Mobile communication is based on two digital units that communicate to a cell cite. For effective communication, there should be smooth transmission of frequency with little or no interference. Engineers have been trying to improve analogue communication systems to overcome communication barriers. In recent years, frequency access technologies have been developed to improve cellular communication systems to reduce frequencies’ interference for effective communication.
Mobile communication takes place when a mobile unit communicates to a cell cite which has the capacity to respond to many units at the same time. The International Engineering Consortium states that the cell unit contains a channel for each mobile. A single channel contains a forward link and reverse link for transmitting from one cell to another cell (1).
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Mobile communication networks are divided into two, that is, telephone communication network and radio communication network. Radio service functions within a closed network set-up. It does not require telephone network. On the other hand, mobile telephone network can allow connection to a telephone network (The International Engineering Consortium, 1).
Digital systems are mobile communication systems that have integrated wireless technologies to overcome the shortcomings of wire line (landline) communication technologies (The international Engineering Consortium, 12). Digital systems have evolved from basic radio communication to advanced cellular systems used today.
According to First generation, cellular systems are similar to the 1970s cellular radio communication network. This network was based on frequency division multiple access (FDMA) and analog FM technology. Second generation cellular systems include GSM/DCS1800/PCS1900 standard in Europe, the PDC standard in Japan, the IS 54-/136 and IS-95 standards in the United States. Another cellular communication system is the cordless telephones. These are used in domestic cordless phones, wireless private access business exchange, (PABX) and radio drops of wireless local loops. Third generation communication system involves adoption of International Mobile Telephone. This system has a characteristic introduction of wireless wide-band packet switched data for wireless access to internet with upto 2 Mb/s. Others include Wireless Area Network systems and Personal Area Networks.
According to The international Engineering Consortium 1, this involves replacement of traditional analogue communication systems with their improved digital substitutes. Improvement of cellular frequency access is done by adopting multiple frequency access techniques. There are several multiple frequency access techniques that include frequency-division multiple access (FDMA), Time-division multiple access (TDMA), and Code-division multiple access (CDMA),
The concept divides frequency band into frequency channels. The strategy was adopted for the first-generation analog mobile systems. The application of this is in the broadcast, that is, for radio and television. In this case, a channel is given a range of frequency either in amplitude modulation (AM) or frequency modulation (FM) over which to broadcast. The reason behind unique frequency range is to prevent interference (Schwartz 2005, 137).
These are second-generation systems with circuit systems similar to switched telephone systems. The systems are advanced than digital signals, which are sent out on a particular frequency band that is transmitted in specified time slots in a repetitive frame structure (Schwartz 138). Usually, these frequencies operate at the carrier frequency allotted to that channel. The advantage is that each user can take longer on a call, because they are assigned time slots in a given frame.
This access technique is found in second-generation system IS-95. However, the technology has been adopted for third generation cellular systems. Schwartz (142) maintains that this scheme based spectrum technology is similar to traditional military communication systems. A user is assigned a unique digital code.
Because of high data transmission demand, there is an increased demand for integration of multiple transmission technologies in the digital systems (Ghazi-Maghrebi, 2278). Orthogonal Frequency-division Multiplexing (OFDM) is defined as a block modulation scheme that involves transmission of data in a parallel pattern that engages multiple orthogonal sub-carriers (Stüber 175). Ghazi-Maghrebi (2278), defines OFDM as a digital MCM technique that can transmit several narrowband-modulated signals at a slower rate than one wideband fast-modulated signal. He further states that Orthogonal Frequency-division Multiplexing (OFDM) is a special type of multicarrier transmission because of three things (2278). According to Ghazi-Maghrebi, there is an improvement of the traditional OFDM. Firstly, OFDM has a unique spectral efficiency. Secondly, it is strong in various multipath propagation environments. Thirdly, it has the capacity to stop inter-symbol interference (ISI).
OFDM has been suggested to be incorporated in cellular radio, digital audio broadcasting and wireless LAN systems such as MMAC, HIPERLAN and IEEE 802.11 because of its efficiency (Stüber 175). This information is in line with Swartz (129), who confirmed the proposed use of OFDM in advanced cellular systems. Swartz (129), however gives new findings that reveal that OFDM technology has been integrated as part of IEEE standards 802.11g as well as 802.11a for high rate, 54 Mbps, and transmission of data over wireless LANS (WLANs).
Digital systems have evolved from basic radio communication to advanced cellular systems used today. For effective communication, there should be smooth transmission of frequency with little or no interference. Engineers have been trying to improve analogue communication systems to overcome communication barriers. In the recent years, frequency access technologies have been developed to improve cellular communication systems to reduce frequencies interference for effective communication. Mobile communication networks are divided into two, that is; telephone communication network and radio communication network. Radio service functions within a closed network set-up. It does not require telephone network.
