## 2 The Pulsed Coding Technique: Theory, Modelling and Methods of the Optical Approach  
Figure 1 illustrates the top-level complete architecture of an optical wireless transcutaneous biotelemetry system. The dashed black square box highlights the scheme implementing the proposed optical pulsed UWB modulation technique performing the data coding process. The overall system typically includes two units, implantable and external, each one consisting of a main sub-block: the transmitter and the receiver. The conditioning and processing circuitry block in the implantable unit handles analogue signals related to neural signals (e.g., coming from neural recording systems) [21; 22]. The analogue signals  
Figure 1: Complete scheme of an optical wireless transcutaneous biotelemetry system: in the dashed box, the architecture implementing the proposed data coding approach based on optical pulsed Ultra-wideband (UWB) modulation technique. are converted into a serial digital data stream \(S_{T}(t)\) to be transmitted, after its coding in pulsed signal \(U_{T}(t)\), from the implantable to the external unit. The receiver in the external unit provides the decoded/regenerated serial digital data stream \(S_{R}(t)\) to the related conditioning and processing circuitry block. The latter, in this case, generates analogue and/or digital signals associated to the transmitted neural analogue signals for the control of external actuators as well as for neural data monitoring. More in detail, the transmitter takes, as input signals, the main clock \(\mathrm{CLK}_{T}\) and the data stream \(S_{T}(t)\) to be coded and transferred, while the receiver provides the recovered clock signal \(\mathrm{CLK}_{R}\) and the decoded data stream \(S_{R}(t)\). In the transmitter, the coding system generates the analogue pulsed voltage signal \(U_{T}(t)\) converted by the bias and drive circuits into current pulses that generate the laser pulses (i.e., gain switching operation of the laser). In the receiver, the photodiode provides photocurrent pulses converted by the biasing and signal conditioning circuitry into analogue voltage pulses \(U_{R}(t)\) processed by the decoding system that recovers the clock \(\mathrm{CLK}_{R}\) and the data stream \(S_{R}(t)\). The transmitter uses a high speed Vertical Cavity Surface Emitting Laser (VCSEL) generating sub-nanosecond optical pulses and the receiver employs a wide bandwidth Si photodiode. Figure 2 illustrates an example of a timing diagram, with the corresponding laser operating conditions, describing the proposed data coding process that allows the optical pulsed UWB modulation performed by the transmitter to be achieved. During each bit period \(T\), the VCSEL driving current is maintained at a value \(I_{\mathrm{min}}\) just above the VCSEL threshold current \(I_{\mathrm{th}}\) (corresponding to a minimum laser power \(P_{\mathrm{min}}\)) to provide high output power stability, few picoseconds rise times and negligible jitter respect to the driving current pulses. In order to simultaneously transmit the \(\mathrm{CLK}_{T}\) and \(S_{T}(t)\), short current pulses with a maximum value \(I_{\mathrm{max}}\) drive the VCSEL that generates optical pulses with a power \(P_{\mathrm{max}}\). Therefore, only during the laser pulse generation the transmitter achieves the maximum power consumption. Depending on the receiver performances, the current pulse amplitude and duration can be adjusted to obtain the optimal trade-off between power consumption, data rate and BER. More in detail, referring to Figure 2, at the start of each bit period \(T\) a short laser pulse is generated, independently from the symbol being transmitted, so allowing for the clock signal to be simultaneously transmitted with the symbol. This is needed to synchronise the transmitter and the receiver (i.e., the clock recovery procedure).