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- What is UWB communications?
Ultra-wideband communications is fundamentally different from all
other communication techniques due to employing extremely short-time
pulses (sub-nanosecond) to communicate between transmitters and receivers.
The short duration
of UWB pulses instantaneously generates very wide bandwidth (GHz) in
frequency domain. Return to top
- Is UWB a new technology?
UWB is not a new technology. In fact it was first used by Guglielmo
Marconi in 1901 for transmitting the Morse code sequences across the
Atlantic ocean using spark gap radio transmitters. However, the benefit
of a large
bandwidth and the capability of implementing multi-user systems provided
by electromagnetic pulses were never considered at that time. Return
to top
- How is UWB communications different
from narrowband communications?
Narrowband communication systems modulate continuous wave RF signals
with a specific carrier frequency to transmit and receive information.
A continuous waveform has a well-defined signal energy in a narrow
frequency band (KHz)
that makes it vulnerable to intercept and detect. However, UWB systems
use carrierless, short duration pulses with very low duty cycle
(<0.5%)
for transmission and reception of the information. The energy of such
signals is spread over a very wide range of frequencies (GHz); therefore,
each frequency has very low power which makes UWB
signals very difficult to detect and intercept. Return to top
- What is the definition
of a UWB signal?
As defined by the Federal Communications Commission's (FCC) First
Report and Order, UWB signals must have bandwidths of greater than
500 MHz or a fractional bandwidth larger than 20% at all times of transmission.
Fractional
bandwidth is the ratio of a signal's actual bandwidth to its center
frequency. Return to top
- Is UWB concept
the same as spread spectrum technology?
Although UWB and spread spectrum techniques share the same advantage of
the expansion in bandwidth, the method of achieving the large bandwidth
is the main distinction between the two technologies. In conventional
spread
spectrum techniques, information bits are broken into "smaller bits" or chips
and the chips are modulated with either a fixed carrier frequency
or for frequency-hopping spread spectrum, a set of carrier frequencies.
While
in UWB communications, there is no carrier frequency and the short
duration of the pulses directly generates the extremely wide bandwidth.
Spread spectrum techniques can offer MHz of bandwidth, while UWB
pulses provide several
GHz of bandwidth. Return to top
- How are the RF signals classified based
on their fractional bandwidth?
The classification of signals based on their fractional bandwidth is as
follows:
Narrowband signal |
Bf<1% |
Wideband signal |
1%<Bf<20% |
Ultra-wideband signal |
Bf>20% |
Return to top
- What are the advantages of UWB communications?
The main advantages and benefits of UWB systems over narrowband
wireless technologies are summarized in the following table.
Advantage |
Benefit |
Co-existence with current narrowband and wideband radio services |
Avoids expensive licensing fees |
High channel capacity |
High bandwidth can support real-time HD video streaming |
Ability to work with low signal-to-noise-ratios |
Offers high performance in noisy environments |
Low transmit power |
Provides low probability of detection and intercept |
Resistance to jamming |
Reliable in hostile environments |
High performance in multipath channels |
Delivers higher signal strengths in adverse conditions |
Simple transceiver architecture |
Enables ultra-low power, smaller form factor at a reduced cost |
Return to top
- How can UWB signals share the frequency
spectrum with legacy radio services?
Since UWB signals energy is spread over a very wide range of
frequencies, it has very low power spectral density, meaning that each
frequency has a very low power. The PSD of a UWB signal is below the
noise floor of a
typical narrowband receiver, therefore UWB signals look like noise
to coexisting radio services.
Return to top
- Can narrowband signals cause harmful
interference to UWB systems?
Because of their very large bandwidth, UWB signals have a certain
level of resistance to interference and jamming. However, depending
on the strength of the narrowband interferer, UWB pulses can be degraded
severely.
Therefore, interference mitigation techniques are crucial to successful
UWB receiver design.
Return to top
- Why UWB communications systems have
high data rates?
According to Shannon's capacity formula C=B log2(1+SNR);
where C the channel capacity (bits per second) increases linearly
with bandwidth, B. Therefore, having several GHz of bandwidth available
for
UWB signals, a
data rate of Gbps can be expected. Return to top
- What is the typical range for UWB communications?
Due to the FCC's current power limitation on UWB transmissions,
the high data rate is only available for short ranges up to 10 meters.
This makes UWB systems perfect candidates for short range, high data
rate wireless
applications such as wireless personal area networks (WPAN). Return
to top
- Are UWB signals immune to multipath
phenomenon?
The short duration of UWB pulses makes them less sensitive to
multipath effect compared to narrowband signals. This is because with
transmission of pulses shorter than a nanosecond in duration, the reflected
pulse has an extremely
short window of opportunity to collide with the line of sight (LOS)
pulse to cause signal degradation. However, this relative immunity
to multipath is only in comparison with narrowband signals and dense
indoor channels can certainly
degrade UWB communications too. Return to top
- Why UWB transceivers have simpler architecture
compared to narrowband transceivers?
The transmission of low powered pulses eliminates the need for
a power amplifier (PA) in UWB transmitters. Also, since UWB transmission
is carrierless, there is no need for mixers and local oscillators to
translate the
carrier frequency to the required frequency band and consequently
there is no need for carrier recovery stage at the receiver end. In
general, the analog front end of a UWB transceiver is noticeably less
complicated and cheaper to
build than a narrowband transceiver. Return to top
- Why is it difficult to detect UWB signals?
Because of the low duty cycle, UWB signals have very low transmission
average power, therefore an eavesdropper has to be very close to the
transmitter (about 1 meter) to be able to detect the transmitted information.
In addition,
UWB pulses are time modulated with codes unique to the transmitter/receiver
pairs. This time modulation adds more security to UWB transmission,
since detecting picosecond pulses without the prior knowledge of their
time of
arrival
information is next to impossible. Return to top
- Is UWB communications possible in low
signal-to-noise-ratio environments?
The Shannon's theory for maximum capacity (refer to Q.10) also
indicates that the channel capacity is only logarithmically dependent
on SNR. Therefore, UWB communication systems are capable of working
in harsh communication
channels with low signal-to-noise ratio and still offer a large channel
capacity due to their large bandwidth.
Return to top
- How can UWB signals penetrate through
walls?
Unlike narrowband technology, UWB systems can penetrate effectively
through different materials. The reason is that the low frequencies
covered in the broad range of UWB frequency spectrum have long wavelengths
and
allow UWB
signals to penetrate through different materials including walls. Return
to top
- What are the main applications of UWB
communications?
The trade off between data rate and range in UWB systems holds
great promise for a wide variety of applications both in military,
civilian, and commercial sectors. The following table summarizes the
UWB
applications in data communications, radar, and localization.
|
Applications |
UWB Technology Category |
Military & Government |
Commercial |
Data Communications |
1. Secure communications
2. Wireless sensor networks (battlefield operations |
1. Local and personal area networks
2. Wireless streaming video distribution (home networking)
3. Wireless sensor networks (health and habitat monitoring, home automation) |
Radar |
1. Through-wall imaging (law enforcement, firefighters)
2. Ground penetrating radars (rescue operations)
3. Surveillance and monitoring |
1. Medical imaging (remote heart monitoring)
2. Ground penetration radars (detection of electrical wiring, studs,
etc. in construction sites)
3. Automotive industry (collision avoidance, roadside assistance)
4. Home security (proximity detectors) |
Localization |
1. Personnel identification 2. Situational awareness for soldiers 3.
Prisoner tracking |
1. Inventory tracking 2. Tagging and identification 3. Asset management |
Return to top
- What are the FCC regulations on UWB
emissions?
In order to protect the existing radio services from UWB interference,
the FCC has assigned conservative emission masks between 3.1 GHz to
10.6 GHz for commercial UWB devices. The maximum allowed power spectral
density
for these devices is -41.3 dBm/MHz equal to 75 nW/MHz that places
them at the same level as unintentional radiators (FCC part 15 class)
such as TV and computer monitors. In general the FCC ruling per application
with part 15
classification of -41.3 dBm for both outdoor and indoor operations
can be summarized as shown in the following table.
|
|
Operation Band (GHz) |
|
Application |
0.96 to 1.61 |
1.61 to 1.99 |
1.99 to 3.1 |
3.1 to 10.6 |
10.6 to 22.0 |
22.0 to 29.0 |
EIRP (dBm) |
Communications
|
Indoor |
-75.3 |
-53.3 |
-51.3 |
-41.3 |
-51.3 |
-51.3 |
Outdoor |
-75.3 |
-63.3 |
-61.3 |
-41.3 |
-61.3 |
-61.3 |
Imaging |
-53.3 |
-51.3 |
-41.3 |
-41.3 |
-41.3 |
-51.3 |
Vehicular radar |
-75.3 |
-63.3 |
-63.3 |
-63.3 |
-41.3 |
-41.3 |
Return to top
- How is UWB communications compared to
Bluetooth and WLAN technologies?
The following table compares the UWB technology to the other
currently available data communication standards.
|
|
|
WLAN |
Bluetooth |
WPAN |
UWB |
Zigbee |
|
802.11a |
802.11b |
802.11g |
802.15.1 |
802.15.3 |
802.15.3a |
802.15.4 |
Operational Frequency |
5 GHz |
2.4 GHz |
2.4 GHz |
2.4 GHz |
2.4 GHz |
3.1-10.6 GHz |
2.4 GHz |
Maximum Data Rate |
54 Mbps |
11 Mbps |
54 Mbps |
1 Mbps |
55 Mbps |
>100 Mbps |
250 Kbps |
Maximum Range |
100 meters |
100 meters |
100 meters |
10 meters |
10 meters |
10 meters |
50 meters |
Return to top
- What are the challenges of UWB communications?
There are many challenges involved in using nanosecond duration
pulses for communications. Some of the main difficulties of UWB communications
are discussed in the following table.
Challenge |
Problem |
Pulse shape distortion |
Low performance using conventional pulse detection technique (i.e., classical
matched filter receivers) |
Channel estimation |
Difficulty in predicting the template signals |
High frequency synchronization |
Very fast ADCs required |
Multiple access interference |
Detecting the desired user's information is more challenging than in
narrowband communication |
Low transmission power |
Information can only travel to short distances |
Return to top
- Why is pulse shape distortion a problem
in UWB communications?
The shape of the signal at the receiver is distorted by multipath,
diffraction, and other scattering effects. This limits the performance
of UWB receivers that correlate the received pulses with a pre-defined
template such as
classical matched filters. Return to top
- Why is time synchronization a challenge
in UWB communications?
Sampling and synchronization of nanosecond pulses pose a major
limitation in the design of UWB systems. In order to sample these
narrow pulses, very fast (on the order of GHz) analog to digital converters
(ADCs)
are
needed. Moreover, the strict power limitations and short pulse duration
make the performance of UWB systems highly sensitive to timing errors
such as jitter and drift. This can become a major issue in the success
of pulse position
modulation (PPM) receivers that rely on detecting the exact position
of the received signal.
Return to top
- What are the common UWB modulation techniques?
Some of the common UWB modulation techniques with their advantages
and disadvantages are summarized in the following table:
UWB Modulation |
Encoding Method |
Advantages |
Disadvantages |
On Off Keying (OOK) |
Presence and absence of a pulse represents digital bits "1" and "0" respectively. |
Simple and low power transmitter. |
Highly susceptible to noise. UWB synchronization becomes even more challenging
if a stream of zeros is transmitted. |
Pulse Amplitude Modulation (PAM) |
A pulse with higher amplitude represents a data bit "1" and a pulse with
lower amplitude represents a data bit "0". |
Simple transmitter, since pulses with only one polarity are needed to
represent data. |
Attenuation in wireless channel can convert them to OOK case. |
Pulse Position Modulation (PPM) |
Signals are pseudo randomly encoded based on the position of the transmitted
pulse trains by shifting the pulses in a predefined window in time. |
Less vulnerable to false detection due to noise compared to OOK and PAM
pulses. |
Requires very strict timing synchronization. Time uncertainties such
as drift and jitter cause unreliable detection. |
Bi-Phase Modulation |
Polarity of the pulse changes to represent digital data bits. |
Less susceptible to distortion since the difference between the two pulse
levels is twice the pulse amplitude. |
More complexity in physical design of the transmitter to generate two
polarities. |
Return to top
- What is the common detection technique
for UWB pulses?
All aforementioned UWB modulation techniques share the same conventional detection method called matched
filter receivers. In order to detect the data, matched filters correlate the received pulse with a predefined
template locally generated at the receiver. Return to top
- What are the challenges of traditional pulse detection techniques?
The main problem with matched filters is that the received UWB pulse is significantly distorted by the
wireless channel and does not resemble the locally generated template. In order to achieve high performance,
matched filters need to estimate the wireless channel very accurately and generate a template pulse that shows
the same type of distortion that the received pulse has experienced. However, channel estimation is a very
challenging task for UWB pulses. Therefore, lack of similarity between the pulses and high frequency
synchronization are the main drawbacks of conventional detection technique. Return to top
- How long has LLNL been active in the field of UWB?
LLNL has been active in UWB research and development for many years, and in "micropower" UWB for 15 years.
Return to top
- What is the transmitted-reference methods (TR) and what are the advantages of the TR
method?
TR modulation technique has the advantage of sending the same
pulse twice through an unknown channel where both pulses experience
the same type of channel distortion and detection becomes easier with a
correlation receiver. Therefore, instead of correlating the distorted received
pulse with a "clean" template pulse as in PPM, both the data
pulse and so called template ("reference pulse") are distorted and show high
correlation at the TR receiver. Therefore, there is no need for channel
estimation in TR receivers.
Furthermore, a TR receiver is self-synchronized and eliminates the need for individual pulse synchronization with locally
generated templates that exists in PPM scheme. The reason is that each "reference pulse" acts as a preamble for its "data
pulse" and has the advantage of providing rapid synchronization. Moreover, synchronization in TR receivers occurs after
correlation between the "data pulses" and "reference pulses," thus the sampling requirements are relaxed to baseband
signals. This way, the need for synchronization of the received short duration RF pulses and very fast ADCs are eliminated.
Another advantage of TR modulation to the other UWB modulation schemes is its high performance in multipath environments.
TR receivers exploit multipath phenomenon to improve their performance in dense multipath and indoor channels. This is
because the reference and data pulses are correlated with each other, and the multipath channel introduces a longer
duration in the signal component of the received signal, thus increasing the overall signal energy at detection stage.
Return to top
- At what stage is LLNL's UWB communications system?
LLNL's UWB communications system is at prototype stage. This prototype has been extensively evaluated in
harsh propagation channels such as heavy concrete and heavy metallic environments through real field experiments
conducted in collaboration with Naval Postgraduate School (NPS). Return to top
- What is the communications range and power consumption of LLNL's current UWB prototype
communications system?
In a heavy concrete environment, with 200 mwatts of power, LLNL's UWB communications prototype is capable
of providing successful real time video communications through 3 to 4, 12" concrete walls and 400 ft range.
Return to top
- What are the capabilities of future generations of LLNL's UWB communications systems?
LLNL engineers are working on a UWB system that can work in the presence of multiuser interference, as well
as improving their communications range. Return to top
- Can LLNL's UWB communications technology be licensed by other organizations?
Yes, see Partnering. Return to top
- Can private companies partner with LLNL on commercializing the UWB communications
prototype system?
Yes, see Partnering. Return to top
- Can other organizations partner with LLNL in R&D projects for future improvements
to the current UWB prototype system?
Yes, see Partnering. Return to top
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