This applied engineering reference covers a wide range of wireless communication design techniques; including link budgets, error detection and correction, adaptive and cognitive techniques, and system analysis of receivers and transmitters. Digital modulation and demodulation techniques using phase-shift keyed and frequency hopped spread spectrum systems are addressed. The title includes sections on broadband communications and home networking, satellite communications, and global positioning systems (GPS). Various techniques and designs are evaluated for modulating and sending digital signals, and the book offers an intuitive approach to probability plus jammer reduction methods using various adaptive processes. This title assists readers in gaining a firm understanding of the processes needed to effectively design wireless digital communication systems.
Inspec keywords: transceivers; automatic gain control; phase locked loops; multipath channels; jamming; broadband networks; probability; interferometers; radio direction-finding; cognitive radio; demodulation; satellite navigation
Other keywords: cognitive systems; global navigation satellite systems; pulse theory; receiver; multipath; AGC design; satellite communications; jammers; transmitter; broadband communications; demodulation; transceiver design; direction finding; PLL comparison; basic probability; interferometer analysis
Subjects: Telecommunication applications; Modulation and coding methods; Electromagnetic compatibility and interference; Other topics in statistics; Satellite communication systems; General electrical engineering topics; Radionavigation and direction finding; Modulators, demodulators, discriminators and mixers
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 transmitter is responsible for formatting, encoding, modulating, and upconverting the data communicated over the link using the required power output according to the link budget analysis. The transmitter section is also responsible for spreading the signal using various spread spectrum techniques. Several digital modulation waveforms are discussed in this chapter. The primary types of digital modulation using direct sequence methods to phase modulate a carrier are detailed, including diagrams and possible design solutions. A block diagram showing the basic components of a typical transmitter is shown.
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.
Automatic gain control (AGC) is used in a receiver to vary the gain to increase the dynamic range (DR) of the system. AGC also helps deliver a constant amplitude signal to the detectors with different radio frequency (RF) signal amplitude inputs to the receiver. AGC can be implemented in the RF section, the intermediate frequency (IF) section, in both the RF and IF portions of the receiver, or in the digital signal processing circuits. Digital AGCs can be used in conjunction with RF and IF AGCs. Most often the gain control device is placed in the IF section of the receiver, but placement depends on the portion of the receiver that limits the DR. The detection of the signal level is usually carried out in the IF section before the analog-to-digital converter (ADC) or analog detection circuits. Often the detection occurs in the digital signal processing (DSP) circuitry and is fed back to the analog gain control block. The phase-locked loop (PLL) is analyzed and compared with the AGC analysis, since both processes incorporate feedback techniques that can be evaluated using control system theory. The similarities and differences are discussed in the analysis. The PLL is used for tracking conditions and not for capturing the frequency or when the PLL is unlocked.
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.
To achieve a better understanding of digital communications, some basic principles of probability theory need to be discussed. This chapter provides an overview of theory necessary for the design of digital communications and spread spectrum systems. Probability is used to calculate the link budget in regards to the error and required signal-to-noise (SNR) ratio and to determine whether a transceiver is going to work and at what distances. This is specified for digital communications systems as the required Eb/No.
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.
The receiver is open to reception of not only the desired signal but also all interfering signals within the receiver's bandwidth, which can prevent the receiver from processing the desired signal properly (Figure 8-1). Therefore, it is crucial for the receiver to have the ability to eliminate or reduce the effects of the interfering signals or jammers on the desired signal. This chapter discusses in detail three solutions to reduce the effects ofjammers: a method to protect the system against pulse or burst jammers; an adaptive filter to reduce narrowband jammers such a continuous wave (CW); and a jammer reduction technique called a Gram-Schmidt orthogonalizer (GSO). Other techniques to reduce the effects ofjammers are antenna siting, which mounts the antenna away from structures that cause reflections and potential jammers, and the actual antenna design to prevent potential jammers from interfering with the desired signal. In addition, in some systems, the ability for another receiver to detect the transmitted signal is important. These types of receivers are known as intercept receivers. A discussion is presented on the various types of intercept receivers, and the advantages and disadvantages of each are evaluated.
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.
Broadband and home networking will shape the future. New standards are being reviewed as new technologies are developed and as the data rates increase. With several incoming signals to a home, such as voice, data, and video, there is a need to provide optimal distribution throughout to allow for easy access. Networking is becoming important on the military battlefield, and JTRS and Link 16 play important roles in the interoperability of communication devices. The development of these and other new technologies will provide the military with a network for all communication devices 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.
The last few years have shown an increased interest in the commercialization of the global navigation satellite system (GNSS), which is often referred to as the global positioning system (GPS) in various applications. A GPS system uses spread spectrum signals-binary phase-shift keying (BPSK)-emitted from satellites in space for position and time determinations. Until recently, the use of GPS was essentially reserved for military use. Now there is great interest in using GPS for navigation of commercial aircraft. The U.S. Federal Aviation Administration (FAA) has implemented a wide area augmentation system (WAAS) for air navigation to cover the whole United States with one system. There are also applications in the automotive industry, surveying, and personal and recreational uses. Due to the increase in popularity of GPS, and since it is a spread spectrum communication system using BPSK a brief introduction is included in this text.
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 diagrams showing the relevant coordinate conversions for motion including heading, roll, pitch and yaw. These conversions are for the alternate method using the x-axis on the port-starboard plane and the y-axis on the bow-stern 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.
The elevation effects on the azimuth error are geometric in nature and were evaluated to determine if they are needed in the azimuth determination and to calculate the magnitude and root mean square azimuth error. A simulation was done using three different angles 10°, 25°, and 45°. The simulation sweeps from 0° to 24° in azimuth angle, and the error is plotted in degrees. The results of the simulations are shown. The azimuth error is directly proportional to the elevation angle; the higher the angle the greater the error. Also, the azimuth error is directly proportional to the azimuth angle off interferometer boresight. This analysis was performed with a horizontal interferometer baseline with no pitch and roll.
Presents a calculation of the desired elevation angle for the curved Earth, using the law of cosines.