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Thursday, September 8, 2011

MIMO Transmission Schemes for LTE and HSPA Networks


For over a decade universities and wireless research labs have been combining multiple antenna transmission techniques with advanced signal processing algorithms to create what is sometimes called smart-antenna and is also known as multi-input multi-output (MIMO) technology. These schemes are now moving into mainstream communication systems. Indeed, MIMO technologies can already be found in wireless local area network access points (e.g. 802.11n based solutions). This has led to MIMO being standardized in WiMAX as well as in 3GPP Rel-6 and Rel-7 of the UTRAN (HSPA) specifications. Further, Rel-8 of the E-UTRAN (LTE) 3GPP specifications, completed in March 2009, included the most advanced forms of MIMO in any standard in the industry. And even more advanced MIMO enhancements are currently being studied for inclusion in 3GPP Rel-9 and Rel-10.




T
here are many MIMO schemes standardized in 3GPP systems, and the base station scheduler has the capability to optimally select the MIMO scheme that suits the channel conditions of the mobile. A fundamental MIMO scheme is that of precoded spatial multiplexing (SM) where multiple information “streams” are transmitted simultaneously from the base station to the mobile. These techniques are appropriate in high SINR areas with rich scattering environments, in combination with suitable antenna configurations.






Measurements at the base station receiver and feedback signals by the mobile help the base station

determine the number of information streams that can be supported across single or multiple users. SM is augmented with techniques such as beamforming and open loop transmit diversity that can be used as the channel conditions become less favorable to spatial multiplexing. The ability to dynamically adapt to the channel optimal MIMO scheme as channel conditions change is a key focus of the LTE Rel-8 specifications.



Antenna configurations at the tower have an impact on the types of MIMO schemes that are available to the base station. Narrowly spaced antennas are ideal for supporting beamforming, while widely spaced or cross pole antennas are ideal for spatial multiplexing and transmit diversity. The choice of antenna at the base station will depend on several factors including the expected performance benefits for the particular

deployment environment as well as real estate and cost considerations at the tower. At the terminal, the device form factor as well as operation at lower frequency bands further challenges designers in terms of achieving the required antenna efficiencies. Performance benefits of various MIMO schemes are shown through system simulation results, and demonstrate that both peak throughput as well as system and edge spectral efficiency are increased with MIMO techniques. In the paper, a technology-oriented approach was followed, leaving many real-world deployment and MIMO BS antenna design issues untreated. These include issues ranging from antenna sharing at the tower, to antenna beam design, to wind loading and many others worth of a separate white paper dedicated to this subject.

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