UCLA

The first part of this work covers the effect of the troposphere on

Ka band (20-30 GHz) satellite signals. The second part deals with

the estimation of the capacity and outage probability for

terrestrial links when constrained to quadrature amplitude

modulations.

The desire for higher data rates and the need for available

bandwidth has pushed satellite communications into the Ka band

(20-30 GHz). At these higher carrier frequencies the effects of

scintillation and rain attenuation are increased. In regards to the

effects of scintillation, the first part of this work quantifies,

through the use of a multiple phase screen simulation model, the

benefits of using two receive antennas to mitigate

tropospheric-induced scintillation on Ka band satellite downlinks.

Two representative turbulence profiles are considered, and

cumulative distribution curves for scintillation-induced attenuation

are generated for selection and maximal ratio combining schemes and

compared to those for a single antenna. The results indicate that

there can be significant diversity gains achieved by combining two

antennas separated by only a short distance. Also, a comparison of

simulation results with the results predicted by the \emph{basic

Rytov} approximation shows that at elevation angles greater than 10

degrees, Rytov theory can accurately predict performance benefits of

antenna combining, but at elevation angles less than 10 degrees it

is better to use multiple phase screen simulations to make

performance predictions. In addition, the effects of

scintillation-induced phase perturbations on the output power of

large aperture antennas is examined. It is found that the output

power degradation due to scintillation-induced phase perturbations

is generally negligible and can be countered by the simple means of

antenna tracking if necessary.

In regards to rain attenuation, this work developed simple methods

for estimating the outage probability and outage capacity and

ergodic capacity of satellite links due to rain fades. The

rain-induced fades of a satellite link are often modeled with a

log-log-normal distribution. Researchers have determined methods for

calculating the outage probability for Shannon capacity for

log-log-normal channels. However, in practical communications

systems, the input signal is constrained to a discrete signalling

set such as finite-size quadrature amplitude modulations. Under

these conditions the outage probability with regards to the

constrained capacity is a more accurate measure. A method is

detailed in this work for tightly estimating the outage probability

and outage capacity of satellite links with quadrature amplitude

modulations. In addition this work derives a lower bound for the

ergodic constrained capacity of log-log-normal channels. To date, no

other method for calculating the outage probability, outage

capacity, or a lower bound for the ergodic capacity for a

log-log-normal channel with a finite-size quadrature amplitude

modulation has been published. Also, this portion of the work

quantifies the benefit of using receive diversity to mitigate rain

fades, providing the gains in outage capacity due to the use of

diversity for a tropical region and a fairly dry region under the

constraint that practical constellations are transmitted. The above

information and analysis methods provide useful tools for satellite

system planners.

The second part of this work examines terrestrial communication

links, which can suffer greatly from channel fading or shadowing.

Two common statistical models for channels are the Rayleigh

distribution and the log-normal distribution. The goal of this

second part of the work was to develop a simple method for tightly

estimating the ergodic capacity and outage probability of these two

channel types when used with quadrature amplitude modulated

signalling sets. Specifically an innovative method was developed for

estimating the ergodic constrained capacity for Rayleigh and

log-normal channels with and without antenna combining. The

expressions facilitate straightforward computation of outage

probability as well. Researchers have determined methods for

calculating the ergodic Shannon capacity for log-normal and Rayleigh

channels for single and multiple receive antenna systems. However,

in practical communications systems, the input signal is constrained

to a discrete signalling set such as finite-size quadrature

amplitude modulation constellations. Under these conditions the

ergodic constrained capacity is a more accurate measure. The method

detailed in this work provides a uniform expression for computing

the ergodic capacity, both Shannon and constrained, of Rayleigh and

log-normal channels with and without antenna combining. The

expressions facilitate straightforward computation of outage

probability as well. Both the noise-limited and

interference-limited cases are studied. To date, no other method

for estimating the outage probabilities for the constrained capacity

of Rayleigh or log-normal channels has been published for either the

noise-limited case or interference-limited case. Also, no method

for estimating the ergodic constrained capacity of a log-normal

channel or of an interference-limited Rayleigh channel has appeared

in the literature. The analysis methods and information for

terrestrial links developed in the second part of this work provide

useful tools for the designers of wireless communication systems in

general and have particular application to cellular mobile and

ultra-wideband systems.