CN102970268B - A kind of OFDM adaptive modulation system for EOC equipment - Google Patents

Abstract

The invention discloses a kind of OFDM adaptive modulation system for EOC equipment, first coaxial cable channel quality is estimated in receiving demodulation end channel quality estimation module, obtain channel frequency transfer function and subband signal-to-noise ratio (SNR) estimation value; Then subband signal-to-noise ratio (SNR) estimation value channel quality estimation obtained sends modulating mode to and selects module, modulating mode selects module antithetical phrase band signal-to-noise ratio (SNR) estimation value and desirable snr threshold to compare, obtain modulating mode, modulating mode modular converter is changed the pattern that next time modulates according to the modulating mode received; Self-adaptive modulator carries out corresponding modulation according to the mode value that modulating mode modular converter sends, make modulating system can adopt modulating mode adaptively according to channel conditions, when channel conditions is good, adopt the modulating mode of high-order, to improve the transmission rate of system; When channel conditions difference, adopt the modulating mode of low order to ensure the requirement of system to the error rate.

Description

A kind of OFDM adaptive modulation system for EOC equipment

Technical field

The present invention relates to modulation /demodulation field, especially a kind of OFDM adaptive modulation system for EOC equipment.

Background technology

Along with the propelling of integration of three networks process, promotion cable TV digitalization and the Two-way Reconstruction in CATV Network work of China's policy start comprehensively.Coaxial cable, as the resource of cable TV network, has high broadband, broadband and good noise suppressive characteristics, can transmit data, voice and image simultaneously.EOC (transmission of Ethernet over Coax coaxial Ethernet) technology as a kind of low cost, high performance coaxial Ethernet access solution, generally accept by various places radio and TV operator, become market focus.

Modulation EOC mainly adopts OFDM(OFDM) technology, OFDM is a kind of special multi-carrier transmission scheme, and it can be counted as a kind of modulation technique, also can be taken as a kind of multiplex technique.Multi-carrier transmission resolves into some sub data flows data flow, each like this sub data flow will have than originally much lower bit rate, the low rate multimode symbol formed with these low bit rates goes to modulate corresponding subcarrier, just forms the transmission system of multiple low-rate symbol transmitted in parallel.OFDM is to multi-carrier modulation (McM, Multi carrier Modulation) one improve, feature is that each subcarrier is mutually orthogonal, frequency spectrum after band spectrum modulation can be overlapped, not only reduce the mutual interference between subcarrier, also substantially increase the availability of frequency spectrum.

The input signal of ofdm system is serial binary code element, sequence of symhols is first divided framing by system transducer, F code element is had in every frame, then this F code element is divided into N group, code element number often in group can be different, N group serial code element is converted to N road paralleled code element through serial/parallel conversion device, is used for carrying out different MQAM(M-ary Quadrature Amplitude modulation for N number of subcarrier to the N road paralleled code element obtained) modulation; IFFT(inverse discrete Fourier transform) data in subcarrier in frequency domain are transformed into the IFFT data block of time domain by module, and be converted to serial data through parallel/serial transducer, be converted to OFDM analog signal through D/A converter; Receiving terminal is the inverse process of transmitting terminal, repeats no more here.

The problems such as when the existing OFDM modulation technology for EOC exists influx noise interference, spectrum efficiency low, bad channel quality the error rate is higher.

Summary of the invention

The object of the present invention is to provide a kind of OFDM adaptive modulation system for EOC equipment, this adaptive modulation system can solve the problem of the aspects such as the network insertion broadband of existing EOC system existence is low, poor stability, poor anti jamming capability, can utilize hfc plant bandwidth resources fully.

For achieving the above object, present invention employs following technical scheme.

This adaptive modulation system comprises the channel quality estimation module of receiving demodulation end, modulating mode selects module and the modulating mode modular converter of modulation transmitting terminal and self-adaptive modulator; First coaxial cable channel quality is estimated in receiving demodulation end channel quality estimation module, obtain channel frequency transfer function and subband signal-to-noise ratio (SNR) estimation value; Then subband signal-to-noise ratio (SNR) estimation value is sent to modulating mode and selects module by channel quality estimation module, modulating mode is selected module subband signal-to-noise ratio (SNR) estimation value and desirable snr threshold to be compared to select modulating mode afterwards, then the selection result of modulating mode is sent to the modulating mode modular converter of modulation transmitting terminal, modulating mode modular converter is changed according to the modulating mode of the selection result of modulating mode to self-adaptive modulator; The modulation of corresponding system is next time carried out after the modulating mode conversion of self-adaptive modulator.

Described adaptive modulation system uses and carries out transfer of data with the frequency of cable TV (CATV) system compatible, and frequency range is 5 ~ 65MHz.

In described adaptive modulation system, data-signal uplink and downlink transmission channel adopts the full-duplex communication mechanism of 7 two-way independent frequency sub-band, each frequency allocation 128 subcarriers.

First described channel quality estimation module obtains the channel frequency transfer function of each OFDM subcarrier of current transmission, for the signal that the demodulation of receiving demodulation end is currently received; Then according to the present data transmission error rate, by the relation of the error rate and signal to noise ratio under different modulating pattern, reverse goes out the signal to noise ratio of each OFDM subcarrier current, again according to the signal to noise ratio of each OFDM subcarrier current, averaging method is adopted to obtain the signal to noise ratio of subband, finally utilize the signal to noise ratio of the mean square deviation of subband sub-carrier signal-noise ratio to subband to revise, obtain subband signal-to-noise ratio (SNR) estimation value.

Described modulating mode selects module subband signal-to-noise ratio (SNR) estimation value and desirable snr threshold to be compared, and obtains the modulating mode value set { 0,2 corresponding with modulating mode according to comparative result, 4,16,64, a value in 256}, then sends modulating mode modular converter to this modulating mode value.

Described modulating mode modular converter judges the modulating mode value that modulating mode selects module to transmit, and changes according to the modulating mode of judged result to self-adaptive modulator.

The modulating mode of described self-adaptive modulator comprise do not transmit, BPSK modulation, QPSK modulation, 16QAM modulation, 64QAM modulation and 256QAM modulation.

Described subband signal-to-noise ratio (SNR) estimation value r _kwhen≤9.2, the modulating mode of self-adaptive modulator is not for transmit; Work as 9.2<r _kwhen≤12.6, the modulating mode of self-adaptive modulator is BPSK modulation; Work as 12.6<r _kwhen≤16.3, the modulating mode of self-adaptive modulator is QPSK modulation; Work as 16.3<r _kwhen≤19.9, the modulating mode of self-adaptive modulator is 16QAM modulation; Work as 19.9<r _kwhen≤24.1, the modulating mode of self-adaptive modulator is 64QAM modulation; Work as 24.1<r _kwhen≤27.3, the modulating mode of self-adaptive modulator is 256QAM modulation.

The present invention has the following advantages relative to the existing OFDM modulation demodulation techniques tool for EOC system: 1) the present invention can adopt BPSK(binary phase shift keying adaptively according to channel conditions), QPSK(quaternary phase shift keying), 16QAM(16 kind symbol QAM modulation), 64QAM(64 kind symbol QAM modulation), 256QAM(256 kind symbol QAM modulation) one in 5 kinds of digital modulation pattern, when channel conditions is good, adopt the modulating mode of high-order, to improve the transmission rate of system; When channel conditions difference, adopt the modulating mode of low order to ensure the requirement of system to the error rate.

2) the present invention adopts adaptive OFDM modulation pattern, when Communication Jamming occurs, automatically uses the modulating mode compared with low rate, and makes Data Modulation on other undisturbed subcarriers, thus have stronger antijamming capability.

3) the present invention adopts 5-65MHz low-frequency range, adopts the OFDM modulation technology of 7 two-way independent frequency sub-band, solves influx noise problem.

4) the present invention can resist the interference between signal waveform effectively, when in channel because Multipath Transmission and the frequency of occurrences Selective intensity time, the subcarrier and its information of carrying that only drop on frequency band recess are influenced, other subcarrier unimpaired, the bit error rate performance that therefore system is total is far better.

Accompanying drawing explanation

Fig. 1 is the existing OFDM transmission system figure for EOC system;

Fig. 2 is OFDM adaptive modulation system theory diagram of the present invention;

Fig. 3 is modulating mode transfer principle block diagram in the present invention.

Embodiment

Below in conjunction with drawings and Examples, the invention will be further described.

The present invention is a kind of adaptive modulation system adaptive modulation technology and OFDM technology combined, and effectively can improve the spectrum efficiency of system.Its transmitting terminal present channel quality situation on the estimation selectes optimal modulation pattern, carries out transfer of data.The quality of present channel directly can replace (to becoming channel slowly) by the channel quality status transmitting feedback last time, also according to the channel quality information of front N transmission feedback, present channel quality situation (for change continuous print Quick-Change channel) can be gone out according to its Changing Pattern pre-estimation; The selection of modulating mode can be considered from bit error rate and throughput two aspect, when channel conditions is good, adopts the modulating mode of high-order, to improve the transmission rate of system; When channel conditions difference, adopt the modulating mode of low order to ensure the BER(error rate of system) requirement.

Because the transmission data rate of system requirements is very high, and band system band resource is limited, therefore only has and uses high order modulation just can satisfy the demands.Although the transmission environment of hfc transport system has certain advantage relative to wireless transmitting system, when using high order modulation, noise margin is very low, stricter to the designing requirement of system.In outband channel, because there is frequency selective fading, the data on each subcarrier are subject to decline in various degree, cause the signal to noise ratio in different sub carrier also different.Especially cause in the channel of violent frequency selective fading in the serious reflection of existence, the signal to noise ratio difference in different sub carrier is very large.If each subcarrier-modulated pattern is identical, if be as the criterion with the serious sub-carrier signal-noise ratio that declines, use the modulating mode of low order, as 16QAM etc., be then exactly a kind of very large waste concerning the high subcarrier of signal to noise ratio; If slight sub-carrier signal-noise ratio is as the criterion to decline, then the error rates of data on the subcarrier of low signal-to-noise ratio can be very high.In this case, each subcarrier adopts different modulating modes to be a kind of scheme taking into account efficiency and performance.Adopt the cost of adaptive modulation scheme to be need to estimate the carrier-to-noise ratio on effective subcarrier, and need entire system to regulate and control message transmission rate.

According to the channel conditions of each subcarrier place frequency, in native system, each subcarrier pair data adaptive ground adopts BPSK, QPSK, 16QAM, 64QAM, 256QAM one in totally 5 kinds of digital modulation pattern.Because the noise of coaxial cable outband channel is less, therefore under non-severe frequency Selective Fading Channel, some frequency signal to noise ratio is higher likely adopts 256QAM modulating mode.

Specifically described below.

With reference to figure 2, channel quality estimation module, modulating mode select module, modulating mode modular converter and self-adaptive modulator to form adaptive modulation system; The data-signal of encoder to input carries out forward error correction coding, self-adaptive modulator adopts corresponding QAM and OFDM modulation multiplex technique coded data signal madulation in the frequency range of 5-65MHz according to the modulating mode that modulating mode modular converter feeds back, then generate OFDM analog signal by waveform generating module, and be sent on coaxial cable channel.

Receiving terminal modulation system is extracted and is adopted the method for command transmitting to inform the corresponding demodulation modes of demodulation module.Symbol synchronization is that the commutator pulse sequence in moment is adjudicated in sampling for demodulation module provides one to determine.Demodulation is that ofdm modulation signal is demodulated into base-band data signal.Equilibrium compensates channel frequency.

Channel quality estimation is the channel frequency transfer function of each OFDM subcarrier in order to obtain current transmission, with the signal helping receiving terminal demodulation to be currently received; Also be used for obtaining the signal to noise ratio (snr) of each OFDM subcarrier current, and then obtain the estimated value of subband signal to noise ratio:

Suppose that the channel frequency transfer function of i-th subcarrier is H _i, then the signal R i-th subcarrier received _ir can be represented _i=S _i.H _i+ n _i.

Wherein S _ibe the symbol of i-th sub-carriers carry, n _iit is the white Gaussian noise on i-th subcarrier.In order to the convenience stated and compare, assuming that the energy of each subcarrier is normalized, so signal to noise ratio snr of i-th subcarrier _ican be expressed as:

SNR _i=|H _i/n _i| ²

For coaxial cable channel, when once transmitting the modulating mode of employing on selecting, this snr value SNR can be thought _iit is exactly the signal to noise ratio of i-th subcarrier.

The OFDM adaptive modulation system of this EOC equipment, its data transfer mode is single user carrier way.For single user single-carrier system, suppose that the frequency domain impulse response function of channel is H, additive white Gaussian noise is n, passes through SNR=|H/n| ²calculate signal to noise ratio, coaxial cable channel can be mapped as H _i/ n _igaussian white noise channel, namely think that they have close channel conditions.Due to the orthogonality between ofdm system and carrier wave, for each subcarrier, can regard single carrier case as, therefore each subcarrier coaxial cable channel is mapped as SNR _i=| H _i/ n _i| ²gaussian white noise channel.Like this, the threshold interval of various modulating mode just can be determined under Gaussian white noise channel.

The modulating mode that Adaptive Modulation adopts for not transmitting, BPSK, QPSK, 16QAM, 64QAM and 256QAM, M={0 is expressed as by set mode value, 2,4,16,64,256}, provide this several modulating mode below at AWGN(additive white Gaussian noise) under channel BER about the computing formula of bit signal to noise ratio, order: r _b=E _b/ N ₀, wherein E _bfor bit energy, N ₀for power spectral density, then:

${\mathrm{BER}}_{\mathrm{BPSK}}\left(r_b\right)=\frac{1}{2}\mathrm{erfc}\left(\sqrt{r_b}\right)$

${\mathrm{BER}}_{\mathrm{QPSK}}\left(r_b\right)=\frac{1}{2}\mathrm{erfc}\left(\sqrt{r_b}\right)$

${\mathrm{BER}}_{16\mathrm{QAM}}\left(r_b\right)=\frac{1}{8}[3\mathrm{erfc}\left(\sqrt{\frac{{2r}_b}{5}}\right)+2\mathrm{erfc}\left(3\sqrt{\frac{{2r}_b}{5}}\right)-\mathrm{erfc}\left(5\sqrt{\frac{{2r}_b}{5}}\right)]$

${\mathrm{BER}}_{64\mathrm{QAM}}\left(r_b\right)=\frac{1}{24}[7\mathrm{erfc}\left(\sqrt{\frac{r_b}{7}}\right)+6\mathrm{erfc}\left(3\sqrt{\frac{r_b}{7}}\right)-\mathrm{erfc}\left(5\sqrt{\frac{r_b}{7}}\right)+\mathrm{erfc}\left(9\sqrt{\frac{r_b}{7}}\right)-\mathrm{erfc}\left(13\sqrt{\frac{r_b}{7}}\right)]$

${\mathrm{BER}}_{256\mathrm{QAM}}\left(r_b\right)=\frac{1}{64}\left[\begin{array}{c}15\mathrm{erfc}\left(\sqrt{\frac{{5r}_b}{48}}\right)+13\mathrm{erfc}\left(3\sqrt{\frac{{5r}_b}{48}}\right)-\mathrm{erf}c\left(5\sqrt{\frac{{5r}_b}{48}}\right)+5\mathrm{erfc}\left(9\sqrt{\frac{{5r}_b}{48}}\right)+4\mathrm{erfc}\left(11\sqrt{\frac{{5r}_b}{48}}\right)-5\mathrm{erfc}\left(13\sqrt{\frac{{5r}_b}{48}}\right)-4\mathrm{erfc}(15\\ \sqrt{\frac{{5r}_b}{48}})+5\mathrm{erfc}\left(17\sqrt{\frac{{5r}_b}{48}}\right)+4\mathrm{erfc}\left(19\sqrt{\frac{{5r}_b}{48}}\right)-3\mathrm{erfc}\left(21\sqrt{\frac{{5r}_b}{48}}\right)-2\mathrm{erfc}\left(23\sqrt{\frac{{5r}_b}{48}}\right)+\mathrm{erfc}\left(25\sqrt{\frac{{5r}_b}{48}}\right)-\mathrm{erfc}\left(29\sqrt{\frac{{5r}_b}{48}}\right)\end{array}\right]$

The signal to noise ratio obtained from channel estimating is subcarrier bit signal to noise ratio, when once transmitting the modulating mode of employing on judging, needs to compare with sub-carrier snr threshold.Order: r=E _s/ N ₀, E _sfor symbol energy, N ₀power spectral density, r and r _bpass be E _s/ N ₀=m.E _b/ N ₀, wherein m represents order of modulation.By relational expression E _s/ N ₀=m.E _b/ N ₀substitute into above formula, under obtaining AWGN, the relation of several modulating mode BER and symbol signal to noise ratio, abbreviation is also converted into Q function and can obtains:

${\mathrm{BER}}_{\mathrm{BPSK}}\left(r\right)=Q\left(\sqrt{2r}\right)$

${\mathrm{BER}}_{\mathrm{QPSK}}\left(r\right)=Q\left(\sqrt{r}\right)$

${\mathrm{BER}}_{16\mathrm{QAM}}\left(r\right)=\frac{1}{4}[3Q\left(\sqrt{\frac{r}{5}}\right)+2Q\left(3\sqrt{\frac{r}{5}}\right)-Q\left(5\sqrt{\frac{r}{5}}\right)]$

${\mathrm{BER}}_{64\mathrm{QAM}}\left(r\right)=\frac{1}{12}[7Q\left(\sqrt{\frac{r}{21}}\right)+6Q\left(3\sqrt{\frac{r}{21}}\right)3-Q\left(5\sqrt{\frac{r}{21}}\right)+Q\left(9\sqrt{\frac{r}{21}}\right)-Q\left(13\sqrt{\frac{r}{21}}\right)]$

${\mathrm{BER}}_{256\mathrm{QAM}}\left(r\right)=\frac{1}{32}\left[\begin{array}{c}15Q\left(\sqrt{\frac{5_r}{192}}\right)+13Q\left(3\sqrt{\frac{5r}{192}}\right)-Q\left(5\sqrt{\frac{5r}{192}}\right)+5Q\left(9\sqrt{\frac{5r}{192}}\right)+4Q\left(11\sqrt{\frac{5r}{192}}\right)-5Q\left(13\sqrt{\frac{5r}{192}}\right)-4Q(15\\ \sqrt{\frac{5r}{192}})+5\mathrm{Qc}\left(17\sqrt{\frac{5r}{192}}\right)+4Q\left(19\sqrt{\frac{5r}{192}}\right)-3Q\left(21\sqrt{\frac{5r}{192}}\right)-2Q\left(23\sqrt{\frac{5r}{192}}\right)+Q\left(25\sqrt{\frac{5r}{192}}\right)-Q\left(29\sqrt{\frac{5r}{192}}\right)\end{array}\right]$

Wherein $Q\left(x\right)=\frac{1}{\sqrt{2\mathrm{\&pi;}}}\underset{x}{\overset{+\&infin;}{\&Integral;}}{e}^{{-t}^2}\mathrm{dt}=\frac{1}{2}\mathrm{erfc}(x/\sqrt{2})=\frac{1}{2}[1-\mathrm{erfc}(x/\sqrt{2})].$ After having had the relation of the error rate (BER) and sub-carrier signal to noise ratio, next step just will be able to be negated release snr threshold according to BER.

Threshold value is under certain restrictive condition, adopts the lowest signal-to-noise value needed for certain modulating mode.The present invention selects to ensure that certain BER requires and makes transmission rate maximized threshold value determination criterion, and according to above-mentioned BER formula, under calculating various modulating mode, BER equals snr value during target BER, using they bottom threshold as modulating mode.

Based on the characteristic of coaxial cable channel, when meeting data-signal transmitting, native system chooses target error rate BER=1 × 10 ^-7.Six kinds of transmission rates are in error rate BER=1 × 10 ^-7signal-to-noise theory threshold interval corresponding to modulating mode under condition is as shown in table 1:

Table 1BER=1 × 10 ^-7desirable snr threshold under condition is interval

Modulating mode	Desirable snr threshold is interval
		Do not transmit	≤9.2
BPSK	[9.2，12.6]
		QPSK	[12.6，16.3]
16QAM	[16.3，19.9]
		64QAM	[19.9，24.1]
256QAM	[24.1，27.3]

After threshold value is determined, just need the problem solving how estimator band signal to noise ratio.Subband signal to noise ratio reasonably must embody the situation of each subcarrier in subband, and must improve the transmission performance of whole subband as far as possible with the modulating mode that it is Standard Selection, comprises BER and throughput performance.

According to the relation of the error rate and sub-carrier signal to noise ratio, the error rate reverse of each subcarrier utilizing current system receiving terminal to add up goes out sub-carrier signal to noise ratio snr _{k, i}, be compared on average obtaining each sub-carrier noise, and use this average signal to noise ratio as subband:

${\stackrel{\_}{r}}_k=\frac{1}{N_s}\underset{i=1}{\overset{N_s}{\mathrm{\&Sigma;}}}{\mathrm{SNR}}_{k,i}$

SNR _k,imiddle k represents a kth subband, and i represents subchannel, and Ns represents subband sub-channels number;

The basic point of departure of averaging method is the mutual compensation of BER and throughput: for the subcarrier that situation is good, the BER adopting the modulating mode selected by average to transmit can be lower than target BER, the part BER that part causes because the subcarrier of situation difference adopts too high modulating mode " can be neutralized ", higher throughput can be ensured again simultaneously.

The mean square deviation of recycling subband sub-carrier signal-noise ratio is revised, and the mean square deviation (sample standard deviation variance) of subband sub-carrier signal-noise ratio is defined as:

${\mathrm{\&sigma;}}_k=\sqrt{\frac{1}{N_s-1}\underset{i=1}{\overset{N_s}{\mathrm{\&Sigma;}}}{({\mathrm{SNR}}_{k,i}-\stackrel{\&OverBar;}{r_k})}^2}$

The signal-to-noise ratio (SNR) estimation value of mean square deviation as subband is deducted by average:

Modulating mode selects module by the set { 9.2,12.6,16.3 of desirable snr threshold, 19.9, the set { 0,2 of 24.1,27.3} and modulating mode value, 4,16,64,256} sets up one-to-one relationship, signal-to-noise ratio (SNR) estimation value and the desirable snr threshold of the subband then channel quality estimation sent compare, as the signal-to-noise ratio (SNR) estimation value r of subband _kwhen≤9.2, exporting modulating mode value is 0; Work as 9.2<r _kwhen≤12.6, export modulating mode value 2; Work as 12.6<r _kwhen≤16.3, export modulating mode value 4; Work as 16.3<r _kwhen≤19.9, export modulating mode value 16; Work as 19.9<r _kwhen≤24.1, export modulating mode value 64; Work as 24.1<r _kwhen≤27.3, export modulating mode value 256; Finally, modulating mode value is sent to modulating mode modular converter.

Shown in figure 3, self-adaptive modulator comprise do not transmit, BPSK modulator, qpsk modulator, 16QAM modulator, 64QAM modulator and 256QAM modulator, modulating mode modular converter judges the modulating mode value that modulating mode selects module to transmit, if 0, selection mode 0, does not transmit; If 2, selection mode 2, BPSK modulator is started working; If 4, selection mode 4, qpsk modulator is started working; If 16, selection mode 16,16QAM modulator is started working; If 64, selection mode 64,64QAM modulator is started working; If 256, selection mode 256,256QAM modulator is started working.

In the present invention, OFDM adaptive modulation system implementing procedure is specially, first determine according to target error rate the desirable snr threshold that various modulating mode is corresponding, then receiving terminal counts the error rate of each channel sub-carrier, utilize the error rate to carry out channel estimating and obtain sub-carrier signal-noise ratio estimated value, then sub-carrier signal-to-noise ratio (SNR) estimation value is carried out process and is obtained subband signal-to-noise ratio (SNR) estimation value, then antithetical phrase band signal-to-noise ratio (SNR) estimation value and desirable snr threshold compare, obtain modulating mode, then transmitting terminal is changed next modulating mode according to the modulating mode received.

By adopting the present invention:

Adopt this adaptive modulation technology, make EOC data transmission system can adopt optimum modulating mode according to link channel conditions, improve efficiency of transmission and the stability of link.By adopting 5-65MHz low-frequency range, adopting the OFDM modulation technology of 7 two-way independent frequency sub-band, solving influx noise problem.Make data-signal when the 5-65MHz low frequency single channel transmission speed based on coaxial cable, reach stable 30Mbps.

Claims (4)

1. for an OFDM adaptive modulation system for EOC equipment, it is characterized in that: this adaptive modulation system comprises the channel quality estimation module of receiving demodulation end, modulating mode selects module and the modulating mode modular converter of modulation transmitting terminal and self-adaptive modulator; First coaxial cable channel quality is estimated in receiving demodulation end channel quality estimation module, obtain channel frequency transfer function and subband signal-to-noise ratio (SNR) estimation value; Then subband signal-to-noise ratio (SNR) estimation value is sent to modulating mode and selects module by channel quality estimation module, modulating mode is selected module subband signal-to-noise ratio (SNR) estimation value and desirable snr threshold to be compared to select modulating mode afterwards, then the selection result of modulating mode is sent to the modulating mode modular converter of modulation transmitting terminal, modulating mode modular converter is changed according to the modulating mode of the selection result of modulating mode to self-adaptive modulator; The modulation of corresponding system is next time carried out after the modulating mode conversion of self-adaptive modulator;

Described adaptive modulation system uses and carries out transfer of data with the frequency of cable television system compatibility, and frequency range is 5 ~ 65MHz;

In described adaptive modulation system, data-signal uplink and downlink transmission channel adopts the full-duplex communication mechanism of 7 two-way independent frequency sub-band, each frequency allocation 128 subcarriers;

Described subband signal-to-noise ratio (SNR) estimation value r _kwhen≤9.2, the modulating mode of self-adaptive modulator is not for transmit; Work as 9.2<r _kwhen≤12.6, the modulating mode of self-adaptive modulator is BPSK modulation; Work as 12.6<r _kwhen≤16.3, the modulating mode of self-adaptive modulator is QPSK modulation; Work as 16.3<r _kwhen≤19.9, the modulating mode of self-adaptive modulator is 16QAM modulation; Work as 19.9<r _kwhen≤24.1, the modulating mode of self-adaptive modulator is 64QAM modulation; Work as 24.1<r _kwhen≤27.3, the modulating mode of self-adaptive modulator is 256QAM modulation.

2. a kind of OFDM adaptive modulation system for EOC equipment according to claim 1, it is characterized in that: first described channel quality estimation module obtains the channel frequency transfer function of each OFDM subcarrier of current transmission, for the signal that the demodulation of receiving demodulation end is currently received; Then according to the present data transmission error rate, by the relation of the error rate and signal to noise ratio under different modulating pattern, reverse goes out the signal to noise ratio of each OFDM subcarrier current, again according to the signal to noise ratio of each OFDM subcarrier current, averaging method is adopted to obtain the signal to noise ratio of subband, finally utilize the signal to noise ratio of the mean square deviation of subband sub-carrier signal-noise ratio to subband to revise, obtain subband signal-to-noise ratio (SNR) estimation value.

3. a kind of OFDM adaptive modulation system for EOC equipment according to claim 1, it is characterized in that: described modulating mode selects module subband signal-to-noise ratio (SNR) estimation value and desirable snr threshold to be compared, the modulating mode value set { 0 corresponding with modulating mode is obtained according to comparative result, 2,4,16,64, a value in 256}, then sends modulating mode modular converter to this modulating mode value.

4. a kind of OFDM adaptive modulation system for EOC equipment according to claim 3, it is characterized in that: described modulating mode modular converter judges the modulating mode value that modulating mode selects module to transmit, and changes according to the modulating mode of judged result to self-adaptive modulator.