Modified Pulse Repetition Coding Boosting Energy Detector
Performance in Low Data Rate Systems
Florian Troesch, Frank Althaus, and Armin Wittneben
Swiss Federal Institute of Technology (ETH) Zurich
Communication Technology Laboratory, CH-8092 Zurich, Switzerland
Abstract—We consider Ultra-Wideband Impulse Radio (UWB-IR) Low
Data Rate (LDR) applications where a more complex Cluster Head (CH)
communicates with many basic Sensors Nodes (SN). At receiver side,
noncoherent Energy Detectors (ED) operating at low sampling clock, i.e.,
below 300kHz, are focused. Drawback is that EDs suffer from significant
performance losses with respect to coherent receivers. Pulse Repetition
Coding (PRC) is a known solution to increase receiver performance at
the expense of more transmit power. But in LDR systems known PRC is
very inefficient due to the low receiver sampling clock. Boosting transmit
power is not possible due to Federal Communications Commission’s
(FCC) power constraints. Hence, we present a modified PRC scheme
solving this problem. Modified Repetition Coded Binary Pulse Position
Modulation (MPRC-BPPM) fully exploits FCC power constraints and
for EDs of fixed integration duration is optimal with respect to Bit Error
Rate (BER). Furthermore, MPRC-BPPM combined with ED outperforms
SRAKE receivers at the expense of more transmit power and makes ED’s
performance robust against strong channel delay spread variations.
Recently, Ultra-Wideband Impulse Radio (UWB-IR) technology
has gained strong interest as a very promising technology for future
indoor wireless communication. Key applications for which UWB-IR
technology is considered an interesting candidate are Low Data Rate
(LDR) communication systems requiring rates below 1Mbps .
UWB-IR transmitters produce very short time domain pulses
of up to 7.5GHz bandwidth without the need for an additional
Radio Frequency (RF) mixing stage due to their essentially baseband
nature. This leads to significant complexity reduction at transmitter
and receiver side with respect to conventional radio systems. This
advantage makes UWB-IR a well suited candidate for low cost
LDR applications. On the other hand, channel investigations 
show that UWB-IR indoor channel energy is spread over a large
number of multipath components. This highly increases complexity of
coherent receivers as energy has to be re-combined by a large number
of RAKE fingers. Furthermore, UWB-IR systems are intended to
operate over a large bandwidth, overlaying bands of many other
services. They are thus rigorously power constrained by regulations,
as e.g., by the Federal Communications Commission (FCC), to
minimize interference to victim receivers. These regulations impose
hard performance limits to UWB-IR communication systems as
energy per pulse is restricted very stringently.
In this work, we focus on UWB-IR LDR applications where a more
complex Cluster Head (CH) communicates with many basic Sensor
Nodes (SN). An example could be a wireless control system where
only very small amount of data is transmitted from and to the SNs.
At sensor side, only simple hardware structures are affordable. While
the design of simple UWB-IR transmitters seems a minor problem,
this is not the case for simple receivers. Only non-coherent receivers
seem reasonable, which suffer from significant performance losses
with respect to coherent receivers as channel energy is spread over a
large number of multipath components.
Hence, we consider non-coherent Energy Detectors (ED) operating
at very low sampling clock, i.e., below 300 kHz, as a reasonable
choice and investigate signaling schemes to efficiently increase
performance of ED. The low sampling clock is applied to relax
requirements on receiver sampling accuracy and to reduce power
Pulse Repetition Coding (PRC) is a known solution in asymmetric
sensor networks to increase receiver performance of SNs at the
expense of more transmit power at CH side. With PRC a bit is loaded
on several consecutive pulses, as e.g., it is often applied in Time-
Hopping (TH) Pulse Position Modulation (PPM). In LDR systems,
classic PRC has two major drawbacks. First, throughput is further
decreased and secondly, it does not exploit FCC power constraints
In this paper, we present a Modified PRC (MPRC) coding scheme
for LDR systems with receiver sampling rates of below 300kHz. This
MPRC scheme maximizes transmit power, if FCC power constraints
have to be respected. For an ED of fixed integration duration, men-
tioned precoding scheme is optimal, i.e., it minimizes Bit Error Rate
(BER) by fully exploiting FCC power constraints and transmitting
maximized power most efficiently. Furthermore, it is well known
that performance of EDs strongly depends on the appropriate choice
of the integration duration. MPRC, which requires Channel State
Information (CSI) neither at transmitter nor at receiver side, mainly
decouples receiver performance from integration duration. This has
major advantages. First, performance of the ED becomes extremely
robust against strong delay spread variations. Secondly, constraints
on the integration duration, e.g., fixed large size due to circuit
design aspects, can be compensated. Finally, jitter robustness can be
increased by choosing a large integration duration. Presented MPRC,
without any CSI, achieves performance of a complex Selective RAKE
(SRAKE), at the expense of more transmit power. Presented results
are based on BER performance analysis incorporating simulations
using UWB channels from different measurement campaigns. Al-
though, FCC power constraints are considered, only, results are easily
adaptable to other regulations.
Applied Modified Pulse Repetition Coded Binary Pulse Position
Modulation (MPRC-BPPM) scheme equals an orthogonal BPPM
scheme of equivalent pulses, where each equivalent pulse consists of a
sequence of equidistant copies of a basic pulse waveform, as shown in
Fig. 1. The extension to dithered temporal pulse separation is straight
Fig. 1. Principle difference between BPPM (Left) and MPRC-BPPM (Right)
forward, but was omitted for convenience. The different copies are
multiplied by an arbitrary phase in order to flatten the spectrum of the
transmit signal and to minimize interference to other users, i.e., MAC.
NP = 1, NR = 20
NP = 1, ED
NP = 2, ED
NP = 3, ED
NP = 4, ED
duration and 20 finger SRAKE.
BER comparison between ED of optimal (adjusted) integration
2468 1012 1416 18 20
NP = 1, NR = 20
NP = 1 , ED
NP = 2 , ED
NP = 3 , ED
NP = 4 , ED
BER comparison between ED of fixed integration duration and 20
taken from NLOS measurements performed at IMST. While there are
major performance gains, if the number of transmitted MPRC pulses
is increased, there are hardly any gains, if bandwidth is increased.
E.g., if NP = 3 MPRC pulses of B = 500 MHz are transmitted,
SNR performance of the ED at BER = 10−3increases by 5 dB
with respect to a single transmit pulse of 500 MHz bandwidth.
This is achieved at the expense of three times more power. But
if three times more power is radiated by increasing bandwidth of
the transmit pulse to 1.5 GHz, there is at most a gain of 2 dB in
SNR. This phenomenon mainly occurs due to frequency dependent
pathloss effects. While noise power at the receiver, increases linearly
with B, this is not the case for the received signal power, due to
stronger pathloss at higher frequencies. Hence, as shown in , there
exists an optimal bandwidth. Although broader bandwidth increases
diversity gains, with respect to BER, one might prefer spending
energy in additional MPRC pulses, if NP < Nmax
to increasing pulse bandwidth. Further advantages of MPRC-BPPM
is satisfied, prior
Pulse Bandwidth [B] (GHz)
Nbr. Precoding Pulses [NP]
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5
SNR in dB at BER = 10−3plotted as a function of NP and B.
are that with increasing number of MPRC pulses, BER performance
of the ED of fixed integration duration becomes more and more
robust against delay spread variations of the channel. This is because
MPRC artificially increases delay spread such that the importance
of the real channel delay spread is significantly reduced. Hence,
outage probability can be drastically reduced. This has the major
advantage, that involved integration duration adaption can be omitted.
Furthermore, if hardware constraints do not allow realization of
integrators with extreme short TI, MPRC is a helpful approach to
compensate for possibly too large integration windows. It is evident
that MPRC also increases jitter robustness and that it helps to
significantly relax synchronization requirements.
We have presented a simple modified pulse repetition coding
scheme, that fully exploits FCC power constraints and signifi-
cantly improves performance of EDs in LDR systems. It has been
demonstrated that for fixed integration duration TI, MPRC-BPPM
is optimal. MPRC-BPPM was shown to outperform the SRAKE
at expense of more transmit power. It was discussed that it is
preferable to distribute power over several MPRC pulses than over
huge bandwidth and that MPRC-BPPM can be used to make ED
performance almost independent of the adequate integration window.
This makes EDs robust against delay spread variations and reduces
The authors would like to thank all partners of the PULSERS
project (www.pulsers.net), that is partially funded by the European
Commission and the Swiss Federal Office for Education and Science,
for their contributions and constructive discussions.
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