Demodulator

Details Demodulator Demodulator

1999-08-19

2002-08-27

Pham, Chi (Department: 2631)

Pulse or digital communications

Receivers

Particular pulse demodulator or detector

C375S341000, C375S342000, C375S346000, C375S348000, C375S324000, C375S285000, C375S278000, C375S284000, C370S500000, C370S522000, C370S525000, C329S349000, C329S353000

Reexamination Certificate

active

06442218

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to demodulators which receive a signal transmitted via phase modulation or multi-value QAM modulation, and derive transmitted data from the received signal. The present invention particularly relates to a demodulator used in a receiver of a digital-data-transfer system which transfers data, digital audio signals, digital image signals, or the like.
In the transfer of data, digital audio signals, digital image signals, or the like, symbol data representing binary values or multi-values as digital data is transmitted as carrier-wave modulated signals. The symbol data arrives at a receiver via a transmission line, and demodulator of the receiver decides the symbol data as digital data. The decided data is output from the receiver.
The transmitted signals suffer phase fluctuations as well as magnitude fluctuations because of interference from other communication and/or interference between paths along the transmission line on which a plurality of paths are established. The transmitted signals thus can be distorted before arriving at the receiver. Further, the received signals can further be distorted by noise inside the receiver.
If the distortion of the received signals generated is large, the demodulator cannot ascertain the transmitted symbol data correctly, and ends up outputting incorrect digital data. In order to insure high-quality communication of digital information, the demodulator needs to achieve higher reliability in deciding the symbol data, and needs to decrease data-error rates.
Transmission of digital data is performed by transmitting a signal with a carrier wave thereof modulated in terms of its amplitude or phase in accordance with the digital data to be transmitted. When the transmitted signal is received, however, the actually received signal has distortion introduced along the transmission line.
In consideration of this, a demodulator compensates for the distortion based on the estimation of characteristics of the transmission line, and, then, decides the digital data from the amplitude or phase of the received data. The decided data is supplied as a demodulator output. A well-known method of estimating characteristics of the transmission line includes transmitting predetermined pilot symbols and estimating the characteristics of the transmission line based on the received pilot symbols. This is disclosed in Seiichi Sampei, “Rayleigh Fading Compensation Method for 16 QAM Modem in Digital Land Mobile Radio Systems”, Transaction of the Institute of Electronics, Information and Communication Engineers, Vol. J72-B-II, No. 1, pp.7-15 (1988), for example.
The above-described transmission-line-estimation method based on use of pilot symbols transmits at least one of N symbols as a pilot symbol when transmitting the N symbols in one slot. In particular, one pilot symbol is inserted and transmitted as (N−1) data symbols. The receiver then uses the pilot symbol to estimate the characteristics of the transmission line.
A complex envelope of a transmitted pilot symbol is denoted as (3+j·3), and a complex envelope characteristic (i.e., phasing distortion introduced during passage through the transmission line) is denoted as c(k). Further, a received complex base-band signal of the pilot symbol in the receiver is u(k), and a Gaussian white noise is represented as n(k). Then, the following relation is satisfied.
u
(
k
)=
c
(
k
)·(3
+j·
3)+
n
(
k
)  (1)
On the receiver side, an operation as shown in the following (2) is performed on the received complex base-band signal u(k) by using the transmitted pilot symbol 3+j·3, so as to obtain an estimate of the transmission line c{circumflex over ( )}(k).
c
^
⁡
(
k
)
=
u
⁡
(
k
)
/
(
3
+
j
·
3
)
=
c
⁡
(
k
)
+
n
⁡
(
k
)
/
(
3
+
j
·
3
)
(
2
)
When the data symbol transmitted from the transmitter is s(k) and the data symbol received at the receiver is r(k), then, the received data symbol r(k) under the effect of the transmission-line characteristic c(k) is represented by the following.
r
(
k
)=
c
(
k
)·
s
(
k
)+
n
(
k
)  (3)
An operation as shown in the following (4) is performed on the received data symbol r(k) by using the transmission-line estimate c{circumflex over ( )}(k), so as to obtain the received data symbol r{circumflex over ( )}(k) having the effect of the transmission line removed therefrom.
r
(
k
)=
r
(
k
)/
c{circumflex over ( )}
(
k
)  (4)
Here, r{circumflex over ( )}(k) is equal to the transmitted data symbol s(k) if the Gaussian white noise n(k) is zero. In this case, the receiver side can reconstruct the transmitted data symbol s(k) without any effect of the transmission line.
The transmission-line estimate c{circumflex over ( )}(k) becomes identical to the transmission-line characteristic c(k) if there is no Gaussian white noise n(k). Because of the effect of the Gaussian white noise n(k), however, there is a difference between the transmission-line estimate c{circumflex over ( )}(k) and the transmission-line characteristic c(k). In consideration of this, the transmission-line estimate is obtained many times by using a plurality of pilot symbols, and an average is calculated to derive a transmission-line estimate that is highly accurate. Based on the highly accurate transmission-line estimate c{circumflex over ( )}(k), which is obtained by averaging transmission-line estimates corresponding to the plurality of pilot symbols, a received data symbol is decided by removing the phasing deformation of the received data symbol through the operation as shown in the above equation.
As described in the above, a plurality of pilot symbols are required for the averaging operation in order to raise accuracy of the transmission-line estimate c{circumflex over ( )}(k). Namely, p pilot symbols are inserted each time (N−p) data symbols are transmitted in one slot, and p transmission-line estimates are then averaged. An area where the p pilot symbols occupy is referred to as a pilot block.
An increase in the number p of pilot symbols relative to the number N of transmitted symbols, however, results in a decrease in the number of symbols transmitted as digital data, thereby ending up decreasing a data-transfer rate. Because of this, the number p of the pilot symbols cannot be unconditionally increased.
To cope with this situation, a CDMA demodulating device disclosed in Japanese Patent laid-open Application No. 10-51424 employs a scheme in which a weighted average is calculated with respect to estimates of a channel (transmission-line characteristic) after these estimates are obtained from pilot symbols contained in pilot blocks of a plurality of slots. Another scheme is to obtain an average between a channel estimate obtained from pilot blocks of a plurality of slots and an average of information symbols having polarization thereof corrected.
The method of obtaining a weighted average of transmission-line estimates with respect to pilot blocks of a plurality of slots is accompanied by a processing delay until transmitted symbols of the plurality of slots are received. This is because the estimates of the transmission line are obtained after the pilot blocks of the plurality of slots are received.
There is a need for an accurate estimation of the transmission line without increasing the number p of pilot symbols in one slot and without incurring a processing delay over the plurality of slots. To this end, data symbols placed between pilot symbols may be decided tentatively as digital data, and the transmission-line characteristics may be estimated from the decided data symbols by treating the decided data symbols as pilot symbols. This scheme brings about the same effect as increasing the number of pilot symbols, thereby enhancing an accuracy regarding the estimation of transmission-line characteristics.
In comparison to the transmission-line estimate obtained from known pilot symbols inserted on the transmitter side, when a transmission-line estimate is der

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