Pulse or digital communications – Synchronizers – Synchronizing the sampling time of digital data
Reexamination Certificate
2000-03-20
2001-10-30
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Synchronizers
Synchronizing the sampling time of digital data
C375S261000
Reexamination Certificate
active
06310926
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to a multicarrier QAM system and a QAM receiver as well as a method for receiving and demodulating an analog multicarrier QAM signal into a digital output signal consisting of successive bits. In particular, the invention relates to the adjustment of a phase of a sample frequency at the receiver side of the multicarrier QAM system. A receiver, a multicarrier QAM communication system for transmitting information and a method according to the preamble of claims
1
,
16
,
27
is known from EP 0 656 706 A2. Here, explicitly the phase shift value of each QAM value is determined and the timing is adjusted on the basis of the explicitly determined phase shift value. An adjustment of the timing merely on the basis of the an averaged direction value as in the present invention is not performed here.
Multicarrier QAM Systems
Multicarrier QAM systems are widely used in general switched telephone networks, cellular radio technology and transmissions between individual modems using DMT or OFTM (DMT: discrete multi-tone modulation; OFTM: orthogonal frequency division multiplexing). Generally, multicarrier QAM transmission has advantages over single-carrier transmission, e.g. that a multicarrier QAM signal can be processed in a receiver with little noise or interference which is e.g. caused by linear equalization of a single-carrier signal, and secondly that the long symbol time used in the multicarrier QAM transmission produces a much greater immunity to pulse noise and fast fading effects.
On the other hand, the recovery of the sample rate with a correct phase and the recovery of the symbol rate and its start position from the received analog multicarrier QAM signal at the receiver must be very accurate to not cause a wrong decoding of the data.
BACKGROUND OF THE INVENTION
The background of multicarrier modulation/demodulation of data will first be explained wish reference to the attached
FIG. 1
to
4
. A general review article, which describes such basic aspects and which relates to the preamble of the attached claims
1
,
14
,
23
, is published in IEEE Communications Magazine, May 1990, pages 6 to 14, “Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come” by John A. C. Bingham.
Multicarrier Modulation
As shown in
FIG. 1
, input data T
x
at Mf
s
bits per second are grouped into blocks of M bits at a block (“symbol”) rate of f
s
. As shown in
FIG. 3
, the input data T
x
may e.g. be obtained from a video signal coder
13
, generally as bits from a data source. The successive bits of the input data T
x
have a certain bit rate, i.e. a certain number of bits per second. As also shown in
FIG. 3
, a block of bits M correspond to one symbol in the multicarrier transmission technology. In a serial-to-parallel converter
1
, the input data T
x
is grouped into groups of m
1
bits, m
2
bits, . . . , m
n
bits. In a modulator
2
, m
n
bits for the carrier at f
c,n
are used to modulate N
c
carriers, which are spaced &Dgr;f apart across any usable frequency band. That is
f
c,n
=n
&Dgr;f for
n=n
1
to
n
2
(1)
and
M
=
∑
n
=
n
1
n
2
m
n
with
N
c
=n
2
−n
1
+1,
&Dgr;f: carrier spacing,
n
1
: lowest index of carriers,
n
2
: highest index of carriers,
N
c
: total number of carriers considered.
The carriers which have been modulated by the individual bits are summed for transmission on a transmission line TM and must in principle be separated in the receiver before demodulation. As will be seen in the following, a separation and demodulation can be done at the same time using an FFT.
Multicarrier Demodulation
Multicarrier demodulation techniques conventionally used are described in the above prior art document. Nowadays, most systems will use a demodulation method where the carriers are “keyed” by the data using the so-called quadrature amplitude shift keying (QAM). The individual spectra are now sinc functions and are not band-limited. The separation is then carried out by baseband processing and not by bandpass filtering which leads to the advantage that both the transmitter and the receiver can be implemented using efficient Fast Fourier Transform (FFT) techniques.
QAM System Using Fast Fourier Transforms
FIG. 2
shows such a basic multicarrier QAM system using Fast Fourier Transforms in the means
4
,
10
at the transmitter and receiver side. Modulation is performed on M bits (symbol or block) of data at a time—preferably using an inverse FFT (FFT
−1
)—and samples of the transmitted signal on the transmission line TM (an analog signal) are generated at the sampling rate f
sampT
in the digital/analog converter
5
. For greatest efficiency, f
sampT
should be equal to &Dgr;f multiplied by an integer power of 2.
If f
samp
=2*N
tot
*&Dgr;f, then N
tot
carriers are available for modulation, but the channel will usually be such that only N
c
(N
c
<N
tot
) carriers can be used. If these are at frequencies n
1
&Dgr;f to n
2
&Dgr;f, as defined in the above equation (1), modulation of a total of M bits, m
n
at a time, is most easily accomplished by calculating N
c
complex numbers (each selected from a constellation with 2
m
n
points), augmenting them with n
1
−1 zeros in front and N
tot
−n
2
zeros behind and performing a FFT
−1
using N
tot
points. Thus, the modulation via FFT
−1
is equivalent to a multicarrier QAM in which the fundamental baseband pulse shape is a rectangle.
In the receiver
7
-
12
, the received analog signal is demodulated by assembling N
tot
samples into a block and performing a real-to-complex FFT in the means
10
. This is equivalent to demodulating each subband separately and then doing an integrate-and-dump procedure on each product.
That is, after the serial-to-parallel conversion in the converter
1
, the encoder
3
generates a plurality of complex values whose number corresponds to the number of carriers used in the multicarrier system. The shift keying is then essentially performed by the FFT
−1
in the means
4
to result in a number of separate real sample values. The digital/analog converter
5
composes these digital samples into the analog transmission signal using a predetermined sample rate or sample frequency f
sampT
.
The demodulation at the receiver
7
-
12
is completely analogous to the modulation in the transmitter
1
-
6
. The analog signal is converted into digital samples using the analog/digital converter
8
employing a sample rate f
sampR
. The digital samples are then applied to the serial-to-parallel buffer
9
which outputs these values to the FFT means
10
that performs the real-to-complex FFT transform. In the output of the FFT
10
, one finds N
CR
complex values that should ideally be identical to those input to the FFT
−1
means
4
in the transmitter. The decoder
11
and the parallel-to-serial buffer
12
carry out a respective inverse operation as the parts
1
,
3
in the transmitter to yield a digital output signal R
x
consisting of successive bits which should correspond to the input bitstream T
x
.
Decoding the Complex QAM Data Values
FIG. 4
shows the complex QAM data values Y
00
, Y
01
, Y
10
, Y
11
of the digital frequency domain multicarrier QAM signal output by the FFT means
10
in the receiver. As an example,
FIG. 4
shows the situation using one carrier transmitting two bits. Also shown in
FIG. 4
are the complex default QAM data values C
00
, C
01
, C
10
, C
11
which were actually transmitted from the transmitter. As can be seen, the transmitted data symbols C
k
do not necessarily coincide with the received data symbols Y
k
in the complex plane. Such deviation is primarily caused by the fact that the symbol rate and/or the sample rate cannot be fully synchronized to each other in the transmitter and the receiver. Furthermore, there are distortions on the transmission line TM (i.e. the transmission channel)
However, the transmitted data symbols 00, 01, 10, 11 can still be decoded because the decoder
11
will estimate or sel
Ghebretinsae Temesghen
Nixon & Vanderhye P.C.
Telefonaktiebolaget LM Ericsson (publ)
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