Rats go wireless in an analog fashion

Rat carrying wireless systemThis little image on the left shows a rat carrying a wireless system, partly mounted on his head, partly realized as a kind of a backpack. It has been developed by researchers at Harvard University in close collaboration with colleagues at California Institute of Technology and is being used for neurological research on rats in the wild. According to Nature (Febr. 25, online) the entire systems comprises "a tetrode microdrive, for chronic positioning of electrodes in the brain; an integrated circuit for high channel-count neural recordings; and a radio-frequency wireless transmitter. The device takes up to 64 analog voltage signals from neurons in the brain and muliplexes them into one signal that appears in a temporally interleaved fashion, one after the other. Then that signal is transmitted by analog FM radio to a receiver."

The article further reports that good old FM (frequency modulation) transmission has been used as it outperforms digital wireless communication on weight, power drain, throughput and distance.

So why does analog FM outperform its digital counterparts, such as FSK, QPSK, QAM and OFDM? Before answering this question it is important to realize that from a channel-capacity perspective (as defined by Shannon) there is no preference for analog modulation over digital modulation. The answer thus has to follow from practical considerations. Digital modulation implies that the information is transmitted over the wireless radio channel in a digital fashion. As all information in nature, also neural signals are analog in nature and thus, in order to prepare the neural signals for their wireless journey, they have to be converted to the digital domain by analog-to-digital conversion (ADC). This thus requires at least one ADC. Often, depending on the digital modulation type used, channel coding is performed prior to the digital modulation. As a consequence, with these additional blocks, the entire transmitter becomes more complex, which, in turn, entails a larger power consumption and, when battery-operated, a larger battery and thus a larger weight on the head or back of the rat.

Another reason why analog FM may outperform its digital counterparts lies in the frequency spectrum of the transmitted radio signal. FM produces an almost flat frequency spectrum. As a consequence, it is relatively immune to frequency-selective fading, which is good for radio communication over relative long distances. Also, FM transmission does not require a highly linear power amplifier. This is good for its power efficiency and thus for the overall energy efficiency of the transmitter.

One final remark. From the picture it looks like the rat is loaded with a transmitter that has been implemented using discrete components, rather than with a single chip (or integrated circuit, IC). Many of the blocks needed for digital transmission could be implemented with a much smaller form factor and consuming less power when realized on-chip, rendering the antenna the largest component and the largest power consumer. In such a case the choice for either analog or digital could have just as well turned upside down.


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