the orthogonal action of this subcarrier is lost.

The OFDM method therefore requires precise time and frequency synchronization. The task of time synchronization is to find for each OFDM symbol a group of N consecutive samples which belong to this OFDM symbol and which are as free as possible from inter-symbol interference so that they can then be de-coded by means of a Fast Fourier transformation , The task of frequency synchronization is to determine the spectral deviation between the local oscillators in the transmitter and receiver in order to be able to compensate for this before demodulation.

The usual way to ensure a good synchronization today is to provide the data transmission with a preamble, for example by using more than one fixed or known OFDM symbol [IEEE 802.11]. This preamble is designed in such a way that it is possible to achieve very good time and frequency synchronization using simple correlation methods. In addition, the synchronization preamble provides a basis for correlation in order to carry out channel measurements and to estimate the channel-induced phase and amplitude changes of each individual carrier.

For a relatively small number of individual carriers (e.g. N = lβ or 64), a preamble has a size of two to four OFDM symbols, followed by a long data packet, which results in a small synchronization signal superstructure of the channel capacity compared to the channel capacity is provided for data transmission results. But if future systems for very high data rates are designed, the number of individual carriers increases, as is already proposed today, to N = 64 or more (for example in the case of the standards IEEE 802.15.3A, IEEE 802.11n in preparation). In the event that a short data signal packet is to be transmitted on a large number of individual carriers, the signal data then easily fit into a single OFDM symbol. A typical example of such a "short packet" is the response to a data packet by sending an acknowledgment packet.

However, it is necessary to prepend this short data signal packet with several OFDM symbols for synchronization and for channel estimation. This can easily take up most of the channel connection capacity. Most of the signal transmission energy is also used for synchronization signals.

The object of the invention is now to minimize the signal superimposition of symbols or the effort for the synchronization and the channel estimation.

According to the invention, the object is achieved in a method for synchronization in the transmission of OFDM signals of the type mentioned at the outset in that a known second synchronization symbol is transmitted in parallel with the transmission of the OFDM data symbols in that the first and the second synchronization symbols are not full to one another are correlated and the synchronization and the channel estimation is carried out using the two synchronization symbols.

According to the invention, a known first and second synchronization symbol are transmitted in parallel with an OFDM data symbol, for example in synchronism with it. By supplying the synchronization symbols with sufficient signal energy, the channel estimation and synchronization can be carried out first. The OFDM data symbol is recognized as a small noise when the common energy of the synchronization symbol spreads across all individual carriers known data symbols for which the synchronization process can be bundled.

After the synchronization and the channel estimation, the known synchronization symbols can be subtracted from the sampled signal again, which in principle leaves an interference-free OFDM data symbol for demodulation to a demodulator.

The chronological sequence of the data symbols forms the data signal, the chronological sequence of the known first synchronization symbols forms the first synchronization signal and the chronological sequence of the known second synchronization symbols forms the second synchronization signal. In addition to the known first and second synchronization symbols, it is also possible to transmit a known third and / or fourth synchronization symbol, a third and / or fourth synchronization signal also being formed. These additional synchronization symbols make it possible to estimate further parameters such as, for example, channel coefficients, nonlinear distortions, caused by transmitter and / or receiver modules (amplifier), and I-Q imbalances.

Within a data symbol period T it is possible to have one or more synchronization symbols with the synchronization symbol period T _s in one

Transfer synchronization signal, where T _{s is} an integer

Is a multiple of T or vice versa and can be synchronized with it in time.

One embodiment of the invention provides that one synchronization symbol has a correlation maximum in time and the other synchronization symbol has a correlation maximum in frequency.

These synchronization symbol properties make the Synchronization and channel estimation effort reduced.

In a further embodiment of the invention it is provided that the synchronization symbols are orthogonal to one another.

In a special form of the requirement for the synchronization symbols that the first and the second synchronization symbols are not fully correlated to one another, it is also possible that these are orthogonal to one another.

In a special embodiment of the invention it is provided that the synchronization symbols are transmitted simultaneously in one symbol cycle.

In one embodiment of the invention, it is provided that the synchronization symbols are transmitted independently of one another in different symbol cycles.

According to the invention, it is possible to transmit the known first and the known second synchronization symbol, which each have its own synchronization symbol period T _s , at the same time to one another and to the data symbol. Another possibility is to transmit the two known synchronization symbols in different symbol cycles, that is to say in directly or indirectly successive data symbol periods T. Furthermore, it is also possible to arrange a plurality of synchronization symbol period T _s in one or more synchronization signals within one data symbol period T. Each synchronization signal for each variant described can consist of a continuous sequence of synchronization symbols and also have synchronization symbol gaps.

One embodiment of the invention provides that the same synchronization symbols are superimposed on each OFDM data symbol in each symbol period. The synchronization symbols assigned to a data symbol, a data symbol period T, can be the same as those which are assigned to a subsequent data symbol period T. This results in an unchanged sequence of successive synchronization symbols in the synchronization signals.

In one embodiment of the invention it is provided that a synchronization _{symbol has} a change in frequency during its synchronization symbol period T _s .

In a special embodiment of the invention, it is provided that each synchronization symbol has a change in frequency in its synchronization symbol period T _s that is opposite to the other synchronization symbol of the same symbol clock.

The known synchronization symbols to be transmitted can have a change in the frequency parameter within their synchronization symbol period T _s . This change may be a change in frequency from a lower to an upper frequency, or the reverse, a change in frequency from an upper to a lower frequency. This change can take place according to a linear or non-linear function. In a special embodiment, the two known synchronization symbols transmitted at the same time as a data symbol period T have a different and / or opposite change in the frequency parameter.

In another embodiment of the invention it is provided that a first synchronization symbol has a change in frequency during its synchronization symbol period T _s and a subsequent second synchronization symbol has a change in frequency opposite to the first synchronization symbol in its synchronization symbol period T _s having .

The two known synchronization symbols that include within their Synchronisationssybol _s-period T, a different and / or opposite change of the parameter frequency may be transmitted in the same or in different data symbol periods T.

In a further embodiment of the invention it is provided that the transmission with embedded pilots, i.e. with individual known data symbols on a single carrier.

The method known from the prior art, embedding pilots in the data signal, can also be used in addition to the transmission of two known synchronization symbols and can lead to a further simplification or improvement of the synchronization.

Another embodiment of the invention provides that the first, the second or both synchronization symbols are subtracted from the OFDM data symbol in a receiver after the synchronization and channel estimation.

After the synchronization and channel estimation have taken place, the known synchronization symbols can be subtracted from the sampled signal again, which in principle leaves an interference-free OFDM data symbol for demodulation to a demodulator.

Another embodiment of the invention provides that when OFDM data symbols are transmitted via multi-antenna systems, so-called MIMO systems (MIMO = Multiple-Input Multiple-Output), two synchronization symbols, which are at least partially decorrelated to one another, are transmitted per antenna , The method according to the invention can also be used on systems with several antennas. In this case, each known antenna is assigned two known synchronization symbols which have the properties described above, such as, for example, “not fully correlated with one another”.

In a special embodiment of the invention it is provided that a synchronization symbol in turn consists of two synchronization symbols.

According to the invention, it is possible for a known synchronization symbol to be constructed from two or more known synchronization symbols.

In a special embodiment of the invention, it is provided that the synchronization symbols receive a power distribution that is adapted to the radio channel relative to the data signals.

One embodiment of the invention provides that a synchronization symbol has a power distribution that is the same as that of the other synchronization symbol.

In a special embodiment of the invention, it is provided that a synchronization symbol has a power distribution that differs from the respective other synchronization symbol.

The transmission power available in the transmitter can be divided between the data signal to be transmitted, which contains the OFDM data symbols, and the synchronization signals, which contain the known synchronization symbols, to be transmitted. This adjustment of the power can take place in such a way that the signals to be transmitted each receive an identical or different share of the total power.

It can help improve the Synchronization process in the receiver may be advantageous to transmit the synchronization signals with more signal energy than the data signal.

A dependent power distribution that adapts to the current state of a radio channel is also possible and improves the quality of the synchronization.

The invention will be explained in more detail below using an exemplary embodiment. In the accompanying drawings

1 shows a schematic diagram of a data signal which is overlaid by m synchronization symbols,

2 shows a representation of different possible combinations of the assignment of two synchronization symbols to one data symbol,

3 shows various structuring options for the synchronization symbols,

4 shows a representation of a synchronization symbol which changes in its frequency parameter over time,

5 is an extract of a representation of transmitter and receiver assemblies for implementing the method,

6 shows a basic illustration of a baseband unit for implementing the method,

7 shows a schematic diagram of a radio transmission channel from FIG. 6,

Fig. 8 shows a schematic diagram of a receiver with Interference cleanup

Fig. 9 is a schematic diagram of a transmitter structure for transmitters with multiple antennas and

10 shows a basic illustration of a further transmitter structure for transmitters with several antennas.

1 shows the principle of superimposing a data signal 2 consisting of data symbols 1 with a plurality of synchronization symbols 3. The representation extends over a time of two data symbol periods T 5 of the data signal 2. In this case, a data symbol 1 is an OFDM data symbol with N subcarriers 7. The synchronization signals 1, 2,... M 4 are sequences of Synchronization symbols 3, the period T _s 6 here equal to the data symbol period

T 5 are and occupy the same bandwidth as the data signal 2. In synchronization signal 1, the synchronization symbol B follows the synchronization symbol A, in synchronization signal 2 D follows C and in synchronization signal m follows F E. The symbols are so different that they are not completely correlated with one another. This applies both between the different synchronization signals 4 and within a synchronization signal 4.

FIG. 2 shows that individual synchronization symbols 3 can be repeated within a synchronization signal 4, for example. This is shown in a first example for the synchronization symbol A within the synchronization signal 1 and in a second example for the synchronization symbol C within the synchronization signal 2. In addition, it is also possible to repeat synchronization symbols 3 from a first synchronization signal 4 at another time in another synchronization signal 4, for example for the synchronization symbol B shown in Fig. 2.

Another possibility for the transmission of two synchronization symbols 3 according to the invention is shown in FIG. It is shown here that the synchronization symbols A and C of the synchronization signals 1 and 2 do not occupy the entire bandwidth of the data signal 2 and only partially or not superimpose one another on the frequency. Furthermore, it is shown that the synchronization symbol B of the synchronization signal 1 partially occupies a frequency range outside the frequency range of the data signal 2. It is also possible that a synchronization signal 4 is not sent all the time, as shown in the example of the synchronization signal 3 in FIG. 3. The example of the synchronization symbol A also shows that the synchronization symbol 3 can be repeated at another time in another synchronization signal 4 in a different frequency range.

FIG. 4 shows an example of a change in the synchronization symbol parameter frequency over two synchronization symbol period durations T _s 6 within a synchronization signal 4. Here, the first synchronization symbol A has a change in frequency from lower frequency f ₀ to an upper frequency f ₀ linearly with time within its period T _s . The following symbol B experiences a reverse linear change in frequency from an upper frequency f ₀ to a lower frequency f _υ within its period T _s . The time of the period T _s can be the same or different from the data symbol period T of a data symbol. The frequency profile within or over the data symbol period T can run continuously or discontinuously.

FIG. 5 shows an example of a transmitter arrangement for implementing the method according to the invention. In this, the data sequence 8 provided is initially carried out a baseband unit 9, which contains, for example, modules for coding and modulation, is converted into a sequence of data symbols 1, that is to say into a data signal 2. Each of these data symbols 1 can in turn consist of OFDM symbols, for example. The various synchronization signals 4, which are formed by a sequence of synchronization symbols 3, are generated in the synchronization signal generators 10 in such a way that the synchronization symbols 3 are not fully correlated with one another. Various possibilities for generating the synchronization signals 4 have already been shown in FIGS. 1 to 4. The timing of the signal processing in units 9 and 10 is controlled by a clock signal which is generated by a clock generator 11. However, it is also possible for the units 9 and 10 to be controlled by different clock signals in each case.

The data signal 2 and the synchronization signals 4 are each amplified by amplifier arrangements 12 and added to the transmission symbol sequence 14 on the summing element 13. The amplification factors a, b and c of the amplifiers 12 can be different from one another. It is advantageous to adapt a, b and c to the current state of the radio channel in such a way that the data can be transmitted in high quality or with low transmission power.

The transmission symbol sequence reaches a receiver 17 via a transmission channel 16.

An example of a baseband unit 9 for generating OFDM symbols is shown in FIG. 6. The input data sequence 8 is first encoded, punctured, scrambled by an encoder 18 and then modulated in the downstream modulator 19, for example by phase shift keying (PSK = phase shift keying) or quadrature amplitude modulation (QAM). The serial-to-parallel converter 20 forms a signal group, which represents the OFDM symbol, from a number of M symbols modulated in this way. These are subsequently from an N-point IFFT unit 21 with N • M, a downstream parallel-serial converter 22 processed into a form suitable for further processing, it also being possible to insert a cyclic prefix through the arrangement 23.

It is also possible to spectrally spread the output symbols of the modulator. The energy of the symbols that occupy a bandwidth B at the output of the modulator 19 is spread over a frequency range that is greater than the bandwidth B. This can be done, for example, using the MCSS (Multi-Carrier Spread Spectrum) spreading method.

FIG. 7 shows the transmission channel 16 from FIG. 5 using the example of a radio channel. In this case, the transmission symbol sequence 14, that is to say the signal in the so-called baseband, is adapted in a high-frequency transmission unit 24 to the transmission via this channel and is emitted by a transmission antenna 25. Adaptation of the signal is understood to mean, for example, digital-to-analog conversion, mixing to the required center frequency and high-frequency signal amplification. At the receiver, the signal transmitted via the radio channel first reaches the radio-frequency receiver unit 27 via the receiver antenna 26, which converts the radio-frequency reception signal into a baseband signal, the reception symbol sequence 15.

An exemplary embodiment for the receiver 17 shown in FIG. 5 is shown in FIG. 8. The received symbol sequence 15 first arrives in an evaluation unit 28, which uses the transmitted synchronization symbols 3 to determine estimated values of the symbol clock and the frequency offset between the transmitter and receiver and to make them available at their output , With a channel estimation unit 29, it is additionally possible, based on the received signal, to obtain estimated values for the influence of the channel on the transmitted signal. This channel estimate can advantageously be included, for example of the transmitted synchronization symbols 3. The estimated values for symbol clock, frequency offset and channel influence can be used in a calculation unit 30 to regenerate the synchronization symbols 3 including their distortions impressed by the channel. It is advantageous if the receiver knows the synchronization symbols 3 used by the transmitter and this knowledge is also used in the units 28, 29 and 30. If this knowledge is not available, so-called blind estimation methods must be used. The regenerated synchronization signals 4 are subtracted from the received symbol sequence 15 using a summing element 31. This creates an interference-cleaned reception symbol sequence 32, which can be supplied to a demodulation unit 33 for recovering the data signal 2 or for determining estimated values of the data sequence.

FIG. 9 shows a multiple antenna system consisting of two antennas 25 with an associated transmitter arrangement. For each antenna 25, one data signal 2 and at least two synchronization signals 4 each are broadcast, provided that any pair of the synchronization signals 4 of all antennas 25 is not completely correlated. The signal structures can be selected, as already described above using the example of m synchronization signals 4 for a transmitting antenna 25.

The data signals 2 and the synchronization signals 4 for each transmission antenna 25 can be generated independently of the signals of the other transmission antenna 25, as shown in FIG. 9, or also from a common transmission unit for several or all transmission antennas 25, as shown in FIG. 10 , For example, a large number of so-called MIMO techniques (MIMO = multiple input multiple output) are known for generating the data signals 2 for a plurality of transmit antennas 25. These known methods include, for example, VBLAST (Vertical Bell Labs Layered Space Time) or STC (Space-Ti e-Coding). This principle shown can be extended to any number of transmission antennas 25. Synchronization method for the transmission of OFDM signals

List of reference symbols 1 data symbol 2 data signal 3 synchronization symbol 4 synchronization signal 5 data symbol period T 6 synchronization symbol period ^ 7 subcarrier 8 data sequence 9 baseband unit 10 synchronization signal generator 11 clock generator 12 amplifier 13 summing element 14 transmission symbol sequence 15 reception symbol sequence 16 transmission channel 17 receiver 18 encoder 19 modulator 20 serial -Parallel converter 21 N-point IFFT unit 22 Parallel-serial converter 23 Cyclic prefix 24 Radio frequency transmitter unit 25 Transmitter antenna 26 Receiver antenna 27 Radio frequency receiver unit Evaluation unit, channel estimation unit, calculation unit, summing element, interference-adjusted reception symbol sequence, demodulation unit

Claims

Synchronization method for the transmission of OFDM signals

1. A method for synchronization in the transmission of OFDM signals, in which an OFDM data symbol is transmitted over a transmission channel with several individual carriers, and a synchronization of the received signals and a channel estimate is carried out at a receiver, parallel to the OFDM -Data symbol a known first synchronization symbol with the same symbol clock and in the same frequency band is transmitted and with the aid of the known synchronization symbol a synchronization and a channel estimation is carried out, characterized in that a known second synchronization symbol (3) is transmitted in parallel with the transmission of the OFDM data symbols (1). is transmitted that the first and the second synchronization symbol (3) are not fully correlated with each other and the synchronization and the channel estimation is carried out using the two synchronization symbols (3).

2. The method according to claim 1, characterized in that the one synchronization symbol (3) has a correlation maximum in time and the other synchronization symbol (3) has a correlation maximum in frequency.

3. The method according to claim 1 or 2, characterized in that the synchronization symbols (3) to each other are orthogonal.

4. The method according to any one of claims 1 to 3, characterized in that the synchronization symbols (3) are transmitted simultaneously in one symbol cycle.

5. The method according to any one of claims 1 to 3, characterized in that the synchronization symbols (3) are transmitted independently of one another in different symbol cycles.

6. The method according to claim 4, characterized in that the same synchronization symbols (3) are superimposed on each OFDM data symbol (1) in each symbol period.

Method according to one of claims 1 to 6, characterized in that a synchronization symbol (3) has a change in frequency during its synchronization symbol period T _s (6).

8. The method according to any one of claims 1 to 7, characterized in that each synchronization symbol (3) to the other synchronization symbol (3) of the same symbol clock opposite change in frequency in its synchronization symbol period T _s (6).

9. The method according to claim 5 and 8, characterized in that a first synchronization symbol (3) a change the frequency during its synchronization symbol period T _s (6) and a subsequent second synchronization symbol (3) has a change in frequency opposite to the first synchronization symbol (3) in its synchronization symbol period T _v (6).

10. The method according to any one of claims 1 to 9, characterized in that the transmission with embedded pilots, i.e. with individual known data symbols on a single carrier.

11. The method according to any one of claims 1 to 10, characterized in that in a receiver after the synchronization and channel estimation the first, the second or both synchronization symbols (3) are subtracted from the OFDM data symbol (1).

12. The method according to any one of claims 1 to 11, characterized in that in the transmission of OFDM data symbols (1) via multiple antenna systems, so-called MIMO systems (MIMO = multiple-input multiple-output), per antenna (25) two synchronization symbols (3), which are at least partially decorrelated to one another, are transmitted.

13. The method according to any one of claims 1 to 12, characterized in that a synchronization symbol (3) in turn consists of two synchronization symbols (3).

14. The method according to claims 1 to 13, characterized in that the synchronization symbols (3) receive a power distribution adapted to the radio channel (16) relative to the data signals (2).

15. The method according to claim 14, characterized in that a synchronization symbol (3) has the same power distribution as the respective other synchronization symbol (3).

16. The method according to claim 14, characterized in that a synchronization symbol (3) has a different power distribution than the other synchronization symbol (3).