Asymptotic behavior of error exponents in the wideband regime

Xinzhou Wu, R. Srikant

Research output: Contribution to journalArticlepeer-review


In this paper, we investigate the fundamental tradeoff between rate and bandwidth when a constraint is imposed on the error exponent. Specifically, we consider both additive white Gaussian noise (AWGN) and Rayleigh-fading channels where the input symbols are assumed to have a peak constraint. For the AWGN channel model, the optimal values of Rz(0) and Rz(0) are calculated, where Rz (1/B) is the maximum rate at which information can be transmitted over a channel with bandwidth B when the error-exponent is constrained to be greater than or equal to z. The computation of Rz(0) follows Gallager's infinite-bandwidth reliability function computation, while the computation of Rz(0) is new and parallels Verdu's second-order calculation for channel capacity. Based on these calculations, we say that a sequence of input distributions is near optimal if both Rz(0) and Rz(0) are achieved. We show that quaternary phase-shift keying (QPSK), a widely used signaling scheme, is near optimal within a large class of input distributions for the AWGN channel. Similar results are also established for a fading channel where full channel side information (CSI) is available at the receiver.

Original languageEnglish (US)
Pages (from-to)1310-1325
Number of pages16
JournalIEEE Transactions on Information Theory
Issue number4
StatePublished - Apr 2007


  • Error exponent
  • Low signal-to-noise ratio (SNR)
  • Near-optimality signaling
  • Wideband

ASJC Scopus subject areas

  • Information Systems
  • Computer Science Applications
  • Library and Information Sciences


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