| Multi-carrier modulations have generated strong interest recently because of the challenging characteristics of important contemporary channels. While the idea of partitioning spectrally using orthogonal frequency division multiplexing (OFDM) was developed over thirty years ago, its implementation as a practical solution to a communications system problem is much more recent due to advances in digital signal processing and VLSI. Implementation has brought about a simultaneous increase in developing analysis that allow the performance characteristics of the approach to be understood in practical channels. Example channels where OFDM is being strongly considered are data communications over CATV channels and wireless applications such as digital terrestrial broadcasting. Widespread consideration of OFDM as a primary transport mechanism has occurred to date only for high speed data over the twisted pair channel. In this application, the technology is referred to as the Asymmetric Digital Subscriber Line (ADSL). Unlike that application, however, CATV and wireless systems represent RF channels. As such, one important characteristic of each is the phase jittery nature of low-cost, high frequency synthesized carriers. The research will present unique results indicating how these impairments degrade the OFDM system. The approach taken is to model the untracked jitter variation as a lowpass process, note the breakdown of traditional assumptions associated with single carrier modulations, and proceed with analytical solutions from this practical model. The single carrier solutions required are developed, including solutions for slow and rapid phase noise for standard and offset 16-QAM. Results for 64-QAM are also developed, and can be easily extended to any rectangular constellation, such as 256-QAM or 1024-QAM. A comparison of the various simplifying analysis assumptions is made to contrast with exact performance curves when phase noisy RF channels are encountered. A straightforward technique to improve link margin on phase-noise limited, slow jitter channels is presented. Finally, system design guidelines are developed for the difficult moderately varying jitter situation, critical for multi-carrier systems that rely on multiple subchannels of individually low symbol rate. |
