Impact of transmit power on throughput performance in wireless ad hoc networks with variable rate control
Introduction
It is well known that ad hoc networks can be constructed with neither fixed infrastructures nor central controllers. After the development of nearly 30 years, ad hoc networks, nowadays, are required to support different high quality of services (QoS) demanded by many applications, such as audio, video streaming and time-sensitive information transportation [1]. For these kinds of QoS to be guaranteed, the total network capacity, in terms of throughput, is considered as one of the most important parameters [2]. Many factors contribute in network throughput enhancement, such as transmission power, data rate, interference and spatial reuse.
Many researches have been done to evaluate the power (range) control required to improve network throughput. Early research could be traced in [3], where an optimum transmission radius was derived to maximize the throughput of a stationary packet radio networks using S-ALOHA. This work was extended to 802.11 MAC protocols in mobile ad hoc networks [4]. Common ranges were used in [3], [4]. In [5], a variable power control scheme was proposed, that every node has an identical number of neighbor nodes. Authors in [6] investigated the impact of variable-range power transmission control on network capacity. Physical and network connectivity, as well as power saving in wireless multi-hop networks were also evaluated.
On the other hand, when multiple modulating and coding schemes became available as well as hardware implemented, the multiple-rate control was considered as a promising scheme to increase the network throughput. Enhancing network’s capacity was investigated in [7] using variable data rate. Selection of the appropriate modulation/coding scheme (MCS) in fixed data rate networks may greatly improve throughput performance level [8].
The study impact of the transmission power combined with variable rate on throughput of ad hoc networks is sparse [9]. In [9], variable power and rate control is used in Spatial TDMA to adapt traffic variability and to improve throughput. In addition, both transmission power control scheme and data rate control scheme could also improve energy efficiency in ad hoc networks [14], [15].To our knowledge, there have been no prior works which investigate the optimum power combined with variable rate control in ad hoc networks to maximize the network throughput.
All nodes in the network are assumed to use the same transmission power. Increasing transmitted power will reflect to higher signal power obtained at the receiver, thus providing higher Signal and Interference to Noise Ratio (SINR), and consequently offering higher data rate, which intuitively enhances network throughput. However, high transmission power will also increase the interference conducted by potential transmitters, which will result in low spatial reusing as the number of simultaneous transmission links reduced. Therefore, it is required to tradeoff between adequate transmission rate and accepted interference level in order to find an optimum transmitted power (optimum range) that achieves maximum network throughput.
This paper is organized as follows: the first section was devoted for studying previous articles in this field as well as presented the core idea of our research. Through the second section, our system model will be introduced. The third section will be directed to illustrate and analyze our simulation results. Finally, the last section will offer evaluation and conclusion of our research.
Section snippets
System model
We assumed n randomly distributed nodes over a two dimensional square area (a × a) and that all nodes use both same frequency band and identical transmission power to construct communication links with their neighbors using pure ALOHA protocol as MAC protocol. Each node forwards data packets to its desired destination in a multi-hop fashion and contains one packet buffer. The choice of variable data rate is based on link distance, transmitted power and interference level. Idle nodes generate new
Performance evaluation
A common topology network is proposed, which is constructed by 200 nodes uniformly distributed in a 100 m × 100 m square area. The probability of nodes falling into the area [0, Rmin] (Rmin = 1 m) as calculated from Eq. (8) is found to be 0.03%. Therefore the impact of these nodes can be neglected. The OPNET simulator was used with simulation parameters as listed in Table 1.
Both simulation results and mathematical expressions are used to evaluate ad hoc network performance. The network throughput is
Conclusions
In this paper, using different transmission power levels (range) to improve ad hoc network throughput with variable data rate control was investigated. Mathematical models were derived to analyze ad hoc networks capacity. Capture probability approximation was also revised. Theoretical and simulation performance were evaluated to achieve an optimal transmission power (range) that provides maximum network throughput. This optimal transmission power (range) will be helpful in future design
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Key Program) under Grant No. 60532030, National Science Fund for Distinguished Young Scholars under Grant No. 60625102, and Aviation Fund under Grant No. 2007ZD51049.The authors express their gratitude and regards to all reviewers for their insightful comments that improved and enhanced this paper.
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