In this paper, we analyzed the error rate of the sphere decoders with reduced search space (SD-RSS) and proposed a formula for estimating their error rate. With the instantaneous error rate information, an adaptive SD-RSS was designed that could adjust the search space on-the-fly to minimize the complexity while meeting the error rate requirement specified by the standard. We focused on the ultradense network with severe inter-cell interference, which increases the detection complexity and

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... icates the error rate analysis. The accuracy of the error rate performance estimation was demonstrated by several simulations, and one numerical example was presented to show that the proposed adaptive SD-RSS can significantly reduce its average computational complexity while meeting the LTE block error rate requirement. INDEX TERMS Adaptive receiver, error rate analysis, multiple-input multiple-output (MIMO), sphere decoder.