*6.2.3. M-sequence feedback-sampling*

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

corresponding input levels of the measurement signal.

signal power (see [48] for details).

new signal peaks which leave an apparently chaotic mark (see [2] and Fig. 1 for details). While the cross-talk and the internal clutter may be removed by device calibration [50] since they are caused by linear effects, the non-linear distortions should be avoided by respecting

The level diagram at the bottom of Fig. 53 refers to the non-compressed receiver signal. It shows the strength of the linear, quadratic and cubic signal parts in dependency from the

**Figure 53.** Top: Sensor pulse response of system as a function of the input level. Bottom: Level diagram (see also Fig. 1) of raw data (i.e. without impulse compression). In the shown case, the input related 1dB

compression point is -14 dB below the transmitter power.

This sub-chapter gives an example of the usefulness of the modular experimental device. It deals with feedback sampling. Feedback loops have been used for a long time in sampling circuits. However, they were usually restricted to sequential sampling having very large Nyquist rates so that only minor signal variations between consecutive samples appear. Only these variations are captured by that approach (see [2] for details).

In our case, this simple method cannot be applied since the voltage steps between two consecutive samples may cover the full receiver input range as we firstly apply Nyquist sampling and secondly, the natural order of the data samples may be disrupted due to interleaved sampling. Hence, we need some modifications of the principle which pose some challenges to the practical implementation.

For the purpose of feedback sampling, the data capturing & control unit was additionally equipped with a digital-to-analog converter which has to provide the feedback signal. The principle and the device structure are depicted in Fig. 55. The idea behind the digital

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

feedback sampling implementation is to deal with high-speed signals (analog and digital) of low dynamic range (i.e. low amplitude) and to exploit the fact that the temporal variations of the scenarios under test are of the orders smaller than the measurement speed. This implies for the radargram (see Fig. 55, on the left) that adjacent samples at a horizontal line undergo only minor variations (instead of consecutive samples in sequential sampling). Thus, it will be possible to predict the measurement values along the observation time axis. This is the reason to insert a DAC into the feedback loop which converts the predicted digital values into analog ones. If the predicted signal levels are subtracted from the received signal, only the prediction error has to be captured by the ADC and processed by a digital high-speed system.

HaLoS – Integrated RF-Hardware Components for Ultra-Wideband Localization and Sensing 427

**Figure 56.** Digital feedback sampling. Above: open loop; below: closed loop.

Fig. 57 shows a photograph of a primary (1Tx 2Rx) M-sequence RF board and corresponding ADC PCB with PC Interface (USB). The RF board is designed for assembly with the *HaLoS*-project originating ICs. Each of the board layouts corresponds to the architecture shown in Fig. 4, so that both boards connected together represent the basic Msequence working unit. This unit is considered as main integral part of the UWB devices

provided for partner projects within the UKoLoS- and other scientific projects.

some device examples are depicted in Fig. 58 as well in the chapter 11.

**Figure 57.** Photograph of the primary 1Tx 2Rx RF board (left-hand side) and corresponding ADC board

While in the early project phase the sensor devices were finalized in cooperation with Meodat GmbH (Ilmenau, Germany), the final device assembly was performed by ILMSENS (TU Ilmenau service GmbH, Ilmenau, Germany) later. To give the reader an impression,

**6.3. Prototype devices** 

(right-hand side).

**Figure 55.** Basic structure of an M-Sequence feedback approach.

Fig. 56 gives an example of the output signals of the T&H-circuit. The constant voltage during the hold phase must be captured by the ADC. In the open loop example (above), we can observe that the hold voltage jumps from sample to sample. Hence, the ADC must be able to convert voltages within a large range. The second example shows the closed loop operation. Now, the predicted value is subtracted before AD-conversion, and we actually get a voltage during the hold phase which is always at about the same level. Under optimum conditions, the magnitude of the prediction error is determined by the strength of random noise which is usually quit weak. Therefore the requirements onto the dynamic range of the receive electronics can be relaxed.

Under optimum conditions, the magnitude of the prediction error is in the same order as random noise. Therefore, the demands made on the dynamic range of the receive electronics can be relaxed.

**Figure 56.** Digital feedback sampling. Above: open loop; below: closed loop.
