Pulse or digital communications – Transmitters – Plural diversity
Reexamination Certificate
2001-12-21
2004-09-28
Tran, Khai (Department: 2684)
Pulse or digital communications
Transmitters
Plural diversity
C370S342000
Reexamination Certificate
active
06798847
ABSTRACT:
PRIORITY
This application claims priority to an application entitled “Transmit Antenna Diversity Apparatus and Method for Base Station in a CDMA Mobile Communication System” filed in the Korean Industrial Property Office on Dec. 21, 2000 and assigned Serial No. 2000-79713, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a communication apparatus and method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a forward transmit antenna diversity apparatus and method in a CDMA mobile communication system.
2. Description of the Related Art
An existing CDMA mobile communication system that mainly supports voice service, has been developed into a future (CDMA mobile communication system which provides high-speed data service as well as voice service. The future CDMA mobile communication system supports voice, moving image and Internet search services. In the mobile communication system, communication links existing between a base station and a mobile station are classified into a forward link for transmitting a signal from the base station to the mobile station, and a reverse link for transmitting a signal from the mobile station to the base station.
The mobile communication system must resolve a fading problem in order to transmit high-speed data. The fading causes a reduction in the amplitude of a received signal from several dB to several tens dB. In order to solve the fading problem, a variety of diversity techniques are used.
One of the techniques used in the CDMA system employs a Rake receiver, which receives a signal on a diversity basis using delay spread of a channel and the Rake receiver supports a reception diversity technique for receiving a multi-path signal. However, this diversity technique is disadvantageous in that it is not operable when the delay spread is low in level.
Also, a time diversity technique utilizing interleaving and coding is used in a Doppler spread channel. However, this technique is not effective in a low-speed Doppler spread channel. It is possible, though, to effectually solve the fading problem using a space diversity technique, in an indoor channel with a low Doppler spread level and a pedestrian channel, a low-speed Doppler channel.
The space diversity technique uses two or more antennas. In this technique, even though a signal transmitted through one antenna is attenuated due to the fading, it is possible to compensate for the attenuation using a signal transmitted through the other antennas. The space antenna diversity technique is divided into a reception antenna diversity using a plurality of reception antennas and a transmit (transmission) antenna diversity using a plurality of transmission antennas. It is hard to install the reception antenna diversity in the mobile station in light of its size and cost. Thus, the use of the transmit antenna diversity for the base station is recommended.
The transmit antenna diversity includes a “closed loop transmit diversity” transmitting a signal based on forward channel information fed back from the mobile station, and an “open loop transmit diversity” receiving no feedback information from the mobile station. In the closed loop transmit diversity scheme, the base station applies weights to transmission signals of the respective transmission antennas based on the channel information measured and fed back by the mobile station to maximize a signal-to-noise ratio (SNR) of an antenna at the mobile station. In the open loop transmit diversity scheme, the base station transmits the same signal through two quadrature (or orthogonal) paths without using the feedback information. The quadrature paths can be provided by time division, frequency division or code division.
FIG. 1
illustrates a structure of a base station transmitter using an open loop transmit diversity scheme according to the prior art. Referring to
FIG. 1
, an input bit stream is encoded by a channel encoder
101
, and an output sequence of the channel encoder
101
is mapped into an M-ary symbol by an M-ary symbol modulator
102
. The M-ary symbol modulator
102
serves as a QPSK (Quadrature Phase Shift Keying), 8-PSK (8-ary Phase Shift Keying) or 16-QAM (16-ary Quadrature Amplitude Modulation) modulator according to its data rate, and its modulation mode can be changed in a physical layer packet unit where the data rate can be changed. I and Q sequences of the M-ary symbol output from the M-ary symbol modulator
102
are modulated into two different complex symbols by an STTD/STS (Space-Time Transmit Diversity/Space Time Spreader) modulator
103
. A detailed description of the STTD/STS modulator
103
will be made with reference to
FIGS. 4 and 5
. Walsh cover parts
104
and
105
orthogonally spread their input symbols using a Walsh orthogonal code W
N
i
assigned to the mobile station. A detailed structure of the Walsh cover parts
104
and
105
is illustrated in FIG.
2
. The two complex symbols spread by the Walsh cover parts
104
and
105
are subject to complex spreading by their associated complex spreaders
106
and
107
, respectively. An internal operation of the complex spreaders
106
and
107
is illustrated in FIG.
3
. The output signals of the complex spreaders
106
and
107
are shifted to RF (Radio Frequency) band signals by associated RF parts
108
and
109
, and then radiated through first and second antennas ANT
1
and ANT
2
.
FIG. 2
illustrates a detailed structure of the Walsh cover parts
104
and
105
illustrated in FIG.
1
. Each Walsh cover part
104
and
105
spreads its input complex symbol to a transmission bandwidth, using a Walsh code assigned to a transmission channel.
FIG. 3
illustrates an internal operation of the complex spreaders
106
and
107
shown in FIG.
1
. Each of the complex spreaders
106
and
107
complex-spreads its input complex signal into an I-channel (or I-arm) signal and a Q-channel (or Q-arm) signal, using a spreading sequence comprised of an I-channel spreading sequence PN
I
and a Q-channel spreading sequence PN
Q
.
FIG. 4
illustrates an internal operation of the STTD/STS modulator
103
of
FIG. 1
when it operates in an STTD (Space-Time Transmit Diversity) mode. In the STTD mode, the STTD/STS modulator
103
operates as shown in Table 1.
TABLE 1
Input to
Antenna #1
Antenna #2
Time t
S
0
S
0
−S*
1
Time t + T
S
1
S
1
S*
0
In Table 1, S
0
and S
1
represent complex symbols, and are represented by
S
0
=Si
0
+jSq
0
S
1
=Si
1
+jSq
1
If symbols S
0
and S
1
are input to the STTD modulator
103
at a specific time t and a time t+T, respectively, then the STTD modulator
103
outputs the symbol S
0
for the first antenna ANTI and a minus conjugate of the symbol S
1
for the second antenna ANT
2
at the time t, and outputs the symbol S
1
for the first antenna ANT
1
and a conjugate of the symbol S
0
for the second antenna ANT
2
at the time t+T.
FIG. 5
illustrates an internal operation of the STTD/STS modulator
103
of
FIG. 1
when it operates in the STS (Space Time Spreader) mode. Referring to
FIG. 5
, a serial-to-parallel (S/P) converter
501
converts each of its input complex symbols comprised of an I-phase symbol and a Q-phase symbol into two ½-rate complex symbols comprised of an I-phase symbol and a Q-phase symbol. The two complex symbols I
1
/Q
1
and I
2
/Q
2
are provided to symbol repeaters
511
-
518
, where they are repeated. For example, the symbol I
1
is input to the symbol repeaters
511
and
515
. The symbol repeater
511
(++) repeats the input symbol I
1
, while the symbol repeater
515
(+−) repeats the input symbol I
1
. The outputs of the symbol repeaters
511
-
518
are provided to four summers
519
-
522
and then converted to two complex symbols. Herein, the STTD/STS modulator will be referred to as a “diversity modulator” for simplicity.
FIG. 6
illustrates a structure of a base station transmitter using a
Hwang Jong Yoon
Kim Sung Jin
Lee Ju Ho
Lee Yong Suk
Dilworth & Barrese LLP
Samsung Electronics Co,. Ltd.
Tran Khai
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