Introduction Asymmetric Digital Subscriber Line (ADSL) systems are the most popular form of consumer DSL systems currently deployed. It can support data rates up to 12 Mbps in a short loop. When the loop is extended to 5.5Km or longer, a meaningful data rate can still be achieved by adjusting its rate and frequency spectrum. However, many interferences reduce the coverage of ADSL, which creates loopholes in the network and cannot achieve acceptable service rates. Figure 1 Example of a modem architecture optimized for longer loops ADSL basics Interference compensation Figure 2 Modem architecture optimized for short loop (ADSL2+). Please note that the combination of the two paths of the modem optimized for long loops constitutes TEQ and FFT operations Performance improvement based on transmission spectrum Since ADSL is based on DMT modulation, it has great flexibility in forming transmission bands. We can use this flexibility to improve the coverage of ADSL systems, handle mixed CO and RT deployments, and minimize crosstalk. Flexible architecture For compensation of various interferences in long loops, two paths can be used according to the observed ISI on different parts of the channel. EC can be used for a path designed for the transition zone (where the echo is the most), while a receiver window or TEQ design that reduces RFI can be used for another path, because the above-mentioned interference will appear outside the transition zone (see Figure 1) . Conclusion By combining flexible modem design with the new ADSL standard, the coverage of ADSL modems has been expanded. This article summarizes some methods of compensating for general interference, coupled with the flexibility in the ADSL transmission spectrum, which can greatly increase the data rate for all loop lengths.
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This article will introduce some ADSL modem design techniques to compensate for the above interference, including inter-symbol interference (ISI), bridge taps and radio frequency interference (RFI), etc., as well as improvements based on the transmission spectrum, which can achieve longer coverage and minimize crosstalk and solve Some new standards for mixed central office (CO) and remote terminal (RT) deployment issues. With the continuous development of new standards and the introduction of flexible modem architectures, the combination of the two enables operators to expand the field of DSL services provided at a certain target rate.
ADSL uses discrete multi-tone (DMT) modulation to convert a single broadband communication channel into multiple narrowband sub-channels.
If the channel memory does not exceed the length of the cyclic prefix, the use of the cyclic prefix (tailing the signal after IFFT before the block to be transmitted) can achieve simplified equalization of the receiver. In this case, equalization is just a complex multiplication of sub-channels to eliminate channel influence.
For the case where the channel memory is larger than the cyclic prefix, a channel shortening filter (called a time domain equalizer or TEQ) is usually used in the designed receiver, so that most of the power of the TEQ and the channel series is limited to the cyclic prefix length +1 . This is the opposite of the single-channel case. In the single-channel case, linear equalizers usually design the minimum mean square error to flip the channel.
The typical ADSL deployment is Frequency Division Duplex (FDD), a single channel (twisted pair) is used to carry two signals, and is connected to the transmitter and receiver of each modem through a hybrid circuit. Broadly speaking, a hybrid circuit uses an isolation transformer for four-to-two line conversion. More specifically, the hybrid circuit performs analog echo cancellation to reduce the transmission signal (or echo) reflection in the receiver. Hybrid echo suppression depends on the impedance of the reflected line (through the transformer), and it varies according to different loop topologies.
The ADSL channel is affected by various interferences, and sometimes the data rate is even as low as ADSL cannot be used from a commercial point of view (for example, it is difficult for us to introduce the 32 kbps rate as broadband to the market). This part will study ISI, bridge taps and RFI, and analyze how to use appropriate modem design to limit the impact of interference on ADSL systems.
ISI
The ISI in the ADSL channel is the result of the combination of the twisted pair medium and the FDD filter. TEQ, which works better when ISI is strong near the transition zone, may produce notches in frequency. As noise propagates from the FFT, the notch will cause a loss of SNR, which will reduce the data rate.
One of the solutions to the above problem is to use multiple receive paths with independent channel shortening equalizers, each of which optimizes different parts of the channel. For example, one TEQ can be designed for the transition band with strong ISI, and another TEQ can be designed for the rest of the frequency band, where the ISI will be weaker, which helps to obtain a smoother frequency response. After FFT, the outputs of the two channels are combined to form a single output of the sub-channel (this is as simple as choosing between the two channels).
Bridge taps Bridge taps refer to the connection of multiple distribution cables to a single feeder cable. Only one power distribution cable is connected, the others remain open. Although this architecture allows operators to flexibly allocate lines, bridge taps can cause impedance matching problems and reflection problems in the channel. Depending on the architecture, the wiring in the home will have a similar impact.
The reflection of the transmitted signal caused by the bridged tap leads to an increase in the echo component of the received signal. Even if the ADSL system is operating in an FDD configuration, the enhancement of the echo (if not compensated) will cause the data rate to decrease. This is because in the case of a long loop, the echo power will be greater than the received signal power, which actually limits the gain setting of the receiver, which increases the effective noise level of the modem. Moreover, the diffusion from the FFT allows one frequency band to spread to other frequency bands, as if there are additional noise sources. Although the use of a sensitive band separation filter can help reduce the amount of diffuse echo, its disadvantage is that it will bring equalization problems to other receivers. In addition, it cannot solve the problem of modem noise level. Therefore, when dealing with the extra echo caused by the bridged tap, a more reasonable method is to proceed in two steps.
First, in order to optimize the dynamic range of the receiver, the hybrid circuit must be adjusted to adapt to the different reflected line impedance caused by the changing loop topology. In the simplest implementation, multiple hybrid circuits can be used for different loop topologies to achieve this.
For echo components that are not removed by the hybrid circuit matching, an echo canceller (EC) can be used to remove the remaining echo signals. ADSL systems can be designed to use traditional EC in the time domain, or to perform echo cancellation in the frequency domain (using some form of cyclic echo synthesis).
RFI
RFI is caused by the coupling of radio frequency signals in the ADSL frequency band (0~1104 or 2208KHz). For example, the AM radio is coupled to the signal due to the incomplete balance between the twisted pair and the modem front end. The spread of the sinusoidal interference signal of the FFT may cause the data rate to drop in many sub-channels. Therefore, it is necessary to develop some algorithms for processing RFI.
The main criterion of TEQ design is to shorten the channel, and the TEQ design based on MMSE is to deal with zero at the position of a strong RFI source. Although zeroing will reduce the speed, in general, the noise diffusion will be greatly reduced, and the cost of the speed reduction is reasonable. In this way, if there is an RFI source when calibrating TEQ, TEQ can be used to compensate for RFI.
Receiver windowing is the second method that can be used for RFI compensation. The window of the receiver uses the information in the cyclic prefix to form a window, which will affect noise. As long as the channel memory is shortened to the cyclic prefix minus the length of the window, the signal will not be affected. In this way, what we get is a window with side lobes, which attenuates much faster than a rectangular window. Therefore, even if the RFI appears after the modem is calibrated, the modem still shows high immunity to the harmful effects of RFI. The price it pays is the additional restriction (lower degree of freedom) brought about by the shortening of the channel. 
Wider spectrum shaping
The common shape of DSL channels makes high frequency attenuation greater than low frequency. In addition, the channel attenuation increases as the loop length increases. Since the FDD ADSL system allocates higher frequencies to the downstream to improve the performance of ADSL on longer loops, it is usually necessary to increase the downstream data rate.
ADSL2 is the second generation of ADSL. It uses a specific accessory (range extension ADSL2) to solve the above problems, that is, spectrum shaping is used to place the power in a better place of the channel or overlap the uplink and the downlink. The former can reduce the range of the downlink frequency or increase the power Implementation, while the latter requires an EC. In addition, the uplink power can reduce the frequency to avoid crosstalk and reduce the incoming downlink echo.
ADSL2+ deals with CO and RT
Mixed deployment situation The local loop unbundling makes it possible for one operator to provide services for a certain area from the CO while another operator can provide services for the same area from the RT. Since RT may be much closer to the end user than the CO, the crosstalk caused by RT will seriously affect the performance of the ADSL system running on the CO. Depending on the distance between the CO, RT and the end user, as well as the coupling between different lines, the impact on performance is also different.
ADSL2+ is a new high-speed version of the ADSL standard. Its downlink bandwidth is doubled, providing a possible solution for the mixed deployment of CO and RT. The basic idea is to make the formation of the ADSL2+ spectrum (maybe just close the sub-channels) to minimize the crosstalk caused by lower frequencies. Due to the short loop, the ADSL2+ system deployed by RT can achieve a more reasonable rate even if only higher sub-channels are used. Correspondingly, if the ADSL system deployed by CO is limited to lower sub-channels by the loop attenuation on the long loop, the crosstalk of the system from the ADSL2+ system deployed by RT will be less, so it can still achieve more reasonable s speed.
Minimize crosstalk
ADSL2 follows the principle of being a good neighbor of the existing system (co- and RT-mixed deployment is an example of it) and provides more ways to minimize crosstalk. This includes the use of an upper limit-based power reduction mechanism to remove power while still maintaining the same data rate; also includes the L2 mode that reduces the transmission power when there is less data, and can be repeated by fully controlling the bit loading change of the playback time Iterative (iteraTIve waterfilling) to minimize crosstalk.
For shorter loops where the above-mentioned interference causes less problems, the two paths can be combined to double the number of sub-channels that the system can handle. Assuming that the number of multiplications per cycle that TEQ can provide is constant, each of the two lengths L TEQ works at R sampling rate per second. Combining the two with the smallest logic, you can get a single working at 2R rate. Length L TEQ (split the filtering operation into two parts and use delay). In addition, the odd and even samples of the TEQ output can be routed to an independent FFT of size N, and the two FFT outputs can be combined together, plus an additional butterfly stage to generate a size of 2N FFT (basically follow the time FFT derivative Extraction). In this way, a modem that can achieve high-efficiency and high-speed operation of short loops is obtained, and can handle a large amount of interference of long loops (see Figure 2).