1.1 An Overview of Wireless Communications
The development of wireless communication has gone through four generations. The first-generation (1G) wireless communication system is analog cellular wireless communication based on analog signal, which mainly provides voice services. It was the booming period for the development of 1G from the 1980s to 1990s. In 1978, Bell labs developed the advanced mobile phone service systems with the operation frequency 900 MHz. It was firstly put into commercial application in Chicago, Illinois, USA. After that, other industrialized countries developed their own cellular wireless communication systems such as total access communication system, Nordic Mobile Telephone and high capacity mobile telecommunication system, etc. The reason why the development of cellular wireless communication systems was so fast is due to: (1) multichannel sharing and frequency reuse technique; (2) complete system functions such as handover, wander, etc.; and (3) availability to civil telephone network. 1G employs the frequency division multiple access (FDMA) technique and voice signals are analogy modulated. Although they achieved great commercial success, some drawbacks that existed in these systems became more and more fatal as the number of users increased, such as low spectral efficiency, limited business lines, low rate data service, low security, high-cost equipment, etc.
To overcome the previous drawbacks of 1G, the second-generation (2G) wireless communication system was proposed and achieved fast development. Compared with 1G, 2G improves the spectral efficiency and supports several business lines (i.e., voices and low rate data services); hence it can be also called narrow-band digital wireless communication systems. Two typical cases, global system for mobile communication (GSM) and interim standard 95 (IS-95) were proposed by Europe and the United States from the middle 1980s, respectively. GSM originated from Europe and was designed for the global digital cellular communications. It employs time division multiple access (TDMA) and its data rate achieves 64 Kbps, working on 900 MHz. As the digital cellular standard of North America, IS-95 adopts code division multiple access (CDMA) technique, working on the frequency 900 or 1800 MHz.
Since 1990s, the number of Internet users has increased explosively with the rapid development of Internet. People wanted to access the Internet not only at home and the office, but also from moving locations. Thus, it was an urgent requirement to combine Internet and wireless communication technique. However, 1G and 2G cannot meet the requirement due to the low data rate. To settle the conflict between the huge wireless communication market and limited spectrum resources, the third-generation (3G) wireless communication system was developed. All over the world, three famous standards of 3G are wideband CDMA (WCDMA) in Europe, CDMA 2000 in the United States, and time-division synchronous CDMA (TD-SCDMA) in China. Compared with 1G and 2G, 3G systems employ more frequency bands up to more than 5 MHz and the data rates are 384 Kbps at least and 2 Mbps at most. 3G can transmit either voice or data. Thus, it can provide fast and convenient wireless application such as accessing to the Internet wirelessly. 3G can combine high-speed mobile access and Internet services and achieve the goals of global coverage and seamless connection among different wireless networks.
To meet the requirement of high data rate, the international telecommunications union specified a set of requirements for 4G standards in 2008, that is, the international mobile telecommunications advanced (IMT-Advanced), setting the peak speed to be 100 Mbps for high mobility communications such as in cars and trains. Orthogonal frequency division multiplexing (OFDM) and multi-input multi-output (MIMO) techniques constitute the basis of 4G standards. Compared with CDMA, OFDM has a stronger ability to fight against the frequency selective fading channel, especially...