Abstract
Hierarchical modulation multiplexes layers of different robustness into one stream. Usually being exploited in broadcast systems such as digital video broadcasting, hierarchical modulation can also be applied to unicast communications in a cooperative manner. This article deals with novel multi-hop cooperative transmission and routing strategies with hierarchical modulation. In multi-hop networks, when intermediate nodes receive different sets of streams, they cooperatively forward streams toward the final destination. We first show how hierarchical modulation reduces the total number of transmissions in tandem networks. We then model an optimization problem that determines proper modulation schemes and routing strategies as a stationary infinite-horizon Markov decision process (MDP). The MDP problem requires exponential computations forcing us to delve for heuristic solutions. We simplify the optimization problem by restricting hierarchical modulation consisting of only two fixed constellations. Even with this approximation, the routing problem with hierarchical modulation is still too complex because we must consider all possible paths and hierarchical modulation schemes at the same time. We introduce an approximation scheme that determines the most efficient hierarchical modulation scheme for each three node subgraph assuming subgraphs are isolated each other. Based on the three node subgraph approximation, we map original graphs to virtual graphs. We then apply one of shortest path algorithm to the virtual graphs to find the best route with hierarchical modulation. We perform simulations with various network topologies to evaluate the efficiency of hierarchical modulation in multi-hop unicast communication. Our results show that hierarchical modulation enhances the throughput about 25–30% compared to non-hierarchical modulation. However, optimal solution derived by solving the MDP is also 5% better than heuristic solutions suggesting the necessity to develop better heuristics in future.
Highlights
In wireless networks, bandwidth is a scarce resource and should be efficiently used
This article deals with unicast communications with hierarchical modulation in multi-hop wireless network
The optimization problem is to find the most efficient end-to-end path that realizes the potential benefits of hierarchical modulation
Summary
Bandwidth is a scarce resource and should be efficiently used. As the demands for mobile communications increase explosively, the importance of careful use of radio resources increases and the last decade has witnessed rigorous research on this topic. One approach that improves the efficiency of wireless networks is cooperative communication. Multiple nodes collaborate to transfer information to final destinations. Hierarchical modulation multiplexes layers of different robustness in one symbol. Receivers in good channel conditions can decode all layers including less robust layers
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