COMMITTEE T1 TELECOMMUNICATIONS Working Group T1E1.4 (DSL Access) Arlington, VA; April 20, 1999

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1 COMMITTEE T1 TELECOMMUNICATIONS Working Group T1E1.4 (DSL Access) Arlington, VA; April 20, 1999 T1E1.4/ CONTRIBUTION TITLE: SOURCE: AUTHOR(S): PROJECT: Measured Spectral Compatibility of T1.413 FDD PSD Mask Compliant Non-Stationary Service with DMT Elastic Networks Inc. Patrick Stanley Jack Terry T1E1.4, Spectral Compatibility ABSTRACT This paper presents some measurements supporting the spectral compatibililty of a T1.413 FDD PSD mask compliant non-stationary service with a DMT service that is using bit swapping. EtherLoop technology modems use a Spectrum Manager function to monitor the line between bursts, and upon detecting significant coupling with an ADSL type service, switch to a safe mode of operation [1]. This safe mode of operation imposes a maximum output power limit for each EtherLoop baud rate that conforms to the T1.413 Issue 2 FDD PSD mask [2]. First, this paper points out a possible error in the test setup used in Selected Measurement on Non-Stationary and Stationary Crosstalk Effects upon FDM ADSL [3]. Next this paper presents test data which confirm that non-stationary services whose output power exceeds the T1.413 FDD PSD mask [2] can impact a DMT system that uses bit swapping. Finally, this paper presents test data which show that a non-stationary service whose power levels conform to the T1.413 FDD PSD mask [2] has no negative impact on a DMT system that uses bit swapping. NOTICE This contribution has been prepared to assist Accredited Standards Committee T1 Telecommunications. This document is offered to the Committee as a basis for discussion and is not a binding proposal on Elastic Networks Inc. (Elastic), Northern Telecom (Nortel), or any other company. The requirements are subject to change in form and numerical value after more study. Elastic and Nortel specifically reserve the right to add to, amend, or withdraw the statements contained herein. CONTACT: Patrick Stanley; pstanley@elastic.com; Tel: ; Fax:

2 1. Introduction EtherLoop is a new technology that incorporates a number of new concepts. EtherLoop transmits data in packetized bursts in one direction at a time, and is quiet between bursts. EtherLoop is adaptive and can adapt to support the highest bit rates that it can achieve on an individual basis. The rate adaptation at each end of the loop is independent and proceeds separately based on the SNR and BER conditions at each end. The details of the rate adaptation mechanism of EtherLoop are not discussed in this contribution. The maximum allowed transmit power adaptation is independent and proceeds separately under the control of the Spectrum Manager, based on the coupled near end crosstalk (NEXT) environment at each end. Each EtherLoop modem has its own Spectrum Manager. There are three transmission parameters that EtherLoop may adapt: speed, number of constellation points, and transmit signal power. The speed can be one of a dozen different values with increasing speeds using higher baud rates and higher frequencies. At each speed, any one of three modulation densities are supported: QPSK (2 bits per Hz), 16-QAM (4 bits per Hz), and 64-QAM (6 bits per Hz). The total average transmit power of EtherLoop can be between 14dBm and -31dBm, dependent on both loop loss, and the crosstalk environment, which is determined by the systems that have significant NEXT coupling with the EtherLoop system s loop. The training mechanism determines how much transmit power is required to overcome the loop loss, but this transmit power is limited to the maximum power specified by the Spectrum Manager. In other words, on a short loop, with little loss, the transmission power will be low. As the loop loss increases, the transmission power will increase, until the maximum allowed transmit power has been reached. This maximum allowed transmit power is what is controlled by the Spectrum Manager. Once significant coupling has been detected with an ADSL type service[1], the Spectrum Manager imposes maximum transmit levels on each baud rate that conform to the T1.413 Issue 2 FDD PSD mask[2]. This paper presents test data that show the non-stationary burst transmissions of EtherLoop under this safe mode of operation having no impact on a bit swapping DMT system. T1E1.4/ [1] presented some measurements of the performance of a downstream DMT ADSL modem in the presence of both stationary and non-stationary NEXT with a PSD equivalent to an EtherLoop modem transmitting at full power with a center frequency of 250kHz. The spectrum manager feature of the EtherLoop modem[1], would not allow this power level at this center frequency in the presence of significant coupling with ADSL. Furthermore, the testing performed by Alcatel used a 2MHz sample rate for the Wavetek 395 arbitrary waveform generator[1]. The lowest corner frequency available for the integrated reconstruction filter of this unit is 10MHz. Therefore, unless the Alcatel coupling box contained a 1MHz low pass filter, the test signals generated contained an improper image reflected about the Nyquist frequency of 1MHz. While the images of the baud rates used in [1] should have be out of band for the ADSL modems, as the baud rate of the simulated EtherLoop signal increases, these images start moving in band. This testing has been redone, using the TI TNETD2000 ADSL/G.Lite chipset EVM, with valid operating conditions of the EtherLoop modem, and with a sample rate of 20MHz to allow use of the reconstruction filter available within the Wavetek 395. This data is presented, along with test data from every baud rate of EtherLoop. Under these valid operating conditions, the nonstationary NEXT resulting from EtherLoop did not cause interference with the operation of the bit swapping DMT system. 2. Assumptions used in T1E1.4/ [1] The T1E1.4/ [1] assumed that the output power used at the 125kHz center frequency was the relaxed +14dBm. The safe mode PSD mask, which will be used by the EtherLoop modem if 1

3 significant coupling with an ADSL service has been identified[1], allows only 1dBm of transmit power at this frequency in order to conform to the T1.413 non-overlapped PSD mask. Furthermore, the testing performed by Alcatel used a 2MHz sample rate for the Wavetek 395 arbitrary waveform generator[1]. The lowest corner frequency available for the integrated reconstruction filter of this unit is 10MHz. Therefore, unless the Alcatel coupling box contained a 1MHz low pass filter, the test signals generated contained an improper image reflected about the Nyquist frequency of 1MHz. While the images of the baud rates used in [1] should have be out of band for the ADSL modems, as the baud rate of the simulated EtherLoop signal increases, these images start moving in band. This testing has been redone, using the TI ADSL/G.Lite chipset EVM, with valid operating conditions of the EtherLoop modem, and with a sample rate of 20MHz to allow use of the reconstruction filter available within the Wavetek Test Setup EtherLoop may transmit a dozen different symbol or baud rates, and each symbol rate can support three different levels of modulation, allowing a wide variety of transmitted bit-rates. Additionally, EtherLoop can adaptively vary its transmit power level from 14 dbm to -31 dbm depending on loop conditions. The bit-rates, frequency bands and other transmission parameters of simulated EtherLoop are presented here. The upstream (subscriber to central office) and downstream (central office to subscriber) channels both use square-root raised cosine shaping with nominal 30% excess bandwidth. An EtherLoop signal has a passband bandwidth equal to the baud rate, and the 30% excess bandwidth extends the nominal start and stop frequencies up to 1.30 times the baud-rate. The carrier frequency is equal to the baud rate, which allows a transceiver implementation with a simple and high performance modulator and demodulator. Table 1 shows the 12 possible baud rates of EtherLoop, with the corresponding safe mode maximum power levels. Table I: EtherLoop Safe-Mode Powers Center Frequency (khz) Max Power To Protect ADSL Up (dbm) Max Power To Protect ADSL Down (dbm) NA NA NA NA NA NA NA NA NA NA means that transmission at these center frequencies is Not Allowed. Figure 1 shows the aggregate PSD of all of the allowed Upstream EtherLoop buad rates in safe mode. Only one baud rate is active at any given point in time. 2

4 EtherLoop Safe Mode Upstream PSD(dBm/Hz) igure Freq (khz) EtherLoop Safe Mode Upstream T1.413, Issue 2, Upstream Figure 1: EtherLoop Safe Mode Aggregate Upstream PSD 3

5 Figure 2 shows the aggregate PSD of all of the allowed Downstream EtherLoop buad rates in safe mode. Only one baud rate is active at any given point in time. EtherLoop Safe Mode Downstream PSD(dBm/Hz) Freq (khz) EtherLoop Safe Mode Downstream T1.413, Issue 2 FDD Downstream Figure 2: EtherLoop Safe Mode Aggregate Downstream PSD 4

6 TI EVM ATU-C Consultronics DLS400A TI EVM ATU-R HP 8110A Wavetek 395 Figure 3. Test Setup Figure 3 shows a block diagram of the test setup. A PN sequence was used to generate a QAM 16 waveform at the appropriate baud rate. The waveform was filtered by the 30% excess bandwidth square root raised cosine shaping filter. This signal was scaled to the appropriate power level, and filtered by the Unger 99% worst case NEXT coupling model. This signal was generated, with a 20MHz sampling rate, by the Wavetek 395 arbitrary waveform generator. The Wavetek s internal 10MHz Bessel reconstruction filter was used to filter out any images. The HP8110A pulse generator was set to generate the following pattern [3]: Followed by 300 more 0 s. This pattern was used as the external trigger input of the Wavetek 395. The period of the pulses was setup to match the equivalent of a 64 byte QAM 16 burst at the specified baud rate, as shown in Table 2. So, each 1 above represents 64 bytes worth of QAM 16 transmission, and each 0 represents a period of silence with a duration equivalent to 64 bytes worth of QAM 16. This was designed to reproduce Pattern 2 of [3]. 5

7 Table 2: 64 Byte QAM16 Burst Center Frequency Period (msec) (khz) The output of the Wavetek 395 was connected to the Port B external NEXT port of the Consultronics DLS400A. The NEXT noise generator of the Consultronics was left on, and the output enable of the Wavetek 395 was used to apply and remove the NEXT. The Consultronics was programmed to simulate 15,000 ft of 26 AWG wire. The TNETD2000 EVM was programmed for G.Lite operation. The impact of CPE NEXT would be seen in the lower tones of the downstream signal, which are common to full rate FDD ADSL and G.Lite. The impact of CO NEXT would be on the upstream tones, which are also common to full rate FDD ADSL and G.Lite. Bit swapping was enabled in this G.Lite load. As a baseline, the DMT modems were allowed to train, with the NEXT noise generator enabled, but with the Wavetek disabled. The various baud rates and power levels were then applied to the line, and the DMT modems were allowed to train in the presence of this NEXT. When the 125kHz, +14dBm signal, triggered by the pulse pattern described above, was applied at the ATU-R, the bit swapping DMT modems trained to a throughput level that was lower than the baseline. While these particular DMT modems did not get locked into an infinite training loop by this condition, the fact that the throughput was impacted opens up the possibility for dynamic instability in the presence of non-stationary interference of this level at this frequency. Table 3 shows these results, as well as several other cases. It is seen that whenever the power level exceeds T1.413 Issue 2 FDD PSD mask,[2] the trained throughput is impacted, and whenever the power level conforms to T1.413 Issue 2 FDD PSD mask [2], the trained speed is not impacted. If the presence of the NEXT does not impact the bit swapping DMT system, then the timing of the burst is irrelevant. Whenever EtherLoop s Spectrum Manager detects significant coupling with an ADSL type system, the maximum allowed power levels are reduced to the values found in Table 1 [1], which conform to the T1.143 Issue 2 FDD PSD mask[2]. These levels, as measured here, had no impact on a bit swapping DMT system. 6

8 Baud Rate (khz) Power Level (dbm) Table 3: ATU-R Injected NEXT Test Results Port B Port B Exceeds T1.413 Mask Downstream (kbps) Upstream (kbps) Below Baseline X X X X X X X X Baud Rate (khz) Power Level (dbm) Port B Table 4: ATU-C Injected NEXT Test Results Port B Exceeds T1.413 Mask Downstream (kbps) Upstream (kbps) Below Baseline X X X X X X X X X X X X X An Example of Real World Spectral Compatibility A second test setup was created to allow a pair EtherLoop modems and the DMT modems to operate on a pair of coupled loops within the same binder. This test setup is shown in Figure 4. The coupling profile of the pair of 3kft 24 AWG segments attached to the ATU-C and the EtherLoop CO is shown in Figure 3. The coupling profile of the pair of 3kft 24 AWG segments attached to the ATU-C and the EtherLoop CO is shown in Figure 4. The Consultronics boxes were each set to 8kft of 24AWG wire. The EtherLoop modems were setup according to the Table 5. The EtherLoop modems were transmitting, and then the G.Lite bit swapping DMT modems were allowed to train. The G.Lite modems achieved the throughputs shown in Table 5. The EtherLoop modems were transmitting with QAM 16 modulation, and continued operating at the same baud rate and modulation level after the G.Lite was applied. 7

9 Table 5: Example Results Using EtherLoop Modems EtherLoop Downstream Baudrate EtherLoop Downstream Power Level EtherLoop Upstream Baudrate EtherLoop Upstream Power Level G.Lite Downstream G.Lite Upstream 357kHz 14dBm 89kHz 14dBm 1536kHz 512kHz TI EVM ATU-C Consultronics DLS400A TI EVM ATU-R EtherLoop CO Consultronics DLS400A EtherLoop CPE Characterized 3kft, 24 AWG Characterized 3kft, 24 AWG Figure 4: Second Test Setup ATU-C Pair to Pair Coupling Profile Coupling Loss (db) Fequency (khz) Coupling (db) Unger 99%, N=1 Figure 5 ATU-C Pair-to-Pair Coupling 8

10 ATU-R Pair to Pair Coupling Profile Coupling Loss (db) Fequency (khz) Coupling (db) Unger 99%, N=1 Figure 6 ATU-R Pair-to-Pair Coupling 5. Recommendations Based on the spectral compatibility demonstrated by these measurements, it is recommended that any non-stationary service that can be shown to be spectrally compatible with guarded services be allowed within the ANSI spectral compatibility document. Elastic Networks plans to provide additional information in the form of future contributions. 6. References [1] Telcordia (formerly Bellcore), Elastic Networks, Spectral Compatibility and EtherLoop s Spectrum Manager, ANSI T1E1.4/99-191R1 [2] "Network and Customer Installation Interfaces, Asymmetric Digital Subscriber Line (ADSL) Metallic Interface," T1.413 Issue 2 (DRAFT), T1E1.4/97-007R5, American National Standards Institute, Inc. [3] Jim Sticia and Dan Johnson, "Selected Measurements of Non-Stationary and Staionary Crosstalk Effects on FDM ADSL,", ANSI T1E1.4/99-040, February 1, [4] Bellcore, Elastic Networks, Analytical Study of the Spectral Compatibility of ADSL, HDSL, ISDN, T-Carrier and POTS with EtherLoop,, ANSI T1E1.4/98-334, December