الفهرس 
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Abstract
In this thesis we are interested in spatial holes or extra spatial dimensions and how to exploit these holes in different ways to achieve a more reliable communication system. Throughout this work we have considered two different scenarios for utilizing the extra spatial dimensions. In the first scenario we consider spatial holes in a cognitive radio network, while the second scenario was concerned about spatial holes in a decode-and-forward relaying network. Our contribution can be summarized in two points. The first is investigating a spectrum sensing algorithm for detecting spatial dimension holes in Multiple Inputs Multiple Outputs (MIMO) transmissions for OFDM systems using Compressive Sensing (CS) tools. This extends the energy detector to allow for detecting transmission opportunities even if the band is already energy filled. We show that the task described above is not performed efficiently by regular MIMO decoders (such as MMSE decoder) due to possible sparsity in the transmit signal. Building on successful activity detection by the CS detector, we show that the use of a CS-aided MMSE de¬coders yields better performance rather than using either CS-based or MMSE decoders separately. The second is proposing an incremental decode-and-forward (DF) relaying scheme; in our proposed scheme, we make use of the free spatial dimensions over which the relay node will forward the source node data. The relay node will help in providing an automatic repeat request (ARQ) service for the source node that is completely trans¬parent to the source node. The proposed ARQ service will not require any additional complexity at the source node transmitter and will not incur any rate loss. We provide a complete analysis for the symbol error rate (SER) of our proposed scheme. In addition, we derive SER upper-bounds to calculate the diversity order achieved by our scheme. The results show the significant performance gains of our proposed scheme, in terms of the system SER and the diversity order, compared to the system where no relay is deployed.