| 內容/備註 |
The performance of a peer-to-peer communication system where the transmitted signal undergoes fading and is subject to multipath propagation may be improved through the usage of Walsh-Hadamard codes when there are bandwidth restrictions in place. If no bandwidth restrictions existed, good performance could be achieved by using traditional spread spectrum techniques. On the other hand, if such restrictions existed, it could be infeasible to transmit a high data rate signal over a fading channel with multipath propagation, and achieve good performance at the same time, since the data signal may occupy the entire channel bandwidth. This problem can be partially solved by subdividing the original bit stream into several lower-rate bit streams and then use direct sequence spread spectrum techniques on each substream, similar to the downlink of a cellular system. If orthogonal sequences such as Walsh-Hadamard are used, then the resulting signal is a multi-level PAM signal with an odd number of levels. In order to reduce the average symbol energy and to optimize transmission over a fading channel, a non-traditional QAM constellation is designed. Approximate closed-form expressions of symbol error probability are presented and performance improvement is verified via computer simulation. |