JP2924536B2 - Soft handoff method in spread spectrum communication system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spread spectrum communication system, for example, a cellular telephone system employing a code division multiple access (CDMA) system and a multi-spot beam satellite communication system. In particular, the present invention relates to a method for switching (handoff) a cell in charge of communication and a spot beam.

[0002]

2. Description of the Related Art In a cellular telephone system and a multi-spot beam satellite communication system, a service area is divided into areas called cells and spot beams, respectively, and terminals communicate through at least one cell and spot beam. FIG. 8 is a conceptual diagram of a terminal, a cell or a spot beam (hereinafter, a cell or a spot beam is described as a “cell / spot beam”), and a base station / gateway earth station that is in charge of each cell / spot beam. Show.

In FIG. 1, reference numeral 29 denotes a terminal, and 30 denotes a terminal 2.
9 is in the service area and therefore is in charge of the cell /
Spot beam, 31, 32, 33, 34, 35, 36
Is the cell / spot beam adjacent to cell / spot beam 30; 37, 38, 39 and 40 are base stations / gateway earth stations serving cell / spot beams 31, 32, 29 and 33, 34, 35 and 36, respectively. is there.

[0004] Here, each cell / spot beam and the base station / gateway earth station in charge of the cell / spot beam do not always correspond one-to-one, and one base station / gateway earth station transmits a plurality of cell / spot beams. Sometimes I am in charge. The terminal 29 changes its position within the serving cell / spot beam 30. The change in the position of the terminal within the cell / spot beam is caused by the movement of the terminal itself. In a multi-spot beam satellite communication system using orbiting satellites, even if the terminal stays in one place, the position of the terminal within one spot beam relatively changes because the satellite orbits the earth. become.

When the terminal 29 is located at the edge of the cell / spot beam 30, the quality of the communication via the cell / spot beam 30 is inferior to that obtained at the center of the cell / spot beam 30. Also, the position of the terminal 29 may subsequently change into the cell / spot beams 31 to 36, in which case the cell / spot beam via communication must be switched. 2. Description of the Related Art In a spread spectrum communication system, for example, a cellular telephone system and a multi-spot beam satellite communication system employing a code division multiple access (CDMA) system, a soft hand-off technique is used in order to prevent an instantaneous interruption of a communication line due to cell / spot beam switching. .

More specifically, at least one of the adjacent cell / spot beams 31 to 36 is used while the quality of the communication is degraded due to a change in the position of the terminal 29 that has been communicating via the cell / spot beam 30. For example, the same information is transmitted from the cell / spot beams 33 and 34, and these are diversity-received at the terminal 29. That is, the quality of communication accompanying a change in the position of the terminal is maintained by temporarily transmitting information to the terminal via a plurality of cell / spot beams.

[0007] When a terminal receives information signals transmitted simultaneously via a plurality of cell / spot beams in a diversity manner, each signal must be separable. In the related art, a signal received by a terminal can be separated by using a code having at least one of a spreading code and a frequency different from that of another cell / spot beam for each cell / spot beam.

FIG. 7 shows an example of a terminal receiver system configuration. In the figure, 1, 1 ′ and 1 ″ are matched filters corresponding to different spreading codes, and 2 ′ and 2 ″ are 1
Symbol time delays, 3, 3 'and 3 "are multipliers, 4 and 5 are delays corresponding to the arrival time of each signal,
Reference numeral 6 denotes a combining / switching device.

As shown in FIG. 7, in the terminal, a receiver comprising a matched filter, a delay unit, and a multiplier is provided with a number of systems (numbers) corresponding to the number of cells / spot beams used for performing diversity reception. (3 systems in the example). As described above, when information signals are simultaneously transmitted using different spreading codes or frequencies through a plurality of cell / spot beams, the terminal requires a plurality of receivers, so that the terminal can be designed to be small. It will be difficult.

When a different spreading code is used for each cell / spot beam at the time of simultaneous transmission, one terminal needs a number of spreading codes corresponding to the number of cells / spot beams to be simultaneously transmitted. It is not preferable from the viewpoint of effective use of the spreading code.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems of the prior art, and simultaneously transmits an information signal using the same spreading code and the same frequency via a plurality of cell / spot beams. It is an object to transmit and provide soft handoff of a cell / spot beam using a terminal having only one system of receiver.

[0012]

A feature of the present invention is that when the quality of communication via one cell / spot beam is degraded due to a change in the position of a terminal, the cell / spot which has been used for communication until then is degraded. An information signal spread and modulated using the same spreading code and the same frequency as those used in the beam is transmitted from at least one cell adjacent to the cell which has been in charge of the communication in accordance with a predetermined transmission time offset. In a terminal, an information signal transmitted simultaneously via a plurality of cells / spot beams is diversity-received by a single receiver to obtain an output information signal.

[0013]

According to the present invention, as described above, soft handoff can be performed by a terminal having only one receiver, so that the hardware scale of the terminal can be reduced. Also,
The information signals transmitted via the cell / spot beam used for simultaneous transmission use the same spreading code, which leads to effective use of the spreading code.

[0014]

Embodiment 1 A first embodiment of the present invention will be described with reference to the drawings.

In this embodiment, it is assumed that the information signal is differentially encoded.

FIG. 1 shows a configuration example of a terminal receiver system using the present invention. In the figure, 1 is a matched filter,
2 is a delay unit corresponding to one symbol time of the information signal, 3 is a multiplier, 4 and 5 are delay units corresponding to an arrival time difference generated based on a transmission time offset in each cell, and 6 is a combiner / decoder.
It is a switch. FIG. 2 shows an output example of the multiplier 3 in FIG. In the figure, reference numerals 7 and 8 denote arrival time differences of modulated signals,
9 is one symbol time of the information signal, 10, 11, 12, 1
3, 14 and 15 are detected information signals. Also,
FIG. 3 shows an example of a transmission time offset of a modulated signal transmitted via a cell / spot beam using the present invention. In the figure, reference numerals 7 'and 8' denote transmission time offsets of modulated signals transmitted via cell / spot beams, 11 ', 1', and 1 '.
Reference numerals 2 ', 13', 14 ', and 15' indicate times at which modulated signals are transmitted via cell / spot beams.

A terminal 29 in FIG. 8 communicates information modulated by a certain spreading code via a cell / spot beam 30. Terminal 29 is located at cell / spot beam 3
If the edge changes to 0, the terminal 29 and the base station / gateway earth station 38 know that the communication quality has deteriorated due to, for example, a decrease in the received signal strength. The terminal 29 or the base station / gateway earth station 3
8 transmits a control signal requesting handoff. According to the handoff request control signal, the same information signal is simultaneously transmitted via at least one of the cell / spot beams 31 to 36 adjacent to the cell / spot beam 30 where the terminal 29 is currently located.

For example, if simultaneous transmission is performed via cell / spot beam 33 and cell / spot beam 34, an information signal directed to terminal 29 via cell / spot beam 33 and 34 is transmitted as follows. You. Of the base stations / gateway earth stations 38 and 39 that are in charge of the cells / spot beams 33 and 34, respectively,
The signal line is connected to the base station / gateway earth station 39 to which the signal line for the nine directions is not connected. Further, the base station / gateway earth station 39 is assigned the same spreading code and the same frequency as those used in communication via the cell / spot beam 30. The base station / gateway earth stations 38 and 39 modulate the information signal according to the assigned spreading code, frequency and pre-assigned transmission time offsets △ 1 'and △ 2' in FIG. Transmit at 14 'and 15'.

The transmission time offset is predetermined for all cells / spot beams and for all cells adjacent thereto. However, the value of each transmission time offset is set to a value in which the delay spread caused by multipath or the like in the cell / spot beam is large. By the way, the terminal 29
Receives information signals transmitted simultaneously through cell / spot beams 30, 33 and 34.

Since these information modulated signals are modulated by the same spreading code and the same frequency, correlation can be obtained by one matched filter 1 and detection can be performed by one delay unit 2 and multiplier 3. is there. Multiplier 3
Are transmitted via cell / spot beam 33 and output 1 obtained via cell / spot beam 30
A composite signal of the outputs 11 and 14 delayed from the outputs 0 and 13 by the arrival time difference △ 1, and the outputs 12 and 15 transmitted via the cell / spot beam 34 and delayed from the outputs 10 and 13 by the arrival time difference △ 2. Is obtained as

Further, it is conceivable that noise is mixed in the process until the terminal outputs an information signal. In FIG. 2, △ 1 and △ 2 are arrival time differences corresponding to transmission time offsets △ 1 ′ and △ 2 ′ that differ depending on the cell / spot beam, respectively. Although the arrival time differences △ 1 and △ 2 do not match the values of the transmission time offsets △ 1 'and △ 2', the distance between the antenna and the terminal provided for each cell is equal, and the When the influence of path fading is low, for example, in the case of a multi-spot beam satellite communication system, the arrival time differences △ 1 and △ 2 can be easily estimated from the transmission time offsets '1 ′ and' 2 ′.

The output of the multiplier 3 is further distributed to branches having delay units 4 and 5 corresponding to the arrival time differences, respectively. In each branch having the delay units 4 and 5, the information signal is extracted with the arrival time differences △ 1 and △ 2, respectively, and synthesized in the synthesizing / switching unit 6 together with the output of the multiplier 3.
The resulting information signal output is a signal whose noise-to-noise ratio is significantly improved compared to the output obtained via one cell / spot beam.

As described above, the soft handoff in the spread spectrum communication system is based on the fact that an information signal modulated using the same spreading code and the same frequency is transmitted via a plurality of cells / spot beams in accordance with different transmission time offsets. This can be done by sending. Also, only one receiver is required in the terminal.

[0024]

Embodiment 2 A second embodiment of the present invention will be described with reference to the drawings.

In this embodiment, it is assumed that the information signal is differentially encoded.

FIG. 4 shows an example of the configuration of a terminal receiver system using the present invention. In the figure, 1 is a matched filter,
2 is a delay unit corresponding to one symbol time of the information signal, 3 is a multiplier, 4 and 5 are delay units corresponding to an arrival time difference generated based on a transmission time offset in each cell, and 6 is a combiner / decoder.
Switchers, 16, 17 and 18 are signal strength determiners, 1
Reference numeral 9 denotes a signal strength reference threshold, and reference numeral 20 denotes a device that controls the output of the combining / switching unit 6 according to the signal strength.

FIG. 3 shows an output example obtained from the multiplier 3 in FIG. In the figure, 7 and 8 are the arrival time differences of the modulated signal, 9 is one symbol time of the information signal, 10, 11, 1
2, 13, 14, and 15 are information signals. FIG. 3 shows an example of a transmission time offset of a modulation signal transmitted via a cell / spot beam using the present invention. In the figure, reference numerals 7 'and 8' denote transmission time offsets of modulated signals transmitted via cell / spot beams, 11 'and 12'.
, 13 ', 14', and 15 'indicate the times at which the modulated signal is transmitted via the cell / spot beam.

In accordance with the position change of the terminal 29 in the cell 30 / spot beam, information modulated by the same spread code and the same frequency via the cell / spot beams 33 and 34 as in the first embodiment. The signal is transmitted according to the predetermined offsets 7 'and 8'
9, the cell / spot beams 30, 33 and 3
4 are respectively 0, △
1 and 受 信 2.

The output of the multiplier 3 is also distributed to a branch having delays 4 and 5 corresponding respectively to the arrival time differences. The output extracted by the output of the multiplier 3 and the arrival time differences △ 1 and △ 2 in each branch having the delay units 4 and 5, respectively, is:
The signal strength is determined by the signal strength determiners 16, 17 and 18 and input to the control device 20. The output of the multiplier 3 and the output via the delay unit are combined in the combining / switching unit 6 when the threshold value 19 is satisfied.

In the receiver system shown in the present embodiment, the intensity of the information signal obtained via the cell / spot beams 30, 33 and 34 is not less than the threshold value except when all the intensity is small. No information signal is input to the combiner / switch 6. In the receiver system according to the first embodiment, all signals are combined regardless of the signal strength of the received information signal, that is, the information signal having a small strength is also subjected to synthesis. The noise-to-noise ratio may not be significantly improved.

In the receiver system according to the present embodiment, only those having a large signal strength are synthesized, so the first embodiment
As a result, an information signal having a higher signal-to-noise ratio is obtained.

As described above, the soft handoff in the spread spectrum communication system uses the same spread code and the same frequency to modulate an information signal through a plurality of cell / spot beams according to different transmission time offsets. This can be done by transmitting and only one receiver is required at the terminal.

[0033]

Embodiment 3 A third embodiment of the present invention will be described with reference to the drawings.

In this embodiment, it is assumed that the information signal is differentially encoded.

FIG. 5 shows a configuration example of a terminal receiver system using the present invention. In the figure, 1 is a matched filter,
2 is a delay unit corresponding to one symbol time of the information signal, 3 is a multiplier, 21 is an information signal integrator related to a predetermined time Tm, and 22 is a signal strength determiner. FIG. 6 shows an output example obtained from the multiplier 3 in FIG. In the figure, reference numerals 7 and 8 denote arrival time differences of modulated signals corresponding to different transmission time offsets depending on cell / spot beams, reference numeral 9 denotes one symbol time of an information signal, reference numerals 10, 11, 12, 13, 14 and 15 denote via a cell. Information signals obtained from the modulated signals reaching the terminal at the shortest distance, 23, 24, 25, 26, 2
Numerals 7 and 28 are information signals obtained from modulated signals that arrive at the terminal via the multipath propagation path.

FIG. 3 shows an example of a transmission time offset of a modulated signal transmitted via a cell / spot beam using the present invention. In the figure, 7 'and 8' are transmission time offsets of a modulated signal transmitted via a cell / spot beam, and 11 ', 12', 13 ', 14' and 15 'are modulated signals via a cell / spot beam. Is transmitted.

As the position of the terminal 29 in the cell / spot beam 30 changes, an information signal is transmitted via the cell / spot beams 33 and 34 as in the first and second embodiments. Signals transmitted via cell / spot beams 33 and 34 and received via the shortest path are assumed to have been received with arrival time differences △ 1 and △ 2, respectively.

The output of the multiplier 3 is also input to an integrator for adding all signals arriving within a predetermined time Tm. In the integrator 21, all signals arriving within a predetermined time, that is, those arriving directly at the terminal via the cell / spot beam, are transmitted from one cell / spot beam and transmitted through several different propagation paths. Signals 23, 24, 25, 26, 27 and 28 corresponding to so-called multipath propagation arriving at the terminal via the terminal are all added.

As the value of the time Tm is larger, more signals included in the time can be added in the integrator 21. However, since noises in a section where there is no signal are also added at the same time, care must be taken in setting the value. I have to do it. Here, the value of the transmission time offset is determined by the delay spread in one cell / spot beam in order to separate and diversity information signals received via multipaths in different cells / spot beams and each cell / spot beam. Need to be set large, but it is not necessary for the terminal to know the value of the transmission time offset in the arrival time difference of each signal.

The output of the integrator 21 and the output of the multiplier 3 are input to the signal strength determiner 22, and the direct output of the multiplier 3 and the output of the integrator 21 having the larger signal strength are selected and output. You. The receiver system shown in this embodiment is effective when the influence of multipath fading is large.

As described above, the soft handoff in the spread spectrum communication system uses the same spread code and the same frequency to modulate an information signal via a plurality of cells / spot beams according to different transmission time offsets. This can be done by transmitting and only one receiver is required at the terminal.

[0042]

According to the present invention, when an information signal is transmitted simultaneously via a plurality of cells / spot beams, a signal obtained by modulating an information signal using the same spreading code and the same frequency is transmitted at a predetermined transmission time offset. , Diversity reception by one receiver is enabled. Therefore, it is possible to reduce the hardware of the terminal and effectively use the spreading code.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration example of a receiver system of a terminal to which the present invention has been applied.

FIG. 2 is a diagram showing an example of differential detection output in an example of a receiver system configuration of a terminal according to the present invention.

FIG. 3 is a diagram showing a conceptual diagram of transmitting an information signal via a plurality of cells / spot beams according to a predetermined time offset when the present invention is applied.

FIG. 4 is a diagram illustrating a configuration example of a receiver system of a terminal to which the present invention has been applied.

FIG. 5 is a diagram illustrating a configuration example of a receiver system of a terminal to which the present invention has been applied.

FIG. 6 is a diagram showing an example of differential detection output in a terminal receiver system configuration example according to the present invention.

FIG. 7 is a diagram showing a configuration example of a cell / spot beam and a base station / gateway earth station in a spread spectrum communication system / multi-spot beam satellite communication system.

FIG. 8 is a diagram showing a configuration example of a terminal receiving system according to a conventional technique.

[Explanation of symbols]

1, 1 ', 1 "Matched filter 2, 2', 2" Delay device 3, 3 ', 3 "Multiplier 4, 5 Delay device 6 Combining / switching device 7, 8 Modulation transmitted from adjacent cell / spot beam Signal arrival time difference 7 ', 8' Time offset for transmitting modulation signal via adjacent cell / spot beam 9 One symbol time of information signal 10, 11, 12, 13, 14, 15 Via cell / spot beam Information signal 10 ', 11', 12 ', 13', 14 ', 15' obtained from the modulated signal reaching the terminal in the shortest distance Time at which the modulated signal is transmitted via cell / spot beam 16, 17, 18 signal Intensity judging device 19 Reference threshold 20 Synthesizing / switching device control device 21 Information signal integrator for a predetermined time Tm 22 Signal intensity judging device 23,24,25,26,27,28 Information signal obtained from a modulated signal transmitted through a communication system and arriving at a terminal via a propagation path different from that of the shortest distance 29 terminal and its users 30, 31, 32, 33, 34, 35, 36 cells / spot Beam 37,38,39,40 Base station / gateway earth station

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Yasuhiko Ito 2-3-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation (56) References JP-A-5-268128 (JP, A) JP 64-7825 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H04Q 7/00-7/38 H04B 7/24-7/26 102

Claims (4)

(57) [Claims] 1. A soft handoff method in a spread spectrum communication system in which a terminal communicates an information signal using a spreading code assigned in a cell or a spot beam, comprising: a relative position change between the cell or the spot beam and the terminal; The same information signal is transmitted via at least one cell adjacent to the cell or at least one spot beam adjacent to the spot beam when the cell or the spot beam is switched via communication caused by the And a soft handoff method for switching a cell in charge of communication or a spot beam by receiving an information signal via at least one adjacent cell or the spot beam and the at least one adjacent spot beam, Adjacent to An information signal transmitted via at least one cell or at least one spot beam adjacent to the spot beam is modulated with the same spreading code and the same frequency as those transmitted via the cell or the spot beam; The terminal transmits an information signal transmitted through the cell and at least one adjacent cell or the spot beam and the adjacent at least one spot beam.
Soft handoff method characterized by receiving by a system receiver. 2. The method according to claim 1, wherein all information signals arriving at the terminal within a predetermined time longer than a transmission time offset predetermined to be allocated to an adjacent cell or an adjacent spot beam are combined. Item 1
4. The soft handoff method according to 1. 3. The method according to claim 1, wherein some of the information signals arriving within a predetermined time longer than a transmission time offset predetermined to be assigned to an adjacent cell or an adjacent spot beam are combined. The soft handoff method according to claim 1. 4. An information signal arriving within a predetermined time longer than a transmission time offset predetermined as previously assigned to an adjacent cell or an adjacent spot beam is extracted. 2. The soft handoff method according to 1.