Indoor Slotted-ALOHA Protocols Using a Smart Antenna Basestation

Abstract

In this paper we consider the reverse-linkcapacity performance of indoor Slotted-ALOHA protocolsadapted for use with a multibeam smart antenna (SDMA)basestation. When SDMA is used in this application, the basestation attempts to transmit or receivemultiple packets simultaneously using its antenna array.S-ALOHA protocols are well suited for this purpose sincethey permit simultaneous station transmission more readily compared with conventional LANprotocols such as CSMA. It is assumed that the portablestations have omnidirectional antennas and share asingle radio link to the basestation. Versions of the protocols are considered where initial stationaccess occurs in the data slots directly, and when botha single-beam and multibeam reservation channel is usedprior to data slot access. Two slot allocation algorithms are proposed which can be used withthe reservation channel protocols to dynamically assignportable stations to the data channel. It is shown thatsignificant improvements in system capacity are possible when more sophisticated dynamicslot allocation is employed. In the results presented,we optimize the design of each system to maximize thecapacity achieved. The comparisons thus give an indication of the relative trade-offs betweencapacity and complexity possible in such a system. Ofthe cases considered, multibeam operation in bothreservation minislots and data slots can achieve the highest theoretical capacity. However, we findthat when operating under low SNR, and especially forlong packet lengths, only very marginal improvements incapacity are achieved compared with other methods. This is important to note, since operating thesystem with multibeam minislot contention is expected tobe highly complex due to the difficulties of dynamicacquisition. We find that when packet lengths are less than about 2 kbits, there is verylittle capacity advantage in using a single-beamreservation protocol over multibeam slotted ALOHA(S-ALOHA).