Analog-to-digital conversion

Since digitizing the overall SDR bandwidth at a rate of several gigahertz is expensive and power consuming, many current solutions are considering the use multiple sub-band processing for high data rates. In fact, even a set of 500 or 250 MHz bands to digitize is not compatible with a realistic UWB solution because it imposes a sampling rate of few
hundreds of megahertz. It is necessary to extract from the overall signal available on the channel a sub-part of
information which is sufficient to compute the detection issue of concern. W opted, therefore, for an energy detection shceme.
Consequently, in our system, it only requires us to sample the received signal every time the delay spread is a few tens of megahertz, typically 30 MHz. The use of multiple subbands in this case is only justified to speed up the data rate of the information, and to avoid narrow band interference. Each band must only keep sufficient wideband characteristics
to provide the receiver with enough multipath to benefit from the diversity offered by the channel. Channel analysis has shown that a 500 MHz, or even 250 MHz band, respects these requirements.
Moreover, only a reduced resolution in terms of number of bits is necessary, which eases the digital processing of the SDR system.
The number of bits required depends on the accuracy expected when estimating the received energy (required to set the threshold). A number of four to eight bits can be used during the estimation procedure. Once the threshold is set, an analog decision could be even envisaged, that is to say, having a one-bit resolution. However, an ADC conversion on a few bits during this stage can be useful to adapt the threshold according to the channel variation or to make soft decoding.
As a conclusion, the proposed architecture for high data rates only requires a simultaneous digitization on each sub-band (24 as an example) at a rate of a several tens of megahertz with a resolution of one to a few bits. A reduced system can cope with lower data rates while adjusting the number of sub-bands to each case.

Synchronization and MAC

Relaxed synchronization methods based on asynchronous time-hopped signal detection have also been investigated to enable fast channel delay spread estimation. From a system point of view, usual medium access control (MAC) is
being re-visited, taking benefit from energy detection techniques while FDMA division may favor the inherent frequency separation properties in such a system.

Performance

Analytical performances of the system discussed have been computed and show remarkable results [1]. These results
are obtained without channel coding techniques, which can be added later for even better performance. As shown in Table 1, corresponding throughputs are 150 Mbps at 10 meters for a 10-5 bit error rate on different types of IEEE 802.15.3a channel models (CM) under FCC transmission mask. Data rates of 600 Mbps at 3 meters are affordable
with the use of 24 sub-bands of 250 MHz. Note that the proposed system can be dimensioned accordingly to the usage demand in terms of data rate.
Indeed, comparison with coherent systems show that, to compete with our on-off keying scheme, a classical rake receiver for a coherent BPSK should collect up to 40% of the whole available energy. This is challenging due to severe multipath characteristics for typical UWB impulse radio signals and should hardly be achievable at the same cost of the
solution proposed in this article.