JP5515494B2 - Transmission power control method - Google Patents

Description

  The present invention relates to a closed loop transmission power control method in a wireless transmission system such as a mobile phone and a wireless LAN.

  The wireless transmission technology is an indispensable technology for the network function of information communication devices because it eliminates the hassle of signal wiring. As for wireless transmission technology, not only wireless LAN devices such as IEEE802.11a / b / g / n but also wireless personal area networks for realizing a ubiquitous network are being actively studied. Such wireless transmission is expected to continue to require higher bit rates for wireless transmission devices in order to exchange high-definition video data and large-capacity data as digital signal technology advances.

  In general, due to the characteristics of radio wave space propagation, the intensity of radio waves attenuates according to the propagation distance. Loss due to propagation distance is called path loss. As for the path loss, the strength of the received signal becomes weaker as the propagation distance is longer. For this reason, in order to receive a reception level that varies greatly depending on the distance at an appropriate level, measures to increase or decrease transmission power according to the reception power and normalization of the reception signal by AGC are adopted. The problem of optimization of transmission power according to path loss is known as a near-far problem, and has become an indispensable technique for reducing interference in a cell in the field of mobile communication.

  As a technique for avoiding this perspective problem, for example, there is a closed loop transmission power control method. In this method, uplink transmission power control data is determined based on a signal-to-interference ratio (SIR) measurement value based on a downlink despreading result. Here, “Signal” indicates the reception power of the desired wave after despreading, and “Interference” indicates the power of the interference wave after despreading, and is one of indexes indicating the reception likelihood in CDMA communication. Then, the received SIR measurement value is compared with a reference SIR given in advance, and if the received SIR measurement value is low, a transmission power control bit is generated to instruct to increase the transmission power of the base station, When the received SIR measurement value is high, a command (transmission power control bit) for instructing to reduce the transmission power is generated. The transmission power control bit is embedded as uplink transmission power control data and transmitted on the uplink. The base station controls the downlink transmission power transmitted in the next slot according to the transmission power control bit sent to the base station on the uplink. A typical closed-loop transmission power control method described here is described in, for example, (Patent Document 1).

  However, in wireless transmission devices that are used indoors, such as wireless LANs and femtocells for mobile phones that are expected to become popular in the future, where the transmitter and receiver are both installed indoors, the propagation distance may vary depending on the installation case. It can be considered to be shorter. In this case, the path loss becomes small and the reception level becomes excessive. Further, in an inexpensive reception AGC having a narrow reception dynamic range, amplitude normalization cannot be performed correctly due to saturation of the reception AGC due to an excessive reception level, resulting in distortion. As a result, there is a problem that a bit error occurs during demodulation and the data throughput is lowered.

  As a prior art for avoiding this problem, for example, there is a transmission power control technique described in (Patent Document 2). According to this, the receiver measures the reception level and the reception quality. If the reception level is high and the reception quality is not good, the receiver is instructed to lower the reception level, thereby causing excessive reception. The level state is avoided. However, even in the transmission power control described in (Patent Document 2), in the case where the path loss is extremely small and the gain control cannot be normally performed due to the saturation of the reception AGC or the like, the correct reception amplitude cannot be obtained. (RSSI value) cannot be measured correctly. As a result, there is a problem that even if the reception likelihood is lowered, the transmission power control cannot be controlled in the direction to be properly narrowed, and the instruction to lower the transmission level from the receiver does not operate normally. That is, if the characteristics of the throughput vs. propagation distance are taken as shown in FIG. 3, there is a problem that the throughput deteriorates in the region II that is relatively close.

JP 2001-308787 A JP 2006-50216 A

  The present invention has been made in view of such problems, and an object of the present invention is to eliminate deterioration of a region (region II in FIG. 3) having a short transmission / reception propagation distance. That is, it solves the problem that the receiving unit cannot saturate the linear amplifier due to excessive input and the reception level cannot be measured accurately. An object of the present invention is to obtain a wireless transmission device that is not affected by the overrange.

The transmission power control method of the present invention is a transmission power control method in a wireless transmission system having a reception unit and a transmission unit, which is transmitted with a predetermined transmission power and measured by the reception unit and a reception error rate estimation value. The transmitter receives the signal strength, creates a correlation distribution between the reception error rate estimated value and the received signal strength, and searches for a minimum value of the reception error rate estimated value in the correlation distribution in subsequent transmission and reception If the received signal strength is larger than the received signal strength corresponding to the minimum value of the reception error rate estimated value , control is performed to reduce the transmission power. Compared with a predetermined threshold value, only when the received signal strength is equal to or higher than the threshold value, it is incorporated into the creation of the correlation distribution .

  The present invention detects the saturation of the received AGC on the transmission side and appropriately sets the transmission power when the transmitter and the receiver are close to each other and the path loss is small in a wireless system (such as a wireless LAN or a femtocell) used indoors. Control. As a result, it is possible to prevent a decrease in throughput by preventing saturation at an inexpensive reception AGC with a small gain control range. Further, by preventing an increase in transmission power due to erroneous transmission power control, it contributes to a reduction in system power consumption and interference power.

The block diagram which showed the transmission power control method in Embodiment 1 of this invention Diagram showing characteristics of received signal strength RSSI _n and received error rate estimate En Characteristic diagram showing the relationship between throughput and distance between transmission and reception The figure which showed one structure of RF receiving part of a receiving side The block diagram which shows the structure of the radio | wireless transmission apparatus which comprises the transmission power control method of this invention

Example 1
(When the received signal strength RSSI _n exceeds the threshold RSSI_th, it is suppressed to reduce the excess.)
In the first embodiment, distribution characteristics of the received signal strength RSSI _n and the reception error rate estimation value En (reciprocal of reception likelihood) are generated based on the transmission power level and the report value of the reception level, and based on the result. The first point is to appropriately control the downlink power control value.

  First, FIG. 5 shows a configuration block diagram of a radio transmission apparatus including the transmission power control method described in the first and second embodiments. Reference numeral 100 denotes a transmitter-side terminal device, 110 denotes a transmitter antenna, 200 denotes a receiver-side terminal device, and 210 denotes a receiver-side antenna. In this figure, user bits are wirelessly transmitted from the terminal device 100 on the transmitter side by the antenna 110 and received by the antenna 210 and the terminal device 200 on the receiver side. The terminal device 200 shows an operation of demodulating user bits and wirelessly transmitting new user bits from the antenna 210 to the transmission antenna 110 together with the received signal strength RSSI and the reception error rate estimation value obtained at that time.

  Next, a transmission power control method in this wireless transmission system will be described with reference to FIG.

  FIG. 1 shows the configuration of the transmission power control method of the present invention. The wireless transmission device includes a portion collectively referred to as the terminal device 100 and a portion collectively referred to as the terminal device 200. Also, the parts collectively referred to as the terminal device 100 are a channel encoder 101, a modulation unit 102, an RF transmission unit 103, an RF reception unit 104, a demodulation unit 105, a channel decoder 106, a distribution characteristic generation unit 107, a transmission power control unit 108, The transmitter antenna 110 is included. The channel encoder 101 performs predetermined convolutional encoding, puncturing, and interleaving processing.

  Also, the parts collectively referred to as the terminal device 200 include an RF receiver 201, a demodulator 202, a channel decoder 203, a received signal strength (RSSI) calculator 204, a channel encoder 205, a modulator 206, an RF transmitter 207, and an antenna 210. Consists of. Here, the terminal device 100 corresponds to a transmission device, and the terminal device 200 corresponds to a reception device. The channel decoder 203 includes a deinterleave process, a depuncture process, and a predetermined error correction process.

  FIG. 4 is a diagram illustrating a configuration of the RF receiving unit 201. The movement of the reception AGC will be described with reference to FIG. Reference numeral 2016 denotes an RF front-end low-noise amplifier, 2017 denotes a down-converter composed of a local oscillator LO and a mixer, 2018 denotes a low-pass filter (LPF) for extracting a baseband signal, and 2019 denotes an input of the A / D converter 2015. A reception AGC unit for controlling the amplitude to be a predetermined target value, 2011 is a variable gain amplifier, 2012 is a power detection unit that calculates power from the signal amplitude of the variable gain amplifier 2011, and 2013 is an A / D conversion unit A difference calculator for calculating the difference between the target value of the input level 2015 and the power value of the power detector 2012. 2014 is a gain for calculating the gain control value to be set in the variable gain amplifier 2011 from the difference of the power value of the difference calculator 2013 It is a calculation part.

  In the transmission unit 100 of FIG. 1, the channel encoder 101, the modulation unit 102, the demodulation unit 105, the channel decoder 106, the distribution characteristic generation unit 107, and the transmission power control unit 108 are collectively referred to as a transmission side baseband processing unit 120. In the reception unit 200, the demodulation unit 202, the channel decoder 203, the channel encoder 101, the bit comparison unit 208, the RSSI calculation unit 204, the channel encoder 205, and the modulation unit 206 are collectively referred to as a reception side baseband processing unit 220.

  Hereinafter, the operation of the wireless transmission device will be described in order from the transmission side.

The terminal device 100 performs a transmission operation. First, user data to be transmitted is input to the channel encoder 101. The channel encoder 101 encodes user data to be transmitted and further performs error correction coding. The data subjected to error correction coding is modulated by the digital modulator 102 and sent to the RF transmitter 103. The RF transmitter 103 amplifies the power to a corresponding modulated signal to transmit power instruction value TxPow _n, transmission via antenna 110.

On the other hand, in the terminal device 200, the RF reception unit 201 receives a reception signal obtained via the antenna 210. In the RF receiving unit 201, the variable gain amplifier 2011 is controlled so as to have a target amplitude so that the A / D converter 2015 is not saturated, so that a predetermined target value is obtained. The configuration of the reception AGC is as described above with reference to FIG. The demodulator 202 is intended to demodulate in accordance with the modulation scheme of the modulator 102 and demodulates the received signal. The channel decoder 203 performs deinterleaving, depuncture, and error correction decoding based on a predetermined encoding specification based on the demodulated signal, and decodes user bits. Further, the decoded user bits are re-encoded by the channel encoder 101 based on a predetermined encoding specification. That is, the reception-side baseband processing unit 220 also generates a bit string that has undergone convolutional coding, puncturing, and interleaving. The bit comparison unit 208 obtains a reception error rate estimation value En by comparing the input bit string of the channel decoder 203 with the re-encoded bit string. On the other hand, the RSSI calculator 204 calculates the received signal strength RSSI _n at the antenna 210 by subtracting the gain value of the RF receiver 201 from the output amplitude of the RF receiver 201. The reception AGC detects the power of the output signal of the variable gain amplifier 2011, feeds back a difference from a predetermined target value, and controls the gain of the variable gain amplifier, thereby making the input level of the A / D converter 2015 constant. There is. The target value here refers to an AGC output or an input value of the A / D converter. However, if the path loss is extremely small and a large signal is added to the variable gain amplifier 2011, the power cannot be correctly detected if an extremely large signal is input to the power detector 2012. Or, if the control range of the variable gain amplifier itself is narrow (in the direction of narrowing the gain), or if the lower limit value of the gain that can be set so that the power detection does not extremely reduce the gain of the variable gain amplifier, In this case, the output signal becomes larger than the target value. In that case, the A / D converter is overranged. Generally, since the received signal strength RSSI _n is calculated from the output of the power detection unit 2012 or the amplitude value of the digital baseband after A / D conversion, saturation occurs.

The channel encoder 205 multiplexes the uplink user bits, the received signal strength RSSI _n obtained by the RSSI calculation unit 204, and the reception error rate estimation value En of the packet signal obtained by the channel decoder 203, thereby error correcting code. Turn into.

  As a method for obtaining the reception error rate estimation value En, a method of creating a bit sequence of an expected value by re-encoding the bit sequence that has been subjected to error correction decoding shown in Reference 1 and collating can be used. The signal subjected to the error correction coding is transmitted to the terminal device 100 via the modulation unit 206, the RF transmission unit 207, and the antenna 210.

In the terminal device 100, the radio wave radiated from the antenna 210 is received by the antenna 110, and reception processing is performed. The signal received by the antenna 110 is amplified and normalized by the RF receiving unit 104 having a reception AGC function, and sent to the demodulation unit 105. The demodulation unit 105 performs demodulation corresponding to the modulation scheme of the modulation unit 206. Subsequently, the demodulated signal is subjected to error correction decoding by the channel decoder 106, and the received signal strength RSSI _n and the reception error rate estimation value En of the packet signal obtained by the channel decoder 203 are decoded.

Subsequently, the distribution characteristic generation unit 107 compares the received signal strength RSSI _n with the threshold RSSI_th. The threshold RSSI_th is set to be lower than RSSI _max obtained below. The receiver samples the received signal strength RSSI _n exceeds the threshold RSSI_th, a received signal strength RSSI _n, reception error rate En is associated, the distribution characteristics shown in FIG. 2 as time series samples are generated. If the received signal strength RSSI _n is less than or _equal to the threshold value, it is considered that there is no saturation in the receiving unit and discarded without reflecting the sample. This processing makes it possible to reduce the amount of memory used for associating distribution characteristics. Hereinafter, it will be referred the transmission power TxPow _n, and the received signal strength RSSI _n, a distribution characteristic Xn combined reception error ratio En. Here, n represents the index number of the received sample corresponding to the transmitted sample.

The distribution characteristic Xn is developed on the memory, and the distribution characteristic generation unit 107 holds from the latest result to a predetermined past sample. Then the minimum value of the reception error ratio En (E _min of FIG. 2) and the corresponding received signal strength RSSI _max, the transmission power TxPow _n is updated by every sample search. Here, the relationship between the transmission power TxPow _n from the transmission power control unit 108 and the received signal strength RSSI _n is related to Equations 1 and 2 below. The distribution characteristic generation unit 107 outputs to the transmission power control unit 108 the result of where the current result is on the distribution characteristic shown in FIG. Subsequently, in the transmission power control unit 108, based on the result of the current position from the distribution characteristic generation section 107, the transmission power correction value ΔTxPow is calculated by detecting the increase of the reception error ratio E _n. First, the current received signal strength RSSI _n is compared with the received signal strength RSSI _{max at} which the minimum value E _min is obtained. As a result, if RSSI _n > RSSI _max , it is determined that the point is on the extension of the point B from the point U, and becomes the target of the transmission power correction value ΔTxPow. In this case, the transmission power correction value ΔTxPow is calculated by (Equation 1).

ΔTxPow _n = k * (RSSI _max -RSSI _n ) (Formula 1)
Here, k represents a coefficient of 1 or more. Expression 1 calculates the difference between the minimum value U of the reception quality and the current Pathloss _n in the sections B to U. Then, the coefficient k (≧ 1) is multiplied to obtain a correction value ΔTxPow _n . Then, control is performed so as to reduce the transmission power by ΔTxPow _n as shown in Expression 2 for the instruction value TxPow _{n + 1} in the next transmission power control.

TxPow _{n + 1} = TxPow _{n + 1} −ΔTxPow _n (Expression 2)
By setting k ≧ 1, the state of point B is reliably controlled on the extended line from point U to point A.

(Example 2)
(Change TxPow _n to large or small to find the minimum point)
In the second embodiment, in order to search for the minimum value E _min in the distribution characteristic Xn of the first embodiment, before performing the transmission gain control of the present invention described in the first embodiment, the transmission power instruction value TxPow _n (RSSI _The point is to generate the distribution characteristics by changing _n ) to a larger or smaller value. This is a wireless LAN and indoor wireless device, the mobile is small semi-fixed communications transmitter and receiver, for variations in path loss is small, the distribution of the distribution characteristics of the RSSI _n vs E _n in the distribution characteristic generation section 107 This is a measure for cases that are difficult to obtain. The feature of this embodiment is that the TxPow _n is changed in advance in order to search for the minimum value E _min to create the RSSI distribution spread, so that the minimum value can be clearly defined even if the distribution characteristic Xn is not sufficiently wide. It can be discriminated and can be controlled without providing a threshold. Even when the minimum value E _min varies from terminal to terminal, optimal control can be performed individually.

  A method for controlling transmission power in this wireless transmission system will be described with reference to FIG.

In the initial steps of the communication via the channel search, after the communication path with the terminal apparatus 100 and the terminal device 200 is established, the transmission power controller 108 increases the transmission power TxPow _n from the predetermined level 1 to a predetermined level 2. Then, by the operation described in the first embodiment, the distribution characteristic generation unit 107 generates the distribution characteristic Xn including the reception signal strength RSSI _n (transmission power TxPow _n ) and the reception error rate En. After that, normal communication operation is performed. By this function, even if the terminal device 100 and the terminal device 200 are fixedly installed and it is difficult to obtain the spread of the distribution characteristic Xn, the profile of the point B can be known from the point U in FIG.

  In the AGC configuration of FIG. 4 used in the first and second embodiments, the input signal of the power detection unit 2012 may be obtained from the digital baseband unit after the output of the A / D converter 2015. .

  In the first embodiment and the second embodiment, the reception quality input to the channel encoder 205 has been described with the bit error rate estimation value En of the received packet. However, in addition to this, the ACK / NACK of the packet or the channel decoder 203 A reception likelihood value such as a path metric obtained from error correction processing can be used. In CDMA communication, a despread SIR measurement value in demodulation can be used. A configuration in which the reception quality comes from the demodulation unit 202 is also conceivable.

  In the first and second embodiments, the process is simplified by using the path metric value obtained by the channel decoder 203 and the result of the packet error detection code as the reception likelihood instead of the bit error rate estimation value En. Can be

  The transmission power control method according to the present invention can reduce throughput reduction, reduce system power consumption, and improve the efficiency of repeated use of the same frequency by suppressing transmission power with an inexpensive reception AGC with a small control range. It is useful for a wireless transmission system such as a wireless LAN.

DESCRIPTION OF SYMBOLS 100 Terminal apparatus 101 Channel encoder 102 Modulation part 103 RF transmission part 104 RF reception part 105 Demodulation part 106 Channel decoder 107 Distribution characteristic generation part 108 Transmission power control part 110 Transmission part antenna 120 Transmission side baseband processing part 200 Terminal apparatus 201 RF reception Unit 2011 variable gain amplifier 2012 power detection unit 2013 difference calculator 2014 gain calculation unit 2015 A / D converter 202 demodulation unit 203 channel decoder 204 RSSI calculation unit 205 channel encoder 206 modulation unit 207 RF transmission unit 208 bit comparison unit 210 antenna 220 Receiver baseband processor

Claims (2)

A transmission power control method in a wireless transmission system having a reception unit and a transmission unit, wherein the transmission unit receives a reception error rate estimation value and a received signal strength that are transmitted with a predetermined transmission power and measured by the reception unit. And creating a correlation distribution between the reception error rate estimate and the received signal strength, and searching for a minimum value of the reception error rate estimate in the correlation distribution in subsequent transmission and reception, When the received signal strength is larger than the received signal strength corresponding to the local minimum value, control is performed to reduce the transmission power , and the reception signal strength is compared with a predetermined threshold in creating a correlation distribution, A transmission power control method, which is incorporated into the creation of the correlation distribution only when the signal strength is equal to or greater than the threshold value . 2. The transmission power control method according to claim 1, wherein a path metric value of an error correction process of a channel decoder is used in the correlation distribution in place of the reception error rate estimation value in the receiving unit .

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