JPH06276176A - Cdma communication system - Google Patents

Abstract

PURPOSE:To reduce intra-signal interference at the time of demodulating signals from respective remote stations and to solve a perspective problem by preparing plural chip rates and appropriately allocating them for the respective remote stations. CONSTITUTION:When the power level of reception signals initially detected by a reception power detection part 5 is as showh by a figure (b) for the signals from the remote stations RS1 and RS2 inputted to the spectrum inverse spread demodulation part 4 of a base station BS3, a chip rate deciding blade 6 judges that the reception power level of the RS1 is to be strong interference in the inverse spread demodulation of the signals of the RS2. Then, the present chip rate Cj of the RS1 is changed and decided to be Ci lower than the Cj and the RS1 is informed from a chip rate informing part 7. In the RS1, a spreading code is generated corresponding to the chip rate Ci informed from the BS by a spreading code generation part 8, is supplied to a spectrum spread modulation part 9 to perform spectrum spread modulation and is transmitted to the BS. Thus the reception power level from the RS1 and RS2 in the BS is turned to be as shown by the figure (c,) the BS performs an inverse spread processing by the chip rate Cj and the interference is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a base station and a remote station,
C between a mobile station or a slave station connected via a satellite
The present invention relates to a CDMA communication system for improving the line quality and increasing the system capacity in a wireless communication system that performs multiple access by the DMA system.

[0002]

2. Description of the Related Art In a CDMA communication system, a transmitting station digitally modulates a carrier wave with digital data (baseband signal) of a modulated signal, and further spread spectrum modulation is performed by using a spreading code having a different code sequence for each channel. The station on the receiving side performs inverse spread spectrum demodulation on the received signal using the spread code corresponding to the channel, and further digitally demodulates to restore digital data.
Multiple stations are multiple-accessed by multiple channels with different spreading codes. The remote stations are connected to each other via the base station.

In a radio communication system in which a base station and a plurality of remote stations communicate by making multiple connections by a CDMA communication system, the transmission power of each remote station is the same in the uplink for transmitting from the remote station to the base station. However, due to the difference in the distance from each remote station to the base station, the received electric field strength at the base station varies depending on the remote station. This problem is called the perspective problem, and an example is shown in FIG.

FIG. 9A shows a physical arrangement relationship between the base station BS and the two remote stations RS1 and RS2. Here, RS1 is arranged at a position closer to BS than RS2. In this case, the transmission power level of the spread spectrum signal in each of BS, RS1 and RS2 is set to the same +10, and the radio wave propagation loss of the link between RS1 and BS is -2, R.
If the radio wave propagation loss of the link between S2-BS is -6,
As shown in (b) of FIG. 9, in the downlink having RS1 as a reception point, RS1 having BS as a transmission source and RS2 for BS1 as a transmission source.
Received field strength at RS1 of incoming radio waves (incoming level)
Are +8 respectively. Similarly, as shown in (c) of FIG. 9, in the downlink having RS2 as a reception point, BS, R
RS for S1 and RS2 radio waves with S1 as the transmission source
The received electric field strength at 2 is +4. On the other hand, as shown in (d) of FIG. 9, in the uplink where the BS is the reception point, the received electric field strengths at the BS of the power sources whose transmission sources are RS1 and RS2 are +8 and +4, respectively.

FIG. 10 shows the remote stations RS1 and RS2 and the base stations BS which are received in FIG. 9 (b), (c),
It shows the band and level of the demodulated signal when the signal of the received electric field intensity shown in (d) is subjected to spectrum despread demodulation.

[0006] FIG. 10A shows a signal obtained by demodulating a spread spectrum signal with a reception level of +8 transmitted from the BS by the RS1 and a reception level of a radio wave for RS2 serving as interference noise +8.
And a signal as a result of despreading the spread spectrum signal of FIG. FIG. 10B shows a signal in which RS2 demodulates a spread spectrum signal of reception level +4 transmitted from BS and R.
The result of despreading the spread spectrum signal of the reception level +4 of the radio wave for S1 is shown. In FIG. 10C, BS is R
Received level +8 transmitted from S1 demodulated spread spectrum signal and received level transmitted from RS2 +
4 shows the result of despreading the spread spectrum signal of FIG. (D) of FIG. 10 is obtained by replacing the signal of (c) of FIG. 10 with the demodulated signal of the spread spectrum signal of the reception level +4 transmitted from RS2 and the spectrum of the reception level +8 transmitted from RS1. The result of despreading the spread signal is shown.

Assuming that the transmission power of each station is the same, as shown in FIG. 9 (d), the received electric field strength of the signal from the remote station RS1 located near the base station BS is far from the base station BS. It becomes stronger than the received electric field strength of the signal from the remote station RS2 located. Therefore, when demodulating a signal of RS2 having a weak received electric field strength, a signal of RS1 having a strong received electric field strength has a small level difference with respect to the demodulated signal as shown in FIG. Become.

The near-far problem is that, as described above, signals of various power levels exist in the band, and when demodulating a low power signal, the interference power is large and the line quality deteriorates. By the way, the line quality is determined by the ratio of the signal power of the desired signal to the interference noise power (hereinafter referred to as SIR) after despreading the received signal on the receiving side. Here, when the signal after despreading is called an information modulation signal, the SIR of the signal after band limitation by the signal band of the information modulation signal is the processing gain, that is, the bandwidth of the information modulation signal and the band of the spread signal. It is higher by the width ratio. This is because, as shown in FIG. 10, only the desired wave is returned to the original information modulated signal by despreading and the level is increased, but the undesired wave remains diffused.

Originally, in the spread spectrum system, the SIR of the spread signal is very low, but the SIR of the information modulated signal is improved by the processing gain by despreading. In the downlink, since the base station broadcasts to all remote stations at once, the absolute reception level differs depending on the distance between each remote station and the base station, but the relative level of the level to each remote station is large. Is constant. Therefore, the SIR of the information-modulated signal after despreading (the demodulated signal in FIG. 10) is equal in all remote stations. However, in the uplink, the distance relationship between the base station and the remote station is reflected, and in the example of FIG.
Is higher than the reception level of RS2. Therefore, the SIR of the information-modulated signal of each remote station does not become constant, and as shown in the example of FIG. 10D, the signal of the remote station far from the base station strongly interferes with the remote stations near the base station. Therefore, the line quality is deteriorated.

By the way, the CDMA communication system is considered to have the maximum accommodation capacity when the received electric field strengths of all remote stations are equal. Therefore, it is necessary to solve the near-far problem in order not to reduce the accommodation capacity.

Conventionally, in order to solve this problem, the signal power reaching the base station is kept constant at each remote station.
Each remote station was controlling the transmission power. FIG. 11 shows an example thereof, (a) of FIG. 11 is the same as (d) of FIG. 9, and the remote station RS1 in the base station BS is shown.
And the received electric field strength of the signal transmitted from RS2. FIG. 11B shows a state in which control is performed to increase the transmission power of RS2 and the reception electric field strengths of signals from RS1 and RS2 are made equal.

However, in many CDMA systems, the required transmission power control amount may reach as high as 60 to 80 dB, which exceeds the control capability of the transmission power amplifier of a normal remote station and is highly accurate. Since it is difficult to control, it has been a great obstacle in realizing the CDMA communication system.

[0013]

SUMMARY OF THE INVENTION In a CDMA wireless communication system including a base station and a plurality of remote stations, the present invention is for demodulation based on an imbalance of received electric field strength of signals from each remote station in the base station. It is an object of the present invention to provide an effective means other than the transmission power control of a remote station in order to reduce interference between signals and solve the near-far problem.

[0014]

The present invention is based on the following facts. That is, for a signal having a high signal power, the SIR can satisfy the reference line quality even if the processing gain is not large. Conversely, for signals with low signal power, the channel quality cannot meet the standard unless the processing gain is increased. Therefore, in a CDMA communication system sharing the same frequency band,
It can be said that it is meaningful to adaptively change the processing gain according to the received electric field strength at the base station.

On the other hand, the processing gain depends on the chip rate (code rate) of the spread code. That is, as the chip rate increases, the pulse width of the spread code bit becomes narrower and the spread spectrum bandwidth becomes wider. On the contrary, if the chip rate decreases, the pulse width of the spread code bit becomes wider and the spread spectrum bandwidth becomes smaller.

Therefore, by assigning a low chip rate to a remote station having a large signal power and assigning a high chip rate to a remote station having a small signal power,
It is possible to improve the SIR of the signal after despreading and solve the near-far problem.

FIG. 1 is a diagram for explaining the principle of the present invention. Figure 1
(A) shows a basic configuration diagram of the present invention, and 1 is a remote station RS1.

Reference numeral 2 is a remote station RS2. 3 is a base station BS. Reference numeral 4 is a spectrum despread demodulation unit.

Reference numeral 5 is a received power detector for detecting the power of the received signal. Reference numeral 6 denotes a chip rate determination unit that determines an appropriate chip rate Ci in the remote station according to the detected received power.

Reference numeral 7 denotes a chip rate notifying unit for notifying the remote station of the determined chip rate Ci. 8 is a plurality of chip rates C1, C2, which are prepared in advance.
, Ci, ..., Cn is a spreading code generation unit that selects the notified chip rate Ci and generates a spreading code corresponding to the channel at the selected chip rate Ci.

Reference numeral 9 is a spread spectrum modulation unit that performs spread spectrum using the generated spread code. Base station B
RS1, R input to the S spectrum despread demodulation unit 4
For the signal from S2, if the power level of the received signal first detected by the received power detection unit 5 is as shown in FIG. 1 (b), the chip rate determination unit 6 determines that the RS
If it is determined that the received power level of 1 will cause strong interference in the despreading demodulation of the RS2 signal, a decision is made to change to a chip rate Ci lower than the current chip rate Cj of RS1. To notify.

In RS1, the spread code generator 8 determines the chip rate Ci according to the chip rate Ci notified from the BS.
, A spread code is generated and supplied to the spread spectrum modulator 9 to perform spread spectrum modulation.
A spread spectrum signal with a narrower spreading band than before is transmitted to the BS.

As a result, RS1 and RS2 in BS are
The received power level of the signal from is as shown in FIG. 1 (c), and the BS can reduce the interference by despreading at the original chip rate Cj.

[0024]

The function of the present invention will be described with reference to specific examples. Also here, the arrangement of the base station BS and the remote stations RS1 and RS2 shown in FIG. 9A is used.

Actually, an example in which two chip rates are used only for the uplink is shown in FIGS. 9 and 10 of the conventional example.
2 and 3 in a form corresponding to The downlink chip rate is fixed. The chip rate for the remote station RS1 is C1, and that for RS2 is C2
In the following description, C2 = 2 × C1 (1). Further, for simplification, as shown in FIG. 2C, the reception levels L1 and R of RS1 at the base station BS are
It is assumed that the reception level L2 of S2 has a relationship of L2 = L1 / 2 (2). When the information from RS1 is obtained, the power of the information-modulated signal of the signal from RS1 is equal to the processing gain G1 given by G1 = C1 / R (3) due to despreading by C1, where R is the information bit rate. Only grows. At this time, the level and band of the interference wave from RS2 remain unchanged. Therefore, the SIR of the demodulated signal from RS1 is SIR1 = L1 G1 / L2 = L1 C1 / (R L2) = L1 C1 / (R L1 / 2) = 2 C1 / R (4) On the other hand, when obtaining information from RS2, the processing gain G2 by despreading by C2 is G2 = C2 / R (5). Further, the interference wave from RS1 is diffused by C2, so that its level drops to half. Therefore,
The SIR of the demodulated signal from RS2 is SIR2 = L2 G2 / (L1 / 2) = (L1 / 2) C2 / (R L1 / 2) = (L1 / 2) (2 C1) / (R L1 / 2) = 2 C1 / R (6), and the communication quality in the uplink can be made the same even if the transmission powers are different.

3C and 3D are respectively shown in FIG.
The demodulated signal obtained by applying the chip rates C1 and C2 to the received signals from RS1 and RS2 shown in (c) is shown.

[0027]

EXAMPLE An example of the present invention will be described with reference to FIGS. FIG. 4 is a block diagram of an embodiment of the base station. In the figure, 11 is an SS modulator that performs spread spectrum modulation at a specified chip rate, 12 is an SS demodulator that performs spectrum despreading at a specified chip rate, and 13 is a received power detector that detects the level of a received signal. , 14 are control devices, 1
Reference numeral 5 is a chip rate determination device that determines the chip rate based on the reception level. Controller 14 is MPU and RA
The chip rate determination device 15 is a program control device configured by M or the like, and can be configured by a logic circuit or a control table stored in a ROM.

FIG. 5 is a block diagram of an embodiment of a remote station. In the figure, 16 is a control device, 17 is an information modulator for primary modulating a carrier wave by transmission information, and 18 is a spread code at an instructed chip rate. , An SS modulator that performs spread spectrum using the spread code of the designated chip rate, 20 is an SS demodulator with a fixed chip rate, and 21 detects the level of the received signal. It is a level detector. The control device 16 is composed of an MPU, a RAM and the like, like the control device 14 of FIG.

In the base station of FIG. 4, the transmission information is taken in by the control device 14, spread spectrum modulated by the SS modulator 11 and transmitted. The received signal input to the base station is despread demodulated by the SS demodulator 12, and the demodulated signal is output by the control device 14 as received information. At this time, the reception power detector 13 detects the reception level.

Also in the remote station shown in FIG. 5, the transmission information is sent to the information modulator 17 by the control device 16, is primary-modulated, then is spread spectrum by the SS modulator 19, and is transmitted to the base station. The reception signal from the base station is despread and demodulated by the SS demodulator 20, and is output as reception information by the control device 16.

In the remote station, the level detector 21 detects the level of the signal received from the base station. Control device 1
Reference numeral 6 denotes the level information of the detected received signal, which is information modulator 1
7. The SS modulator 19 modulates and transmits, and reports to the base station. Note that the spread code generation unit 18 is first provided with the control device 16
The spread code is generated at the chip rate initialized by the above, and when the controller 16 instructs another chip rate thereafter, the spread code is generated accordingly.

The control device 14 of the base station shown in FIG. 4 operates on the basis of the reception level at the remote station reported from each remote station and the reception level at the base station detected by the reception power detector 13. The reception level of the station is obtained and sent to the chip rate determination device 15. Assuming that the system has two chip rates C1 and C2 as in the above example, the chip rate determination device 15 sets a threshold value for the reception level, and sets a low chip rate C1 to a remote station above the threshold value. , A high chip rate C2 is assigned to the remote stations below the threshold.

The embodiment in which the present invention is applied to the mobile communication system of the CDMA communication system can be realized by the base station having the configuration shown in FIG. 4 and the mobile station having the configuration shown in FIG. However,
Depending on the size of the radio zone covered by the base station and the changing speed of the reception level determined by the moving speed of the mobile station,
It is necessary to determine the time interval for the mobile station to report the reception level to the base station, the cycle for changing the chip rate, etc. so as to suit the system.

For this reason, the control device 14 of the base station manages the reception level fluctuation data for each mobile station according to the time,
The time interval of these reports and the chip rate change period are dynamically determined and controlled.

FIG. 6 shows an outline of an embodiment in which the present invention is applied to a satellite communication system of a CDMA communication system. It is assumed that the base station among them has the configuration of FIG. 4 and the slave station has the configuration of FIG. In a satellite communication system using geostationary satellites, fluctuations in signal level between each slave station in a base station are mainly caused by rain attenuation. Therefore,
In the CDMA satellite communication system according to the present invention, when rain attenuation is detected due to a decrease in signal level, a slave station which is suffering rain attenuation is assigned a higher chip rate than when there is no rain attenuation. This control is performed by the control device 14 of the base station.

When the present invention is applied to a CDMA communication system in which a plurality of types of remote stations having different maximum transmission powers are simultaneously present, such as a small power portable device, a medium power vehicle-mounted device, and a large power fixed station, a base station is used. The configurations of the station and the remote station are the same as those of FIGS. 4 and 5, but it is necessary to further have the following functions. That is, the remote station notifies the base station of information about the maximum transmission power of the local station, and the base station determines the chip rate to be assigned to the remote station from the reception level and the maximum transmission power. 7 and 8 show an example in which the maximum transmission power of each remote station and the ID number of the remote station have a one-to-one correspondence.

The configurations of the base station shown in FIG. 7 and the remote station shown in FIG. 8 are basically the same as the configurations of the base station shown in FIG. 4 and the remote station shown in FIG. 5, respectively. However, in the base station of FIG. 7, the ID-maximum transmission power table 22
Is provided so that the maximum transmission power corresponding to the ID of the remote station can be identified. The remote station of FIG. 8 has the ID information 23, and the ID is set when the communication starts.
Notify the base station of. The control device 14 of the base station refers to the ID-maximum transmission power table 22 by using the notified ID of the remote station RS, and when it is identified that the mobile device has a small transmission power, the chip rate determination device 15 chips. Make the rate change. For a remote station with a large transmission power, the transmission power of the remote station can be controlled within a controllable range, and when the control limit is exceeded, the chip rate can be changed.

[0038]

As described above, according to the present invention, a plurality of chip rates are prepared and appropriately allocated to each remote station, so that the CDM can be viewed from a different viewpoint from the transmission power control.
It is possible to solve the near-far problem in the A communication system.

Since the switching of the chip rate can be realized by a digital circuit, it is highly accurate and highly stable as compared with the transmission power control by the conventional analog system, and is easy to realize. Further, by combining the method of the present invention and the transmission power control, the control range and accuracy can be optimized.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is an explanatory diagram of an example of changing a chip rate according to the present invention.

FIG. 3 is an explanatory view of the operation of changing the chip rate according to the present invention.

FIG. 4 is a configuration diagram of a base station according to an embodiment of the present invention.

FIG. 5 is a block diagram of a remote station according to an embodiment of the present invention.

FIG. 6 is a schematic diagram of a CDMA satellite communication system according to an embodiment of the present invention.

FIG. 7 is a block diagram of a base station according to another embodiment of the present invention.

FIG. 8 is a block diagram of a remote station according to another embodiment of the present invention.

FIG. 9 is an explanatory diagram of a perspective problem in a CDMA communication system.

FIG. 10 is an explanatory diagram of interference of demodulated signals caused by a near-far problem in a CDMA communication system.

FIG. 11 is an explanatory diagram of a transmission power control method that is a conventional solution to the near-far problem.

[Explanation of symbols]

 1 Remote Station RS1 2 Remote Station RS2 3 Base Station BS 4 Spectrum Despreading Demodulation Section 5 Received Power Detection Section 6 Chip Rate Determining Section 7 Chip Rate Notification Section 8 Spreading Code Generation Section 9 Spread Spectrum Modulation Section

Claims (4)

[Claims] 1. In a wireless communication system comprising a base station and a plurality of remote stations, and performing multiple access between the base station and the plurality of remote stations by a CDMA system, the chip rate of the CDMA can be controlled in a plurality of steps. A CDMA communication system characterized in that a chip rate is selected based on a radio wave propagation loss in wireless communication between a base station and a remote station, and CDMA chip rate control is performed. 2. A mobile communication system comprising a base station and a plurality of mobile stations, wherein multiple access between the base station and the plurality of mobile stations is performed by a CDMA system, and a CDMA chip rate can be controlled in a plurality of steps. A CDMA communication system characterized in that a chip rate is selected based on a radio wave propagation loss according to a distance between a base station and a mobile station, and the chip rate is adaptively controlled. 3. A satellite communication system comprising a base station connected via a satellite and a plurality of slave stations, and performing multiple access between the base station and a plurality of slave stations by a CDMA system, wherein the chip rate of CDMA is A CDMA communication system characterized in that it can be controlled in multiple stages and that the chip rate is adaptively selected by selecting the chip rate based on the radio wave propagation loss due to rainfall between the slave station and the satellite. 4. The maximum transmission power of a chip-rate-controlled remote station or slave station when a plurality of types of maximum transmission power of a remote station, a mobile station, or a slave station exist in any one of claims 1 to 3. A CDMA communication system characterized by identifying and selecting an appropriate chip rate.