Synchronization symbol structure using OFDM based transmission method

Abstract

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.

Description

C_i−1=±C_1−i,

• • n being the number of symbols of the predefined symbol sequence,
  • m being an integer larger than one,
  • C being the symbol value, and
  • i being an integer running from 1 to m.

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:

• • A A A -A -A -A -A A -A -A A -A,
    wherein A is a complex value.

Alternatively the predefined symbol sequence can be:

• • A -A A A -A A A A A -A -A -A,
    wherein A is a complex value.

Alternatively the following predefined symbol sequence can be used:

• • A B -A B -A -B B A -B A -B -A,
    wherein A, B are complex values.

As a further alternative the following sequence can be used:

• • A -B -A -B -A B -B A B A B -A,
    wherein A, B are complex values.

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.

FIG. 1 shows schematically a transmitter according to the present invention,

FIG. 2 shows an alternative embodiment for a transmitter according to the present invention,

FIG. 3 shows an alternative mapping scheme according to the present invention,

FIGS. 4a to 4d show the time domain signal properties achieved with the synchronization symbol structure using OFDM based transmission according to the present invention,

FIGS. 5a to 5d show the time domain signal properties of synchronization symbol structures according to alternative embodiments of the present invention,

FIG. 6 shows a synchronization preamble structure known from the prior art,

FIG. 7 shows an IFFT mapping according to the prior art, and

FIGS. 8a to 8d show the time domain properties of the synchronization symbol structure according to the prior art,

FIGS. 9a and 9b show the time domain properties, particularly the dynamic range of the synchronization symbol structure according to the prior art, and

FIGS. 10a and 10b show the time domain properties of the synchronization symbol structure according to further alternative embodiments of the present invention,

According to the present invention the time domain synchronization burst structure as shown in FIG. 6 is maintained. The IFFT mapping as shown in FIG. 7 can be maintained or alternatively the IFFT mapping according to FIG. 3 can be used. The symbol sequences mapped to the subcarriers are optimized to sequences which result in a lower PAPR.

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 $A=\exp \left(j*2+\pi *{\phi }_A\right)\mathrm{and}$ $B=A*\exp \left(j\frac{\pi }{2}\right)=\exp \left(j\mathrm{2\pi }*{\phi }_A+j\frac{\pi }{2}\right)\mathrm{and}0.0\le {\phi }_A<1.0.$

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,
C_i−1=±C_n−i

• • wherein n is a number of symbols of the predefined symbol sequence,
  • m is an integer larger than 1,
  • c is the symbol value, and
  • i is an integer value running from 1 to m.

In the following the time domain signal properties of the new sequences according to the present invention will be shown with reference to FIGS. 4a to 4d and FIGS. 5a to 5d.

For simplicity we use in our demonstration the classical quadriphase symbol alphabet, $S=\sqrt{\frac{1}{2}}\left(\pm 1\pm j\right),$
(this corresponds to φ_A=0.125)

Symbol
A	$\exp \left(j\frac{\pi }{4}\right)$	$\sqrt{\frac{1}{2}}\left(+1+j\right)$
−A	$-\exp \left(j\frac{\pi }{4}\right)=\exp \left(j\frac{5\pi }{4}\right)$	$\sqrt{\frac{1}{2}}\left(-1-j\right)$
B	$\exp \left(j\frac{\pi }{4}+j\frac{\pi }{2}\right)=\exp \left(j\frac{3\pi }{4}\right)$	$\sqrt{\frac{1}{2}}\left(-1+j\right)$
−B	$-\exp \left(j\frac{3\pi }{4}\right)=\exp \left(j\frac{7\pi }{4}\right)$	$\sqrt{\frac{1}{2}}\left(+1-j\right)$

Table 1: Complex symbol mapping

FIGS. 5a and 5b thereby show the time domain signal (magnitude) when using the optimized sequence according to the present invention in the case of no oversampling/8-times oversampling is effected.

PAPR (in decibel) is limited to 2.059 (even when using a time domain oversampling to capture the actual peak).

FIGS. 5c and 5d show the in-phase and quadrature-phase component, respectively, of the resulting wave form. It is clearly visible that the full symbol consists of four repetitions of a short sequence.

FIGS. 5a to 5d show graphics corresponding to FIGS. 4a to 4d for the other proposed sequences S1, S2 and S3.

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:

• • S=2*{C00, 0, 0, 0, C01, 0, 0, 0, C02, 0, 0, 0, C03, 0, 0, 0, C04, 0, 0, 0, C05, 0, 0, 0, 0, 0, 0, 0, C06, 0, 0, 0, C07, 0, 0, 0, C08, 0, 0, 0, C09, 0, 0, 0, C10, 0, 0, 0, C11}

	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*φ_A) and $B=A*\exp \left(j\frac{\pi }{2}\right)=\exp \left(j\mathrm{2\pi }*{\phi }_A+j\frac{\pi }{2}\right)$
and 0.0≦φ_A<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 FIGS. 10a and 10b.

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 FIG. 1 and 2 possible implementations of a transmitter according to the present invention will now be explained.

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 FIG. 2 alternatively the time domain signal can be precomputed once in a computation unit 7 and then be stored in a memory 6 for the precomputed sample for the time signal. Then the time domain signal of the synchronization burst can be sent to the modulator 5 directly from the memory 6.

With reference to FIG. 3 a modified IFFT mapping scheme will now be explained.

According to this scheme, the principle of setting only every fourth subcarrier of the OFDM system to a non-zero amplitude (see FIG. 7) is abandoned. Therefore the time domain signal achieved according to the mapping scheme of FIG. 3 will not present a periodic nature.

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 FIG. 7 to the other burst t2, t3, . . . t6. Obviously the mapping scheme according to FIG. 3 reduces the computing effort necessary for the IFFT. The periodic nature of the time domain signal of the SYNCH bursts is therefore no longer achieved by the IFFT step, but by copying the burst t1 generated with the simplified IFFT mapping scheme.

The mapping scheme shown in FIG. 3 is also advantageous in combination with the precomputing technique shown in FIG. 2.

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:

• • An OFDM based SYNCH symbol with a reduced Peak-to-Average-Power-Ratio (PARP) is proposed,
  • Improved synchronization performance (compared to the state of the art proposal),
  • Reduced AGC (automatic gain control) pull-in time due to reduced dynamic range of the SYNCH burst,
  • Improved AGC settlement (AGC has to adjust to a incoming signal level that later on now overflow/underflow in the AD happens. The reduced dynamic range of the SYNCH burst help to find this reference level more accurate),
  • Reduced synchronization detection complexity on the receiver (reduced resolution necessary due to reduced envelope fluctuation).

Claims

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

2. A method for synchronizing wireless ofdm systems, characterized by the steps of

generating a synchronization burst according to a method according to claim 1, and

transmitting the synchronization burst.

3. A method according to claim 2, characterized in that

the time domain signal of the synchronization burst is precomputed and stored in a memory.

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

5. An ofdm transmitter according to claim 4, characterized by

a time extension unit copying the burst part to achieve a periodic nature of the time domain signal.

6. An ofdm transmitter according to claim 4, characterized by

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.

7. A mobile communications device, comprising a transmitter according to claim 4.

8. A synchronization burst signal for synchronizing ofdm systems generated by a method according to claim 1.

9. A method for synchronizing an ofdm receiver, comprising the steps of generating a synchronization signal by:

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 symbols sequence represent subcarriers of the ofdm system with non-zero amplitude,

generating a synchronization signal by inverse fourier Transforming the subcarriers of the ofdm system mapped with the symbols of said predefined sequence, wherein respective symbols of the symbol sequence have the value $\sqrt{\left(\frac{1}{2}\right)}\left(+1+j\right),\sqrt{\left(\frac{1}{2}\right)}\left(+1-j\right),\sqrt{\left(\frac{1}{2}\right)}\left(-1+j\right),\mathrm{or}\sqrt{\left(\frac{1}{2}\right)}\left(-1-j\right),$

transmitting the synchronization signal, and

receiving said synchronization signal to synchronize a receiver.

10. The method of claim 9, wherein an AGC in the receiver is locked in using the received synchronization signal.

11. The method of claim 9, wherein a reference value of an A/D converter in the receiver is adjusted using the received synchronization signal.

12. A method for synchronizing a receiver in an ofdm communication system, comprising the steps of:

receiving data signals and a synchronization signal exhibiting periodicity, the data signals and synchronization signal being transmitted by using a plurality of subcarriers, said synchronization signal being based on a predefined symbol sequence having a predefined number of symbols, respective symbols having the value $\sqrt{\left(\frac{1}{2}\right)}\left(+1+j\right),\sqrt{\left(\frac{1}{2}\right)}\left(+1-j\right),\sqrt{\left(\frac{1}{2}\right)}\left(-1+j\right),\mathrm{or}\sqrt{\left(\frac{1}{2}\right)}\left(-1-j\right)$

 and

wherein said predefined symbols are periodically mapped on every fourth subcarrier of said plurality of subcarriers so that the periodicity of the synchronization signal contains a predefined number of repetitions of one synchronization signal in the time domain, and

performing time and frequency synchronization in accordance with said periodicity of the synchronization signal.

13. A method for synchronizing a receiver in an ofdm communication system, comprising the steps of:

receiving data and synchronization signals transmitted from a transmitter side by using a plurality of subcarriers, and

performing time and frequency synchronization in accordance with said synchronization signal,

wherein said synchronization signal is generated based on a predefined symbol sequence and respective symbols have the value $\sqrt{\left(\frac{1}{2}\right)}\left(+1+j\right),\sqrt{\left(\frac{1}{2}\right)}\left(+1-j\right),\sqrt{\left(\frac{1}{2}\right)}\left(-1+j\right),\mathrm{or}\sqrt{\left(\frac{1}{2}\right)}\left(-1-j\right).$

14. A method for synchronizing a receiver in an ofdm communication system, comprising the steps of:

receiving data and synchronization signals transmitted from a transmitter by using a plurality of subcarriers, and

performing time and frequency synchronization in accordance with said synchronization signal,

wherein said synchronization signal is generated based on a predefined symbol sequence comprising non-zero symbols with respective symbols having the value $\sqrt{\left(\frac{1}{2}\right)}\left(+1+j\right),\sqrt{\left(\frac{1}{2}\right)}\left(+1-j\right),\sqrt{\left(\frac{1}{2}\right)}\left(-1+j\right),\mathrm{or}\sqrt{\left(\frac{1}{2}\right)}\left(-1-j\right)$

 and other symbols are set to zero, so that said non-zero symbols are arranged with periodicity in said predefined symbol sequence in the frequency domain.

15. A method for synchronizing a receiver in an ofdm communication system, comprising the steps of:

receiving data and synchronization signals transmitted from a transmitter side by using a plurality of subcarriers, and

performing time and frequency synchronization in accordance with said synchronization signal,

wherein said synchronization signal is generated based on a predefined symbol sequence comprised of a predefined number of symbols and respective symbols have the value $\sqrt{\left(\frac{1}{2}\right)}\left(+1+j\right),\sqrt{\left(\frac{1}{2}\right)}\left(+1-j\right),\sqrt{\left(\frac{1}{2}\right)}\left(-1+j\right),\mathrm{or}\sqrt{\left(\frac{1}{2}\right)}\left(-1-j\right),$

wherein the predefined symbols are arranged such that every fourth subcarrier among said plurality of subcarriers has non-zero amplitude.

16. An ofdm communication system comprising a transmitter and a receiver, the transmitter comprising:

means 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 a predefined number of the symbols of the symbol sequence define subcarriers with non-zero amplitude, and

means for 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 predefined symbols of the symbol sequence have the value $\sqrt{\left(\frac{1}{2}\right)}\left(+1+j\right),\sqrt{\left(\frac{1}{2}\right)}\left(+1-j\right),\sqrt{\left(\frac{1}{2}\right)}\left(-1+j\right),\mathrm{or}\sqrt{\left(\frac{1}{2}\right)}\left(-1-j\right);$

 and

the receiver receives said synchronization signal to perform time and frequency synchronization in accordance with said synchronization signal.

17. An ofdm communication system comprising a transmitter and a receiver, the transmitter comprising:

means for 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

means 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 said predefined symbols have the value $\sqrt{\left(\frac{1}{2}\right)}\left(+1+j\right),\sqrt{\left(\frac{1}{2}\right)}\left(+1-j\right),\sqrt{\left(\frac{1}{2}\right)}\left(-1+j\right),\mathrm{or}\sqrt{\left(\frac{1}{2}\right)}\left(-1-j\right);$

 and

the receiver receives said synchronization signal to perform time and frequency synchronization in accordance with said synchronization signal.