CN1838539A

CN1838539A - Application method for frequency synthesizer in OFDM system

Info

Publication number: CN1838539A
Application number: CN 200610039843
Authority: CN
Inventors: 刘进; 田玲; 蒋政波; 徐文虎; 严频频
Original assignee: NANJING DONGDA BROADBAND COMMUNICATION TECHNOLOGY Co Ltd
Current assignee: NANJING DONGDA BROADBAND COMMUNICATION TECHNOLOGY Co Ltd
Priority date: 2006-04-24
Filing date: 2006-04-24
Publication date: 2006-09-27

Abstract

This invention relates to the application method of frequency synthesis device in OFDM system, which is applied extensively in OFDM communication system, and it uses DDS combining with phase-locked loop to provide radio-frequency radical frequency for the frequency circuit in OFDM system. The frequency part directly in OFDM communication system adjusts carrier frequency deviation of the sender and collector. The advantage in this invention is that it has higher frequency differential rate, better phase noise character and wider frequency restricted portion and so on; it changes the output frequency rate of the frequency synthesis device to decrease the communication system carrier frequency deviation and lower the complexity of the synchronous algorithm of the base strip portion. The frequency synthesis device's trimming is obtained by changing DDS's frequency control character with microprocessor. This invention also designs elliptic low pass filter and low loop filter with fountain to provide better phase noise and frequency restricted portion character.

Description

The application process of ofdm system medium frequency synthesizer

Technical field

But the present invention is a kind of frequency synthesizer of extensive use ofdm communication system.This frequency synthesizer has high frequency resolution and phase noise characteristic preferably, can regulate carrier frequency offset in the communication system according to client's demand, reduces the complexity of ofdm communication system baseband portion synchronized algorithm.

Background technology

OFDM (orthogonal frequency division multiplexi) is the high speed transmission technology under a kind of wireless environment.As everyone knows, the frequency response curve of wireless channel is non-flat forms mostly, and the main of OFDM technology is divided into many orthogonal sub-channels with given channel exactly in frequency domain, uses a subcarrier to modulate on each subchannel, and each subcarrier parallel transmission.Like this, although total channel be non-flat forms, have frequency selectivity, but each subchannel is a relatively flat, what carry out on each subchannel is narrow band transmission, and signal bandwidth is less than the respective bandwidth of channel, so the just interference between the erasure signal waveform greatly.Because the carrier wave of each subchannel is mutually orthogonal in ofdm system, so their frequency spectrum is overlapped, has so not only reduced the phase mutual interference between subcarrier, improved the availability of frequency spectrum simultaneously again.But also there is defective in the OFDM technology: very sensitive to frequency shift (FS) and phase noise; Peak value and average power are relatively large, and the increase of this ratio can reduce the efficient of radio frequency amplifier.

In mobile communication system, particularly in ofdm system,, thereby caused the bigger carrier frequency offset of the existence in the communication system owing to the different reference source of use, if frequency departure exceeds certain scope, communication system can't operate as normal.Simultaneously, because the existence of frequency departure, the baseband portion of communication system also need spend more resource and be used for system synchronization, and this has just increased the complexity and the cost of system greatly.In order to reduce the influence of carrier frequency offset to communication system performance, normally by adjusting sending and receiving system reference clock VCXO output frequency separately, reducing frequency departure, but this adjustment technology, frequency resolution is not high.

The synthetic DDS of numerical frequency is that the sampling address that the counting sequence generator by a linearity is produced scans the sine waveform memory that has obtained, and then periodically read data in the wave memorizer, its output just can be synthesized a sine wave signal complete, that have certain frequency by digital to analog converter and low pass filter.It mainly is made of standard reference frequency source, phase accumulator, wave memorizer, digital to analog converter, low pass smoothing filter.Under the control of clock pulse, frequency control word K obtains corresponding sign indicating number mutually by accumulator, sign indicating number addressing waveforms memory carries out the different amplitude coding of sign indicating number-width of cloth code conversion output mutually mutually, obtain corresponding staircase waveform through digital-to-analog converter again,, promptly obtain continually varying output waveform by frequency control word K decision after low pass ripple device carries out smoothly staircase waveform.Wherein, reference frequency source generally is the crystal oscillator of a high stable, and its output signal is used for each parts synchronous working of DDS.Therefore, the frequency stability of the composite signal of DDS output is the same with crystal oscillator.Phase accumulator is the core that realizes DDS, and it is made up of binary adder and the N position phase register by fixed clock pulse sampling of a N position word length.The output of phase register links to each other in inside with an input of adder, and another input of adder is the frequency control word K of outside input.Like this, when each clock pulse arrives, phase register the sample value and the frequency control word K sum of phase register in the last clock cycle, and as the output of phase accumulator in this clock cycle.

When the frequency synthesizer operate as normal, under the control of standard frequency reference source (frequency control word K has determined the respective phase increment), phase accumulator then constantly carries out linearity to this phase increment and adds up, when amassing will produce once when completely measuring, phase accumulator overflows, thereby finish the action of one-period, this action cycle promptly is a frequency period of DDS composite signal.So the frequency of signal output waveform and frequency resolution can be expressed as follows:

fout＝Kfc/2` fmin＝fc/2`

In the formula: fout is an output signal frequency; Fmin is an output signal resolution; K is a frequency control word; N is the phase accumulator word length; Fc is a standard reference frequency source operating frequency.

By following formula as can be known, the DDS output signal frequency depends primarily on frequency control word K, the frequency resolution of phase accumulator word length N decision DDS.When K increased, fout can constantly improve, and by sampling theorem, the highest output frequency must not be greater than fc/2, but the work output frequency is when reaching the 40%fc left and right sides, and the phase jitter of output waveform is just very big.But the digitized processing of DDS has also been brought unfavorable factor, and abundant is spuious along with basic frequency is exported together, makes to reduce the spuious subject matter that becomes.

The spuious source of DDS mainly contains the factor of following three aspects:

(1) error introduced of D/A converter, it is that non-ideal characteristic by the D/A converter causes.The non-ideal characteristic of DAC has: peak current, the switching rate in non-linear, the D/A transfer process of difference, integration is limited etc.;

(2) error caused by finite word length of ROM stored data.Because the figure place of ROM storage is limited D, so will produce quantization error in the amplitude quantization process;

(3) error that causes of phase place truncation.In DDS, the figure place L of general phase accumulator is much larger than the addressing bit number W of ROM, so during the output addressing ROM of accumulator, its L-W low level just must be cast out, and so just produces phase error inevitably, is called Phase Truncation Error.

Spuious in order to reduce, can not utilize the very wide advantage of DDS relative bandwidth fully, can only select one section spuious lower finite bandwidth among the DDS, expand its upper limiting frequency by frequency multiplication.Here it is adopts DDS to add the reason of frequency multiplication in order to obtain the broadband signal waveform.The method of spread bandwidth has a lot, can utilize the direct frequency multiplication of frequency multiplier, the multiplier frequency multiplication, utilize the image-reject mixer branch to get upper and lower sideband, utilize the output of DDS quadrature synthetic, DDS and mixer combination, DDS and phase-locked loop combination, and the method for multidiameter delay DDS.

Because the index of DDS frequency shift (FS) and phase noise is also bad, so the frequency synthesizer of ofdm system generally adopts conventional simulation circuit (phase-locked loop circuit) that the frequency source of radio frequency or fundamental frequency is provided.

CN98810942.5 adopts Direct Digital quadrature modulation synthetic and up-conversion to disclose a kind of transmitter subsystem (100 and 200), and the DDS device (202) that is used in transmitter subsystem is covered with the frequency that obtains big radio frequency band by frequency multiplication.In an embodiment, the DDS device constitutes like this: the DDS output signal spectrum can be converted directly to radio frequency or intermediate frequency by direct frequency multiplication.Requirement to the DAC in the DDS device has reduced like this, because the sampling frequency of DAC (108 and 116) is more much lower than the prior art.In addition, because the big frequency difference that produces between the output of needs and mirror image frequency band, the requirement that is used for the band pass filter (114,122,208) that the mirror image of up conversion suppresses has also reduced.In a second embodiment, DDS device (302) is to constitute like this: the multiple of baseband frequency spectrum can be finished at intermediate frequency.Therefore, according to the present invention, can be used on high-frequency with more uncomplicated or cost DAC and produce broadband signal but still can keep high linearity.

CN03813952.9 quadrature modulator amplitude imbalance compensation discloses a kind of reflector that is used to modulate the RF carrier signal with input digital data stream.Described reflector has: processor is used for producing the digital phase signal and the digital amplitude signal that characterize described input digital data stream; Signal generator is used for producing the RF carrier signal that has by the phase characteristic of the digital phase signal definition that is produced; Amplifier is used for receiving described RF carrier signal as input signal; And two level switch circuit, it responds to the digital amplitude signal that is produced and changes between two level, is used for the voltage signal of powering to amplifier with generation.

Such scheme does not relate to the frequency synthesizer of this ofdm system.

The key technology of some 4G and 3.5G as OFDM technology, MIMO technology, smart antenna and software radio etc., begins to be applied in the WLAN (wireless local area network), to promote the performance of WLAN.Adopt the OFDM modulation technique as 802.11a and 802.11g, improved transmission rate, increased network throughput.802.11n plan adopts MIM0 to combine with OFDM, and transmission rate is significantly improved.In addition, antenna technology and transmission technology make the transmission range of WLAN (wireless local area network) increase greatly, can reach several kilometers (and can ensure 100Mbps transmission rate).In order further to promote the message transmission rate of WLAN (wireless local area network), realize wired and seamless combination WLAN (wireless local area network), IEEE has set up IEEE 802.11n work group, to formulate a new high-speed radio local area network standard.IEEE802.11n plan increases to the transmission rate of the WLAN 54Mbps from 802.11a and 802.11g more than the 108Mbps, and flank speed can reach 320Mbps, becomes another opera involving much singing and action after the 802.11b/a/g.802.11 standard differences in the past, 802.11n agreement are two-frequency operation pattern (comprising two working frequency range of 2.4GHz and 5.8GHz).802.11n has guaranteed and in the past 802.11a/b/g operating such like this.The HiperLAN/2 physical layer has been used OFDM and link adaptation techniques, medium access control (MAC) layer adopts towards the TDMA/TDD mode and the wireless ATM technology that connect, pooling of resources is controlled, flank speed reaches 54Mbps, and practical application is minimum also to remain on about 20Mbps.In addition, IEEE 802.11 WLAN (wireless local area network) work in ISM and exempt from the licence frequency range, have defined 1EEE 802.11a and the IEEE 802.11g standard that adopts the OFDM technology at 5.8GHz and two frequency ranges of 2.4GHz respectively, and its maximum data transmission rate is brought up to 54Mbps.

The OFDM technology is applicable to the high-speed transfer under the wireless environment, not only is applied to WLAN (wireless local area network), also is applied in broadband wireless access (BWA).IEEE 802.16 working groups are responsible for the technical work of BWA aspect specially, and it has developed the standard one IEEE 802.16a of a 2GHz～11GHz BWA, and physical layer has just adopted the OFDM technology.This standard is not only the wireless access technology of a new generation, and also significant to the development of following cellular mobile communication.Therefore design a kind of frequency synthesizer that is applied to ofdm system, have higher frequency resolution and preferably phase noise characteristic be very useful.

Summary of the invention

The objective of the invention is to: in order to reduce in the communication system carrier frequency offset the influence of systematic function, use the frequency synthesizer of DDS+PLL, adopted the DDS+PLL technology, by adjusting the control word of DDS, change the output frequency of reference clock, thereby can effectively reduce the carrier frequency offset of system in the radio frequency part of communication system, reduce the complexity of baseband portion synchronized algorithm.Can be widely used in the every field of communication system, especially frequency departure be required in the high ofdm communication system, as using in the systems such as UWB, WiMAX, WLAN.

The object of the present invention is achieved like this: be used for the method for ofdm system applying frequency synthesizer, DDS and phase-locked loop combination provide the radio frequency fundamental frequency are provided in the ofdm system.

The present invention directly adjusts the carrier frequency shift of sender and receiver in the radio frequency part of communication system, and can guarantee still have phase noise characteristic preferably under the condition of high frequency resolution, thereby can reduce the complexity of baseband portion synchronized algorithm.Frequency synthesizer mainly comprises Direct Digital Frequency Synthesizers (DDS), phase-locked loop chip, filter, voltage controlled oscillator, microprocessor, with reference to compositions such as crystal oscillators.Filter is active low-pass filter and/or loop filter.The present invention is applied to the frequency synthesizer of ofdm system, and frequency resolution can reach 0.05Hz or higher.

A) phase noise of carrier frequency shift and frequency synthesizer all can cause the net synchronization capability variation of ofdm system.Simultaneous techniques is the difficult point of ofdm system, and a lot of solution OFDM synchronized algorithms are arranged aspect Base-Band Processing, mainly is to carry out at cyclic extensions and special training sequence and pilot signal.

B) microprocessor is realized the control to Direct Digital Frequency Synthesizers (DDS), according to the requirement of user to frequency error, microprocessor is suitably revised the control word that flows to DDS, and to reach the purpose that reduces the communication system carrier frequency departure, fine steps can reach 0.05Hz;

C) because the output signal of DDS is spuious more,, abandoned traditional Butterworth LPF in order to guarantee the phase noise index of local oscillator, adopted transition band precipitous oval low pass filter, improved the phase noise performance of system;

D) simultaneously, also design active low-pass filter, further enlarge the frequency output area of radio-frequency (RF) local oscillator, to satisfy different communication systems to radio-frequency (RF) local oscillator reference frequency output demand, can be widely used in the ofdm communication systems such as UWB, WiMax, WLAN, also can be applicable in the business-like at present 3G communication system, as WCDMA, cdma2000, TD-SCDMA etc.

E) output of Direct Digital Frequency Synthesizers is imported the PLL circuit after connecting active low-pass filter again.The local frequency of the voltage controlled oscillator output radio frequency of PLL circuit.

The invention has the advantages that: the present invention is directed to ofdm communication system, designed a frequency synthesizer, proposition can effectively solve a kind of method of stationary problem in communication system radio circuit part, thereby reduce the influence of frequency departure, effectively reduce the complexity of baseband portion synchronized algorithm systematic function.The output frequency of frequency synthesizer can be realized by the control word of adjusting DDS, and output frequency has high frequency resolution, can reduce in the ofdm communication system the frequency departure sensitive issue, reduce the complexity of baseband portion algorithm, improve the performance of communication system.

Description of drawings

Fig. 1 is the theory diagram that the present invention is applied to the frequency synthesizer of ofdm system.

Fig. 2 is the frequency synthesizer circuit schematic diagram that the present invention is applied to ofdm system

Fig. 3. frequency synthesizer of the present invention is applied to the ofdm system schematic diagram

Fig. 4 is that the present invention is that output frequency resolution rate of the present invention is the characteristic of 200KHz

Fig. 5 is that output frequency resolution rate of the present invention is the characteristic of 0.05Hz

Fig. 6 the present invention is applied to the elliptic filter simulation result among the DDS

Embodiment

With reference to figure 1 block diagram and the disclosed design circuit schematic diagram of the present invention of Fig. 2, the frequency synthesizer that is applied to ofdm system is by direct synthesizer, low pass filter, phase-locked loop chip, active low-pass filter, voltage controlled oscillator, compositions such as microprocessor.Wherein, microprocessor is realized Direct Digital Frequency Synthesizers is controlled, according to the requirement of user to frequency departure, suitably revise the control word that (increase or reduce) is defeated by DDS, to reach the purpose that output frequency is adjusted, when exporting the local oscillator scope, when frequency resolution is 0.05Hz, still has phase noise characteristic preferably in the 2GHz left and right sides.Because the output signal of DDS is spuious more, in order to guarantee the phase noise index of local oscillator, designs well behaved oval low pass filter.Simultaneously, in order to satisfy the requirement of bigger reference frequency output, designed active low-pass filter, further enlarge the frequency output area of radio-frequency (RF) local oscillator, to satisfy the demand of different communication systems, can be widely used in the various 3G and the 4G communications fields such as UWB, WiMAX, WLAN, WCDMA, TD-SCDMA, cdma2000 to the radio-frequency (RF) local oscillator output area.

Frequency synthesizer is applied to the schematic diagram of ofdm system and the test result of phase noise under the high frequency resolution condition.

Adopt the frequency synthesizer of this programme design to be successfully applied in the MB-OFDM UWB system, as shown in Figure 2.In Fig. 2 circuit diagram, adopt the frequency synthesizer of DDS chip+PLL amplifying circuit design local oscillation signal to be provided for respectively sender and receiver, when there is bigger carrier frequency offset in system, suitably adjust the frequency control word of sender or receiver, can reduce the problem of intercarrier frequency departure, the resolution of frequency can be chosen the DDS chip of higher model and realize.Can improve the performance of MB-OFDM UWB system.

Among Fig. 2, U01 is direct synthesizer (DDS) chip, comprise the DDS circuit, U02 is a microprocessor, be used to control the output of DDS chip, comprise simultaneously in the U01 chip that the IC circuit of PLL constitutes the PLL circuit with the voltage controlled oscillator of being made up of U12, wherein U09, U10 are amplifying circuit, be used to constitute the PLL loop and amplify, U04 (LMV771) and related resistors electric capacity (R22, R24, R25, R45, C38, C39, C53, C50, C55, C56) have been formed the active loop filter.U05-08 is a power supply chip.Being connected between U01 chip 5 to 6 pin is the elliptic filter circuit, and by capacitor C 17-23, C67-68, C71-77, inductance L 01-03, L06-08, resistance R 41 constitutes the standard ellipse filter circuits.

Constitute the reference frequency circuit of Direct Digital Frequency Synthesizers by low pass filter especially elliptic filter circuit.Export the local frequency of radio frequency by active low-pass filter and voltage controlled oscillator in the output of PLL circuit.

The Figure 4 and 5 test result shows still have phase noise characteristic preferably under the higher frequency resolution condition.

In Fig. 3, the sender of ofdm system has adopted direct modulation scheme, baseband signal is launched through direct modulator and power amplifier, the radio-frequency (RF) local oscillator frequency range that the invention provides to direct modulator is 2.0～2.4GHz, the receiver of corresponding ofdm system has adopted the scheme of direct demodulation, the radiofrequency signal that receives is given baseband circuit through low noise amplifier, direct demodulator and baseband amplifier and is handled, and the radio-frequency (RF) local oscillator scope that offers demodulator still is 2.0～2.4GHz.

Fig. 4 frequency synthesizer is applied to the ofdm system schematic diagram, and Fig. 5 output frequency resolution rate is the characteristic of 200KHz.Fig. 4 and Fig. 5 illustrate and have adopted the frequency synthesizer of the present invention's design still to have phase noise performance preferably under the condition of higher frequency resolution (0.05Hz).

Fig. 6 is the simulation result with the oval low pass filter S parameter of ADS emulation, is that the harmonic wave of 19.2MHz has good supression to reference clock as seen, simultaneously also effective filtering reference clock spuious component on every side.This compares with traditional Butterworth filter, and circuit performance is had greatly improved.

WCDMA, the cdma2000, the TD-SCDMA that are applied to UWB, WiMax, WLAN or 3G communication system of applying frequency synthesizer in the ofdm system of the present invention.

Claims

1. the application process of ofdm system medium frequency synthesizer, it is characterized in that adopting DDS and phase-locked loop to make up to the radio circuit in the ofdm system provides the radio frequency fundamental frequency.

2, the application process of ofdm system medium frequency synthesizer according to claim 1 is characterized in that directly at the radio frequency part adjustment sender of ofdm communication system and the carrier frequency shift of receiver.

3, the application process of ofdm system medium frequency synthesizer according to claim 1 and 2, the frequency resolution that it is characterized in that the DDS that adopts and phase-locked loop combinational circuit is more than 0.05Hz.

4, the method for ofdm system medium frequency synthesizer according to claim 1 and 2, it is characterized in that the control of microprocessor realization to Direct Digital Frequency Synthesizers (DDS), suitably revise the control word that flows to DDS by microprocessor, reduce the communication system carrier frequency departure.

5, the method for ofdm system medium frequency synthesizer according to claim 1 and 2 is characterized in that low-pass filter circuit constitutes the reference frequency circuit of Direct Digital Frequency Synthesizers.

6, the application process of ofdm system medium frequency synthesizer according to claim 5 is characterized in that using after the output of PLL circuit active low-pass filter and voltage controlled oscillator to export the local frequency of radio frequency.

7, the application process of ofdm system medium frequency synthesizer according to claim 1 and 2 is characterized in that described low pass filter is oval low pass filter.

8, be applied to WCDMA, cdma2000, the TD-SCDMA of UWB, WiMax, WLAN or 3G communication system according to the described ofdm system medium frequency of claim 1 to 7 synthesizer.