| To meet the challenges of broadband access technologies and to meet new business demands, 3G (3rd Generation) long term evolution (LTE) standardization was launched at the end of 2004 by the 3GPP (3rd Generation Partnership Project). LTE seeks to increase the data rate and to reduce the system latency and to increase system capacity and coverage. This paper studies the key technologies of LTE downlink physical layer.Firstly, this paper describes the background of the project and its objectives, and analyses the proposed six basic physical layer transmission schemes.Secondly, this paper describes LTE unique time-frequency resource block allocation methods, and verifies the LTE downlink physical reference signal pattern through simulations, whose result shows that the pattern is justifiabla. Orthogonal Frequency Division Multiplexing is used as the basic transmission scheme of LTE downlink, which determines that the mapping between resources and LTE pilot structure is different from Code Division Multiple Access (CDMA) for 3G systems. In term of resource mapping, LTE downlink user data is separated into physical resource block, and multiplex multi-user's physical resource block data in a single shared channel, relying on a high efficient scheduling mechanism. In terms of the pilot structure, LTE does not use the same pilot signal as used in CDMA to help complete channel estimation, but utilizes some specific reference signal inserting in the physical resource block for transmission.Then, this paper analyses several space diversity techniques and linear precoding technology utilized in LTE downlink And, verifies their influence to the BER performance through simulation. Spatial diversity techniques can take advantage of multiple-input multiple-output (Multiple Input Multiple Output, MIMO) channel diversity gain to improve the BER performance. LTE Downlink two transmitting antennas used in space-frequency block coding transmit diversity as part of its program, this program is able to obtain good diversity gain. In order not to reduce the spectrum efficiency, LTE downlink transmission antennas is 4, the use of space-frequency block codes and antenna diversity switching combining transmit diversity program, combined with channel coding in this program compared to space-frequency block code can be further improve BER performance. LTE downlink precoding in support of the closed-loop linear space multiplexing, linear precoding techniques to gainFinally, concerning to the difficulties of in-door coverage brought about by the higher LTE transmitting frequency, this paper proposes a Femtocell technology utilized in LTE. Analysis shows that Fentocell can solve problems of indoor coverage, increasing system capacity, and etc, while there also lie many technology challenges, including radio interference, frequency allocation and synchronous. According to the synchronization of GPS, this paper concludes that Femtocell needs new technological solution. Precise time synchronization protocol is very suitable for a synchronization program, the analysis of the agreement based on the application of this agreement on LTE Femtocell system to achieve time synchronization framework. And then, according to the traditional beam forming technology, the pattern is spindly using point-to-point model of centralized antenna arrays and mobile. This paper proposes a research of cooperative multi-antenna broadband focus. |
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