Design And Implementation Of Downlink Spatial Multiplexing Based On GNU Radio In FDD-LTE

With people’s growing demand for bandwidth,mobile communication technology has entered the 4G era with the feature of low time delay,high transmission rate and all IP,etc.MIMO,as a critical technology in LTE specification,combined with OFDM can effectively improve the reliability and transmission speed of the system.Faced with service,especially video service that demands high transmission rate,spatial multiplexing has become one of the key technologies to deal with the bandwidth problem.In this paper,GNU Radio is utilized as the software platform and USRP as the hardware platform.The LTE 2×2 MIMO spatial multiplexing mode is designed and implemented based on the LTE specification.First of all,the basic principle of MIMO and its classification is introduced in this paper,including transmission diversity and spatial multiplexing.At the same time,how the MIMO technology improves the system capacity is analyzed in detail.The SFBC method is applied in transmission diversity technology while large CDD method is applied in spatial multiplexing technology,which is also called precoding technique.Both schemes are introduced in this paper.And then the design and implementation of MIMO spatial multiplexing mode is clarified in detail.Based on the existing LTE single-antenna implementation,the schedule scheme of MAC layer is re-designed.The system transmits two TBs instead of one TB at a TTI time.After scrambling,modulating,precoding and so on,these two TBs are both transmitted by two antennas.Large CDD is adopted in PDSCH to realize spatial multiplexing.Meanwhile,SFBC is adopted in PBCH / PCFICH / PDCCH.This paper makes full use of the multiple antennas by combining transmission diversity technology with spatial multiplexing technology,which achieves the goal of guaranteeing the system reliability and improving the system transmission rate.The signal from the first antenna can be extracted to synchronize the frame in order to reduce the computational complexity.After the synchronization,the number of antennas can be detected by different CRC masks of PBCH.And then the four channel coefficients can be figured out and the channel can be equalized.Meanwhile,PCFICH /PDCCH data is extracted from two channels.After the SFBC data of PCFICH/PDCCH is decoded,the downlink control information can be obtained.And then the precoding data of PDSCH is decoded from two channels and submitted to the data link layer.At last,the spatial multiplexing mode is tested in both simulation environment and real wireless environment,compared with single-antenna mode and transmission diversity mode.According to the packet error rate result,the transmission diversity mode performs the best and spatial multiplexing mode is second to it,which can reduce the packet error rate by one order of magnitude compared with single-antenna mode.Intuitively,the video blurs and stutters when the system is in the single-antenna mode and the transmission diversity mode.In contrast,when the system is in the spatial multiplexing mode,the video can be played clearly and smoothly,since it can improve the transmission rate up to two times of singleantenna mode averagely.