Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-12-04
2001-10-30
Bocure, Tesfaldet (Department: 2731)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S267000
Reexamination Certificate
active
06310907
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spread-spectrum system, and more particularly to a spread spectrum diversity transceiver making use of code-division multiplexing, time-diversity, and error-correcting decoding, especially in digital radio transmission having difficulties in multipath fading.
2. Description of Related Art
Regarding a diversity system to be applied to digital communication, there has been disclosed a diversity system for preventing losses of data caused by fading in Japanese Patent Application Laid-open Publication No. 07-143101 (hereinafter, to be referred to as a first publication).
According to this first publication, after a predetermined length of transmission data has been coded into a code by an error-correcting encoder, the bit information of the code is transmitted simultaneously through a plurality of channels formed by transmitters of different frequencies and a transmission antenna.
At a receiver side, the bit information of the above one code is received simultaneously by a plurality of receivers through a receiving antenna and the received code information is subjected to error-correcting and decoding by an error-correcting decoder.
In such a radio transmission under channel fading, as described above, diversity receiving is usually necessary. Fading is broadly classified into frequency-nonselective (flat) fading and frequency-selective fading. The flat fading involves no occurrence of multipath propagation but directly causes variations in amplitude and phase of a receiving wave itself in the middle of its propagation. On the other hand, the frequency-selective fading involves an occurrence of multipath propagation and independently causes variations in amplitude and phase of each of a plurality of arrival waves due to multipath.
In the case of frequency-selective fading, a receiving signal becomes a combined wave of a plurality of multipath waves and, therefore, the received signal may have an inverse phase combination in a certain frequency depending on the status of phase variation. In other words, frequency-selective fading (a notch) occurs in the received spectrum.
On the other hand, in the case of flat fading, variations in the received signal level become a problem and the received waveform itself is not distorted. However, in the case of the frequency-selective fading by multipath, there occurs a distortion in the received waveform in addition to the variations in the received signal level.
For the above-described fading channel, there have been conventionally used diversity receiving and adaptive equalizer techniques. There are various types of system available for the diversity receiving and the adaptive equalizer technique. A spread spectrum technique which is considered to be effective for a multipath distortion will be explained below as an example of a prior-art technique.
The spread spectrum communication has so far been used for military purpose to achieve robust communication against interference waves.
In the case of multipath waves with a long delay time, however, correlation with a desired wave is reduced. In this case, if the spread spectrum technique is applied, a multipath wave is not correlated with a spreading code and is suppressed by a despreading operation. In other words, it can be said that the spread spectrum is a kind of an equalizer for regarding a multipath wave as an interference wave as well. However, a multipath wave with a short delay time has high correlation with the desired wave and this multipath wave cannot be suppressed easily by the despreading.
In this case, when there exists an inverse phase relation between the multipath wave and the desired wave, a signal level might be lowered or a fade-out could occur. In order to cope with such a fade-out, diversity receiving making use of non-correlation of a plurality of propagation paths becomes essential.
Referring to
FIG. 1A
, it is assumed that a transmitter
10
carries out transmission by using one non-directional antenna and a receiver
11
receives multipath propagating waves. Let us consider a model where multipath propagation occurs in which a wave radiated from the non-directional antenna of the transmitter
10
is propagated directly through a diversity path
12
and through diversity paths
13
and
14
including a reflection respectively.
FIGS. 1B-1D
show variations of receiving electric field levels of the case where waves propagated through the diversity paths
12
to
14
respectively are received by the receiver
11
. More specifically, variations in the received electric field of the waves that have propagated through each diversity path, that is, through the diversity path
13
in case of
FIG. 7
, through the diversity path
12
in case of
FIG. 8
, and through the diversity path
14
in case of
FIG. 9
, respectively, where the horizontal axis shows time and the vertical axis shows the levels of received electric fields or received power. In this case, since the propagation paths are different from each other in space, each fading becomes independent and the respective received signal levels vary as shown in
FIGS. 1B
to
1
D.
In this model, the diversity receiving is performed by selecting or combining non-faded-out portions of the respective diversity branches to thereby reduce probabilities of fade-out.
The diversity like this is called a space diversity or a path diversity as the diversity utilizes non-correlation of the propagation paths. As a means for achieving this path diversity, an adaptive array using a plurality of antennas is usually employed. In other words, it is possible to achieve diversity combining by extracting a plurality of multipath arriving waves and performing maximum-ratio combining by the directional control of the adaptive array.
In general, however, the space diversity requires a plurality of antennas and is disadvantageous in cost. Particularly, in microwave communication, the number of antennas cannot be increased easily as the costs of antennas are high and a large-scale apparatus is required.
In order to improve the drawbacks of this space diversity, there has been disclosed a diversity system utilizing the spread spectrum code multiplexing and the time diversity in Japanese Patent Application Laid-open Publication No. 08-191289 (hereinafter to be referred to as a second publication).
This prior-art technique will be explained with reference to
FIGS. 2A and 2B
.
Referring to
FIG. 2A
, in a transmitter, transmission data is input to an error-correcting encoder
21
and the corded data is branched into N branches. The coded data is input to an interleaver
23
(
1
) and is also input it interleavers
23
(
2
)-
23
(N) through delay elements
22
(
1
)-
22
(N-
1
) each providing the corresponding branch with a different delay.
The N-branch data with the delay step are independently interleaved by the interleavers
23
(
1
)-
23
(N) and then the respective interleaved data are modulated by modulators
24
(
1
)-
24
(N). Output signals of the modulators
24
(
1
)-
24
(N) are sent to spectrum spreaders
25
(
1
)-
25
(N), respectively. After having been spectrum-spread by the spreaders
25
(b
1
)-
25
(N), the N-branch signals of the same frequency band are output to a combiner
26
.
The combiner
26
combines the N-branch signals into a code-multiplexed signal, which is converted into radio frequency by a radio transmitter
27
. The radio-frequency signal is then transmitted as a radio wave through a transmission antenna
28
.
Referring to
FIG. 2B
, in a receiver, a radio signal received by a receiving antenna
29
is input to a radio receiver
30
, which converts the radio-frequency received signal into the spectrum-spreading frequency band. Then, the received signal is branched into N branches by a branch circuit
31
. The N-branch received signals are input to despreaders
32
(
1
)-
32
(N) corresponding to the N transmitting branches.
After have been spectrum-despread by the despreaders
32
(
1
)-
32
(N) the respective signals are domodulated by demodul
Bocure Tesfaldet
McGinn & Gibb PLLC
NEC Corporation
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