This chapter presents the factors to be considered when designing a two-way communications link. The discussion includes the operation frequency, radio transmitter, radio channel, radio receiver, and link budget.

The receiver is responsible for downconverting, demodulating, decoding, and unformatting the data received over the link with the required sensitivity and bit error rate (BER) according to the link budget analysis of Chapter 1. The receiver is responsible for providing the dynamic range (DR) to cover the expected range and power variations and to prevent saturation from larger power inputs and provide the sensitivity for low-level signals. The receiver provides detection and synchronization of the incoming signals to retrieve the data sent by the transmitter. The receiver section is also responsible for despreading the signal when spread spectrum signals are used. The main purpose of the receiver is to take the smallest input signal, the minimum detectable signal (MDS), at the input of the receiver and amplify that signal to the smallest detection level at the analog-to-digital converter (ADC) while maintaining a maximum possible signal-to-noise ratio (SNR). A typical block diagram of a receiver is shown. Each of the blocks will be discussed in more detail.

The demodulation process is part of the receiver process that takes the downconverted signal and retrieves or recovers the data information that was sent. This process removes the carrier frequency that was modulated in the transmitter to send the digital data and detects the digital data with the minimum bit error rate (BER). It also removes the high-speed code used generate spread spectrum for jammer mitigation. This process was performed in the past using analog circuitry such as mixers and filters to remove the carrier frequency and spread spectrum codes, but today the process is incorporated in the digital circuitry using applicationspecific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), and digital signal processing (DSP) integrated circuits. The basic system concepts are presented in this chapter, and the method of implementation is left to the designer.

Multipath is a free-space signal transmission path that is different from the desired, or direct, free-space signal transmission path used in communications and radar applications. The amplitude, phase, and angle of arrival of the multipath signal interfere with the amplitude, phase, and angle of arrival of the desired or direct path signal. This interference can create errors in angle of arrival information and in received signal amplitude in data link communications. The amplitude can be larger or smaller depending on whether the multipath signals create constructive or destructive interference. Constructive interference is when the desired signal and the multipath signals are more in-phase and add amplitude. Destructive interference is when the desired signal and the multipath signals are more out of phase and subtract amplitude. Angle of arrival errors are called glint errors. Amplitude fluctuations are called scintillation or fading errors. Therefore, the angle of arrival, the amplitude, and the phase of the multipath signal are all critical parameters to consider when analyzing the effects of multipath signals in digital communications receivers. Frequency diversity and spread spectrum systems contain a degree of immunity from multipath effects since these effects vary with frequency. For example, one frequency component for a given range and angle may have multipath that severely distorts the desired signal, whereas another frequency may have little effect. This is mainly due to the difference in the wavelength of the different frequencies.

Cognitive systems are used to adapt the operating system to the changing environment. This can be accomplished by many methods suggested in this chapter, and many additional methods will be available in the future. The optimal cognitive system solution evaluates a system's available capabilities as well as all of the available knowledge about the changing environment, and then it calculates, makes trade-offs, and determines the best solution for the system to adapt to these environmental changes with minimal impact to performance. In addition, the cognitive system contains a learning capability that uses past experiences and impact/results of changes and applies this information to make smart decisions in the future.

Satellites are now providing extensive coverage for communications and data link operations in remote areas. The satellite connection consists of a remote earth station, a satellite, and another earth station. This triangle forms a two-way communications link to provide the remote earth station with access to all types of communications, including data links, military operations, Internet, video, voice, and data at high data rates. Satellite and ground systems use five bands: L, C, X, Ku, and Ka. The latter is becoming popular for both commercial and military sectors. A geostationary orbit is used so that the ground station tracks a fairly stationary transceiver and the satellite appears to be stationary. A link budget is used to determine the power, gains, and losses in a system and also determines the figure of merit. Multiple access schemes are used to allow multiple users on the same band. Costs are associated with the type of system used and the length of use.

Direction finding is a method to determine the direction of a transmitted signal by using two antennas and measuring the phase difference between the antennas, as shown in Figure 13-1. This process is called interferometry. In addition to using a static interferometer, further analysis needs to be done to calculate the direction when the interferometer baseline is dynamic; that is, the interferometer is moving and rotating in a three-dimensional plane. Thus coordinate conversion processes need to be applied to the nonstabilized antenna baseline to provide accurate measurement of the direction in a three-dimensional plane.

Presents a formula to be used for correcting from true north and also a formula for correcting from true north to magnetic north. Phase ambiguities are also mentioned.

Presents a calculation of the desired elevation angle for the curved Earth, using the law of cosines.