JPH09284212A - Spread spectrum communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce interference onto its own station and adjacent cells and to increase a subscriber capacity while eliminating a remote/near problem by adjusting a process gain depending on a distance of a mobile station from a center of a cell. SOLUTION: An information transmission section 10 of a transmission section 1 applies primary modulation to information data consisting of a voice signal, data and an image or the like based on a data clock from a data clock generator 9 to be information data having a prescribed data transmission speed and the resulting data are given to a spread modulation section 11. Furthermore, a PN clock from a PN clock generator 13 is given to a PN generator 12, from which a PN signal 14 with a prescribed spread speed is generated and given to the spread modulation section 11. Then a distance from a base station is estimated, each mobile station adjusts a spread speed based on the estimated distance. The spread speed is adjusted by controlling a clock frequency from a PN clock generator 13 to be supplied to the PN generator 12.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to spread spectrum.
(SS: Spread Spread) communication system, in particular, by CDMA (Code Division Multiple Acccess) communication based on direct spread (DS: Direct Spread) modulation method, the transmission power control method on the upper link is reduced, and The present invention relates to a technique for suppressing interference with adjacent base stations.

[0002]

2. Description of the Related Art Conventionally, in SS communication systems, a direct spread (DS) modulation system in which a spreading code is directly spread and a frequency hopping system in which a carrier is switched by a code string to spread.
(FH) modulation method. In particular, CDMA communication using direct sequence (DS) modulation (hereinafter referred to as DS / CDMA) has a large subscriber capacity, is resistant to multipath fading, and is capable of high quality communication. It has the features of

By the way, the above subscriber capacity is limited by the amount of interference. In particular, in the DS / CDMA system, all users share the same frequency at the same time, so that the so-called near-far problem that occurs from the distance between the base station and the mobile station limits the subscriber capacity and communication quality.

Such a near-far problem is mainly caused by the uplink.
Since it occurs in (transmission from the mobile station to the base station), in order to solve the near-far problem, the transmission power of each mobile station is adjusted so that the signal transmitted by each mobile station becomes the same reception level at the base station, So-called power control is essential.

This will be described more specifically below.

FIG. 11 shows a cell structure in mobile communication, which is usually made up of many cells, but here, in order to simplify the explanation, two cells, cell A and cell B, are shown. It is assumed that it consists of cells. Within the cell A, there is one base station BS _A and two mobile stations MS ₁ and MS ₂ under the control of that base station BS _A , and one mobile station M
S ₁ is located near the base station BS _A and the other mobile station MS ₂
Are located near the service area near the boundary of another cell B, far away from the base station BS _A.

Here, for example, when the mobile stations MS ₁ and MS ₂ both transmit with the same power, the power received by the base station BS _A is propagated based on the distance difference between the mobile stations MS ₁ and MS _2. Due to the loss, the signal from _one mobile station MS ₁ is attenuated less than the signal from the other mobile station MS ₂ . Therefore, when the base station BS _A of the cell A receives a transmission signal from the other mobile station MS ₂ that is far away from the base station BS _A, a strong signal from _one mobile station MS ₁ side causes interference. It becomes a wave.

Therefore, in order to solve such a perspective problem, the base station BS _A of the cell _A has the mobile stations MS ₁ and MS _{2 respectively.}
So that the received power from both mobile stations MS ₁ ,
Transmission power control is performed to control the transmission power of MS ₂ .

That is, in this example, the other mobile station MS
_The transmission power control is performed so that 2 transmits at a higher power than _one mobile station MS ₁ .

Regarding such a method, "Influence of transmission power control error on channel capacity in CDMA mobile communication" (1991 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers B-2)
45), and “Channel Capacity of CDMA Land Mobile Communication System—Study” (IEICE Technical Report RC892-2).

[0011]

By the way, DS / CD
Even when the transmission power control is performed in the MA method, the reception power greatly changes due to multipath fading or the like, or the reception power sharply changes with time due to the moving speed of the mobile stations MS ₁ and MS ₂ , and so on. It is extremely difficult to accurately perform transmission power control in a short time in order to follow such fluctuations, and errors are likely to occur.

If there is such a transmission power control error, the subscriber capacity is greatly reduced as shown in the above-mentioned respective documents. For example, if there is a control error of 1 dB, the subscriber capacity is reduced. Is reduced by about 25%, and a control error of 2 dB reduces subscriber capacity by about 55%.

Therefore, the transmission power control is required to have an accuracy of 1 dB or less, and a very complicated transmission power control method must be established, which is not easy to put into practical use.

Further, when the above transmission power control is performed, the problem of interference arises.

That is, as shown in FIG. 11, when attention is paid to the base station BS _A of one cell A, the mobile station MS ₂ far from the base station BS _A has a large power. When the transmission power control is performed so as to transmit, the reception power from each of the mobile stations MS ₁ and MS ₂ becomes the same, so the near-far problem is solved, but when attention is paid to the base station BS _B of the other cell B , The base station BS _B is a mobile station MS ₂
Are not under control. And this mobile station MS
_{If 2} transmits with a large power, it becomes a strong interference wave to a mobile station (not shown) existing in this cell B, the base station BS _B of the cell B increases the amount of interference, and the subscriber capacity decreases. I will end up.

The present invention has been made to solve the above problems, and in a spread spectrum communication system of the DS / CDMA system, simplifies the transmission power control in the uplink and at the same time as the own station and adjacent cells. It is an object of the present invention to reduce interference with and increase subscriber capacity.

[0017]

In order to solve the above-mentioned problems, the present invention changes the process gain according to whether the mobile station is located near the center of the cell or near the boundary of the cell. The point is that it was done.

More specifically, in the uplink, the cell is divided into a plurality of areas according to the distance from the base station located in the center of the cell to the periphery, and for mobile stations located in the area near the base station, , The process gain is increased by increasing the spreading rate of direct spreading while keeping the data transmission rate constant to widen the spreading bandwidth, while directly increasing the process gain for mobile stations located in the area far from the base station. The feature is that the process gain is increased by reducing the data transmission rate while keeping the diffusion rate of diffusion constant.

As a means for solving the problem, the present invention adopts the following configuration in a spread spectrum communication system of CDMA communication based on the direct sequence modulation method as described in the claims.

That is, in the invention according to claim 1,
In the cell of its own station, provided with a process gain adjusting means for adjusting the process gain according to the distance from the base station located in the center of the cell to the mobile station, to the base station of its own station, and to the adjacent base station It is characterized by suppressing interference.

According to a second aspect of the present invention, in the configuration of the first aspect, the process gain adjusting means directly directs a mobile station located in the vicinity of the base station with a constant data transmission rate. The feature is that the process gain is increased by increasing the diffusion speed of diffusion and widening the diffusion bandwidth.

According to a third aspect of the invention, in the configuration of the first aspect, the process gain adjusting means makes the diffusion rate of direct diffusion constant for mobile stations located far from the base station. The feature is that the process gain is increased by reducing the data transmission speed as it is.

According to a fourth aspect of the present invention, in the configuration of the first aspect, the process gain adjusting means directly applies to a mobile station located in the vicinity of the base station while keeping the data transmission rate constant. While increasing the spreading speed of spreading to widen the spreading bandwidth, by reducing the data transmission speed while keeping the spreading speed of direct spreading constant for mobile stations located far from the base station, It is characterized by increasing the process gain.

According to a fifth aspect of the invention, in the configuration according to any one of the second to fourth aspects, in addition to the process gain adjusting means, a means for selecting a plurality of defined transmission powers is provided. It is characterized by that.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION In this embodiment, for simplification of description, as shown in FIG. 11, two cells, cell A and cell B, are targeted, and a base station is included in one cell A. BS
Two mobile stations MS ₁ , M under the control of _A and its base station BS _A
S ₂ is present, one mobile station MS ₁ is located near the base station BS _A and the other mobile station MS ₂ is far away from the base station BS _A and the service area near the border of another cell B. It is assumed to be located in the vicinity.

However, the present invention is not limited to such an example, and even when a large number of cells exist and a large number of mobile stations having different distances from the base station exist in each cell. The present invention is applicable.

Embodiment 1 FIG. 1 is a block diagram of a DS / CDMA spread spectrum communication apparatus according to Embodiment 1 of the present invention.

The spread spectrum communication apparatus having the configuration shown in FIG. 1 is provided in the base station BS _A and the mobile stations MS ₁ and MS ₂ and comprises an SS transmitter 1 and an SS receiver 2.

In the SS transmitter 1, voice, data,
The information data including an image or the like is first-modulated by the data clock from the data clock generator 9 in the information transmission section 10 to be generated as information data having a predetermined data transmission rate, and then the diffusion modulation section 11 in the next stage. Entered in.

The PN from the PN clock generator 13
When the clock is input to the PN generator 12, P
The N generator 12 generates a PN signal 14 having a predetermined spreading speed, and the PN signal is input to the spread modulator 11.

In the spread modulator 11, the above-mentioned information data 10
Is directly spread-modulated by a PN signal, and the direct-spread modulated signal (hereinafter referred to as SS signal) 15 is converted into a radio frequency by a frequency conversion unit 16 and then amplified by a power amplifier 17 and transmitted from an antenna 18. It

On the other hand, in the SS receiving section 2, the SS signal received by the antenna 19 is amplified by the amplifying section 20 and then converted by the frequency converting section 21 into an intermediate frequency or a baseband frequency, and then the correlation is obtained. After being correlated and synchronized by the section 23, the information is demodulated as information data in the information demodulating section 24 at the next stage.

FIG. 2 shows the base station B in the first embodiment.
The frequency spectrum of the SS signal from the mobile stations MS ₁ and MS ₂ reaching S _A is shown.

In FIG. 11 described above, the base station B of the cell A
The mobile station MS ₁ located near S _A and the mobile station MS ₂ located far from the base station BS _A have the same data transmission rate D ₁
If the data is transmitted at the same spreading rate C ₁ (chip / sec) at (bps), the cell A receives the propagation loss depending on the distance between the base station BS _A and the mobile stations MS ₁ and MS _2. Base station B
Although the power spectrums from the mobile stations MS ₁ and MS ₂ received at S _A are the same in that the bandwidth of the received SS signal is 2C ₁ as indicated by the symbols P ₁ and P ₂ in FIG. 2, respectively. , The power spectrum is from the base station BS _A to each mobile station M
Since it depends on the distance to S ₁ and MS _2, there is a large difference ΔP between both P ₁ and P ₂ .

Therefore, conventionally, the power spectrums received by the base station BS _A should be the same, that is,
The transmission power of each mobile station MS ₁ , MS ₂ is controlled so that ΔP≈0. That is, the other mobile station MS ₂ transmits at a higher power than the _one mobile station MS ₁ .

On the other hand, in the first embodiment, in the mobile station MS ₁ located near the base station BS _A ,
With the data transmission rate D ₁ kept constant, the clock frequency of the PN clock generator 13 is changed from C ₁ (chip / sec) to C.
_By changing to ₂ (chip / sec) (> C ₁ ), the spread modulator 1
In item 1, the diffusion speed when the information data is directly diffused is increased.

In general, the power spectrum is inversely proportional to the spread rate, and the spread bandwidth is proportional to the spread rate. Therefore, increasing the spread rate as described above means Means spread.

This means that, as shown in FIG. 3, a frequency spectrum F (f) obtained by Fourier transforming a rectangular wave f (t) having a one-chip time Tc of a PN sequence (FIG. 3 (a)) (see FIG. b)),
And can be understood from the following equation.

That is, f (t) ← → F (f) = Tc {sin (πfTc) / (πfTc)} (1) Tc = 1 / C (2) where f (t): time width Tc Square wave F (f): Fourier transform of f (t) f: Frequency Tc: Chip time C: Spreading speed (chip / sec), so spreading speed of direct spreading is from C ₁ (chip / sec) to C ₂ If the speed is changed by changing to (chip / sec), the chip time Tc becomes smaller from the equation (2), so that the amplitude of the frequency spectrum F (f) becomes smaller as can be seen from the equation (1) and FIG. 3 (b). As a result, the power spectrum density decreases, while the spreading bandwidth widens from 2C ₁ to 2C ₂ as shown in FIG.

The process gain PG is given by the following equation.

PG = C / D (3) where C: spread rate (chip / sec) D: data transmission rate (bps) Now, the data transmission rate of any mobile station MS ₁ , MS ₂ is D ₁ ( bps), PG ₁ = C ₁ / D ₁ PG ₂ = C ₂ / D ₁ from the equation (3), but since C ₁ <C ₂ , PG ₁ <PG ₂
Equivalently, the process gain has increased from PG ₁ to PG ₂ .

Therefore, in this example, if the spreading speed of the mobile station MS ₁ located near the base station BS _A is changed from C ₁ to C ₂ to increase the speed, the spreading bandwidth is widened and the power spectrum density is increased. Therefore, at the base station BS _A , the received power spectrum from the mobile station MS ₂ existing on the boundary of the cell A and the mobile station M located near the base station BS _A are received.
It is possible to make the received power spectrum from S ₁ almost at the same level and reduce the near-far problem.

As described above, in order to adjust the direct spread rates for the mobile stations MS ₁ and MS _{2 so} that the received power spectrum at the base station BS _A is almost the same, the following adjustments are made. It is possible to adopt means.

Pilot signal transmitted by base station BS _A (or CDMA signal directed to mobile stations MS ₁ and MS ₂ )
Is received by the mobile stations MS ₁ and MS ₂ and the received signal level is detected, so that the distance from the base station BS _A is estimated. Then, based on the estimated value of this distance, the diffusion rate C (c
Provide a means to adjust hip / sec).

The base station BS _A receives the signals transmitted by the mobile stations MS ₁ and MS ₂ , and the base station BS _A determines that the mobile stations MS ₁ and MS ₂ are the base stations B based on the received signal level.
It is determined whether it is located near S _A or at the boundary of cell A. Then, the result of this determination is used as the base station BS _A.
Informs each mobile station MS ₁ , MS ₂ that
Each mobile station MS ₁ , MS ₂ is provided with means for adjusting the diffusion rate C (chip / sec).

Here, the case where the adjusting means is adopted will be described more specifically.

[0047] One base station BS _A in the cell A of transmits (CDMA signals towards or mobile station MS _1, MS ₂₎ pilot signal, the signal is the mobile station MS _1, M
Received at S ₂ .

FIG. 4 is a block diagram showing details of the frequency converter 21 and the correlator 23 of the SS receiver 2.

The frequency conversion unit 21 is composed of a frequency conversion local oscillator 26, a mixer 25, a filter 27, an amplifier 28, and a detector 29. And the detector 2
The output 35 from 9 is the power of the combined wave of all the received signals.
(Received signal power before correlation) is shown.

The output 22 of the frequency conversion unit 21 is an intermediate frequency or baseband frequency SS (CDMA) signal, and is input to the correlation unit 23. Further, the PN code 34 corresponding to the pilot signal or the code unique to the target mobile stations MS ₁ and MS ₂ is generated by the PN code generator 30 and similarly input to the correlation unit 23.

The correlator 31 constituting the correlator 23 correlates the input received SS (CDMA) signal 22 with the PN code 34 generated by the PN code generator 30 to obtain a desired SS signal. Select and output. Then, the output 35 of the correlator 31 is input to the correlation level detector 32, where the signal level of the desired received signal is detected.

Then, based on the result of monitoring the received signal level detected by the correlation level detector 32,
In the SS receiving unit 2, the base station BS _A and each mobile station MS ₁ , M
Estimate the distance to S ₂ .

This distance estimation is performed as follows.

Generally, since the propagation loss in free space is inversely proportional to the square of the distance, the received power is expressed by the following equation.

P ₁ = AP ₀ r ⁻² (4) where P ₀ : transmission power P ₁ : reception power r: distance between transmission and reception (here, distance between base station and mobile station) A: proportional constant In a multipath environment such as mobile communication, it is said that, for example, it is inversely proportional to the fourth power of the distance.
The following equation is applied instead of the equation (4).

P ₁ = AP ₀ r ⁻⁴ (5) Then, in this equation (5), the received power P ₁ can be found by detecting the signal level of the received signal input to the correlation level detector 32, and the transmitted power P ₀ is from base station BS _A to mobile station M
If they are included in the information to be transmitted to S ₁ and MS ₂ in advance, since both P ₀ and P ₁ are known, the distance r between the base station BS _A and the mobile stations MS ₁ and MS ₂ can be calculated. Can be estimated.

In this way, when the distance r from the base station BS _A is estimated, the mobile stations MS ₁ and MS ₂ adjust the spreading speed based on the estimation result of the distance r. As described above, the adjustment of the spreading speed is performed by the PN clock generator 1 to be supplied to the PN generator 12 in the SS transmitter 1 shown in FIG.
3 by controlling the clock frequency.

The above description is for the spread rate control performed by the mobile stations MS ₁ and MS ₂ by an open loop, but the following feedback loop can be further added for more accurate control. .

That is, the base station BS _A receives the SS signal transmitted by the mobile station (here, MS ₁ ) whose spreading rate is controlled as described above. Here, in order for the base station BS _A to receive the SS signal from the mobile station MS ₁ whose spread rate has changed, it is necessary to recognize the changed spread rate.

Therefore, for example, when the mobile station MS ₁ transmits an SS signal to the base station BS _A , as shown in FIG. 5, data is transmitted at a newly changed spreading rate C ₂ (chip / sec). Data A is added to the stage before transmitting B, and the original diffusion rate C ₁ (ch
Send with ip / sec). And for this area A,
The diffusion speed C ₂ (chip / se
c) The information such as the distinction between the mobile stations MS ₁ and MS ₂ and the time length of the area A is included. In this case, it is desirable that the time length of the area A is as short as possible within a range where the information demodulation in the area A is possible and the information demodulation of the area B is not hindered.

When the base station BS _A receives the SS signal from the mobile station MS ₁ , the area A shows the original spreading rate C ₁ (chip
/ Sec), the power spectrum is
It is at a high level as indicated by P ₁ in FIG.
The base station BS _A can sufficiently demodulate the signal in the area A by the normal correlation synchronization.

For example, the mobile station MS ₁ , which uses the PN code used in the area A in the area B after changing the spreading rate,
If it is set to be the same as the PN code unique to MS ₂ , the base station BS _A generates the PN code used in the area A from the PN code generator 30 shown in FIG. Information can be demodulated by correlating at 31.

[0063] Then, because they contain information diffusion rate C ₂ after the change to be used in the area B (chip / sec) in this area A, the base station BS _A is the changed diffusion rate C ₂ ( chip / se
c) can be sufficiently recognized, and the signal level of the SS signal in the area A can be monitored by the correlation level detector 32. The area A that gives information for changing the spreading rate may be a PN code of a type different from the PN code of the mobile station.

Then, the base station BS _A specifies the spreading rate to be set for each mobile station MS ₁ , MS ₂ based on the monitored received signal level. In this example, the PN code unique to each mobile station MS ₁ and MS ₂ is used, and for one mobile station MS ₁ , C ₂ (> C ₁ ) is set as the spreading rate,
The other mobile station MS ₂ is notified of the information of C ₁ as the spreading rate. Alternatively, all mobile stations MS ₁ , MS ₂ such as pilot signals, control signal channels, paging signals, etc.
Each mobile station MS by the channel (PN code) common to
Information can also be sent to ₁ and MS ₂ .

Thus, new spreading rates C ₁ and C ₂ (chip / se) to be set by the mobile stations MS ₁ and MS ₂ from the base station BS _A are obtained.
After obtaining the information of c), the mobile stations MS ₁ and MS ₂ show the adjusted diffusion rates C ₁ and C ₂ (chip / sec) in P shown in FIG.
By setting the N clock generator 13, the final diffusion speed is set.

By the way, in mobile communication, fading occurs due to the existence of multipath. And FIG.
Since one mobile station MS ₁ is located near the base station BS _A , the delay time of the main multipath wave tends to be small.

If the diffusion rate of direct diffusion in _one mobile station MS ₁ is increased from C ₁ to C ₂ as described above, one chip time is from Tc ₁ (= 1 / C ₁ ) to Tc ₂ (= 1 / C). It shortens to ₂ ). Since the time width of the main rope of the autocorrelation waveform corresponds to the time resolution, the time resolution is improved by shortening the one-chip time Tc.

FIG. 6 shows a correlation waveform when despreading is performed in the base station BS _A.

S ₁ (solid line) in FIG. 7A is the diffusion rate C ₁ (chip /
sec) is an example of a deteriorated correlation waveform when multipath fading is present, and is larger than the correlation waveform of only the direct wave shown by S ₂ (broken line) in FIG. You can see that it has collapsed.

On the other hand, FIG. 6B shows the diffusion rate as C ₂ (chip / se
This is an example in which the multipath wave is separated by increasing the speed to c) and increasing the time resolution. S ₃ (broken line) is the direct wave, S ₄₁ , S ₄₂ , S
₄₃ (solid line) is a delayed wave in which multipath fading exists. As described above, the mobile station MS ₁ located in the vicinity of the base station BS _A has improved time resolution because the diffusion speed is increased, and multipath waves S ₄₁ ,
S ₄₂ and S ₄₃ can be separated. In this way, if the multipath waves S ₄₁ , S ₄₂ , and S ₄₃ can be separated, they can be removed, or can be used to compensate for the insufficient strength of the direct wave S ₃ .

On the other hand, since the reception signal from the mobile station MS ₂ located far from the base station BS _A has a long propagation distance, the delay time of the main multipath waves tends to be large. Therefore, it is possible to sufficiently separate the multipath waves without increasing the diffusion speed and keeping the original diffusion speed C ₁ (chip / sec).

A specific example will be given below in which, if the diffusion speed of direct diffusion is increased, the time resolution is enhanced, multipath waves with a short delay time can be separated, and the power spectrum is also reduced.

Here, the data transmission rate D = 50 kbp,
If diffusion rates C ₁ = 5 Mchip / sec and C ₂ = 2OMchip / sec, the chip time lengths Tc ₁ and Tc ₂ can be calculated from the relationship of equation (2): Tc ₁ = I / C ₁ = ₂₀₀ nsec Tc ₂ = 1 / C ₂ = 50 nsec. Therefore, C ₂ = 2OMchip / sec is C ₁ =
It has 4 times the time resolution of 5 Mchip / sec.

Similarly, from the relationship of FIG. 3 and the above equation (1), since the power spectrum is proportional to the one-chip time length Tc (inversely proportional to the diffusion rate C), the diffusion rate is from 5 Mchip / sec to 20 Mchip / sec. By speeding up to sec, the power spectrum becomes 1/4 (6 dB reduction).

As described above, in the first embodiment, the spreading speed is increased for the mobile station MS ₁ located near the base station BS _A , and as a result, the spreading bandwidth is widened and the power spectrum density is reduced. Get lower. Therefore, this mobile station M
SS signal transmitted from the S _1, despite being located in the vicinity of the base station BS _A, the received power spectral density at the base station BS _A is lowered, as a result, each mobile station MS
₁ , the received power spectrum from MS ₂ becomes almost the same level, and the near-far problem is reduced.

In addition, in the first embodiment, since the process gain is increased by adjusting the spreading speed, it is not necessary to perform the transmission power control with high accuracy as in the conventional case. Therefore, multipath fading and mobile station Since the influence of the transmission power control due to the fluctuation of the reception power due to the moving speed of MS ₁ and MS ₂ etc. is reduced, a sufficient subscriber capacity can be secured.

Embodiment 2 FIG. 7 relates to Embodiment 2 of the present invention, in which the base station BS
Power spectrum of SS signals from mobile stations MS ₁ and MS ₂ reaching _A (solid line), and SS received at base station BS _A
The power spectrum (dashed line) after despreading the signal is shown.

Mobile station MS ₂ located at the cell boundary of cell A
Is the data transmission rate D ₁ (bps) and the diffusion rate C ₁ (chip / sec)
When the SS signal is transmitted by the base station BS _A and the SS signal is received by the base station BS _A , the power spectrum thereof is the power spectrum P _{2 of} FIG.
Will be the same as

On the other hand, in the mobile station MS ₂ located at the boundary of the cell A, while keeping the spreading rate C ₁ constant,
When the data transmission rate is reduced to D ₂ (bps) (<D ₁ ) by lowering the clock frequency of the data clock generator 9 in the SS transmission unit 1 shown in FIG. 1, the base station BS _A
The power spectrum P ₄ of the SS signal received at is the same as that of the power spectrum P _{2 described} above, but the process gain PG ₂ is larger than the process gain PG ₁ when the data transmission rate is D ₁ (bps). growing. That is, the above (3)
From the relationship of the equation, PG ₁ = C ₁ / D ₁ PG ₂ = C ₁ / D ₂ , but since D ₂ <D ₁ , PG ₁ <PG ₂
Equivalently, the process gain has increased from PG ₁ to PG ₂ .

As a result, although the power spectrum of the SS signal received by the base station BS _A is the same for both P ₂ and P ₄ , the signal whose data transmission rate is delayed to D ₂ (bps) is despread. The resulting power spectrum P ₄ ′ is larger than the power spectrum P ₂ ′ obtained by despreading the signal having the data transmission rate D ₁ (bps) due to the difference in the process gain and the data transmission rate. That is, the required signal carrier power to noise power ratio characteristic (hereinafter referred to as C / N characteristic) at the base station BS _A is improved.

Therefore, the mobile station MS _{2 in} the vicinity of the boundary of the cell A away from the base station BS _A has conventionally increased the transmission power depending on the distance or the propagation loss and the reception at the base station BS _A. The power is set to be at the same level as the power spectrum P _{1 of one} mobile station MS ₁ , but in the present invention, it is not necessary to transmit a large amount of power to the mobile station MS ₂ , so the base of the cell B is used. The interference with the station BS _B can be greatly reduced.

Therefore, not only the subscriber capacity in the target cell A can be increased, but also the interference with the adjacent cell B is reduced, so that the subscriber capacity in the adjacent cell B is not suppressed. Furthermore, since the C / N characteristic is improved,
It is also possible to strengthen the error correction capability.

As described above, in order to increase the process gain at the base station BS _A by adjusting the data transmission rates D ₁ and D ₂ for the mobile stations MS ₁ and MS ₂ , as described in the first embodiment. It is conceivable to provide the following means as in the case of.

Pilot signal transmitted by base station BS _A (or CDMA signal directed to mobile stations MS ₁ and MS ₂ )
Is received by the mobile stations MS ₁ and MS ₂ and the received signal level is detected, so that the distance from the base station BS _A is estimated. Then, means for adjusting the data transmission rate D (bps) based on the estimated value of this distance is provided.

The base station BS _A receives the signals transmitted by the mobile stations MS ₁ and MS ₂ , and the base station BS _A determines that each of the mobile stations MS ₁ and MS ₂ has a base station B based on the received signal level.
It is determined whether it is located near S _A or at the boundary of cell A. Then, the result of this determination is used as the base station BS _A.
Informs each mobile station MS ₁ , MS ₂ that
Each mobile station MS ₁ , MS ₂ is provided with means for adjusting the data transmission rate D (bps).

Here, the means will be described more specifically.

[0087] The base station BS _A in one cell A towards pilot signal (or mobile station MS _1, MS ₂ C
DMA signal), and this signal is transmitted to each mobile station MS ₁ , MS.
Received in ₂ .

As already shown in FIG. 4, the frequency converter 2
A desired signal of the SS signal having the frequency of 1 is selected by the correlator 23 at the next stage, and subsequently, the signal level of the desired received signal is detected by the correlation level detector 32. Then, based on the result of monitoring the received signal level detected by the correlation level detector 32, the distance between the base station BS _A and the mobile station MS ₁ or MS _{2 in} the SS receiver ₂ is calculated by the above formula (5). Estimate based on.

In this way, when the distance r from the base station BS _A is estimated, each mobile station MS ₁ , MS ₂ adjusts the data transmission rate based on the estimation result of the distance r. As described above, the adjustment of the data transmission rate is performed by controlling the clock frequency of the data clock generator 9 in the SS transmitter 1 shown in FIG.

The above description is the data transmission rate control performed by the mobile stations MS ₁ and MS ₂ by the open loop, but the following feedback loop may be added for more accurate control. .

That is, the base station BS _A receives the SS signal transmitted by the mobile station (here, MS ₂ ) whose data transmission rate is controlled as described above. Here, in order for the base station BS _A to receive the SS signal from the mobile station MS ₂ whose data transmission rate has changed, it is necessary to recognize the changed data transmission rate.

Therefore, for example, when the mobile station MS ₁ transmits the SS signal to the base station BS _A , as shown in FIG. 8, the data F is transmitted at the newly changed data transmission rate D ₂ (bps). The data E is added to the stage before the transmission of the data, and the data E is transmitted at the original data transmission rate D ₁ (bps) before the data transmission rate changes. The area E is preliminarily included with information such as the changed data transmission rate D ₂ (bps) used in the area F, the distinction between the mobile stations MS ₁ and MS ₂ , and the time length of the area E. Keep it. In this case, it is desirable that the time length of the area E is as short as possible within a range in which the information demodulation of the area E is possible and the information demodulation of the area F is not hindered.

Since the area E includes information on the changed data transmission rate D ₂ (bps) used in the area F, the base station BS _A sets the changed data transmission rate D ₂ on the base station BS _A. It can be sufficiently recognized, and the signal level of the SS signal in the area E can be monitored by the correlation level detector 32. The area E that gives information for changing the data transmission rate can be a PN code of a type different from the PN code of the mobile station. Also, the area E
Is an SS signal at the original data transmission rate D ₁ (bps), its power spectrum is at a low level as indicated by P ₂ in FIG. 7, so that the base station BS _A can easily demodulate the signal in the area E. to do the, as shown in FIG. 9, the mobile station MS ₂
May increase the power level of the SS signal to be transmitted in the area E. In this case, since the time length of the area E is short, interference with other cells can be small.

The base station BS _A specifies the data transmission rate to be set for the mobile stations MS ₁ and MS ₂ based on the monitored received signal level.

Thus, the new data transmission rates D ₁ and D ₂ (bp) to be set by the mobile stations MS ₁ and MS ₂ from the base station BS _A are set.
s), the mobile stations MS ₁ and MS ₂ set the changed data transmission rates D ₁ and D ₂ (bps) in the data clock generator 9 shown in FIG. Set the final data transmission rate.

As described above, in the second embodiment, the mobile station MS ₂ located far from the base station BS _A lowers the data transmission rate by keeping the spreading rate constant. The C / N characteristic at the station BS _A is improved. Therefore, even though the mobile station MS ₂ is far away from the base station BS _A, there is no need to output a signal with large power, and the near-far problem can be mitigated. Moreover, as described above, the high-power signal output is unnecessary, so that the mobile station MS ₂ located near the boundary of the cell A also greatly reduces the interference with the base station BS _B of another adjacent cell B. be able to.

Moreover, in the second embodiment, since the process gain is increased by adjusting the data transmission rate, it is not necessary to perform highly accurate transmission power control as in the prior art, and therefore multipath fading and movement are eliminated. Station M
Since the influence of the transmission power control due to the fluctuation of the reception power due to the moving speed of S ₁ and MS ₂ etc. is reduced, a sufficient subscriber capacity can be secured.

Embodiment 3 In Embodiment 1 described above, for the mobile station MS ₁ located in the vicinity of the base station BS _A , the spreading speed of direct spreading is made high while keeping the data transmission speed constant, and the spreading bandwidth is increased. To increase the process gain, and in the second embodiment, for the mobile station MS ₂ located far away from the base station BS _A , the data transmission rate is kept constant while maintaining the direct diffusion rate constant. Although the process gain is increased by reducing the speed of the process, it is also possible to combine both adjusting means.

[0099] That is, in the mobile station MS ₁ located in the vicinity the base station BS _A cell A, the data transmission rate D ₁ (b
ps) to increase the spreading rate C ₂ (chip / sec) —on the other hand, the mobile station M located at the boundary of the cell A away from the base station BS _A.
In S ₂ , the data transmission rate is set to D ₂ (bps) (<D ₁ ) and the spreading rate is set to C ₁ (chip / sec) (<C ₂ ).

[0100] Figure 10, in the third embodiment of the present invention, the power spectrum of the SS signal from the mobile station MS _1, MS ₂ to reach the base station BS _A (solid line), and the base station BS _A
Power spectrum after despreading SS signal received by
(Broken line) is shown respectively.

The power spectrum of the SS signal transmitted from the mobile station MS ₁ located in the vicinity of the base station BS _A of the cell A is P _3, which is the same as the power spectrum P ₃ of FIG. On the other hand, the power spectrum of the SS signal transmitted from the mobile station MS ₂ located at the cell boundary is P _4, which is the same as the power spectrum P _{4 of} FIG. 7.

If these power spectra P ₃ and P ₄ are despread at the base station BS _A , each power spectrum P ₃ ′,
Since P ₄ ′ and the process gain is increased in both cases, more reliable signal reproduction is possible and the amount of interference can be reduced as compared with the cases of the first and second embodiments.

In each of the above-described first to third embodiments, the base station BS
_The process gain is adjusted by changing the diffusion rate and the data transmission rate according to the distance from _A to the mobile stations MS ₁ and MS _{2. In} addition to this process gain adjustment means, Although the means for varying the transmission power at the stations MS ₁ and MS ₂ or the means for selecting a plurality of specified transmission powers is added, the transmission power control is much simpler than the conventional one. Therefore, the amount of interference can be reduced.

[0104]

According to the present invention, the following effects can be obtained.

(1) In the invention according to claim 1, DS
In the / CDMA system, since the process gain is adjusted according to the distance from the center of the cell, there is no need to perform highly accurate transmission power control as in the conventional art. Therefore, multipath fading, the moving speed of the mobile station, etc. The influence of fluctuations in the received power caused by is reduced. Moreover, since it is possible to reduce interference with the own station and adjacent cells, it is possible to solve the near-far problem and increase the subscriber capacity.

(2) In the invention according to claim 2, particularly for mobile stations located in the vicinity of the base station, the spreading rate of direct spreading is made high while keeping the data transmission rate constant and the spreading band is increased. Since the process gain is increased by increasing the width, even when the mobile station is located near the base station, the received power spectrum density at the base station becomes low and the near-far problem can be mitigated. it can.

(3) In the invention according to claim 3, in particular, for a mobile station located far away from the base station, the data transmission rate is reduced while keeping the diffusion rate of direct diffusion constant. Since the process gain is increased, the C / N characteristic is improved, and even a mobile station far away from the base station does not need to output a signal with large power, and the near-far problem can be reduced. Moreover, since it is not necessary to output a high power signal, it is possible to greatly reduce interference with base stations of other adjacent cells.

(4) In the invention according to claim 4, since claims 2 and 3 are combined, both effects of claims 2 and 3 can be obtained at the same time.

(5) Since the invention according to claim 5 is provided with a means for selecting a plurality of specified transmission powers, even if the transmission power control is far simpler than in the past, the amount of interference is reduced. Can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram of a DS / CDMA spread spectrum communication apparatus according to a first embodiment of the present invention.

FIG. 2 is a frequency spectrum of a direct spread signal from each mobile station that reaches a base station in the first embodiment.

FIG. 3 is an explanatory diagram showing a relationship between a rectangular wave and a frequency spectrum obtained by Fourier transforming the rectangular wave.

FIG. 4 is a block diagram showing details of a frequency conversion unit and a correlation unit of the SS reception unit in the spread spectrum communication device having the configuration of FIG. 1.

FIG. 5 is an explanatory diagram showing a configuration of a signal transmitted from a mobile station to a base station in the first embodiment.

[Fig. 6] Fig. 6 shows a correlation waveform due to multipath when despreading is performed. Fig. 6 (a) shows the correlation waveform deteriorated by multipath, and Fig. 6 (b) shows the correlation waveform by increasing the spreading speed. The correlation waveform which isolate | separated the pass wave is shown.

FIG. 7 is a power spectrum of a direct spread signal from each mobile station that reaches a base station and a power spectrum of a direct spread signal received by the base station after despreading in the second embodiment.

FIG. 8 is an explanatory diagram showing a configuration of a signal transmitted from a mobile station to a base station in the second embodiment.

FIG. 9 is an explanatory diagram showing another configuration of the signal transmitted from the mobile station to the base station in the second embodiment.

FIG. 10 is a power spectrum of a direct spread signal from each mobile station reaching a base station and a power spectrum of a direct spread signal received by the base station after despreading in the third embodiment.

FIG. 11 is an explanatory diagram showing a cell structure in mobile communication.

[Explanation of symbols]

1 ... SS transmitter, 2 ... SS receiver, 9 ... data clock generator, 10 ... information transmitter, 12 ... PN generator, 13 ...
PN clock generator, 21 ... Frequency converter, 23 ... Correlator, 30 ... PN code generator, 31 ... Correlator.

Claims (5)

[Claims] 1. In a spread spectrum communication system for CDMA communication based on a direct sequence modulation method, a process gain is adjusted in a cell of its own station according to a distance from a base station located at the center of the cell to a mobile station. A spread spectrum communication system, characterized in that a process gain adjusting means is provided to suppress interference with its own base station and adjacent base stations. 2. The spread spectrum communication system according to claim 1, wherein the process gain adjusting means directly spreads the mobile station located near the base station while keeping the data transmission rate constant. A spread spectrum communication system characterized in that the process gain is increased by increasing the processing speed and widening the spreading bandwidth. 3. The spread spectrum communication system according to claim 1, wherein the process gain adjusting means, with respect to a mobile station located far from the base station, maintains a constant direct spreading rate and a data transmission rate. Spread spectrum communication system characterized in that the process gain is increased by reducing the speed. 4. The spread spectrum communication system according to claim 1, wherein the process gain adjusting means directly spreads a mobile station located near the base station while maintaining a constant data transmission rate. While increasing the spread bandwidth by increasing the speed, the mobile station located far away from the base station increases the process gain by reducing the data transmission rate while keeping the direct diffusion rate constant. A spread spectrum communication system characterized by the following. 5. The spread spectrum communication system according to claim 2, further comprising means for selecting a plurality of defined transmission powers in addition to the process gain adjusting means. Spread spectrum communication system.