# FM-UWB: Towards a Robust, Low-Power Radio for Body Area Networks
## 2 Review of FM-UWB Transceivers  
### FM-UWB Receivers  
Different FM-UWB receiver implementations can be found in the literature. A simplified block diagram of each of the proposed architectures is shown in Figure 3. The wide-band FM demodulator based on a delay-line (DL) demodulator (Figure 3a) was initially proposed in [8]. Assuming that the delay is relatively small, the demodulated signal can be represented as [8]  
Figure 2: Principle of FM-UWB signal generation. \[V_{dem}=(-1)^{(N+1)/2}\,\frac{A^{2}}{2}\sin(N\frac{\pi}{2\omega_{c}}\frac{d\phi(t )}{dt}),\text{ for }\tau=N\frac{\pi}{2\omega_{c}},\ N=1,3,5,..., \tag{1}\]  
where \(A\) is the amplitude of the demodulator input signal, \(\omega_{c}\) is the carrier frequency and \(\phi(t)\) is the useful frequency-modulated signal. The delay \(\tau\) must be chosen to be an odd multiple of the carrier frequency in order to cancel out the offset in the demodulated signal [8]. The choice of \(N\) is a trade-off between the conversion gain and the bandwidth of the demodulator. Lower value of \(N\) leads to lower conversion gain, but also increases the useful frequency range. In practice, it will be very difficult to achieve high precision of the delay (due process variation), and so a small offset in the demodulated signal will always be present. However, since the transmitted signal is at least \(500\,\mathrm{MHz}\) wide, this small offset should not have a major impact on the receiver performance. The first fully integrated FM-UWB receiver based on a DL demodulator was described in [6]. It achieves a sensitivity of \(-88\,\mathrm{dBm}\) while consuming \(9.4\,\mathrm{mW}\). The demodulator itself consumes around \(5.8\,\mathrm{mW}\), while the Low Noise Amplifier (LNA) consumes \(3.6\,\mathrm{mW}\). An LNA that provides high gain across a large bandwidth inevitably requires more power compared to a narrowband LNA. In order to reduce the power consumption, a narrow-band regenerative receiver was proposed in [9]. This approach allowed to preserve high gain and relatively good noise figure, while minimizing the power consumption. The high-Q band-pass filter behaves as a frequency discriminator that converts the input FM signal into an AM signal, which is then converted to Intermediate Frequency (IF) using an envelope detector. Due to the high-Q factor of the filter that results in a very nonlinear Frequency Modulation to Amplitude Modulation (FM-AM) conversion characteristic, the demodulated signal will be a train of pulses whose frequency corresponds to the sub-carrier frequency (Figure 4). The receiver from [9] consumes \(2.2\,\mathrm{mW}\) while achieving -\(84\,\mathrm{dBm}\) sensitivity. A later implementation presented in [10] introduced several improvements at the circuit level (most notably current reuse among several blocks), which resulted in power consumption of only \(560\,\mathrm{\SIUnitSymbolMicro W}\) and a slight reduction of sensitivity. Although the regenerative receiver achieved significant power savings, there are some downsides to this architecture. Narrow-band interferer rejection mostly relies on the high-Q input filtering. If the interferer falls inside the pass-band, it could easily saturate  
Figure 3: FM-UWB receiver architectures reported in literature: (**a**) delay-Line demodulator at RF; (**b**) regenerative demodulator; (**c**) baseband delay-Line demodulator and (**d**) dual-band-pass-filter demodulator (balanced frequency discriminator). the stages following the LNA and prevent reception. The second downside comes from the nonlinear FM-AM conversion. If several FM-UWB signals were to occupy the same RF band, the weaker signals would be attenuated in the nonlinear conversion process, which would prevent correct demodulation. This is known as the capture effect [11] and limits the regenerative receiver to cases where only one FM-UWB signal is transmitted in a given RF band. In an attempt to improve the linearity of the regenerative demodulator, a modified architecture was proposed in [12].