Mobile WiMAX has been developed for high-speed data rate applicaitons especially in mobile environments compared with other IEEE 802 series standards. Because of scarce frequency resources, it is unavoidable to interfere the neighboring commercial mobile networks when mobile WiMAX system is deployed.
Wireless broadband targets to bring high-speed data to multitudes of people in various geographical locations where wired transmission is costly, inconvenient, or unavailable. WiMAX is one kind of technologies devoted to make broadband wireless commercially available to the market. Compared with fixed version WiMAX, mobile WiMAX, based on IEEE 802.16e amendment, is intended primarily for both stationary and mobile deployments and designed specifically to optimize performance of wireless links in the outdoor environment over the next couple of year.
Mobile WiMAX defines interoperable system profiles of base stations and terminals, which are targeted for licensed and licensed-exempt frequency bands around the world. WiMAX channel bandwidths are 1.25MHz, 5MHz, 10MHz and 20MHz, etc. Data bursts can be transmitted through either FDD or TDD mode. Furthermore, TDD mode is more attractive than FDD mode in WiMAX system because TDD mode WiMAX system will not require a pair of frequency bands. On the other hand, the frequency resources are very limited around the world and it is hard to find frequency band pairs to deploy FDD mode WiMAX system.
In China, more than 150MHz frequency bands, located in 400MHz, 1.8GHz and 2.3GHz, have been allocated to TDD mode wireless access technologies. Among these frequency bands, nearly 140MHz frequency bands are reserved to 3G TDD systems. And in the left frequency bands, only 1.8GHz frequency band is adjacent to the commercial mobile networks, DCS1800 DL, which were deployed by China Mobile and China Unicom (DCS1800 is based on GSM technology which is deployed in 1800MHz frequency band). To avoid the interference between mobile WiMAX and the existing system, it is assumed that mobile WiMAX is deployed in this frequency band, the most important thing is to do the analysis on co-existence problem.
Some research results show that the interference between these two systems is very small and no additional protection is requierd when the BSs of the two systems are not co-sited. When the BSs of the two systems are co-sited, the inter-system interference except in WiMAX UL is negligible. The WiMAX UL capacity loss due to GSM BS can be alleviated to a satisfied level through increasing either ACIR (Adjacent Channel Interference Ratio) or antenna isolation. If the method of increasing ACIR is used, the recommended value is 85dB. On the other hand, if the method of increasing antenna isolation is used, the recommended value is about 60dB. Furthermore, these two methods can be used simultaneously and the required values for ACIR and antenna isolation are lower than the above-mentioned value.
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