By K. Laudel, E. Tsui, J. Harp, A. Chun, and J. Robinson, Applied Signal Technology, Inc.
The premise of this paper is to show that 256-QAM is a viable modulation type for the cable television environment. Performance results for various cable system impairments such as AM hum, FM phase noise, residual FM, SNR, and microrejlections are presented for an actual prototype 256-QAM Equalizer/ Demodulator that verify this premise. The prototype demodulator has also undergone extensive testing at the Advanced Television Test Center (ATTC) in support of the cable portion of the HDTV testing (January 1994).
All prototype testing results conclude that, with a well conceived architecture and design, 256-QAM is an eminently practical format for the transmission and reception of high-data-rate cable television signals. QAM and VSB modulations are compared. There are advantages in the use of 256-QAM over 16-VSB in areas involving carrier and phase tracking and blind equalization which are analyzed here and also supported by results obtained in carrier offset, residual FM, phase noise, and channel change acquisition time testing [8]. Cable microreflection environments are modeled and equalizer length trades are presented that indicate that 16-32 taps (possibly 64 taps in some cases) are generally required to overcome the effects of cable microrejlections.