Case study

A classical example of high-speed single carrier demodulators is DOCSIS downstream J.83 – Annex B demodulator. Critical for performance of these demodulators is a good design of processing units for downstream equalizer and carrier recovery for 64/256 QAM constellations. The DOCSIS center frequency of the channels range from 111 to 999 MHz ±30 KHz with power range of -15 dBmV to +15 dBmV per channel. The symbol rates are 5.056941 Msym/s (64 QAM) and 5.360537 Msym/s (256 QAM) with appropriate shaping filters, alpha = 0.18 Square Root Raised Cosine shaping for 64 QAM and alpha = 0.12 Square Root Raised Cosine shaping for 256 QAM).

Solution

Bitgear specializes in designing of QAM single carrier modems. In order to have a quick convergence and excellent steady state performance, equalizer operates in dual mode. The equalizer operation starts in the Constant Modulus (CMA) mode, as CMA error signal does not depend on frequency shift. The CMA has fast convergence time but it has a large error in the steady state. To overcome CMA large error, when error monitor unit detects integrated error below threshold, it switches equalizer to Decision Feedback mode (DF). The DF mode has long convergence time but very low steady state error, so DF is used in tracking mode.

Added Value

• Implementation of standard singe- and multicarrier modems
• Burst or streaming
• FPGA implementation
• Hardware integration