Research On Adaptive Bit Allocation For ADSL-DMT System

1.ADSL-DMT system and modulationADSL( Asymmetric Digital Subscriber Line) is a kind of asymmetric copperlines access network technology, it can achieve higher data rates by using thedigit coding technique from the existing twisted-pair telephone lines, at the sametime ,it does not affect the common voice service in the same line. The reason whywe call it asymmetric technique is that it assigns most part of its bandwidth toprovide the path for subscribers download information and uses a little part of itsbandwidth receive the information which subscriber upload, thus it appears a kindof asymmetric transmission mode like this. Adopting this kind of the transmissiontechnique is not only applicable to obtain high-speed data for the consumer to visitthe Internet and use the vod system, but also can let up the near the crosstalk ofport consumedly ( NEXT).In fact that the ADSL line carries the end to endperformance under the influence of following factor: The circuitry length, loadingcoil, the tap of bridging, the end to end line concatenation quality, the specificationof line complication, the cross talk jam and so on.There are three kind of ADSL modulation technique which are widely used:QAM(Quadrature Amplitude Modulation), CAP( Carrierless Amplitude/ PhaseModulation), DMT.( Discrete Multi-tone Modulation).Among them the DMTmodulation technique was adopted as the national standard which is established byANSI standardization group T1E1.4 .The modulation of DMT is to divide the bandwith into n independent and thesame wide subchannels, according to the performance( such as signal noise ratio,attenuation etc.) of the channel namely the capability of the transport data assignseach statures channel to the self-adapting of input data last. If some statureschannel can't load the data then the simple way is to close it; But for thosesubchannel who can transport the data, then deliver 1~15 bit informations in a codeenvelope according to its moment characteristic. In 1 to n statures subchannels,the DMT adopts QAM which has different constellstions, the output of theconstellation drawing coder will still be the different from the range of sine wave,but each constellation drawing coder use different sine and cosine frequency. Allthe sine and cosines waves overlap together and are transported through thechannel.The key technique that DMT modulation demand to solve is self-adapting bitallocation: the outstanding advantage that DMT owns is to test the channelcharacteristic by the channel initialization and estimate noise power. From this,itcan self-adapting assign the data to different subchannels. Generally, the DMT canget the best bit allocation by testing the SNR of subchannels, and the result isalways decimal fraction. But in fact, the result must be integer. The DMT modulation method belongs to many carriers modulation technique,it self-adaptingly assigns the bits into each subchannel. In each subchannel it adoptQAM as modulation.2.Research on Adaptive Bit Allocation for ADSL-DMT System Bit allocation problem is that while the sending out main power P and theerror rates are a certain number, it can obtain the best n distribution and make Rblargest from the channel function |H(f)| 2 . Correspond the best power( or energy)allocation at the same time. For a parallel multi-channels system, how to assign bits in subchannels can getthe best result is a problem we should pay attention to. The aim of bit allocationalgorithm is to get the highest transmission rate by properly selecting the number ofbits and the coefficient of power alloction when the BER and transport power arecertained. Such as bit allocation algorithms ——" water flooding streaming"algorithms. The procedure of "Water flooding streaming" bit allocation algorithm is asfollows: 1. The initialization of energy: Given that the input signal energy of eachstatures channel is equal, namely εi = ε/ N ;( the N is a sub-channel number); 2. Noise power: From the type(3.17), type(3.18) andεi determining eachstatures channel noise Zi ; 3. Bit allocation: Doing bit and energy allocations according to water floodingstreaming algorithms, and determining the total bit number Rtotal and total bit rateRb ; 4. Discrimination: The judgment is to compare the total bit number Rtotal atthe first and the next times. If the difference is less than 1 bit, then ending theprocedure and outputting allocation result. If it is not less than 1 bit , then assigningaccording to the present εi , returning the 2 steps. The computer simulation by using the " water flooding streaming" algorithmare shown in figure1,figure 2 and figure 3.The energy of the system and bit allocation change along with the differentΓ.When Γ=1, the channel has the highest transmission rate;( namely shannoncapacity);When Γis enlarged, the number of bits in each carrier transmission willreduce with the number. For comparing with the traditional water floodingstreaming algorithm , figure 4, figure 5 will give the new algorithm and simulationcurve of the algorithm with the water flooding when Γ=1.15.From the figure 5 we can see the water flooding algorithm tends to compressthe whole acquired energy to the subchannels which are in the low frequency oftransmission bardwith, it does not meet the limit of peak energy. But the algorithmwhich is introduced in 3.3 not only meet the energy limited term, but also trying toassign the energy to in the higher frequency of the sub-channel. However therestrict of the peak value energy with throughput reduces many carrierstransmission system as cost of. From figure 6 the speed allocation curve, we canthat see, forΓ=1.15 in the CSA circuitry the peak value limits the throughput loses2% or so. By further calculating when Γ=1.3, throughput loss is near to 6%,when the Γ〉1.3 , throughput losses would be larger. When Γ=1.15, the system can be realize and the performance is the best.Figure 7, figure 8 and figure 9 give the MATLAB simulate results which isdertermined the two new algorithm when Γ=1.15. Among them the dispersecurve simulation is come from integer bit algorithm, continue curve is conductedfrom the non-integer bit algorithm.From figure 7 to figure 9 we can see that the integer bit allocation algorithmnot only overcome the defect that speed assign of non-integer bit allocationalgorithm, but also the loss is limited for the allocation project in chapter 3.3.Thethroughput reduce 1% or so by calculating, therefore it is a viable project.3.Conclusion This article discusses the bit allocation problem according to ADSL systemwhich is based on DMT. Primarily introducing the " water flooding streaming"allocation method, then discussing the problems of subchannel energy and bitallocation when the total input energy and peak energy exist. In the condition ofobtaining the best solution of curve speed function circumstance, it is applicatedin difference in amount look like of circumstance. The calculation complexity of the water flooding streaming algorithm iso(N 2). The new algorithm which this article discusses do not need to resemble thesignal to noise ratio of the sub-channel, as the water flooding streaming algorithm,its calculation complexity is o(N) , therefore this kind of new bit allocation thealgorithm can consumedly lower calculation complexity. The highest transmission rate is lower than the channel capacity no matterwhat you choose between the water folding streaming algorithm and new bitallocation algorithm. So the DMT is a sub-optimum multi-carries modulationtechnology. The water flooding streaming algorithm tends to compress overall energy tothe low frequency of the transmission bandwidth in several sub-channels, itdeviates the restrict of the constraint condition εmax of the peak value energy, but j...