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We describe results from a channel measurement and modeling campaign for the airport surface environment in the 5-GHz band. Using a 50-MHz bandwidth test signal, thousands of power delay profiles (PDPs) were obtained and processed to develop empirical tapped-delay line statistical channel models for large airports. A log-distance path loss model was also developed. The large airport surface channel is classified into three propagation regions, and models are presented for each of the regions for two values of bandwidth. Values of the median root-mean-square (RMS) delay spread range from 500 to 1000 ns for these airports, with the 90th percentile RMS delay spreads being approximately 1.7 ms. Corresponding correlation bandwidths (i.e., correlation value 1/2) range from approximately 1.5 MHz in non-line-of-sight (NLOS) settings to 17.5 MHz in line-of-sight (LOS) settings. Two types of statistical nonstationarity were also observed: 1) multipath component persistence and 2) propagation region transitions. We provide the multipath component probability of occurrence models and describe Markov chains that are used for modeling both phenomena. Channel tap amplitude statistics are also provided, using the flexible Weibull probability density function (pdf). This pdf was found to best fit fading tap amplitude data, particularly for frequently observed severe fading, which is characterized by fade probabilities that are worse than the commonly used Rayleigh model. Fading parameters equivalent to Nakagami-m-model values of m near 0.7 were often observed (with ra = 1 being Rayleigh and m < 1 being worse than Rayleigh). We also provide channel tap amplitude correlation coefficients, which typically range from 0.1 to 0.4 but occasionally take values greater than 0.7. Index Terms--Channel impulse response (CIR), fading, radio propagation.

In this paper, a new modulation scheme that is called spectrally shaped generalized multitone direct sequence spread spectrum (SSG-MT-DS-SS) is proposed. This scheme enables moderate shaping of the transmitted spectrum without the use of spectral-shaping filters; instead, the spectrum is shaped by appropriate selection of the parameters of each subcarrier. These parameters are amplitude, data rate, processing gain, and the frequency separation between adjacent subcarriers. Since these parameters are selectable, this scheme is a completely generalized MT-DS-SS approach. Analysis and simulation results show that by properly choosing these parameters, the level of the spectral sidelobes can be reduced, while slightly changing the width of the mainlobe and simultaneously obtaining a good error-probability performance, on both additive white Gaussian noise and dispersive fading channels. For even a small number of subcarriers (e.g., 3-5), spectral sidelobes can be reduced by 3-6 dB from those of the conventional MT-DS-SS spectrum, with little or no degradation in error-probability performance. For larger numbers of subcarriers, additional sidelobe reductions are attainable. In addition, the SSG-MT-DS-SS system has a smaller peak-to-average power ratio than the conventional MT-DS-SS when the amplitudes of each subcarrier are not identical. Index Terms--Bit error ratio (BER) performance, multitone, spectrum shaping, spread spectrum (SS).