A key challenge in sharing satellite uplink spectrum with terrestrial users is to protect the on-orbit satellite receiver from the aggregate interference of a large population of terrestrial emitters. Solving this challenge requires improved understanding of the characteristics of aggregate interference. We report here novel measurements of interference at a low-earth-orbit satellite receiver from a large population of unlicensed terrestrial emitters. We describe an aggregate interference model that incorporates the number and duty-cycle of outdoor access points, as well as their antenna characteristics and RF operating environment. We use the measurement data to validate and parameterize the model for estimating aggregate interference in similar situations, and show a method to calculate interference based on the number of terrestrial users sharing the uplink spectrum.

A detection module has been prototyped for cognitive radio usage in the industrial, scientific and medical (ISM) band using time domain as well as frequency domain detection. Duty cycle and pulse width characteristics are used for detection in the time domain. Fast Fourier transform (FFT) signatures and spectral occupancy information is used to confirm the detection in the frequency domain. The detection module is designed to behave dynamically. Varying the input parameters to the detector module adjusts the detection overhead. The overhead varies depending on the relative usage of a fast detection algorithm versus a slower but more accurate scheme; the number of device types to be detected; and whether frequency-selective scanning or wide-band detection is needed. The prototype is built on a software radio platform and is targeted for future software defined radios (SDR), and can be adapted to current and future orthogonal frequency division multiplexed (OFDM) systems.

Current portable and land-mobile radio systems employ frequency modulation which exhibits a distinct capture effect. Furthermore, multipath propagation, shadowing, and the spatial distribution of the terminal units create data packets with varying signal strength. We investigate the performance of nonpersistent inhibit sense multiple access (ISMA) applied to the forward channel of packet radio communication networks. We show that the aggregate capacity of ISMA substantially increases compared to the noncapture case for small capture ratios. The mean packet delay of ISMA is also reduced with capture. The effect of different signal strength distribution is explored, and the capacity and delay improvements discussed.

The performance and optimization of switched diversity systems are considered. First, the one-dimensional distribution and probability density functions of the envelope of the received signal are obtained for three different switching strategies. This information is used to obtain the average probability of bit error for the case of non-coherent detection of binary FSK signals with Rayleigh fading envelopes and additive white Gaussian noise. The optimization of two of these switching strategies is then considered, and it is shown that by proper selection of switching thresholds, the average probability of bit error during detection can be minimized. It is also shown that these optimized switching strategies yield a significant improvement in performance over non-diversity systems and can approach the performance of more complex receivers such as maximal ratio combining. Computer simulations of switched diversity systems using a practical field model are used to verify the analysis.