# FM-UWB: Towards a Robust, Low-Power Radio for Body Area Networks
## 2 Review of FM-UWB Transceivers
### FM-UWB Transmitters  
Unlike the FM-UWB receivers, the architecture of the FM-UWB transmitters has remained unchanged over the past several years. Considering its simplicity (Figure 2), it is clear that there is not a lot of potential for improvement on the architectural level. In fact, the reduction of power on the transmitter side is mainly a result of improvements on the circuit level. Every FM-UWB transmitter consists of three blocks, the sub-carrier generator, the Voltage Controlled Oscillator (VCO) (sometimes as a part of a PLL or a Frequency Locked Loop (FLL)) and a power amplifier (PA). The sub-carrier generator generates the triangular waveform that is used to drive the VCO. As the sub-carrier frequencies are rather low (typically 1-2 MHz), this block does not contribute significantly to the overall transmitter consumption. One way to implement it is a Direct Digital Synthesis (DDS) as described in [20]. The advantages of digital implementation are the simple and precise frequency control without the need for calibration. The drawback of the fully digital approach becomes apparent at higher data rates, where higher sub-carrier frequencies are needed. In [21], 51 MHz sub-carrier frequency is used. Since roughly 20 points per period are needed to generate a reliable sub-carrier waveform, a DDS would need to operate at a clock speed of more than 1 GHz, which would be difficult to implement and would consume a significant amount of power. Instead, a relaxation oscillator is used within a PLL, as a more optimal solution. Another interesting approach that leads to very low power consumption is a free-running relaxation oscillator, which is periodically calibrated using an \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline
**Reference** & [6] & [9,16] & [17] & [12] & [13] & [10,18] & [7,19] \\ \hline Year & 2009 & 2010 & 2012 & 2013 & 2014 & 2014 & 2016 \\ Demodulator & RF-DL & Reg & RF-DL & DBPF & DBPF & Regen. & BB-DL \\ Frequency [GHz] & 7.5 & 3.75 & 3.8 & 3.75 & 8 & 4 & 4 \\ Power cons. [mW] & 9.4 & 2.2 & 7.2 & 3.8 & 0.6/4 * & 0.58 & 0.42 \\ Supply [V] & 1.8 & 1 & 1.6 & 1 & 1 & 1 & 1 \\ Data rate [kb/s] & 50 & 100 & 50 & 100 & 1000 & 100 & 100 \\ Sensitivity [dBm] & \(-88\) & \(-84\) & \(-70\) & \(-78\) & \(-76\) & \(-80.5\) & \(-70\) \\ NB SIR [dB] & \(-25\) & \(-30\) & - & -23 & - & \(-18\) & \(-17\) \\ SC-FDMA & Yes & No & - & No & No & No & Yes \\ Efficiency [mJ/b] & 188 & 2.2 & 144 & 38 & 1 & 5.8 & 4.2 \\ Tech. node [nm] & 250 & 65 & 180 & 65 & 65 & 65 & 65 \\ \hline \hline \end{tabular}  
* Power consumption is 0.6 mW with duty-cycling and 4 mW without duty-cycling.  
\end{table} 
Table 1: Performance summary of state-of-the-art Frequency Modulated Ultra-Wideband (FM-UWB) receivers. FLL [22]. Digital frequency control is provided through a capacitor bank; however, this approach is usually not precise enough if multiple sub-carrier channels are to be used. The two parts of the FM-UWB transmitter that essentially determine its power consumption are the VCO and the PA. In the case where transmitted power is \(10\,\mathrm{dBm}\) or more, the transmitter efficiency was dominated by the PA, however at lower output powers, such as, at \(-10\,\mathrm{dBm}\), the contribution of the VCO becomes quite significant. In some of the earlier implementations, the RF carrier was synthesized using an LC VCO within a PLL [21; 23]. To decrease power, the frequency synthesizer is duty cycled, making the frequency dividers active for only 10% of the time. Although this allowed some savings, the power consumption was still on the order of \(10\,\mathrm{mW}\). A significant improvement was made when the LC oscillator was replaced with a ring oscillator [22; 24]. This was possible owing to the loose phase noise constraints of the FM-UWB modulation. Additionally, instead of the quasi-continuous PLL, an FLL calibration loop was used [22]. Since the FM-UWB spectrum is very wide, the center frequency can deviate slightly without a major impact on performance and it does not need to be monitored continuously.