Abstract
In this thesis, we present a system design for large set of user uplink codes which are suit- able under the scenario of large amount of users ranging in the machine-to-machine (M2M) communications. To distinguish from different users, spreading each user’s signals with spec- ified codes is one of the practical solutions. This idea is similar to the concept of code division multiplexing access (CDMA) system. The zero correlation zone (ZCZ) sequences and Gold sequences are taken as spreading sequences in the proposed CDMA-like system. The spread- ing procedures for each user’s signals are divided into two levels and the above codes are applied according to their different correlation properties to fight against different kind of in- terferences. Compared with previous works of traditional CDMA systems, this work supports a quantity of simultaneously uplink users up to 256 and is with better performance than that of the system accommodating only 64 users that traditional binary sequences such as Kasami or pseudo noise (PN) sequences are taken as spreading codes. The proposed system requires no additional data scrambling after two layer spreading. Instead, we take a much larger spreading factor. The simulation results obtained from C language programming show that the bit error rate of the system with proposed code assignments improves with 2 dB under SNR equals to 10^−3 compared with the work that supports only 64 simultaneous users and requires channel equalization and data descrambling at the receiver end. The architecture and logic design of this two layer spreading system are also presented. The Gold sequence generator is implemented by two m-sequence generators and the ZCZ sequence generator is designed since there are no related works before. Besides, because of ZCZ sequences being taken as second layer spreading codes, the code set size of ZCZ sequences will decide the number of users supported which means that the code length will relatively become much larger. Therefore, the logic design of ZCZ sequence generator plays an important role in the size of transmitter since it takes part of 65% in the transmitter. The ZCZ generator design is required to be optimized to reduce unnecessary costs. The total gate counts of the whole system are roughly estimated about 11.k which is suitable for low complexity and low power consumption of M2M communications requirements, and the total synchronization time for the proposed code system is about 200 ms operating in chip rate of 2.5 Mcps. Compared with IEEE 802.16e, IEEE 802.16m, and cdma2000, the synchronization cycles of the proposed system design are 33%, 16.7%, and 83.9% fewer.