A Brief History of Wireless Telecommunications

Radio is defined as the transmission and reception of electrical impulses or signals by means of electromagnetic waves without the use of wires. Basically, radio waves are electromagnetic radiation transmitted through the air to a receiver.

The history of wireless technology goes back over 200 years. It began in 1747, when Benjamin Franklin illustrated that electricity can move through air and proposed a model of electricity that proved to be correct. In 1819, Hans Christian Oersted, a Danish physicist, observed that a compass needle would move in the presence of an electric field. This established the fundamental relationship between electricity and magnetism and gave rise to the field of study we know as electromagnetics. Then, between 1865 and 1873, Scottish physicist James Maxwell identified the mathematical relationships between electricity and magnetism, developing four equations that describe the movement of electromagnetic waves through space. He illustrated the basic principle of radio transmission by showing that an oscillating electric field would produce an oscillating magnetic field that would in turn produce an oscillating electric field.

The next step in the development of wireless communications came in 1887, when German physicist Heinrich Rudolf Hertz invented the oscillator (an alternating-current generator) and was credited with the discovery of radio waves. Essentially, a radio wave is made of two fields: one electric and one magnetic. These two fields are perpendicular to each other, and the sum of the two fields is called the electromagnetic field. Energy transfers back and forth from one field to the other, and this is known as oscillation. The invention of the first radio transmitter, in 1895, is credited to Italian Guglielmo Marconi, who sent the first radio telegraph transmission across the English Channel in 1895 and across the Atlantic Ocean in 1901. In 1896, Marconi submitted an application for a patent for the world's first wireless telegraph using Hertzian waves. The use of public radio began in 1907.

Since the beginning of radio communications, there have been two key issues to address, and they are still the main goals of the industry: expanding capacity and maintaining quality.

Radio waves are classified by their frequency, which describes the number of times a signal cycles per second, commonly referred to as Hertz (Hz), in honor of Heinrich Hertz. The wavelength is the distance between repeating units of a wave pattern (see Figure 13.1). In a sine wave, the wavelength is the distance between any point on a wave and the corresponding point on the next wave in the wave train. As shown in Table 13.1, there is an inverse relationship between frequency and wavelength: As the frequency increases, the wavelength decreases.

Figure 13.1. Wavelength

There are performance differences between radio frequencies. Low frequencies can travel much further without losing power (i.e., attenuating), but they carry much less information because the bandwidth (i.e., the difference between the highest and lowest frequency carried in the band) is much lower. High frequencies (those in the HF band, from 3MHz to 30MHz) offer much greater bandwidth than lower frequencies, but they are greatly affected by interference from a variety of sources. Very high frequencies (those in the SHF band, from 3GHz to 30GHz) suffer greatly from adverse weather conditions, particularly precipitation. This problem is even greater in the extremely high frequencies (EHF band, from 30GHz to 300GHz). Above 300GHz, the earth's atmosphere greatly absorbs electromagnetic radiation, rendering the atmosphere basically opaque to higher frequencies of electromagnetic radiation, until it once again becomes transparent in the infrared and optical frequency ranges.

Table 13.1. Radio Spectrum

Class of Frequency (Abbrev.)	Frequency	Wavelength	ITU Band	Key Applications
Extremely low (ELF)	<3Hz-30Hz	100,000-10,000 km	1	Communication with submarines
Super low (SLF)	30Hz -300Hz	10,000-1,000 km	2	Communication with submarines
Ultra low (ULF)	300Hz-3KHz	1,000-100 km	3	Communication within mines
Very low (VLF)	3KHz-30KHz	100-10 km	4	Submarine communication, avalanche beaconing, wireless heart rate monitoring
Low (LF)	30KHz-300KHz	10-1 km	5	Navigation, time signaling, AM long-wave broadcasting
Medium (MF)	300KHz-3MHz	1,000-100 m	6	AM medium-wave broadcasting
High (HF)	3MHz-30MHz	100-10 m	7	Short-wave broadcasting and amateur radio
Very high (VHF)	30MHz-300MHz	10-1 m	8	FM and television broadcasting
Ultrahigh (UHF)	300MHz-3GHz	100-10 cm	9	Television broadcasting, mobile phones, WLANs, and ground-to-air and air-to-air communications
Super high (SHF)	3GHz-30GHz	10-1 cm	10	Microwave devices, mobile phones (W-CDMA), WLANs, and most modern radars
Extremely high (EHF)	30GHz-300GHz	1-0.1 cm	11	Radio astronomy, high-speed microwave radio relay

Beyond EHF, no class of frequency or ITU band is specified, but in the frequencies above 300GHz, the wavelengths are smaller than 1 mm. One application for this region is night vision. The majority of wireless data applications today are therefore handled in the band between 2GHz and 6GHz (the UHF and SHF bands). However, there is also great interest in making use of the higher bands, where the bandwidth is much greater, driven by the emergence of bandwidth-intensive multimedia applications. There is currently a great deal of discussion about what to do with the analog television channels as broadcasting is required to move to digital. That spectrum is coveted by many, especially the wireless industry.