Previous work by the authors presents a novel class of signal space codes called collision resistant modulations (CRM) for bandwidth efficient transmission on a random access collision channel, as in packet-radio applications. Results show that CRMs achieve a significant gain over traditional modulation formats under a slotted ALOHA protocol. We investigate the potentials of using CRM under a partially ordered access protocol like packet reservation multiple access (PRMA). A system based on Reed-Solomon coded CRM is compared with a more traditional system using the same Reed-Solomon coding concatenated with simple one-dimensional modulation schemes. Results show that the proposed coded PRMA yields significant improvements in terms of both the blocking probability and number of established calls (served users), at the expense of a slight increase in the average transmitted power.