An Adaptive MIMO System Using Incremental Diversity

In the recent years, Multiple Input Multiple Output (MIMO) systems have emerged as a promising technology that offers reliability and spectral efficiency. Since then a great concern was devoted to enhance the performance of MIMO systems through efficient encoding and decoding techniques. Two famous MIMO approaches have emerged; Spatial Multiplexing (SM), and Space Time Block Coding (STBC). On one hand, SM offers high data rates. On the other hand, STBC presents transmission fidelity. This actually imposed a fundamental tradeoff between capacity and reliability. For a more efficient utilization of the MIMO resources, a combination between SM and STBC grasps both spectral efficiency and reliability. Hybrid SM/STBC was introduced as a MIMO system technology that offers the advantages of the two systems simultaneously. In this paper, an adaptive MIMO system that incrementally switches from multiplexing towards diversity or hybrid diversity/multiplexing is proposed. The system is initialized with a pure multiplexing mode. An error detection check is performed at the receiver side and successful detection is fed back to the transmitter via an acknowledgement. A negative acknowledgement due to false detection triggers the transmitter to incrementally switch towards the diversity mode where it sends a part of the redundancy symbols constituting an STBC or a hybrid SM/STBC - diversity mode. The incremental diversity symbols are transmitted until a positive acknowledgment is received or the transmitter exhausts its full diversity capabilities. Results indicate that, the proposed scheme combines the transmission reliability offered by MIMO diversity, while maintaining a gradual increase in the spectral efficiency in correspondence to the increase in the SNR.