Ebook Variable-Rate Two-Phase Collaborative Communication Protocols for Wireless Networks
In many wireless networks, the power consumption of communication nodes is a critical issue. In addition, typical wireless channels suffer from signal fading which, for a given average transmit power, significantly reduces communication capacity and range. If the channel is slow and flat fading, channel coding does not help and spatial diversity may be the only effective option that can either reduce the average transmit power or increase communication range.
Results on space-time coding (STC) have shown that the use of antenna arrays at the transmitter and receiver can significantly educe transmit energy. However, for many applications with low-cost devices such as wireless sensor networks, deployment of multiple antennas at each node is too costly to implement due to severe constraints on both the size and power consumption of analog devices.
The recently proposed collaborative (or cooperative) diversity approaches demonstrate the potential to achieve diversity or enhance the capacity of wireless systems without deploying multiple antennas at the transmitter. Using nearby collaborators as virtual antennas, significant diversity gains can be achieved.
These schemes basically require that the relay nodes share the information data of the source node, and this data sharing process is generally achieved at the cost of additional orthogonal channels (in frequency or in time). In a companion paper, we have shown that for a given fixed rate and under suitable node geometry conditions, there are collaborative coding schemes that can nearly achieve the same diversity as if all the relay node antennas were connected to the source node, without any additional orthogonal channels or bandwidth. The construction of such codes, however, appears to be challenging.
Among many approaches in the literature, Laneman analyzes several low-complexity relaying protocols that can achieve full diversity, under realistic assumptions such as half-duplex constraint and no channel state information (CSI) at the transmitting nodes. It has been shown that in the low-spectral-efficiency regime, the SNR loss relative to ideal transmit diversity system with the same information rate is 1.5dB. Multiple-relay cases are also considered in and bandwidth-efficient STC-based collaborative protocols are proposed.
Collaborative diversity protocols are largely classified into amplify-and-forward and decode-and-forward schemes. In the following, we will restrict our attention to decode-and-forward schemes since these may provide some salient advantages. First, there is no error propagation if the relay transmits information only when it decodes correctly. Otherwise, the relay remains silent and thus an unnecessary energy transmission can be saved1. Second, the information rate per symbol does not need to be the same for each phase. In other words, the relative duration of each phase can be changed according to node geometry.
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