The OOK radio uses noncoherent communication to reduce circuit and architectural complexity. When there is sufficient margin in the link budget, choosing a noncoherent modulation instead of a coherent modulation can result in significant energy savings. Noncoherent communication does not encode information in the carrier phase and instead encodes information in the carrier frequency and/or amplitude. This allows for significantly reduced architectural complexity at the cost of reduced link margin. For highly energy constrained systems where power consumption is dominated by transceiver fixed power consumption rather than transmitter output power, the power savings afforded by noncoherent communication is often worthwhile (89).
To minimize power consumption while achieving fast start-up time, the OOK receiver has an envelope detection-based architecture with a highly scalable radio-frequency (RF) front-end. By using an envelope detector, no high-frequency oscillator or phase-locked loop is required by the receiver in contrast to a traditional direct conversion receiver architecture. This allows for reduced power consumption and a fast turn-on time of 2.5 μs. The receiver power consumption can be scaled depending on link requirements, scaling from 0.5 mW to 2.6 mW, with an associated sensitivity of −37 dBm to −65 dBm at a bit error rate (BER) of 10−3. The transmitter consists of an oscillator, mixer, and power amplifier and avoids the need for a phase-locked loop by stabilizing the oscillator with a surface acoustic wave (SAW) resonator.
A key challenge for implanted radios is to minimize the overall volume of the antenna and circuits while still achieving low-energy operation. The limited size of the antenna combined with the lossy human body that surrounds it result in a poor antenna gain. A typical MICS band implanted antenna has a modeled gain of −35 dBi resulting from human body losses (90). To minimize the volume of the circuits, the transceiver must be as integrated as possible, requiring few if any off-chip components other than a crystal oscillator. Realizing compact, low-power transceivers is difficult because their high-frequency circuits often require off-chip filters, resonators, and passive components. Off-chip components, such as bulk acoustic wave (BAW) resonators, can allow for ultralow-power RF circuits but at the cost of increased volume (91); however, advanced packaging techniques, such as system-in-package or chip stacking, can mitigate this problem. A recent trend that promises to increase integration while decreasing power consumption is the emergence of the highly digital radio. A highly digital radio minimizes the use of external components and instead leverages advanced CMOS devices to realize equivalent functionality in less area and power.
A highly digital ultrawideband transceiver chipset has been recently demonstrated that can be adapted for use in biomedical devices (92, 93). A block diagram of the transceiver architecture is shown in Figure 14. Wireless communication is via the transmission and reception of short, 2-ns ultrawideband pulses in the 3-to-5 GHz frequency band. These ultrawideband (UWB) pulses are much wider in bandwidth and narrower in time than traditional narrowband signals. These wide bandwidths with relaxed frequency tolerances allow for highly digital architecture, as they are amenable to low-power integration in advanced digital CMOS processes (94). UWB signals can be efficiently amplified and processed with wide-bandwidth, low-Q circuits, which can be easily integrated on-chip with minimal area.