As the current video and data transmission services increase cable bandwidth requirements, the downlink data rate is increasing at a rate of 30% to 40% year by year. In addition, consumers also want to use the ever-increasing number of connected devices at home at the same data rate. From the long-term goal, the currently used analog downlink modem is difficult to meet the cost requirements. Service providers have also noted that it is expensive to upgrade existing access platforms to meet growing bandwidth demands.
As you can see, users and service providers face the same problem: analog transceivers are no longer able to meet ever-increasing bandwidth demands. Instead, a new generation of digital RF modulators provides a high-density, low-cost solution for future bandwidth needs. The digital RF modulator uses a direct conversion architecture that enables the Converged Access Platform (CCAP) to support Quadrature Amplitude Modulation (QAM) transmission over the entire frequency band. These digital RF modulators can have up to 32 times the capacity of analog modulators, and each QAM transmit channel consumes only about one-twentieth of the analog technology.
This paper describes the principles and advantages of implementing a digital QAM modulator for a CCAP system using a direct conversion architecture.
Reasons to replace analog transceivers with direct conversion transceivers
Cable TV (CATV) CCAP platform (Figure 1) integrates two downstream service transmission methods: one for edge QAM devices for video and the other for cable modem termination systems (CMTS) for high-speed Internet access. . QAM modulated digital carriers include broadcast television and narrowcast services such as video on demand (VoD), switched digital video (SDV) and high speed internet. These carriers are in the downstream CATV spectrum with a bandwidth of 50 MHz to 1000 MHz. Up to 158 (6 MHz bandwidth) QAM carriers (channels) occupy the entire spectrum of each RF port on the CCAP front end. Each line card can accommodate up to 8 to 12 RF ports, and each 13RU CCAP chassis can accommodate up to 5 downstream cards.
The downstream CCAP physical layer (PHY) requires highly dense RF modulators, so these QAM modulators must have low power consumption, scalability, and QAM carrier frequency agility. The pre-RF front-end equipment combines the QAM carriers from multiple super-heterodyne analog transmitters in the CATV spectrum (Figure 2). Each CCAP RF port power in this scheme may need to exceed 300 W. Direct conversion transmission The up-conversion (DUC) and modulation of the QAM carrier is easily implemented in the digital domain and can be implemented using an ASIC or FPGA (Figure 3). Since the entire spectrum of the QAM carrier is transmitted over a single RF link, this digital architecture can only be achieved with a wideband RF digital-to-analog converter (RF DAC).
Direct conversion transmitters have significant advantages in CCAP systems: the entire signal processing is implemented in the digital domain, benefiting from the CMOS process architecture. The CMOS process allows for very high channel densities with a small footprint and low power consumption, and the advantages of this approach are easily understood by the following examples.
The MAX5880 is a 128-channel DUC and QAM modulator that drives an RF DAC that receives forward error correction (FEC) encoded symbols from the FPGA, performs QAM modulation, pulse shaping, and resampling each QAM channel, then 128 channels. The QAM channel is combined, interpolated, and modulated to drive the RF DAC. The RF DAC must have a sample rate higher than 2Gsps for synthesizing the entire CATV band signal. It must also meet stringent DOCSIS RF specifications. This design uses a 14-bit 4.6Gsps MAX5882 RF DAC.
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