JP2004134924A - Link establishment method in spread spectrum communication - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a link establishment method in spread spectrum communication, capable of improving performance of link establishment without executing transmission power control, even when each distance between a radio base station and a plurality of mobile stations differs greatly, in performing communication between the base station and the plurality of mobile stations by using spread spectrum communication. <P>SOLUTION: In a system where the radio base station communicates with the plurality of mobile stations by using spread spectrum communication, in the link establishment method in spread spectrum communication for establishing a communication path between each of the mobile stations and the radio base station, a data rate of information included in a spread spectrum signal transmitted from the mobile station to establish a communication path between the mobile station and the radio base station or a chip rate of a spread spectrum code is automatically changed when a prescribed condition is satisfied, thereby changing a spread spectrum processing gain in the radio base station for a signal to be transmitted from the mobile station. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a link establishment method in spread spectrum communication, and more particularly to control for establishing a link in direct spread spectrum communication, that is, DS-CDMA communication.
[0002]
[Prior art]
In a mobile station or a base station communicating in the DS-CDMA system, for example, a circuit having a configuration as shown in FIG. 3 is used to generate a direct spread modulated signal used for transmission. In this example, it is assumed that PSK (Phase Shift Keying) data modulation is performed.
[0003]
That is, in the case of two-phase PSK modulation, the data modulator 11 multiplies the sine wave carrier by the digital data d (t) to obtain a PSK modulated signal. In the case of the direct spreading method, a direct spreading modulated signal is obtained by multiplying the PSK modulated signal output from the data modulator 11 by a spreading code P (t) as shown in FIG.
[0004]
Here, the change of the spreading code P (t), that is, the chip rate is much faster than the change of the digital data d (t), that is, the bit rate of information. Therefore, as shown in FIG. 7, the spectrum Bp of the direct spread modulation signal has a wider band than the spectrum Bd of the PSK modulation signal.
Although the case of the PSK modulation is shown here, the spectrum (Bd) of the modulation signal output from the data modulation unit is generally narrow even when another modulation method is used, and the modulation signal of the spread processed modulation signal is generally used. The spectrum (Bp) has a wide band.
[0005]
The spread spectrum processing gain Gp when performing spread spectrum communication using such a signal is expressed by the following equation.
Gp = Bp / Bd
When receiving the transmitted direct spread modulation signal and demodulating the data contained therein, first perform a process called despreading to return the wideband spectrum to the spectrum of the PSK modulation signal, and then perform PSK demodulation, The transmitted digital data d (t) can be obtained.
[0006]
In a DS-CDMA communication system, the same frequency band is shared and used by all mobile stations. Therefore, the SI ratio (desired wave reception power to interference power ratio) required to obtain a required reception error rate is obtained. The mobile station capacity that can be accommodated in the base station is determined.
Also, the received signal level of a signal received by the base station varies greatly depending on the location of the mobile station. Generally, a signal from a mobile station closer to the base station is received by the base station at a higher received signal level.
[0007]
When a large number of mobile stations at different distances from the base station transmit signals at the same time, a received signal having a high received signal level at the base station causes great interference with other received signals. , An interference due to the difference in the distance, that is, a “far perspective problem” occurs.
Conventionally, transmission power control has been performed in order to solve this “far-far problem”. That is, as shown in FIG. 5, even when the distance between the base station and each mobile station is different, the transmission power of each mobile station is controlled such that the level of all the received signals received by the base station becomes constant.
[0008]
In this case, as shown in FIG. 5, for any mobile station, the SI ratio of the received signal of the base station after the despreading process is maintained at a constant level. Therefore, the base station can demodulate all the received signals from each of the plurality of mobile stations.
In a DS-CDMA communication system, one or more channels for link establishment are conventionally provided. The mobile station, that is, each user randomly selects an available spreading code from among a plurality of spreading codes that can be used in a control channel for link establishment notified from the base station using a channel for link establishment. One is selected and a link is established (communication path is established) using the selected spreading code.
[0009]
The technique of “far problem” and transmission power control are disclosed in the following “Patent Document 1” and “Non-Patent Document 1”.
[Patent Document 1] JP-A-2001-308787
[Non-Patent Document 1] "Spread Spectrum Communication and Its Application", Hajime Marubayashi, Masao Nakagawa, Ryuji Kono, (IEICE pp180-182 ISBN 4-88552-153-X)
[0010]
[Problems to be solved by the invention]
When the above-described transmission power control is not performed, as shown in FIG. 6, a signal from a mobile station whose distance to the base station is smaller than a signal from a mobile station whose distance to the base station is larger is smaller than that of the base station. Since the received signal level at the station increases, when the base station tries to receive a signal from a mobile station having a large distance to the base station, a sufficient SI ratio cannot be secured after the despreading process.
[0011]
Therefore, there is a high possibility that a link cannot be established even if a mobile station having a large distance to the base station attempts to establish a link. As a result, the capacity of the mobile station that can be accommodated in the base station is greatly limited.
Such a problem can be solved by performing the above-described transmission power control. However, in order to perform transmission power control, the configuration of the wireless communication system is inevitably complicated, and there is a problem that the cost of the system increases and the power consumption increases.
[0012]
The present invention provides a method for transmitting data between a radio base station and a plurality of mobile stations by spread spectrum communication even when the distance between the radio base station and the plurality of mobile stations is significantly different. An object of the present invention is to provide a link establishment method in spread spectrum communication that can improve the performance of link establishment without performing power control.
[0013]
[Means for Solving the Problems]
According to a first aspect of the present invention, in a system in which a wireless base station and a plurality of mobile stations communicate using spread spectrum communication, a link is established in spread spectrum communication for establishing a communication path between each mobile station and the wireless base station. A method for automatically changing a data rate of information included in a spread spectrum signal transmitted by a mobile station to establish a communication path between the mobile station and a radio base station when a predetermined condition is satisfied. And, it is characterized in that the spread spectrum processing gain in the radio base station for the signal transmitted by the mobile station is changed.
[0014]
The spread spectrum processing gain Gp in the radio base station that receives the spread spectrum signal transmitted from the mobile station is expressed by the following equation.
Gp = Bp / Bd
Bp: bandwidth of spread spectrum signal Bd: bandwidth of modulated signal before spreading Bp = 1 / Tp
Bd = 1 / Td
Tp: time corresponding to the change rate (chip rate) of the spreading code p (t) Td: time corresponding to the bit rate (data rate) of the information d (t) to be transmitted The spread spectrum processing gain Gp in the base station is determined according to the data rate, and the spread spectrum processing gain Gp increases as the data rate is reduced.
[0015]
If the spread spectrum processing gain Gp is increased, the SI ratio is improved, so that link establishment is facilitated. However, if the data rate is low, it takes time to transmit information, so it is not desirable to reduce the data rate from the beginning.
According to the first aspect, the data rate is automatically changed when a predetermined condition is satisfied. Therefore, when the link cannot be established due to the shortage of the SI ratio, the spread spectrum processing gain Gp is increased by changing the data rate. However, since the SI ratio is improved, a link can be established.
[0016]
In any case, since it is not necessary to control the transmission power of the mobile station, it is possible to simplify the wireless system, reduce costs, reduce power consumption, and the like.
According to a second aspect of the present invention, in the method for establishing a link in the spread spectrum communication according to the first aspect, the mobile station first allocates a predetermined initial data rate to a data rate of information included in the spread spectrum signal to be transmitted. If a response from the radio base station cannot be detected even after a lapse of a predetermined time from the transmission of the spread spectrum signal, the mobile station changes the data rate to a value smaller than before. .
[0017]
According to the second aspect, when a link cannot be established within a predetermined time, the data rate is changed to a small value, so that the data rate gradually decreases. Therefore, the SI ratio is gradually improved, and a link can be established over time.
According to a third aspect of the present invention, in the link establishment method in the spread spectrum communication according to the second aspect, if a response from the radio base station cannot be detected even if a predetermined minimum data rate is assigned to the data rate, the mobile station performs the spread spectrum communication. And re-selecting a spreading code to be used for the above, and assigning the initial data rate to a data rate of information included in a spread spectrum signal to be transmitted.
[0018]
If two mobile stations simultaneously attempt to establish a link with the base station using the same spreading code, the link may not be established even if the data rate is changed.
According to the third aspect, if a link cannot be established even when the minimum data rate is reached, the spread code is selected again. Therefore, even if two mobile stations use the same spread code at the same time, the time is shortened. Different spreading codes will be used over time, and a link can be established.
[0019]
According to a fourth aspect of the present invention, in a system in which a wireless base station and a plurality of mobile stations communicate using spread spectrum communication, a link is established in spread spectrum communication for establishing a communication path between each mobile station and the wireless base station. A method for automatically setting a chip rate of a spread code used for spread spectrum of a spread spectrum signal transmitted by a mobile station to establish a communication path between the mobile station and a radio base station when a predetermined condition is satisfied. And changing the gain of the spread spectrum processing in the radio base station with respect to the signal transmitted by the mobile station.
[0020]
The spread spectrum processing gain Gp in the radio base station that receives the spread spectrum signal transmitted from the mobile station is expressed by the following equation.
Gp = Bp / Bd
Bp: bandwidth of spread spectrum signal Bd: bandwidth of modulated signal before spreading Bp = 1 / Tp
Bd = 1 / Td
Tp: time corresponding to the change rate (chip rate) of the spreading code p (t) Td: time corresponding to the bit rate (data rate) of the information d (t) to be transmitted The spread spectrum processing gain Gp in the base station is determined according to the chip rate, and the spread spectrum processing gain Gp increases as the chip rate is increased.
[0021]
If the spread spectrum processing gain Gp is increased, the SI ratio is improved, so that link establishment is facilitated.
In the fourth aspect, the chip rate is automatically changed when a predetermined condition is satisfied. Therefore, when a link cannot be established due to a shortage of the SI ratio, the spread spectrum processing gain Gp is increased by changing the chip rate. However, since the SI ratio is improved, a link can be established.
[0022]
In any case, since it is not necessary to control the transmission power of the mobile station, it is possible to simplify the wireless system, reduce costs, reduce power consumption, and the like.
According to a fifth aspect of the present invention, in the method for establishing a link in the spread spectrum communication according to the fourth aspect, the mobile station first allocates a predetermined initial chip rate to a chip rate of a spreading code used for spread spectrum of a spread spectrum signal to be transmitted. If the mobile station does not detect a response from the radio base station after a predetermined time has elapsed after transmitting the spread spectrum signal, the mobile station changes the chip rate to a value larger than before. Features.
[0023]
In claim 5,
If a link cannot be established within a predetermined time, the chip rate is changed to a large value, so that the chip rate gradually increases. Therefore, the SI ratio is gradually improved, and a link can be established over time.
[0024]
According to a sixth aspect of the present invention, in the method for establishing a link in spread spectrum communication according to the fifth aspect, if a response from the radio base station cannot be detected even if a predetermined maximum chip rate is assigned to the chip rate of the spread code, Is characterized by redoing selection of a spreading code used for spread spectrum and assigning the initial chip rate to a chip rate of a spreading code used for spread spectrum of a spread spectrum signal to be transmitted.
[0025]
If two mobile stations simultaneously attempt to establish a link with the base station using the same spreading code, the link may not be established even if the chip rate is changed.
According to the sixth aspect, if the link cannot be established even when the maximum chip rate is reached, the selection of the spreading code is performed again. Therefore, even if two mobile stations are using the same spreading code at the same time, the time is shortened. Different spreading codes will be used over time, and a link can be established.
[0026]
BEST MODE FOR CARRYING OUT THE INVENTION
(First Embodiment)
One embodiment of a link establishment method in spread spectrum communication according to the present invention will be described with reference to FIG. 1 and FIGS. This embodiment corresponds to claims 1 to 3.
[0027]
FIG. 1 is a flowchart showing the link establishment operation of the mobile station in this embodiment. FIG. 3 is a block diagram showing a configuration of a circuit for generating a direct spread modulation signal. FIG. 4 is a block diagram showing the despreading process of the base station. FIG. 5 is a graph showing the SI ratio when transmission power control is performed. FIG. 6 is a graph showing the SI ratio when transmission power control is not performed. FIG. 7 is a spectrum diagram showing the power spectrum of each signal.
[0028]
This embodiment assumes a wireless communication system provided with one base station and n mobile stations as shown in FIG. In addition, DS-CDMA communication is performed between the base station and each mobile station.
That is, the transmitting station generates a direct spread modulated signal using a circuit as shown in FIG. 3, and transmits the direct spread modulated signal as a radio signal. In the receiving station, the received signal is despread and then demodulated to obtain the transmitted digital data d (t).
[0029]
In this example, a signal frequency used in communication (uplink) transmitted from the mobile station to the base station and a signal frequency used in communication (downlink) transmitted from the base station to the mobile station are assigned to different frequency bands. .
When one mobile station establishes a link with the base station to form a new wireless channel, the mobile station performs the operation shown in FIG. This operation will be described below.
[0030]
In the first step S11, the mobile station randomly selects one of a plurality of spreading codes prepared in advance for link establishment as a spreading code to be used.
In step S12, a predetermined maximum data rate Wd is assigned as a data rate (bit rate) DR of digital data (transmission data) used for modulation of the data modulation unit 11 shown in FIG.
[0031]
In step S14, the mobile station uses the one spreading code selected in step S11 to generate a direct-spread modulated signal, that is, a spread-spectrum signal, according to the current data rate DR according to the processing shown in FIG. Transmit to base station for establishment. In this case, the chip rate of the spreading code may be constant.
After transmitting the spread spectrum signal in step S14 for link establishment, the mobile station waits for a response from the base station. At this time, the mobile station performs despreading in the despreading processing unit of the receiving circuit using the spreading code for link establishment selected by itself, and waits for a response from the base station.
[0032]
On the other hand, the base station holds all of the plurality of link establishment spread codes possessed by the mobile station, and the base station is always in a standby state for all the link establishment spread codes.
That is, the base station performs the despreading process in the despreading processing unit 21 for each of the spread codes (1) to (n) as shown in FIG. 4, and generates a spread spectrum signal from each mobile station for each spread code. Attempt to receive.
[0033]
Therefore, the base station can also perform despreading on the spread spectrum signal spread using the spreading code randomly selected by the mobile station, and reproduce the modulated signal before spreading.
For example, when one mobile station (a) attempts to establish a link with a base station using a spreading code (3), one or more other mobile stations (b) have already established a connection with the base station. Assuming that a link has been established between the base stations, a signal as shown in FIG. 4 is obtained at the output of the despreading processing unit 21 of the base station.
[0034]
That is, since the other mobile station (b) that has already completed the link establishment transmits a spread spectrum signal using a spreading code different from that of the mobile station (a), the base station transmits a plurality of mobile stations (a), Although the spread spectrum signal is received at the same time from (b), in the signal despread by the despreading processing unit 21 using the spreading code (3), only the signal from the mobile station (a) is converted into a narrow band signal. The demodulated signal from the other mobile station (b) appears while being spread over a wide band.
[0035]
At this time, if the SI ratio between the signal from the mobile station (a) and the signal from the other mobile station (b) is sufficiently large in the output of the despreading processing unit 21, the base station is switched from the mobile station (a). Can be demodulated to the original data, and a response can be made to the mobile station (a).
That is, if the demodulation of the received data is successful, the base station transmits a spread spectrum signal using the same spreading code as that used by the mobile station (a) for transmission, and transmits the spread spectrum signal to the mobile station (a). Reply for link establishment.
[0036]
At this time, the base station notifies the mobile station that the link has been established, and specifies the spreading code to be used for communication.
Since a signal transmitted by the base station uses a different frequency band from that of the mobile station, a difference in distance does not matter when separating a signal transmitted by the base station from a signal transmitted by the mobile station.
[0037]
Upon detecting a response from the base station in step S16, the mobile station (a) attempting to establish a link proceeds to step S19, and uses the communication spreading code notified from the base station to transmit the current data rate DR. , Transmission and reception of the spread spectrum signal are started.
On the other hand, for example, the distance between the mobile station (a) attempting to establish a link and the base station is larger than the distance between the other mobile station (b) that has already completed the link establishment and the base station. In such a case, since the transmission power control of the mobile station is not performed, a sufficient SI ratio may not be obtained.
[0038]
When the S / I ratio is insufficient, the base station cannot demodulate the transmission signal of the mobile station (a) and does not respond to the mobile station (a).
In this case, the mobile station (a) cannot detect a response from the base station even if the time to wait for a response in steps S15 to S17 has passed a predetermined time Tw, so the process proceeds to step S18.
[0039]
In step S18, the value of the data rate DR is updated to a value smaller than the current value (low speed). In this example, the value of the data rate DR changes to Wd-0.9Wd-0.8Wd-0.7Wd-... 0.01Wd every time step S18 is executed.
When the mobile station changes the data rate DR to a low speed and transmits the spread spectrum signal again, the SI ratio at the output of the despreading processing unit 21 of the base station is improved according to the difference in the data rate DR.
[0040]
For example, if the data rate DR of the mobile station (a) trying to establish a link is reduced to 1/10, the spread spectrum processing gain Gp (= Bp / Bd) at the base station increases by 10 dB, and the SI ratio also increases. It is improved by 10 dB.
As described above, when the link cannot be established, the mobile station gradually changes the data rate DR to a low speed and tries to establish the link again, so that the SI ratio is gradually improved, and the base station receives the signal from the mobile station (a). Can be demodulated. That is, a state is established in which a link can be established.
[0041]
In step S19, communication is started at the data rate DR at the time when the link was established. Therefore, if the data rate DR is changed at the time of link establishment, the data rate DR used in actual communication is also changed.
By the way, when two mobile stations try to establish a link using the same spreading code at the same time, the link cannot be established in either one or both mobile stations even if the data rate DR is changed to a low speed. There is.
[0042]
Therefore, in this embodiment, if a link cannot be established even at the lowest data rate DR, the selection of a spreading code used for link establishment is performed again.
That is, if the condition of (DR <0.01Wd) is satisfied in step S13, the process returns to step S11, and one spread code to be used is randomly selected again from a plurality of spread codes.
[0043]
As described above, since the link is attempted by gradually changing the data rate DR to a low speed, it is possible to establish a link with the base station at the maximum data rate that can be transmitted at that time without performing transmission power control. .
(Second embodiment)
One embodiment of a link establishment method in spread spectrum communication according to the present invention will be described with reference to FIG. This embodiment corresponds to claims 4 to 6.
[0044]
FIG. 2 is a flowchart showing the link establishing operation of the mobile station in this embodiment. This embodiment is a modification of the first embodiment. In FIG. 2, processes corresponding to those in FIG. 1 are denoted by the same reference numerals. The changed parts will be described below. In this embodiment, the above-mentioned data rate DR is kept constant, and control is performed so as to improve the SI ratio at the base station by changing the chip rate CR of the spread spectrum signal transmitted by the mobile station when establishing a link.
[0045]
That is, in the mobile station which is to establish a link, first, a predetermined minimum chip rate Wch is assigned to the chip rate CR in step S22 shown in FIG. 2, and the chip rate CR is reduced.
In step S24, using the spreading code selected in step S11, a direct spread-spectrum modulated signal is generated by a circuit as shown in FIG. 3 according to the current value of the chip rate CR, and this spread spectrum signal is used as a radio signal by the base station. To establish a link.
[0046]
If a response from the base station cannot be detected even after the lapse of the predetermined time Tw, the mobile station updates the value of the chip rate CR to a value larger than the current value in step S28. That is, the chip rate CR is made higher than before.
In this example, every time step S28 is executed, the value of the chip rate CR changes to Wch-1.1Wch-1.2Wch-1.3Wch -...- 100Wch. That is, when the mobile station cannot detect a response from the base station, it updates the value of the chip rate CR to a successively larger value, and transmits a spread spectrum signal at a different chip rate CR each time it is updated.
[0047]
Therefore, the SI ratio at the output of the despreading processing unit 21 of the base station is improved each time the chip rate CR is updated, and when the SI ratio becomes sufficiently large, the base station succeeds in demodulating the data transmitted from the mobile station. Link establishment becomes possible.
By the way, when two mobile stations try to establish a link using the same spreading code at the same time, the link cannot be established in either one of the mobile stations or both mobile stations even if the chip rate CR is changed to a low speed. There is.
[0048]
Therefore, in this embodiment, if the link cannot be established even at the maximum chip rate CR, the selection of the spreading code used for link establishment is performed again.
That is, if the condition of (CR> 100 Wch) is satisfied in step S23, the process returns to step S11, and one spread code to be used is selected again randomly from a plurality of spread codes.
[0049]
As described above, since the link is attempted by gradually changing the chip rate CR at a high speed, it is possible to establish a link with the base station at the minimum chip rate that can be transmitted at that time without performing transmission power control. .
In each of the above embodiments, control is performed such that one of the data rate and the chip rate is fixed and the other is sequentially updated. However, both the data rate and the chip rate are changed to change the data rate and the chip rate. It is also possible to improve the SI ratio.
[0050]
【The invention's effect】
As described above, according to the present invention, it is possible to improve the SI ratio at the base station without performing transmission power control of the mobile station, and establish a link between the mobile station and the base station. Accordingly, transmission power control can be omitted, and simplification of the system configuration, cost reduction, suppression of power consumption, and the like can be achieved.
[Brief description of the drawings]
FIG. 1 is a flowchart illustrating a link establishment operation of a mobile station according to a first embodiment.
FIG. 2 is a flowchart illustrating a link establishment operation of a mobile station according to a second embodiment.
FIG. 3 is a block diagram illustrating a configuration of a circuit that generates a direct spread modulation signal.
FIG. 4 is a block diagram showing a despreading process of the base station.
FIG. 5 is a graph showing an SI ratio when performing transmission power control.
FIG. 6 is a graph showing an SI ratio when transmission power control is not performed.
FIG. 7 is a spectrum diagram showing a power spectrum of each signal.
[Explanation of symbols]
11 data modulation section 12 diffusion processing section 21 despreading processing section

Claims (6)

In a system in which a wireless base station and a plurality of mobile stations communicate using spread spectrum communication, a link establishment method in spread spectrum communication for establishing a communication path between each mobile station and the wireless base station,
The mobile station automatically changes the data rate of the information included in the spread spectrum signal transmitted by the mobile station to establish a communication path between the mobile station and the radio base station when a predetermined condition is satisfied, A method for establishing a link in spread spectrum communication, comprising changing a spread spectrum processing gain in a radio base station for a signal to be transmitted. The link establishment method in spread spectrum communication according to claim 1,
The mobile station initially allocates a predetermined initial data rate to a data rate of information included in the spread spectrum signal to be transmitted,
If the mobile station cannot detect a response from the radio base station after a predetermined time has elapsed after transmitting the spread spectrum signal, the mobile station changes the data rate to a value smaller than before. Characteristic link establishment method in spread spectrum communication. The link establishment method in spread spectrum communication according to claim 2,
If a response from the radio base station cannot be detected even if a predetermined minimum data rate is assigned to the data rate, the mobile station reselects a spread code used for spread spectrum and transmits the initial data rate. A link establishment method in spread spectrum communication, characterized by allocating to a data rate of information included in a signal. In a system in which a wireless base station and a plurality of mobile stations communicate using spread spectrum communication, a link establishment method in spread spectrum communication for establishing a communication path between each mobile station and the wireless base station,
Automatically change the chip rate of a spreading code used for spread spectrum of a spread spectrum signal transmitted by the mobile station to establish a communication path between the mobile station and the radio base station when a predetermined condition is satisfied, A method for establishing a link in spread spectrum communication, comprising changing a spread spectrum processing gain in a radio base station for a signal transmitted by a mobile station. The link establishment method in spread spectrum communication according to claim 4,
The mobile station initially allocates a predetermined initial chip rate to a chip rate of a spreading code used for spread spectrum of a spread spectrum signal to be transmitted,
If the mobile station does not detect a response from the radio base station after a predetermined time has elapsed after transmitting the spread spectrum signal, the mobile station changes the chip rate to a value larger than before. Characteristic link establishment method in spread spectrum communication. The link establishment method in spread spectrum communication according to claim 5,
If a response from the radio base station cannot be detected even if a predetermined maximum chip rate is assigned to the chip rate of the spreading code, the mobile station reselects a spreading code used for spectrum spreading and transmits the initial chip rate. A link establishing method in spread spectrum communication, wherein the link rate is assigned to a chip rate of a spreading code used for spread spectrum of a spread spectrum signal to be transmitted.

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