CN1165170C - Burst signal detecting and AGC method for orthogonal frequency division multiplexing system - Google Patents

Burst signal detecting and AGC method for orthogonal frequency division multiplexing system Download PDF

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CN1165170C
CN1165170C CNB021376174A CN02137617A CN1165170C CN 1165170 C CN1165170 C CN 1165170C CN B021376174 A CNB021376174 A CN B021376174A CN 02137617 A CN02137617 A CN 02137617A CN 1165170 C CN1165170 C CN 1165170C
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signal
power
burst signal
digital
agc
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CN1404309A (en
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军 周
周军
丁颢
周跃峰
罗汉文
宋文涛
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Shanghai Jiaotong University
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Abstract

The present invention relates to a burst single detection and automatic gain control(AGC) method for an orthogonal frequency division multiplex system, which is characterized in that after simulating the operation of orthogonal demodulation and analog-to-digital conversion, the operation of burst signal detection for baseband digital signals is carried out, and the results of the operation of burst signal detection control a digital switch. Meantime, the operation of AGC is also controlled by the results of the operation of the burst signal detection so as to regulate the whole gain of the system. The present invention has the advantages of simple implement method, high detection precision and little computational complexity, and realizes rapid AGC adjustment. The present invention can be used in various OFDM systems, such as digital audio broadcast, digital television ground broadcast, orthogonal frequency division multiplex multi-address systems of wireless local area networks and other systems.

Description

The burst signal detection of ofdm system and auto gain control method
Technical field:
The present invention relates to a kind of burst signal detection and auto gain control method of ofdm system, is a kind of method of carrying out input and carrying out power control in OFDM (OFDM) communication system, belongs to digital communication technology field.
Background technology:
Orthogonal frequency division multiplexi begins to be applied to military domain the sixties in last century, just progresses into civil area after the seventies.Because this modulation system has the very high availability of frequency spectrum, so more application is arranged in wireless transmitting system.OFDM is adopted by digital audio broadcasting in Europe (DAB) and Digital TV broadcasting standard for terrestrial (DVB-T), it has also become the core technology of WLAN (wireless local area network) (ETSIHiperlan/2 and IEEE802.11a) and broadband wireless access (IEEE 802.16) simultaneously, and probably becomes one of key technology of the 4th third-generation mobile communication.
Ofdm system is an a kind of Packet Service system, and system is in wait state when idle, in case there is user data arrival system just to change the transmission and the processing procedure of data over to.Data are received with decision so ofdm system just need carry out the detection of system burst signal, this process is so-called burst signal detection (BurstDetection).
The burst signal detection of OFDM receiving system is in the past finished in radio frequency part, and it is realized by envelope detector and network of electrical bridge.Because the RF envelope detector has certain time delay, when a certain reference voltage of RF envelope detector output greater than network of electrical bridge, when promptly having detected the signal transmission, the useful training signal of some frame header is lost, and is unfavorable for the carrying out of OFDM receiving system frame synchronization, Frequency Synchronization.Simultaneously, RF analogue device ratio is easier to be interfered, and device itself has certain stalling characteristic, causes the burst signal detection accuracy of detection not high, and is affected easily.
Summary of the invention:
The objective of the invention is at the deficiencies in the prior art, propose a kind of burst signal detection and auto gain control method of ofdm system, realize improving the performance of system to the detection of system signal transmission and to the control of entire system power.
For realizing such purpose, in the technical scheme of the present invention, at first signal is received by antenna, transfer the radiofrequency signal that receives to baseband digital signal by modules such as amplifier, analog quadrature demodulation and analog-to-digital conversion then.After analog quadrature demodulation and analog to digital conversion, carry out the burst signal detection of baseband digital signal.Carry out power statistic, calculate the power of signal by digital signal base band, power to digital signal carries out conversion generation derivation sequence then, judge that to deriving from sequence just can detect no signal transmits, and the result who utilizes burst signal detection comes the control figure switch, the work of AGC simultaneously also is subjected to burst signal detection result's control, to reach the effect of Adjustment System entire gain.
Deriving from sequence can have the method for a lot of selections, not only can be relevant with the signal energy of current time, and can be relevant with the signal energy of previous moment.In input, the result of detection also delivers to automatic gain control AGC unit to realize the adjustment to system power.
The present invention includes following concrete steps:
1, received signal and transfer signal to baseband digital signal.
Receiving terminal is sent into band pass filter by the signal that antenna receives,, and signal is amplified, then the radiofrequency signal that receives is converted to baseband signal by first order amplifier by the band pass filter noise filtering that passband is outer.The method of conversion has a lot, in ofdm system, can adopt the method for analog quadrature demodulation (DAQM), radiofrequency signal is directly changed into be baseband signal, again by analog to digital conversion (ADC), with the analog signal conversion that receives is digital signal, to realize digital signal detection.
2, signal power is added up.
With signal power as the detection variable of the burst signal detection statistical property of reaction signal better.Making the digital signal of analog to digital conversion ADC output is S, and this signal is a complex signal, and the power K that obtains transmission signals is as detection variable.When rated output, calculate the power of N data at every turn.N promptly detects data length.
3, signal power is carried out conversion and produce the derivation sequence.
Signal power is carried out proper transformation, with the new derivation sequence U (i) that obtains as the input variable, thereby make input become more simple and clear, also can carry out the detection of signal more effectively by transform sequence simultaneously.
Make U that (i) is for the derivation sequence of K (i), have
U(i)＝K(i)： (1)
U ( i ) = 10 log ( K ( i ) K ( i - 1 ) ) ; - - - ( 2 )
When employing formula (1) produces sequence U (i), to calculate comparatively simply, amount of calculation is very little.Can guarantee that the overall gain of wireless transmission channel remains unchanged substantially by the gain amplifier of adjusting receive channel.This detection mode can obtain good testing result under the little situation of physical transfer environmental change.When used physical transmission channel changes when very fast, this adjustment will expend a large amount of system resource.When employing formula (2) produced sequence U (i), the generation of U (i) was not only relevant with current K value, and also relevant with the K value in the previous moment, amount of calculation is relatively large.But this detection mode has suitable adaptability to the variation of physical channel, can obtain comparatively ideal testing result.
It should be noted that the value that detects data length N is quite important.If the value of N is too small, when existing of short duration multipath phase cancellation or spike to disturb in the signal, system will judge by accident.And the N value is also better to the adaptability of channel variation when big.But the N value directly influences the time-delay of burst signal detection process, if this process is oversize, will cause the AGC adjustment process longer.For overcoming this problem, can be with formula (1), (2) are modified as follows:
U(i)＝K(i-1)+K(i)； (3)
U ( i ) = 10 log ( K ( i - 1 ) + K ( i ) K ( i - 1 ) + K ( i - 2 ) ) ; - - - ( 4 )
So just can address this problem preferably.
4, detection has the no signal transmission.
Can adopt following judgment rule detection signal:
Make W (i) for testing result, represent that this segment signal has transfer of data when W (i)=1, represented this segment signal free of data transmission at W (i)=0 o'clock.The initial value of W (i) is 0 promptly not have signal transmission.Make E 1And E 2Be the upper lower limit value that configures, E is arranged 1＞E 2E 1And E 2The gain of setting and wireless transmission channel relevant.If U (i) 〉=E 1Then W (i)=1, U (i)＜E else if 2W (i)=0 so, otherwise W (i) remains unchanged.
5, utilize the result of burst signal detection to control the AGC process.
Two processes of burst signal detection and AGC are to be mutually related.The startup of burst signal detection process control AGC process and stopping, the AGC process is dynamically adjusted the power that reports of burst signal detection according to collection of letters channel gain, has influence on the work of burst signal detection conversely again.The gain adjustment of AGC output is divided into two-stage, and one-level is as analog gain control, and another level is as digital gain control.The main foundation of two-stage gain allocation is a device property, and wherein main that considers be the gain adjustment range of adjustable gain device, effectively utilize the sensitivity of the word length and the choose reasonable received signal of ADC device for simulation part.
The AGC process is determined to carry out when having begun to receive information in burst signal detection.For simplifying the AGC process, can utilize the signal power of in burst signal detection, adding up.Control strategy can adopt following mode:
1, statistical average power is greater than prescribing a time limit on the power controlling
Then report to the police less than least gain and gain-adjusted step-length sum as adjustable gain, otherwise adjustable gain reduces the gain-adjusted step-length;
2, statistical average power is less than prescribing a time limit under the power controlling
Then report to the police greater than the difference of maximum gain and gain-adjusted step-length as adjustable gain, otherwise adjustable gain increases the gain-adjusted step-length.
AGC process of the present invention has adopted the adjustment mode of variable step.If adopt the adjustment mode of fixed step size, the step-length of adjustment is G Step, convergence rate is slow, generally need power could be controlled within the scope of required control through repeatedly adjusting.The adjustment of variable step then has convergence rate faster, can reach the purpose of adjustment in step seldom.This method is applicable to that some signal power is more steady, and it is less to adjust number of times, and regulating the speed requires occasion faster.
The base-band digital burst signal detection that the present invention carries out, the position in wireless receiver be after analog to digital converter and unlike the RF analog detection after low noise amplifier.
The problem includes: some problems that the burst signal detection of baseband digital signal of the present invention has avoided the conventional method problem, it can not lose useful training signal.Because it is based on digital signal, the precision of detection only is subjected to the long restriction in digital hardware device position.Simultaneously, when the numeric field detection signal, need the statistical signal average power, can be controlled the utilization of AGC adjustment process, reduce AGC and adjust the required time by automatic gain.This its design difficulty of detection method based on baseband digital signal also designs far below the RF analogue detection circuitry.AGC can carry out adaptive control to the power of signal in the system by automatic gain control, makes simulating adjusting range and the precision that part can make full use of ADC, to improve the performance of ofdm system.
The present invention can be applicable to the single carrier ofdm communication system, also can be applicable to the multi-carrier OFDM communication system.
Description of drawings:
Fig. 1 is used for the block diagram of single carrier OFDM receiving system for the present invention.
As shown in the figure, the signal that receives from antenna is through a band pass filter, the noise filtering that filter passband is outer, through the first order power amplifier analog signal is amplified then, through analog quadrature demodulation (DAQM) signal is transferred to base band from radio frequency immediately, analog to digital conversion module then becomes digital signal with analog signal conversion.Next will carry out burst signal detection.At first add up the power of N hits, utilize the power results of statistics to obtain the derivation sequence then, judge again that according to the derivation sequence that produces the no signal transmission is arranged.Control the digital switch of back according to the result who judges and control subsequent conditioning circuit: power amplifier, synchronously and the work of FFT etc. whether, can make subsequent conditioning circuit when no signal, be in resting state like this and reach the purpose of economize on electricity.Simultaneously, according to the result who judges to AGC module reported result.AGC adjusts the result with power and gives former and later two power amplifiers, and AGC just reaches the purpose of pair amplifier power control according to reported result like this.Second level power amplifier is given amplifying signal synchronously/down-converter unit again, and then through FFT with each carrier signal separately.At last signal is sent to follow-up circuit such as deinterleave.
Correlation when Fig. 2 controls AGC for burst signal detection in the ofdm system between each parameter.
As shown in the figure, P_s is the average power of short training sequence after adjusting through AGC.E_up, E_low are respectively power upper limit and the lower limits that system can bear.Scope between P_up and the P_low is for adjusting later dynamic range of signals through AGC.
Embodiment:
Below in conjunction with accompanying drawing and single carrier ofdm communication system Application Example technical scheme of the present invention is further described.
The present invention can be applied in the receiving system of WLAN (wireless local area network) IEEE802.la, and functional block diagram as shown in Figure 1.At receiving terminal, after signal received through antenna, with the noise signal filtering outside the filter passband, filtered signal input first order power amplifier amplified signal through band pass filter.Will pass through analog quadrature demodulation DAQM module through the signal after amplifying, this module functions directly is demodulated to base band with signal from intermediate frequency exactly, this method than in the past multistage demodulation make things convenient for to such an extent that Duo and also efficient also high a lot.In order to carry out digital signal detection in base band, will use analog-to-digital conversion module that analog signal is changed into digital signal, so just can realize that the burst of digital signal detects.In the burst signal detection stage, the digital signal of reception is S (nT), is designated as S (n), and order:
S(n)＝I(n)+jQ(n)
I in the formula (n), when Q (n) is respectively sampling instant nT, I road and Q road output signal.Detection variable K is as follows:
K ( i ) = Σ m = 0 N - 1 S ( i * N + m ) S * ( i * N + m )
Wherein N is each length that detects data when carrying out burst signal detection, consider that from the 802.11a system frame structure selecting N is 8, be that per 8 data are carried out input one time, this helps the detection of the short training sequence beginning with the specified data frame has also been guaranteed the promptness of input simultaneously in the 802.11a system.
For simplicity, the derivation sequence U (i) that is adopted here is: U (i)=K (i).Just think and detected signal when the value of U (i) has surpassed certain threshold value Th.Burst signal detection institute can detected signal strength signal intensity be-70dbm in the system here.Simultaneously, burst signal detection can be opened the reception that digital switch carries out Frame.Synchronously at ordinary times, module such as FFT all is in dormancy shape body, their reception data of just starting working at this moment.
When carrying out input, burst signal detection sends testing result to AGC, and the power of the digital signal that obtains by burst signal detection is controlled the startup of AGC and stopped.AGC makes the power of system be controlled in certain scope according to the power amplifier that the result of power statistic controls on the link.The AGC process adopts the adjustment strategy of variable step.The power statistic length N is 24 triple speed points, and total loop time-delay of AGC is no more than 81 times of fast sampling points.By analog gain control signal short training sequence power is controlled at E_s_up in the scope of E_s_low.In the specific implementation process, there are several important parameters to determine in advance.
1、E_up，E_low
E_low has embodied the sensitivity of system to input.E_low is P_in_low+G, and G is for receiving the gain of path.E_up is the power upper limit that system can bear.
2、P_s，P_s_low，P_s_up
P_s is the average power of short training sequence after adjusting through AGC.P_s_low and P_s_up are the precision that AGC adjusts short training sequence.The foundation that its is set is the error that the AGC gain is adjusted, and mainly considers the error that gain adjustment that the sum of errors of controllable gain amplifier itself provides is instructed.
3、G_max，G_min，G_init
G_max is the maximum gain that system can regulate, and G_min is the least gain that system can decay.G_init is the initialization gain of default.
The relation of these parameters as shown in Figure 2.P_s is the average power of short training sequence after AGC adjusts.And the scope between P_up and the P_low is for adjusting later dynamic range of signals through AGC.
The result of AGC will give dual-stage amplifier respectively to adjust their power.Second level power amplifier is given amplifying signal synchronously/down-converter unit again, and then through FFT with each carrier signal separately.At last signal is sent to follow-up circuit such as deinterleave.

Claims (4)

1, a kind of burst signal detection of ofdm system and auto gain control method is characterized in that comprising the steps:
1) with received signal by the band pass filter noise filtering that passband is outer, and after by the first order power amplifier signal being amplified, adopt analog quadrature demodulation DAQM method, transfer radiofrequency signal to baseband signal, by analog to digital conversion ADC, be digital signal with analog signal conversion;
2) baseband digital signal to analog to digital conversion ADC output carries out power statistic, calculates the power K (i) of signal, when rated output, calculates the power of N data at every turn, and N promptly detects data length;
3) signal power is carried out conversion, produces according to U (i)=K (i) and derive from sequence U (i), and with U (i) as the input variable;
4) detect the no signal transmission is arranged, make W (i) be testing result, represent that this segment signal has transfer of data when W (i)=1, represented this segment signal free of data transmission at W (i)=0 o'clock, the initial value of W (i) is 0, as U (i) 〉=E 1W (i)=1 then is as U (i)＜E 2W (i)=0 then, otherwise W (i) remains unchanged, wherein E 1And E 2Be the upper lower limit value that configures, E is arranged 1＞E 2, burst signal detection result is in order to the control subsequent conditioning circuit, and send automatic gain control AGC module;
5) automatic gain control AGC process adopts the adjustment mode of variable step, and power is adjusted the result give the front and back stages power amplifier, and control amplifier power.
2,, when it is characterized in that signal power carried out conversion, produce according to U (i)=K (i-1)+K (i) and to derive from sequence U (i) as the burst signal detection and the auto gain control method of the said ofdm system of claim 1.
3, as the burst signal detection and the auto gain control method of the said ofdm system of claim 1, when it is characterized in that signal power carried out conversion, according to U ( i ) = 10 log ( K ( i ) K ( i - 1 ) ) Produce and derive from sequence U (i).
4, as the burst signal detection and the auto gain control method of the said ofdm system of claim 3, when it is characterized in that signal power carried out conversion, according to U ( i ) = 10 log ( K ( i - 1 ) + K ( i ) K ( i - 1 ) + K ( i - 2 ) ) Produce and derive from sequence U (i).
CNB021376174A 2002-10-24 2002-10-24 Burst signal detecting and AGC method for orthogonal frequency division multiplexing system Expired - Fee Related CN1165170C (en)

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