Wireless Communications
Maja Bystrom
I. Introduction
A. History
The field of wireless communications has been in existence since the first humans learned to communicate. In early days of civilization humans would transmit notices of important events, such as enemy invasions or royal births, through the sounding of horns or the lighting of fires. While simple messages could be effectively transmitted in this manner, in order to communicate over long distances the manpower expense was great, since watchtowers had to be built within sight of each other and continually manned, and the number of messages was small. It was not until the 1800’s that wireless communications became what we know it as today. Now we are able to use radio frequencies to communicate information over long distances (think of the Cassini mission to Saturn), we can send voice or video at rates of more than hundreds of megabits per second, and the associated technology has become so inexpensive that many people are able to afford a mobile phone in order to be in constant contact with others.
We often attribute the beginnings of wireless communication to Guglielmo Marconi (1874-1937); however, to paraphrase Isaac Newton, “he stood on the shoulders of giants”. In Marconi’s case these giants were scientists such as James Clerk Maxwell who proved that radio waves existed, although he could not produce them, and Heinrich Hertz whose name is now used as a unit of frequency, who transmitted the first man-made radio waves. Besides being the first to use the antenna, Marconi did not in fact invent anything new. Instead, he was a remarkable engineer who combined the work of many others to produce something that was known theoretically to be feasible. It took him through his adolescence and into his early twenties to develop a wireless system which would even transmit as far as several miles, but after that point the scaling up of radio systems to longer transmission ranges was rapid. By 1897 Marconi and his associates had established a 14.5-mile fixed wireless link over water and the Italian navy had begun to use his invention for ship-to-shore communication. These first communications were digital, using Morse code, which was already widely established for wireline telegraphy. However, the communication rate was slow on the order of 12 words per minute (wpm). The early transmission systems operated at wavelengths of few thousands of meters up to 10,000 meters; this corresponds to 3-30 kHz. At this time many of the great communications companies, which still exist today in various forms, were founded: American Telephone and Telegraph, Marconi Company, Westinghouse, the Radio Corporation of America.
At the same time as Marconi was laboring on his systems, others were racing to build improved ones. Development came quickly despite the setbacks of fierce storms that repeatedly destroyed many of the transatlantic antennas. On Christmas Eve 1906 Reginald Fessenden transmitted the first voice and music that was heard by many wireless operators in the northeast. He was then granted a US patent for voice transmission. At the same time Lee DeForest developed his Audion tube, which could amplify signals. Edwin Armstrong, a Columbia University graduate student, made use of this vacuum tube to develop a system that made long-distance voice transmission possible. During the early 1900’s voice transmission across oceans and continents was proven; however, many were in doubt of the usefulness of the “radio telephone”; since there was no way of ensuring privacy, anyone with a wireless receiver could listen in. This remains one of the concerns with many of the wireless systems in use today.
Many of the developments of radio came during the two world wars. Spurred by the necessity of creating effective military communications, the U.S. government forced the communications companies and scientists to work together. At the same time many scientists in other countries were working to develop systems for their militaries. WW I saw the first air-to-ground communication. Marconi was the first to recognize the usefulness of short waves, these 1-100 meter waves would use less power and travel less far and thus could hide information from a distant enemy as well as reduce interference with neighboring transmitters. Actually, short waves ended up being much more efficient than longer waves, used less power, and were not reflected off of the ionosphere (which prohibits daytime transmission) but were reflected off of a higher layer. More importantly, short waves could transmit information faster than 100 wpm. Several researchers also noted that ultrashort radio waves could be reflected from objects in their path, thus laying the basis for radar, a technology perfected during WWII.
While there were sporadic radio broadcasts of music and news to the public in both the US and Europe prior to WW I, these broadcasts, and indeed all amateur radio operation was shut down in the US during WW I for reasons of national security. Broadcasts were resumed in the fall of 1919 and the first radio station KDKA in Pittsburgh opened on Nov. 2, 1920 to begin daily broadcasts. The total number of amateur radio operators in the US at that time was perhaps 30,000, so to ensure a listening audience the Westinghouse company manufactured cheap radio sets and when news of the broadcast spread the general public hastily bought parts to build their own sets. After the rapid success of broadcast radio, manufactures quickly improved their receivers, but yet, these sets cost on the order of $25-$400, a month’s wages, and needed frequent replacement of vacuum tubes. Compare this to the price and quality of a Walkman today! Introduction of the analog color and digital television sets saw the same problems. In 1954 the US saw its first color TV sets for sale for $1300, which was near the price of a car at that time. Currently, you can purchase an HDTV set for $5000-$7000, still a significant amount of money.
The number of telecommunications innovations grew rapidly during the last half of the 20th century. Currently there is widespread and growing use of cellular phones, cordless phones, digital satellite systems, and personal mobile radio networks. Wireless communications occurs at many different frequencies, from underwater communication at extremely low frequencies on the order of tens or hundreds of Hertz, to infrared at Hertz. See Fig. 1 for a partial diagram of the radio frequency (RF) spectrum. In the United States the spectrum is allocated by the Federal Communications Commission (FCC).
Fig. 1 A section of the RF spectrum showing some of the frequency assignments in MHz.
A significant development in telecommunications in the United States was the 1996 Telecommunications Act. This act was written in part to promote competition (telecommunications had hitherto been controlled mainly by a group of monopolies), promote integration of advanced services to all Americans and development of the underlying infrastructure. Furthermore, it created measures, such as a rating code, to deal with violence and obscenities, and laid out punishments for misuse, such as harassing phone calls, of the telecommunications systems.
The area of wireless communications will continue to grow for many reasons. People are becoming accustomed to immediate access to information wherever their locations, and technological improvements have made providing universal telecommunications access feasible. There currently is an expansion in the number of personal mobile radio networks that are the systems used by law enforcement groups, ambulance services, and on the floor of factories. The signals are meant to be relatively short-range and communication takes place on designated frequency ranges where they will not interfere with other applications such as wireless or mobile phones. In the near future there will be significant growth in wireless for the office, such as wireless local area networks and wireless private branch exchanges. New developments in personal communications systems (PCS) include integrated phone/paging/email/data transmission. Currently handheld units are offered by the major wireless industries with many of these features. These units range from cell phones with email capability, wireless pen tablets (low-end laptops without keyboards – interaction is via a pen), PDAs, and personal organizers, At the moment these have low-rate internet service on the order of 10 kbps, however speed and interconnectivity will be increased.
Television and radio broadcasts, while still in analog, are rapidly changing to digital. One example of this is the direct broadcast satellite (DBS) systems that send a digital TV signal from a satellite to an antenna at each subscriber’s home. The digital signal is a composite of many television channels. The home antenna is connected to a set-top box that extracts the desired channel and converts it to an analog signal for display on the television set. Now the terrestrial broadcast of digital TV is mandated with switch to all digital required in 2006. These signals are typically called digital television (DTV) or high-definition television (HDTV). Similarly, digital radio is an area of recent significant research and development. It is currently deployed and growing in popularity in Europe and will likely become a standard in the U.S. The motivation behind digital transmission is that the quality is better and there is no slow degradation as the receiver is moved farther from the transmitter. The aspect ratio, ratio of width to height, is different than in analog television, so that movies can be shown without truncation of the sides or being displayed in “letterbox” format. Also, DTV allows for easier video manipulation such as split screens or display of video in video. The drawbacks of digital systems are an increase in required bandwidth and the “cliff effect” in which either reception is good or no reception is possible.
B. Communications Systems Overview
All of the systems mentioned previously, regardless of frequency or purpose, are communications systems. A communications system necessarily consists of three parts: a transmitter, a receiver, and a channel. The transmitter takes a signal, whether analog or digital, and formats it for transmission over the channel. A wireless channel can be water, air, or vacuum, and may contain obstructions such as buildings, terrestrial features, or planets, depending on the medium. The receiver captures the transmitted signal and performs signal processing, changing it from a form that can be transmitted over the channel into a form that can be viewed, heard, or stored. All of these system components introduce degradation to the transmitted signals; furthermore each system has a limit on the number of signals that can be transmitted. By carefully studying and compensating for the degradation caused by the system components, and by carefully designing the signal processing within a communications system, the number of signals that can be transmitted at one time can be maximized while the signals’ degradation can be minimized. In the following sections the signals and system design tradeoffs are briefly considered.
II. Introduction to Signals
When we listen to radio or the telephone, or watch television, we are observing analog signals, that is, signals that are continuous in amplitude and time. Fig. 2 illustrates a segment of speech. From this figure we can see how the signal is able to take on any amplitude value within the range [-1,1].
Fig.2 A sample of speech with a section extracted.
As was mentioned in the previous section, one of the fundamental constraints to our transmission systems is the available bandwidth. The FCC only allocates a limited amount of bandwidth for each application, and no one is allowed to exceed his or her limitation. Therefore, we need a method of determining the bandwidth or frequency content of a signal; this bandwidth is measured in cycles per second, commonly called Hertz.
One of the greatest mathematical discoveries of the 19th century was made by Jean Baptiste Joseph Fourier, who determined that most aperiodic signals could be represented by summing their frequency components. That is, for most signals we are interested in the equation
holds. This means that a time-domain signal, , such as our speech of Fig 2., can be represented by the different frequencies in it. In the frequency domain our signal is represented by . This is perhaps best illustrated by an example. Fig. 3 shows the Fourier transform of extracted segment of speech from Fig. 2. This figure demonstrates that the speech sample is composed of frequencies between 0 Hz and 16 kHz. This implies that if the channel or, correspondingly, the bandwidth allowance is greater than 16 kHz, then the signal can be transmitted over the channel and not interfere with any other transmitters. Thus, the Fourier transform is a very powerful tool in communications system design. Note that this is not a typical speech sample, since it has high-frequency noise; speech is typically bandlimited to 300-4000 Hz.
Fig 3. The discrete Fourier transform of the speech sample.
Rather than transmitting an analog signal, we may instead wish to transmit a digital signal. Digital signals are signals that are discrete in both time and frequency and may arise in many ways. For instance, to transmit information stored in the memory of computer such as an email, a stream of bits (1’s and 0’s), called a bitstream, is formed.
We can also change our analog signals into digital signals through sampling and quantization. Fig. 4 illustrates the process of converting the voice signal of Fig. 2 to a digital signal. First, the signal is sampled periodically, that is every seconds we record the amplitude of the signal. Nyquist proved that the sampling frequency, , must be at least twice the maximum frequency in the signal. Typically, voice signals are sampled at a rate of 8400 samples/second. For compact-disk quality music, which is typically limited to the range 0 to 15 kHz, the sampling rate is 44,100 samples/second.