Читать книгу Spatial Multidimensional Cooperative Transmission Theories And Key Technologies - Lin Bai - Страница 15

Along with the rapid development of the first three generations of mobile communication systems and intelligent mobile terminals, the users’ demand for services has changed from voice-based to Internet-based communication modes based on high-speed data streams. As users’ demand for transmission rate continues to increase, people are beginning to develop the next-generation system based on the first three generations of mobile communication systems to better support high-speed broadband mobile communication services. The World Radio Conference in 2007 allocated spectrum for IMT-Advanced, and IMT-Advanced standards began to be collected in March 2008. By October 2009, a total of six candidate proposals were collected, which can be classified into 3GPP LTE-Advanced³ and IEEE802.16m.⁴ At present, international standards for 4G mobile communication technologies mainly include FDD-LTE, FDD-LTE-Advance, TD-LTE, and TD-LTE-Advanced. Among them, TD-LTE and TD-LTE-Advanced are 4G international standards led by China.

LTE is an evolution of 3G. It improves 3G air access technology, using orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) technologies as its wireless evolution technology. The LTE mobile communication system can provide a downlink rate of 100Mbit/s (TD-LTE) or 150Mbit/s (FDD-LTE), an uplink rate of 50 Mbit/s (TD-LTE) or 40 Mbit/s (FDD-LTE), and peak rates in a 20-MHz spectrum bandwidth. TD-LTE is the 4G international standard led by China, and it is adopted by China Mobile.

LTE-Advanced can be divided into FDD-LTE-Advanced and TD-LTE-Advanced. It is optimized for indoor environments and uses technologies such as carrier aggregation. It flexibly allocates spectrum for a wider spectrum bandwidth and effectively supports new frequency band and large-bandwidth applications. It can provide a downlink rate of 1 Gbit/s and an uplink rate of 500 Mbit/s in a 100-MHz spectrum bandwidth.

WiMax is an IEEE 802.16 standard that provides a maximum access rate of 70 Mbit/s and a working frequency range of 2–66 GHz without authorization. The main advantages of WiMax are as follows: (1) it is beneficial to avoid known interference, (2) it is beneficial to save spectrum resources, (3) flexible bandwidth adjustment capability is beneficial for operators to coordinate spectrum resources, and (4) wireless signal transmission distance can be up to 50 km. However, it cannot meet the seamless connection of wireless networks under high speed in terms of mobile performance. Therefore, WiMax is not a wireless mobile communication technology, but only a wireless broadband LAN technology.

Wireless MAN-Advanced is an upgraded version of WiMax, namely the IEEE 802.16m standard, which has the ability to seamlessly switch under high speed. It can effectively solve mobile performance problems of WiMax. IEEE 802.16m is compatible with 4G networks. Its advantages are as follows: (1) expand network coverage to achieve seamless network connectivity, (2) improve spectrum efficiency, (3) provide 1 Gib/s wireless transmission rate in roaming mode or high-efficiency/strong signal mode.