Introduction – Wireless communication, like their wired counterparts rely on the manipulation of electrical charge to enable communication between devices, however they use a specific type of signal known as radio frequency or RF. Following the demonstration by Marconi, led to the widespread use of radio waves for communication. Radio communication is hence the process in which radio waves are used as a carrier for transmitting and receiving intelligent information through atmosphere or deep space and sky (Poole, 2006, p. 8). This article explains the fundamentals of radio communication starting from the basics of radio signals, then going on to explain how radio waves can be used for communication. In the later part of the article, the popular cellular radio communication schemes are discussed.
1. The nature of radio signals
a. What radio signals are?
Radio Signals are electromagnetic waves, almost exactly similar to visible and infrared light except for their frequency and wavelength. The frequency of radio waves is much lower than the visible and infrared light. And in consequence there wavelength is much longer (Poole, 2006, p. 19). This is because of the fact that the wavelength of a radiation is inversely proportional to its frequency; this can be represented as below:
Here, represents the wavelength of electromagnetic radiation, represents the frequency of electromagnetic radiation, and is the constant speed at which any electromagnetic wave travels equal to 3×108 m/s.
b. How radio signals propagate over a wireless medium?
Radio waves travel outwards much like waves in a water pond and become weaker as they travel longer distances. However, the distances covered by the radio waves are very large and are in fact the signals that scientist look for from the farthest galaxies to find the outer extremities of the universe. Since they are electromagnetic waves, radio signals do not need any medium in order to propagate. Light waves, like radio waves travel, in straight lines and are subject to reflection, refraction, diffraction, absorption and scattering. The entire radio spectrum is divided into very low, low, medium, high, very high, ultra high, super high and extremely high frequencies, and a group frequencies is called a band (Haslett, 2008, p. 87).
c. Factors that adversely affect radio signal propagation?
There are several factors that affect the radio transmission. At different frequencies some of these factors affect radio waves more than others. Many of the factors lead to attenuation, which means the weakening of a radio signal as it passes through the atmosphere. All the radio signals are attenuated as they pass through rain or any type of water in the air, such as cloud, snow sleet etc, but radio signals at higher frequencies are attenuated more than the radio signals at lower frequencies (Xiao, 2007, p. 352). In addition to this radio communications are also affected by the ionosphere layer present in the earth’s atmosphere. This layer affects the radio waves due to the presence of a large number of electrons and other ions. This layer causes two major disturbances in the transmission of radio waves:
i. Faraday Rotation – This phenomena is whereby the polarization plane of radio waves rotates due to the interaction between radio waves and earth, especially magnetic flux lines and electrons in the ionosphere. This interference causes a lot of problems chiefly in the satellite communication, especially when plane-polarized waves are used in the 850 MHz band (Haslett, 2008, p. 141)
ii. Ionosphere Scintillation – This phenomenon arises from spatial and temporal fluctuations in the ionosphere electrical characteristics, and is observed as irregular transient fluctuations in the received signal strength. This phenomenon occurs more often in summer months during the periods of intense solar activity and in equatorial regions. The level of ionosphere scintillation is inversely proportional to frequency of radio waves (Haslett, 2008, p. 142).
2. Using radio signals to carry information
a. How information can be added to a radio signal?
A radio wave carries information bearing signals through space. Each carrier may have information encoded directly on it by periodically interrupting its transmission as in Morse code telegraphy, or encoded on it by what is known as a modulation technique. There are many modulation techniques used to encode the messages the most common ones being amplitude modulation, frequency modulation and phase modulation (Haslett, 2008, p. 145).
b. How a number of different radio signals can be sent from a transmitter to a receiver simultaneously by employing modulation and multiplexing?
Modulation and multiplexing are principles of moving information in progressively more efficient methods. Modulation schemes are used to maximize the limited resources of the spectrum. Multiplexing is a technique whereby several message signals are combined into a composite signal for transmission over a common channel. To transmit a number of these signals over the same channel, the signals must be kept apart so that they do not interfere with each other and thus they can be separated at the receiver’s end. The two basic techniques of multiplexing are frequency division multiplexing FDM and time division multiplexing TDM. As the name suggests, in the former case the signals are separated in frequency, while in latter the signals are separated in time (Haslett, 2008, p. 148).
3. Using radio signals for mobile communication
a. The basic principle of cellular radio communication systems
The purpose of cellular radio is to provide communication between two subscribers separated by thousands of miles. No satellite or microwave link is established between them, but a frequency band of 800-900 MHz, assigned by Federal Communications Commission FCC, is used for the transmission of radio waves (Bedell, 2005, p. 2). Cellular radios enhance the use of mobile-radio telephone systems. Their main purpose is to provide voice communications in mobile systems. There are five components of a cellular telephone system: “the mobile telephone or station MS, the cell base station BS, the mobile switching centre MSC, the fixed network or the transmission systems, and connectivity to the Public Switched Telephone Network PSTN“ (Bedell, 2005, p. 4). The figure below shows the operation of a basic Cellular Radio communication System.
Fig -1 Diagram of a basic cellular radio telephone System (http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Computer%20networks/pdf/M5L9.pdf)
b. The Advanced Mobile Phone Service
This is the American analog Cellular standard and was invented by AT&T Bell laboratories in order to allow moiré number of users to access the systems simultaneously. This system was first deployed in Chicago in 1983. When more capacity was needed, the area served by each transmitter could be divided again to create a new base station. The development of this concept was the birth of wireless technology as we know it today. In AMOS system voice is transmitted using frequency modulation scheme, and uses the Manchester-coded binary shift keying at 10Kbps (Bedell, 2005, p. 7). The AMPS system as given originally by Bell Laboratories in 1984 is as given below.
Fig -2 Diagram of AMPS system (http://www.privateline.com/mt_cellbasics/x_appendix/c_early_bell_system_overview_of_amps/)
The earlier configurations of AMPS did not anticipate the dynamic growth of the market, and hence were later replaced y multiple access techniques which will be discussed in the later sections.
c. Time Division Multiple Access
Time Division Multiple Access or TDMA technique is one mature method of sharing the radio spectrum and is used extensively in data systems due to its simplicity. The communication channels are divided here according to the time slot. When a transmit/receive pair is given permission to communicate, it is assigned to a specific time slot in which to do so (Buehrer, 2006, p. 4). Every time frame, each transmit/receive pair may communicate during its slot. TDMA systems can be used with both centralized and de-centralized systems. The frequency used here is the 800 or the 1900 MHz, with frequencies separated by 30 KHz divided into 3 time slots each 6.7ms long (Buehrer, 2006, p. 5). The figure below shows the operation of a basic TDMA system.
Figure 3 TDMA System
d. Code Division Multiple Access
In the Code Division Multiple Access or CDMA technique, the channels are not defined by time or frequency but by code. CDMA is based on spread spectrum techniques that originated in the military communications. The channels here, as is mentioned earlier, are defined by spreading waveforms or the spreading codes that underline these waveforms. As there are various forms of spread spectrum, there are also various forms of CDMA such as: direct-sequence CDMA i.e. DS-CDMA, frequency hopped CDMA i.e. FH-CDMA and time-hopped CDMA i.e. TD-CDMA. CDMA has now become the most common transmission methods for mobile handsets in the United States (Buehrer, 2006, p. 15). For cellular telephony, CDMA is a digital multiple access technique specified by the Telecommunications Industry Association TIA under Intermediate Standard IS-95 which occupies 1.25 MHz of radio spectrum. However, this technology is more complex and has a higher operation cost then that of TDMA. Also this technique is incompatible with the GSM technology (Buehrer, 2006, p. 17). The figure below shows the operation of a basic CDMA system.
Figure 4 CDMA System
e. The Global System for Mobile Communications
Before the introduction of the Global System for Mobile Communication GSM, mobile networks implemented in different countries were usually incompatible. To overcome this problem, GSM standards were developed in the year 1989. By the year 2000, these initiatives were so successful that the networks compliant with GSM standards were developed worldwide having a total of 1048.6 million subscribers by the year 2004 (Goransson & Greenlaw, 2007, p. 42). A GSM network is characterized by digital voice communication and support of low-rate data services. The basic subsystems used in GSM are similar to other cellular systems mentioned above. The GSM air interface is based on TDMA system and the transfer of data is carried out over circuit-switched connections. GSM service primarily provides voice, low-speed-data, and short messaging services. One of the most popular GSM services is the Short Message Service SMS, which allows the subscribers to exchange short text messages (Goransson & Greenlaw, 2007, p. 43).
4. The future of radio wireless mobile communication
a. The Universal Mobile Telecommunication System
The Universal Mobile Telecommunications Systems or UMTS is a 3G cellular technology. The system has been defined by the International telecommunications Union ITU, and has evolved from the corresponding 2G GSM and 2.5G General Packet Radio Services or GPRS technologies. UMTS is subject to a continuous worldwide standardization effort under the umbrella of 3G partnership Project 3GPP (Berndt, 2008, p. 24). The system builds on the 2G infrastructure, but introduces a new radio transmission technology by applying CDMA. The higher data rate of the UMTS transmission give rise to the data applications that were not possible or practical with the GSM. In particular GSM can browse the web and exchange multimedia enriched message using Multimedia Messaging Services MMS (Berndt, 2008, p. 25).
Conclusion – The article presented a discussion of the use of radio signals of communication along with giving a description of the various commercial protocols used presently in the cellular radio communication. As can be seen, radio communication finds many applications not only for home entertainment and military applications but also for commercial communications In fact wireless communication applications have exploded in popularity over the last decade, with the onslaught of products such as pagers, cellular phones, radio navigation and wireless data networks. The radio based communication system is expected to have a strong hold in future too with many advances on various fronts such as global coverage, improved interoperability, wireless-wireline integration for broadband services, and the support of high bit rate data, the internet and increased multimedia usage (Meites, Zuman and Golio, 2008, p. 4)
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