The present invention proposes a method for generating synchronization bursts for ofdm transmission systems. The symbols of a predefined symbol sequence are mapped according to a predefined mapping scheme on subcarriers of the ofdm systems by a mapping unit (2), wherein the symbols of the predefined symbol sequence represent subcarriers of the ofdm system with nonzero amplitudes. A synchronization burst is generated by a inverse fast Fourier transforming unit (3) transforming the subcarriers of the ofdm system mapped to said predefined symbol sequence. The mapping (2) of the symbols of the predefined symbol sequence is set such that the resulting time domain signal of the synchronization burst represents a periodic nature. According to the invention the predefined symbol sequence is set such that the envelope fluctuation of the time domain signal of the synchronization burst is minimized. Therefore advantageous symbol sequences reducing said the envelope fluctuation of the time domain signal are proposed.
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0. 13. A method for transmitting data signals in an ofdm transmission system, comprising the steps of:
receiving a plurality of subcarriers on which a predefined symbol sequence is mapped, said predefined symbol sequence having a predefined number of symbols set to non-zero values and other symbols set to zero values, wherein respective symbols of the predefined symbol sequence have the value
generating a synchronization signal in the time domain by performing Inverse Fourier Transformation on said plurality of subcarriers, and
transmitting said synchronization signals and said data signals.
0. 9. A method for generating a synchronization signal for frequency synchronization of ofdm systems, comprising the steps of:
mapping symbols of a predefined symbol sequence in the frequency domain in accordance with a predefined mapping scheme on a plurality of subcarriers of the ofdm system, wherein the symbols of the symbol sequence define subcarriers with non-zero amplitude, and
generating a time domain synchronization signal by Inverse Fourier Transforming the subcarriers of the ofdm system mapped with the symbols of the symbol sequence,
wherein respective symbols of the symbol sequence have the value
0. 11. Apparatus for generating a synchronization signal for an ofdm transmission system, comprising:
a unit for mapping, in the frequency domain, symbols of a predefined symbol sequence in accordance with a predefined mapping scheme on a plurality of subcarriers of the ofdm system, wherein the symbols of the symbol sequence define subcarriers with non-zero amplitude, and
a unit for generating the time domain synchronization signal by Inverse Fourier Transforming the subcarriers of the ofdm system mapped with the symbols of the symbol sequence,
wherein respective symbols of the symbol sequence have the value
0. 16. Apparatus for transmitting data signals in an ofdm transmission system, comprising:
a unit for receiving a plurality of subcarriers on which a predefined symbol sequence is mapped, said predefined symbol sequence having a predefined number of symbols set to non-zero values and other symbols set to zero values, wherein respective ones of said predefined symbols of the symbol sequence have the value
a unit for generating a synchronization signal in the time domain by performing Inverse Fourier Transformation on said plurality of subcarriers, and
a transmitter for transmitting said synchronization signals and said data signals.
0. 17. Apparatus for transmitting data signals in an ofdm transmission system, comprising:
a unit for receiving a plurality of subcarriers on which a predefined symbol sequence is mapped,
a unit for generating a synchronization signal in the time domain by performing Inverse Fourier Transformation on said plurality of subcarriers, and
a transmitter for transmitting said synchronization signals and said data signals,
wherein respective ones of said predefined symbols of the symbol sequence have the value
and
wherein said predefined symbols are arranged periodically in said predefined symbol sequence in the frequency domain.
0. 15. A method for transmitting data signals in an ofdm transmission system, comprising the steps of:
generating synchronization signals in the time domain by performing Inverse Fourier Transformation on a plurality of subcarriers in which a predefined symbol sequence is mapped in accordance with a predefined mapping scheme, and
transmitting said synchronization signals and said data signals,
wherein the symbols of the symbol sequence comprise a number of predefined symbols and respective symbols of the symbol sequence have the value
wherein said predefined symbols are mapped on every fourth subcarrier of said plurality of subcarriers.
0. 18. Apparatus for transmitting data signals in an ofdm transmission system, comprising:
a unit for generating synchronization signals in the time domain by performing Inverse Fourier Transformation on a plurality of subcarriers on which a predefined symbol sequence is mapped in accordance with a predefined mapping scheme, and
a transmitter for transmitting said synchronization signals and said data signals,
wherein the symbols of the symbol sequence comprise a number of predefined symbols and respective ones of said predefined symbols have the value
wherein said predefined symbols are mapped on every fourth subcarrier of said plurality of subcarriers.
0. 14. A method for transmitting data signals in an ofdm transmission system, comprising the steps of:
receiving a plurality of subcarriers on which a predefined symbol sequence is mapped,
generating a synchronization signal in the time domain by performing Inverse Fourier Transformation on said plurality of subcarriers, and
transmitting said synchronization signals and said data signals,
wherein the symbols of the symbol sequence comprise a predefined number of symbols and respective symbols of the symbol sequence have the value
and
wherein said predefined symbols are arranged periodically in said predefined symbol sequence in the frequency domain.
0. 10. A method for generating a synchronization signal for frequency synchronization of ofdm systems, comprising the steps of:
generating a predefined symbol sequence having a predefined number of symbols corresponding to respective pre-selected ones of a plurality of subcarriers of the ofdm system, and
generating said synchronization signal in the time domain by performing Inverse Fourier Transformation on said pre-selected ones of said plurality of subcarriers,
wherein said predefined symbols are set to non-zero having complex values and others of said symbols are set to zero, such that said predefined symbols are arranged periodically in said predefined symbol sequence in the frequency domain and
wherein respective symbols of the symbol sequence have the value
0. 12. Apparatus for generating a synchronization signal for frequency synchronization of ofdm systems, comprising:
a unit for generating a predefined symbol sequence having a predefined number of symbols corresponding to respective pre-selected ones of plural subcarriers of the ofdm system, and
a unit for generating said synchronization signal in the time domain by performing Inverse Fourier Transformation on said pre-selected ones of said plurality of subcarriers,
wherein said predefined symbols are set to non-zero having complex values and others of said symbols are set to zero, such that said predefined symbols are arranged periodically in said predefined symbol sequence in the frequency domain and
wherein respective symbols of the predefined symbol sequence have the value
0. 1. A method for generating synchronization bursts for ofdm transmission systems, comprising the following steps:
mapping the symbols of a predefined symbol sequence according to a predefined mapping scheme on subcarriers S of the ofdm system, wherein the symbols of the predefined symbol sequence represent subcarriers of the ofdm system with non-zero-amplitude, and
generating a synchronization burst by Inverse Fourier Transforming the subcarriers S of the ofdm system mapped with the symbols of said predefined symbol sequence,
characterized in that
the predefined symbol sequence is set such that the envelope fluctuation of the time domain signal of the synchronization burst is minimized and the symbols of the predefined symbols sequence can be expressed as
A −A A −A −A A −A −A A A A A
A being a complex value.
0. 2. A method for synchronizing wireless ofdm systems, characterized by the steps of
generating a synchronization burst according to a method according to
transmitting the synchronization burst.
0. 3. A method according to
the time domain signal of the synchronization burst is precomputed and stored in a memory.
0. 4. An ofdm transmitter, comprising:
a unit for mapping the symbols of a predefined symbol sequence according to a predefined mapping scheme on subcarriers of the ofdm system, wherein the symbols of the predefined symbol sequence represent subcarriers of the ofdm system with non-zero-amplitude, and
a unit for generating a synchronization burst by Inverse Fourier Transforming the subcarriers of the ofdm system mapped with the symbols of said predefined symbol sequence,
characterized in that
the mapping unit is designed to modulate the subcarriers such that the envelope fluctuation of the time domain signal of the synchronization burst is minimized by using the following predefined symbol sequence:
A −A A −A −A A −A −A A A A A
A being a complex value.
0. 5. An ofdm transmitter according to
a time extension unit copying the burst part to achieve a periodic nature of the time domain signal.
0. 6. An ofdm transmitter according to
a processing unit for precomputing the time domain signal of the synchronization burst
and a memory for storing the precomputed time domain signal of the synchronization burst.
0. 7. A mobile communications device, comprising a transmitter according to
0. 8. A synchronization burst signal for synchronizing ofdm systems generated by a method according to
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Ci-1=±C1-i,
The mapping of the symbols of the predefined symbol sequence and the Inverse Fast Fourier Transform can be set such that the resulting time domain signal of the synchronization burst represents a periodic nature.
Alternatively the mapping of the symbols of the predefined symbol sequence and the Inverse Fast Fourier Transform is set such that one burst part of the synchronization burst in the time domain is generated and the periodic nature of the synchronization burst in the time domain is achieved by copying the one burst part.
The number of symbols of a symbol sequence (n) can for example be 12.
The above equations define generally the symbol sequences according to the present invention. The predefined symbol sequence can therefore be for example:
Alternatively the predefined symbol sequence can be:
Alternatively the following predefined symbol sequence can be used:
As a further alternative the following sequence can be used:
According to the present invention furthermore a method for synchronizing wireless OFDM systems is provided, wherein a synchronization burst is generated according to a method as set forth above and the synchronization burst is transmitted respectively before the transmission of data fields.
Thereby the time domain signals of the synchronization burst can be precomputed and stored in a memory, such that the computation of the time domain signal of the burst is only effected once.
According to the present invention furthermore a OFDM transmitter is provided comprising a mapping unit for mapping the symbols of a predefined symbols sequence according to a predefined mapping scheme on subcarriers of the OFDM system, wherein the symbols of a predefined symbols sequence represent the subcarriers of the OFDM system with nonzero amplitudes. Furthermore an inverse fast Fourier transforming unit is provided for generating a synchronization burst by inverse fast Fourier transforming the subcarriers of the OFDM mapped with said predefined symbols sequence. The mapping unit thereby is designed such that the resulting time domain signal of the synchronization burst represents a periodic nature. The mapping unit according to the present invention uses a predefined symbol sequence which is such that the envelope fluctuation of the time domain signal of the synchronization burst is minimized.
According to the present invention furthermore a mobile communications device such as set forth above is used.
With reference to the figures of the enclosed drawings referred embodiments of the present invention will now be explained.
According to the present invention the time domain synchronization burst structure as shown in
According to the present invention a short OFDM symbol (t1, . . . t6) consists of 12 phase-modulated subcarriers.
C00
C01
C02
C03
C04
C05
C06
C07
C08
C09
C10
C11
Seq0
A
A
A
−A
−A
−A
−A
A
−A
−A
A
−A
Seq1
A
−A
A
A
−A
A
A
A
A
−A
−A
−A
Seq2
A
B
−A
B
−A
−B
B
A
−B
A
−B
−A
Seq3
A
−B
−A
−B
−A
B
−B
A
B
A
B
−A
with
Generally the predefined symbol sequence therefore is chosen such that the envelope fluctuation of the time domain signal of the synchronization burst is minimized.
Therefore generally the predefined symbol sequence is set such that the following equations are satisfied for all symbols for the predefined symbol sequence:
n=2m,
Ci-1=±Cn-i
In the following the time domain signal properties of the new sequences according to the present invention will be shown with reference to
For simplicity we use in our demonstration the classical quadriphase symbol alphabet,
(this corresponds to φA=0.125)
Symbol
A
−A
B
−B
Table 1: Complex symbol mapping
PAPR (in decibel) is limited to 2.059 (even when using a time domain oversampling to capture the actual peak).
Further simulations have shown that not only the PAPR can be optimized but also the dynamic range of the signal should be minimized. Therefore another four sequences, with achieve a small PAPR and at the same time a small overall dynamic range are proposed further below.
Using the sequence as proposed in the state of the art the PAPR is 3.01 dB and the dynamic range (defined as the ratio of the peak power to the minimum power) is 30.82 dB (see FIGS. 9a and 9b).
Using the sequences according to the present invention and as described above the PAPR is reduced to 2.06 dB, however, the dynamic range is increased as the signal power is ‘0’ at some points.
Therefore the following four sequences are proposed as a further embodiment of the present invention:
The symbol sequence is C0, C1, . . . C11 and the mapping is:
C00
C01
C02
C03
C04
C05
C06
C07
C08
C09
C10
C11
Seq-Alt0
A
A
A
A
−A
−A
A
−A
−A
A
−A
A
Seq-Alt1
A
−A
A
−A
−A
A
−A
−A
A
A
A
A
Seq-Alt2
A
B
−A
−B
−A
−B
−B
−A
−B
−A
B
A
Seq-Alt3
A
−B
−A
B
−A
B
B
−A
B
−A
−B
A
with A=exp (i*2*π*φA) and
and 0.0≦φA<1.0.
Using these sequences the PAPR is reduced to 2.24 dB and the dynamic range is limited to 7.01 dB as it is shown in
The advantages are the same as described before, however, the clipping problem is further reduced due to the very limited dynamic range of the signal.
With reference to
In the transmitter the sync symbol data 1 are prepared and mapped in a IFFT mapping unit 2 to the appropriate IFFT points. The subcarriers of the OFDM system are transformed by a IFFT unit 3 and then the time domain signal is extended in a time extension unit 4 by copying parts of the signals (for example, t1, t2 are copied to t5, t6). The time extended signal is then sent to the I/Q modulator 5.
As shown in
With reference to
According to this scheme, the principle of setting only every fourth subcarrier of the OFDM system to a non-zero amplitude (see
The IFFT size is now only 16 (instead of 64 as it is the case in FIG. 7). Only one of the bursts t1, t2, . . . t6 will be generated. The other bursts can be generated by copying to retain the periodic nature of the synchronization time domain signal necessary for the correlation and synchronization on the receiving side. Therefore for example the time extension unit 4 can perform the copying of the 16-sample burst t1 generated by the IFFT 16 according to
The mapping scheme shown in
According to the present invention therefore a synchronization burst structure to be used in high speed wireless transmission systems is proposed. The synchronization burst is constructed using especially designed OFDM symbols and time domain repetitions. The resulting synchronization burst achieves a high timing detection and frequency offset estimation accuracy. Furthermore the burst is optimized to achieve a very low envelope fluctuation (Low peak-to-average-power-ratio) to reduce the complexity on the receiver and to reduce time and frequency acquisition time at the receiver.
Therefore the synchronization performance can further be improved. As with the scheme according to the present invention the envelope of the OFDM based synchronization burst in the time domain is reduced, the AGC pool-in speed at the receiver can be improved and an accurate time and frequency synchronization can be achieved. Furthermore the synchronization complexity on the receiver side can be reduced due to the reduced resolution requirements necessary due to reduced envelope fluctuation.
The advantages of the present invention can be set forth as following:
Böhnke, Ralf, Dölle, Thomas, Puch, Tino
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