Pulse or digital communications – Transceivers
Reexamination Certificate
1998-09-11
2003-12-02
Corrielus, Jean B. (Department: 2631)
Pulse or digital communications
Transceivers
C375S316000, C370S491000, C370S500000
Reexamination Certificate
active
06658050
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention is directed toward improving output of cellular communication units, and more particularly toward improving the quality of channel estimates used to convert received signals to usable output in cellular communication units.
2. Background Art
Signals for wireless systems are subjected to varying conditions which can degrade the signal received by the mobile units or mobile stations (“MS”) using the system. Such conditions can also degrade the signal received by base transceiver stations (“BTS”) from mobile stations.
For example, MS and BTS can receive signals from multiple directions (e.g., a specific signal can be received by an MS directly from a BTS, and reflected off of many different ground objects), with the varying signal sources potentially being out of phase and thereby tending to cancel each other out to some degree, reducing signal strength. Such signal fading, generally known as Rayleigh fading, occurs spatially over the area of the system, with specific areas potentially having significant fading which could cause the mobile unit to lose the signal entirely.
The net result of such factors is that the signal which is transmitted by the transmitter (e.g., a cell tower) will be distorted by the time it reaches the receiver (e.g., a cellular telephone). In a cellular telephone call, for example, this can result in distortion objectionable to the ear, or even a lost signal.
In order to account for this distortion, channel estimates have been used to determine the signal distortion at known pilot symbols in the data bursts and channel coefficients (correction factors used to derive channel estimates) at other symbols in the data bursts have been interpolated based on the channel estimates at the pilot symbols. As an example, data bursts have been transmitted in the IS-136 System with 162 symbols, each symbol comprising two bits. In a proposed extension of the IS-136 System, the data bursts of 162 symbols at predetermined, known locations P
i
in the data bursts are predetermined, known pilot symbols S
Pi
(where i=1 to n, n being the number of pilot symbols used). In the proposed extension of the IS-136 System, each symbol contains three bits.
The channel coefficients determined from the pilot symbols have been used to estimate the most likely value for each data symbol in a data burst. That is, the channel coefficients determined from the pilot symbols have been interpolated to determine the channel coefficients at the other symbols (i.e., data symbols) in the data burst by using an interpolator or filter suited to work under the conditions most likely to be encountered by the communication unit.
A more detailed discussion of the prior art use of channel estimates is included below in context with the Description of the Preferred Embodiment. While such prior art channel estimates have enabled the received signal to be more accurately demodulated to provide improved output, such channel estimates are still susceptible to error and resulting degradation of the output.
The present invention is directed toward overcoming one or more of the problems set forth above.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a transceiver adapted to communicate via signals with another transceiver is provided. The one transceiver includes a receiver adapted to receive from the other transceiver a signal having multiple symbols therein including predetermined pilot symbols, and a transmitter adapted to transmit signals having multiple symbols therein including power control bits, the power control bits instructing the other transceiver to establish selected power levels for its signal to the receiver. The transceiver also includes a processor adapted to interpolate channel coefficients for received symbols as a function of both (1) the selected power levels of the other transceiver signal and (2) the difference between the predetermined pilot symbols and the pilot symbols as received by the receiver. A demodulator demodulates all received symbols based on the interpolated channel coefficients.
In one preferred form, pilot symbol channel coefficients C
Pi
are determined by the processor so as to minimize the following summation for selected pilot symbols:
&Sgr;
E{|R
Pi
−C
Pi
*S
Pi
*{square root over (W)}
Pi
|
2
}
where: E is the expectation value for the power level at pilot position P
i
,
W
Pi
is the known power level at pilot position P
i
,
R
Pi
is the pilot symbol as received at pilot position P
i
, and
S
Pi
is the pilot symbol known to have been sent at pilot position P
i
.
With this form, the processor includes an interpolation filter which interpolates channel coefficients for other than the pilot symbol channel coefficients based on the determined pilot symbol channel coefficients C
Pi
.
In a second preferred form, the processor is adapted to calculate an interpolation filter as a function of the selected power levels, and channel coefficients for received symbols other than the pilot symbols are interpolated using the calculated interpolation filter.
In this second preferred form, when the transceiver is subjected to conditions in which the power levels can be set accurately and the power control bits will be received correctly by the other transceiver, the processor uses the following auto-correlation function R
c
, in calculating the interpolation filter:
R
c′|W
i
W
k
(
i,k
)=
{square root over (W)}
i
{square root over (W)}
k
J
O
(2
&pgr;f
d
(
i−k
)
T
s
)
where: J
O
is the Bessel function of the first kind;
f
D
is the Doppler spread of the channel;
T
s
is the symbol duration, and
W
i
and W
k
are known power levels at positions i and k in the received signal.
Alternatively, in this second preferred form, when the transceiver is subjected to conditions in which the power control bits are always received correctly and changes in the power level are independent and identically distributed, the-processor uses the following auto-correlation function R
c
, in calculating the interpolation filter:
R
c
′
(
i
,
k
)
|
y
1
,
y
2
,
…
,
y
L
=
E
[
W
i
]
E
[
s
1
]
y
1
,
y
2
,
…
,
y
L
J
o
(
2
π
f
D
(
i
-
k
)
T
s
)
=
AE
[
s
1
]
y
1
,
y
2
,
…
,
y
L
J
o
(
2
π
f
D
(
i
-
k
)
T
s
)
where: J
O
is the Bessel function of the first kind;
f
D
is the Doppler spread of the channel;
T
s
is the symbol duration;
i and k are positions in the received signal;
P
i
is the known power level at position i in the received signal;
s
1
, is the incremental change in signal power resulting from a non-zero power control bit;
y
i
is either 1 or −1, where at position k there have been L incremental power steps;
E is expectation;
W
i
is the known power level at positions i of the received signal; and
A is the average power level of the received signal.
In another alternative of the second preferred, form, when the transceiver is subjected to conditions in which the transmitted power control bits are received by the other transceiver with very poor signal to noise ratio (SNR) and incremental changes in the power level of the received signal are accurate, the processor uses the following auto-correlation function R
c′
in calculating the interpolation filter:
R
c
′
(
i
,
k
)
=
AJ
0
(
2
π
f
D
(
i
-
k
)
T
s
)
(
s
+
s
-
1
2
)
L
where: J
O
is the Bessel function of the first kind;
f
D
is the Doppler spread of the channel;
T
s
is the symbol duration;
i and k are positions in the received signal;
s is the incremental change &Dgr;W
i
in signal power resulting from a non-zero power control bit;
L is the number of incremental p
Ramesh Rajaram
Wang Yi-Pin Eric
Coats & Bennett P.L.L.C.
Corrielus Jean B.
Ericsson Inc.
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