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Optimal resource allocation for cooperative cognitive radio networks with opportunistic access to the licensed spectrum is studied. Resource allocation is based on minimizing the symbol error rate at the receiver. Both the cases of all-participate relaying and selective relaying are considered. The objective function is derived and the constraints are detailed for both scenarios. It is then shown that the objective functions and the constraints are nonlinear and nonconvex functions of the parameters of interest, that is, source and relay powers, symbol time, and sensing time. Therefore, it is difficult to obtain closed-form solutions for the optimal resource allocation. The optimization problem is then solved using numerical techniques. Numerical results show that the all-participate system provides better performance than its selection counterpart, at the cost of greater resources.

The ever increasing wireless communication networks have put great stress on the already limited spectrum. Due to the fixed spectrum allocation policy, only the licensed users, otherwise known as primary users, are able to access the licensed spectrum. Additionally, the Federal Communications Commission (FCC) task force highlighted in their report the fact that at any given time only 2% of the spectrum is being used [

Cognitive radios have been proposed to resolve this issue [

In this paper, optimal resource allocation is discussed to minimize the SER. In order to achieve minimum SER, cooperation is introduced into the system as it decreases the SER due to diversity [

The rest of the paper is organized as follows. Section

Consider a cognitive radio network in which the secondary source utilizes

Primary and secondary networks.

Based on the spectrum sensing result, there are two possible received signal models.

In this scenario, with probability

Therefore, the received signal at the destination from the

Writing the

In this case, with probability

Taking into account the fact that the source and relays have no knowledge of the interfering signal and adopting the same approach as previously, one can write

Again in matrix form one has

Spectrum sensing is performed, by means of an energy detector, for the first

In this section, an all-participate (AP) system is discussed. In such a system, all the relays forward the signal to the destination. Firstly, the optimization problem is formulated. Then the constraints on the objective function are derived. The SER at the destination is given by

After substituting (

Now we form the different constraints on the problem. First, we consider both individual power constraints at the source and the relay and a global power constraint on the whole system. Therefore, the constraints are given by

In the other scenario, the constraints are given by

The individual power constraints are set to limit the interference suffered by the primary user in the case of missed detection. As there is no individual power constraint, the interference caused to the user in the global power constraint only case, where the primary user is only protected by spectrum sensing, is greater.

The problem with optimizing (

A special case of importance is the absence of the direct link between source and destination, because the relays take on a more prominent role in the presence of no direct link. In this case, the SER is can be obtained by setting

The drawback of the all-participate (AP) scheme discussed in the previous section is that to avoid causing interference, the source and the relay transmit on orthogonal channels. Hence, consuming a considerable amount of resources. In our discussion of a time orthogonal systems,

To overcome these problems, a selection scheme is proposed in this section in which only one relay is selected to take part in forwarding the signal from the source. Now only 2 time slots are used in transmitting one frame of data and thus decreasing the likelihood of primary becoming active again during relay transmission. In the selection case, the SER is

It is again evident that, even in the selection case, the SER is still a nonlinear and nonconvex function. Therefore, one has to resort to numerical techniques to find the optimal solution. The special case of no direct link is again of particular interest and considered separately.

In this section, numerical results are provided for the optimization problems discussed. First, the proposed AP system with optimal resource allocation is discussed and it is shown that the proposed AP schemes give better performance than the uniform power allocation (UPA) scheme. In UPA, the power is uniformly distributed among the source and the relays and the sensing time and the symbol time are set so that the inequality

Glossary.

Acronym | Full name |
---|---|

AP-ORA | All-participate with optimal resource allocation |

AP-ORA-GL | All-participate optimal resource allocation with global constraint only |

AP-ORA-Ind | All-participate optimal resource allocation with individual constraints only |

UPA | uniform power allocation |

AP-ORA-NDL | All-participate optimal resource allocation with no direct link |

AP-ORA-GL-NDL | All-participate optimal resource allocation with global constraint only and no direct link |

AP-ORA-Ind-NDL | All-participate optimal resource allocation with individual constraints only and no direct link |

UPA-NDL | uniform power allocation with no direct link |

Sel-ORA | selection with optimal resource allocation |

Sel-ORA-GL | selection optimal resource allocation with global constraint only |

Sel-ORA-Ind | selection optimal resource allocation with individual constraints only |

Sel-UPA | selection with uniform power allocation |

Sel-ORA-NDL | selection optimal resource allocation with no direct link |

Sel-ORA-GL-NDL | selection optimal resource allocation with global constraint only and no direct link |

Sel-ORA-Ind-NDL | selection optimal resource allocation with individual constraints only and no direct link |

Sel-UPA-NDL | selection uniform power allocation with no direct link |

An interior-point algorithm was used to perform the optimization. The MATLAB function

The relationship between the number of samples (

SER as a function of the number of samples (

Figure

SER as a function of the symbol time (

Figure

Comparison of SER performance of an all-participate under different schemes and constraints with

In Figure

Comparing the different constraints, AP-ORA gives the worst performance in both scenarios of direct and no direct link. This is due to the fact that AP-ORA is constrained both globally and individually. Thus, even if one relay has more favourable conditions, the power allocated to it cannot exceed

First, the global constraints only and individual only scenarios are compared in the no direct link case. Here, AP-ORA-Ind-NDL provides lower SER than AP-ORA-GL-Ind for all values of

Now consider the direct link case. Here, AP-ORA-GL outperforms AP-ORA-Ind at low values of

Figure

SER as a function of number of layers (

Figure

Comparison of SER performance of a selection system under different schemes and constraints with

SER performance for selective relaying as a function of the number of relays is shown in Figure

SER as a function of number of relays (

Figures

Comparison of AP vs Sel with

Comparison of AP versus Sel as a function of

As one can see, the difference in performance between the respective AP schemes and Sel schemes increases with increase in number of relays. As discussed earlier, the Sel schemes look to be bounded by a minimum threshold. Due to this, Sel with direct link scenarios even fall below the AP with no direct link scenarios for a large number of relays.

In this paper, ORA for a cognitive relay network has been discussed. It has been shown that for an AP system that ORA improves SER performance and the discussed schemes outperform the UPA schemes. The importance of the direct link between the source and the destination has also been demonstrated. Among the different constraints on the system, the case of both individual and global constraints gives the worst performance while global constraints only is the best for low

It was then noted that the AP scheme consumes considerable resources and is spectrally inefficient. Therefore, a simple relay selection scheme has been proposed. Optimal resource allocation was then discussed for the selection scheme. The performance comparison of the AP and Sel shows that while AP provides better SER performance, it comes at the cost of considerable resources.

This work was supported by King Abdullah University of Science and technology (KAUST).