Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1999-11-12
2003-08-05
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S341000, C375S130000
Reexamination Certificate
active
06603823
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to channel estimators in wireless communication systems generally and to such channel estimators that operate on data and pilot signals in particular.
BACKGROUND OF THE INVENTION
In practical digital communication systems, the frequency response of the underlying channel from the transmitter to the receiver is seldom known at the receiver side. For example, in digital communication over the dial-up telephone network, the communication channel will be different every time a number is dialed, because the channel route will be different. In this example, the characteristics of the channel are unknown a priori. There are other types of channels, e.g. wireless channel such as radio channels and underwater acoustic channels, whose frequency response characteristics are time varying. Thus coherent communications for such channels require the utilization of adaptive algorithms, known as “channel estimators”, for tracking/estimating the varying characteristics of the channel.
Traditionally, channel estimators are divided into two categories: data aided and non-data aided (blind) estimators. Data aided channel estimators operate on a pre-specified set of transmitted symbols that are known to the receiver. These symbols do not convey any information and are often called “pilot symbols” or “training sequences”. Data aided channel estimators are typically simple to implement and relatively robust. Their major disadvantage is that they lead to an overall reduction in system throughput, since some of the transmitted symbols (the pilot symbols) do not carry any information.
Non data aided channel estimators, on the other hand, do not reduce the system throughput. However, they are typically quite complicated to implement as they are often based on higher order moments/cumulants of the received signal, and they most often suffer from high statistical variability, i.e. they suffer from large estimation errors.
The article “Maximum A Posteriori Multipath Fading Channel Estimation for CDMA Systems” by Mohamed Siala and Daniel Duponteil,
Proceedings of Vehicular Technology Conference
, Houston, Tex., May, 1999, describes a channel estimation algorithm which combines both approaches. This algorithm uses both pilot and data symbols to construct a channel estimator. However, this algorithm requires that the joint statistical probability distribution of the channel multipaths be known to the receiver. In practice, a complete statistical description of the channel characteristics is seldom known to the receiver. Moreover, these characteristics may be time varying.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved channel estimator without using any a priori statistical information about the channel. Instead, the present invention uses a priori probabilities of the received symbols, be they pilot, data, power control, etc. symbols.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a channel estimator based on the values of received data and on a priori probabilities only of received symbols. The channel estimator includes a symbol probability generator, a noise variance estimator and a channel tap estimator. The symbol probability generator generates a priori probabilities only of transmitted symbols found in the received signal(s). The noise variance estimator estimates at least one noise variance corrupting the received signal(s). The channel tap estimator generates channel estimates from the received signal(s), the a priori probabilities and the noise variance(s).
Additionally, in accordance with a preferred embodiment of the present invention, the channel tap estimator solves the following equation:
h
^
ML
=
1
2
T
·
∑
t
=
1
T
y
_
(
t
)
·
z
(
t
;
h
^
ML
)
*
Moreover, in accordance with a preferred embodiment of the present invention, the channel tap estimator includes a z-unit, a combiner and a channel tap unit. The z-unit generates a z-value for z(t;ĥ
ML
) from the a priori probabilities, the noise variance(s) and the channel estimates. The combiner combines the z-value with the received signal(s). The channel tap unit determines channel tap values from the output of the combiner.
Further, in accordance with a preferred embodiment of the present invention, the z-unit calculates the following equation for a quadrature phase shift keying (QPSK) channel:
z
(
t
;
h
_
)
≡
[
p
1
(
t
)
(
1
+
j
)
ⅇ
2
σ
2
Real
{
<
y
_
(
t
)
,
h
_
*
>
(
1
-
j
)
}
+
p
2
(
t
)
(
1
-
j
)
ⅇ
2
σ
2
Real
{
<
y
_
(
t
)
,
h
_
*
>
(
1
-
j
)
}
+
p
3
(
t
)
(
-
1
+
j
)
ⅇ
2
σ
2
Real
{
<
y
_
(
t
)
,
h
_
*
>
(
-
1
-
j
)
}
+
p
4
(
t
)
(
-
1
-
j
)
ⅇ
2
σ
2
Real
{
<
y
_
(
t
)
,
h
_
*
>
(
-
1
-
j
)
}
]
×
[
p
1
(
t
)
ⅇ
2
σ
2
Real
{
<
y
_
(
t
)
,
h
_
*
>
(
-
1
-
j
)
}
+
p
2
(
t
)
ⅇ
2
σ
2
Real
{
<
y
_
(
t
)
,
h
_
*
>
(
-
1
+
j
)
}
+
p
3
(
t
)
ⅇ
2
σ
2
Real
{
<
y
_
(
t
)
,
h
_
*
>
(
-
1
-
j
)
}
+
p
4
(
t
)
ⅇ
2
σ
2
Real
{
<
y
_
(
t
)
,
h
_
*
>
(
-
1
+
j
)
}
]
-
1
where {overscore (y)}(t) is a vector of the received signal(s), {overscore (h)} is a vector of the channel tap estimates and {overscore (p)}(t) is a vector of the symbol probabilities.
Still further, in accordance with a preferred embodiment of the present invention, the channel tap unit is a summer over a window of received symbols.
Additionally, in accordance with a preferred embodiment of the present invention, when s(t) is a pilot symbol, then {overscore (p)}(t)={overscore (e)}
i
where the {overscore (e)}
i
's are the normal basis vectors having a one (1) in their i-th component and zero otherwise. When s(t) is a data symbol, then
p
1
(
t
)=
p
2
(
t
)=
p
3
(
t
)=
p
4
(
t
)=0.25
When s(t) is a transmit power control (TPC) symbols, then
p
1
(
t
)=
p
4
(
t
)=0.5
, p
2
(
t
)=
p
3
(
t
)=0
Moreover, in accordance with a preferred embodiment of the present invention, the channel tap unit includes an anchor unit, an averager and an interpolator. The anchor unit determines a pilot anchor ĥ
p
using N
p
pilot symbols of one time slot n and a data anchor ĥ
s
using N
s
data symbols of the time slot n. The averager averages the pilot and data anchors to produce a slot anchor ĥ
anchor
(n) and the interpolator interpolates between adjacent anchors ĥ
anchor
(n−1) and ĥ
anchor
(n) to obtain channel estimates for the n-th slot.
Further, in accordance with a preferred embodiment of the present invention, N
s
=2N
p
and N
p
of the data symbols can be taken from before the pilot symbols of the time slot and N
p
of the data symbols can be taken from after the pilot symbols.
Still further, in accordance with a preferred embodiment of the present invention, the unit for linearly interpolates includes unit for separately interpolates amplitudes and phases of the adjacent anchors.
Additionally, in accordance with a preferred embodiment of the present invention, the channel estimator also includes a unit that averages slot anchors as follows:
H
^
anchor
(
n
-
1
)
=
∑
k
=
-
M
M
h
^
anchor
(
n
-
1
-
k
)
·
β
k
∑
k
=
-
M
M
β
k
where H
anchor
is the average value of slot anchors h
anchor
in a slot ranging between the values −M to M for k, and &bgr;
k
is a user defined weight factor.
Moreover, in accordance with a preferred embodiment of the present invention, the z-unit includes a lookup table unit.
Further, in accordance with a preferred embodiment of the present invention, the at least one received si
Ashkenazi Rony
Rainish Doron
Yellin Daniel
Chin Stephen
Eitan, Pearl, Latzer & Cohen-Zedek, LLP.
Yeh Edith
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