Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-03-23
2001-08-14
Le, Amanda T. (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S152000, C370S342000, C370S479000
Reexamination Certificate
active
06275521
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a demodulating apparatus and a demodulating method, and more particularly, is applicable to a portable telephone.
2. Description of the Related Art
There exists a portable telephone as a mobile communication system which has spread remarkably. Various communication methods of the portable telephone have been proposed. The typical one is the code division multiple access (CDMA) method. The CDMA method has been proposed and put to practical use as the IS-95 (Interim Standard-95) Standards in the U.S. Recently, in the standardization project of the next generation mobile communication system called IMT2000 (International Mobile Telecommunication 2000) which is being carried out by the International Telecommunication Union (ITU), the CDMA method is also watched as a strong communication method in the next generation.
The CDMA method is a communication method using so-called spread spectrum communication method, in which at a transmitting side, a code sequence to be transmitted is multiplied by a spread code sequence having higher speed than that of the code sequence to be transmitted so as to spread the code sequence, and at a receiving side, the code sequence is inversely spread by using the same spread code sequence as the spread code sequence used at the transmission. The maximum period code sequence is generally used as the spread code sequence. The maximum period code sequence is a code sequence having the characteristics that a correlation between different codes is independent and the auto-correlation function is like impulse. Therefore, at the receiving side, the received code sequence can not be demodulated if it is not multiplied by the same spread code sequence as that of the transmitting side and having the same timing as that of the transmitting side. Thus, the CDMA method is a superior communication method in a confidentiality since the transmitted contents are difficult to be heard by a third party. Moreover, the CDMA method assigns a different spread code sequence for each mobile station, so that an interference problem will not occur even if the same frequency is used.
Hereinafter, the principle of the spread spectrum will be explained. “s
i
” denotes a transmission signal here. The transmission signal s
i
is a signal sequence of the complex number comprising a symbol length T
sym
. “c
j
” denotes a spread code. The spread code c
j
is a signal sequence of the complex number comprising a tip length T
chip
. “j” denotes a time series symbol number and the maximum value is “J”. The spread code c
j
is the maximum period code sequence having the period J
MAX
, and the auto-correlation is like impulse. That is, the auto-correlation is obtained by the following equation (1):
∑
j
=
0
J
c
j
c
(
j
+
k
)
%
J
MAX
*
=
δ
(
k
)
=
{
A
(
k
=
0
)
0
(
k
≠
0
)
(
1
)
Note that “c*” denotes the conjugate of c and “%” denotes a residue arithmetic.
Further, the spread code c
j
is independent of the spread code c
j
′ having the same period J
MAX
as that of the spread code c
j
. This is indicated by the following equation (2):
∑
j
=
0
J
c
j
c
(
j
+
k
)
%
J
MAX
′
*
=
0
(
2
)
At the transmitting side, the spreading of the transmission signal si which is performed by using the spread code c
j
is indicated by the following equation (3):
x
(
iI
sym
+jT
chip
)=
s
i
c
i
(3)
In this case, as shown in
FIGS. 1A
to
1
C, because the symbol length T
sym
(
FIG. 1A
) of the transmission signal s
i
is extremely longer than the tip length T
chip
(
FIG. 1B
) of the spread code c
j
, the spread transmission signal x
j
is spread into a very wide area comparing to the original transmission signal s
i
(FIG.
1
C).
On the contrary, at the receiving side, by using the spread code sequence which is the same code sequence as that of the transmitting side and has the same timing, the received code sequence is inversely spread by the following equation (4):
y
(
iT
sym
)
=
∑
j
=
1
J
x
(
iT
sym
+
jT
chip
)
c
j
*
=
s
i
∑
j
=
1
J
c
j
c
j
*
=
A
θ
i
(
4
)
and the transmission signal s
i
is demodulated. If the timing of the spread codes generated at the recording side deviates, the receiving side performs the inverse spreading by the following equation (5):
y
(
iT
sym
)
=
∑
j
=
1
J
x
(
iT
sym
+
jT
chip
)
c
j
*
=
s
i
∑
j
=
1
J
c
j
+
kc
j
*
=
0
(
5
)
and the transmission signal s
i
can not be demodulated. In this way, in the spread spectrum communication, the inversely spread codes which has the same code sequence as the code sequence used at the transmission side and has the same timing is convoluted into a reception signal, so as to demodulate th e reception signal.
In the portable telephone, the multipath fading where the reception level changes moment by moment occurs. Hereinafter, the multipath fading will be explained with referring to FIG.
2
and FIG.
3
. An electric wave from a base station is reflected and diffracted by the buildings and then is transmitted as plural scattered waves. The portable telephone receives the plural scattered waves transmitted through respective transmission paths. For example, as shown in
FIG. 2
, the plural scattered waves
1
are received by an antenna
3
of an automobile
2
which has a portable telephone. The portable telephone then combines these plural scattered waves and demodulates them.
The transmission paths of the scattered waves respectively have the different transmission time so that the transmission characteristics respectively have a predetermined frequency response. This produces the linear distortion called inter-code interference in the reception signal. The inter-code interference is a phenomenon that occurs when at the receiving timing of a predetermined symbol, the influence of symbols before and after the symbol is added in accordance with the impulse response of the transmission path, so that a code decision error increases. Generally, the code decision error remarkably increases when the delay time &tgr; of each scattered wave is the same degree and over as the tip length T
chip
of the spread code. Such transmission path of the scattered wave is indicated by the following equation (6):
h
(
t
)
=
a
1
δ
(
t
-
τ
1
)
+
a
2
δ
(
t
-
τ
2
)
+
a
3
δ
(
t
-
τ
3
)
…
=
∑
n
=
1
N
a
n
δ
(
t
-
τ
n
)
(
6
)
Note that “N” is the number of the scattered waves, “a
n
” is the complex gain showing the attenuated amount of each scattered wave and the phase rotation, and “&tgr;
n
” is the delay time of each scattered wave. These values of N, a
n
, and &tgr;
n
change at random. Thereby, the linear distortion also changes moment by moment.
As a receiving apparatus for compensating the linear strain caused by the multipath fading, there is a RAKE receiver. The RAKE receiver divides the multiwaves due to the multipath fading for each transmission path to generate plural scattered waves, and inversely spreads each of the scattered waves to combine them, thereby decreasing the linear distortion. Hereinafter, the RAKE receiver will be explained with referring to FIG.
3
.
The RAKE receiver
10
inputs a reception signal S
1
received by an antenna
11
to a high frequency amplifier
12
. The high frequency amplifier
12
amplifies the reception signal S
1
and outputs the resultant high frequency signal S
2
to a frequency converter
13
. The frequency converter
13
frequency-converts the high frequency signal S
2
and outputs the resultant intermediate frequency signal S
3
to an intermediate frequency amplifier
14
. The intermediate frequency amplifier
14
amplifies the intermediate frequency signal S
3
and outputs the intermediate frequency signal S
4
to a filter
15
. The filter
15
removes
Le Amanda T.
Maioli Jay H.
Sony Corporation
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