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ABSTRACT

The communication of voice over wireless local area networks (WLANs) is influenced by the choice of speech codec, packetization interval and PHY layer bit rates. These choices affect the number of voice users that can be supported on the WLAN as well as the speech quality experienced by each user. We investigate the effect of different combinations of these parameters for a 802.11a WLAN in different channel conditions with the objective of maximizing the number of voice users supported on the WLAN subject to a quality constraint. We use an indicator for assessing the speech quality experienced by a single user in a WLAN, based on a Perceptual Evaluation of Speech Quality (PESQ) Mean Opinion Score (MOS) constraint and the probability of a voice user achieving this constraint. The contributions of this paper are three-fold. First, a PHY layer rate adaptation scheme is proposed, in which the operating rate for each Signal to Noise Ratio (SNR) is chosen as the one that maximizes the capacity given a quality constraint. Second, based on the PHY layer rate adaptation scheme, we evaluate the effect of the choice of codec and voice payload size on the capacity values obtainable at different SNRs. Finally, we show the effect of channel conditions and the tightening of quality constraints on the capacity values at different SNRs.

REFERENCES

• 1: ITU-T Recommendation G.711. Pulse Code Modulation for voice frequencies, Nov. 1988.
• 2: ITU-T Recommendation G.729. Coding of speech at 8 kbit/s using conjugate-structure algebraic-code-excited linear-prediction (CS-ACELP), Mar. 1996.
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• 7: IEEE 802.11a WG. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification: High-Speed Physical Layer in the 5 GHz Band. 1999.
• 8: ITU-T Recommendation G.107. The E-model: A computational model for use in transmission planning. Feb. 2001.
• 9: ITU-T Recommendation G.114. One-Way Transmission Time. May 2000.
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• 11: ITU-T Recommendation P.862. PESQ: An objective method for end-to-end speech quality assessment of narrowband telephone networks and speech codecs. Feb. 2001.
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INDEX TERMS

Primary Classification:
  C. Computer Systems Organization
  C.2 COMPUTER-COMMUNICATION NETWORKS
      C.2.1 Network Architecture and Design
          Subjects: Wireless communication

General Terms:
Design, Performance

Keywords:
IEEE 802.11a, frequency selective multipath fading, rate adaptation, voice capacity, voice over WLANs, voice quality