Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
2000-02-07
2002-08-20
Vo, Don N. (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S224000
Reexamination Certificate
active
06438173
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method and a configuration for determining a start of a first symbol and a carrier-frequency shift at a receiver end when a multicarrier signal containing a sequence of data symbols, which form a data block, is received once. The determination of the symbol start controls a symbol clock for demodulation of the individual symbols. The estimated value of the carrier-frequency shift is used as a manipulated variable for frequency correction in the receiver both during reception and during subsequent transmission of data signals. In this case, a test signal is transmitted from the transmission end at an uncertain time together with a data block, and is looked for and evaluated by a configuration at the receiver end. A design rule for test signals is specified, which allows the combined estimation of the start of the first data symbol and the carrier-frequency shift between the transmitter and receiver.
The invention is suitable for forward-acting digital synchronization of wire-free or wire-based receivers which process orthogonal frequency division multiplexing (OFDM) signals for transmission of individual data blocks, which are transmitted irregularly. The invention relates to the general case of single-shot synchronization, which can be carried out for each individual data block irrespective of preceding or future synchronization attempts. Its accuracy is suitable for high-rate OFDM signals, which may use higher-level modulation (for example 8-DPSK or 16-QAM) for high bandwidth efficiency. At present, OFDM is regarded as a suitable modulation technique for future broadband multimedia mobile radio systems and broadband wire-free networks.
Published, British Patent Application GB 2 307 155 A describes a synchronization method for OFDM, which uses guard intervals that are present in the signal.
The synchronization of OFDM signals has furthermore been dealt with, inter alia, in Published, European Patent Application No. 92113788.1, in a reference by F. Classen, titled “Systemkomponenten für eine terrestrische digitale mobile Breitbandübertragung” [System Components For A Terrestrial Digital Mobile Broadband Transmission], in a dissertation at the RWTH Aachen, Shaker Verlag, Aachen 1996 and in conference publications by M. Schmidl, D. Cox, titled “Low-Overhead, Low-Complexity [Burst] Synchronization For OFDM”, Conference Proceedings, IEEE International Conference on Communications '96, pages 1301-1306, and by M. Sandell, J. Beek, P. Börjesson, titled “Timing And Frequency Synchronization In OFDM Systems Using The Cyclic Prefix”, Conference Proceedings, International Symposium on Synchronization, Essen, Germany, December 1995, pages 16-19.
A number of previous works relating to the synchronization of OFDM receivers have proposed the transmission of a test signal of specific length at cyclic times, whose periodicity is evaluated by the receiver and is used to determine the start of a data block or of any carrier-frequency shift between the transmitter and receiver. Methods for this evaluation have been specified both for before and after the calculation of he fast fourier transformation (FFT) used for demodulation of FDM signals.
A disadvantage of the known methods and configurations is that they are each characterized by at least one of the below recited features. First, only a portion of the total synchronization of the receiver is dealt with, in which case the remaining synchronization tasks are presupposed to have been completed ideally; one example is the description of a method for estimation of the carrier-frequency shift, presupposing ideal symbol-clock synchronization. Second, regular repetition of test signals for receiver synchronization is stipulated, and/or averages over a plurality of synchronization sequences and test signals are required for adequate synchronization accuracy. While this approach is advantageous for broadcast radio applications, it is impossible, or feasible only with great complexity, for irregular transmission of data blocks in two transmission directions the computation operations to be carried out per synchronization sequence do not have the aim of minimum hardware processing complexity.
OFDM is a multicarrier modulation method. The transmitted OFDM signal s(t) in baseband includes a time sequence of individual OFDM symbol signals g
i
(t) of duration T
S
:
s
(
t
)
=
∑
i
g
i
(
t
-
iT
s
)
where
(
1
)
g
i
(
t
)
=
∑
k
S
i
,
k
ⅇ
j2π
kF
Δ
t
b
(
t
)
b
(
t
)
=
{
1
,
T
G
≤
t
≤
T
0
,
else
The summation index i represents the symbol clock, and k represents the subcarrier of the frequency kF&Dgr;. The OFDM symbol signal g
i
(t) contains the superposition of M (for example M=49) subcarriers e
j2&pgr;F
&Dgr;
t
which are modulated independently of one another by the complex data symbols S
i,k
. The vector of all the symbols S
i,k
for a fixed symbol clock value i is referred to as the symbol block s
i
. The superposition, also called modulation, is carried out digitally by an inverse fast fourier transformation (IFFT) of length N
FFT
. N
FFT
>M where M input values of the IFFT are identical to S
i,k
, and the remaining (N
FFT
−M) input values are set to zero. The demodulation of the OFDM signal is carried out by an FFT of length N
FFT
. The following parameters are also defined:
T—symbol duration used,
T
G
—guard interval, which is at least as long as the maximum channel echo, and
F
&Dgr;
—subcarrier separation
The relationships T
S
=T+T
G
and F
&Dgr;
=I/T apply. For practical applications, T
G
<0.25T
s
.
A data block contains a sequence of at least one OFDM symbol g
i
(t). This is provided with a test signal, which is positioned either in front of the data block or in the middle of the data block. In the former case, the test signal is referred to as a preamble, and in the second case as a midamble. In a practical implementation of a multicarrier transmission system, it can be stated, as a precondition, that: the time characteristics of the transmission channel are approximately constant for the duration of the test signal 2T
S
; and the frequency characteristics of the transmission channel are approximately constant for a frequency interval of at least 2F
&Dgr;
.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method and a configuration for a combined measurement of the start of a data block and a carrier-frequency shift in a multi-carrier transmission system for irregular transmission of data blocks that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for receiving a multicarrier signal, including a single transmission of a data block, in which case a cyclic multicarrier test signal having a cyclic time layout is transmitted together with the data block, and the cyclic multicarrier test signal being used for a combined measurement of a start of a first data symbol of the data block and of any carrier-frequency shift between a transmitter and a digital receiver, the cyclic multicarrier test signal being evaluated in the digital receiver performing the steps which includes:
performing a coarse measurement of the start of the data block by evaluating the cyclic multicarrier test signal without any fast fourier transformation (FFT) being calculated in the step;
determining a fine carrier-frequency shift, which may be present between the transmitter and the digital receiver, by isolation and evaluation of a part of the cyclic multicarrier test signal without any FFT being calculated in the step;
performing isolation and digital frequency correction of N
FFT
sample values from the cyclic multicarrier test signal and calculation of an FFT of length N
FFT
from the sample values resulting in calculated FFT val
Al-Beshrawi Tony
Infineon - Technologies AG
Locher Ralph E.
Vo Don N.
LandOfFree
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