Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2001-02-01
2002-11-05
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C370S210000
Reexamination Certificate
active
06477210
ABSTRACT:
FIELD OF INVENTION
The invention relates generally to communications and particularly to a method and apparatus for near optimal joint channel estimation and data detection to improve channel tracking and, thus, improve link robustness.
BACKGROUND OF THE INVENTION
The rapid growth in the use of the Internet and the increasing interest in portable computing devices have triggered the desire for high-speed wireless data services. One of the more promising candidates for achieving high data rate transmission in a mobile environment is Orthogonal Frequency Division Multiplexing (OFDM), which divides the wide signal bandwidth into many narrow-band subchannels, which are transmitted in parallel. Each subchannel is typically chosen narrow enough to eliminate the effects of delay spread. Coded OFDM (COFDM) systems, which combine both OFDM and channel coding techniques, are able to improve the performance further by taking advantage of frequency diversity of the channel.
Though both differential and coherent demodulation can be applied in a COFDM system, the latter leads to a performance gain of 3 to 4-dB in signal-to-noise ratio (SNR) with accurate channel estimation. Channel estimation techniques realized by a frequency-domain filter using Fast Fourier Transform (FFT), followed by time-domain filters for a COFDM system with Reed-Solomon (RS) coding have been proposed. These channel estimation techniques, while good, did not provide the near optimal channel estimation required for data-decoding with improved channel tracking capability for reliable link performance even under high user mobility and/or high RF carrier frequency.
SUMMARY OF THE INVENTION
The physical layer configuration is shown in FIG.
1
A. At the transmitter, the encoded data stream is sent to an OFDM transmission branch. The data stream may, for example, be convolutionally encoded. The encoded data stream may then be optionally interleaved. If the encoded data stream is interleaved in the transmitter then the receiver must correspondingly deinterleave the data stream. After interleaving, the transmitter modulates the encoded data stream. By way of example, QPSK modulation is used. The signal is then subjected to inverse Fast Fourier Transformation and transmitted, in the present invention over the air.
Correspondingly, a receiver accepts multicarrier transmitted signals and subjects these received signals to Fast Fourier Transformation. The transformed signals are concurrently fed into a channel estimator and demodulators. The demodulated signals are combined in a maximum ratio combiner, optionally deinterleaved and decoded.
An OFDM signal is divided into a number of subchannels. By way of example, an OFDM signal bandwidth is divided into 120 6.25-kHz subchannels with QPSK modulation on each subchannel. At the receiver, the demodulated signals from two receiving branches are combined using maximal ratio combining and then decoded. With a symbol period of 200 &mgr;s (including a 40-&mgr;s guard interval) and ½-rate coding, a maximum information rate of 600 kbps can be achieved in a 750-kHz bandwidth (about 800 kHz including guard bands). The information rate is calculated by dividing the 120 subchannels (tones) by the 200 &mgr;s period to obtain 600 kbps.
For purposes of example for the present invention, ½-rate convolutional codes (CC) are considered. The results with ½-rate Reed-Solomon (RS) code based on Galois-Field (64) (GF(64)) are compared. The size of a code word is the same as that of an OFDM block (an OFDM symbol of 200 &mgr;s and 120 subchannels). To achieve coding gain with inherent frequency diversity in OFDM, a simple interleaving scheme is applied. For both RS and CC cases, the first 120 bits of a code word are assigned to the in-phase component and the rest to the quadrature component. To gain additional randomness within a code word for the CC case, each 120-bit group is interleaved over subchannels by an 11-by-11 block interleaver (without the last bit).
In the simulations, the wireless channel, as a Rayleigh-fading channel, with a two-ray multipath delay profile is modeled. Good performance for impulse separation as high as 40-&mgr;s can be achieved; a 5-&mgr;s impulse separation in the numerical results is considered.
For the performance with respect to channel variations, maximum Doppler frequency up to 200 Hz, which is reasonable for most vehicular speeds, for a possible RF carrier frequency around 2 GHz is considered. To demonstrate the advantage of the proposed joint detection methods, results at a maximum Doppler frequency as high as 500 Hz corresponding to a scenario in which the wireless system uses a higher carrier frequency, e.g. 5 GHz are presented.
In the medium access control (MAC) layer, a frequency reuse is considered with dynamic resource management, e.g., Dynamic Packet Assignment (DPA), to achieve high spectral efficiency for packet data access.
A simple analysis to highlight the ideal or optimal joint channel estimation and maximum likelihood (ML) decoding scheme indicated in
FIG. 1A
for the case of M=2 receiving antennas is now presented.
At a diversity receiver, the signal from the m th antenna at the k th subchannel and the n th block can be expressed as
x
m,n,k
=h
m,n,k
a
n,k
+w
m,n,k
, (1)
where a
n,k
, h
m,n,k
and w
m,n,k
are the transmit signal, channel response and additive Gaussian noise, respectively.
For convolutional codes, because the size of a code word is the same as that of the OFDM block, (1) can be rewritten as
x
m,n
=H
m,n
c
n
+w
m,n
, (2)
where, if there are K
f
subchannels,
H
m,n
=diag
(
h
m,n,1
, h
m,n,2
, . . . , h
m,n,K
f
),
c
n
is the transmitted code word at time epoch n, and the rest of the vectors are similarly defined.
Assume that the number of code words is N, we introduce the following notations,
c=[c
1
T
, c
2
T
, . . . , c
N
T
]
T
,
H
m
=diag
(
H
m,1
, H
m,2
, . . . , H
m,N
),
x
m
=[x
m,1
T
, x
m,2
T
, . . . , x
m,N
T
]
T
. (3)
At the receiver, the objective is to solve a maximum likelihood (ML) problem
c
^
=
arg
min
c
[
min
H
m
∑
m
&LeftDoubleBracketingBar;
x
M
-
H
m
c
&RightDoubleBracketingBar;
2
]
,
(
4
)
with a constraint on channel response
L
(
H
m
)=0, (5)
where L() is a constraint function. In a wireless environment, this constraint can be simplified to be
∑
l
=
-
K
m
K
m
B
n
,
l
d
(
H
m
,
n
-
l
)
=
0
,
(
6
)
where the length of the channel memory is K
m
OFDM symbol durations, B
n,l
are coefficients determined by the correlation between channel responses at the time epochs n and n−1, which is a function of the Doppler spectrum of the channel, and d() is a vector function defined by
d
(
H
m,n
)=[
h
m,n,1
, h
m,n,2
, . . . , h
m,n,K
f
]
T
.
The optimal solution of this ML problem can be obtained by exhaustive search. It requires solving the mean square error (MSE)
MSE
(
c
)
=
min
H
m
∑
m
&LeftDoubleBracketingBar;
x
m
-
H
m
c
&RightDoubleBracketingBar;
2
,
(
7
)
for any possible c with the channel constraint (6). Then,
c
^
=
arg
min
c
MSE
(
c
)
.
(
8
)
After obtaining MSE(c), the corresponding channel estimate H
m
(c) can be found. Consequently, the optimal approach for estimating channel response requires the knowledge of the entire set of x and c.
Another observation from this ML receiver is that the channel estimation results H
m
is not a direct output of the detection process and hence, channel estimation which calculates H
m
explicitly may not be necessary in theory. However, for other required parameter estimation, such as timing and frequency synchronization, a known data sequence is usually transmitted in the beginning of a group of OFDM blocks. This known data sequence, also called a synch word or a unique word, can be used as a training sequence in (7) to obtain initial channel estimate explicitly without resorting to blind detectio
Chuang Justin C.
Li Ye
Lin Lang
AT&T Corp.
Banner & Witcoff , Ltd.
Emmanuel Bayard
Pham Chi
LandOfFree
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