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Unlike point-to-point cognitive radio, where the constraint imposed by theprimary rigidly curbs the secondary throughput, multiple secondary users havethe potential to more efficiently harvest the spectrum and share it amongthemselves. This paper analyzes the sum throughput of a multiuser cognitiveradio system with multi-antenna base stations, either in the uplink or downlinkmode. The primary and secondary have $N$ and $n$ users, respectively, and theirbase stations have $M$ and $m$ antennas, respectively. We show that an uplinksecondary throughput grows with $\frac{m}{N +1}\log n$ if the primary is adownlink system, and grows with $\frac{m}{M +1}\log n$ if the primary is anuplink system. These growth rates are shown to be optimal and can be obtainedwith a simple threshold-based user selection rule. Furthermore, we show thatthe secondary throughput can grow proportional to $\log n$ while simultaneouslypushing the interference on the primary down to zero, asymptotically.Furthermore, we show that a downlink secondary throughput grows with $m\log\log n$ in the presence of either an uplink or downlink primary system. Inaddition, the interference on the primary can be made to go to zeroasymptotically while the secondary throughput increases proportionally to $\log\log n$. Thus, unlike the point-to-point case, multiuser cognitive radios canachieve non-trivial sum throughput despite stringent primary interferenceconstraints.

Capacity Limits of Multiuser Multiantenna Cognitive Networks