Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1999-05-07
2002-07-16
Ghayour, Mohammad H. (Department: 2734)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S354000
Reexamination Certificate
active
06421401
ABSTRACT:
TECHNICAL FIELD
The invention relates in general to the synchronization of a radio receiver to a received signal. In particular the invention relates to the realization of symbol synchronization in a system wherein the received signal contains a certain guard interval the timing of which has to be right in the reception, so that potential multi-path components in the received signal can be utilized in an optimal manner.
BACKGROUND OF THE INVENTION
Abbreviation OFDM (Orthogonal Frequency Division Multiplex) refers to a modulation method in which the transmitting apparatus divides and combines the transmitted signal into several subcarriers which are located on the frequency axis at regular intervals on a certain frequency band and which are sent simultaneously. Known radio-frequency communications systems that employ OFDM modulation include the DAB (Digital Audio Broadcasting) and DVB (Digital Video Broadcasting) systems. The former is specified in general outline in the standards drawn up by the European Broadcasting Union (EBU) and the European Telecommunications Standards Institute (ETSI), and the latter is specified in general outline in a draft standard by the same organizations. In these systems, a section of a digital signal to be transmitted on a certain subcarrier is encoded into phase and/or amplitude changes with respect to a certain known phase. That time slice of the transmitted signal during which the modulating phase state is constant separately at each subcarrier frequency is called an OFDM symbol, or a symbol in short.
Successful OFDM reception requires that the receiver maintains the correct symbol synchronization and sampling frequency. Symbol synchronization means that the receiver knows at which point in time each symbol begins and times the symbol detection correspondingly. Sampling frequency refers here to the frequency at which the A/D converter in the receiver takes samples from the received analog oscillation in order to convert the signal into digital form, whereby the A/D converter and subsequent circuits can interpret to which bits or bit combinations in the digital data flow the signal phase changes refer. In addition, the receiver has to maintain frequency synchronization, i.e. tune the reception and mixing circuits so that the detected frequency band covers all subcarriers of the OFDM signal at an accuracy which is less than half of the difference between two adjacent subcarriers. Maintaining the symbol synchronization, sampling frequency and frequency synchronization is especially difficult if the transmitter and receiver are moving with respect to each other. The receiver may be located in a car, for example, and as the car moves around in an urban environment, the propagation path of the radio signal changes constantly, resulting in attenuation and reflections. The receiver may also be located in a satellite, and as the satellite moves, the speed difference between the receiver and the satellite changes, being possibly up to several kilometres per second. This patent application is especially concerned with achieving and maintaining symbol synchronization.
An adjustment method for symbol synchronization and sampling frequency in an apparatus receiving OFDM-modulated transmissions as well as an apparatus realizing such a method is known from Finnish patent application Ser. No. 963649. The method disclosed is based on utilizing time-domain correlation characteristics of the reference signal in an OFDM transmission. In the DAB system, the reference signal means a phase reference symbol, and cross-correlation between the received format and the known format of that symbol yields the instantaneous impulse response. In the DVB system, the impulse response is estimated from scattered pilot subcarriers for four consecutive symbols. The required changes in the symbol synchronization and sampling frequency can be deduced by monitoring how the impulse response changes from one measurement to another. The symbol synchronization is preferably set such that the guard interval between the symbols coincides with the beginning of the correlation function representing the impulse response. A sampling frequency error shows between the measurements as a slow and monotonously continuous shift of the maximum of the correlation function representing the impulse response. By correcting the sampling frequency the receiver attempts to eliminate said change.
From publication “Low-Complex Frame Synchronization in OFDM Systems” by J-J. van de Beek, M. Sandell, M. Isaksson, P.O. Börjesson, IEEE International Conference on Universal Personal Communications, Tokyo 1995, a method for achieving symbol synchronization by utilizing characteristics of data transmitted in an OFDM system is known. This method is briefly explained below.
FIG. 1
shows a simple OFDM system model wherein complex numbers X
k
, k&egr;[1,N] taken from a fundamental set, or constellation, are to be transmitted (cf. allowed points in phase-amplitude coordinate system in quadrature amplitude modulation, QAM). The complex numbers x
k
are used for modulating N subcarriers by means of an inverse discrete Fourier transform (IDFT) in block
101
. The result is N samples s, the last L of which are copied to the beginning of the sample set. After the copying, the number of samples is N+L and a given sample can be marked s
k
, where k&egr;[1,N+L]. The samples copied to the beginning of the sample set constitute a so-called guard interval because in time domain they appear as a period in the beginning of the symbol the contents of which are a copy of the end of the symbol.
A parallel-to-serial converter
102
is used to generate an OFDM symbol, marked s(k). When traveling from the transmitter to a receiver through a given channel the symbol s(k) is affected by the impulse response h(k) of the channel and noise n(k) is added to it. The receiver sees the received sample sequence, marked r(k). The latter undergoes a serial-to-parallel conversion in block
103
producing samples r
k
, where still k&egr;[1,N+L]. Only the last N samples are independent of each other, so they are taken to block
104
where a discrete Fourier transform takes place. Symbol synchronization is the same as determining out from which location in the received sample sequence said last N samples will be taken. The end result are complex numbers y
k
, k&egr;[1,N]. If reception was fully successful, those complex numbers are the same as the transmitted complex numbers x
k
.
In said method, a copy r(k−N) is made of the received symbol r(k) and said copy is delayed by N samples with respect to the original received symbol. A correlation function is defined between the copy and the original:
j
(
k
)=
r
(
k
)
r
*(
k−N
) (1)
where * stands for complex conjugation. Then we can calculate the moving sum using a window of L samples
u
(
k
)
=
∑
i
=
0
L
-
1
j
(
k
-
1
)
(
2
)
and sliding the window over the received sample sequence for the length of 2N+L samples.
FIG. 2
shows a received sample sequence r(k) with a guard interval
201
for a symbol and the corresponding period
202
at the end of the symbol, a copy r(k−N) of the received sample sequence, sequence portions
201
′ and
202
′ corresponding to the guard interval
201
and original samples
202
, and a correlation result j(k) with a period
203
that represents high correlation. In addition,
FIG. 2
shows the value of the moving sum u(k) in such a way that the value on the u(k) curve corresponds to the sum according to equation 2 in that window the right edge of which coincides with the value in question. The figure shows that the u(k) curve has a distinct correlation peak
204
the top of which coincides with the end of a given symbol in the original sample sequence. In said method according to the prior art symbol synchronization is based on the detection of the top of the correlation peak
204
.
The methods according to the prior art described above are applicable i
Ghayour Mohammad H.
Nokia Corporation
Ware Fressola Van Der Sluys & Adolphson LLP
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