Project Scope

The DOCSIS upstream is an example of burst TDMA bandwidth constrained communications, where cable headend demodulator receives packets from many downstream modulators. Specific design is needed in cable network test equipment application, when upstream demodulator has to self-synchronize to TDMA schedule.

DOCSIS upstream demodulator consists of the following modules:

• packet detection (for synchronization)
• automatic gain control
• timing recovery
• carrier offset estimation and compensation
• phase offset estimation and compensation
• equalizer.

Spectral shaping is by square-root Nyquist shaping filter with rolloff factor 0.25, with the following data rates:

Required MER performance is according to DOCSIS 3.0 standard specifications, and upstream demodulator has to support QPSK, 8 QAM, 16 QAM, 32 QAM, and 64 QAM modulations for TDMA.

Solution

The packet detection is based on crosscorrelation between preamble and incoming signal, the peak is detected using CFAR (Constant False Alarm Rate) algorithm.

The efficient designs are based oversampled multirate polyphase filter banks, conceptually explained in the paper “Multirate Digital Filters for Symbol Timing Synchronization in Software Defined Radios”, fred harris at al.

The timing acquisition can be implemented using feed forward Oerder and Meyr, and tracking with feedback loop with Gardner timing error detector. The 1/64 symbol resampling of polyphase filter bank gives a good smoothness of timing recovery loop for 64-QAM.

A combination of Non-Data-Aided (NDA) and Data Aided (DA) feedforward frequency synchronizer followed by feedforward Non-Data-Aided (NDA) carrier phase estimation is a standard approach. After zero-forcing (ZF) 8 tap equalizer processing, the residual phase offset between equalized and detected symbols can be measured and fed to external frequency loop.

Added Value

• Implementation of standard singe- and multicarrier modems
• Burst or streaming
• Design of custom signal detection algorithms
• FPGA implementation
• Hardware integration