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Narrowband cellular systems require no bandwidth expansion for spectrum sharing. This attractive property is offset by the need to separate cochannel cells in order to reduce mutual interference. The net effect is a larger cluster size and a smaller system capacity than can be obtained by wideband cellular systems that operate with a cluster size of one. We propose to use a joint maximum-likelihood detection in the uplink of a narrowband system as a method to also allow it to operate with a unit cluster size. Unlike previous research, we jointly perform the detection on the desired and other-cell users. The focus of this multicell multiuser detection is more on cluster-size reduction than additional same-cell users, although the latter is also achieved. We address the problem of computational complexity by including only the strongest interferers in the joint detection. We have shown that, with a small computational cost, a narrowband system can operate with a cluster size of one and, thereby, obtain many times the spatial reuse efficiency of conventional narrowband systems.