Sub-sampling receiver

376 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications

By assumption, these signals have a bandwidth in the GHz range requiring often Nyquist rates of the measurement receivers above 10 GHz. Disregarding the device costs, this is hardly to achieve with the limited power budget and the restricted means of data handling (see section 2.1 – *Figure of Merit* and *Data throughput*) which a sensor usually has at its disposal. Hence, all these devices must reduce their data rates at the expense of receiver efficiency, which is reflected by a reduced dynamic range *D*max (see(7)). The data rate

Fig. 2 refers to three possible device conceptions for illustration. The two upper approaches require periodic sounding signals. Here, the signal shaper may be a pulse generator, a binary PN-generator or an arbitrary waveform generator. The most often found device implementations apply sub-nanosecond pulse generators. Indeed, the concept allows the implementation of very cost-effective and power saving sensors. However, their system performance often suffers from reduced dynamic range due to the large crest factor of the sounding signal (compare (7)); they do not provide jitter suppression (see also sub-chapter 2.3) and they are not robust against jamming. Wideband PN-generators are an interesting alternative to pulse generators since they provide powerful signals of low magnitude (i.e. of low crest factor). Arbitrary waveform generators are able to provide signals which can flexibly be adapted to the measurement problem. However, they are quite expensive, power hungry and limited with respect to the bandwidth. Hence, they have not been found in

*Sub-sampling receiver*: It is the most often applied UWB concept. It supposes periodic sounding signals ( *Pt* - signal period). Typically, the measurement signals are captured by sequential sampling, providing one data sample per period whose time position is stepwise shifted over the whole signal. The actual sampling interval is *Pt t* , while the equivalent sampling interval which has to meet the Nyquist criteria is *t* . Newer concepts apply interleaved sampling permitting higher sampling rates since more than one point per period is taken. The classical concept of time shift control uses the fast ramp-slow ramp approach which, however, tends to non-linear time axis representation, sampling jitter and time drift. A second method deals with two stable sine wave generators (e.g.

reduces time drift and avoids time axis non-linearity. However, it still keeps the sampling jitter quite high since the trigger events launching the sounding pulse and activating the sampling gates are based on relative flat edges of the two sine waves of (comparatively low) frequency 1*f* and 2*f* . Timing control based on digital counters for coarse timing exploits steep trigger edges improving the jitter performance. Then, the fine tuning is typically done by programmable delay chips which consist of hundreds of delay gates. As these gates are not absolutely identical, the delay line cannot ensure equidistant sampling.

<sup>1</sup> *<sup>P</sup> f t*

 ; <sup>1</sup> <sup>2</sup> *<sup>P</sup> <sup>f</sup> t t*

). This

Direct Digital Synthesizers of slightly different frequency ( <sup>1</sup>

reduction is either achieved by sub-sampling or by serializing the data recording.

multi-carrier signals (also assigned as multi-sine), and

practically applicable sensor concepts recently.

white random noise.

**Figure 2.** Three possible sensor structures exploiting signals of large instantaneous bandwidth.

*Analog correlator*: Due to the lag of programmable analog wideband delay lines, one applies two wideband sources (pulse or PN-sequence) providing two identical signals which are shifted in time. The time shift may be controlled by the same approaches as mentioned above. One of these signals stimulates the DUT, and the other one acts as reference in a correlator. Even if the mixer and the integrator do not waste signal energy, the correlator has about the same efficiency as a sequential sampling receiver as long as one does not deal with parallel correlation stages. We can find from eq. (7) that the correlation principle will provide the best dynamic range due to the large time-bandwidth product. But this benefit will be gambled away if sounding signals of large crest factors are applied.

HaLoS – Integrated RF-Hardware Components for Ultra-Wideband Localization and Sensing 379

which can be transformed via IFFT into the impulse response function. Simple implementations (e.g. many FMCW-radars) abstain from vector receivers. They only deal

Measurement principles applying sine waves provide the best suppression of noise and harmonic distortions due to narrowband filtering before signal capture. Their receiver efficiency tends to one as long as the settlement of resolution filters and signal source are negligible against the recording time. Hence, such devices often suffer from long measurement duration which leads to a strong range-Doppler coupling. The recording time can be reduced either by simultaneous measurements at different frequencies [7] (requiring complex parallel receiver and synthesizer) or by renouncing the narrowband filters (giving

Under the assumption of Pseudo-Noise (PN)-codes for sounding, Nyquist sampling for data capture and embedded pre-processing for data reduction, the principle depicted on the top of Fig. 2 seems to be the most promising if one trades the pros and cons of the various UWB principles with respect to monolithic integration, system performance, MIMO-capability and power consumption. Fig. 4 represents the modified structure adapted to the conditions mentioned above. The use of two receiver channels yields the best performance with respect to different application aspects like synchronous measurement of stimulus and reaction signal, opportunity of device calibration, difference or interferometric measurements as well

A stable microwave oscillator controls the whole system. It has to provide only a single

Data transfer Pre-processing Shift register

2 *<sup>n</sup> <sup>s</sup> <sup>c</sup> f f*

> Binary divider

pushes a high-speed shift register. Depending on its feedback structure, it provides any binary sequence. Preferentially, M-sequences are used due to their favorable autocorrelation function. Other options could be Golay-codes [8] or Gold-codes if cross-

*f* which allows the use of simple and stable generator concepts. The oscillator

*cf*

order *m*

2 *cf*

2 *cf*

2 *cf*

ADC T&H

ADC T&H

up the sensitivity benefits compared to the wideband approaches).

with the in-phase component.

**2.3. UWB pseudo-Noise Concept** 

as long term sensor stability.

**Figure 4.** Basic structure of UWB PN-device.

correlation properties are in the foreground of interest.

frequency *<sup>c</sup>*

*Sub-sampling correlator*: Here, we can use also random noise as stimulus. The time lag between measurement and reference signal is performed by shifting the sampling time as explained before. The correlation is done in the numerical domain. The approach is quite time consuming since the averaging time must be high in order to achieve a stable estimation.
