Adsl modem technology
Abstract: Introduce the characteristics of ADSL and the three modulation technologies QAM, CAP and DMT, and compare the three modulation technologies.
Keywords: ADSL; QAM; CAP; DMT; modulation technology
ADSL and its characteristics
ADSL (Asymmetric Digital Subscriber Line, Asymmetric Digital Subscriber Line) is an asymmetric rate copper wire access network technology, which uses digital coding technology to obtain a larger data transmission rate from the existing copper telephone line, while not Interfere with regular voice services on the same line. ADSL creates three channels on the copper telephone line that can work simultaneously, namely High Speed ​​Downstream (High Speed ​​Downstream), Medium Speed ​​Duplex (Medium Speed ​​Duplex) and POTS channel (it is used to ensure that even if the ADSL connection fails At this time, voice communication can still operate normally). ADSL uses advanced digital signal processing technology and new algorithms to compress data, so that a large amount of information can be transmitted on the Internet at high speed.
In order to separate the effective bandwidth on the telephone line and generate multiple channels, ADSL modems are generally implemented in two methods, frequency division multiplexing (FDM) or echo cancellation (Echo CancellaTIon) technology. FDM allocates a frequency band in the existing bandwidth as a data downlink channel, while allocating another frequency band as a data uplink channel. Downlink channels are further divided into multiple high-speed channels and low-speed channels by time division multiplexing (TDM) technology. Similarly, the upstream channel is also composed of multiple low-speed channels. The re-echo cancellation technology superimposes the upstream frequency band and the downstream frequency band, and distinguishes the two frequency bands by local echo cancellation. Of course, no matter which of the two technologies is used, ADSL will separate the 4KHz band for telephone service (POTS).
ADSL's asymmetry is manifested in the bidirectional data rate asymmetry. Generally, the downstream rate is much larger than the upstream rate. It is very absorbing for Internet surfing and remote LAN user access. ADSL can provide end users with 8 Mbit / s downlink transmission speed and 1 Mbit / s uplink transmission speed, which is much higher than the speed of traditional Modem or ISDN.
2 ADSL modulation technology
At present, there are three ADSL modulation technologies widely used in the world: Quadrature Amplitude Modulation (QAM), Quadrature Amplitude Modulation (CAP), and Carrierless Amplitude-plase ModulaTIon (Discrete MulTItone).
2.1QAM modulation technology
In QAM modulation, the transmitted data is divided into two signals of the original 1/2 rate in the bit / symbol encoder, respectively multiplied by a pair of quadrature modulation components, summed and output. The receiving end completes the reverse process, orthogonally demodulates two opposite code streams, the equalizer compensates for the distortion caused by the channel, and the decider recognizes the complex signal and maps it back to the binary signal. With QAM modulation technology, the channel bandwidth must be at least equal to the symbol rate. For timing recovery, additional bandwidth is required, which is generally increased by about 15%. Compared with other modulation technologies, QAM modulation technology has the characteristics of full use of bandwidth and strong anti-noise. But the main problem of QAM modulation technology used in ADSL is how to adapt to the large performance differences between different telephone lines. To obtain more ideal operating characteristics, the QAM receiver needs an input signal with the same frequency spectrum and corresponding characteristics as the transmitter for decoding. The QAM receiver uses an adaptive equalizer to compensate for the distortion generated by the signal during transmission, so QAM is used. The complexity of ADSL system comes from its adaptive equalizer.
2.2CAP modulation technology
CAP modulation technology is developed based on QAM modulation technology. It can be said that it is a variant of QAM technology. The difference between it and QAM is the digitization of its implementation means. The input data is sent to the encoder. In the encoder, the m-bit input bits are mapped to k = 2m different complex symbols, and the k-CAP line coding is composed of k different complex symbols. After encoding, the sum is sent to the in-phase and quadrature digital shaping filters, after summing, it is sent to the D / A converter, and finally sent out through the low-pass filter.
CAP demodulation uses soft-decision technology and uses an equalizer for adaptation. The spectrum shape produced by CAP is the same as QAM.
The CAP code is a two-dimensional redundant linear modulation code, the power spectrum is a band-pass type, the upper limit is 180KHz, and the low frequency cut-off frequency is less than 20KHz. The CAP is less affected by impulse noise with rich low-frequency energy and high-frequency near-end crosstalk. There is no low-frequency delay distortion, and the intersymbol interference caused by group delay distortion is also small. The main technical difficulty of CAP technology for ADSL is to overcome the interference of near-end crosstalk to the signal. Generally, this problem is solved by using a near-end crosstalk canceller or near-end crosstalk equalization. Compared to QAM, CAP is easier, more convenient, and more flexible to implement.
2.3DMT modulation technology
DMT modulation technology divides the 0-1104KHz frequency band of copper cable lines, of which 0-4KHz is the voice segment, used for the transmission of ordinary telephone services, the other frequency bands are divided into 255 subcarriers, and the frequency interval between the subcarriers is 4.3125 KHz. QAM modulation is performed on each subcarrier to form a subchannel. A part of subcarriers in the low frequency part is used for uplink data transmission, and the remaining subcarriers are used for downlink signal transmission. The separation point of the uplink and downlink carriers is set by a specific device.
The range of DMT modulation capability is 0-15bit / s. The DMT modulation system uses these 255 sub-channels according to the situation, and the transmission rate of the sub-channel can be determined according to the instantaneous attenuation characteristics, group delay characteristics, and noise characteristics of each sub-channel. In the middle frequency sub-channel with excellent performance, the general modulation capability is greater than 10bit / s / Hz, while in the low-frequency or high-frequency sub-channel, DMT technology can adaptively reduce the modulation capability to 4bit / s / Hz according to the channel performance. Channels that cannot transmit data will be closed.
If N sub-channels are actually used, each sub-channel uploads a bit, and maps to a DMT complex sub-symbol (j = 1, ... N), using 2N point IFFT (inverse fast Fourier transform) to convert N in the frequency domain A complex sub-symbol change number of 2N real samples (j = 1, ..., 2N) is output on the line after digital-to-analog conversion and low-pass filtering. The receiving end performs the opposite transformation, performs FFT transformation on the sampled 2N time-domain samples, and obtains N complex sub-symbols in the frequency domain. After decoding, the original input bit stream is restored.
3 Conclusion
The frequency band of 0-1.1MHz of the telephone twisted pair is non-linear, different frequency attenuation is different, noise interference is different, and the delay is also different. If the whole frequency band is used as a channel, a single frequency noise interference will affect the entire transmission performance . The DMT modulation method divides the entire frequency band into many channels. Each channel has a narrow frequency band and can be considered as linear. Each channel can automatically adjust the transmission bit rate according to interference and attenuation to obtain better transmission performance. Compared with CAP and QAM, DMT has advantages in signal-to-noise ratio, communication rate, bandwidth utilization, frequency compatibility, and actual performance.
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