Telecommunications – Radiotelephone system – Including private cordless extension system
Reexamination Certificate
2000-06-02
2003-09-23
Urban, Edward F. (Department: 2685)
Telecommunications
Radiotelephone system
Including private cordless extension system
C455S405000, C455S516000
Reexamination Certificate
active
06625459
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to communications systems in general and, more specifically, to a method and apparatus for efficiently determining a channel estimate and a baud frequency offset estimate for a communications channel between a transmitter and a receiver.
BACKGROUND OF THE INVENTION
FIGS. 1-2
respectively show models of a typical Quadrature Amplitude Modulation (QAM) transceiver
10
and a typical Carrierless Amplitude-Phase (CAP) transceiver
12
on a communications channel. Assuming perfect synchronization of the transmitter and receiver, a system may be modelled by the equation v=u*h+n (shown diagrammatically in FIG.
3
), wherein v is the complex received signal, u is a transmitted complex symbol sequence, h is a complex channel model, and n is a complex noise signal. The complex noise signal is not assumed to be a white noise signal.
One goal of a characterization of a channel is to find a channel estimate, ĥ, that minimizes the expectation value (EV) of the difference between any given received signal and the expected signal. One formula for EV is shown as Equation 1.
EV
=
E
{
&LeftBracketingBar;
v
-
u
*
h
^
&RightBracketingBar;
2
}
(Equ. 1)
Once known, channel estimate Ii can be used by a decoder for determining the appropriate decoder parameters, such as equalizer coefficients.
Further, in a real system, the clocks at the transmitter and receiver are not perfectly synchronized. The difference between the frequency of the transmitter's clock and the receiver's clock is called the baud frequency offset. Typically, the receiver would use some type of a timing recovery loop to track the differences in clock rates. To optimize performance in a packet-based demodulator, it's valuable to also have an estimate of the baud frequency offset to initialize the timing recovery loop before starting demodulation of the packet.
Optimal sequences for obtaining channel estimates have long been understood. See, for example, J. Letaief and R. D. Murch's, “Complex Optimal Sequences with Constant Magnitude for Fast Channel Estimation Initialization”, IEEE Trans. Comm., vol. 46, no3, p. 305-308, March 1998, and Simon Haykin's “Adaptive Filter Theory, 3
rd
Edition”, Prentice-Hall, Inc. 1996, p 498. However, many methods for channel estimation require high computational complexity or a long preamble, or they don't offer a convenient means of also obtaining an estimate of the baud frequency offset.
Therefore, a need exists for an efficient method and apparatus for determining both the channel estimate and the baud frequency offset estimate. The present invention provides such method and apparatus, wherein a type of preamble sequence and a computational structure obtains both a channel estimate and a baud frequency offset estimate using a minimal number of preamble symbols and having a very low computational complexity.
SUMMARY OF THE INVENTION
In accordance with the present invention, an efficient method and apparatus for obtaining a channel estimate and a baud frequency offset estimate for a communications channel in a communications system is provided. The communications system has a transmitter for transmitting to a receiver over the communications channel signals representing data appended to a preamble signal. The preamble signal is provided as a periodic plurality of preamble sequences, each preamble sequence being generated in accordance with:
1
32
∑
15
k
=
0
b
k
b
mod
(
k
+
n
,
16
)
*
=
{
1
,
n
=
0
0
,
n
≠
0
.
A transmission signal representing the periodic plurality of preamble sequences prepended to the data is sent by a transmitter over the communications channel to a receiver and is received at the receiver as a received signal. The received signal is processed to determine from the received signal both:
a) a channel estimate in accordance with:
h
^
=
1
64
B
H
(
y
1
+
y
2
)
wherein B represents a matrix of preamble symbol values, upsampled by four and zero-filled, and y
1
and y
2
are column vectors of received samples; and
b) a baud frequency offset estimate &Dgr;f
b
in accordance with:
S
=
∑
63
k
=
0
y
k
y
k
+
64
*
Δ
f
b
≅
real
(
S
)
·
f
b
imag
(
S
)
·
f
c
·
32
π
wherein f
b
is the baud frequency and f
c
is the center frequency of the transmission signal.
REFERENCES:
patent: 5255291 (1993-10-01), Holden et al.
patent: 5343498 (1994-08-01), Toy et al.
patent: 5432821 (1995-07-01), Polydoros et al.
patent: 5450456 (1995-09-01), Mueller
patent: 5790598 (1998-08-01), Moreland et al.
patent: 6084862 (2000-07-01), Bjork et al.
patent: 6130894 (2000-10-01), Ojard et al.
patent: 6331976 (2001-12-01), Sriram
patent: 6418175 (2002-07-01), Pukkila et al.
patent: WO 98/10531 (1996-09-01), None
Sandell, “Iterative channel estimation using soft decision feedback”, Sep. 1998, 0-7803-4984-9/98 IEEE.*
John C. L. Ng, et al., “Complex Optimal Sequences with Constant Magnitude for Fast Channel Estimation Initialization,”IEEE Trans Comm, Mar. 1998, pp. 305-308, vol. 46, No. 3.
Simon Haykin, “Chapter 11, Method of Least Squares,”Adaptive Filter Theory, 3rdEdition,1996, pp. 483-501, Prentice-Hall, Inc.
Alfred Baier, “Correlative and Iterative Channel Estimation in Adaptive Viterbi Equalizers for TDMA Mobile Radio Systems,”ITG Fachberichte, VDE Verlag,Apr. 12, 1989, pp. 363-368, No. 107, Berlin, Germany. (XP-002077505).
Magnus Sandell, et al., “Iterative Channel Estimation Using Soft Decision Feedback,”IEEE Global Telecommunications Conference,Nov. 8, 1998, pp. 3728-3733, New York, New York. (XP-000805320).
Khalid A. Hamied, “A New Channel Estimator for Fast Start-Up Equalization,”IEEE Transactions on Communications,Feb. 1, 1991, pp. 177-181, vol. 39, No. 2, New York, New York. (XP-000225299).
Corry Alan
Ojard Eric
Broadcom Corporation
Chow C.
Christie Parker & Hale LLP
Urban Edward F.
LandOfFree
Method and apparatus for efficient determination of channel... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for efficient determination of channel..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for efficient determination of channel... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3037160