Analysis of LTE system core technology and discussion of eNodeB test plan 1 Introduction The wide application of UMTS (Universal Mobile TelecommunicaTIons System) system satisfies users' demands for data services and effectively improves call quality and data rate. However, the emergence and popularization of broadband access technology, Wi-Fi, WiMAX system high data rate advantages, have a great impact on the UMTS system. This makes the UMTS system's shortcomings such as low data rate, long delay, and complicated network structure. Therefore, the long-term evolution plan (LTE) of UMTS proposed by the 3GPP (3rd Genera Tion Partnership Project) guarantees the competitiveness and leadership of UMTS in the next 10 years by providing a packet optimization system characterized by high rates and low latency. To achieve this goal, the LTE system has introduced a number of key new technologies relative to the UMTS system, which has resulted in corresponding improvements in the physical layer technology, network structure and protocol architecture of the LTE system, and the core network also needs to be upgraded accordingly To support the LTE system. Therefore, the LTE system is not only an evolution of the UMTS system. The test work of eNodeB equipment in the LTE system also has higher challenges. As an important link in the mobile communication industry chain, the test is located upstream of the industry chain and is the fundamental guarantee for the normal operation and maintenance of the entire wireless communication system. Therefore, research on test methods and test cases of eNodeB equipment is imperative. 2 The core new technology of LTE system LTE is a new mobile broadband access standard proposed by 3GPP to meet the needs of the times. To this end, 3GPP stipulates various technical indicators of the LTE system and introduces a number of core new technologies. The main technical indexes of LTE system and HSPA system are compared in Table 1. Table 1 Main technical indicators of LTE system In order to achieve high data rates and high spectrum utilization, the LTE system uses SC-FDMA and OFDM modulation techniques in the uplink and downlink respectively. They split the entire system bandwidth into a large number of subcarriers and support multiple modulation methods such as QPSK, 16QAM and 64QAM. The LTE system also specifies different modes of MIMO technology to adapt to different signal-to-noise ratio conditions. The LTE operating frequency is from 700MHz to 3GHz, and the channel bandwidth is from 1.5MHz to 20MHz, which provides network operators with flexible frequency band configuration. The core new technologies introduced by the LTE system are summarized as follows: 2.1 OFDM / OFDMA The transmission technology in LTE adopts OFDM modulation technology. Its principle is to distribute high-speed data streams through serial-parallel conversion and distribute them to several mutually orthogonal sub-channels with lower transmission rates for parallel transmission. Since the symbol period in each sub-channel will increase relatively, the impact of the time dispersion caused by the multipath delay spread of the wireless channel on the system can be reduced. Insert a guard interval between OFDM symbols to make the guard interval greater than the maximum delay spread of the wireless channel, thereby maximizing the elimination of inter-symbol interference (ISI) caused by multipath. In the LTE system, a cyclic prefix CP (Cyclic Prefix) is used as a guard interval, and the length of the CP determines the anti-multipath capability and coverage capability of the OFDM system. Long CP helps to overcome multipath interference and supports a wide range of coverage, but the system overhead will increase accordingly, resulting in a decline in data transmission capacity. 3GPP defines two sets of long and short cyclic prefix schemes, which are selected according to specific usage scenarios; the short CP scheme is the basic item, and the long CP scheme is used to support large-scale coverage and multi-cell broadcast services in the LTE system. LTE stipulates a multiple access scheme that uses OFDMA for the downlink and SC-FDMA for the uplink, which ensures orthogonality between users who use different spectrum resources. One transmission symbol in OFDMA includes M orthogonal sub-carriers transmitted in parallel, while in SC-FDMA mechanism M orthogonal sub-carriers are transmitted in a serial manner, which reduces the large amplitude fluctuation of the signal and reduces the peak power ratio. In addition, in order to ensure the orthogonality between uplink multi-users, the uplink signals of each user are required to reach the eNodeB at the same time within the error range of the CP length. Therefore, the eNodeB needs to adjust the transmission time of each user according to the distance between the users. The LTE system also has more flexible scheduling methods for OFDM subcarriers, with two types: centralized and distributed, and can flexibly convert between these two methods. In addition to using this scheduling mechanism for the uplink, a contention (ContenTIon) mechanism can also be used. 2.2 MIMO MIMO technology is the main method to increase the system rate. The LTE system supports MIMO technology adapted to various environments such as macro cells, micro cells, and hot spots. The basic MIMO model is a downlink 2 × 2, uplink 1 × 2 antenna array, and a 4 × 4 antenna configuration will be supported in the later stages of LTE development. Currently, the downlink MIMO mode includes beam formation, transmit diversity and spatial multiplexing. These three modes are suitable for different signal-to-noise ratio conditions and can be converted into each other. Beamforming and transmit diversity are suitable for scenarios where the signal-to-noise ratio is not high, and are used for cell-edge users to improve the coverage of the cell; spatial multiplexing mode is suitable for scenarios with high signal-to-noise and are used to improve the user's Peak rate. The maximum number of code streams simultaneously transmitted in the spatial multiplexing mode is 4; the spatial multiplexing mode also includes SU-MIMO (single user) and MU-MIMO (multi-user), and the switching between the two modes is determined by the eNodeB. In the uplink MIMO mode, open-loop or closed-loop transmission modes are respectively set according to whether feedback information of the eNodeB is required.
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