Digital RF modulators for CCAP
They will help enable the evolution to new network architectures.
The demand for cable broadband digital video and data is increasing downstream data rates at 30 percent to 40 percent per year. Meanwhile, consumers expect to keep spending on an increasing number of connected devices at home, helping to assure future growth in downstream demand.
The current generation of analog downstream modulators will not be able to satisfy bandwidth demand in a Converged Cable Access Platform (CCAP) architecture in a cost-efficient way. Even before CCAP, it is becoming too expensive to meet the growing demand for bandwidth by using incremental upgrades to existing access platforms.
For performance reasons, analog transmitters are no longer the answer. Instead, a new generation of digital RF modulators provide high-density and low-cost solutions to meet future bandwidth requirements. Using the direct RF architecture, digital RF modulators enable CCAP for full-spectrum quadrature amplitude modulation (QAM) transmissions. These digital RF modulators also have up to 32x the capacity of analog modulators at about one-twentieth of the power dissipated per transmitted QAM channel.
CCAP combines the downstream services transmitted by edge QAM equipment for video and by cable modem termination systems (CMTSs) for high-speed Internet access. The transmitted QAM-modulated digital carriers include broadcast TV and narrowcast services like video-on-demand, switched digital video and high-speed Internet (see Figure 1).
These carriers fill the downstream cable TV spectrum in the 50 MHz to 1,000 MHz bandwidth. Up to 158 (6 MHz wide) QAM carriers (channels) occupy the entire spectrum from a single RF port in a CCAP headend. Up to eight to 12 RF ports are present in a single line card, and more than five downstream line cards can be present in a single 13RU CCAP chassis.
The downstream CCAP physical layer (PHY) requires highly dense RF modulators. These QAM modulators, in turn, must have low-power dissipation, scalability and QAM carrier agility.
In older-generation headends, QAM carriers from multiple superheterodyne analog transmitters were combined to fully populate the cable TV spectrum (see Figure 2). However, that methodology would potentially require more than 300 watts for a single CCAP RF port.
A direct RF transmitter (see Figure 3), however, easily performs digital upconversion (DUC) and modulation of QAM carriers in the digital domain, and it can be implemented in an ASIC or an FPGA. This type of digital architecture is enabled only by a wideband RF digital-to-analog converter (RF DAC) as the entire spectrum of QAM carriers is transmitted by a single RF chain.
There is a significant advantage with the direct RF transmitter in a CCAP system: The entire signal processing, now implemented in the digital domain, can benefit from CMOS process geometry. CMOS processes allow much higher channel densities in a small footprint and at low power dissipation. The benefits of this approach are easily seen with an example.
The MAX5880 is a 128-channel DUC and QAM modulator that drives an RF DAC. It accepts forward error correction (FEC)-encoded symbols from an FPGA and performs QAM modulation, pulse shaping and re-sampling of each QAM channel. It then combines, interpolates and modulates up to 128 QAM channels to drive an RF DAC. The sample rate for the RF DAC must be more than 2 giga samples per second (GSPS) to synthesize the entire cable TV band; it must also satisfy the stringent DOCSIS RF performance requirements. This design uses the 14-bit 4.6 GSPS MAX5882 RF DAC.
This DUC/QAM modulator oversamples the 1 GHz band at an update rate of more than 4 GSPS. Note that according to the Nyquist theorem, slightly more than a 2 GHz sample rate is required to synthesize the 1 GHz band. However, if a 2.5 GSPS DAC is used, the dominant harmonic distortion products like the second harmonic (HD2) and third harmonic (HD3) can fold back into the 1 GHz cable spectrum due to aliasing (see Figure 4a).
These distortion products can violate the in-band RF performance requirements for DOCSIS transmitters. However, if a 4 GSPS DAC is used (see Figure 4b), the HD2 and HD3 can never fold back into the cable TV band.
The RF output of the digital RF QAM modulator chipset is shown in Figure 5 for 128 channels that are 6 MHz wide in a 1 GHz span. The RF performance is fully compliant with DOCSIS RF requirements. The DUC and DAC dissipate a total of about 6 watts for transmitting 128 QAM channels. That translates to about a 95 percent savings in power dissipated per QAM channel compared with traditional analog RF modulators. Each of the chips is 17 mm x 17 mm in a 256-ball CSBGA package, thus enabling the RF port density required by downstream CCAP line cards.
The new digital RF modulator takes advantage of modern technologies like high-performance wideband digital-to-analog conversion and CMOS technology scaling. The digital RF modulator is a highly integrated solution that also satisfies stringent DOCSIS RF performance requirements. Telecommunications companies now have the opportunity to supply cable service providers with the technology to meet the broadband requirements of tomorrow – cost-effectively – today.