Multiplex communications – Generalized orthogonal or special mathematical techniques
Reexamination Certificate
2000-01-06
2003-11-25
Vincent, David (Department: 2732)
Multiplex communications
Generalized orthogonal or special mathematical techniques
C370S208000
Reexamination Certificate
active
06654339
ABSTRACT:
The present invention relates to a method for generating synchronization bursts for OFDM transmission systems, a method for synchronizing wireless OFDM systems, an OFDM transmitter as well as to a mobile communications device comprising such a transmitter.
The present invention relates generally to the technical field of synchronizing wireless OFDM (orthogonal frequency division multiplexing) systems. Thereby it is known to use a synchronization burst constructed using especially designed OFDM symbols and time domain repetitions.
Particularly from the document IEEE P802.11a/d2.0 “Draft supplement to a standard for telecommunications and information exchange between systems—LAN/MAN specific requirements—part 1: wireless medium access control (MAC) and physical layer (PHY) specifications: high-speed physical layer in the 5 GHz band” a synchronization scheme for OFDM systems is proposed. This document is herewith included by reference as far as it concerns the synchronization including the proposed implementation. Said known scheme will now be explained with reference to
FIG. 6
to
8
of the enclosed drawings.
FIG. 6
shows the structure of the known synchronization field. As shown in
FIG. 6
the synchronization field consists of so-called short symbols t
1
, t
2
, . . . t
6
and two long symbols T
1
, T
2
. In view of the present invention particularly the short symbols t
1
, t
2
. . . t
6
are of interest. Among the short symbols t
1
, t
2
, . . . t
6
used for the amplifier gain control (t
1
, t
2
, t
3
) and the course frequency offset and timing control only the symbols t
1
, t
2
, t
3
and t
4
are actually generated, whereas the symbols t
5
, t
6
are cyclic extensions (copies of the symbols t
1
and t
2
, respectively). It is to be noted that
FIG. 5
shows only the synchronization preamble structure as the structure of the following signal field indicating the type of baseband modulation and the coding rate as well as the structure of further following data fields are not of interest in view of the present invention. For further details reference is made to said prior art document.
The symbols t
1
, t
2
, t
3
, t
4
are generated by means of an OFDM modulation using selected subcarriers from the entire available subcarriers. The symbols used for the OFDM modulation as well as the mapping to the selected subcarriers will now be explained with reference to FIG.
6
.
Each of the short OFDM symbols t
1
, . . . t
6
is generated by using 12 modulated subcarriers phase-modulated by the elements of the symbol alphabet:
S={overscore (2)}(±1±j)
The full sequence used for the OFDM modulation can be written as follows:
S
−24,24
=2*{1+j,0,0,0,1+j,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1−j,0,0,0,−1−j,0,0,0,0 0,0,0,1+j,0,0,0,1+j,0,0,0,−1−j,0,0,0,1+j,0,0,0,−1+j,0,0,0,1+j}
The multiplication by a factor of 2 is in order to normalize the average power of the resulting OFDM symbol.
The signal can be written as:
r
SHORT
(
t
)
=
w
SHORT1
(
t
)
∑
k
=
-
N
2
/
2
N
s
/
2
S
k
exp
(
j
2
π
k
Δ
F
t
)
The fact that only spectral lines of S
−24, 24
with indices which are a multiple of 4 have nonzero amplitude results in a periodicity of T
FFT
/4=0.8 &mgr;sec. The interval T
TSHORT1
is equal to nine 0.8 &mgr;sec periods, i.e. 7.2 &mgr;sec.
Applying a 64-point IFFT to the vector S, where the remaining 15 values are set to zero, four short training symbols t
1
, t
2
, t
3
, t
4
(in the time domain) can be generated. The IFFT output is cyclically extended to result in
6
short symbols t
1
, t
2
, t
3
, . . . t
6
. The mapping scheme is depicted in FIG.
7
. The so called virtual subcarriers are left unmodulated.
The way to implement the inverse Fourier transform is by an IFFT (Inverse Fast Fourier Transform) algorithm. If, for example, a 64 point IFFT is used, the coefficients 1 to 24 are mapped to same numbered IFFT inputs, while the coefficients −24 to −1 are copied into IFFT inputs
40
to
63
. The rest of the inputs, 25 to 39 and the 0 (DC) input, are set to zero. This mapping is illustrated in FIG.
7
. After performing an IFFT the output is cyclically extended to the desired length.
With the proposed inverse fast Fourier transform (IFFT) mapping as shown in
FIG. 7
the resulting time domain signal consists of 4 periodically repeated short symbols t
1
, t
2
, t
3
, t
4
, and cyclically extended by a copy of t
1
, t
2
, which copy is depicted in
FIG. 5
as t
5
, t
6
. Note that in the present case only spectral lines with indices which are a multiple of 4 have nonzero amplitude. Other periodic natures can be generated by setting other multiples of the spectral lines to nonzero amplitudes.
Though the known synchronization scheme is very effective, it provides for disadvantage regarding the time domain signal properties.
For OFDM (or in general multicarrier signals) the signal envelope fluctuation (named Peak-to-Average-Power-Ratio=PAPR) is of great concern. A large PAPR results in poor transmission (due to nonlinear distortion effects of the power amplifier) and other signal limiting components in the transmission system (e.g. limited dynamic range of the AD converter).
For synchronization sequences it is even more desirable to have signals with a low PAPR in order to accelerate the receiver AGC (automatic gain control) locking and adjusting the reference signal value for the A/D converter (the whole dynamic range of the incoming signal should be covered by the A/D converter resolution without any overflow/underflow).
FIGS. 8
a
,
8
b
show the “absolute” (sqrt{In*+Quad *Quad}) value of the resulting time domain signal waveform with the sequences proposed by Lucent Technologies. Oversampling (8*) was considered in order to ensure the peak was captured correctly using the limited 64-point IFFT.
FIGS. 8
c
,
8
d
show the real and imaginary part of the resulting transmitted time domain waveform. The resulting PAPR is 2.9991 dB (no oversampling) and 3.0093 dB (with 8 times oversampling).
Therefore it is the object of the present invention to provide for a synchronization technique which bases on the known synchronization technique but which presents improved time domain signal properties to reduce the requirements for the hardware.
The above object is achieved by means of the features of the independent claims. The dependent claims develop further the central idea of the present invention.
According to the present invention therefore a method for generating synchronization bursts for OFDM transmission systems is provided. Symbols of a predefined symbol sequence are mapped according to a predefined mapping scheme on subcarriers of the OFDM system wherein the symbols of the predefined symbol sequence represent subcarriers with nonzero amplitudes. A synchronization burst is generated by inverse fast Fourier transforming the subcarriers mapped with a predefined symbol sequence. According to the present invention the predefined symbol sequence is optimized such that the envelope fluctuation of the time domain signal (Peak-to-average-power-ratio) is minimized.
The predefined symbol sequence can be chosen such that the following equations are satisfied for all symbols of the predefined symbol sequence:
n
=2
m,
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 F
Böhnke Ralf
Dölle Thomas
Konschak Tino
Frommer William S.
Frommer & Lawrence & Haug LLP
Mahr Hans R.
Sony International (Europe) GmbH
Vincent David
LandOfFree
Synchronization symbol structure using OFDM based... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Synchronization symbol structure using OFDM based..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Synchronization symbol structure using OFDM based... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3137697