Performance Assessment of Mobile OFDM-based Systems: Variability within Given Wideband Channel Categories

Abstract: An ongoing challenge in communications is to provide ever fast er, broadband multi-media access to mobile terminals. In the mobile multipath environment, performance is affected by both frequency-selectivity and Doppler spread. Realistic channel simul ators are required to compare competing systems on a fair basis. The RACE code-division testbed (CODI T) provides a realistic model ba sed on extensive wideband measurements, ideal for OFDM (Qrthogonal Frequency Division Multiplexing) testing, which defines a range of indoor and outdoor environment categories. However, there remains considerable variability within any given category. To enable fair and reliable comparison, the variability within a given channel type must be quantified, since this determines the length of simulation time and the number of independent instances of a channel type to be averaged before stable and repeatable results can be inferred. This paper explains how the chan nel model was implemented, providing the im portant environment parameters. Variabilit y is then quantified in terms of statistics of BER (Bit Error Ratio) from simulation runs of differing lengths for many different instances of nominally the same environment. An accurate estimate (~5%) of mean BER for a given CODIT environment typically requires averages over simulations of around 90-150 instances of the channel, with runs of at least 700 OFDM symbols.

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