Energy Efficiency in ARQ-Based Multi-Hop Systems and the Tradeoff with Throughput

The minimum energy per bit(EPB)as the energy efficiency(EE)metric in an automatic retransmission request(ARQ)based multi-hop system is analyzed under power and throughput constraints.Two ARQ protocols including type-I(ARQ-I)and repetition redundancy(ARQ-RR)are considered and expressions for the optimal power allocation(PA)are obtained.Using the obtained optimal powers,the EE-throughput tradeoff(EETT)is analyzed and the EETT closed-form expressions for both ARQ protocols and in arbitrary average channel gain values are obtained.It is shown that how different throughput requirements,especially the high levels,affect the EE performance.Additionally,asymptotic analysis is made in the feasible high throughput values and lower and upper EETT bounds are derived for ARQ-I protocol.To evaluate the EE a distributed PA scenario,as a benchmark,is presented and the energy savinggain obtained from the optimal PA in comparison with the distributed PA for ARQ-I and ARQ-RR protocols is discussed in different throughput values and node locations.

Solar Energy Harvesting Using a Timer-Based Relay Selection

In this paper,the throughput and delay of cooperative communications are derived when solar energy is used and relay node is selected using a timer.The source and relays harvest energy from sun using a photo voltaic system.The harvested power is used by the source to transmit data to the relays.Then,a selected relay amplifies the signal to the destination.Opportunistic,partial and reactive relay selection are used.The relay transmits when its timer elapses.The timer is set to a value proportional to the inverse of its Signal to Noise Ratio(SNR).Therefore,the relay with largest SNR will transmit first and its signal will be detected by the other relays that will remain idle to avoid collisions.Harvesting duration is optimized to maximize the throughput.Packet’s waiting time and total delay are also computed.We also derive the statistics of SNR when solar energy is used.The harvested power from sun is proportional to the sum of a deterministic radiation intensity and a random attenuation due to weather effects and clouds occlusion.The fixed radiation intensity depends on season,month and time t in hour.The throughput of cooperative communications with energy harvesting from sun was not yet studied.

Cognitive Radio Networks Using Intelligent Reflecting Surfaces

In this article,we optimize harvesting and sensing duration for Cognitive Radio Networks(CRN)using Intelligent Reflecting Surfaces(IRS).The secondary source harvests energy using the received signal from node A.Then,it performs spectrum sensing to detect Primary Source PS activity.When PS activity is not detected,The Secondary Source SS transmits data to Secondary Destination SD where all reflected signals on IRS are in phase at SD.We show that IRS offers 14,20,26,32,38,44,50 dB enhancement in throughput using M=8,16,32,64,128,256,512 reflectors with respect to CRN without IRS.We also suggested to add a second IRS between A and SS to increase the harvested energy.The use of 2 IRS with M1=8 reflectors in the first IRS and M2=8 reflectors in the second IRS offers 18 dB gain(respectively 32 dB)gain with respect to a single IRS with M2=8 reflectors(respectively without IRS).The use of 2 IRS with M1=16 reflectors in the first IRS and M2=8 reflectors in the second IRS offers 28 dB gain(respectively 42 dB)gain with respect to a single IRS with M2=8 reflectors(respectively without IRS).Our results are valid for Nakagami channels of fading figure m.We also provide the throughput of IRS with energy harvesting.We have studied packet waiting time and total delay in the presence and absence or IRS.At Signal to Noise Ratio(SNR)per bit equal to 0 dB,packet waiting time is 0.9 ms when there is no IRS and 0.5 ms when there is an IRS with M=8 reflector.At SNR per bit equal to 0 dB,total transmission delay is 54 ms when there is no IRS and 1.5 ms when there is an IRS with M=8 reflectors.We show that the energy efficiency is larger when both harvesting and sensing durations are optimized.The maximum of energy efficiency is 260 Mbit/s/Hz/J when harvesting and sensing durations are optimized while the maximum is 80 Mbit/s/Hz/J when harvesting and sensing durations are not optimized.