By Hongbiao Zhang & Peter Wolff, Casa Systems
In a cable plant, it is essential to manage upstream spectrum and mitigate the impact of interferences, especially for lower spectrum bands that are susceptible to various types of noise ingress. The introduction of OFDMA in DOCSIS 3.1 (D3.1) with different bit-loadings at different minislots provides the benefits of enhanced capacity, as well as all flexibility in managing the spectrum usage. Yet surprisingly, not many MSOs have taken full advantage of DOCSIS 3.1 capabilities. Part of the reason is, accompanying the enhancements in capacity and flexibility, comes the complexity in computation, as we have to deal with finer granularity in both detecting the noise levels and reacting to them fast enough.
D3.1 provides an OFDMA profile, which defines, among other properties, a bit-loading pattern that could be adopted by a group of cable modems (CMs). Further, a Profile Management Application (PMA) utilizes the power of offline servers to tackle the computation complexity. A PMA server may take performance measurements on an OFDMA channel for a considerably long time in order to calculate OFDMA profiles. However, in a typical cable plant, especially in its lower band, we may observe many random bursts of noise that vary in time, frequency and power amplitude, and such noise might come and go swiftly. Figure 1 below gives a snapshot of upstream interference in an HFC network. Therefore, using PMA alone is not enough. As upstream interference is naturally observed at the CMTS, it makes sense to augment PMA with a CMTS-based solution that adapts quickly to interference levels measured locally.
This paper presents a novel approach that uses various OFDMA profiles with different bit-loading configurations to generate a dynamic profile. We consider upstream impairment with a duty cycle of one or a few seconds, and target a CMTS-based adaptive solution that yields optimal throughput across all CMs. To achieve the objective, we measure upstream channel impairments at a per-minislot level through constantly monitoring the receptions at the CMTS burst receiver. Based on the results, we could dynamically upgrade or downgrade IUCs. The decisions to upgrade or downgrade IUCs are considered for each individual minislot, and can take place locally and automatically without notifying the affected modems.
This approach is referred to as “Dynamic IUC” in the rest of the paper. The generated dynamic profile is referred to as “Dynamic scheduling IUC”, or “DS-IUC” in short.
This approach could be combined with any existing profile management mechanisms. For example, an operator could manually configure 2 OFDMA profiles on an OFDMA channel, or alternatively, a PMA server could elect 2 such profiles based on its calculation. The CMTS could then utilize these 2 profiles, plus NULL bit-loading as a special case, to generate a DS-IUC, thus providing a 3-tier adaptation for each minislot, at an interval much shorter than that of the PMA updates.