Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2000-02-01
2001-03-06
Ghebretinsae, Temesghen (Department: 2734)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S355000, C375S326000, C370S208000
Reexamination Certificate
active
06198782
ABSTRACT:
This invention relates to communication systems and particularly to those employing orthogonal frequency division multiplexing (OFDM) modulation methods.
OFDM is a technique whereby information symbols are communicated from a transmitter to a receiver over a plurality of sub-carriers.
Carrier and clock frequency offsets can produce large degradations of the Bit Error Rate (BER) performance of Orthogonal Frequency Division Multiplex (OFDM) systems. See for example Proc. IEEE 83(6) 982-996 June 1995 and Proc. COST254 1997. Indeed they not only produce extra-noise due to Inter Carrier Interference (ICI) but also a parasitic rotation of the symbols which also increases the BER.
Herein, carrier frequency offset means any difference in frequency between the carrier frequency generators in the transmitting and receiving circuitry and clock frequency offset means any difference between the sampling rates of the transmitting and receiving circuitry.
There already exist several methods for estimating and compensating a carrier frequency offset. See for example, U.S. Pat. No. 5,450,456 (Mueller) and Elec. Lett. 33(2) 113-114 January 1997. However, using either of these known methods, the residual carrier frequency offset can rise up to a few percent of the sub-carrier spacing and there still remains a degradation. As regards clock frequency offset, the degradation depends on the number of sub-carriers. If this number is large, this may prevent the designer from relaxing constraints on the receiver's clock generation mechanism.
To illustrate the problem, consider a typical OFDM transmission system such as Digital Audio Broadcasting (DAB) mode
1
. In FIG.
1
and
FIG. 2
, the degradation due to ICI is plotted. After correction with the classical algorithms, the carrier frequency offset can reach 3% of the sub-carrier spacing (in
FIG. 2
, this corresponds to f=3% where f is the carrier frequency offset expressed in number of sub-carrier spacings, N is the number of sub-carriers and T is the reciprocal of the sampling rate.) A low-price ageing clock oscillator can have a frequency offset of 50 ppm (in
FIG. 1
, this corresponds to a normalised clock frequency offset e=5.10
−5
). Then at a Signal to Noise Ratio (SNR) of 20 dB, the ICI degradation can represent more than 1 dB. Moreover, the parasitic rotation due to f can represent 15% of the angle separating neighbour points of the QPSK (Quadrature Phase Shift Keying) constellation. The parasitic rotation due to e can represent 20% of this angle. Therefore, the error probability rises up severely.
Known methods which exist for reducing clock and carrier frequency offsets work on the assumption that time synchronization is achieved and that carrier frequency offset has been compensated up to a certain precision. However, there is still a need for a method with reduced complexity and better performance.
In cable transmission, a sub-carrier is dedicated to clock recovery, which implies a loss of useful capacity of the system.
In U.S. Pat. No. 5,345,440 (Gledhill) a method is presented for estimating the carrier frequency offset and the clock offset. But only a formula for straight carrier frequency offset estimation is provided, whereas no formula for directly estimating the clock frequency offset is given. Besides, carrier and clock are estimated separately. Both phenomenona produce similar effects, so that a joint method for estimating both would have an improved efficiency and a reduced complexity.
In Elec. Lett. 34(6) 520-521 March 1998, a method for jointly estimating carrier and clock frequency offsets is proposed. However, this method has the disadvantage of poor performance on a frequency-selective channel, such as the radio-mobile channel or cable channels.
In U.S. Pat. No. 5,802,117 (Ghosh), a joint method working on any channel is presented. However, it is not a ‘blind’ technique ie. It relies on a specific training signal. Therefore, it cannot be applied on any existing standardised communication system.
There is subsequently a need for jointly and blindly estimating carrier and clock frequency offsets on any channel, including frequency selective ones and it is this need that the present invention addresses.
Accordingly, the present invention comprises apparatus for estimating carrier frequency offset and sampling frequency offset between transmitter circuitry and receiver circuitry which communicate over a channel of an OFDM system, the apparatus including in a receiver circuit;
a local oscillator having a frequency fb for converting a received OFDM modulated signal s(t), representing information symbols S(k) having components Sm(k) and sampled at a first sampling rate ft and modulated onto a carrier of frequency fc, to a base-band signal, an analogue to digital converter having a second sampling rate fr for sampling the baseband signal,
a demodulator for performing a discrete Fourier transform on the sampled baseband signal to generate blocks of symbols R(k) having components Rm(k) representing the information symbols S(k) of components Sm(k), and characterised by; a first module for removing parasitic effects of the channel on R(k) to generate at least one block Y(k) of components Ym(k),
a second module for removing modulation effects from Y(k) to generate at least one block Z(k) of components Zm(k) and to compute V of components Vm where Vm=/Ym/,
and a third module for estimating terms A and B relating respectively to the carrier frequency offset /fc−fb/ and the sampling frequency offset /ft−fr/ by performing a joint maximun likelihood estimation (MLE) such that;
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∑
m
=
-
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=
K
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[
-
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sin
(
A
+
m
B
)
+
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cos
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=
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=
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=
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sin
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V
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The invention thus provides a method and apparatus for jointly estimating carrier and clock frequency offsets on any channel, including frequency selective ones.
The invention computes the estimates {circumflex over (ƒ)} of f and {circumflex over (&egr;)} of e, by analyzing the rotations produced by clock and carrier frequency offsets on the received signals. The estimate {circumflex over (ƒ)} can be added to that obtained with known algorithms, and may be used to compensate the carrier frequency offset on a time-domain signal. The estimate {circumflex over (&egr;)} can be used to steer the receiver clock frequency (e.g: using a Voltage Controlled Crystal Oscillator VCXO). Thus, the effect of carrier and clock frequency offsets is brought down to a negligible level, at the expense of slightly higher complexity.
Advantages of the method compared with existing techniques are:
The complexity required is low (a few times K, for a typical precision, where K is the number of useful sub-carriers).
The implementation of the invention allows either to reduce the cost of the receiver by relaxing constraints on the clock oscillator and on the carrier synchronization mechanism, or to save power (the gain can then be greater than 1 dB)
Since the method used is blind, it can be applied on existing standards such as ADSL, DAB or DVB-T. It works either on coherent or on differential modulation schemes;
The method is flexible. The ability to track f and e depends on the number of symbols over which the estimation is performed. The more symbols, the more accurate and precise the estimate, but the lower the ability of tracking. Furthermore, the symbols picked up for the estimate do not need to be consecutive. For instance, in a time-varying channel, the chosen symbols can be spaced out. Thus, the complexity decreases and the estimation takes benefit from diversity effects. To summarize, it
De Courville Marc
Settineri Veronique Buzenac
Simoens Sebastien
Fekete Douglas D.
Ghebretinsae Temesghen
Motorola Inc.
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