Pulse or digital communications – Repeaters – Testing
Reexamination Certificate
1997-12-12
2002-05-14
Tse, Young T. (Department: 2734)
Pulse or digital communications
Repeaters
Testing
C370S335000, C370S342000
Reexamination Certificate
active
06389060
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a receiving unit suitable for a CDMA (Code Division Multiple Access) type cellular telephone system, a receiving method thereof, and a terminal unit for use with a radio system thereof.
2. Description of the Related Art
In recent years, a CDMA type cellular telephone system has become attractive. In the CDMA type cellular telephone system, a pseudo-random code is used as a spread code. A carrier of a transmission signal is spectrum-spread. The pattern and phase of each spread code in the code sequence are varied so as to perform a multiple access.
In the CDMA system, the spectrum spread method is used. In the spectrum spread system, when data is transmitted, the carrier is primarily modulated with the transmission data. In addition, the carrier that has been primarily modulated is multiplied by a PN (Pseudorandom Noise) code. Thus, the carrier is modulated with the PN code. As an example of the primarily modulating method, a balanced QPSK modulating method is used. Since the PN code is a random code, when the carrier is modulated by the PN code, the frequency spectrum is widened.
When data is received, the received data is multiplied by the same PN code that has been modulated on the transmission side. When the same PN code is multiplied and the phase is matched, the received data is de-spread and thereby primarily modulated data is obtained. When the primarily modulated data is demodulated, the original data is obtained.
In the spectrum spread method, to de-spread the received signal, the same PN code that has been modulated on the transmission side is required for both the pattern and the phase. Thus, when the pattern and the phase of the PN code are varied, the multiple access can be performed. The method for varying the pattern and the phase of each spread code in the code sequence and thereby performing the multiple access is referred to as CDMA method.
As cellular telephone systems, an FDMA (Frequency Division Multiple Access) system and a TDMA (Time Division Multiple Access) system have been used. However, the FDMA system and the TDMA system cannot deal with a drastic increase of the number of users.
In other words, in the FDMA system, the multiple access is performed on different frequency channels. In an analog cellular telephone system, the FDMA system is usually used.
However, in the FDMA system, since the frequency use efficiency is bad, a drastic increase of the number of users tends to cause channels to run short. When the intervals of channels are narrowed for the increase of the number of channels, the adjacent channels adversely interfere with each other and thereby the sound quality deteriorates.
In the TDMA system, the transmission data is compressed on the time base. Thus, the use time is divided and thereby the same frequency is shared. The TDMA system has been widely used as a digital cellular telephone system. In the TDMA system, the frequency use efficiency is improved in comparison with the simple FDMA system. However, in the TDMA system, the number of channels is restricted. Thus, it seems that as the number of users drastically increases, the number of channels runs short.
On the other hand, the CDMA system has excellent interference resistance. Thus, in the CDMA system, adjacent channels do not interfere with each other. Consequently, the frequency use efficiency improves and more channels can be obtained.
In the FDMA system and the TDMA system, signals tend to be affected by fading due to multi-paths.
In other words, as shown in
FIG. 5
, a signal is sent from a base station
201
to a portable terminal unit
202
through a plurality of paths. In addition to a path P
1
in which a radio wave of the base station
201
is directly sent to the portable terminal unit
202
, there are a path P
2
, a path P
3
, and so forth. In the path P
2
, the radio wave of the base station
201
is reflected by a building
203
A and sent to the portable terminal unit
202
. In the path P
3
, the radio wave of the base station
201
is reflected by a building
203
B and sent to the portable terminal unit
202
.
The radio waves that are reflected by the buildings
203
A and
203
B and sent to the portable terminal unit
202
through the paths P
2
and P
3
are delayed from the radio wave that is directly sent from the base station
201
to the portable terminal unit
202
through the path P
1
. Thus, as shown in
FIG. 6
, signals S
1
, S
2
, and S
3
reach the portable terminal unit
202
through the paths P
1
, P
2
, and P
3
at different timings, respectively. When the signals S
1
, S
2
, and S
3
through the paths P
1
, P
2
, and P
3
interfere with each other, a fading takes place. In the FDMA system and the TDMA system, the multi-paths cause the signal to be affected by the fading.
On the other hand, in the CDMA system, with a diversity RAKE method, the fading due to the multi-paths can be alleviated and the S/N ratio can be improved.
In the diversity RAKE system, as shown in
FIG. 7
, receivers
221
A,
221
B, and
221
C that receive signals S
1
, S
2
, and S
3
through the paths P
1
, P
2
, and P
3
are disposed, respectively. A timing detector
222
detects codes received through the individual paths. The codes are set to the receivers
221
A,
221
B,
221
C corresponding to the paths P
1
, P
2
, and P
3
, respectively. The receivers
221
A,
221
B, and
221
C demodulate the signals received through the paths P
1
, P
2
, and P
3
. The received output signals of the receivers
221
A,
221
B, and
221
C are combined by a combining circuit
223
.
In the spectrum spread system, signals received through different paths are prevented from interfering with each other. The signals received through the paths P
1
, P
2
, and P
3
are separately demodulated. When the demodulated output signals received through the respective paths are combined, the signal intensity becomes large and the S/N ratio improves. In addition, the influence of the fading due to the multi-paths can be alleviated.
In the above-described example, for simplicity, with the three receivers
221
A,
221
B, and
221
C and the timing detector
222
, the structure of the diversity RAKE system was shown. However, in reality, in a cellular telephone terminal unit of diversity RAKE type, as shown in
FIG. 8
, fingers
251
A,
251
B, and
251
C, a searcher
252
, and a data combiner
253
are disposed. The fingers
251
A,
251
B, and
251
C obtain demodulated output signals for the respective paths. The searcher
252
detects signals through multi-paths. The combiner
253
combines the demodulated data for the respective paths.
In
FIG. 8
, a received signal as a spectrum spread signal that has been converted into an intermediate frequency is supplied to an input terminal
250
. This signal is supplied to a semi-synchronous detecting circuit
255
. The semi-synchronous detecting circuit
255
is composed of a multiplying circuit. The semi-synchronous detecting circuit
255
multiplies a signal received from the input terminal
250
by an output signal of a PLL synthesizer
256
. An output signal of the PLL synthesizer
256
is controlled with an output signal of a frequency combiner
257
. The semi-detecting circuit
255
performs a quadrature detection for the received signal.
An output signal of the semi-synchronous detecting circuit
255
is supplied to an A/D converter
258
. The A/D converter
258
converts the input signal into a digital signal. At this point, the sampling frequency of a controller
254
is much higher than the frequency of the PN code that is spectrum-spread. In other words, the input signal of the A/D converter
258
is over-sampled.
An output signal of the controller
254
is supplied to the fingers
251
A,
251
B, and
251
C. In addition, the output signal of the controller
254
is supplied to the searcher
252
. The fingers
251
A,
251
B, and
251
C de-spread the signals received through the respective paths, synchronize the signals, acquire the synchronization of the received signals, demodula
Maioli Jay H.
Tse Young T.
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