Sum-Rate Maximization of Wireless Powered Primary Users for Cooperative CRNs NOMA or TDMA at Cognitive Users

Abstract : Recently, wireless powered cooperative cognitive radio networks (CRNs), which combine the technologies of radio frequency (RF) energy harvesting and CR, have drawn great attention. In such networks, energy cooperation between the cognitive users (CUs) and the wireless powered primary users (PUs) can be performed, where the CUs can charge the PUs wirelessly in exchange for the spectrum access. Specifically, energy cooperation and information transmission is executed in two phases, where the CUs transmit their data signals and the PUs harvest energy from these signals in the first phase, and the PUs transmit their data using the harvested energy in the second phase. In particular, we consider two multiple access schemes for the CUs, namely non-orthogonal multiple access (NOMA) and time-division multiple access (TDMA). For both NOMA and TDMA, the PU sum-rate maximization problems under the minimum CU sum-rate constraint are first simplified by exploring particular problem structure, then are transformed to convex problems, and finally are solved optimally. The PU sum-rates of the two schemes are compared theoretically as well as numerically. It is revealed that the circuit power consumption at the CUs, the required minimum CU sum-rate, and the PU energy harvesting sensitivity and saturation thresholds play key roles in the PU performance comparison of the two schemes.
 EXISTING SYSTEM :
 ? In general, the existing NOMA schemes can be classified into two categories: power-domain NOMA and code-domain NOMA. ? The former assigns a unique power level to a user and multiple users transmit their signals sharing the same time-frequency-code resources, each using its allocated power. ? Moreover, VLC is particularly useful in some sensitive environments such as aircraft cabins and hospitals, where the interference to existing radio frequency (RF) systems is by nature problematic. ? Despite major differences between the VLC channel and the RF channel, the application of NOMA to downlink VLC case has the potential to further increase the performance of VLC networks without affecting light emitting diode (LED) lighting quality.
 DISADVANTAGE :
 ? To tackle non-convexity of the sum-rate maximization problem, we propose an iterative algorithm where the time allocation and the transmit power are updated based on the weighted minimum mean square error criteria and the gradient projection method, respectively. ? In this case, due to the absence of interference in the UL, the sum-rate maximization becomes a convex problem, and thus we readily find a closed-form solution for time allocation. ? To tackle this problem, we provide an alternating algorithm which first optimizes power allocation utilizing the weighted minimum mean square error (WMMSE) approach, and then computes time allocation based on the gradient projection method.
 PROPOSED SYSTEM :
 • In, a layered transmission scheme is proposed based on QR factorization. • Under instantaneous CSI, an approach to maximize the sum rate of MIMO-NOMA with layered transmissions is proposed after showing that the sum rate is concave in allocated powers to multiple layers of users. • Both optimal and low-complexity suboptimal power allocation schemes are proposed under total transmit power constraint and minimum rate constraint of the weak user. • It is shown that if the broadcast channels are quasi-degraded, the proposed optimization algorithm in combination with superposition coding and SIC can achieve system capacity.
 ADVANTAGE :
 ? This is quite different from traditional communication systems, where the interference signal reaching at unintended users usually deteriorates the performance. ? The asynchronous protocol presented in the previous section provides good sum-rate performance. ? Due to a trade-off between the amount of the harvested energy and the WIT interference, it is observed that the sum-rate performance of the proposed algorithm for the asynchronous protocol slightly changes as ?˜ increases. ? In contrast, the performance of the TDMA protocol continuously enhances as the interference strength ?˜ grows, whereas that of the synchronous protocol and the non-cooperation scheme constantly decline for both PT = 15 dBm and 30 dBm.

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