The conventional design approach for software-defined radio (SDR) receivers tends to minimize the RF analog frontend
by transposing RF processing in the digital domain as much as possible. However, this approach is no longer appropriate for impulse ultra-wideband (UWB) systems because of bandwidth constraints and consequently the required sampling and processing frequencies as well as associated power consumption.
For this reason, it is necessary to rethink the problem. In addition, UWB imposes specific constraints for features like low-cost designs and optimized run-time performance (i.e. low power consumption).
The best way to address these issues is to perform fast and complex processing in an analog passive front end and provide the digital SDR part with reducedbandwidth signals. Hence, the following characteristics are required for the front
end:

1) Passive analog frontend (low cost and low power consumption).
2) Easy-to-integrate.
3) Pre-processing that only extracts the necessary metrics (i.e. sufficiently informative statistics to solve essential questions such as detection and transmission issues).
4) Adequate bandwidth for low-cost analog-to-digital converter (ADC) and realistic digital processing speed.
5) Relaxed synchronization means.
6) Multiple usage of the same RF front-end output for several applications managed by SDR.
7) Multiband support for high data rates (each band having convenient characteristics for SDR capabilities) in a way that allows for parallelization of the subsequent digital processing.

Thus, based on a non-coherent energy detector, an original demodulation scheme was designed and investigated for a multiband on-off keying modulation system. For this receiver, channel estimation constraints were relaxed and suitable signal processing schemes were developed, resulting in a simple hardware architecture. Only approximate delay spread and energy levels are needed, and the associated optimum demodulation turns out to be a non-trivial energetic threshold comparison. We have analytically computed the solution to demonstrate its feasibility.

Physical constraints imposed by the transmission channel For a high data rate impulse radio scheme, the elementary symbol information was carried within a single pulse duration of Tw, which is around one nanosecond. To achieve a high data rate at low-cost, complex equalization processes were eliminated to avoid inter-symbol interference. Thus, the symbol repetition period Tr is chosen such as Tr$ Td, where Td is the delay spread of the channel. Favoring non-coherent demodulation, and thus a receiver working as an energy detector, information is preferably carried by signal amplitude rather than its phase. Consequently, it leads us to consider pulse amplitude modulation (PAM). In that case, considering a non-coherent demodulation, an on-off keying (OOK) modulation appeared to be a suitable candidate. Consequently, to increase the system capacity while preserving these properties, we propose to duplicate this basic scheme on several separate subbands (in practice from eight to 24 bands of 250 to 500 MHz each).Accordingly, the adopted non-coherent receiver structure per sub-band is provided in Figure 1, where Ti denotes the energy integration time devoted to a symbol demodulation.