Brief on xdsl interference

Brief on xdsl interference Prepared by C. Szabo - EUROCONTROL 1. Introduction The xdsl acronym includes the following Digital Subscriber Lines (DSL): - High Data-Rate Digital Subscriber Line (HDSL) - Single-Line Digital Subscriber Line (SDSL) - Asymmetric Digital Subscriber Line (ADSL) - Very-high-Data-rate Digital Subscriber Line (VDSL). HDSL is simply a better way of transmitting T1/E1 over copper wires, using less bandwidth without repeaters. It uses more advanced modulation techniques to transmit 1.544Mbps over lines up to 12,000 feet long. SDSL is a single-line version of HDSL, transmitting T1/E1 signals over a single twisted pair, and able to operate over the plain old telephone service (POTS), so that a single line can support POTS and T-1/E-1 at the same time. It fits the market for residence connection which must often work over a single telephone line. However, SDSL will not reach much beyond 10,000 feet. At the same distance, ADSL reaches higher rates. Asymmetric Digital Subscriber Line (ADSL) is a new modem transmission technology that converts existing twisted-pair telephone lines into access paths for high-speed communications of various sorts. ADSL can transmit more than 6 Mbps to a subscriber - enough to provide Internet access, video-on-demand, and LAN access. Very-high-data-rate Digital Subscriber Line (VDSL) is the technology capable of delivering full-service up to 52 Mbps of bandwidth on a single twisted-pair copper loop. Although these are wired systems, a by-product is energy radiated from the lines, the amount of which is largely dependent upon the balance and quality of the cable. 2. System description xdsl is intended to complete the connection with the customer's premise. ADSL in interactive mode can transmit 6 Mbps. This increases the existing access capacity by more than fifty times enabling the transformation of the existing public network. No longer is it limited to voice, text, and low-resolution graphics. It promises to be nothing less than an ubiquitous system that can provide multimedia (including fullmotion video) to the entire country. It transmits two separate data streams with much more bandwidth devoted to the downstream leg to the customer than returning. It is effective because symmetric signals in many pairs within a cable (as occurs in cables coming out of the central office) significantly limit the data rate and possible line length. ADSL succeeds because it takes 1

advantage of the fact that most of its target applications (video-on-demand, home shopping, Internet access, remote LAN access, multimedia, and PC services) function perfectly well with a relatively low upstream data rate. MPEG movies require 1.5 or 3.0 Mbps down stream but need only between 16 kbps and 64 kbps upstream. The protocols controlling Internet or LAN access require somewhat higher upstream rates but in most cases can get by with a 10 to 1 ratio of downstream to upstream bandwidth. VDSL is the highest-rate DSL technology available. Running at speeds of up to 52 Mbps, VDSL is the next generation of DSL, with higher throughput and simpler implementation and requirements than ADSL. VDSL is both symmetric and asymmetric. VDSL is nearly ten times faster than ADSL and is over thirty times faster than HDSL. The tradeoff for increased speed is loop length: VDSL has a shorter reach in the loop. The frequency band used by VDSL is shown in the following figure: 3. The Modem Market Sales in the modem business started out slowly until customers started buying PCs. Likewise, costs were high until the volumes picked up. When the 14.4-kbps modem was first introduced, it cost $14,400 or one dollar per bit. Today, a much faster consumerlevel modem with many more features costs only $100 $300, making it unusual for a home PC to be without a modem. Over the years, customers watched modem vendors evolve their products on a standards basis. This technique, although somewhat time consuming, was very important and led to significant feature enhancement. Initially, several modulation schemes were in use, but by the time the V.34 modem came out all of the major modem-modulation schemes were combined in that standard - giving the customer one modem that could be used in many applications. As the modem market matured, customers became less concerned with the internals of standards and more concerned with features, size, and flexibility. As a result of the progress in analog-modem technology and with the advent of massmarket consumer-level PCs, there are over 500 million modems in the world today. The xdsl modem market will follow similar market patterns. Today, things like modulation schemes, the type of protocol supported to the home or small business, and costs of the units are the main topics. As the xdsl market matures, most likely in a fashion similar to that of the analog modem, customers will become less concerned with modulation and protocols. On the other hand, they will look for vendors that provide plug-and-play interoperability with their data equipment, ease of installation, the best operating characteristics on marginal lines, and minimalist size and power requirements. 2

Electronic mail and access to the Internet represents a major market at the dawn of the 21 st century. Even in Western Europe, domestic penetration is far from the saturation point. Further market growth mainly depends on equipment and access prices. Many existing and new upcoming services in the future are rapidly increasing the telecommunications market, e.g. telelearning, teleworking, e-commerce, value added services (remote meter reading, customer profiling, security), telephony, multimedia, file transfer, multicasting (special advertising). 4. Threat for Aviation The Civil Aviation has raised a concern that the xdsl radiation has the potential to adversely affect, or even deny ADF operation and HF communications. Civil Aviation uses the HF band for en-route and off route, flying at high and low level over long distances, such as communications across the Atlantic route. xdsl could interfere with the ground-receiver installation and possibly with airborne use at low altitude. It is also used for long range maintenance purposes and reporting major in-flight failures; the receive site could be situated in the domestic environment and have a high gained antenna system. Under some conditions this would intensify the effects of xdsl on the communication system. Non Directional Beacons (NDBs) and the associated Automatic Direction Finder (ADF) avionics are widely used to support en-route navigation and airport approach procedures. After tuning to a particular NDB, the pilot uses the audible ident as a confirmation that the correct beacon has been selected and the ADF instrument shows the heading compared with the NDB. Other users/services that can be affected: The Navy, The Air Force, Military mobile, MoD, Radio Broadcast Service, Radio Amateurs, News Agencies, Embassies etc. 5. Discussion 5.1. NDB-ADF issues One potential source of interference within the NDB frequency range (255kHz to 526.5kHz) is the xdsl. xdsl is considered to be a distributed source and always present because xdsl is distributed in frequency and geographically, it is likely that there will be many on and off frequency undesired signals arriving at the ADF antenna from all angles and with varying signal strength. It is the cumulative effect of all of those signals, and their effect upon ADF, which needs to be determined. Theoretical studies to date have predicted that the cumulative effect at an airborne platform from cabled transmissions could be significant, [Ref 7]. Non Directional Beacons (NDBs) and the associated Automatic Direction Finder (ADF) avionics are widely used to support en-route navigation and airport approach procedures. After tuning to a particular NDB, the pilot uses the audible ident as a confirmation that 3

the correct beacon has been selected and the ADF instrument shows the heading compared with the NDB. The ADF-NDBs are planned for use as an aid to general and commercial aviation navigation for many years to come. It is important therefore that the accuracy, integrity and availability of those systems are maintained to the levels defined in the appropriate standards. ( Ref. SE35(00)37). NDB-ADF systems must be protected from unwanted interference. The maximum accepted interference can be determined by the following criteria: The change in indicated bearing shall not exceed 3 degrees when an undesired signal is present. In order to allow the ADF accuracy requirement to be met, the signal strength of one onfrequency undesired signal that is at 90 degrees to the desired signal must be at least 15dB below the desired signal strength. The minimum desired field strength in which the ADF is specified to operate is 70µV/m, (i.e. 37 dbµv/m). Therefore, the maximum allowable field strength at the ADF antenna from any single source of on-frequency undesired signal at 90 degrees to the desired signal is 22 dbµv/m). Field strengths greater than this could cause the accuracy of the ADF to fall below acceptable levels. Since the existing standards define the undesired signal as a single source, it is important that the cumulative xdsl signal can be described in terms that will facilitate comparison. The effect upon ADF from the cumulative xdsl signal could then be predicted. It should be recognised that it will only be possible to specify meaningful protection requirements once the cumulative effect has been determined. In establishing the acceptable interference level from xdsl, at least a 6 db safety margin and the multiple entry factor has to be considered, due to interference from many other sources. 5.2. HF communications issues The HF communication receivers could also be affected by xdsl. Aeronautical HF communications - Technical parameters - The HF band in which 12 sub-bends are used: 2850-22000 khz - The aeronautical HF sub-bands: 2.850-3.025, 3.400-3.500, 4.650-4.700, 5.450-5.480, 5.480-5.680, 6.525-6.685, 8.815-8.965, 10.005-10.100, 11.275-11.400, 13.260-13.360, 17.900-17.970, 21.294-22.000 - Emission type: J3E - RX Bandwidth: 3kHz - Receiver sensitivity for 6dB S+N/N: 2uV - Co-channel protection ratio: 15 db used for frequency planning (ITU-R S27/25) - Maximum level of interfering signal: 0dBuV/m (This value is considered as a limitation of the atmospheric noise. The aeronautical HF receiver sensitivity would allow lower level signal detection with acceptable S/N. The 0dBuV/m is dependent on the atmospheric conditions, season of the year etc, it is proposed as a maximum acceptable value for interference computation.) 4

6. Conclusions Studies and/or practical trials are undertaken to quantify the ADSL signal that could be present at an ADF and HF receiver. These studies should enable a radiation limit to be set for xdsl emissions (single line and accumulated over a densely populated area) that would protect NDB/ADF operation and HF communications. Any harmful interference to the aviation bands has to be avoided. Aviation is contributing to the CEPT PT SE35 studies with the goal to propose adequate protection limits and the associated measurement method for all types of cable transmissions. References 1 "International Standards and Recommended Practices - Aeronautical Telecommunications - Annex 10 to the Convention on International Civil Aviation. Volume 1 (Radio Navigation Aids)" International Civil Aviation Organisation (ICAO) 5th Edition of Volume 1 - July 1996 2. "International Standards and Recommended Practices - Aeronautical Telecommunications - Annex 10 to the Convention on International Civil Aviation. Volume 5 (Aeronautical Radio Frequency Spectrum Utilisation)" International Civil Aviation Organisation (ICAO) 1st Edition of Volume 5 - July 1996 3. "Clarification of Aeronautical Radiobeacon Protection Criteria and Consideration of the Radiation Limits Proposed in MPT 1570." Civil Aviation Authority Ref: 8AP/88/08/04 - February 2000 4. "Minimum Performance Specification for Airborne Automatic Direction Finding Equipment." European Organisation for Civil Aviation Electronics (EUROCAE) ED-51 October 1983 5. "Minimum Operational Performance Standards for Automatic Direction Finding (ADF) Equipment." Radio Technical Commission for Aeronautics (RTCA) DO-179 May 1982 6. "Aircraft Radio Equipment." 5

CAP 208. Volume 1: Minimum Performance Requirements Civil Aviation Authority 7. "Protection of "Sensitive" Receiving Sites - Paper for RA Working Group on HF Mains Signalling." Jonathan Stott, BBC October 1999 8. Darren Roberts "Description of NDB and ADF Operation and Definition of Protection Requirements." 9. ViaGate Tehnologies: Very-High-Data-Rate Subscriber Line (VDSL) Tutorial on www.iec.org 6