Optimal Power allocation in OFDM based CR system

Vaibhav KumarY13PG052

The LNM IIT, Jaipur

Outline Introduction Cognitive Radio System Model Spectrum of SU in OFDM based CR system Transceiver diagram of SU Mathematical modeling of Interference constraint Formulation of Power allocation problem Power increment water-filling Power- decrement water-filling Comparison of the proposed algorithm with IPWF Conclusion References

Introduction In CR systems the transmission of SU is required to satisfy the PU's interference limit to protect the PU's performance

Constraint 1 : Maximum allowable collision probability which is related to spectrum sensing.

Constraint 2 : Interference power limit which introduces power restrictions on the channel output which is different form conventional wireless systems.

Cognitive Radio System Model

Spectrum of SU in OFDM based CR system

Assumption : The distance between the SU and the nearest undetectable PU transmitter is known in advance. The PU transmitter is assumed to be just at the margin of the reliable sensing region

Transceiver diagram of SU in OFDM based CR system

Mathematical modeling of interference constraintAssume Gj is the power constraint in the jth sub-channel after the sensing, then

Rj = radius of the protection region of PUj

dj = Distance between the SU's transmitter and the nearest undetectable PUj transmitterηj = Interference power limit of PUj

βj = Path attenuation factor

Formulation of Power Allocation Problem

: Channel-to-Noise Ratio

Power Increment Water Filling Consider the problem with total power constraint Ptotal + Δ

The channels are divided into three sets

Lemma 1 : If all channel satisfy or

Then the new water level becomes

The new powers in each channel are for Pi Є Q U R and

Power Decrement Water Filling

Lemma 2 : Assume that Pi Є C satisfies the following condition

Then the new water level is

and

A comparison of the proposed algorithm and the IPWF (Iterative partitioned water filling) algorithm

Conclusion : A new low-complexity optimal power allocation algorithm for OFDM based CRN is proposed

By exploiting some properties of the water-filling algorithm, a new power-increment and power-decrement water filling algorithms are proposed

In future, the algorithm may be extended to be applicable to realistic Cognitive Radio Networks

References [1] P. Wang et al. , “Power allocation in OFDM based cognitive radio system” in Proc. IEEE Global Telecommunication Conference (Globecom 2007), Washington, DC, Nov. 2007, pp.- 4061-4065

[2] Qi, Quilin and Yang, Yaoqing Lamar, “An efficient Water-filling algorithm for Power-allocation in OFDM based CR system” (2012). CSE conference and workshop papers