Evolution of the Modulation Method for HDSL2

Numerous modulation methods (or line codes) were considered as possible candidates for HDSL2 prior to the selection of sixteen-level trellis code pulse amplitude modulation (TC-PAM). In order to gain significant performance over the 2B1Q line code for HDSL, the general approach taken for performance improvement in the modem signal processing is to use a high-state trellis code with channel precoding. In addition, special spectral shaping would be required to find the proper balance between performance and spectral compatibility with other signals in the cable. The upstream and downstream channels would share common frequencies so the duplexing method for HDSL2 would be echo cancellation. Figure 4.14 shows the generalized functional block diagram of the transceiver structure for HDSL2. Note that this block is applicable for baseband systems such as PAM and single-carrier passband systems such as QAM or CAP.

Figure 4.14. General transceiver functional block diagram for HDSL2.

Early on in the development of HDSL2, the line codes considered for HDSL2 were multilevel PAM and multilevel CAP (or QAM). At first, all of the systems considered had used symmetric upstream and downstream spectra with the bandwidth set to one-half the symbol rate for the PAM-based systems and equal to the symbol rate for the CAP/QAM systems. The performance of all these systems was limited by self near-end crosstalk (SNEXT), and the best configuration of each system fell about 2 to 3 dB short of the requirements [38], [39], [41], [42]. The SNEXT limitation needed to be overcome by some method in order to meet or get close to meeting the objective requirements.

An alternative to using fully overlapped symmetric spectra is to use frequency-division multiplexing (FDM) of the upstream and downstream signal spectra. If the upstream and downstream spectra were totally nonoverlapping, then there would be no SNEXT. The performance limitation would be

NEXT from other systems or self far-end crosstalk (SFEXT) from other similar systems. Although the latter case is certainly preferable, in a multiservice cable environment there will be a mixture of overlapped and nonoverlapped signal spectra in the cable. Also, the higher frequency band in an FDM system would be susceptible to greater loop attenuation because of its placement at higher frequencies. Crosstalk coupling is also greater at the higher frequencies, so crosstalk impact to other services needs to be considered. Hence, the spectrum design of HDSL2 must be such that it operates in numerous NEXT and FEXT crosstalk environments. An early proposal that considered these issues was that of a "staggered FDM" scheme described in T1E1.4/96-340 [25].

In T1E1.4/97-073 [26], partially overlapped echo-canceled transmission (POET) was proposed, which involved overlapping nonidentical upstream and downstream spectra. The spectra were carefully shaped so as to maximize performance in the presence of crosstalk and to minimize the spectral compatibility impact into other services in the cable. Other approaches working off of the same core principle included POET-PAM [26], OverCAPed (oversampled CAP/QAM) [27], OPTIS [28], [29], and MONET [30], [31].

Most of the modulation schemes later proposed had PSDs with boosted energies at higher

frequencies that were above their nominal low-frequency values. Testing of HDSL systems in the presence of crosstalk from systems using the OPTIS shaping showed significant impact on the performance of the HDSL. Based on this testing, the OPTIS spectra were modified such that their spectral compatibility impact on HDSL would be less than 2-dB of degradation. The network

operators in North America unanimously agreed upon this degree of degradation. The resulting PSDs adopted for HDSL2 are shown in Figure 4.23 for the downstream channel and Figure 4.24 for the upstream channel.

Figure 4.23. Downstream channel spectrum.

Figure 4.24. Upstream channel transmit spectrum.

One element introduced in the evaluation of the above proposals is that of decoupling of the transmit signal bandwidth from the symbol rate, so that the bandwidth of the PSD could have significant energy in bandwidth greater than that determined directly by the symbol rate. An implication of such an approach would require the use of oversampling in the transceiver design, particularly in the receiver where at least a 2x or 3x sampling would be required, depending on the modulation

approach chosen. The benefits of this decoupling were found in T1E1.4/97-237 [28] when measuring the performance of proposed HDSL2 spectra in the presence of mixed crosstalk for PAM-based systems. This extension of the bandwidth showed particular benefit to the PAM approach under certain conditions of mixed crosstalk in the downstream channel. With a conventional DFE, the alias terms of the PAM spectrum reflect back at half the PAM symbol rate. With the excess bandwidth of the downstream OPTIS spectrum (i.e., the portion of the downstream spectrum above the overlapped

upstream-downstream region), the folding is such that this region fills the area of poor SNR due to NEXT from the upstream region.

The CAP/QAM system is processed as an analytic signal (i.e., a complex signal having only positive frequency components), and it folds in a nonreflective manner at the symbol rate. With the OPTIS spectrum, the folding does not benefit the passband system as it does the baseband PAM system because the frequencies that fold into the SNR-poor high end of the passband are substantially higher in frequency. The result is that with the OPTIS spectrum, the downstream channel administers 3 to 4.5 dB better performance in mixed crosstalk containing T1 AMI disturbers for the PAM system than for the CAP/QAM-based system. It was for this region that the PAM line code was chosen over CAP/QAM for HDSL2.

In summary, the core of the HDSL2 transceiver uses the OPTIS PSD with the PAM line code. The shape of the transmit spectrum is decoupled from the symbol rate to allow for flexible use of the excess bandwidth. The excess bandwidth provides improved performance in the downstream channel against mixed crosstalk that contains T1 AMI. The selected PAM configuration uses 3 information bits per symbol, which corresponds to an eight-level signal. The unique spectral shaping of OPTIS is the best compromise found in optimizing the performance against crosstalk in the loop plant and maintaining spectral compatibility with other systems. The nominal transmit power for the upstream channel is 16.5 dBm and that for the downstream channel is 16.8 dBm. This modulation technique was shown (using ideal DFE calculations) to have a minimum of 1.0 dB of margin on the worst-case test loop. In order to achieve the highly desired objective of 6-dB margin against worst-case

crosstalk, an advanced coding mechanism with a minimum coding gain of 5 dB is required. It was agreed to use a high order trellis code to help achieve the objective margin. The following sections provide an overview of the configuration defined in HDSL2 standard T1.418 [8].