CN107634926A - High-order modulation soft decision method combining channel information - Google Patents
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Abstract
The invention relates to a high-order modulation soft decision method of combined channel information. Specifically, in the implementation process of the OFDM system, equalization is not performed separately, but channel equalization is performed in combination with channel information. The method is simpler to realize, and the resources of the divider are saved; secondly, the method is not influenced by the equalization process, and particularly under the condition of low signal-to-noise ratio or deep fading channel, the influence of noise cannot be amplified, so that the sensitivity of the receiver can be improved.
Description
Technical field
The invention belongs to GSM technical field, more particularly to a kind of soft-decision device of combined channel information and
Method.
Background technology
In the transmission plan of system of broadband wireless communication, OFDM (orthogonal frequency division
Multiplexing) technology is a kind of typical transmission system, and it is due to, the spy such as availability of frequency spectrum height relatively easy with realizing
Point, and it is widely used to a variety of communication standards.Usually, in OFDM transmission technology, when CP (cyclic prefix) length
Degree is not less than the maximum delay of channel, and the starting point of OFDM symbol FFT (fast Fourier transform) window determines to exist
In its protection interval, receiver is to the sampled signal that receivesFFT is carried out to be converted to
Wherein, Xl[k]、Yl[k]、Hl[k] and Zl[k] represents transmitting symbol on k-th of subcarrier of l-th of symbol, connect respectively
Receive symbol, the frequency response of channel and Frequency domain noise.Therefore, ofdm system can be considered as to frequency domain as shown in Figure 1 in frequency domain
System, now, reception signal are equal to the product of the frequency response of incoming symbol and channel.In summary, do not having noisy feelings
Under condition, it is only necessary to reception signal divided by channel, you can to detect transmission signal by single tap equalizers.
In the GSM using OFDM transmission system, increase to obtain the higher availability of frequency spectrum and coding
Benefit, data traffic channels typically use advanced channel coding technology (for example, Turbo code and LDPC code) and high order modulation technique
The code modulation system that (16QAM, 64QAM etc.) is combined.In such systems, often it is combined using by decoding with demodulation
Method, demodulator will not go to judge the code word of decoder input, and simply various symbols temporarily be estimated, this side
Formula is referred to as " soft " judgement.It can not lose some useful information for decoder.Soft decision decoding is relative to hard decision
Decoding, has a certain degree of improvement generally in performance.In general, using soft-decision, the SNR of signal can be relative to sentencing firmly
Certainly there is 2dB advantage.
In typical OFDM system implementation process, the estimate of channel is typically obtained first with pilot frequency sequence, in this base
Equilibrium is carried out to data message according to the channel information of acquisition on plinth, the symbolic information finally obtained according to equilibrium treatment carries out soft
Judgement.In fact, this first balanced implementation for then carrying out soft-decision can cause ofdm system performance loss, and increase real
Existing computation complexity, in order to solve this problem, it is proposed that a kind of no longer independent progress is balanced, but Combined estimator obtains
The channel information method that directly carries out soft-decision.
The content of the invention
From the point of view of frequency domain, its any subcarrier can be equivalent to single-input single-output system in an ofdm system, and it is received
The relation of transmitting can be expressed as
Y=HX+Z (2)
Wherein, Y is frequency-domain received signal, and X is frequency domain transmission signal, and H is the frequency domain response of channel, and generally flat fading is believed again
Road gain, Z are Frequency domain noise.Usually, the noise of consideration is white Gaussian noise, and its average is zero, variance is
Traditional method is general first to carry out equilibrium treatment to receiving data-signal, to obtain the symbolic information of transmission signal,
Here illustrate so that relatively easy LS (least square) is balanced as an example
Now,Be zero for average, variance beGaussian random variable.Therefore, under conditions of known transmitting symbol X, connect
Receive symbolCondition PDF (probability density function) be
OrderWithLog-likelihood ratio (the log-likelihood for the glossary of symbols, then l positions that l positions are 1 and 0 is represented respectively
Ratio, LLR) or soft output value be defined as
(i.e. p (X) is a constant) is likely to occur if all of symbol etc., therefore, formula (4) can be approximated to be
Wherein,WithIt is defined as
If in an encoding blockIt is constant, formula (5) can be further simplified as
Different from traditional soft decision method, it is proposed that a kind of soft-decision implementation method of combined channel information, it is no longer
Need single equalization process.According to formula (2), it can obtain under conditions of known transmitting symbol X, receive symbol Y's
Condition PDF is
According to formula (7), no longer consider to carry out equilibrium treatment here, but directly according to reception data-signal and pass through pilot frequency sequence
The channel information for estimating to obtain calculates the LLR or soft output value of l positions, is defined as
Similar, it is likely to occur (i.e. p (X) is a constant) if all of symbol etc., then formula (8) can be approximated to be
It is further assumed that in an encoding blockIt is constant, then formula (9) can be further simplified as
The invention provides a kind of soft decision method of combined channel information to be used for digital communication system, and devises corresponding
The implementation method in OFDM wide-band communication systems.Compared with traditional soft decision method, the present invention mainly has following two aspect
Advantage：First, no longer needing individually to carry out equilibrium treatment in implementation process, divider has been saved in realization, so as to reduce reality
Existing required computation complexity；Second, do not influenceed by balancing procedure, particularly in low signal-to-noise ratio or deep fade channel condition
Under, the influence of noise will not be amplified, so that the sensitivity of receiver can be improved.
Brief description of the drawings
The frequency domain equivalent model of Fig. 1 ofdm systems；
Combined channel information soft-decision realizes block diagram in Fig. 2 ofdm systems；
Traditional soft-decision realizes block diagram in Fig. 3 ofdm systems；
Physical significance of Fig. 4 thresholdings in LLR quantizing process；
The error bit ability curve of Fig. 5 tradition soft decision method and the combined channel information soft decision method of invention.
Embodiment
To make the purpose, technical scheme and advantage of the embodiment of the present invention clearer, below in conjunction with the embodiment of the present invention
Accompanying drawing, clear, complete description is carried out to the technical scheme in the present invention, it is clear that described embodiment be the present invention one
Divide embodiment, rather than whole embodiments.Based on the embodiment in the present invention, those of ordinary skill in the art are not making
The every other embodiment obtained under the premise of creative work, belongs to the scope of the present invention.
The embodiment of the present invention discloses a kind of case study on implementation of soft decision method of combined channel information in an ofdm system, tool
Body realizes that block diagram refers to Fig. 2.In an ofdm system, the signal after receiving terminal synchronization then passes through FFT by serial/parallel conversion
Module obtains frequency-region signal, on this basis, using Preamble signals obtain channel information estimate, then, logarithm it is believed that
Number and channel information carry out parallel/serial conversion, and channel information (CSI, the channel state obtained according to estimation
Information soft-decision acquisition bit-level Soft Inform ation) is carried out with receiving data message, finally decodes Soft Inform ation input channel
Module obtains final decoding output.
Traditional soft-decision realizes that block diagram is as shown in Figure 3 in ofdm system, it can be seen that compared with the present invention, its data exist
Carry out soft-decision before first pass through channel equalization return to send signal carry out soft-decision again.Block-type pilot is used with ofdm system
Exemplified by illustrate channel information process part in two kinds of soft-decision modes shown in Fig. 2 and Fig. 3, in the combined channel letter shown in Fig. 2
Cease in soft decision method, when using least square (LS, Least square) channel estimation and simple linear interpolation method,
Only need to carry out channel estimation according to formula (11)
Wherein,Represent the frequency domain response estimation value of channel, C1And C2The coefficient of linear interpolation, X are represented respectivelyPRepresent pilot tone
Frequency domain transmission signal,WithThe frequency-domain received signal of two neighboring pilot tone is represented respectively, ()*Represent conjugate operation.Notice
In Practical OFDM Systems, the frequency domain transmission signal of pilot tone is often using Chu sequences with permanent mould property etc., therefore, formula
(11) in | XP|2Constant is represented, division is not needed during specific implementation.
In traditional soft decision method shown in Fig. 3, similarly, when using LS channel estimations and simple linear interpolation side
During method, then need to carry out channel estimation and equalization according to formula (12)
Wherein, Y represents the frequency-domain received signal of data, XeqRepresent the frequency domain estimate of data-signal.Analysis mode (12) can be sent out
It is existing,It can be changed over time with transmission environment, therefore, the divider in balanced device shown in formula (12) must
It is indispensable.In summary, the combined channel information soft decision method shown in Fig. 2 has saved divider compared to traditional soft-decision,
Thus realize that the computing resource needed is few.
The core of Soft Inform ation quantifying unit is to select specific thresholding (to be presented as in implementation process at cut position LLR value
Reason), particularly, for multi-system QAM modulation, different bits need to choose different thresholdings.It is specific for LLR value selection
Thresholding can reduce the bit wide quantified required for LLR value to a certain extent, and the physical significance of threshold value is as shown in Figure 4.Threshold value
Obtain it is smaller, corresponding in quantizing process in figure just for circle outside situation can carry out soft-decision, and the situation in enclosing then can
It is hard decision accordingly.Therefore, solid line circle represents that the thresholding of selection is smaller, and dashed circle represents that the thresholding of selection is larger.
In fact, when thresholding chooses smaller, the effect of soft-decision is close to hard decision, and therefore, threshold value should take higher value, and work as
When thresholding exceedes certain value, when representing that current signal value is located near a certain constellation point, it belongs to the constellation point with probability 1.Cause
This, it is rational to choose appropriate thresholding.
The soft decision method of above-mentioned combined channel information is applied to ofdm system, and surveyed in actual environment
Examination, the ofdm system design parameter for test are shown in Table I, and Fig. 5 gives the connection using traditional soft decision method and our inventions
Close the bit error rate curve of channel information soft decision method, it can be clearly seen that, the present invention is compared to traditional soft decision method energy
Enough it is obviously improved the error performance of system.
Table I emulates ofdm system parameter
Claims (1)
1. a kind of high order modulation soft decision method of combined channel information, in ofdm system implementation process, no longer individually carry out
Equilibrium, but combined channel information carries out channel equalization, it is characterised in that it is specially：In an ofdm system, receiving terminal synchronization
Signal afterwards then obtains frequency-region signal by serial/parallel conversion by FFT module, on this basis, utilizes Preamble signals
Channel information estimate is obtained, then, parallel/serial conversion is carried out to data-signal and channel information, and according to channel information and number
It is believed that number modulation system carry out soft-decision obtain Soft Inform ation, finally Soft Inform ation input channel decoding module is finally decoded
Output；
When wherein, using LS channel estimation and linear interpolation method, it is only necessary to carry out channel estimation according to formula (11)
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In formula,Represent the frequency domain response estimation value of channel, C1And C2The coefficient of linear interpolation, X are represented respectivelyPRepresent pilot tone
Frequency domain transmission signal,WithThe frequency-domain received signal of two neighboring pilot tone is represented respectively, ()*Represent conjugate operation.
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