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We extend the study of access schemes for packet-switched radio channels as an alternative to conventional wire communications for data transmission among users. Among the various multiple access schemes previously implemented or proposed, ALOHA presents many advantages, especially for a large population of bursty users. However, more than 60% of the ALOHA channel capacity is wasted. In this paper we introduce a separate large carrier-sensing user who "steals" slots which remain unused by the background of ALOHA users. This leads to a new multiple-access scheme: the Mixed ALOHA Carrier Sense (MACS) access scheme, whose performance We analyze. The total channel utilization is significantly increased with MACS, and the delaythroughput performance of both the large user and the background of ALOHA users is shown to be better with MACS than with a "split channel" mode in which the large user and the ALOHA users are each permanently assigned a portion of the channel.