Hierarchical modulation (HM), which is also known as layered modulation, has been widely adopted across the telecommunication industry. Its strict backward compatibility with single-layer modems and its low complexity facilitate the seamless upgrading of wireless communication services. The specific features of HM may be conveniently exploited for improving the throughput/information-rate of the system without requiring any extra bandwidth, while its complexity may even be lower than that of the equivalent system relying on conventional modulation schemes.
As a recent research trend, the potential employment of HM in the context of cooperative communications has also attracted substantial research interests. Motivated by the lower complexity and higher flexibility of HM, we provide a comprehensive survey and conclude with a range of promising future research directions.
Our contribution is the conception of a new cooperative communication paradigm relying on turbo trellis-coded modulation-aided twin-layer HM-16QAM and the analytical performance investigation of a four-node cooperative communication network employing a novel opportunistic routing algorithm. The specific performance characteristics evaluated include the distribution of delay, the outage probability, the transmit power of each node, the average packet power consumption, and the system throughput.
The simulation results have demonstrated that when transmitting the packets formed by layered modulated symbol streams, our opportunistic routing algorithm is capable of reducing the transmit power required for each node in the network compared with that of the system using the traditional opportunistic routing algorithm. We have also illustrated that the minimum packet power consumption of our system using our opportunistic routing algorithm is also lower than that of the system using the traditional opportunistic routing algorithm.
HISTORIC BACKGROUND OF HIERARCHICAL MODULATION
The evolution and the milestones of HM schemes are portrayed in Table 1. Early in the 1960s, Lucky and Hancock published their research on the optimum performance of non-binary transmission systems. Based on their pioneering work, both combined amplitude and phase modulation (AM-PM) as well as the quadrature amplitude modulation (QAM) had attracted substantial research attention. In 1972, Cover introduced the theory of multi-resolution transmission, while its applications were detailed. It had been shown that if a single source broadcasts its information to multiple receivers associated with different channel conditions, using multi-resolution transmission schemes is beneficial.
OPPORTUNISTIC ROUTING IN ADHOC NETWORKS
In their embryonic era, wireless adhoc networks were referred to as ‘packet radio’ networks, which were primarily designed by the Advanced Research Project Agency (DARPA) during the 1970s. The history of wireless adhoc networks is eloquently portrayed for example inLabiod’s book. The most significant benefit of adhoc networks is their distinctively flexible nature. An adhoc network relies on multiple nodes, which are connected by ‘links’. The communication between any two nodes in the adhoc network may be assisted by all the other nodes in the network relying on a specific transmit power, path-loss, fading and noise characteristics.
In order to simplify the discussions, our analysis provided in this treatise is mainly focussed on a cooperative system assisted by two RNs. The general system model is illustrated in Fig. 6. Although the entire system only has two hops, our transmission algorithm can be readily extended to a larger network. Explicitly, this simple network already characterizes most of the typical situations of a large network having more than three hops.
A similar technique has been conceived for a single-layer transmission scenario. In this treatise, we employ the TTCM aided twin-layer HM scheme of Fig. 7. We firstly generate the FER versus SNR curve for transmission over AWGN channels using simulations and then invoke the schemes designed in to find accurately matching polynomials for characterizing the FER versus SNR curve.
The structure of this long section may be seen at a glance in Fig. 3. Again, the system considered communicates over Rayleigh-distributed block fading channels, where all nodes of the cooperative network are assumed to benefit from perfect Channel State Information (CSI). The related simulation parameters are shown in Table 4.
Note that if the current state is S1, the next state may be any one of all the then legitimate states. We have to mention that if the current state is the same as the following state, this implies that the transmission attempt of the current state has failed, hence the packet has to be retransmitted, as seen in Fig. 9.
The flow chart of the simulation for the single packet’s transmission is illustrated in Fig. 12, which is detailed in the Algorithm 2. Algorithm 2 illustrates the schedule of our simulations in the context of the cooperative communication system considered. To be more specific, the number of sample-packet transmitted is set to NST = 105. As discussed in Section V-B, for every specific state of Table 6, the twinlayer information stream of L1 and L2 will have reached a specific position in the cooperative networks.
The TOR scheme has been lavish by document, for example. Fig. 13 represents the optimized ¶t of each single node in the four-node cooperative network. It may be observed from Fig. 13 that upon increasing Nt, the optimized power ¶t exhibits a slight reduction tendency upto Nt = 6, but for Nt > 6, it will remain near-constant.
Although we use TTCM as the channel code throughout this treatise, the HM based design can be generalized to any arbitrary coding schemes. In this section, we will summarize the general design guidelines of the coded HM based cooperative communication schemes investigated throughout, followed by the design guidelines of coded HM based OR algorithms conceived for ad hoc networks.
In the DCMC capacity was relied upon for analysing the achievable performance of the coded HM based cooperative scheme. However, the results were derived by simulations only, albeit it is desirable to arrive at a theoretical solution. Recall that the CCMC and DCMC capacities were first considered by Cover and Gamal, where the general upper bound of the CCMC capacity of a half- duplex relay-aided system was derived. We have appropriately adapted the approach for deriving the DCMC capacity of our HM aided cooperative communication system.
Source: University of Southampton
Authors: Hua Sun | Chen Dong | Soon Xin Ng | Lajos Hanzo