Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
1999-08-19
2003-05-27
Decady, Albert (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
Reexamination Certificate
active
06571369
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an encoding apparatus for improving transmission characteristics in a fading channel of a communication system.
Various research and developments have occurred in order to realize multimedia communication for practical use. In particular, developments have been made in mobile terminal communication systems, which have become very popular for transmitting information including data and images, etc. as well as voice information for practical mobile terminal communications.
In such mobile terminal communications, one such system is a CDMA (code division multiple access) system. However, a crucial problem in such mobile terminal communications is that the mobile terminal is not fixed in place. Therefore, a system capable of guaranteeing satisfactory communications is required.
For example, when a mobile terminal is used between high buildings, etc., an electric wave transmitted from a base station to the mobile terminal is received by the mobile terminal after being reflected by various obstacles. Such attenuation is referred to as multi-path fading. Through the multi-path fading, the power of an electric wave received by a mobile terminal frequently changes.
When the electric wave is received in a poor electric power state, the bit error rate of data received can become considerably high. When bit errors occur in an uniformly distributed manner, the errors can be easily corrected. However, if the bit errors occur in bursts, it is more difficult to correct such errors.
Especially, in the next-generation mobile terminal communication system, a high-speed and high-quality transmission is indispensable. To attain such quality, a turbo-code is considered to be a probable candidate for an effective error correction code. Using the turbo-code, the transmission characteristic of the communications system can be significantly improved.
FIG. 6
shows an example of a transmitter for a conventional DS-CDMA system. When an input signal of data and/or voice is input, it is received by a turbo-coder
61
, described later, in which a turbo-code is used. In the turbo-encoder
61
, the data signal and voice signal are encoded into turbo-codes to put such signals in a correctable state.
The encoded signals are then input to a channel interleaver
62
, which is provided to prevent the deterioration in transmission characteristic by the error burst through fading. The channel interleaver
62
randomizes the bit array of input signals and outputs the result.
The signals output from the channel interleaver
62
are then input to a multiplexing unit
64
along with a pilot signal
63
for synchronization of the system. The multiplexing unit
64
multiplexes the signals from the channel interleaver
62
and then inputs the multiplexed output to a modulator
65
(for performing a QPSK modulation in FIG.
6
). The signals modulated by the QPSK modulator
65
are transmitted to a spreading unit
66
, for processing by spread spectrum modulation, and then transmitted through an antenna
67
.
FIG. 7
shows an example of conventional turbo-encoder. A data signal u input to the encoder shown in
FIG. 7
is branched. One branched signal is transmitted to a multiplexer
73
, and the other to a convolutional coder
70
-
1
and an interleaver
71
. In the convolutional coder
70
-
1
, a convolutional code is generated using a signal string of input data signals u. In the configuration of
FIG. 7
, an input signal x
k
is added to a one-bit delay value x
k−1
and a two-bit value x
k−2
by an adder. Further, the result is also added again to the 2-bit delay value x
k′−2
, and input as a convolutional code y
1k
to a puncturing unit
72
.
Further, the data signal transmitted to the interleaver
71
is temporarily entered in a matrix, and then read in an order different from the order in which it is written to the matrix. Thus, the data signal output from the interleaver
71
is represented as a bit array different from bit array of the original data signal u. Therefore, after the bit sequence of the data signal is changed at random, the signal is input to a convolutional coder
70
-
2
.
In the convolutional coder
70
-
2
, a similar process as performed in the convolutional coder
70
-
1
is performed, and a convolutional code is generated. However, a convolutional code output from the convolutional coder
70
-
2
is obtained by encoding a data signal having a bit sequence randomized by the interleaver
71
. Therefore, it is input to the puncturing unit
72
as a convolutional code y
2k
different from the code output from the convolutional coder
70
-
1
.
The puncturing unit
72
switches the code y
lk
from the convolutional coder
70
-
1
and the code y
2k
from the convolutional coder
70
-
2
using a predetermined pattern. The switched codes are then input to the multiplexer
73
. A typical puncturing method is to alternately switch the code y
1k
and the code y
2k
. However, the switching of the codes do not necessarily have to be alternated. The user can appropriately determine the switching mode.
The signal x
k
and a signal from the puncturing unit
72
are multiplexed by the multiplexer
73
and the result is then output as an encoded signal. The encoded signal output from the multiplexer
73
is interleaved by an interleaver
62
. In other words, the encoded signal is read to a matrix, read out at random, and then output. The interleaver
71
of
FIG. 7
is provided to make the code y
2k
from the convolutional coder
70
-
2
different then the code y
1k
from the convolutional coder
70
-
1
. The interleaver
71
and the convolutional coder
70
-
2
are assumed to form an encoder. Further, the interleaver
62
is provided to prevent bursts that occur in a transmission signal.
Since a transmitted signal is obtained by the interleaver
62
randomizing a bit array of the signal, an error burst arising in the signal that could span a plurality of bits is distributed. The error burst can be distributed by a receiving side interleaver
74
for distributing the error into scattered bit errors. This is desirable since a plurality of scattered bit errors can be more easily corrected than an error burst. As a result, an error code rate due to error bursts is reduced.
FIG. 8
illustrates an example of a basic interleaving process. An interleaver includes a plurality of memory units for sequentially storing input data. As shown in
FIG. 8
, an input data signal is sequentially written bit by bit from the leading bit. For example, the row direction as indicated by the numbers in FIG.
8
. After a predetermined length of the data signal has been read, the data signal is then read out, for example, in a column direction. Thus, the bits of the data signal is output randomly. It is not always necessary to read the columns sequentially from the left. It may be better to select and read a column randomly to furthermore randomize the data signals to be output.
In order to randomize a data signal, it is desirable that a larger number of data signals are read and output. Thus, the number of signals that can be changed in output order increases. For example, a packet of data signals can be read and interleaved. In this case, the packet is followed by a tail bit, and the tail bit may or may not be interleaved. When the tail bit is interleaved, it can be similarly read with the other signals forming a matrix as shown in FIG.
8
. Unless the tail bit is interleaved, the signals are read from the positions other than those where the tail bit is stored. After all the other data signals have been read, the tail bit is then read in the order it was written, and then added at the end of the output signal.
The interleaver first stores data signals in memory and then reads the data signals out later. Thus, the larger number of data signals to be read, the longer the delay becomes. As a result, when the interleaving process is performed by the interleaver
71
as shown in
FIG. 7
, the output from the convolutional coder
70
-
2
is delayed by the amount
Abraham Esaw
De'cady Albert
Fujitsu Limited
Katten Muchin Zavis & Rosenman
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