Ultra Wide Band (UWB) modulation techniques
are the latest buzzword to hit the industry press. In this feature we
will explore some of the ideas behind these interesting techniques, and
attempt to place them into context.
Most readers will be familiar with established
narrowband and spread spectrum modulation schemes. To best contrast how
different UWB techniques are, it is worth briefly reiterating the basic
principles of narrowband and spread spectrum techniques.
Narrowband modulation techniques are the oldest and
technologically simplest approach, and have also set the precedents via
which bandwidth is allocated commercially. Whether you are or are not a
fan of narrowband techniques, they remain the backbone of modern
wireless communications and form the basis of every regulatory model in
existence. The idea behind narrowband modulation is in principle very
simple. You take a sine shaped carrier wave and you change its
amplitude, phase, or both to impose a signal.
The simplest technique is amplitude modulation, the
basis of AM radio and analogue TV. The instantaneous amplitude of the
carrier wave is proportional to the instantaneous magnitude of the
modulation signal. Modulation is trivial to produce, and trivial to
extract, but also trivial to corrupt with an interfering signal, spike
The alternative is to use either phase or frequency
modulation techniques, whereby the instantaneous phase or frequency of
the carrier is proportional to modulation signal. FM or PM schemes have
the very nice property of being more difficult to interfere with, since
additive noise and signals are more easily rejected by the demodulation
hardware. FM is the basis of FM radio and analogue TV sound channels
(and colour in the SECAM standard).
The final, and least used modulation scheme, is time
based modulation, whereby the carrier wave is pulsed, and the pulses
shifted in time to reflect the modulation signal amplitude. Pulse
Position Modulation (PPM) was experimented with but never found serious
More sophisticated narrowband schemes have become widely
used in recent years, based largely upon variations on the theme of
Quadrature Amplitude Modulation (QAM), whereby the modulation signal is
imposed by varying both phase and amplitude. This is the technique
pioneered in NTSC and PAL-A/B colour subcarriers, and is the basis of
most modems, many cable modems, and with further variations, the US HDTV
transmission standard. Whether the modulation is analogue, or a streams
of ones and zeros, many possible narrowband schemes are available.
The one attribute they all share in common, to greater
or lesser degrees, is a very small ratio of modulation envelope
bandwidth against the frequency of the carrier itself.
Shannon's information theory shows us that the ability
of a modulation scheme to resist noise and interference depends
strongly upon how much bandwidth is used to carry the information. The
wider the modulation bandwidth in relation to the modulation signal
bandwidth, the more resilient the signal becomes.
This idea did not go unnoticed, and by the 1950s spread
spectrum modulation schemes began to appear, intended to exploit this
The simplest spread spectrum technique is Direct
Spreading (DS), whereby the carrier wave is amplitude or phase
modulated with a pseudorandom (pseudo-noise or PN) sequence. A typical
arrangement is where a one or a zero is represented by a PN sequence,
or its inverse. The larger the number of bits (chips) in the PN
sequence, the greater the modulation bandwidth or spreading ratio and
the more resilient the modulation.
The basic alternative to DS is frequency hopping (FH),
whereby the carrier wave is pseudorandomly hopped in frequency, in a
manner analogous to direct spreading. As with direct spreading, the
larger the number of slots across which the carrier is hopped, the
greater the spreading ratio and resilience of the modulation.
Spread spectrum techniques have the very nice property
of being able to share bandwidth between multiple signals concurrently,
by using different PN codes for modulation. Providing these codes have
the important mathematical property of mutual orthogonality in a
cross-correlation operation, then multiple signals can occupy the same
bandwidth. This is called Code Division Multiple Access or CDMA. There
are of course no free lunches here, in the sense that only so many
orthogonal codes may be used concurrently before mutual interference
Security is also improved, since without knowing the
spreading code, you cannot demodulate the signal. The longer the
spreading code, the harder it is to guess it. Another nice property of
spread spectrum techniques is that the power density per bandwidth is
much lower than in narrowband schemes, in proportion to the spreading
ratio. With a very large spreading ratio the signal spectrum becomes
Both DS and FH are incorporated in the 802.11 wireless
LAN standards package, while DS is the basis of CDMA mobile telephony
and GPS satellite navigation.
As with narrowband schemes, hybrids have also become
very popular. The military JTIDS datalink, the backbone of US and NATO
battlefield digital networking, uses a combination of DS and FH
techniques to produce a highly jam resistant signal.
Much of the advantage of spread spectrum techniques over
narrowband techniques stems from their much greater ratio of modulation
bandwidth to signal bandwidth. If we consider also the advantages of
CDMA techniques, there are important gains to be made from using spread
spectrum over narrowband.
Despite this spread spectrum techniques have been slow
to penetrate into the vast commercial marketplace, with the most
important inroads only made in wireless LANs. The military have made
greater gains, but only through the need to deal with eavesdropping and
The great technological enabler for spread spectrum
techniques has been the Monolithic Microwave IC (MMIC), which enables
the complex and fast circuitry needed for spreading modulators and
receiver correlators to be manufactured affordably in large volumes.
Indeed, until recently the cost barriers were the reason why spread
spectrum was used mostly in military designs.
Narrowband techniques still rule the world, but the
advantages of spread spectrum will see it proliferate in coming years
to occupy increasing portions of the total market. Whether it manages
to ever wholly displace narrowband techniques remains to be seen. The
current regulatory models for allocating bandwidth have been structured
around narrowband techniques and hungrily carved up by broadcasters and
other users. Spread spectrum becomes prohibitively expensive to buy
bandwidth for in such a regime, despite its obvious advantages, and
requires a different regime for bandwidth allocation, one in which
codes in a shared band are sold, rather than chunks of the band itself.
Enter UWB techniques, which further extend the ideas
used in spread spectrum.
Ultra Wide Band
UWB techniques aim to increase the spreading ratios used
quite dramatically over established spread spectrum techniques, thereby
exploiting Shannon's model to a greater degree.
Narrowband techniques are inherently a carrier centred
approach, indeed in most such modulations the carrier wave itself is
discrete and is used as a reference signal for demodulating the
modulation signal. Spread spectrum techniques are also tied to a carrier
wave, even if almost all of the energy of the signal lies in the
modulation sidebands carrying the signal.
UWB techniques radically depart from this orthodoxy, as
they do not employ a carrier wave in the conventional sense.
Probably the best conceptual starting point for UWB is
to explore that favourite plaything of electrical engineers, the Dirac
impulse. Infinitely short, the impulse has an infinite bandwidth. It is
a mathematical abstraction which we cannot replicate in the physical
However, if we produce an extremely short pulse, in the
frequency spectrum it will spread its energy over a very large
bandwidth, which gets wider as the pulse gets shorter.
If we are aiming to exploit Shannon, we want as wide a
bandwidth as possible. The question which arises is of course, how to
produce a practical modulation scheme which can exploit the properties
of very short pulses, yet be implementable with affordable hardware.
Several obvious problems arise immediately:
Modulation is then imposed by shifting the monocycles in
time, to arrive either before or after a datum point in time, advanced
or delayed by a fixed increment in time. This is a direct equivalent to
the well known but infrequently used Pulse Position Modulation (PPM)
scheme, indeed it differs from the conventional primarily in the fact
that a burst of carrier wave has been replaced by a monocycle.
In a simple arrangement, an early pulse signals a 1 and
a late pulse a 0. This indeed can be used for transmitting a digital
message. What happens if two links try to occupy the same bandwidth ?
Obviously they interfere.
This is where some clever thinking has been applied by
Time-Domain, and their supporting researchers. Borrowing an important
idea from spread spectrum technology, they use a pseudorandom time delay
rather than a fixed one. In this manner, they combine the properties of
a spread spectrum signal, but achieve a much higher spreading ratio by
using monocycles rather than a continuous carrier wave.
Termed time hopping impulse modulation or time hopping
spread spectrum modulation, this technique combines the attributes of
conventional spread spectrum with those of impulse PPM. Figures 3 and 4
depict the modulation scheme.
By using a PN code modulation, and a correlation
receiver, Time Hopping Spread Spectrum (THSS) acquires the same
capability for code division multiplexing seen in established spread
spectrum techniques. The big difference lies in the additional
spreading effect produced by using a pulse modulation rather than
carrier wave modulation.
Implementation of hardware to perform THSS is not a
trivial chore by any measure.
The PulsON product currently being promoted by Time
Domain Corp uses a 16 bit delay word, to control the timing of a 500
picosecond (0.5 nanosec) monocycle, in a 100 nanosecond frame, with an
accuracy of 10 picoseconds, and required linearity in timing of 10
picoseconds. Figure 5 shows the system block diagram.
The chips containing the modulation hardware and
correlation hardware for the receiver had to be implemented in SiGe
heterojunction MMIC technology, to be capable of achieving such an
ambitious specification. A third chip is in development to provide the
low speed functions required for a complete low cost receiver.
The signal modulation bandwidths published are of the
order of 2 GHz or higher, resulting in spreading ratios of 50 dB for a
20 kilobit/s data rate.
To place this into context, the spreading ratio of
802.11 WLAN is around 10 dB, differing by a factor of 10,000 !
The enormous spreading ratio has two important
Other interesting consequences follow from THSS UWB.
Perhaps the most important one is the issue of multipath
propagation, a genuine plague in mobile telephony and WLANs, especially
indoors. Multipath arises when receivers see multiple, time delayed
copies of the same transmission, produced by reflections from objects
along the transmission path. The carrier wave seen by the receiver is a
sum of these components, which interfere and result in fading. Often
such fading can render a link unusable.
A UWB system using monocycles or wavelets does not
suffer this problem. This is for the simple reason that the monocycle
or wavelet occupies a length in free space of the order of inches.
Consecutive monocycles or wavelets are thus separated in space by tens
to a hundred feet, and any reflected examples will mostly arrive well
after the directly received monocycle or wavelet. Since a PN code is
being used, the correlator will see them as noise and ignore them.
The strategy adopted by Time Domain is common to
established spread spectrum designs, using a rake receiver with
multiple correlators. This ensures that should a destructive
cancellation arise due to multipath, the receiver can find another
signal to demodulate.
Radar and Rangefinding
The monocycle THSS scheme has numerous applications
other than telephony and WLANs. The very accurate timebase and short
duration of the pulses mean that short range GPS-like rangefinding and
positioning schemes with accuracies under a centimetre are feasible.
Indeed, Time Domain Corp papers elaborate on this in some detail.
Such technology has a huge range of industrial
applications, as well as automotive applications. Consider the joys of
Radar applications have also been studied intensively,
both for commercial, law enforcement, burglar alarm and military
applications. Some very ambitious proposals have been floated for UWB
radar, including the ability to image the shape of an object, the
ability to detect stealth aircraft and the ability to produce ultra high
resolution synthetic aperture radar imagery.
While the short range radar applications are feasible in
the near term, antenna bandwidth issues would suggest that any of the
more ambitious ideas will have to wait for some time yet, until antenna
technology can catch up.
UWB has sparked much controversy in various circles, and
is frequently accused of being snake-oil. This is an unfair criticism,
since the THSS scheme is really little more than a fusion of established
spread spectrum techniques, almost archaic pulse position modulation,
and very new monocycle generation techniques. Of the three core ideas
used in THSS, only one is untried, simply because the transistor and
MMIC technology to make it work did not exist until very recently.
UWB is anything but a mature technology, it is well and
truly in the transition from infancy to toddler status, with the first
products emerging in the market very soon.
As yet, the theoretical work on modulation techniques
has only uncovered the tip of the iceberg, much work remains to explore
variations on coding schemes, modulations and antenna technology. Until
this work is done, much of the potential of UWB will not be exploitable.
The MMIC technology for UWB needs a sustained production cycle to gain
Interference effects of UWB upon other signals,
especially GPS, are yet to be comprehensively analysed and a base of
empirical measurements gathered.
The biggest obstacles UWB will face are not
technological in nature. They will revolve around the regulatory issues
of bandwidth usage, and acceptance in a marketplace which has never
been a hotbed of technological radicalism. Indeed, the lukewarm
reception seen by conventional spread spectrum techniques, well
understood for decades, would suggest that UWB will see many hurdles
yet before it becomes a ubiquitous feature of the technological