Degrees of freedom in adaptive modulation: A unified view

Seong Taek Chung, Andrea J. Goldsmith

Research output: Contribution to journalArticlepeer-review

1077 Scopus citations


We examine adaptive modulation schemes for flat-fading channels where the data rate, transmit power, and instantaneous BER are varied to maximize spectral efficiency, subject to an average power and BER constraint. Both continuous-rate and discrete-rate adaptation are considered, as well as average and instantaneous BER constraints. We find the general form of power, BER, and data rate adaptation that maximizes spectral efficiency for a large class of modulation techniques and fading distributions. The optimal adaptation of these parameters is to increase the power and data rate and decrease the BER as the channel quality improves. Surprisingly, little spectral efficiency is lost when the power or rate is constrained to be constant. Hence, the spectral efficiency of adaptive modulation is relatively insensitive to which degrees of freedom are adapted.

Original languageEnglish (US)
Pages (from-to)1561-1571
Number of pages11
JournalIEEE Transactions on Communications
Issue number9
StatePublished - Sep 2001
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Electrical and Electronic Engineering


  • Adaptive modulation
  • Communication systems
  • Fading channels
  • Spectral efficiency


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