Uplink MIMO Technology For TD-LTE System

The technology of Multiple Input Multiple Output (MIMO) for wireless communication, refers to the use of multiple transmit and receive antennas for wireless transmission. With the diversity of radio bearer services, radio spectrum is becoming more and more scarce. How to achieve higher spectral efficiency with limited radio frequency resource, has been urgent currently. MIMO is just a key technology to enhance data transmission rate and quality for future mobile communication systems.In the LTE standard which has been agreed and established, downlink has adopted MIMO technology, and as lots of vendors strive to promote the standard, downlink MIMO has become relatively mature. Although user terminal may have more than one antenna, only one antenna can be used to transmit uplink signal considering PA constraints, while multiple antennas are used for downlink reception. In other words, uplink is a SIMO architecture. With the arrival of mobile internet era and explosive growth of wireless applications, it requires uplink to provide much higher data transmission rate and better service quality increasingly in wireless network. In the evolution of LTE towards LTE-Advanced, uplink MIMO becomes an important part to be discussed in the 3GPP meetings.In this paper, possible uplink MIMO schemes which might be involved were studied. The content includes two parts:spatial diversity and spatial multiplexing.As to spatial diversity, some open-loop MIMO diversity schemes used in downlink were discussed, such as Space Time Block Coding (STBC), Space Frequency Block Coding (SFBC), Cyclic Delay Diversity (CDD), etc. When these schemes are transplanted to uplink, some issues should be noted:(1) influence on the single-carrier property of uplink signal, (2) how about the performance improvement. This paper has done some qualitative analysis about these diversity schemes, and also, proposed Alamouti diversity scheme under 2 transmit antennas and MDC diversity scheme under 4 transmit antennas, and has done quantitative analysis about them along with a performance comparison with SIMO transmission mode.The part of spatial multiplexing mainly studied closed-loop precoding technology, which could be divided into two kinds of operation in detail:codebook based precoding and non-codebook based precoding. About codebook based precoding, we analyzed design principle for uplink codebook first, and then discussed some issues which should be noted, finally proposed a new codebook appropriate for uplink on the basis of downlink codebook. Through performance comparison with downlink codebook, we gave a codebook for 2-antenna scenario and a part of codebook for 4-antenna scenario finally. Non-codebook based precoding is especially applicable to TDD system with channel reciprocity. Simulation results show that non-codebook based precoding performs a bit better than codebook based precoding. However, there are still many practical problems when non-codebook based precoding is applied.In addition, link adaption technology was introduced to uplink, integrated with uplink precoding, to see how adaptive mechanism can improve link performance.