In order to exploit this advantage, the front-end circuits must be designed with well-controlled and relatively fast startup behaviors, such that the stringent timing and phase control requirements can be met. However, the control of the moment at which UWB pulses are generated is a critical aspect of UWB systems as is the control of the phase of the RF carrier.įinally, an important advantage of UWB resides in the possibility to significantly reduce the power consumption of the radio front-end by switching off the transmitter during the relatively long silence periods between UWB pulses. Accurate control of the pulse amplitude and shape, or RF carrier frequency is not of critical importance in UWB. UWB signals consist in extremely short pulses (about 2 ns), or groups of adjacent pulses, separated by long silence periods. The wide range of frequencies covered by UWB also poses significant challenges when designing UWB antennas. Moreover, achieving a maximum absolute frequency of 10 GHz with an effective bandwidth of more than 500 MHz with low power consumption is not straightforward, especially when low-cost devices relying on standard CMOS technologies are considered. The different elements of the transmitter must be designed to offer constant performance across a spectrum of 7 GHz. In contrast, UWB signals are spread over a relatively large bandwidth (500 MHz or more) and the precise control of the carrier is by far less critical than in narrowband radios, which loosens the requirements on the control of the RF frequency reference.Īn important challenge in the design of UWB systems stems from the large range of frequencies that must be covered, i.e. Typical design challenges in that narrowband context are related to the accurate control of the generated frequency references used for modulation of the signal, both in phase and amplitude.
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The challenges associates to the design and implementation of UWB transmitters differ significantly from those encountered in classical low-power radios, typically relying on narrowband air interfaces in the 2.5 GHz ISM band. This poses a serious challenge for the pulse generation of UWB-IR transmitters. Therefore, in order to comply with these regulations and standards, the generated pulses of UWB impulse-radio (UWB-IR) approaches must fulfill stringent spectral masks that can feature such bandwidths. Indeed, various UWB standard proposals have subdivided the entire UWB spectrum in 500MHz sub-bands as a way to mitigate against strong interferers, to improve the multiple access and to compose with the different regulations on UWB emissions worldwide. The minimum bandwidth of a UWB signal is usually 500 MHz. However, the impact of the type of UWB signal chosen on the communication performance and on the complexity of the radio implementation must be carefully analyzed. Moreover, since most of the complexity of UWB communication is in the receiver, it allows the realization of an ultra-low power, very simple transmitter and shift the complexity as much as possible to the receiver in the master.
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In pulse-based UWB, the transmitter only needs to operate during the pulse transmission, producing a strong duty cycle on the radio and the expensive baseline power consumption is minimized. The latter can then be tailored for low hardware complexity as well as low system power consumption.
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Although the regulations on UWB radiation define a power spectral density (PSD) limit of -41dBm/MHz, there are very few regulations on the definition of the time-domain waveform. The Federal Communications Commission (FCC) has authorized UWB communications between 3.1GHz and 10.6GHz. For wireless sensor nodes, a radio is needed which is 1 to 2 orders more power efficient. This leads to a power efficiency of roughly 100 to 1,000mW/Mbps or nJ/bit. Typical chipsets for these radios consume in the order of 10 to 100mW for data rates of 100 to 1,000kbps.