1. Describe at least two strategies for extending the lifetime of a wireless sensor network. Please specify the advantages and the drawbacks of each method (0.5 point).

2. In a multi-hop many-to-one sensor network, the data collection follows a tree structure. Many sources are collecting data, and transmitting packet to a destination sink node via multiple hops. How does this situation affect network lifetime? (0.5 point)

3. The channel bit error rate is 0.00001, and each packets total length is 1024 bytes including all overheads. Assume there are no other overheads. For each data packet transmitted over the wireless channel, what is the Packet Error Rate (PER) for each single round of packet delivery? If one ARQ retry is applied, (i.e. total rounds of transmissions are 2, including 1 initial transmission, and 1 additional retry), what is the final average probability of packet error? (1 point)

4. Scenario description: Assume you got a new wireless sensor device with circuit power consumption of P_elec = 0.015mW. Frequency bandwidth (i.e. symbol rate) is 1MHz. You have a packet with 125 bytes to send. Assume there is no other overhead, and you can ideally turn on and off the transmission without additional power cost. Now you can choose using various data rates to transmit this packet, while maintaining the same BER at 0.0001 with the following assumption: using BPSK, the required TX amplifier power consumption of P_t =0.02mW; using QPSK, the required TX amplifier power consumption of P_t =0.05mW. Receiver energy consumption is not a concern. Questions: Please calculate the energy consumption (in unit of nJ Nano-Joule) of transmitting this packet, using BPSK and QPSK schemes, respectively. Do you prefer slow BPSK transmission to save power, or to transmit quickly with QPSK and then sleep (3 point)?