Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2001-04-03
2003-01-21
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S146000, C375S150000
Reexamination Certificate
active
06510173
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to radio engineering, particularly to transmit-receive diversity methods and devices in code division multiple access (CDMA) communication systems.
DESCRIPTION OF THE RELATED ART
One of the common problems to CDMA cellular radio communications systems is multipath fading that reduces system capacity. The techniques of fading mitigation applied today allow to use only a part of potential non-fading channel capacity. Hence, creation of new anti-fading techniques with the purpose of further CDMA system capacity improvement is very critical at the moment.
The most effective anti-fading technique is diversity reception using M space diversity receiving antennas. The space positions of the antennas are selected to ensure weak correlation of signal fadings in different antennas. A disadvantage to the receive space diversity is high price and increased dimensions of the equipment. This particularly hinders application of the space receive diversity in mobile terminals. Hence, it is presently an important problem to provide an alternative fading combat technique having the same level of efficiency as the receive space diversity systems.
Various transmit diversity methods are known presently where a signal is transmitted from two or more space diversity antennas as shown on FIG.
1
. Two or more diversity antennas are usually installed at a base station to provide the receive diversity. In case of diversity transmission, these antennas are also used as transmitting ones. The known methods of diversity transmission allow to provide mitigation of the adverse fading effect when a signal is received at one antenna. These transmit diversity methods, however, are far less effective than the receive diversity ones.
It is known a transmit-receive method disclosed in U.S. Pat. No. 5,109,390, where a data stream is modulated on the transmitting side and then transmitted via one space diversity channel (via one antenna). On the receiving side, the input signal is demodulated recovering the initial data stream.
Since the data stream is transmitted via one space diversity channel only, the signal at the receiving point may disappear at some time intervals due to the fading in the channel. As a result, reception of such a signal is characterized by high error rate. Decreasing of the error rate requires some measures to be taken to increase signal-to-noise ratio (SNR) at the receiver input and as a result leads to degradation of the communication system capacity.
There is a method for delay transmit diversity (see “Space-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction”. Vahid Tarokh. IEEE Transactions on Information Theory, vol. 44, No.2, March 1998), in which the same wideband signal is transmitted without delay via a first antenna, and with delays relative to one another via the rest of antennas, the delay value being no less than one chip of the spreading pseudo noise (PN) sequence. In this case the signal on the receiving side resembles a multipath signal and can be processed by the conventional Rake-receiver. However, this method has a substantial drawback. The known PN spreading sequences have imperfect autocorrelation properties, so signals arriving at the receiving side with different time delays act as severe interference to one another.
A method for orthogonal transmit diversity (OTD) in a CDMA system and an algorithm for its application is disclosed in the UMTS-2000 standard project for cellular CDMA systems developed by ETSI-SMG2 “Submission of Proposed Radio Transmission Technologies” issued Jan. 29, 1998, pp. 51-52, section 5.6.3.1 “Orthogonal Transmit Diversity”.
The method according to the above solution is as follows.
A stream of user information symbols is split into two sub-streams, each sub-stream having different symbols of the initial stream. Each symbol in each sub-stream is repeated twice, thus doubling its duration. An orthogonal code with repetition period equal to the sub-stream symbol duration is assigned to each symbol sub-stream. Symbols of the first sub-stream are transmitted via a first antenna, and symbols of the second sub-stream are transmitted via a second antenna. Prior to signal transmission a conventional processing is performed which includes PN spreading and analog modulation. That way orthogonality is maintained between two and more output sub-streams. Note, that this OTD method does not require additional base station channelisation codes that represent one of the basic resources. The necessary orthogonal codes could be produced from one orthogonal code assigned to a user. Denote this code by P
k
. Then two new orthogonal codes can be produced as [P
k
, P
k
], [P
k
, −P
k
]. The brackets denote the concatenation operation. The repetition period of the orthogonal codes thus generated is twice the duration of the code assigned to the user and is equal to the duration of a sub-stream binary symbol.
Based on the description of the algorithm, a device on the transmitting side, according to this method, may be accomplished as shown on FIG.
2
. The device according to
FIG. 2
comprises M signal transmit branches
1
-
1
-
1
-M, two modulators
8
1
-
8
2
and two antennas
9
1
-
9
2
. Each transmit branch contains binary symbol stream splitter
2
, the input of which is the input of the device, orthogonal code generator
3
, orthogonal modulator
4
, the first inputs of which are connected to the corresponding outputs of binary symbol stream splitter
2
, and the second inputs—to the outputs of orthogonal code generator
3
, the outputs of orthogonal modulator
4
are linked to the first inputs of PN spreader
5
, the second input of which is joined to the output of the spreading PN code generator, the outputs of PN spreader
5
are connected to the corresponding first inputs of first
8
1
and second
8
2
modulators, the second inputs of first
8
1
and second
8
2
modulators are connected to the outputs of pilot signal generator
7
, the outputs of modulators
8
1
and
8
2
are joined to first
9
1
and second
9
2
antenna, respectively.
For signal receiving, the UMTS standard project proposes a device described in the UMTS-2000 standard project for cellular CDMA systems developed by ETSI-SMG2 “Submission of Proposed Radio Transmission Technologies” issued Jan. 29, 1998, pp. 51-52, section 5.6.3.1 “Orthogonal Transmit Diversity”. This device is accomplished as shown on FIG.
3
.
The known device of
FIG. 3
comprises multiplier
10
, a first input of which is the information input of the device, and a second input is the second input of the device to which PN code is applied, the output of multiplier
10
is connected to the first inputs of multipliers
11
,
13
,
16
and
18
, to the second inputs of which corresponding pilot codes of channels
1
to N are applied, the outputs of multipliers are joined to the inputs of corresponding combiners
12
,
14
,
17
, and
19
, the outputs of combiner
12
and combiner
14
are linked to the first and second inputs of multiplier
15
, respectively, the outputs of combiner
17
and combiner
19
are connected to the first and second inputs of multiplier
20
, respectively, the outputs of multipliers
15
and
20
are joined to the first and the second inputs of soft decision combining unit
21
, respectively, the output of which is the output of the device.
The known device is similar to the Rake-receiver. Multipliers
10
,
13
and combiner
14
represent correlator of the input signal to the pilot (reference) signal of the first antenna, multipliers
10
,
18
and combiner
19
represent correlator of the input signal to the pilot signal of the second antenna.
The outputs of multipliers
15
and
20
are soft decisions on transmitted binary symbols in corresponding sub-streams.
Unit
21
combines soft decisions on symbols of two sub-streams into one stream of soft decisions.
The known device operates as follows.
On the transmitting side, an input binary symbol stream is applied to binary
Garmonov Alexandr Vasillevich
Karpitski Yuri Evgenievich
Abelman ,Frayne & Schwab
Bocure Tesfaldet
Centro de Technologia Vidriera Monterrey, S.A.
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