On The Diversity Multiplexing Tradeoff Analysis Of The Cooperative Diversity Scheme

The mobile wireless channels could be characterized by its multi-path propagation, from which the signal fading and delay spread arise. As an effective anti-fading technology, diversity, has been widely used in wireless communication. According to the way to achieve the diversity, there are time diversity, space diversity, frequency diversity and their combinations. Among different implementations, space diversity is widely utilized due to its effectiveness and impressive performance improvement without additional time and frequency resource. Multi-Input Multi-Output (MEVIO) systems can significantly increase the channel capacity by exploiting the multi-path transmission characteristic in wireless system. However, the MIMO technique may be hard to be implemented directly in some cases duo to the physical constraints of mobile unit. Thus, the idea of cooperative diversity is proposed to solve this problem by using a virtual multi-antenna structure generated by pairs of user with single antenna. It is known that multi-antenna deployment can offer both diversity gain and multiplexing gain. Moreover, it's unveiled that, the increase in one of two gains will definitely result in the decrease in another gain. Therefore, a rational tradeoff between diversity gain and multiplexing gain becomes a new design criterion for multiple antenna diversity. In this thesis, four different types of synchronized and asynchronous diversity schemes will be analyzed in terms of BER performance and the diversity-multiplexing (DM) tradeoff.The thesis begins with the analysis of synchronous cooperative diversity schemes. The Alamouti-type of cooperative diversity scheme is firstly analyzed on its BER performance. It is shown through simulation results that, the BER performance increases with the number of receive antennas and decreases with the modulation order. Then the thesis analyzes the BER performance of the scheme based on orthogonal signaling when the channel conditions between users and the receiver are dissimilar. Then a link-adaptive power control is used to modify the orthogonal signaling diversity scheme. It is validated that, the link-adaptive power control can effectively alleviate the threshold phenomena. Secondly, the DM tradeoffs of the previous two types of cooperative diversity schemes are derived. The tradeoff curves of the Alamouti type of cooperative diversity scheme show that, with the increase of the number of receiver antennas, the diversity gain increases while the multiplexing gain remains unchanged. Analysis of the orthogonal signaling cooperative diversity scheme indicates that link-adaptive technology can only improve the BER performance, but can not bring an enhancement on the DM tradeoff.The thesis continues with the analysis of asynchronous cooperative diversity schemes. The BER performance and DM tradeoff analysis of the delay diversity scheme demonstrates that, the delay diversity can achieve the same DM tradeoff as the repetition coded diversity as long as the relative delay between relays meet certain conditions. The thesis then investigates the BER performance and the DM tradeoff for a kind of asynchronous cooperative diversity based on the distributed delay diversity. Some modifications are made to the scheme by improving the delay selection and the forward scheme of relay nodes. Results indicate that, with certain delays between relays, the asynchronous cooperative diversity can achieve the same, or even better, DM tradeoff as the synchronous cooperative diversity.The BER and DM tradeoff analysis in this thesis could be used as a reference in both the design and evaluation of the cooperative diversity scheme.