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abstract

In this paper, hierarchical modulation is used in conjunction with maximum-weight scheduling to achieve lower transmission delays. Via hierarchical modulation, the scheduled user has the option to transmit to two users simultaneously. In order to reflect service differentiation schemes used in upper layers, a scenario in which each user generates packets with different priority levels is considered. It is assumed that as long as there are high priority packets waiting in the scheduled user’s queue, lower priority packets cannot be transmitted. It is shown that, using hierarchical modulation in the presence of packet prioritization, average delay of low priority traffic can be reduced while achieving higher throughput. In the absence of packet prioritization, using hierarchical modulation lowers packet transmission delays without any loss in throughput. The effect of multiple access interference is also investigated. It is shown that both single and two-layer schemes have similar average spatial reuse factors.

5. Conclusion

In this paper, we study maximum-weight scheduling, where broadcasting to two users is possible in addition to single user transmission. For broadcasting purposes hierarchical modulation and successive cancelation demodulation are used. We consider two scenarios: (1) all generated packets have equal importance and (2) packets can have four different priority levels. It is shown that in both cases hierarchical modulation is utilized for more than 50% of the time, resulting in lower delays and shorter queue lengths when compared with single-user scheduling with singlelayer modulation schemes. Moreover, hierarchical modulation results in the same (scenario 1 and scenario 2 first and second priority level packets) or higher throughput (scenario 2, third and fourth priority level packets) when compared with single-layer transmission. Finally, we incorporate the effects of interference into our simulation and compute the spatial reuse factor for scenario 1. We find that hierarchical modulation does not incur any extra loss at the multiple access layer and attains a similar spatial reuse factor as single-layer transmission. As a conclusion, joint physical layer and multiple access layer design via two-user scheduling improves delay performance with out any loss in total throughput or in average spatial reuse factor. Multiuser modulation schemes have a high potential to improve throughput, transmission delays, buffer occupancy and fairness. These gains are more emphasized when there is asymmetry in the network. In this paper, we only considered the effect of unequal priority levels. Future work includes studying asymmetry in bit error requirements and in packet lengths of the two receiving users.