Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1999-07-30
2003-02-18
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Angle modulation
C375S342000, C375S362000, C329S307000
Reexamination Certificate
active
06522703
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of estimating the frequency offset of a packet of phase-modulated symbols that have been received with phase error.
The invention relates to demodulating signals that have been modulated by phase shift keying (PSK), and more particularly to estimating the frequency and phase offsets that are necessary for demodulating signals in packet form that have been modulated by phase modulation, or by phase-and-amplitude modulation. The invention applies to the field of transmission, for the purpose of demodulating digital signals transmitted in the form of packets. A typical digital transmission channel comprises a binary source, an encoder, a modulator which transmits into the channel, a demodulator at the outlet from the channel, and a decoder which delivers the binary signal. Channel encoding is used to reduce the power needed to achieve a given bit error rate. Modulation and demodulation enable transmission to be performed over the selected channel. Insofar as the invention relates to demodulation, the other elements of the transmission system, such as source encoding and decoding are not described in greater detail.
BACKGROUND OF THE INVENTION
M. Morelli and U. Mengali, “Feedforward frequency estimation for PSK: a tutorial review”, ETT Vol. 9, No. 2, March-April 1998, contains a description of the problem of synchronization in demodulators for digital transmission systems using phase modulation in packet mode. That document also describes the known solutions to the problem.
The problem of demodulating phase modulated digital signals that have been transmitted in packet mode arises particularly in satellite transmission systems using time division multiple access (TDMA), or for land cellular telecommunications systems. This is a major technical problem in the field of telecommunications.
OBJECTS AND SUMMARY OF THE INVENTION
More precisely, the invention provides a method of estimating the frequency offset of a packet of phase-modulated symbols that has been received with phase error, the method comprising:
applying a phase correction to the symbols of the received packet for all possible pairs of frequency offset and phase offset; and
selecting from said offset pairs, the pair which provides the most likely packet of corrected symbols, thereby determining the frequency offset of the received packet.
In an implementation, the method further includes a step of quantizing possible phase corrections.
Preferably, the selection step comprises:
calculating a metric between the packet of symbols as received and phase-corrected and an estimate of the packet of symbols as sent; and
seeking a minimum value of said metric.
In another implementation, the packet of symbols sent is estimated:
for known symbols, by using the transmitted symbol; and
for unknown symbols, by taking a firm decision.
Advantageously, rotation through e
j2&pgr;/k
, where k is a natural integer, leaves the set of possible symbols invariant, and possible phase values are selected from a range of [0, 2&pgr;/k].
In another implementation, the phase correction applied to the symbols, for a frequency offset value f
m
and for a phase offset value &phgr;
k
is written e
−j(2&pgr;f
m
(p−1)T
s
+&phgr;
k
)
, where p is the position of the symbol and T
s
is the symbol time.
In this case, the values of the argument −(2&pgr;f
m
(p−1)T
s
+&phgr;
k
) are preferably calculated in the form of reduced values, e.g. over a range [0, 2&pgr;].
The method can further comprise a prior step of calculating and storing all possible phase corrections to be applied to a symbol.
The method can further comprise a step of estimating the phase offset as being the phase offset value in said pair that provides the most likely packet of corrected symbols.
REFERENCES:
patent: 5376894 (1994-12-01), Petranovich
patent: 5428647 (1995-06-01), Rasky et al.
patent: 5463627 (1995-10-01), Matsuoka et al.
patent: 6038267 (2000-03-01), Oura et al.
patent: 6104767 (2000-08-01), Atarius et al.
patent: 0 380 876 (1990-08-01), None
patent: 0 533 208 (1993-03-01), None
patent: 0 606 163 (1994-07-01), None
patent: 0 851 640 (1998-07-01), None
patent: WO 93/11605 (1993-06-01), None
Thorpe, McLane: “A Hybrid Phase/Data Viterbi Demodulator for Encoded CPFSK Modulation”, IEEE Global Telecommunications Conference, Nov. 28, 1983—Dec. 1, 1983, pp. 754-758 XP002097468.
Cueff Valérie
Terre Michel
Alcatel
Pham Chi
Sughrue & Mion, PLLC
Tran Khanh
LandOfFree
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