Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
2000-01-24
2004-01-06
Chin, Wellington (Department: 2664)
Multiplex communications
Communication over free space
Repeater
C370S342000
Reexamination Certificate
active
06674732
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a transmitting method, a receiving method, a transmitter, and a receiver, and is suitably applicable, for example, to cellular radio communication systems.
BACKGROUND ART
In recent years, a cellular radio communication system that is represented by a portable telephone system has been put to practical use and employed in the field of mobile communication. This cellular radio communication system divides an area providing communication services into cells of a desired size, and installs base stations as fixed stations within the cells, respectively. Communication terminal equipment as a mobile station communicates by radio with a base station whose communication state is believed to be most satisfactory.
As for the communication method between the base station and the communication terminal equipment, a wide variety of methods have been proposed. As the representative one, a time-code-division multiple access method combining a time-division multiple access method (TDMA method) and a code-division multiple access method (CDMA method) together has been proposed.
This time-code-division multiple access method, as shown in
FIG. 1
, divides a band of 1.2 MHz into time cycles of 4.615 ms (this time cycle will hereinafter be called a frame) in a time axis direction. Each frame is further divided at time intervals of 577 &mgr;s, thereby dividing each frame into time slots TS
0
to TS
7
. The divided time slots TS
0
to TS
7
are used as communication channels in a time direction, whereby time-division multiple access is performed.
Also, in this time-code-division multiple access method, as shown in
FIG. 2
, in each time slot a transmission symbol which is an object of transmission is multiplied by a diffusion code and is transmitted. At this time, eight kinds of diffusion codes are prepared per one time slot, whereby eight communication channels by code division are ensured per one time slot.
Thus, in this time-code-division multiple access method, eight communication channels by code division are ensured per one time slot and also eight time slots are formed in a time direction per one frame of 4.615 ms, whereby 64 communication channels are ensured in 1.2 MHz×4.615 ms.
Now, in the case where transmission is actually performed by this method, a transmission symbol with a symbol rate of 135 ksymbol/s is first diffused into eight times as many spectra by the use of a diffusion code with a speed which is eight times the speed so that the chip rate becomes 1080 kchip/s. The diffused transmission signal is transmitted at the timing of time slots allocated to a local station. This makes it possible to realize the communication based on this time-code-division multiple access method.
Note that in each time slot, as shown in
FIG. 3
, 28 transmission symbols (224 chips after diffusion) are transmitted at the first half portion of the interval of 577 &mgr;s as a whole, then 198 chips of predetermined data are transmitted as a training sequence for the characteristic estimation of a transmission line, and then 28 transmission symbols are transmitted again. Incidentally, after the transmission symbol which is last transmitted, a transmission stop interval called a guard interval is provided. With this guard interval, signal collision by a shift in the signal arrival time is prevented.
Now, for the cellular radio communication system which performs communication by the use of a time-code-division multiple access method such as this, a description will be made with FIG.
4
. In
FIG. 4
, reference numeral
1
denotes a transmitter which is provided in a base station, and
2
denotes a receiver which is provided in communication terminal equipment. In the transmitter
1
of the base station, for example if it is assumed that radio communications of eight channels are present, the transmission symbols S
1
to S
8
of the eight channels will be respectively input to a CDMA modulation section
1
A. The CDMA modulation section
1
A has eight kinds of diffusion codes which are in an orthogonal relation with one another, and multiplies the transmission symbol S
1
by the first diffusion code of the eight kinds of diffusion codes, thereby performing spectrum diffusion modulation on the transmission symbol S
1
. Likewise, the CDMA modulation section
1
A multiplies the input transmission symbols S
2
to S
8
by the second to the eighth diffusion codes, respectively, and thereby performs spectrum diffusion modulation on the transmission symbols S
2
to S
8
. Thus, in the CDMA modulation section
1
A, the transmission symbols of the channels thus modulated by spectrum diffusion are combined together, and this is output to a time-division transmitting section
1
B as a transmission signal S
9
.
The time-division transmitting section
1
B converts the frequency of the input transmission signal S
9
to a frequency band allocated to this communication system, and the transmission signal S
10
is transmitted through an antenna
1
C in a burst manner by the use of the time slot TS
0
, for example, among the above-mentioned time slots.
On the other hand, in the receiver
2
, a reception signal S
11
received by an antenna
2
A is input to a time-division receiving section
2
B. The time-division receiving section
2
B fetches the signal component of the time slot TS
0
from this reception signal S
11
and performs a frequency conversion process on the signal component, whereby a reception signal S
12
of the base band corresponding to the transmission signal S
9
of the transmitter
1
is obtained and this is output to a matched filter
2
C.
The matched filter
2
C consists of a shift register, a multiplier, and an adder. For example, if the symbol received by this receiver
2
is taken to be the transmission symbol S
1
, the same code string as the first diffusion code, used when the transmission symbol S
1
is diffused, is used as the coefficient of the multiplier. In the multiplier the chips, output from the taps of the shift register to which the reception signal S
12
was input, are multiplied by respective code sequences. The matched filter
2
C adds the multiplication results by the adder, thereby restoring the transmission symbol S
1
.
Thus, if such a transmitter
1
and receiver
2
are provided in the base station and communication terminal equipment respectively, communication by a time-code-division multiple access method can be realized.
Incidentally, when the transmission signal S
10
multiplexed with such a time-code-division multiple access method is received and from the received signal the transmission symbol of one channel is restored, if a multiple path, etc., are present in the propagation path, the orthogonal relation of the diffusion codes used in the channels will not be entirely held. Therefore, there is a problem that in a simple matched filter such as the matched filter
2
C shown in
FIG. 4
, the signal components of the other
7
channels interfere and the transmission symbol of the channel cannot be accurately restored.
As a method of avoiding this, a method employing a decoding path called a multi-user detection has been proposed. This method, as shown in
FIG. 5
, restores all transmission symbols S
1
to S
8
of eight channels from the reception signal S
12
by a multi-user detection
2
D, taking mutual interference quantities into consideration. Since the transmission symbol is restored by considering the interference quantity which each channel undergoes, this method can reduce the influence of an interference wave in comparison with the method employing a matched filter and restore the transmission symbol more accurately.
However, in this method employing a multi-user detection, not only channels which must be originally decoded but also all channels multiplexed must be decoded and therefore there is a problem that the process associated with decoding becomes complicated and also the amount of processing increases considerably. Specifically, it has been said that the amount of processing increases by about 8
2
times in c
Amir-Alikhani Hamid
Boehnke Ralf
Izumi Seiichi
Odaka Kentaro
Stirling-Gallacher Richard
Chin Wellington
Maioli Jay H.
Pham Brenda
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