Enhancement And Hardware Implementation Of TDCS

The physical radio spectrum resource is running out, and it’s a prominent problem that all wireless communication systems must consider. To address the issue, cognitive radio(CR) has been proposed to improve spectrum efficiency by interacting with and learning from the environment. Besides, CR is also famous its reconfigurable functionality where the transceiver dynamically changes its transmission parameters to adapt to users’ needs. Because of that, CR is often deployed on a software defined radio(SDR) platform.Meanwhile, by employing spectrum sensing and frequency domain spreading, transform domain communication systems(TDCS) is able to achieve spectrum interference avoidance and realiable transmission over spectrum fragments even in the face of complicate and fast changing electromagnetic environment. This makes it a well recognized candidate for CR overlay technology. However, there’re other problems remained to be solved in practical application for TDCS, such as spectrum sensing mismatch(SSM) analysis, multiple access interference(MAI) cancelling, etc.In this paper, the attention is focused on optimizing TDCS performance and building a hardware testbench. Firstly, the SSM is modeled using mathematical set theory, and a SSM factor is proposed to describe the extent of SSM in a deterministic way under any given spectrum pattern. According to our mathematical analysis and computer simulation, it’s found that TDCS’s system performance suffer from SSM, which can be expressed using the SSM factor proposed, and a theoretical bit error ratio(BER) expression is given. Then we move on to address the receiver design of multiple user TDCS. Due to the existence of SSM, TDCS signal can be considered as firstly passing a frequency selective channel, which deteriorate system performance and lead to more severe MAI. In our discussion, QPSK is adopted as the modulation method for TDCS, and a receiver design based on frequency domain oversampling(FDO) is proposed. By projecting the frequency domain signal in a fractional spaced way, more frequency diversity is generated and can be used to conquer frequency selective channel. And because the minimum mean square error(MMSE) detector has involved the process of de-spreading, MAI can be cancelled dramatically. This provide a more robust system performance for multiple user TDCS receiver.Lastly, by the co-design of algorithm, hardware tuning and software programming, a SDR testbench of TDCS is built to using the popolar universal software radio peripheral(USRP) platform to demonstrate that TDCS is a promising wireless communication candidate for both military and civil areas.