Joint Adaptive Modulation and Power Allocation in Cognitive Radio Networks ()
Link adaptation is an important issue in the design of cognitive radio networks, which aims at making efficient use of system resources. In this paper, we propose and investigate a joint adaptive modulation and power allocation algorithm in cognitive radio networks. Specifically, the modulation scheme and transmit power are adjusted adaptively according to channel conditions, interference limit and target signal-to-interference-plus-noise ratio (SINR). As such the total power consumption of cognitive users (CUs) is minimized while keeping both the target SINR of CUs and interference to primary user (PU) at an acceptable level. Simulation results are provided to show that the proposed algorithm achieves a significant gain in power saving.
1. Introduction
With the increasing number of various bandwidth-consuming wireless services, spectrum for available bands becomes more and more scarce. Moreover, these bands are not occupied or underutilized by licensed users most of time, which leads to the waste of bandwidth resources and low spectral efficiency. One solution to this problem is that cognitive (unlicensed) users (CUs) are allowed to have opportunistic access to these idle bands or to the active ones without causing harmful interference to the primary (licensed) user (PU), in order to improve the bandwidth utilization. This technology is called cognitive radio [1,2]. The major advantage of cognitive radio technology is its ability to search for available spectrums in its surrounding environment and adjust its transmit parameters accordingly to enhance the system performance. The transmit parameters, for example, include modulation scheme, beamforming vector, center frequency, transmit power and so on. The whole process can be summarized as “sense-cognition-adaptation”.
In wireless network, a fundamental characteristic is the interference introduced by multi-user or co-channel transmission at the same time or over the same frequency radio channel. It is well-known that power allocation [3,4] is an effective way to mitigate interference by means of updating transmit powers according to the target SINR. Besides, the effective use of transmit power can not only minimize the interference introduced by other transmit nodes to enhance the capacity, but also conserve energy to prolong battery life. In [3], the author proposed a simple distributed power allocation algorithm, in which the power level at next iteration only depends on target and actual SINR as well as current power level. The goal is to minimize the total power consumption subject to the target SINR requirement. Further studies are shown in [5–8]. In [5], the joint optimization of beamforming and power control is studied in the downlink of a cognitive radio network. The objective of the proposed algorithm is to minimize the total transmit power while satisfying the target SINR constraint of CUs and maximum tolerable interference to PU. However, this work can not be extended to the energy-constrained wireless networks, in which there is a constraint of maximum transmit power for each CU. Literature [6] proposes a cross-layer framework for joint scheduling and power control combined with adaptive modulation in ad hoc networks, which can be viewed as the situation where only CUs share the same frequency band with the absence of PU. Therefore, the proposed algorithm can not be applicable to the case of the co-existence of PU and CUs in the same frequency band, since it does not consider the interference introduced to PU caused by CUs. While literature [7] and [8] only consider the problem of adaptive modulation and power control of a single CU in the presence of PU.
In contrast to previous work in [5–8], we consider the scenario where one PU and multiple CUs share the same frequency band in wireless networks. So far as we know, little attention has been paid to the topic of joint adaptive modulation and power allocation in cognitive radio networks, in which the protection of PU and the quality of service (QoS) of CUs are assured. In this contribution, our goal is, therefore, to jointly optimize the modulation schemes as well as transmit powers in order to minimize the total power consumption while keeping both the interference to PU and target SINR of CUs at an acceptable level. More specifically, we perform a two-stage power allocation processing for the proposed algorithm: First, transmit powers are allocated to all CUs with the same modulation scheme, under the constraint of target SINR of CUs and a given interference limit to PU; Second, each CU with adaptive modulation scheme adjusts its transmit power based on the first allocated power, in order to reduce the total power consumption.
The rest of this paper is organized as follows: Section 2 describes the system model and basic assumptions. In Sections 3, we develop the proposed algorithm for joint adaptive modulation and power allocation in cognitive radio networks. Performance analysis of the proposed algorithm is investigated in Section 4. Section 5 concludes this paper.
Notation: All vectors and matrices are denoted in bold letters. 
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identity matrix. 
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represent pseudo inverse, inverse and transpose, respectively.
Figure 1. System model with one PU in dashed line and N CUs in solid line.