research activities in 2007 - CSEM

FM-UWB – A Low Data Rate (LDR) UWB Approach with Short Synchronization Time and
Robustness to Interference and Frequency-Selective Multipath
J. F. M. Gerrits, J. R. Farserotu, M. Hübner, J. Ayadi
Constant-envelope FM-UWB is a true LDR (< 100 kbit/s) UWB system. Instantaneous despreading in the receiver allows for rapid synchronization.
The robustness to interference and frequency-selective multipath make this system a good choice for robust LDR Body Area Network systems.
Ultra Wideband (UWB) communications are poised to enable
short-range applications, such as remote health monitoring (ehealth)
and home or office automation.
Body Area Networks (BANs) [1] are potential candidates for
UWB since the low radiated power of the UWB transmitter
enables low DC power consumption yielding long battery life
and the possibility to use energy scavenging. Size and cost
constraints require a low-complexity approach that allows
multiple users sharing the same RF bandwidth, and offers
robustness to interference and frequency-selective multipath
propagation conditions.
Constant-envelope FM-UWB uses double FM: binary FSK
followed by high modulation index analog FM implementing
analog spreading. The FM-UWB signal is characterized by a
flat spectrum and steep spectral roll-off.
Due to the instantaneous despreading in the receiver,
synchronization time is limited only by the bit synchronizer in
the FSK demodulator. Figure 1 shows measurement results
taken in a 62.5 kbps FM-UWB system operating at 4 GHz.
Transmission starts at the rising edge of the TX_ENABLE
signal. On the receiver side, the raw data RXD is available
almost instantaneously, whereas the bit synchronizer circuit
determines the overall receiver synchronization time. From a
synchronization point of view, the FM-UWB system behaves
like a narrowband FSK system.
Figure 1: Measured FM-UWB receiver synchronization time.
Interference from in-band UWB users benefits from the
receiver processing gain which is equal to the ratio of RF and
subcarrier bandwidth
G
PdB
= 10 log
10
⎛ B
⎜
⎝ B
RF
SUB
⎞
⎟
= 10 log
⎠
10
⎛
⎜
⎝
2Δf
⎞
( ) ⎟ RF
β + 1 R
SUB
In a 100 kbit/s LDR system with a RF bandwidth of 500 MHz a
processing gain of 34 dB is obtained. As a result a 100 kbps
FM-UWB radio can tolerate a 21 dB stronger FM-UWB
interferer. Simulations have confirmed these values and also
⎠
show that Impulse Radio and MBOFDM interference up to
15 dB stronger than the FM-UWB signal can be dealt with.
FM-UWB signals are robust to frequency-selective
multipath [2] . Figure 2 shows MATLAB simulation results of the
RF sensitivity improvement for 1000 realizations of the IEEE
CM4 (strong non line-of-sight) channel. The graph in the
upper part of the figure shows the RF sensitivity improvement
for each channel realization. The histogram in the lower part
of the figure shows the distribution of the receiver sensitivity
improvement. The average and median value both equal 0 dB
meaning that 50% of the strong non line-of-sight channels
yield a performance improvement. The worst-case sensitivity
degradation is only 2.5 dB.
Straightforward by its principles, FM-UWB constitutes a LDR
UWB communication system highly robust to interferences
and multipath.
Figure 2: Sensitivity improvement in CM4 channel.
[1] J. F. M. Gerrits, J. R. Farserotu, "FM-UWB: A Low Complexity
Constant Envelope LDR UWB Communication System", IEEE
P802.15 Working Group for Wireless Personal Area Networks
(WPANs), July 2007, San Francisco, California, USA,
IEEE802.15-07-0778-040ban,
http://www.ieee802.org/15/pub/Sgmban.htm
[2] J. F. M. Gerrits, J. R. Farserotu, J.R. Long, "Multipath Behavior
of FM-UWB Signals", Proceedings of ICUWB2007, Singapore,
September 2007
http://www.wise-project.org/
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