Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2000-11-17
2004-09-21
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S262000
Reexamination Certificate
active
06795512
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to the carrier phase tracking of coded symbols, and, more specifically, to the use of reliability metrics of symbol estimates in the tracking process.
2. Related Art
During the decoding of coded symbols, particularly higher order symbols such as 8-PSK or QAM, there is a risk of cycle slippage of the symbol constellation. Cycle slippage is devastating to receiver performance. When it occurs, the symbol constellation rotates with respect to its true orientation such that another ostensibly valid constellation is interpreted at the receiver. This renders as useless all data decoded afterward. The problem is compounded with low operating E
b
/N
o
values, or with high phase noise in the demodulator oscillator or downconversion circuits. For example, at low E
b
/N
o
values, 8-PSK symbol error rates of 20-30% have been experienced. At such high error rates, decision-oriented phase tracking loops can experience great difficulty in following phase trends induced by instabilities of the demodulator oscillator and downconversion circuitry.
SUMMARY
The invention provides a system for carrier phase tracking of coded symbols in which reliability metrics for the symbol estimates are used. Embodiments with a feed forward or feedback structure are possible.
In an embodiment with the feedforward structure, a received symbol is input to a tracking loop which includes a series combination of a symbol and reliability estimation engine, a tracking loop module, and a symbol derotator. The engine estimates the symbol, and also determines a reliability metric for the estimate.
The symbol estimate and reliability metric are input to the tracking loop module which determines a residual between the received symbol and its estimate. The residual may be a phase residual. The reliability metric for the estimate is used to weight the residual for the symbol. A derotation phase &thgr; is then determined responsive to the weighted residual. Optionally, one or more previous weighted residuals and one or more previous derotation phases are used in this process. A symbol derotator then derotates the symbol by the derotation phase &thgr;. A delay element ensures proper synchronization of this process. The resulting phase adjusted symbol is then output by the derotator. The process may then repeat itself for subsequent symbols.
In one embodiment of the feedforward structure, the operation of this tracking loop may generally be described by the following expression:
θ
k
=
∑
i
=
1
N
a
i
·
θ
k
-
i
+
∑
i
=
0
M
-
1
b
i
·
R
k
-
i
·
z
k
-
i
In this expression, &thgr;
k
is the derotation phase at time k, &thgr;
k−i
represents a previous value of the derotation phase at time k−i, a
i
is a coefficient applied to &thgr;
k−i
, z
k−i
is a residual derived from r
k−i
, the (k−i)th received symbol and s
k−i
, the estimate of that symbol, R
k−i
is the reliability metric for the estimate of the (k−i)th symbol, b
i
is a coefficient applied to R
k−i
·z
k−i
, and N and M are non-negative integers. In one embodiment, the residual z
k−i
is e
k−i
, the phase residual between r
k−i
and s
k−i
. In another embodiment, the residual z
k−i
is the component of r
k−i
orthogonal to s
k−i
.
In an embodiment with the feedback structure, a received symbol is input to a tracking loop which includes a series combination of a symbol derotator, a symbol and reliability estimation engine, and a tracking loop module. The received symbol is pre-rotated by the derotator using the best estimate of the derotation phase which is available at the time. The pre-rotated symbol is then input to the symbol and reliability estimation engine, which estimates the symbol, and also provides a reliability metric for the estimate. In the tracking loop module, a residual between the pre-rotated symbol and the estimate of that symbol is formed. The residual may be a phase residual. The residual is weighted by the reliability metric for the corresponding symbol estimate. A phase offset estimate is then determined responsive to the weighted residual. Optionally, one or more previous values of the weighted residuals, and one or more previous values of the phase offset estimate are used in this process. The phase offset estimate is then added to the value of the derotation phase (the one used to pre-rotate the symbol in the first place) to determine the next value of the derotation phase. Meanwhile, the pre-rotated symbol is output by the derotator. The process may then repeat itself for subsequent symbols.
In one embodiment of the feedback structure, the operation of the tracking loop may generally be described by the following two step process:
Δ
θ
k
+
1
=
∑
i
=
0
N
-
1
a
i
·
Δ
θ
k
-
i
+
∑
i
=
0
M
-
1
b
i
·
R
k
-
i
·
z
k
-
i
θ
k
+
1
=
θ
k
+
Δ
θ
k
+
1
In this expression, &thgr;
k+1
is the derotation phase at time k+1, &thgr;
k+1
is the phase offset estimate at time k+1, &thgr;
k−i
represents a previous value of the phase offset estimate at time k−i, a
1
is a coefficient applied to &thgr;
k−i
, z
k−i
is a residual derived from r
k−i
, the (k−i)th received symbol and s
k−i
, the estimate of that symbol, R
k−i
is the reliability metric for the estimate of the (k−i)th symbol, b
i
is a coefficient applied to R
k−i
·z
k−i
, and N and M are non-negative integers. Note that the coefficients a
1
and b
i
between this embodiment and the feedfoward embodiment.
In one embodiment, the residual z
k−i
is x
k−i
, the phase residual between t
k−i
, the pre-rotated value of the received symbol r
k−i
, and the estimate of that symbol, s
k−i
. In another embodiment, the residual z
k−i
is the component of t
k−i
orthogonal to s
k−i
.
In one example of the embodiment employing the feedback structure, the tracking loop employs a first order loop to determine the derotation phase &thgr;. This first order loop can be expressed as follows:
&Dgr;&thgr;
k+1
=B·R
k
·x
k
&thgr;
k+1
=&thgr;
k
+&Dgr;&thgr;
k+1
where &thgr;
k+1
, &Dgr;&thgr;
k+1,
&thgr;
k
, R
k
, and x
k
are as defined previously, and B is a parameter which is proportional to the nominal loop bandwidth. The parameter B is typically less than 1 and a constant, and is configured to take account of noise. Typically, the more noise in the system, the smaller B is, and vice-versa. Note that the effective loop bandwidth in this example, B·R
k
, can vary from sample to sample, depending on the reliability R
k
of the symbol estimate from which x
k
is computed.
In a second example of the embodiment employing the feedback structure, the tracking loop module employs a higher order loop to determine the derotation phase &thgr;. This higher order loop may be expressed as follows:
Δ
θ
k
+
1
=
B
·
∑
i
=
0
M
-
1
R
k
-
i
·
x
k
-
i
θ
k
+
1
=
θ
k
+
Δ
θ
k
+
1
where the parameters &thgr;
k+1
, &Dgr;&thgr;
k+, &thgr;
k
, B, R
k−i
, x
k−i
and M are as defined previously.
According to a third example of the embodiment employing the feedback structure, a technique is used in which a sinusoid of a function of the incremental phase residual at time k is used to update the derotation phase. In this example, the operation of the tracking loop may be represented by the following two step process:
&Dgr;&thgr;
k+1
=B·R
k
·sin(
C·x
k
)
&thgr;
k+
&thgr;
k
+&Dgr;&thgr;
k+1
where the parameters &thgr;
k+
, &Dgr;&thgr;
k+1,
&thgr;
k
, B, R
k
, and x
k,
are as defined previously and C is a constant such that, when x
k
o
Berggren Magnus H.
Eidson Donald Brian
Krieger Abraham
Murali Ramaswamy
Chin Stephen
Conexant Systems Inc.
Howrey Simon Arnold & White , LLP
Wang Ted
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