CN103326969B - The equalization methods of WLAN receiver - Google Patents
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Abstract
The present invention relates to a kind of equalization methods of WLAN receiver, be characterized in that the coarseness frequency offset correction by feed-forward pattern completes balanced with the fine-grained frequency offset correction of NCO based on ROM.There is good reliability, meet the WLAN (wireless local area network) receiving terminal equalizer architecture of 802.11b standard.Up to the time delay of 100ns root mean square multipath, under the frequency deviation of 80ppm and the state of signal-to-noise of 20Db, this equalizer can reach the Packet Error Ratio close to 10%.This equalizer adopts the compensate of frequency deviation mechanism of the band DFF in two stage, and in chip design, be less than the gate of 50K.
Description
Technical field
The present invention relates to a kind of equalization methods, particularly relate to a kind of equalization methods of WLAN receiver.
Background technology
Along with formulation and the standardization of IEEE802.11 protocol suite, wireless lan (wlan) becomes more and more universal.Although the speed of 802.11g/n is far away higher than 802.11b at present, label (WiFiTag) application based on WLAN (wireless local area network) is also more and more universal.WiFiTag uses battery, so will consider the life-span of battery.Because 802.11b provides the message transmission rate of minimum 1Mbps, so select 802.11b can reach most power saving and least cost as the agreement of WiFiTag.And substantially all WAP (wireless access point) (AP) support 802.11b at present.Because hundreds of WiFiTag is under an AP, so reducing channel impairment is urgent problem.
About channel impairment:
If do not have suitable channel compensation, the channel impairment of wireless channel produces considerable influence by the performance of whole communication system.According to the seriousness to channel impairment, the wireless channel infringement principal element affecting 802.11b WLAN (wireless local area network) receptivity has:
1, multipath attenuation
Wireless signal, by air borne process, can arrive receiving terminal, therefore receiving end signal by multiple different path (due to reasons such as reflection, refractions)
transmitting by different delay can be regarded as
composition, and these transmit and arrive receiving terminal and have different amplitudes and phase place after transmission, can set up receiving end signal model with formula below:
.Index channel model is employed in IEEE802.11 WLAN standard,
.Root mean square (RMS) postpones the standard deviation that expansion is defined as average retardation.The selection of index is that the 5-6 that root mean square postpones to expand doubly is as the criterion with the scope of channel, and this is decided by the discernmible minimum radius of receiving terminal certainly, and in the environment of medium appropriateness, root mean square postpones expanding value is 65-75ns.
2, interchannel noise
Thermal noise is the basic lesions in all communication systems.Mostly count noise source and can regard additive white Gaussian noise (AWGN) as, other noise sources, comprise the quantizing noise that receiving terminal causes due to the A/D converter of finite accuracy, white, but not necessarily Gaussian noise, all these bit error rate on receiving terminal (BER) performance has impact.
3, frequency shift (FS)
Local crystal oscillator 802.11b signal madulation to 2.4GHz frequency time, because the frequency that affects of power supply supply, temperature and other factors has deviation.802.11 criteria limit frequency deviations are at 25ppm(60KHz) in, therefore in the worst case, from transmitting terminal to receiving terminal, have the frequency deviation of 50ppm, standard uses differential phase keying (DPSK) modulation system to make system have stronger reliability when frequency deviation, and does not need relevant detection.The detection but if use is concerned with, multipath performance will improve greatly.
4, path is lost
Because microwave is propagated in idle medium, its power attenuation and propagation distance square be directly proportional (20dB/decade).This mode is in the ideal case, can only be applicable to transmit and receive the very near situation (being no more than 6 meters) of distance.Along with the increase of distance, different barriers causes signal to decay on multipath, under current transmit power restriction, when the distance of transmitting terminal and receiving terminal increases, its effective message transmission rate will decline fast, and usual 802.11 wireless networks can obtain the transmission range being below about 100 meters.This distance is the coefficient result of transmitting power, propagation loss and receiving sensitivity.
Summary of the invention
Object of the present invention is exactly to solve the above-mentioned problems in the prior art, provides a kind of equalization methods of WLAN receiver.
Object of the present invention is achieved through the following technical solutions: the equalization methods of WLAN receiver, and it comprises the following steps:
Use the coarseness frequency offset correction of feed-forward pattern, step 1., use the coarseness frequency offset correction of feed-forward pattern, by the I/Q mixed signal sequence inputting estimating module of 11M/s, utilize Barker de-spreader that I and Q signal are associated with Barker sequence respectively; 2., separate spread spectrum I/Q position by code element collector (symbolpicker) at every 11st and sample, these corresponding preambles are sent in difference two-phase PSK (D-BPSK) decoder step; 3. step, obtains the solution spread spectrum I/Q chip of two-phase PSK (BPSK) constellation by reference to receiver.If there is frequency deviation to occur, will there is phase deviation in the constellation point receiving signal, complete the coarseness frequency offset correction using feed-forward pattern, enter the fine-grained frequency offset correction of NCO based on ROM; The fine-grained frequency offset correction of NCO based on ROM, step 4., chip after step 3. being processed is used for doing the valuation of multi-path channel impulse response (CIR), carries out CIR valuation by using corresponding to the known training sequence of the starting-frame delimiter (SFD) following Physical layer convergence protocol (PLCP) preamble closely; 5., the CIR impulse response coefficient h1 to h7 crossed by pointer regulation does the feedback of DFF to step, at receiving terminal training sequence in advance along moving 2 chips, guarantees the prior pointer search based on the CIR intensity of estimation; (pointer search avoids and uses feedforward filter pulse to carry out non-minimum-phase system in zero forcing equalization channel.); 6., before DFE starts, 7 initial judgements are loaded into feedback limit for length's unit impulse response filter (FIR) to step, and is kept in ROM as DFE part training sequence, and last 7 chips of the last position of SFD are used to initialization and load; Step 7., inquire about by fine-grained frequency offset estimation arc-tangent value, and the phase deviation the calculated moving average filter of 4 pulses is done on average; Step
carry out the NCO particulate frequency departure compensation calculation based on ROM.
The equalization methods of above-mentioned WLAN receiver, wherein: the 3. described phase deviation of step is the mean value of the phase difference of fixed qty code element, the quantity of code element depends on the distribution of preamble sequential budget alloments, and sequential budget alloments obtain function at automatic growth control, carry out between Timed Recovery function and other synchronizing functions needed when antenna receives a bag.
Further, the equalization methods of above-mentioned WLAN receiver, wherein: the 3. described phase deviation of step is the frequency compensation for driving digital controlled oscillator (NCO) to make each chip, calculates the NCO step sizes of normalizing.
The advantage of technical solution of the present invention is mainly reflected in: have good reliability, meets the wireless lan (wlan) receiving terminal equalizer architecture of 802.11b standard.Up to the time delay of 100ns root mean square multipath, in the frequency deviation of 80ppm and the signal to noise ratio (snr) situation of 20Db, this equalizer can reach the Packet Error Ratio (11Mbps) close to 10%.This equalizer adopts the compensate of frequency deviation mechanism of the band DFF in two stage, and in chip design, be less than the gate of 50K.
Accompanying drawing explanation
Object of the present invention, advantage and disadvantage, by for illustration and explanation for the non-limitative illustration passing through preferred embodiment below.These embodiments are only the prominent examples of application technical solution of the present invention, allly take equivalent replacement or equivalent transformation and the technical scheme that formed, all drop within the scope of protection of present invention.In the middle of these accompanying drawings,
Fig. 1, Fig. 2 are that generation is offset schematic diagram by the constellation point receiving signal;
Fig. 3 is NCO circuit structure schematic diagram;
Fig. 4 is chip organigram;
Fig. 5, Fig. 6 are that CIR calculates schematic diagram in serial mode in remaining SFD;
Fig. 7 is that DFE feeds back schematic diagram;
Fig. 8 is that fine-grained frequency offset estimation arc-tangent value inquires about schematic diagram;
Fig. 9, Figure 10 are the NCO schematic diagrames based on ROM;
Figure 11, Figure 12 are equalizer Packet Error Ratio schematic diagrames.
Embodiment
The equalization methods of the WLAN receiver as shown in Fig. 1 ~ 12, is characterized in that comprising the following steps: first, is the coarseness frequency offset correction using feed-forward pattern.Specifically, the input of estimating module is the I/Q mixed signal sequence of 11M/s.Barker de-spreader associates I respectively with Q signal with Barker sequence.
Then, separate spread spectrum I/Q position by code element collector (symbolpicker) at every 11st and sample, these corresponding preambles are sent in difference two-phase PSK (D-BPSK) decoder.
Then, the solution spread spectrum i/q signal of two-phase PSK (BPSK) constellation is obtained by reference to receiver, if there is frequency deviation to occur, to phase deviation be there is in the constellation point receiving signal, complete the coarseness frequency offset correction using feed-forward pattern, enter the fine-grained frequency offset correction of NCO based on ROM.
Subsequently, the fine-grained frequency offset correction process of NCO based on ROM is entered.Specifically, chip after step 3. being processed is used for doing the valuation of multi-path channel impulse response (CIR), carries out CIR valuation by using corresponding to the known training sequence of the starting-frame delimiter (SFD) following Physical layer convergence protocol (PLCP) preamble closely.
And then, at receiving terminal, training sequence, in advance along moving 2 chips, can guarantee the prior pointer search based on the CIR intensity of estimation like this.The CIR impulse response coefficient h1 to h7 crossed by pointer regulation does the feedback of DFF (DFE).Further, before DFE starts, 7 initial judgements are loaded into feedback limit for length's unit impulse response filter (FIR), and is kept in ROM as DFE part training sequence, and last 7 chips of the last position of SFD are used to initialization and load.
Afterwards, inquire about fine-grained frequency offset estimation arc-tangent value, the phase deviation the calculated moving average filter of 4 pulses is done on average.After completing, carry out the NCO particulate frequency departure compensation calculation based on ROM.
With regard to the present invention one preferably execution mode, for the ease of improving equalization performance, described phase deviation is the mean value of the phase difference of fixed qty code element, the quantity of code element depends on the distribution of preamble sequential budget alloments, and sequential budget alloments obtain function at automatic growth control (AGC), carry out between Timed Recovery function and other synchronizing functions needed when antenna receives a bag.。Further, phase deviation is the frequency compensation for driving digital controlled oscillator (NCO) to make each chip, calculates the NCO step sizes of normalizing.
In conjunction with actual service condition of the present invention: frequency offset correction divides 2 stages to perform, and first stage uses the coarseness frequency offset correction of feed-forward pattern.Side-play amount estimates when wrapping and forming preamble.The preamble of a 802.11b comprises 144 processes around 1 (long preamble) frequently or 56 processes around the short preamble of 0(frequently) (referring to IEEEStd802.11b-199918.2.2).In above-mentioned 2 kinds of situations, precedence bits uses D-BPSK modulation and through the Barker sequence spread spectrum of 11 chips (chip).Frequency deviation estimation process is that Barker separates spread spectrum, code element collector, and auto zoom ratio, gets phase difference, gets the mean value of N number of code element, obtains NCO step sizes.
The input of estimating module is the I/Q mixed signal sequence of 11M/s.Barker de-spreader I to carry out with Barker sequence respectively with Q signal associating (in order to and filter match, done time domain upset).We had done Timed Recovery to i/q signal and had known the border of spread symbols supposition before this.Sample in every 11st the solution spread spectrum I/Q position of code element collector (symbolpicker), these corresponding preamble symbols will be sent to D-BPSK decoder.In ideal conditions, parametric receiver will obtain the solution spread spectrum i/q signal of BPSK constellation, and constellation point is spaced apart 180 degree, and owing to not needing relevant detection, therefore these constellation point do not need and I/Q axial alignment.
For the bit stream of D-BPSK modulation, if a code element is above
s n-1 , so next code element
s n be
s n-1 , or be
-S n-1 .If there is frequency deviation to occur, the constellation point receiving signal will offset, as shown in Figure 1 and Figure 2.This phase place drift and frequency deviation proportional, the arc-tangent value be kept in ROM is used for calculating separates the phase value of spread symbols.Detailed arctan function will be discussed in next section.Phase difference is by code element pair
delivery obtains, and as previously described, this method is only adapted to DBPSK modulation.Phase deviation is the mean value of the phase difference of fixed qty code element (such as 16 code elements).The quantity of code element depends on the distribution of preamble sequential budget alloments, and sequential budget alloments obtain function at automatic growth control (AGC), carry out between Timed Recovery function and other synchronizing functions needed when antenna receives a bag.Number of symbols is more, and it is better that frequency deviation is estimated.Under in single circle, arc tangent can differentiate the window size situation of 256 points (corresponding to and differentiating angle is 1.41) and average 16 code elements, when 20dB and frequency deviation 200KHz, energy valuation is to probably in hundreds of Hz.
The maximum drift allowed between continuous code element is depended on by the maximum frequency deviation that this method can reliably be estimated to
.When symbol duration 1 μ s, the maximum frequency deviation that can measure is 250KHz.This is enough good, because the transmission center frequency values excursion of 802.11b standard mandatory provision is being 25ppm, i.e. 60kHz.When 2.4GHz is the worst, transmitting and receiving maximum frequency deviation is 50ppm, i.e. 120kHz.By performing estimation process instead of the code element solution spread spectrum of chip, maximum frequency deviation valuation scope can improve 11 times.But because noise and multipath are by serious reduction phase place drift estimation precision, this method is not recommended.Because Barker code can provide the processing gain of 10.4dB, the estimation of SNR can be improved with chip solution frequency expansion sequence.
Frequency deviation is estimated by the phase deviation mean value of interval 1 μ s code element.Phase deviation is denoted as Δ θ, between zero and one, and the sub-fraction of 2 π angles.Angle is less, and the efficiency of arc tangent ROM is higher.This phase place drift is for driving digital controlled oscillator (NCO) to make the compensate of frequency deviation of each chip.The NCO step sizes of normalizing is:
, wherein coefficient 11 forms 1 code element (Symbol) from 11 chips, and NCO circuit is as Fig. 3, and it is similar to based on following:
, wherein j is
, to be thisly similar to
set up time less, usually set up in situation described herein.This approximate may be used for produces continuous print Sin and Cos value:
.The contrary direction reversion code element of frequency deviation is pressed in the output of NCO:
.
Afterwards, the DFF of band fine granularity frequency offset correction is used.Specifically, according to lowest mean square root error criterion, chip after the frequency offset correction of coarseness is used for doing the valuation of multi-path channel impulse response (CIR), carries out CIR valuation by using corresponding to the known training sequence of the starting-frame delimiter (SFD) following PLCP preamble closely.Length/short preamble SFD is below 16, and launches under 1Mbps speed.The SFD sequence of short preamble and the contrary of long preamble.Training sequence comprises 55 chips (5/code element), starts, as shown in Figure 4 in the 5th position of SFD.SFD starts 3 chips and is used for triggering CIR estimation process.16 bit sequences search for SFD, when search window obtains [1111111111111000] (long preamble) or [0000000000000111] (for short preamble) pattern (rightmost position finally receives), CIR will be triggered and estimate process.
Suppose that the training sequence of the known SFD matrix X of 55x8 represents, wherein
represent training sequence chip
, the channel linearity distortion CIR matrix H of 8x1 represents, receiving sequence is expressed as:
.The Minimum Mean Squared Error estimation of channel calculates with formula the following:
.
Based on known training sequence
,to dissimilar preamble, the matrix P of 8x55 can precalculate out and be kept in receiving terminal ROM, arrange BPSK position and real axis alignment can make matrix P be real matrix, the CIR estimation which reducing receiving terminal calculates, and whole CIR calculates and comprises the 8x55x2MAC cycle.CIR calculates in serial mode in remaining SFD, and as shown in Figure 5, Figure 6, actual multiplication number of times depends on system-clock rate, and in 88MHz situation, CIR calculates and can complete in 2 MAC unit.After passing through awgn channel, the chances are to estimate precision at this algorithm of receiving terminal CIR
.
At receiving terminal, training sequence, in advance along moving 2 chips, can guarantee the prior pointer search based on the CIR intensity (as shown in Figure 5,6) of estimation like this.If use the Barker correlation peak of timing algorithm, when not having prior pointer, the CIR valuation amplitude starting 2 impulse responses is very little, 2 positions this is because training sequence has moved to left.In this case, 2 impulse responses of beginning can be ignored, and finally add that 20 is filled at CIR.If but this channel non-minimum phase, the valuation amplitude starting 2 impulse responses will be considerable.If start 2 pulses within the scope of the 6dB of owner pointer pulse c2, they should be comprised in final CIR valuation.Based on the threshold value of 6dB, c0, c1 and c2 likely become first pulse of CIR valuation.If c2 is first pulse, so the time is not needed to adjust.If c1 is first pulse, chip and code element need the chip that moves to left in time, same, if c0 is first pulse, chip and code element need 2 chips that move to left in time.Amplitude valuation uses following approximate formula
, do not need real range value.
Pointer search avoids and uses feedforward filter pulse to carry out non-minimum-phase system in zero forcing equalization channel.The CIR coefficient h [1] that pointer regulation is crossed is used for doing the feedback of DFE to h [7].Conjugate factor h [0] is used in (according to the constellation of QPSK, the input of cutting chip device (Slicer) has nothing to do with gain, and therefore 1/h [0] is the same with h [0] effect) in forward path.
Before DFE starts, 7 initial judgements are loaded into feedback FIR, and are kept in ROM as DFE part training sequence, and last 7 chips of the last position of SFD are used to initialization and load.Before DFE starts, the initial phase of D-QPSK decoder depends on the phase place of the last position of SFD.DFE exports first chip of conversion after SFD to start.
DFE has 2 road feedback loops to run, as Fig. 7 simultaneously.The balanced multipath distortion of home loop.Because inherent spurious frequency deviation frequency deviation can be comparatively large to DFE multipath performance impact, external feedback compensate for residual frequency deviation.Inherent spurious frequency deviation can cause the constellation point of the input skew QPSK slowly cutting chip device.Even if therefore offset very little (hundreds of Hz), the phase deviation accumulated in sufficiently long bag will cause cut chip device input exceed decision boundaries and cause mistake.Fine-grained frequency compensation offsets slowly.Also can compensate the skew of low frequency (such as comprising the frequency deviation that mains frequency ripple causes) when demodulating data bag.The compensate of frequency deviation similar of basic structure and coarseness.The main distinction is the continuous feedback character in loop and employs high-precision NCO and arctan function.
Fine-grained frequency offset estimation arc-tangent value inquiry as shown in Figure 8.The constellation point of QPSK makes phase place drift valuation very simple.If cut being input as of chip device
(X+jY), so export
, phase deviation is phase vectors
(X+jY) S, just allow the code element of X, Y positive and negative and real-imaginary part reversion.Question blank is with the X of 6 bit wides, and Y address makes input index, is used for calculating arc-tangent value.Owing to can ensure that angle exists
in, the list item number in ROM can be optimized.Calculated value in ROM will do proportional zoom, makes
corresponding to 1.The phase deviation the calculated moving average filter of 4 pulses is done on average.The quantity of pulse did optimization.It is slow that too much pulse makes compensation respond; Very few pulse will cause valuation noise.Every 4th sampling, average phase drift convergent-divergent 4 times is also accumulated.Accumulation is to 2 π deliverys.(1 corresponds to angle after normalization
) be used for overcoming proportional zoom problem and maximize the efficiency of ROM.Accumulated phase
be used for drive based on ROM instead of based on multiplication NCO(as shown in Figure 2).Scheme based on ROM provides higher precision, is not subject to the impact of accumulated phase error and allows constant angle to upgrade.Based on ROM NCO as shown in Figure 9.Arc tangent ROM export corresponding to
the interior angle value of 8.In order to keep precision,
use 10 positions.Start 1/4 quadrant that two MSB positions determine a circle, which 1/8 circle octant the 3rd position determines by.In ROM be an octant [0,
] in save 10 sin/cosine values.Octant position be used for reversion [
,
] address value in angle.Then sin/cosine value in whole circle (comprise and use sign reversing and ROM to export exchange) is decided according to this quadrant information triangle relation, as shown in Figure 10.The output of NCO is used for inputting chip with DFE and is multiplied.This operation carrys out inverted phase value by the skew of contrary direction.
Adopt the equalizer performance after the present invention as follows, the Packet Error Ratio of whole equalizer as is illustrated by figs. 11 and 12, bag size PSDU is respectively 256 bytes (Figure 11) and 1000 bytes (Figure 12), and transmission uses the CCK modulating mode of 11Mbps, and uses short preamble.The AWGNSNR of receiving terminal is from 12 to 24dB changes, and transmitting terminal is 120kHz(50ppm to the whole frequency deviation of receiving terminal).The IEEE index multi-path channel pattern of use standard, except NCO and arc tangent stage use low precision ROMs, other calculate and all use 12 precision.As can be seen from Packet Error Ratio curve, under rational signal to noise ratio and larger frequency deviation, the architecture carried herein can postpone expansion by the balanced root mean square multipath up to 100ns.
Can be found out, after adopting the present invention, to there is good reliability, meet the wireless lan (wlan) receiving terminal equalizer architecture of 802.11b standard by above-mentioned character express.Up to the time delay of 100ns root mean square multipath, in the frequency deviation of 80ppm and the signal to noise ratio (snr) situation of 20Db, this equalizer can reach the Packet Error Ratio (11Mbps) close to 10%.This equalizer adopts the compensate of frequency deviation mechanism of the band DFF (DecisionFeedbackEqualize, DFE) in two stage, and in chip design, be less than the gate of 50K.
Claims (3)
1. the equalization methods of WLAN receiver, is characterized in that comprising the following steps:
1. step, uses the coarseness frequency offset correction of feed-forward pattern, by the I/Q mixed signal sequence inputting estimating module of 11M/s, utilizes Barker de-spreader that I and Q signal are associated with Barker sequence respectively;
2., separate spread spectrum I/Q position by code element collector at every 11st and sample, these corresponding preamble symbols are sent in difference two-phase PSK decoder step;
Step 3., the solution spread spectrum I/Q chip of two-phase PSK constellation is obtained by reference to receiver, if there is frequency deviation to occur, to phase deviation be there is in the constellation point receiving signal, complete the coarseness frequency offset correction using feed-forward pattern, enter the fine-grained frequency offset correction of NCO based on ROM; Described NCO is digital controlled oscillator;
4., the chip after step 3. being processed is used for doing the valuation of multipath CIR to step, and carry out CIR valuation by using corresponding to the known training sequence of the starting-frame delimiter following Physical layer convergence protocol preamble closely, described CIR is multi-path channel impulse response;
5., the CIR impulse response coefficient h1 to h7 crossed by pointer regulation does the feedback of DFF to step, at receiving terminal training sequence in advance along moving 2 chips, guarantees the prior pointer search based on the CIR intensity of estimation;
Step 6., before DFE starts, 7 initial judgements are loaded into feedback limit for length's unit impulse response filter, and is kept in ROM as DFE part training sequence, last 7 chips of the last position of SFD are used to initialization and load, and described SFD is starting-frame delimiter; Described DFE is feedback equalizer;
7., DFE has 2 road feedback loops to run to step simultaneously, the balanced multipath distortion of home loop; Because inherent spurious frequency deviation can be comparatively large to DFE multipath performance impact, external feedback compensate for residual frequency deviation, fine-grained frequency offset estimation arc-tangent value is inquired about, and the phase deviation the calculated moving average filter of 4 pulses is done on average;
8. step carries out the NCO particulate frequency departure compensation calculation based on ROM.
2. the equalization methods of WLAN receiver according to claim 1, it is characterized in that: the 3. described phase deviation of step is the mean value of the phase difference of fixed qty code element, the quantity of code element depends on the distribution of preamble sequential budget alloments, and sequential budget alloments obtain function at automatic growth control, carry out between Timed Recovery function and other synchronizing functions needed when antenna receives a bag.
3. the equalization methods of WLAN receiver according to claim 1, is characterized in that: the 3. described phase deviation of step is for driving digital controlled oscillator to make the frequency compensation of each chip, calculating the NCO step sizes of normalizing.
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