| DS UWB - Direct sequence ultra wideband technology |
DS UWB - Direct sequence ultra wideband technology
- an overview or tutorial about the air interface for direct sequence (DS-UWB)
or impulse format for ultra wideband
Ultra wideband UWB is a revolutionary wireless technology
that enables data to be transmitted at speed well in excess of 100 Mbps. In view
of its capabilities it is likely to become a major presence in the wireless
communications industry.
DS UWB overview
DS UWB, direct sequence format for ultra wideband is often referred to as an
impulse, baseband or zero carrier technology. It operates by sending low power
Gaussian shaped pulses which are coherently received at the receiver. In view of
the fact that the system operates using pulses, the transmissions spread out
over a wide bandwidth, typically many hundreds of Megahertz or even several
Gigahertz. This means that it will overlay the bands and transmissions used by
more traditional channel based transmissions.
Each of the DS UWB pulses has an extremely short duration.
This is typically between 10 and 1000 picoseconds, and as a result it is shorter
than the duration of a single bit of the data to be transmitted. The short pulse
duration means that multipath effects can usually be ignored, giving rise to a
large degree of resilience in ultra wideband UWB transmissions when the signal
path is within buildings.
Energy density
In view of the wide bandwidth over which the DS UWB transmissions are spread,
the actual energy density is exceedingly low. In fact, many of the transmissions
themselves are less that the unintentional or spurious radiation levels from a
typical PC. Typically a DS UWB transmitter might transmit less than 75 nanowatts
per Megahertz. When integrated over the total bandwidth of the transmission, it
means that transmissions may only be around 0.25 milliwatts. This is very small
when compared to 802.11 transmissions that may be between 25 and 100 mW, or
Bluetooth that may be anywhere between 1 mW and 1 W.
This very low spectral density means that the DS UWB
transmissions do not cause harmful interference to other radio transmissions
using traditional carrier based techniques and operating in the existing bands.
Even in the bands that are likely to be more sensitive to interference such as
the Global Positioning System (GPS), it is possible to reduce the UWB
transmission power density levels even further to ensure that there is no
noticeable interference. However as GPS and other satellite based navigational
systems operate on very low received powers, UWB transmissions should not cover
the bands.
DS UWB modulation
There are a number of ways in which DS UWB transmissions can be modulated to
enable data to be carried. The strict power density limits placed on any UWB
transmissions by the FCC means that the form of modulation applied must be
efficient. It must provide the optimum error performance for a given level of
energy per bit. The choice of modulation also affects the UWB transmission
spectrum, and this must be taken into account to ensure that the spectral
density limits are not exceeded.
Two of the most popular forms of modulation used for DS UWB
are pulse position modulation (PPM), and binary phase shift keying (BPSK). These
provide the best performance for in terms of modulation efficiency and spectral
performance.
- Pulse position modulation PPM: This form of modulation
encodes the information by modifying the time interval and hence the
position of the pulses.
- Binary phase shift keying BPSK: This form of modulation
reverses the phase of the pulse to signify the data to be transmitted. This
is a 180 degree reversal. As the pulses consist of an initial upward or
downward voltage, this is easy to reverse. Looking at a pulse on an
oscilloscope it would appear that a pulse is either the right way up, or
upside down.
In addition to this direct sequence codes can be applied to
the transmission. This means that the received signal needs to be correlated
with the correct direct sequence code for it to be demodulated. This has the
advantage that only the intended receiver can demodulate it.
Summary
Although DS UWB, direct sequence ultra wideband is a new technology operating
using a totally different approach to the traditional carrier based
transmissions that are normally used today, UWB with its carrier free technology
offers the possibility of very high data rate transmissions using very low
power. As such it is a technology that cannot be ignored, and one which will
certainly take a significant section of the market.
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