As cable modem and gateway markets evolve to meet the demand for more bandwidth and content, cable operators are increasingly excited over the next-generation cable data standard, namely, DOCSIS 3.1 (D3.1). Even though first silicon samples of D3.1 chipsets are not projected to be available until 2014, I'm seeing cable operators are eagerly making plans for D3.1's throughput enhancements to meet the needs of bandwidth hungry subscribers.
While the D3.1 standard offers backward compatibility to D3.0, it limits headend and modem changes to the physical layer (PHY) only. Despite the changes being limited to the PHY and to the cable frequency plan, the D3.1 standard provides a significant increase in capacity over existing D3.0 cable networks.
In a nutshell, D3.1 calls for expanding the existing upstream cutoff frequency from 42 MHz (or 85 MHz) to 200 MHz while expanding the downstream cutoff from 1 GHz to 1.2 GHz. In addition, D3.1 adds multicarrier orthogonal frequency-division multiplexing (OFDM) channels that can coexist anywhere in the downstream spectrum with legacy single-carrier quadrature amplitude modulation (QAM) channels. The addition of OFDM modulation and other coding techniques results in a 50 percent increase in throughput versus QAM channels across an equivalent bandwidth. For these reasons, D3.1 chipsets potentially offer significant capabilities and value to cable operators in delivering higher data rates relative to today's D3.0 systems.
Given these bandwidth expansions and multiple modulation schemes, agile wideband tuners will be critical to the market success of operators transitioning from D3.0 to D3.1 services. Such flexibility is out of reach of typical narrowband tuners since a unique tuner would be required for each QAM or OFDM channel, thereby crippling the frequency and channel flexibility, as well as the data throughput scalability demanded by cable operators.
Additional challenges emerge in the front-end filtering requirements for D3.1 cable modems and gateways, as cable operators balance expanding bandwidth to consumers against the additional costs of supporting the flexible frequency cutoffs. Each upstream/ downstream frequency cutoff represents a diplexer filter on the front-end. Since D3.1 calls for 42, 85, and 200 MHz cutoffs between upstream and downstream, a need exists for up to three diplexer filter banks.
This could triple the cost of front-end filtering in D3.1 modems compared with today's D3.0 cable modems and gateways, which only require one diplexer filter bank. However, digital-filtering techniques can be employed on the receiver to reduce the bill-of-materials of these diplexers. Integrating adequate signal filtering is one of the key remaining technical challenges to minimizing the cost burden associated with a broad D3.1 roll out.
These are exciting times in the cable modem and gateway markets as consumer demand for bandwidth and over-the-top content is growing rapidly. The D3.1 cable data standard promises to meet this rapidly growing consumer demand with flexible bandwidth and modulation schemes, which will result in significant increases in throughput. Stay tuned for further exciting innovations in D3.1 chipsets as first silicon rollouts are expected in 2014.
— Jim Koutras is product marketing director for MaxLinear, where he manages DOCSIS and Full-Spectrum Capture (FSC) cable tuner product lines.