Mote Hardware and Energy Consumption Communication The most important parameter for a mote radio is the effective energy per delivered bit, but there are a lot of other related and important parameters. We’ll see in later chapters how range is related to transmit power and receiver sensitivity, and how network lifetime is related to range and radio current consumption. For now we’ll just look at the basics. Transmit power, Transmit Current Typical transmit power (the power actually radiated out the antenna as RF energy) is in the 1 to 10mW range. The radio that generates this transmit power can be thought of as having two parts: the modulator, which converts bits into the appropriate time varying (radio frequency) voltage, and the power amplifier that boosts that signal up and delivers it to the antenna. In low power radios, the modulator often burns more power than the power amplifier. With the power amplifier off, essentially no power goes out the antenna, so there’s a minimum “overhead” of current from the power supply just to generate the appropriate voltages. It is difficult to design a power amplifier that is efficient over a wide range of output powers, and power amplifiers are generally designed to be most efficient near their maximum power output capability. The efficiency of the PA is typically between 10% and 80%, and the lower end of that range is most common for low power chips. Figure 1 Transmitter output power versus input current. The result is that a 10x reduction in the output power of the radio is rarely coupled to a corresponding reduction in radio current. Although most sensor network radios do have some type of transmit power control, often over two orders of magnitude or more, the difference in radio current is rarely greater than 2x. Receive sensitivity and current Radios can only receive information if the received signal is strong enough. The minimum detectable signal level for a radio is called the sensitivity. Typical numbers for mote radios are a fraction of a picoWatt. The ratio of the transmit power to the receive Transmitter current Transmitter Output power Typical real PA Constant PA efficiency ITX0 Effective transmitter efficiencysensitivity is called the link margin. For a transmitter putting out a few milliWatts, and a receiver with a sensitivity of a few tenths of a picoWatt, the link margin is around 10 billion! In principle, one transmitter could talk to ten billion receivers, if the transmit power were evenly divided. In practice, we’ll see that sometimes it can be challenging to receive a message from a mote that’s only a few meters away. The mechanism by which the RF energy leaves the antenna, propagates through space, and arrives at the receiver antenna is the topic of the next chapter. Bit rate Motes use digital radios that send information in packets, which are collections of bits. The bit rate varies from thousands of bits per second to millions of bits per second (kbps to Mbps). The transmit current divided by the bit rate yields the amount of charge pulled out of the supply per bit transmitted, QTXbit, since [mA/kbps] = [As/bit] = [C/bit]. Similarly, receive current divided by bit rate yields charge consumed per bit received. Multiplying by the supply voltage yields the energy per transmit ETXbit and receive (RX). The IEEE 802.15.4 standard specifies a 250kbps data rate in the 2.4 GHz band. This is the most common radio standard for wireless sensor networks. The original 15.4 chips burned about 20mA in transmit and receive, for a QTXbit and QRXbit of 20mA/250kbps = 80nC/bit. Operating at 3V, these radios burned 240nJ/bit. 802.11g chips from Atheros run at up to 108Mbps, and burn perhaps 200mA, for a QTXbit of only 2nC/bit. Does this mean that we should use .11g chips for low power networks? The answer is not simple. As we will see later, the medium access protocol will be an important role in the optimum choice of radio, but in addition there is the reality that radios do not turn on instantly. Radio startup Before the first bit can be sent or received on the radio, a long sequence of events must typically take place. From a deep sleep state, this entrails turning on a voltage regulator, waiting for a crystal oscillator to stabilize, and waiting for the radio oscillator to settle (tune) to the proper frequency, among other things. Figure 2 Radio current during startup and TX. Radio current Time Startup charge 1st bit Packet chargeIn a plot of radio current from deep sleep, through startup, to transmission of a packet, the area under the curve before the first bit is transmitted is the startup charge, QTX0, and the total charge for the packet is the startup plus the number of bits in the packet times the charge per bit. QTX = QTX0 + nb * QTXbit Computation Performance Low power microprocessors typically operate with 8, 16, or 32 bit quantities of data. Usually the instruction width is the same as the data width, although there is now a family of 32 bit processors from ARM that use a 16 bit instruction set. In general, the wider the datapath and instruction, the more you can do in a single instruction, and a single clock cycle. So a 20MHz 8 bit processor will be a lot slower than a 20MHz 32 bit processor (sometimes more than 10x), and the code size for the 8 bit processor will be larger than for the 32 bit processor (maybe tens of percent). The energy per cycle is the processor current (mA = mC/s) times the voltage, divided by the clock rate (cycles/s). [mC/s * V / cycles/s = mCV/cycle = mJ/cycle]. It’s usually fairly constant over at least an order of magnitude of clock frequencies, meaning that the current is proportional to clock speed. At low frequencies, the current flattens out at some minimum non-zero value, so the energy per cycle goes up. Typical numbers for well-designed low-power processors are below 1nJ/cycle. For most modern low-power processors there is roughly one cycle per instruction, so we get 1nJ/instruction. Since there is such a wide range of performance per cycle, it’s much more interesting to figure out what the energy per operation or function is. For dedicated hardware blocks such as encryption, this is number is generally fixed and available. For less-well-defined operations like “prepare a data packet for transmission”, the results will depend