Design And Implementation Of FDD-LTE Multi-user Uplink System Based On GNU Radio

In the LTE uplink, the signals are composed of several users' signals with different timing offsets and frequency offsets. So we need to estimate multiple parameters and compensate for the deviations of different users simultaneously to implement uplink synchronization. First of all, the paper states some thesis work-related knowledge about the LTE system, including the system frame structure, modulation techniques, communications channels, also covering key sequences used for the multi-user uplink synchronization, features of the multi-user uplink system, and the subcarrier allocation.Then, based on the implemented LTE demo system, aiming at the problem that only one terminal's data can be transmitted in one uplink subframe, the paper discusses the improved design of the multi-user concurrent function at the base station in detail. The design can transmit user data from multiple terminals simultaneously in one uplink subframe and include the following three parts: uplink scheduling changed from time division multiplexing polling scheduling to frequency division multiplexing subcarrier allocation; “multiway parallel spatial processing + resource demapping” instead of narrow band filter group to separate the multi-user uplink data; the multi-user uplink synchronization scheme for the multi-user uplink system. In the scheme, the timing synchronization is maintained by timing adjustment in UE, in which the adjustment amounts is obtained by DMRS in e NB. Meanwhile, the frequency synchronization is accomplished by frequency offset compensation before FFT. SRS and DMRS are used for coarse frequency offset estimation and residual frequency offset estimation respectively. The performance tests are conducted for synchronous modules in multipath channel simulation. The results show that when five users communicate simultaneously, the performance of timing estimation algorithm in Gauss channel is better than that in the multipath channel, and the performance of joint frequency offset estimation algorithm in Gauss channel is similar to that in the multipath channel.Thereafter, the paper realizes the above design based on the GNU Radio/ USRP platform and the implemented LTE demo system. Firstly, the paper implements the uplink resource allocation, by which a plurality of terminal requests can be responded in the same subframe thus to improve the bandwidth utilization rate and efficiency. Secondly, uplink synchronization scheme is realized, including the achievement of frequency offset compensation and UE timing adjustment. Besides, when SRS and DMRS are used to estimate deviations, SRS correlation algorithm and CRC check are introduced separately to impove the effectiveness and accuracy of the estimation. Thirdly, based on the features of the GNU Radio platform and flow graph mechanism, the paper builds the complete LTE wireless communication system which can support the multi-user uplink synchronization and signal processing.Finally, the system is tested in the indoor wireless environment. The results show that the base station can schedule two terminals' requests, and receive video streams sent by two terminals simultaneously. With the code rate of 0.5 and QPSK modulation under the 3MHz bandwidth wireless environment, the average uplink BER can reach 1.3 × 10-6, and subjective video quality is clear and smooth. Two terminals can reach the total uplink transmission rate of 227.6Kbytes/s, which basically agrees with the theoretical analysis.