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1、dmt调制（DMT modulation）The application of DMT and convolutional code modulation in DSLZhong Xiaojian, pan GUI, Ma Ma, Liang XiaoyuIn(Department of physics, Huazhong Normal University, Wuhan 430079)Abstract to discuss the application of modulation discrete multi tone modulation and encoding grid combin
2、ation in DSL, discrete multi tone modulation DMT 1 is a kind of multi carrier modulation technology, the data transmission based on the sub-band signal-to-noise ratio according to subband bit allocation, so that each sub band width is greater than the symbol multipath delay. If the modulation and er
3、ror correction coding are combined, the bit error rate can be greatly reduced, and it is a modulation method with high bandwidth utilization.Keywords: the ADSL discrete multitone trellis encoding 2 Euclidean distance 3 discrete Fu Liye transform and inverse transform1 IntroductionWith the continuous
4、 development of Internet technology, peoples requirements on data transmission speed, quality is more and more high, in the current in order to effectively utilize existing resources, telephone line, DSL proposed 1 (digital subscriber line) concept, using voice frequency band (4 K above 1.1 MHz) to
5、high speed digital modulation according to a f=4.3125 kHz signal is divided into a subband, because the delta f is just the audio width, so named for the discrete multi tone modulation, DMT is based on discrete Fu Liye transform for modulation and demodulation of the parallel data. With the continuo
6、us progress of VL SI and digital signal processing (DSP) technology, real time DMT modulation with FFT has been put into use. But in the past, the modulation and demodulation system, error correction encoding and modulation are designed and implemented respectively, so decoding and demodulation, thi
7、s demodulator in the received signal is independent of the hard decision signal, then gave the result of hard decision decoder, the hard decision will lead to the receiving end information can not recover the loss of solution this problem is in the receiver with soft decision decoding. DSL technolog
8、y is a comprehensive design of DMT and trellis code, which has greatly improved the error performance of traditional technology in white noise environment. This encoding modulation system is best according to the Euclidean encoding sequence for the design of the distance measure, which requires the
9、encoder and modulator as a whole integrated design, encoding signal sequence makes the encoder and modulator cascaded with the largest Euclidean free distance. From the point of view of signal space, the design of optimal coded modulation is actually an optimal segmentation of signal space. The expe
10、rimental results show that the coding gain of DMT and convolutional coding is 8 dB higher than that of traditional coding. In the2xDSL access device architectureIn ADSL applications, the hardware architecture is generally composed of a line interface, receiving filter, line drive, analog front-end a
11、nd DMT transceiver module. The DMT transceiver in the beginning of the data reuse, cyclic redundancy check, FEC, subband sort, convolutional encoding, constellation mapping and IFFT transform, to transform the analog front-end into analog signals sent out, and in the end is the analog signal through
12、 FFT transform, solution mapping, Vitby decoding and a series of inverse transform submitted to the upper. According to the T1.413 4 standard, Wechsler 16 state 4 dimensional grid code as the inner code, using Reed / Solomon encoding as FEC code, in addition to the grid block encoding noise resistan
13、ce ability is poor, so before the grid data encoding interleaving make noise dispersion. The block diagram of the DMT transceiver for ADSL is shown in figure 1.The principle of 3DMT and convolutional code modulationIn the sending end ADSL, distributing the data to different subbands, the distributio
14、n according to each sub-band signal-to-noise ratio to determine the distribution of bit number. The ADSL system establishes and maintains a list of bits and gains for each subband, which is calculated at one end of the ATU-R and returned to the local end. In order to ensure that the number of bits i
15、n the latter subband is not less than the number of the first subband, the subband is sorted first, i.e., the subband is sorted according to the signal-to-noise ratio.In order to obtain the larger codeword encoding Euclidean free distance, using a four-dimensional TCM mesh encoding, this bit extract
16、ion is based on a subband, because a subband in space is two-dimensional, orthogonal sub bands in space is one of the four, when decoding is one of a pair of Viterbi decoding. Euclidean free distance is calculated in four-dimensional space, which can be combined by the coset four-dimensional form co
17、set two two-dimensional, namely this four-dimensional TCM trellis codes Euclidean free distance can consist of two dimensional constellation and the square of the distance calculated in the decoding system, the error may occur in the case of with the minimum squared Euclidean distance between two se
18、quences an and bn r r, (the former is sending sequence, the latter is the decoding sequence), the minimum squared Euclidean distance is often called square free distance, remember to do:Exergy exergy encoding is designed to make the most of this square free distance.The grid encoding modulation by a
19、 special signal mapping can be turned into a convolutional code form. The principle of this mapping is to divide modulation signals into componentsCut into subsets, where the signals within the subset have larger spatial distances, select the subset of convolutional codes with the encoding efficienc
20、y of k/ (k+1), and select the points in the subset with the remaining bits. In the DSL digital subscriber loop, the encoder structure with 16 states of 4 dimensional trellis code is shown in figure 2.The convolution coding section is shown in figure 3.The number of bits Z =x+y-1 (x) for each two sub
21、bands in Figure 2 (is the number of bands taken by the first subband, and Y is the number of bands taken by the second subbands). uz -1, UZ -2,. U1 the original code, the output is the result of convolution encoding and XOR after the two binary code, namely in the vz-y, VZ -y-1,. V1, v0 and wy-1, wy
22、-2,. W1, w0, these two binary codewords will be mapped into two constellation points. Encoding algorithm makes the constellation points of the two most low decision points of the constellation 2D v0 and w1, v1 coset, w0 is actually the binary representation. For the last two codewords in a frame, th
23、e convolutional coding states s3, S2, S1, s0 are returned to zero state. Let u1, u2=0,0 of the codeword before encoding, and the number of bits Z = x+y-3 of the last two subbands.TheThe encoded signals have two basic features:(1) the number of the signal constellation in the same encoding was greate
24、r than the desired modulation points (doubled), these additional signals provide redundancy for error correction encoding.The (2) using convolutional codes between signal points have been the introduction of some kind of dependence, so only certain signal sequence is allowed. These allowed signal se
25、quence can be modeled as network structure. It can be expressed by grid graph.In the receiver, the Viterbi decoding the received sequence 4, namely the maximum likelihood decoding, can map for the most similar path to describe this algorithm with grid, Markov system which relies on finite state desc
26、ription, including the state transition and output state changes the code word. Based on four TCM encoding f on the decoding to decode the data, calculate the code distance is the Euclidean distance of the four-dimensional space as the standard, take a path of the most similar. The length of grid pa
27、th L+m (L is the length of the sequence information, m said. M 0 vector sequence divergence) receiving all the grid path at time zero in the same initial state, the convergence in the j time (J = L+ m) with a final state. Ideally, for all channels with unlimited storage, all possible paths can be re
28、corded, and then the maximum likelihood function of the log likelihood function is chosen as the decoding result.The logarithmic likelihood function is the logarithm of the conditional probability that the received sequence is determined as a sequence of pathsExergy exergy here log likelihood functi
29、on is maximum, is actually between the sequence and estimation of the received code path distance was calculated based on the minimum Euclidean distance. Here, the kernel of Viterbi decoding algorithm is the viewpoint of rollback,Using dynamic programming method to store data, if each possible path
30、is stored, with the increase of the decoding depth, the storage capacity will be 4 exponential growth, which is impossible under the actual conditions. Because each node has four branches (two input, sixteen state grid graph), we have a certain state of arrival for the j momentDelta I (i=1,2). S-1),
31、 plus - ratio selection, operation is all possible before a moment of the state of the maximum likelihood function of a J-1 (delta P) and sequence current received and the previous state to the likelihood estimate of the current state of adding code, select one of the biggest moments as J I.The maxi
32、mum likelihood function value of the state and the survival sequence J (delta I) on the original basis are added to the optimal path u delta P to delta I. The algorithm given in this way can be expressed as:The variable / storage:The S state number (DSL 16);The T branch - each state number (4);The j
33、 number is the moment, J momentFor the series finished with convolutional encoding can be sent to the digital signal processor IDFT 5 transform into serial data directly. Each of the sub binary code with I can be mapped into complex constellation points on (Zi=ai+jbi), in order to make the output si
34、gnal for real signals, complex frequency domain of subband I values (I = N N =N/2) forExergy exergy K (ZN-i), Zi=conj (I = N N =N/2)In this way, the conjugate complex number is obtained, so that the time domain signal can be obtained by discrete Fu Liye inverse transform:This signal and exergy exerg
35、y after / on transformation, and then through the A / D converter, transmission into analog signals to the line.4 simulation resultsWe in the application of Itexs ADSL solutions, terminal simulation tool IADT provided by the company on the ADSL link performanceThe signal to noise ratio (SNR) of each
36、 subband (from 0255) of ADSL is obtained by simulation, and then the number and gain of each subband are determined according to the predicted value.Thus, a table corresponding to the subband is established, and the signal-to-noise ratio curve of the line prediction is shown in figure 4.As we can se
37、e, the measured uplink speed is 544Kbps, network speed (removing ADSL link overhead) is 448 Kbps, downlink rate is 8160 Kbps, and network speed is 7616 kbps.5 SummaryThe paper describes the use of DMT and convolutional encoding in the limited channel bandwidth combining technology, modulation and er
38、ror correction encoding are integrated, efficient use of the available bandwidth. With the gradual maturity of ADSL technology, the coding technology is also being used in other fields, such as wireless communications, for its channel attenuation characteristics, can obtain higher bandwidth utilizat
39、ion. In the hardware implementation, due to the development of VLSI technology, the hardware is no longer a bottleneck in signal processing, such as Viterbi decoding of the above analysis, the demand of hardware is increased rapidly with the increase of N, realize the one hundred thousand gate circu
40、it needs, Viterbi decoder is now single, or in special the application of digital integrated on a chip, and complete the RS encoding, interleaving, FFT transform etc.Basic principle and key technology of ADSL broadband access networkAbstract this paper introduces the ADSL broadband access network te
41、chnology this transition, discussed the basic principle and the key technology of it; show the status of ADSL development and the prospect of it in the next few years in the future.Key words access network asymmetric digital subscriber loop (ADSL) technology developmentThe user access network (from
42、the local telephone office to the user part) is an important part of the telecommunications network, is the window of the telecommunications network, but also the the last mile of the information superhighway. In order to realize the digitalization and broadband of subscriber access network, using o
43、ptical fiber as subscriber line is the userThe future development of the network is inevitable, but the cost of the optical fiber subscriber network is too high,In the next ten years to decades most network users will still continue to use the existing copper wire loop, have been proposed in recent
44、years a number of transitional broadband access network technology, including ADSL (asymmetric digital subscriber line) andHFC (fiber coax hybrid network) is the two most competitive.ADSL technology can use the existing telephone wire for signal transmission, the highest speed: downlink signal (from
45、 the end to the user) is 9Mbps, the uplink signal (from the user to the end of the Bureau) is 1Mbps. The number of users of the existing local telephone copper network is very large, and ADSL can make full use of the existing telephone line copper wire.In recent years, the development of Internet at
46、 an alarming rate, many Internet users are widely distributed, the user access rate but the existing network users can offer is too low, it is difficult to meet the needs of rapid development of Internet, it provides an opportunity for the development of ADSL. Because the telephone line copper wire
47、is now availableConnecting with all Internet users, once the ADSL technology matures, it will become the preferred solution for telephone companies and Internet users.1. technical performance analysisThe existing subscriber loop is mainly composed of UTP (unshielded twisted pair). The attenuation of
48、 UTP signal is mainly related to the transmission distance and the frequency of the signal. If the signal transmission exceeds a certain distance, the quality of the signal transmission will be difficult to guarantee. In addition, the bridge taps on the line will also increase the attenuation of the
49、 signal. Therefore, line attenuation is the main factor affecting the performance of ADSL. Through asymmetric transmission, ADSL uses frequency division multiplexing (or echo cancellation technology) to separate the upper and lower channels to reduce the influence of crosstalk, so as to achieve high-speed transmission of the sig