ADSL Modulation Technology

1. ADSL and Its Features
Adslasypolicric Digital Subscriber Line (Asymmetric Digital user Line) is an asymmetric copper access network technology. It uses Digital encoding technology to obtain large data transmission rates from existing copper telephone lines, at the same time, it does not interfere with conventional voice services on the same line. ADSL creates three channels that can work simultaneously on copper telephone lines, namely, High Speed Downstream (High Speed Downstream) and Medium Speed Duplex (Medium Speed Duplex channel) it is used with the POTS channel to ensure that voice communication works normally even if the ADSL connection fails ). ADSL adopts advanced digital signal processing technology and new algorithms to compress data, enabling high-speed transmission of large amounts of information on the Internet.

To separate valid bandwidth on a telephone line and generate multi-channel channels, ADSL modem generally adopts two methods, namely, multiplexing of frequency division and multiplexing (OFDM) or Echo elimination (Echo Cancellation) technology. One frequency band is allocated as the downstream data channel in the existing bandwidth, and another frequency band is allocated as the upstream data channel. Downstream channels are divided into multiple high-speed channels and low-speed channels by using the time-division multiplexing TDM technology. Likewise, an upstream channel is composed of multiple low-speed channels. The echo elimination technology overwrites the uplink and downlink frequencies, and uses local ECHO offsets to differentiate the two frequencies. Of course, no matter which of the two technologies is used, ADSL will separate the 4 kHz band for Telephone Service POTS ).

The asymmetry of ADSL is manifested in two-way data rate asymmetry. Generally, the downlink rate is much higher than the uplink rate. It is extremely absorbent for surfing the Internet and accessing remote LAN users. ADSL can provide end users with 8 Mbit/s downstream transmission speed and 1 Mbit/s upstream transmission speed, far greater than the speed of traditional Modem or ISDN.

2 ADSL Modulation Technology

Currently, three ADSL Modulation technologies are widely used internationally: Orthogonal Amplitude Modulation QAMQuadature Amplitude Modulation), CAPCarrierless Amplitude-plase Modulation without carrier Amplitude/phase Modulation, and discrete Multitone DMTDiscrete Multitone ).

2.1QAM Modulation Technology

In QAM modulation, the data sent in the bit/Symbol encoder is divided into two channels with an original rate of 1/2, which are multiplied by a pair of orthogonal modulation components, summed and output. The receiver completes the opposite process. Orthogonal decoding calls up two opposite code streams, and the balancer compensates for channel distortion. The judge recognizes the plural signal and maps it back to the binary signal. By using the QAM modulation technology, the channel bandwidth must at least be equal to the bitwise rate. In order to regularly recover, another bandwidth is required, which is generally increased by about 15%. Compared with other modulation technologies, the QAM modulation technology makes full use of bandwidth and has strong noise resistance. However, the main problem of using the QAM modulation technology in ADSL is how to adapt to the large performance differences between different telephone lines. To achieve ideal performance, the QAM receiver needs an input signal with the same spectrum and characteristics as the sending end for decoding, the QAM receiver uses an adaptive balancer to compensate for the distortion produced by the signal during transmission. Therefore, the complexity of the ADSL system using the QAM comes from its adaptive balancer.

2.2CAP Modulation Technology

CAP modulation technology is developed based on the QAM modulation technology. It can be said that it is a variant of the QAM technology, and the difference between it and the QAM is the digitization of its implementation means. Input data is sent to the encoder. In the encoder, m-bit input bits are mapped to k = 2 m different plural symbols, and k different plural symbols constitute k-CAP line encoding. After encoding and are separately sent to the same phase and orthogonal digital shaping filter, the summation is sent to the D/A converter, and finally sent through the low-pass filter.

The soft decision technique is used for CAP demodulation, And the equalizer are used for adaptation. The spectrum shape produced by CAP is the same as that produced by QAM.

The CAP code is a two-dimensional redundant linear modulation code, and the power spectrum is band-pass. The upper limit is 180 KHz, and the low frequency cut-off frequency is less than 20 KHz. CAP is subject to low-frequency energy-rich pulse noise and high-frequency near-end crosstalk. It has no low-frequency delay distortion and has little inter-code interference caused by group delay distortion. The main technical difficulty of CAP Technology for ADSL is to overcome the signal interference caused by near-end crosstalk. This problem is generally solved by using the near-end crosstalk canceller or near-end crosstalk balancing. Compared with QAM, CAP is easier, more convenient, and more flexible to implement.

2.3DMT Modulation Technology

DMT modulation technology divides the 0-1 kHz band of copper lines. 0-4 kHz is the voice band, which is used for the transmission of common telephone services. Other bandwidths are divided into 255 subcarriers, the frequency interval between subcarriers is 4.3125 KHz. One sub-channel is formed by QAM modulation on each sub-carrier. Some sub-carriers in the low-frequency part are used for uplink data transmission, and other sub-carriers are used for downlink signal transmission, the separation points of upstream and downstream carriers are set by specific devices.

The range of DMT modulation capability is 0-15bit/s. The DMT modulation system uses these 255 sub-channels based on actual conditions. The transmission rate of the sub-channels can be determined based on the instantaneous attenuation, group latency, and noise characteristics of each sub-channel. Generally, the modulation capacity of sub-channels with excellent performance is greater than 10 bit/s/Hz, while that of sub-channels with low or high frequencies, DMT technology can adapt the modulation capability to 4 bit/s/Hz Based on channel performance. Channels that cannot transmit data will be disabled.

If N sub-channels are used, each sub-channel uploads a bit and maps it to a DMT plural sub-symbol j = 1 ,..... n), using the 2N point IFFT fast Fourier inverse transformation), the number of N plural subsymbols in the frequency domain is changed to 2N real number sample values j = 1 ,..., 2N), output online after digital-analog transformation and low-pass filtering. The Receiver performs the opposite transformation and performs FFT transformation on the 2N Time Domain sample values after sampling to obtain N plural subsymbols in the frequency domain. After decoding, The bitstream is restored to the original input bit stream.

3 conclusion

The 0-1.1MHz band of the telephone twisted pair is non-linear. different frequencies have different attenuation, different noise interference conditions, and different latency. If the full band is used as a channel, A single-frequency noise interference will affect the overall transmission performance. The DMT modulation divides the entire band into many channels. Each channel has a narrow band and can be regarded as linear. Each channel can automatically adjust the Transmission Bit Rate Based on interference and attenuation, to achieve better transmission performance. Compared with CAP and QAM, DMT has more advantages in signal-to-noise ratio, communication rate, bandwidth utilization, frequency compatibility, and actual performance.

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